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Patrick Turner Patrick Turner is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

Recently I worked on a Fender Deluxe Re-issue which makes 21Watts at clipping with 2 x 6V6 in output driven by 12AT7 LTP.

The input for clipping at signal input g1 pin2 'AT7= 2.5Vrms, and then you have 0.7Vrms of GNFB from Vo applied to the NFB input g2 pin7 'AT7. The amount of NFB applied 3dB, and makes almost no change to the sound if the 820r is removed to disconnect the GNFB.

The owner wanted variable NFB. So, I installed a linear 1k0 pot on amp rear with small knob. pot bottom goes to 0V, wiper goes to top of junction between 820r and 47r NFB network. 47r was removed, and replaced with 100r. Top of pot taken to Vo, ie, the whole of the single secondary winding on OPT.
The NFB can now be varied between ZERO and about 10dB max.
The center of pot gives the original amp's amount of NFB. But a problem occurs where GNFB exceeds 6dB, with the cabinet speaker connected as a load. Above 6dB NFB the amp breaks into HF oscillation once past the class A to AB threshold. It was to be expected because the Fender OPT is not much interleaved like a Hi-Fi OPT. The oscillations only occur as HF waves appearing on positive going peaks of Vo, and the amplitude grows larger as clipping is reached and the oscillations have amplitude larger than the LF part of waves even when over driven.

So I can only guess that in class B during cut off for one 6V6, the circuit changes to that of an HF oscillator.

The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to anode, placing small C from Vo to pot wiper for phase correction of NFB. None of these worked. Then I remembered the right solution, TWO R&C zobels across each 1/2 primary.
The RLa-a load on two 6V6 is about 7k nominally, so I put 4k7 & 2n2 in series network across each 1/2 primary. The pole between 4k7 and 2n2 = 15khz, so at the F which the amp caters for, 50Hz to 8kHz, the networks cause no loading loss of HF in the wanted region of BW.

So, no matter what sine wave or square wave F I applied at input, there was NO HF oscillations with 10dB FB, so problem solved.

HF oscillations in guitar amps may be more common in over driven output stages than anyone suspects.

With the RF present, a high level 200Hz tone becomes harsh, and you can hear the effective cut off and interrpution to sine waves while RF is present on wave peaks. With RF not present, high overdrive sounds "more grunty", as the amp is wrestling to force the speaker to comply with the applied signals.
with 20dB over drive, with NFB or none at all, sound is similar because it consists mainly of a series of square waves dominated by the lowest note played.
Unfortunately, there is no Master volume control to amplify the preamp signals which have been raised to a desirable level of distortion, and then be able t play the sound in small venues but with a distorty sound.

Now for a tube warmer.

Someone wanted a warmer sound for their guitar amp, so I suggested they have 3 cascaded stages with a 1/2 12AU7, so 3 gain stages and a CF at output is possible. You want warmth? Well, methinks that means chocolatizing the signal with high levels of harmonics produced **without any stage clipping**.
(( nobody has offered any funds for the warmer, but the recipe here is free ))

How? OK, have a normally biased 1/2 12AU7 with say 22k DC carrying anode load and B+ supply about 200V, so Ia = 3.0mAdc. Ea = 130Vdc. Bypass the cathode resistor. Following grid in next stage has 0.27uF and 390k coupling.
Input from a gain control can produce about 42Vrms at onset of grid current and cut off. At onset of g1 I and Co, 2H may be 5% and 3H 2% and other H in declining %.
The voltage gain g1 to a = 11 approx. The maximum g1 input could be approx 4.2Vrms, and easily produced usually by the guitar amp stage just previous to the power amp section.
Now consider you have THREE such stages in series, ( cascaded ). BUT, instead of ending up with gain = 11 x 11 x 11 = 1,331, you use the 390k in a divider network with 11 : 1 signal reduction, using 390k + 39k, and then the gain from g1 to top of 39k is about 1.0. You can have 3 stages in series and still get overall gain of 1. Having 10 stages might be tricky, because if gain is say 1.5, or 0.5, after 10 stages the V0 is either too large of too low.
One thing is for sure, the sound of un-clipped devices becomes very obvious,
and I leave you all to ponder just what that might be like.

Max Vo after 3 stages is taken from top of last 39k and buffered with CF. So max Vo = Vin and perhaps just an input control is all that is needed.

Patrick Turner.
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Guitar Amp RF oscillation, tube warmer recipe


"Patrick Turner"

The center of pot gives the original amp's amount of NFB. But a problem
occurs where GNFB exceeds 6dB, with the cabinet speaker connected as a load.
Above 6dB NFB the amp breaks into HF oscillation once past the class A to AB
threshold. It was to be expected because the Fender OPT is not much
interleaved like a Hi-Fi OPT. The oscillations only occur as HF waves
appearing on positive going peaks of Vo, and the amplitude grows larger as
clipping is reached and the oscillations have amplitude larger than the LF
part of waves even when over driven.

So I can only guess that in class B during cut off for one 6V6, the circuit
changes to that of an HF oscillator.

The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to
anode, placing small C from Vo to pot wiper for phase correction of NFB.
None of these worked. Then I remembered the right solution, TWO R&C zobels
across each 1/2 primary.

** Take a look at the schems on this page for Deluxe Reverb:

http://www.webphix.com/schematic%20h...om/fender.html

Many of them have 1200pF caps from grids to ground on the 6V6s.

I believe this is for the same problem that happens with some combinations
of OT and speakers.

Using caps across the OT primary is dodgy, high voltage ( ie 2 kV ) ceramics
will survive - 600V film caps will not.

275VAC caps " class X " caps should be OK too.



..... Phil


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Patrick Turner Patrick Turner is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

So I can only guess that in class B during cut off for one 6V6, the circuit
changes to that of an HF oscillator.

The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to
anode, placing small C from Vo to pot wiper for phase correction of NFB.
None of these worked. Then I remembered the right solution, TWO R&C zobels
across each 1/2 primary.

** Take a look at the schems on this page for Deluxe Reverb:

http://www.webphix.com/schematic%20h...om/fender.html

Many of them have 1200pF caps from grids to ground on the 6V6s.

I believe this is for the same problem that happens with some combinations
of OT and speakers.

Using caps across the OT primary is dodgy, high voltage ( ie 2 kV ) ceramics
will survive - 600V film caps will not.

275VAC caps " class X " caps should be OK too.

..... Phil

The Deluxe Reverb is a re-issue. In the amp here there are no 1n2 caps between 6V6 g1 and 0V. None on the schematic I have.

I strongly agree caps across 1/2 primaries are a dodgy thing to do.
***BUT***, I have 4k7 5W resistors in series with 2n2 which ARE ceramic and 2kV rated, and muso can now blast away all day and not have tubes making ultra high F oscillations.

Having 1n2 between 6V6 grids and 0V means that output from 12AT7 anodes is effectively shunted and if AT7 anode Rout = 12k, Ra//RLa//Rg, approx, then expect HF pole at 11kHz. But above this F, there is increasing open loop phase
shift making oscillations still likely with lowish levels of GNFB.
Other methods used include a small value cap used between 6V6 anodes and g1, but this also causes ultimate extra phase shift, and only suits old radios where GNFB is frowned upon with a single 6V6 left to muddle away vainly to make horrid sound once above a whisper level.

Using the pair of Zobel networks with 4k7&2n2 gives a pole at 15kHz and above this F the load seen by 6V6 becomes a pure resistance, 4k7, with less phase shift than otherwise. 6V6 tetrode gain is thus curtailed in region where the speaker offers virtually no loading due to its inductive property at HF. Using a Zobel at OPT sec does not stop the problem because the OPT has high LL and when loaded with secondary R load the 6V6 still decouples from sec because of the effectively in series leakage inductance.

Square wave testing reveals a lot about amplifiers. I like to use a 5kHz sq..wv, and have output set to say 1V, low level at first and where OP tube class A operation means the open loop gain is highest. Once some GNFB is added, the waves at OP tube anodes can have far higher overshoots on square waves than at the secondary output. Using higher levels in class B working may bring bursts of HF oscillations on peaks.

High levels of LF, say 100Hz sine waves can cause the same HF crap,
and if very low F at say 10Hz are tried, and OPT saturates then chances are this will also cause bursts of HF oscillations. If the OP tubes overheat and anodes turn red - not at all an unknown phenomena in a guitar amp - then tubes can break into HF oscillations during their self destruction mode of operation, so ALWAYS there must be some way of stopping very high oscillations. And in all amps, not just guitar amps.
The Watt ratings for R in Zobels don't have to be equal to power produced by tubes because their presence STOPS the HF ever becoming present and there is so little average audio signal power above 5kHz.
In old books about old amps, sometimes adjustable spark gaps were recommended, with two pointed screws facing each other and each taken to each PP anode. Two much Vac = Vac arcing, ( NOT DC arcing ) and thus excessive voltage at anodes is controlled somewhat, but methinks the Zobel is better. Some guitar amps have used say 3 x 1N4007 in series from each anode to 0V. Diode anodes are to tube anodes, and if either anode tries to make a huge negative going voltage higher than the B+ value, then diodes conduct, thus shunting the stored energy in leakage inductance and protecting insulation from Va maybe 3 times B+ if signal is turned up with no loading.
But this "excessive Va" function is distinct from the HF oscillations in class B when there is GNFB and the is no loading above say 20kHz.

Patrick Turner.
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Guitar Amp RF oscillation, tube warmer recipe


"Patrick Turner"

" The Deluxe Reverb is a re-issue."


** But no Fender amp is called that.

Kindly post the name printed on the face plate.



..... Phil




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Lord Valve Lord Valve is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

I really hate to give you poor ******s a clue, but...please tell me where
R60 and R61 are *physically* located in your particular unit.

LV



Patrick Turner wrote:

So I can only guess that in class B during cut off for one 6V6, the circuit
changes to that of an HF oscillator.

The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to
anode, placing small C from Vo to pot wiper for phase correction of NFB.
None of these worked. Then I remembered the right solution, TWO R&C zobels
across each 1/2 primary.

** Take a look at the schems on this page for Deluxe Reverb:

http://www.webphix.com/schematic%20h...om/fender.html

Many of them have 1200pF caps from grids to ground on the 6V6s.

I believe this is for the same problem that happens with some combinations
of OT and speakers.

Using caps across the OT primary is dodgy, high voltage ( ie 2 kV ) ceramics
will survive - 600V film caps will not.

275VAC caps " class X " caps should be OK too.

.... Phil

The Deluxe Reverb is a re-issue. In the amp here there are no 1n2 caps between 6V6 g1 and 0V. None on the schematic I have.

I strongly agree caps across 1/2 primaries are a dodgy thing to do.
***BUT***, I have 4k7 5W resistors in series with 2n2 which ARE ceramic and 2kV rated, and muso can now blast away all day and not have tubes making ultra high F oscillations.

Having 1n2 between 6V6 grids and 0V means that output from 12AT7 anodes is effectively shunted and if AT7 anode Rout = 12k, Ra//RLa//Rg, approx, then expect HF pole at 11kHz. But above this F, there is increasing open loop phase
shift making oscillations still likely with lowish levels of GNFB.
Other methods used include a small value cap used between 6V6 anodes and g1, but this also causes ultimate extra phase shift, and only suits old radios where GNFB is frowned upon with a single 6V6 left to muddle away vainly to make horrid sound once above a whisper level.

Using the pair of Zobel networks with 4k7&2n2 gives a pole at 15kHz and above this F the load seen by 6V6 becomes a pure resistance, 4k7, with less phase shift than otherwise. 6V6 tetrode gain is thus curtailed in region where the speaker offers virtually no loading due to its inductive property at HF. Using a Zobel at OPT sec does not stop the problem because the OPT has high LL and when loaded with secondary R load the 6V6 still decouples from sec because of the effectively in series leakage inductance.

Square wave testing reveals a lot about amplifiers. I like to use a 5kHz sq.wv, and have output set to say 1V, low level at first and where OP tube class A operation means the open loop gain is highest. Once some GNFB is added, the waves at OP tube anodes can have far higher overshoots on square waves than at the secondary output. Using higher levels in class B working may bring bursts of HF oscillations on peaks.

High levels of LF, say 100Hz sine waves can cause the same HF crap,
and if very low F at say 10Hz are tried, and OPT saturates then chances are this will also cause bursts of HF oscillations. If the OP tubes overheat and anodes turn red - not at all an unknown phenomena in a guitar amp - then tubes can break into HF oscillations during their self destruction mode of operation, so ALWAYS there must be some way of stopping very high oscillations. And in all amps, not just guitar amps.
The Watt ratings for R in Zobels don't have to be equal to power produced by tubes because their presence STOPS the HF ever becoming present and there is so little average audio signal power above 5kHz.
In old books about old amps, sometimes adjustable spark gaps were recommended, with two pointed screws facing each other and each taken to each PP anode. Two much Vac = Vac arcing, ( NOT DC arcing ) and thus excessive voltage at anodes is controlled somewhat, but methinks the Zobel is better. Some guitar amps have used say 3 x 1N4007 in series from each anode to 0V. Diode anodes are to tube anodes, and if either anode tries to make a huge negative going voltage higher than the B+ value, then diodes conduct, thus shunting the stored energy in leakage inductance and protecting insulation from Va maybe 3 times B+ if signal is turned up with no loading.
But this "excessive Va" function is distinct from the HF oscillations in class B when there is GNFB and the is no loading above say 20kHz.

Patrick Turner.







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John L Stewart John L Stewart is offline
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Quote:
Originally Posted by Patrick Turner View Post
Recently I worked on a Fender Deluxe Re-issue which makes 21Watts at clipping with 2 x 6V6 in output driven by 12AT7 LTP.

The input for clipping at signal input g1 pin2 'AT7= 2.5Vrms, and then you have 0.7Vrms of GNFB from Vo applied to the NFB input g2 pin7 'AT7. The amount of NFB applied 3dB, and makes almost no change to the sound if the 820r is removed to disconnect the GNFB.

The owner wanted variable NFB. So, I installed a linear 1k0 pot on amp rear with small knob. pot bottom goes to 0V, wiper goes to top of junction between 820r and 47r NFB network. 47r was removed, and replaced with 100r. Top of pot taken to Vo, ie, the whole of the single secondary winding on OPT.
The NFB can now be varied between ZERO and about 10dB max.
The center of pot gives the original amp's amount of NFB. But a problem occurs where GNFB exceeds 6dB, with the cabinet speaker connected as a load. Above 6dB NFB the amp breaks into HF oscillation once past the class A to AB threshold. It was to be expected because the Fender OPT is not much interleaved like a Hi-Fi OPT. The oscillations only occur as HF waves appearing on positive going peaks of Vo, and the amplitude grows larger as clipping is reached and the oscillations have amplitude larger than the LF part of waves even when over driven.

So I can only guess that in class B during cut off for one 6V6, the circuit changes to that of an HF oscillator.

The solutions tried were 15r & 0.27uF across Vo, 680pF & 8k2 across anode to anode, placing small C from Vo to pot wiper for phase correction of NFB. None of these worked. Then I remembered the right solution, TWO R&C zobels across each 1/2 primary.
The RLa-a load on two 6V6 is about 7k nominally, so I put 4k7 & 2n2 in series network across each 1/2 primary. The pole between 4k7 and 2n2 = 15khz, so at the F which the amp caters for, 50Hz to 8kHz, the networks cause no loading loss of HF in the wanted region of BW.

So, no matter what sine wave or square wave F I applied at input, there was NO HF oscillations with 10dB FB, so problem solved.

HF oscillations in guitar amps may be more common in over driven output stages than anyone suspects.

With the RF present, a high level 200Hz tone becomes harsh, and you can hear the effective cut off and interrpution to sine waves while RF is present on wave peaks. With RF not present, high overdrive sounds "more grunty", as the amp is wrestling to force the speaker to comply with the applied signals.
with 20dB over drive, with NFB or none at all, sound is similar because it consists mainly of a series of square waves dominated by the lowest note played.
Unfortunately, there is no Master volume control to amplify the preamp signals which have been raised to a desirable level of distortion, and then be able t play the sound in small venues but with a distorty sound.

Patrick Turner.
Perhaps I missed it but what is the frequency of the HF you see? Is it RF?

Good practice in any audio amp is something like 10K, 1/2W non-inductive attached as close as possible to the G1 terminal on the socket. And 100R to 500R in the same way to each G2 lead on the power tubes.

That swamps the Q of the parasitic RF cct formed by the various tubes. Thus, RF is zapped but audio survives in all its glory!

Cheers to all, John L Stewart
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Guitar Amp RF oscillation, tube warmer recipe


"Lord Valve"

I really hate to give you poor ******s a clue, but...please tell me where
R60 and R61 are *physically* located in your particular unit.



** Since most Fender amps have no component numbers LV has decided that PT
has one of these:

" 65 Deluxe Reverb-Amp " sub titled "reissue".

http://www.webphix.com/schematic%20h...erb_manual.pdf

The guts of which look like this:

http://www.pnelectronics.com.au/Uplo...reissue002.jpg

R60 & R61 are 1.5kohm "grid stoppers" fitted on the PCB.

Be dead easy to move them to the 6V6 octal sockets and straddle them across
pins 5 and 1.

Like Fender normally did in the past.



.... Phil




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Patrick Turner Patrick Turner is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

Phil said....

" The Deluxe Reverb is a re-issue."
** But no Fender amp is called that.
Kindly post the name printed on the face plate.
..... Phil

The front plate on amp chassis has Deluxe Reverb in written white lettering.
Silver cloth face has "Fender"
Black cabinet, 12" speaker.
The schematic has '65 Deluxe Reverb' and created and plotted dates on schematic are February and July 1993.

Lord valve calls us all "poor ******s" and I shall leave him confused.

Amp has wear and tear and looks 20 years old, but it sure ain't from 1965.

John Stewart asks me what was the F of the RF oscillations. I don't know,
I didn't measure the F, but appeared to be over 50kHz, or "super sonic"
aka low RF. HF oscillations in audio amps with NFB can be from say 10kHz to 5MHz, depending the amp type and devices and how the circuit is when such oscillations are measured.

The Fender here did not appear to oscillate at RF, super sonic, whatever, when the original 820r + 47r GNFB network was in place and which gives 2dB GNFB.

This tiny amount of NFB does hardly anything to flatten the signal at the speaker. The signal response at has a 6dB peak at 100Hz and +6dB at 2kHz,
relative to 1kHz level. So the Vo merely outlines the speaker Z which is to be expected because the amp Rout is a much higher ohm value than the speaker at 300Hz where speaker Z is the minimum, probably about 8r.

Phil implies moving grid stoppers 1k5 at PCB to tube sockets might do something, because Fender did it in the past, but such a move would do SFA.
Maybe **increasing** grid stoppers to say 4k7, 6k8, 10k0 *might* have an effect.
But what I done loads the 6V6 with 9.4k a-a above about 30kHz, thereby stopping the oscillations.

Possibly this amp could be converted to use EL34, with adjustment of bias and the connection of pin 1 to 8 so suppressor grids are biased at earthy volts.
I dunno if it would make the perceived harsh tone of this particular amp sound any better. Perhaps an Alnico magneted speaker would be better. I have no clue what further mods I could might improve tone. I need to move on to other projects.

BTW, many amps use pin 6 ( not used in octal power tubes ) to bring feed from driver stage coupling caps to OP tubes, AND also the grid bias voltage. Then the stopper R is between pin 6 and
grid pin 5.

If pin 1 is used instead of pin 6, and Summ Eadiot plugs in EL34, watch for smoak, as results could be uncertain, because g3 are being driven like g1.

As the old sage mite siegh, Cood be Farr Khan interress sting.

Patrick Turner.

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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Guitar Amp RF oscillation, tube warmer recipe


"Patrick Turner"
Phil said....

" The Deluxe Reverb is a re-issue."

** But no Fender amp is called that.
Kindly post the name printed on the face plate.

The front plate on amp chassis has Deluxe Reverb in written white
lettering.



** Not " Deluxe Reverb-Amp " ???


Silver cloth face has "Fender"
Black cabinet, 12" speaker.
The schematic has '65 Deluxe Reverb' and created and plotted dates on
schematic are February and July 1993.



** So yes - a "Deluxe Reverb-Amp".


Amp has wear and tear and looks 20 years old, but it sure ain't from 1965.


** Those 3 PCBs are a dead give away.


This tiny amount of NFB does hardly anything to flatten the
signal at the speaker. The signal response at has a 6dB peak
at 100Hz and +6dB at 2kHz, relative to 1kHz level. So the
Vo merely outlines the speaker Z which is to be expected..


** Err - no

Regular guitar ( and most other) speakers don't double impedance with a one
octave change in F above 1kHz.

It takes 2 octaves, at least.



Phil implies moving grid stoppers 1k5 at PCB to tube sockets might do
something, because Fender did it in the past, but such a move would do
SFA.
Maybe **increasing** grid stoppers to say 4k7, 6k8, 10k0 *might* have an
effect.


** What ever it takes....

But what I done loads the 6V6 with 9.4k a-a above about 30kHz, thereby
stopping the oscillations.


** A simple zobel at the speaker output would do as well.


Possibly this amp could be converted to use EL34,


** Sacrilege !!!

Conversion to use Sovtek 5881s is OK, though.

Otherwise, using JJ brand 6V6s is practically mandatory.

SFA difference between the two tubes - really.



.... Phil





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jh[_3_] jh[_3_] is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

On 06.03.2014 10:01, Patrick Turner wrote:
Phil said....

" The Deluxe Reverb is a re-issue."
** But no Fender amp is called that.
Kindly post the name printed on the face plate.
..... Phil

The front plate on amp chassis has Deluxe Reverb in written white lettering.
Silver cloth face has "Fender"
Black cabinet, 12" speaker.
The schematic has '65 Deluxe Reverb' and created and plotted dates on schematic are February and July 1993.

Lord valve calls us all "poor ******s" and I shall leave him confused.

Amp has wear and tear and looks 20 years old, but it sure ain't from 1965.

John Stewart asks me what was the F of the RF oscillations. I don't know,
I didn't measure the F, but appeared to be over 50kHz, or "super sonic"
aka low RF. HF oscillations in audio amps with NFB can be from say 10kHz to 5MHz, depending the amp type and devices and how the circuit is when such oscillations are measured.

The Fender here did not appear to oscillate at RF, super sonic, whatever, when the original 820r + 47r GNFB network was in place and which gives 2dB GNFB.

This tiny amount of NFB does hardly anything to flatten the signal at the speaker. The signal response at has a 6dB peak at 100Hz and +6dB at 2kHz,
relative to 1kHz level. So the Vo merely outlines the speaker Z which is to be expected because the amp Rout is a much higher ohm value than the speaker at 300Hz where speaker Z is the minimum, probably about 8r.

Phil implies moving grid stoppers 1k5 at PCB to tube sockets might do something, because Fender did it in the past, but such a move would do SFA.
Maybe **increasing** grid stoppers to say 4k7, 6k8, 10k0 *might* have an effect.
But what I done loads the 6V6 with 9.4k a-a above about 30kHz, thereby stopping the oscillations.

Possibly this amp could be converted to use EL34, with adjustment of bias and the connection of pin 1 to 8 so suppressor grids are biased at earthy volts.
I dunno if it would make the perceived harsh tone of this particular amp sound any better. Perhaps an Alnico magneted speaker would be better. I have no clue what further mods I could might improve tone. I need to move on to other projects.

BTW, many amps use pin 6 ( not used in octal power tubes ) to bring feed from driver stage coupling caps to OP tubes, AND also the grid bias voltage. Then the stopper R is between pin 6 and
grid pin 5.

If pin 1 is used instead of pin 6, and Summ Eadiot plugs in EL34, watch for smoak, as results could be uncertain, because g3 are being driven like g1.

As the old sage mite siegh, Cood be Farr Khan interress sting.

Patrick Turner.



Hi Patrick,

You're sure the tiny PT of the Fedner can handle the heater current of
EL34s?

I doubt it

regards

Jochen


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Default Guitar Amp RF oscillation, tube warmer recipe


"jh"

Hi Patrick,

You're sure the tiny PT of the Fedner can handle the heater current of
EL34s?


** 6V6s are speced at 0.45amp and EL34s at 1.5 amp.

I know that 5881s are fine and they pull 0.9 amp each.

A couple of watts of extra heat in the PT is not gonna kill it.

Long as the heater voltage is above 6.1V, it should be OK.



..... Phil



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Patrick Turner Patrick Turner is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

Snip.....

I said...
Amp has wear and tear and looks 20 years old, but it sure ain't from 1965..


You said...** Those 3 PCBs are a dead give away.

Well yes, but it is know as a Fender "Deluxe Reverb" reissue.
The full name on schema is "Deluxe Reverb Amplifier". No need to nit pick Phil.

This tiny amount of NFB does hardly anything to flatten the
signal at the speaker. The signal response at has a 6dB peak
at 100Hz and +6dB at 2kHz, relative to 1kHz level. So the
Vo merely outlines the speaker Z which is to be expected..


** Err - no
Regular guitar ( and most other) speakers don't double impedance with a one
octave change in F above 1kHz.

It takes 2 octaves, at least.

OK,at bass F the speaker has a peak in response at around 100Hz and caused by its Fs, due to RESONANT behavior and speaker becomes equivalent to an R damped parallel LC network.
The rise in Z above the minimum Z at about 300Hz is due to increasing XL in series with RL of speaker, and the eventual increases in ZL IS 6dB per octave. The amp with such low NFB and with 6V6 in tetrode is a virtual current source because amp Rout is far more ohms than the speaker minimum Z. So hence the current applied to speaker remains fairly constant and because Z changes, so does the speaker voltage, and the response outlines the ZLs approximately.
Tetrodes like 6V6 need a lot more GNFB if a flat response was to be expected. But in a Guitar amp flatness = Dull & Boring sound, artistic crap quality, like a view on the sound in black and white and no color.

So, the Fender amp with its speaker acts like a tone control with useful
boost at around 100Hz, and above 1kHz. Nobody complains about it, they like it.
The NFB pot I added can increase GNFB to 8dB max which DOES make a change to sound of a tone from a sig gene, and does slightly flatten the response because of reduced Rout. With ZERO GNFB, the sound is kinda sloppy, could be irritating to many ppl, but some may like that. With max FB the sound becomes sort of firm, and maybe liked/disliked, and when driven deep into overload, there's not huge sound change because the signal is a mass of clipped square waves, and twidding the NFB control does nothing much when much overdriven.

But below overdrive, there is considerable sound change.

Using more FB increases the clipping threshold of OP stage so that instead of needing 2.5Vrms input to power amp you need about 8Vrms which means slightly more THD is produced by stages upstream of the power amp.



Phil implies moving grid stoppers 1k5 at PCB to tube sockets might do
something, because Fender did it in the past, but such a move would do
SFA.
Maybe **increasing** grid stoppers to say 4k7, 6k8, 10k0 *might* have an
effect.


** What ever it takes....

But what I done loads the 6V6 with 9.4k a-a above about 30kHz, thereby
stopping the oscillations.


** A simple zobel at the speaker output would do as well.

I tried that, various R and C values, but OPT leakage inductance is so high that OP tubes are not loaded enough at high F to stop oscillations.
It is much better to have critical damping Zobel networks across each 1/2 primary in ALL tube amps where the OPT was designed for LOW bandwidth needed by musicians with electric guitars.

I've not measured the LL, but many years of fixing crappy amps tells me the Fender OPT must have little interleaving, and it probably has just two primary windings with a central single secondary winding, P-S-P interleaving pattern.
Interleaving is generally only used where it needs to be - in good quality hi-fi amps which should have 65kHz BW with a designed resistance load, and be stable unconditionally with no load at all, or with any speaker ever made or with any combination of L, C and R that anyone can devise. To attain this standard for a 22W amp the OPT should weigh twice what is in the Fender and have at least a P-S-P-S-P pattern, with S-P-S-P-S-P-S being best, but mass producing such OPTs makes them cost $10 more, and this would ruin many hi-fi amp makers and they get a marketing BS artist to lie instead about the amp.

Possibly this amp could be converted to use EL34,


** Sacrilege !!!

Why? many ppl like EL34. Musicians are usually Heathens, and follow no God.
They spend as many years as their body will take at getting gigs, getting laid, getting ****ed, avoiding divorce costs, and enjoying the sound, until their ears begin to play up real bad. So they don't much give a stuff what tubes are used, as long as the amp with its speakers makes the magic in whatever circumstances and venue they find.

Conversion to use Sovtek 5881s is OK, though.

Yeah, probably quite OK, biasing is the same, and tubes with the Ea at only 400V approx should last very well. Even NOS 6L6 might survive with 300Vdc screen rating. But 6L6GC, 5881, yeah, sure.

Otherwise, using JJ brand 6V6s is practically mandatory.

SFA difference between the two tubes - really.

Well, not really. 5881 are virtually same as 6L6GC, but have twice the Pda rating than old style 6V6, which I don't recommend in this Fender. But the JJ 6V6 are sort of a poor man's 6L6, and have higher Pda rating and from what I have seen they are good versions of the 6V6.
The Ra, gm of JJ6V6 and Russian 5881/6L7GC are indeed similar and not many ppl would notice the difference in sound. The 5881 would be able to swing anode voltage slightly lower in Fender and thus produce slightly more Po. But if the speaker was changed to lower Z the 5881 will swing the Ea much lower than 6V6 and you get probably 40Watts at clipping, class AB1. But it seems ppl have this devotion to small output tubes because of the sound before overdrive, and because the sound with overdrive is good, but not too loud for some venues.

But I dare to generalize. I once knew a young bloke in a Dark Metal band. He wanted an extra 12AX7 gain stage in a Marshall, and he wanted the pair of 6550 converted from tetrode to triode operation. OK, too easy. When he brought his guitar to try it, I put ear plugs in and ear muffs on, and sound was like a continuous number of 747s crashing on my shed. Ah, Such Music! Such tone! such magic!, and I just agreed and smiled when I got paid, which was a wonder.
Patrick Turner.


.... Phil
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Guitar Amp RF oscillation, tube warmer recipe


"Patrick Turner"

I said...
Amp has wear and tear and looks 20 years old, but it sure ain't from 1965.


You said...** Those 3 PCBs are a dead give away.

Well yes, but it is know as a Fender "Deluxe Reverb" reissue.
The full name on schema is "Deluxe Reverb Amplifier". No need to nit pick
Phil.

** Fender models go by their face plate names.

There IS a "Deluxe Reverb" and also there IS a " Deluxe Reverb-Amp".

Hundreds of models exist with only small name variations between them - eg
there are distinct models called " Twin Reverb", "Twin Reverb-Amp" and "The
Twin" which even owners mix up all the time.

No nit picking happening on my part.


This tiny amount of NFB does hardly anything to flatten the
signal at the speaker. The signal response at has a 6dB peak
at 100Hz and +6dB at 2kHz, relative to 1kHz level. So the
Vo merely outlines the speaker Z which is to be expected..


** Err - no

Regular guitar ( and most other) speakers don't double impedance with a one
octave change in F above 1kHz. It takes 2 octaves, at least.


The rise in Z above the minimum Z at about 300Hz is due to
increasing XL in series with RL of speaker, and the eventual
increases in ZL IS 6dB per octave.


** Not so.

You need to do an impedance test and stop assuming.

The *soft* iron pole pieces of a speaker magnet have LARGE eddy current
losses and so the impedance rises much less rapidly with F than if
transformer lams were used.

Look at almost ANY published speaker impedance curve :

http://www.electrosmash.com/marshall-mg10

Bottom of page:

7.5ohms at 250 Hz ( = minimum) then

9ohms at 1 kHz

18ohms at 5.5 kHz

36ohms at 16 kHz



.... Phil



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Patrick Turner Patrick Turner is offline
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Default Guitar Amp RF oscillation, tube warmer recipe

Phil explained to me that :-

""""You need to do an impedance test and stop assuming.
The *soft* iron pole pieces of a speaker magnet have LARGE eddy current
losses and so the impedance rises much less rapidly with F than if
transformer lams were used.
Look at almost ANY published speaker impedance curve :
http://www.electrosmash.com/marshall-mg10
Bottom of page:
7.5ohms at 250 Hz ( = minimum) then
9ohms at 1 kHz
18ohms at 5.5 kHz
36ohms at 16 kHz

Indeed Phil. I forgot the iron effect, but if you extend the graph at the above address to a higher F and then plot a 20dB/decade line, you can work out that
there is possibly a 0.26mH inductance in series with 7.5 ohms resistance.
So the curve tries t eventually become a line with 6dB / octave slope.
But not between 250Hz and say 1kHz. Rate of Z increase is less than the rate at say between 2.5kHz and 10kHz.

The 6V6 amp output tubes have around 100k anode to anode Ra/ OPT has about typical 7k0 : 8 ZR, 875:1, so the 100k appears at Vo as 114 ohms and because this is 15.2 times higher than 7.5r typical minimum speaker Z then amp is a current source, NOT a voltage source, therefore voltage response across speaker
terminals follows the impedance curve - approximately. The 2dB of NFB does not do much to reduce the amp Rout, and Fender knows all about this. They WANT the speaker to give a bit of boost like a defacto loudness control - boosted highs and lows.
So when lots of series voltage NFB is used, this defacto tone control vanishes because the speaker voltage is held constant at all F because amp Rout might become only 2 ohms with enough NFB. Then many ppl say the sound becomes dull and lifeless. Its OK not having much FB, because the amp and its speaker are part of the instrument which the artist uses to wow an audience, so anything goes with regard to response contours and THD and IMD and noise..

Somewhere along the line of activities, artistic talent comes into play, and some musos get far bigger audiences than others, and make many hits and lotsa dough and despite having bloomin very ordinary sounding amps.
Fact is there ain't many ppl like George Benson, Paul McCartney, or even Mozart born every day.
I wonder what Beethoven would have thought about a Fender Deluxe Reverb had been given one, ( with a convenient power supply from the local windmill). Could have changed the course of musical evolution, and just where would we be now? But possibly the powers that were in Vienna might have sent around a bloke to stop all the damn noise. Trubble wooda bin that Beeto wooda given the bloke a beer, and bloke wooda found he liked twangy rock and roll. So Beeto wooda lived on.
Beeto mighta pulled out a 12AX7 to make himself a hearing aid.

Patrick Turner.




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