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John L Stewart John L Stewart is offline
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Smile Power Frequency Ripple Reduction in B+ Supplies

This note describes a simple, low cost power supply improvement. It allows reduction of the power frequency ripple component (60 Hz in North America & 50 Hz in Europe) by up too 20 db. The problem this modification resolves occurs only in center tapped, full wave rectifier circuits. The conditions described are not found in a full bridge configuration. The experimenter needs to be extremely careful. Deadly voltages are present.

The test bed I used for this modification is an amplifier I built in the early 1960's. It uses a Hammond 273BZ power transformer. Nominal HV is 375 volts each side of center tap. The choke is a Hammond 156R, which along with the 35-microfarad capacitor forms the filter section at the output of the capacitor input rectifier. Load used during testing is a 5k, 50 watt resistor.

We would assume that the ripple would be predominantly even order harmonics ( 2nd, 4th, 6th, etc. However, because power transformers are wound serially, one side of the high voltage winding has a higher resistance then the other. This results in a significant ripple at the power frequency.

The LC filter network following the rectifier is only one-quarter (-12 db) as effective at the power frequency as at main ripple frequency. The DC available at the filter output could contain ripple at the power frequency as large or greater then that of the main ripple frequency.

All of this is of little importance to pushpull amplifiers since they have large immunity to power supply ripple (see Note). However, the single ended amplifier & particularly the triode amplifier (SET & SEUL) are adversely affected.

Depending on the loading conditions of a triode power stage, from 2/3 to 4/5 of this ripple voltage will be applied to the load. Unfortunately, most loudspeaker systems have one or two
resonance’s below 100 HZ. One of these could be at or near the power frequency. Presto!! We have objectionable hum in the listening room.

Figure One illustrates an ordinary center tapped full wave power supply. Two sides of the HV winding measured 38 & 40 ohms. Four resistors have been added. The purpose of R1, R2 & R3 is to apply a small AC voltage in series with the DC output voltage at the break point B1. By selecting the polarity the connection provides series aiding for the high resistance side of the HV winding & series bucking for the low resistance part. R4 is a current sampling resistor used during adjustment. The large dots shown on the transformer secondaries are polarity indicators.

Adjustment for optimum results is made by adjusting R2 while monitoring the current charging pulses with a scope at test point C ( TPC ). R2 must be wirewound since the current pulses are large. I used a Clarostat A4320. Adjust for equal size of the current pulses. An example of how they should appear is shown in Figure 2.

To get consistent information from the test scope it is important to use line triggering. The power cord of the test power supply should be polarized. A X10 probe AC coupled at the scope will keep the trace on the screen.

The diagram shows a solid state rectifier. I used an octal replacement unit from New Sensor Corp for these trials. Several vacuum rectifiers were tried with varying results. Their heaters were connected to the 5 volt winding in the normal way. Socket pin connections are shown.

Similar results could be had by using the 6.3 volt winding. In that case the mod could be inserted at break point B2. These same fixes could be applied to a power supply using a choke input filter.

C3 & C4 are ceramic & must be rated for direct connection to the power line. Mallory makes a capacitor approved by Underwriters Labs for this service. The part number is UN103M.

The ripple components at the input & output of the filter section were measured using an HP302A Wave Analyzer. The results in Table One show the reduction in the 60 Hz component. Higher odd order harmonics are also reduced.

Figure 3 is an FFT comparison of the output of the power supply before and after adjustment. The lower trace shows an improvement of about 20 db of the power frequency ripple.

For some there is an even easier fix. If the power supply uses a directly heated rectifier such as a 5R4WGB then it is possible to simply reverse the 5 volt heater transformer leads. No extra parts are needed. The difference will be very obvious when looking at the trace at test point C. The FFT measured about 15 db improvement. This is shown in Figure 4. By the laws of chance 1/2 of these rectifiers will already be properly connected.

Note- Since doing this work in 1996/1997 I’ve had a closer look at some PP amps. If the PS is unfiltered then 120Hz sidebands are clearly visible on the test signals used. This could explain the so-called mudiness, Etc in the rendering of program by some amplifiers. Some reputed to be high class at that. For those spectrums I was using a Pico Technology ADC-216, so 96 db resolution.


More- A simpler fix is possible when using SS rectifiers or tubes such as 5V4G, 5AR4, Etc with an indirectly heated cathode. Just add enough resistance in the lower R HV lead from the PT to make up the differance.

But I've noticed many of those using DHT such as 2A3, 300B, Etc prefer rectifiers such as 5U4G, 5R4G, Etc. Then the voltage drop across the filament becomes a factor.

The original article was published in Glass Audio, Vol 11, #3, May 1999.
A short version appeared in the June 1998 issue of Electronics World.
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Last edited by John L Stewart : February 17th 14 at 11:01 AM Reason: Additional Information
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Power Frequency Ripple Reduction in B+ Supplies


"John L Stewart"

( snip )

Depending on the loading conditions of a triode power stage, from 2/3 to
4/5 of this ripple voltage will be applied to the load. Unfortunately,
most loudspeaker systems have one or two
resonance's below 100 HZ. One of these could be at or near the power
frequency. Presto!! We have objectionable hum in the listening room.

Figure One illustrates an ordinary center tapped full wave power supply.
Two sides of the HV winding measured 38 & 40 ohms.



** FFS - just add 2 ohms to the 38 ohm winding.

Problem gone.

Dodgy 5U4s etc not withstanding.

SET amp loonies sure are a bunch of neurotic old women.



..... Phil





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Patrick Turner Patrick Turner is offline
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Default Power Frequency Ripple Reduction in B+ Supplies

On Monday, 17 February 2014 08:53:17 UTC+11, John L Stewart wrote:
This note describes a simple, low cost power supply improvement. It

allows reduction of the power frequency ripple component (60 Hz in North

America & 50 Hz in Europe) by up too 20 db. The problem this

modification resolves occurs only in center tapped, full wave rectifier

"John L Stewart"

( snip )

Depending on the loading conditions of a triode power stage, from 2/3 to
4/5 of this ripple voltage will be applied to the load. Unfortunately,
most loudspeaker systems have one or two
resonance's below 100 HZ. One of these could be at or near the power
frequency. Presto!! We have objectionable hum in the listening room.

Figure One illustrates an ordinary center tapped full wave power supply.
Two sides of the HV winding measured 38 & 40 ohms.



** FFS - just add 2 ohms to the 38 ohm winding.

Problem gone.

Dodgy 5U4s etc not withstanding.

SET amp loonies sure are a bunch of neurotic old women.

..... Phil

I'm not so sure an added 2r to a winding with 38r would fix the "problem".
What if there is slight difference in turns of the CT winding? What if there
is Idc imbalance in the two output tubes?

Now, consider Quad-II amps which have about the worst PSU of any amplifier ever made and which looks exactly like an "accountant's delight"

There is a GZ32 charging 16uF and the B+ at approx 140mA Iadc is applied to the anode winding CT on OPT. The Vr at 16uF is approx 18Vrms! However, CMR reduces this to negligible levels and any differential Vr across OPT anode winding appears at cathode windings where the local CFB reduces it. The KT66 screen supply has a 20H choke followed by a second 16uF which then reduces the 18Vrms
by a factor 0.008 to approximately 0.15Vrms. Having a very well filtered screen supply is more important than a well filtered anode supply.
The GNFB then reduces any residual hum further to be what Peter Walker considered low enough, and was less than the THD measured. This assumes that the rectifier has equal Ra anodes, and tubes are new and have equal Idc.

Where the tubes are old and Idc imbalanced, then THD and the hum can rise to be the same as if there was no NFB in the amp.

My solution to the "Quad problem" has been to completely revise the Quad PSU using Si diodes feeding say 150uF then 150ohms to 470uF mounted where tube rectifier once was, then use individual R&C biasing on each KT66, and then the amp works fine with less Idc. The screen choke can be retained, but bypass cap is 100uF. Thus makes the amp dead quiet, even if output tubes are badly matched and a fault is present.

Where there is a CLC filtered supply, the Vr can usually be much reduced by adding a C+R series network across the choke, arranged so that C is resonant with the *working* choke inductance at twice the mains F. R should be about 22r. The damped parallel resonance usually reduces the main harmonic of 2H
in ripple ( 100Hz or 120Hz ) by 12dB. Other H are not much reduced but then the
following C2 of the CLC filter has an increasingly lower XC thus shunting the higher ripple H.

Many Guitar amps have a hum nulling pot so hum from heaters can be nulled but often is has no effect. Some allow balancing of the Idc in each output tube.

Few guitar amps have CLC anode filtering. Hum seems only objectionable if the volume is turned up. Switching noise of diodes can often be heard as pulses at 100Hz or 120Hz. But for hi-fi, owners expect dead quiet when volume is up and methinks one has to go to more trouble to get quietness than either Phil or John suggest. I have also repaired and re-engineered many amps where the earth path has had to entirely re-wired, including position on chassis for green-yellow Earth wire from mains input wiring. a connection lug should be well away from say a bolt holding the PT to chassis.

Noise is not always easily reduced.

Most amps with CT HT windings don't make much more Vr than amps with single HT windings with 4-diode bridges or voltage doublers with single HT winding and two diodes - all things being equal with current to capacitance ratios.
Keeping ancient electrolytic caps and tube rectifiers in ancient amps is not good practice and it is always an up-grade to change to Si diodes and healthy value filter caps. Especially with SE amps, where noise is not reduced be CMRR.
Many SE amps have triode output tubes which means the primary load is in series with lower tube Ra and the B+ rail so that 2/3 to 4/5 of Vr is across the OPT as JS mentions above. So if Vr across OPT primary = 0.5V, and OPT TR = 25:1, then Vr hum at sec = 20mV and 12dB GNFB might reduce it to 5mV but its far too much hum for hi-fi, so the Vr at OPT MUST be much less, so there is no cheap solution and the dopey cost avoiding hobbyist MUST let moths from his wallet.

RDH4 has other solutions of using twin T R&C networks for B+ hum reduction
but none apply to output stages with high Idc.
The other benefits of having a LARGE value C between OPT B+ connection and 0V is to give a very low impedance anchor to the connection and this minimizes IMD generation in tubes caused by having high amounts of LF appearing in series with the higher F. It applies especially to SE amps and to those working in Class AB - including Quad-II, which works in AB with RLa-a only 4k0 when you have a load of 16r connected to the OPT when strapped for 16r.
There is less hum when PP OP tubes work in class A. Nearly all PP amps made now
work in in class AB and owners insist on great bass at high levels and the use of 470uF caps between OPT CT and 0V is very common. 35uF = 91r at 50Hz, 470uF = 6.7r at 50Hz.
Many amps now proudly display the very large C value fancy brand electrolytics above the chassis. But so often that alone does little to change their other dreadful shortcomings with noise and other issues.

Patrick Turner.
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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Power Frequency Ripple Reduction in B+ Supplies


"Patrick Turner"

** FFS - just add 2 ohms to the 38 ohm winding.

Problem gone.

Dodgy 5U4s etc not withstanding.

SET amp loonies sure are a bunch of neurotic old women.


I'm not so sure an added 2r to a winding with 38r would fix the "problem".

** Who cares about that.


What if there is slight difference in turns of the CT winding?

** Not at all common and not mentioned in the OP.

What if there is Idc imbalance in the two output tubes?

** Huh ??

Got nothing to do with the issue of 50Hz PSU ripple.


..... Phil




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Phil Allison[_3_] Phil Allison[_3_] is offline
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Default Another Turneroid red herring


"Patrick Turneroid"


** Another of the Turneroid's famous red herrings:

-------------------------------------------------
" Many Guitar amps have a hum nulling pot so hum from heaters can be nulled
but often is has no effect."

-------------------------------------------------

Actually, very few tube guitar amps have such hum pots.

1970s Fender 50W and 100W combos being the only ones of any note.

The trim pot always works ( if it has not burnt out ) but you have to KNOW
how to do the adjustment.

First step, fit an open circuit 1/4 inch metal sleeved jack plug in the No 1
input.

Second step, turn up the volume till you hear 50/60Hz and harmonics coming
from the speakers.

Then adjust the trim pot to minimise the audible effect.



..... Phil







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John L Stewart John L Stewart is offline
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Smile

Quote:
Originally Posted by Patrick Turner View Post
On Monday, 17 February 2014 08:53:17 UTC+11, John L Stewart wrote:
This note describes a simple, low cost power supply improvement. It

allows reduction of the power frequency ripple component (60 Hz in North

America & 50 Hz in Europe) by up too 20 db. The problem this

modification resolves occurs only in center tapped, full wave rectifier

"John L Stewart"

( snip )

Depending on the loading conditions of a triode power stage, from 2/3 to
4/5 of this ripple voltage will be applied to the load. Unfortunately,
most loudspeaker systems have one or two
resonance's below 100 HZ. One of these could be at or near the power
frequency. Presto!! We have objectionable hum in the listening room.

Figure One illustrates an ordinary center tapped full wave power supply.
Two sides of the HV winding measured 38 & 40 ohms.



** FFS - just add 2 ohms to the 38 ohm winding.

Problem gone.

Dodgy 5U4s etc not withstanding.

SET amp loonies sure are a bunch of neurotic old women.

..... Phil

I'm not so sure an added 2r to a winding with 38r would fix the "problem".
What if there is slight difference in turns of the CT winding? What if there
is Idc imbalance in the two output tubes?

----------------------------------------------------------------

The 2R will help. But if a directly heated rectifer is used then it could be further improved by the pot I shewed across the 5V winding. That way most all the power frequency ripple can be elimnated. But it don't matter much in a guitar amp. Helps a lot for HIFI tho!

------------------------------------------------------------------------

Now, consider Quad-II amps which have about the worst PSU of any amplifier ever made and which looks exactly like an "accountant's delight"

There is a GZ32 charging 16uF and the B+ at approx 140mA Iadc is applied to the anode winding CT on OPT. The Vr at 16uF is approx 18Vrms! However, CMR reduces this to negligible levels and any differential Vr across OPT anode winding appears at cathode windings where the local CFB reduces it. The KT66 screen supply has a 20H choke followed by a second 16uF which then reduces the 18Vrms
by a factor 0.008 to approximately 0.15Vrms. Having a very well filtered screen supply is more important than a well filtered anode supply.
The GNFB then reduces any residual hum further to be what Peter Walker considered low enough, and was less than the THD measured. This assumes that the rectifier has equal Ra anodes, and tubes are new and have equal Idc.

Where the tubes are old and Idc imbalanced, then THD and the hum can rise to be the same as if there was no NFB in the amp.

My solution to the "Quad problem" has been to completely revise the Quad PSU using Si diodes feeding say 150uF then 150ohms to 470uF mounted where tube rectifier once was, then use individual R&C biasing on each KT66, and then the amp works fine with less Idc. The screen choke can be retained, but bypass cap is 100uF. Thus makes the amp dead quiet, even if output tubes are badly matched and a fault is present.

Where there is a CLC filtered supply, the Vr can usually be much reduced by adding a C+R series network across the choke, arranged so that C is resonant with the *working* choke inductance at twice the mains F. R should be about 22r. The damped parallel resonance usually reduces the main harmonic of 2H
in ripple ( 100Hz or 120Hz ) by 12dB. Other H are not much reduced but then the
following C2 of the CLC filter has an increasingly lower XC thus shunting the higher ripple H.

-----------------------------------------------------------------------

The massive CLC filter with resonant L is nice. But it is still 12 db less effective at the power freq than at the main ripple freq. A simple FET regulator or source follower refed to a string of Zeners would do a better job. And you can have that system ramp up slowly so that full PS HV does not appear on the power tube G1s at switch on. And a current limiting NTC Thermistor in the PT primary cct also helps.

http://www.ge-mcs.com/en/temperature...t-limiter.html

-----------------------------------------------------------------------------

Many Guitar amps have a hum nulling pot so hum from heaters can be nulled but often is has no effect. Some allow balancing of the Idc in each output tube.

Few guitar amps have CLC anode filtering.

--------------------------------------------------------------------------

I checked that in Aspen Pitman's ' The Tube Amp Book, 4th Edition'.
About half the amps listed showed some kind of choke in the HV section. But seldom tell us how many 'Heinrichs'. So difficult to tell how effective they are.

If the plates in a PP amp are run straight off the raw DC then ripple side bands on the progam material are a sure thing. But in a guitar amp don't matter much anyway.

---------------------------------------------------------------------------------------

Hum seems only objectionable if the volume is turned up. Switching noise of diodes can often be heard as pulses at 100Hz or 120Hz. But for hi-fi, owners expect dead quiet when volume is up and methinks one has to go to more trouble to get quietness than either Phil or John suggest. I have also repaired and re-engineered many amps where the earth path has had to entirely re-wired, including position on chassis for green-yellow Earth wire from mains input wiring. a connection lug should be well away from say a bolt holding the PT to chassis.

Noise is not always easily reduced.

Most amps with CT HT windings don't make much more Vr than amps with single HT windings with 4-diode bridges or voltage doublers with single HT winding and two diodes - all things being equal with current to capacitance ratios.
Keeping ancient electrolytic caps and tube rectifiers in ancient amps is not good practice and it is always an up-grade to change to Si diodes and healthy value filter caps. Especially with SE amps, where noise is not reduced be CMRR.
Many SE amps have triode output tubes which means the primary load is in series with lower tube Ra and the B+ rail so that 2/3 to 4/5 of Vr is across the OPT as JS mentions above. So if Vr across OPT primary = 0.5V, and OPT TR = 25:1, then Vr hum at sec = 20mV and 12dB GNFB might reduce it to 5mV but its far too much hum for hi-fi, so the Vr at OPT MUST be much less, so there is no cheap solution and the dopey cost avoiding hobbyist MUST let moths from his wallet.

RDH4 has other solutions of using twin T R&C networks for B+ hum reduction
but none apply to output stages with high Idc.
The other benefits of having a LARGE value C between OPT B+ connection and 0V is to give a very low impedance anchor to the connection and this minimizes IMD generation in tubes caused by having high amounts of LF appearing in series with the higher F. It applies especially to SE amps and to those working in Class AB - including Quad-II, which works in AB with RLa-a only 4k0 when you have a load of 16r connected to the OPT when strapped for 16r.
There is less hum when PP OP tubes work in class A. Nearly all PP amps made now
work in in class AB and owners insist on great bass at high levels and the use of 470uF caps between OPT CT and 0V is very common. 35uF = 91r at 50Hz, 470uF = 6.7r at 50Hz.
Many amps now proudly display the very large C value fancy brand electrolytics above the chassis. But so often that alone does little to change their other dreadful shortcomings with noise and other issues.

Patrick Turner.
My two rubles, anyway!

Cheers to all, John L Stewart
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Patrick Turner Patrick Turner is offline
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Default Power Frequency Ripple Reduction in B+ Supplies


Others mentioned.....
I checked that in Aspen Pitman's ' The Tube Amp Book, 4th Edition'.
About half the amps listed showed some kind of choke in the HV section.
But seldom tell us how many 'Heinrichs'. So difficult to tell how
effective they are.


Typically 4H / 100R for a medium powered fenderish or marshallesque PP amp..
Up to 15H for some rare amps, 2H or so for cheaper models (ironically most
are SE and really need good CLC filtering).

Indeed a read of the Book will show lots of anode supply chokes. But in 'export models' they are often left out so that the majority of guitar amps I have serviced in Oz have no anode choke in a CLC type of B+ supply.
As Phil mercilessly points out, 50Hz hum in anode supplies is a largely minor concern not much affected by B+ rectifiers and filtering methods. Usually, the 100Hz ripple F + other H are by far the higher quantity of ripple and most prominent source of hum in amplifiers.

SOME guitar amp makers woke up to the fact that having a well filtered screen B+ supply was more effective at reducing hum than having a clean anode supply.

I have a Fender Deluxe here with a screen supply filter choke that is 1/5 the weight of an anode choke to achieve the same amount of filtering.
Such miniature screen chokes with far less Idc could use very thin wire so lots of turns could be put on a small core. Thus accountants chuckle with delight over reduced costs of production and shipping weight.

Quad was early to realize the benefit of screen supply CLC filtering so they had their Quad-II screens of KT66 well filtered with CLC but little filtering to anode B+. But with class AB operation, the CMR action ceases to obstruct the hum and the hum is in series with the load on 1/2 the OPT at each wave crest so you'll see lots of noise on wave crests as you come up tp clipping. Some guitar amp players like this because it injects mains related GROWL to a tone being created.

But boutique guitar amps often have much better CLC anode filtering so GROWL is not possible. In one, with 2 x EL34 output tubes, I recently was able to add a 1/2 an additional switch to allow B+ to be reduced from +500Vdc with CLC for 50W+ AB to operate as choke input, ie, LC filter, for +320Vdc and have other 1/2 switch to adjust bias for higher Ia. The anode load stays constant but the tubes then produce mainly all class A1 power = 22Watts. The owner was delighted. Is remarkable how many guitar amp users prefer a class A power amp which they say they prefer where huge sound levels are not wanted.

The guitar amp involved had 100uF - 5H - 100uF, and the switch just moved the C1 to be parallel to C2, so 5H - 200uF. 5H was actually a nice choke, not designed by accountants.

However, I did have to add a 68r x 10W resistor in series with HT winding CT and 0V to limit peak current from rectifier Si diodes and thus reduce working B+ to +480Vdc. This is necessary because the working B+ in CLC high B+ mode was +510Vdc, and just above the 500Vdc rating for electrolytics. Just when are amp designers ever going to realize it is bad practice to have B+ above capacitor Vdc ratings? Don't hold your breath.

There was no need to pay any special attention to 50Hz mains hum.

Patrick Turner.
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Lord Valve Lord Valve is offline
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Default Another Turneroid red herring

Phil Allison wrote:

"Patrick Turneroid"

** Another of the Turneroid's famous red herrings:

-------------------------------------------------
" Many Guitar amps have a hum nulling pot so hum from heaters can be nulled
but often is has no effect."

-------------------------------------------------

Actually, very few tube guitar amps have such hum pots.

1970s Fender 50W and 100W combos being the only ones of any note.


Nearly all of the Magnavox-era (and previous) Ampegs had them.

LV

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John L Stewart John L Stewart is offline
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There was no need to pay any special attention to 50Hz mains hum.

Patrick Turner.[/quote]

Don't recall anyone here saying there is a problem with power frequency hum in a guitar amp. Except perhaps by bad grounding.

That is unless someone is using an SET!! Perhaps

Cheers to all, John L Stewart
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Patrick Turner Patrick Turner is offline
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Default Power Frequency Ripple Reduction in B+ Supplies



There was no need to pay any special attention to 50Hz mains hum.

Patrick Turner.

Don't recall anyone here saying there is a problem with power frequency
hum in a guitar amp. Except perhaps by bad grounding.

That is unless someone is using an SET!! Perhaps

Cheers to all, John L Stewart

Hmm, I made a 55Watt SET amp in 2009 which has 2 x parallel 845 tubes which operate from B+ and B- rails, +/- 600Vdc. If ever there was a likelihood of hum, it was in these amps because the B- rail must be especially hum free.
The schematic is at my website.
When completed, the noise total at speaker output with inputs grounded was 0.25mV, without hum being dominant, and without much GNFB. I did use CLC filtered 10Vdc for the 845 cathodes.

SE amps can be very easily be made hum free but you got to get right away from the accountant driven features designed into nearly every crummy amp made since 1920.

Patrick Turner.
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