[Ken]

Any heavy audio output gurus here? Need help bad. I've discussed this on
another NG, so here is my prob along with answers to questions. Any help
appreciated:

This may not be the ng for in depth audio questions, if not, would
appreciate a pointer. I have a Heath AA151 14 watt stereo amp, OT using
screen taps, that probably hasn't seen a good life. I can get 6 watts
each channel, then crossover distortion, BIG TIME, with or without scrn.
taps. Resistance check looks good. Subbed another 6BQ5 P-P OT, no taps,
got 13 watts with a little sine wave dist., but NO crossover dist. Does
this sound like burned OT's? Ken

Running one chan at a time, two sets of tubes used, bias set properly,
both chans. Everything looks good through the phase splitter, wiring has
been checked. I DID find a problem there, though. One 6BQ5 grid had been
shorted to ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this together. So I
checked all the wiring, looks good. Yes, BOTH showing the same
distortion at the same power level makes me wonder. Hate to trash those
OT's though, looks so good at six watts.

The distortion occurs just above the zero crossover point, both pos.
and neg. direction. The screen taps are correct according to the
resistance readings given on the schematic. I did run the orig with no
xfmr taps, applying B+ to the screens, same dist. I recently worked on a
Heath AA100, 25 watts/chan., and the RMS watts was as stated, plus some.
I think when Heath says 14 watts, they're talking RMS. I need to read up
on this, any sites offer this info? Ken

Cathode looks good to me, .05V signal, 15V DC for bias. Ken

[Ian Iveson]

Ken wrote

Any heavy audio output gurus here? Need help bad. I've
discussed this on another NG, so here is my prob along
with answers to questions. Any help appreciated:

This may not be the ng for in depth audio questions, if
not, would appreciate a pointer. I have a Heath AA151 14
watt stereo amp, OT using screen taps, that probably
hasn't seen a good life. I can get 6 watts each channel,
then crossover distortion, BIG TIME, with or without scrn.
taps. Resistance check looks good. Subbed another 6BQ5
P-P OT, no taps, got 13 watts with a little sine wave
dist., but NO crossover dist. Does this sound like burned
OT's? Ken

Running one chan at a time, two sets of tubes used, bias
set properly, both chans. Everything looks good through
the phase splitter, wiring has been checked. I DID find a
problem there, though. One 6BQ5 grid had been shorted to
ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this
together. So I checked all the wiring, looks good. Yes,
BOTH showing the same distortion at the same power level
makes me wonder. Hate to trash those OT's though, looks so
good at six watts.

The distortion occurs just above the zero crossover point,
both pos. and neg. direction. The screen taps are correct
according to the resistance readings given on the
schematic. I did run the orig with no xfmr taps, applying
B+ to the screens, same dist. I recently worked on a Heath
AA100, 25 watts/chan., and the RMS watts was as stated,
plus some. I think when Heath says 14 watts, they're
talking RMS. I need to read up on this, any sites offer
this info? Ken


Mean power = rms voltage times rms current. Power is often
erroneously quoted as rms. Consider P = current squared
times resistance...if you square a square root you don't end
up with a square root.

A circuit diagram for your amp, including the output
transformer winding resistances, is available he

http://mcnally.cc/amps.htm

Ian

Cathode looks good to me, .05V signal, 15V DC for bias.
Ken

[John Byrns]

In article ,
"Ian Iveson" wrote:


Mean power = rms voltage times rms current.

I think you are over simplifying things, isn't the phase angle of the
voltage and current important in power calculations?

Power is often
erroneously quoted as rms. Consider P = current squared
times resistance...if you square a square root you don't end
up with a square root.

Did anyone say you do? You clearly end up with the original number
after squaring a square root.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Ken]

Ian Iveson wrote:
Ken wrote

Any heavy audio output gurus here? Need help bad. I've
discussed this on another NG, so here is my prob along
with answers to questions. Any help appreciated:

This may not be the ng for in depth audio questions, if
not, would appreciate a pointer. I have a Heath AA151 14
watt stereo amp, OT using screen taps, that probably
hasn't seen a good life. I can get 6 watts each channel,
then crossover distortion, BIG TIME, with or without scrn.
taps. Resistance check looks good. Subbed another 6BQ5
P-P OT, no taps, got 13 watts with a little sine wave
dist., but NO crossover dist. Does this sound like burned
OT's? Ken

Running one chan at a time, two sets of tubes used, bias
set properly, both chans. Everything looks good through
the phase splitter, wiring has been checked. I DID find a
problem there, though. One 6BQ5 grid had been shorted to
ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this
together. So I checked all the wiring, looks good. Yes,
BOTH showing the same distortion at the same power level
makes me wonder. Hate to trash those OT's though, looks so
good at six watts.

The distortion occurs just above the zero crossover point,
both pos. and neg. direction. The screen taps are correct
according to the resistance readings given on the
schematic. I did run the orig with no xfmr taps, applying
B+ to the screens, same dist. I recently worked on a Heath
AA100, 25 watts/chan., and the RMS watts was as stated,
plus some. I think when Heath says 14 watts, they're
talking RMS. I need to read up on this, any sites offer
this info? Ken


Mean power = rms voltage times rms current. Power is often
erroneously quoted as rms. Consider P = current squared
times resistance...if you square a square root you don't end
up with a square root.

A circuit diagram for your amp, including the output
transformer winding resistances, is available he

http://mcnally.cc/amps.htm

Ian

Cathode looks good to me, .05V signal, 15V DC for bias.
Ken


I use P-Esq/R where E= peak to peak/2 X .707

[Ian Iveson]

John Byrns wrote:

Mean power = rms voltage times rms current.

I think you are over simplifying things, isn't the phase angle of the
voltage and current important in power calculations?

Simplifying, yes. Over-simplifying if I have managed to confuse anyone...

True for the case in question, mid-band. I'm just trying to
illustrate why power doesn't end up an rms value.

Power is often
erroneously quoted as rms. Consider P = current squared
times resistance...if you square a square root you don't end
up with a square root.

Did anyone say you do? You clearly end up with the original number
after squaring a square root.


Everyone who ends up with an rms value for power. Square rms voltage and you
get mean square voltage. Divide by the resistance and you get mean power. The
root disappears because the rms value is squared.

I think I've made the point I was trying to make.

cheers, Ian

[Ian Iveson]

Ken said

I use P-Esq/R where E= peak to peak/2 X .707

That's OK and easier to measure, generally. Won't upset John either. How do
you measure pk-pk voltage?

Note John's comment about phase difference, which doesn't apply if you assume
a pure resistance, as you do in your equation.

Have you measured your winding resistances yet?

cheers, Ian

[John Byrns]

In article ,
"Ian Iveson" wrote:

Power is often
erroneously quoted as rms. Consider P = current squared
times resistance...if you square a square root you don't end
up with a square root.

Did anyone say you do? You clearly end up with the original number
after squaring a square root.

Everyone who ends up with an rms value for power. Square rms voltage and you
get mean square voltage. Divide by the resistance and you get mean power. The
root disappears because the rms value is squared.

I think I've made the point I was trying to make.

No you haven't, I for one don't have a clue what point you are trying to
make, your explanation is clear as mud. I understand that power is
power and am guessing that may be the point you are trying to make, but
if I didn't already know that I wouldn't have a clue that was your
point, assuming it even is, from what you wrote.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[John Byrns]

In article , Ken
wrote:

Any heavy audio output gurus here? Need help bad. I've discussed this on
another NG, so here is my prob along with answers to questions. Any help
appreciated:

This may not be the ng for in depth audio questions, if not, would
appreciate a pointer. I have a Heath AA151 14 watt stereo amp, OT using
screen taps, that probably hasn't seen a good life. I can get 6 watts
each channel, then crossover distortion, BIG TIME, with or without scrn.
taps. Resistance check looks good. Subbed another 6BQ5 P-P OT, no taps,
got 13 watts with a little sine wave dist., but NO crossover dist. Does
this sound like burned OT's? Ken

Running one chan at a time, two sets of tubes used, bias set properly,
both chans. Everything looks good through the phase splitter, wiring has
been checked. I DID find a problem there, though. One 6BQ5 grid had been
shorted to ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this together. So I
checked all the wiring, looks good. Yes, BOTH showing the same
distortion at the same power level makes me wonder. Hate to trash those
OT's though, looks so good at six watts.

The distortion occurs just above the zero crossover point, both pos.
and neg. direction. The screen taps are correct according to the
resistance readings given on the schematic. I did run the orig with no
xfmr taps, applying B+ to the screens, same dist. I recently worked on a
Heath AA100, 25 watts/chan., and the RMS watts was as stated, plus some.
I think when Heath says 14 watts, they're talking RMS. I need to read up
on this, any sites offer this info? Ken

Cathode looks good to me, .05V signal, 15V DC for bias. Ken


Hi Ken,

It appears that you have checked the output tubes and ruled them out,
from other posts it sounds like you have also replaced the coupling
capacitors or at least checked them for leakage. While you haven't
checked for shorted turns in the output transformers, the fact that both
channels produce the identical 6 Watts, would seem to make it an
unlikely coincidence that both output transformers are shorted in an
identical way.

So what possibilities are we left with? One thing that can cause the
symptom you describe is a too high cathode resistor, have you measured
the value of the cathode resistor? Also you should measure the DC
voltage at the cathode without any signal, which I assume is how you got
the 15 volt reading, you should also measure it at full output power?

Do you know what the turns ratio of the replacement output transformer
that you used is? 6BQ6s are used at a wide range of operating points
and load impedances. IIRC the AA-151 output transformer has a primary
impedance on the low side, if your substitute output transformer had a
high primary impedance that could explain the results you are getting,
especially if the value of the cathode resistor has gone high. A quick
check for this effect would be to measure the power output with the
original output transformers into a 13 Ohm load connected to the 8 Ohm
output tap.

It is normal for some cathode biased class AB amplifiers to develop
crossover distortion of the type you describe when they are tested with
continuous tones. IIRC the AA-100 uses fixed bias rather than cathode
bias like the AA-151, and so isn't subject to this problem.

The AA-151 is rated at an output of 14 Watts per channel using the
³Heath Hi-Fi Rating² method. Does anyone know what the ³Heath Hi-Fi
Rating² method is? I have a vague memory that it may be similar to the
³IHFM Music Power² rating but with a lower distortion level?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Ken]

John Byrns wrote:
In article , Ken
wrote:

Any heavy audio output gurus here? Need help bad. I've discussed this on
another NG, so here is my prob along with answers to questions. Any help
appreciated:

This may not be the ng for in depth audio questions, if not, would
appreciate a pointer. I have a Heath AA151 14 watt stereo amp, OT using
screen taps, that probably hasn't seen a good life. I can get 6 watts
each channel, then crossover distortion, BIG TIME, with or without scrn.
taps. Resistance check looks good. Subbed another 6BQ5 P-P OT, no taps,
got 13 watts with a little sine wave dist., but NO crossover dist. Does
this sound like burned OT's? Ken

Running one chan at a time, two sets of tubes used, bias set properly,
both chans. Everything looks good through the phase splitter, wiring has
been checked. I DID find a problem there, though. One 6BQ5 grid had been
shorted to ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this together. So I
checked all the wiring, looks good. Yes, BOTH showing the same
distortion at the same power level makes me wonder. Hate to trash those
OT's though, looks so good at six watts.

The distortion occurs just above the zero crossover point, both pos.
and neg. direction. The screen taps are correct according to the
resistance readings given on the schematic. I did run the orig with no
xfmr taps, applying B+ to the screens, same dist. I recently worked on a
Heath AA100, 25 watts/chan., and the RMS watts was as stated, plus some.
I think when Heath says 14 watts, they're talking RMS. I need to read up
on this, any sites offer this info? Ken

Cathode looks good to me, .05V signal, 15V DC for bias. Ken


Hi Ken,

It appears that you have checked the output tubes and ruled them out,
from other posts it sounds like you have also replaced the coupling
capacitors or at least checked them for leakage. While you haven't
checked for shorted turns in the output transformers, the fact that both
channels produce the identical 6 Watts, would seem to make it an
unlikely coincidence that both output transformers are shorted in an
identical way.

So what possibilities are we left with? One thing that can cause the
symptom you describe is a too high cathode resistor, have you measured
the value of the cathode resistor? Also you should measure the DC
voltage at the cathode without any signal, which I assume is how you got
the 15 volt reading, you should also measure it at full output power?

Do you know what the turns ratio of the replacement output transformer
that you used is? 6BQ6s are used at a wide range of operating points
and load impedances. IIRC the AA-151 output transformer has a primary
impedance on the low side, if your substitute output transformer had a
high primary impedance that could explain the results you are getting,
especially if the value of the cathode resistor has gone high. A quick
check for this effect would be to measure the power output with the
original output transformers into a 13 Ohm load connected to the 8 Ohm
output tap.

It is normal for some cathode biased class AB amplifiers to develop
crossover distortion of the type you describe when they are tested with
continuous tones. IIRC the AA-100 uses fixed bias rather than cathode
bias like the AA-151, and so isn't subject to this problem.

The AA-151 is rated at an output of 14 Watts per channel using the
³Heath Hi-Fi Rating² method. Does anyone know what the ³Heath Hi-Fi
Rating² method is? I have a vague memory that it may be similar to the
³IHFM Music Power² rating but with a lower distortion level?


Regards,

John Byrns

Thanks, John, I'm going to move that bias around to see what happens. Ken

[Ian Iveson]

John continued:

Did anyone say you do? You clearly end up with the original number
after squaring a square root.

Everyone who ends up with an rms value for power. Square rms voltage and
you
get mean square voltage. Divide by the resistance and you get mean power.
The
root disappears because the rms value is squared.

I think I've made the point I was trying to make.

No you haven't, I for one don't have a clue what point you are trying to
make, your explanation is clear as mud. I understand that power is
power and am guessing that may be the point you are trying to make, but
if I didn't already know that I wouldn't have a clue that was your
point, assuming it even is, from what you wrote.

If you understand the point I am trying to make, and think you can "explain"
it better, don't be shy. Would you have been happier had I simply said "power
is power"? (1)

For my part, I sincerely hope I never "explain" anything (2). "Explanation"
may briefly expand knowledge, but it kills understanding. If you feel that
understanding is a collection of "explanations", you have missed out on the
Meaning of Life.

It is particularly problematic here to explore a misconception without drawing
wholly negative responses from those who think they know everything, and those
who have reasons beyond the group for never admitting to a mistake.

It is always problematic to "explain" why something is *not* true...in this
case why power is *not* an rms value. It is a core problem of atheism,
incidentally.

Consequently newcomers to the art, only too fearful that they are prone to
misconception, daren't raise their hands or open their mouths.

A particularly bad trait, IMO, is to demand that everyone should "learn the
basics" before asking a question or attempting a contribution. That's
protectionism, not education.

I'm here for recreation and have nothing to lose. I can be as slack as I wish
for my purposes. I get a kick out of raising issues and stirring up
educational debate. I get no kick from giving "explanations".

I have made many good things, and taught many people who have later made good
use of they learned, with my guidance, by themselves, because I didn't explain
much then either.

Much more interesting to leap from revelation to revelation, and then to
characterise in retrospect what has become obvious.

I'm a prophet not a dictionary. Don't kill the Spirit.

cheers, Ian

(1)"Should anyone ask me the meaning of good, I would say that good is good,
and let that be the end of the matter." G.E.Moore, in interview for a
philosophy fellowship at Oxford...IIRC

(2) Here as much as anywhere else.

[Phil Allison]

"Ken"
Any heavy audio output gurus here? Need help bad. I've discussed this on
another NG, so here is my prob along with answers to questions. Any help
appreciated:

This may not be the ng for in depth audio questions, if not, would
appreciate a pointer. I have a Heath AA151 14 watt stereo amp, OT using
screen taps, that probably hasn't seen a good life. I can get 6 watts each
channel, then crossover distortion, BIG TIME, with or without scrn. taps.
Resistance check looks good. Subbed another 6BQ5 P-P OT, no taps, got 13
watts with a little sine wave dist., but NO crossover dist. Does this
sound like burned OT's? Ken

Running one chan at a time, two sets of tubes used, bias set properly,
both chans. Everything looks good through the phase splitter, wiring has
been checked. I DID find a problem there, though. One 6BQ5 grid had been
shorted to ground since the thing was built, so the sound must have always
been off. That's why I think an amateur put this together. So I checked
all the wiring, looks good. Yes, BOTH showing the same distortion at the
same power level makes me wonder. Hate to trash those OT's though, looks
so good at six watts.

The distortion occurs just above the zero crossover point, both pos. and
neg. direction. The screen taps are correct according to the resistance
readings given on the schematic. I did run the orig with no xfmr taps,
applying B+ to the screens, same dist. I recently worked on a Heath AA100,
25 watts/chan., and the RMS watts was as stated, plus some. I think when
Heath says 14 watts, they're talking RMS. I need to read up on this, any
sites offer this info? Ken

Cathode looks good to me, .05V signal, 15V DC for bias. Ken



** You real sure the screen and plate tappings are not confused??

When you see that x-over distortion appear - I bet the cathode voltage has
risen well above 15 volts - indicating the tubes are heavily into class B
operation.

Could also be that the secondary windings are haywire, so the 16 ohm output
is labelled as 8 ohm.

Try using a test load of double the value you are now.



........ Phil

[Sander deWaal]

Ken said:


Running one chan at a time, two sets of tubes used, bias set properly,
both chans. Everything looks good through the phase splitter, wiring has
been checked. I DID find a problem there, though. One 6BQ5 grid had been
shorted to ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this together. So I
checked all the wiring, looks good. Yes, BOTH showing the same
distortion at the same power level makes me wonder. Hate to trash those
OT's though, looks so good at six watts.

The distortion occurs just above the zero crossover point, both pos.
and neg. direction. The screen taps are correct according to the
resistance readings given on the schematic. I did run the orig with no
xfmr taps, applying B+ to the screens, same dist. I recently worked on a
Heath AA100, 25 watts/chan., and the RMS watts was as stated, plus some.
I think when Heath says 14 watts, they're talking RMS. I need to read up
on this, any sites offer this info? Ken


"One 6BQ5 grid had been shorted to ground since the thing was built,
so the sound must have always been off."

This leaves open a possibility that's not yet mentioned: a magnetized
core due to a possibly large DC current.
This may show up as a shortage in power, but usually one gets a huge
low frequency roll-off.

A way to cure this is as follows: Pull all cables and tubes, connect a
variac *via an insulation transformer* and a switch to the mains.
The output of said variac goes into the speaker connectors.
Turn the variac all the way down to 0 volts.
Now switch on the tranny combo.

Now slowly turn up the variac to about 15 volts, and back again.
Repeat this several times, but remember to always turn the variac all
the way down to 0 volts.
If the variac starts to growl, turn it down immediately!

Disconnect the variac from the outputs, plug in the tubes and test the
amp.

Be very careful during this operation, you're playing with mains
voltages at the primary side of the isolation tranny/variac!

--

- Maggies are an addiction for life. -

[Phil Allison]

"Sander deWaal"


"One 6BQ5 grid had been shorted to ground since the thing was built,
so the sound must have always been off."

This leaves open a possibility that's not yet mentioned: a magnetized
core due to a possibly large DC current.


** The amp uses cathode bias.

A 100 ohm power resistor serves all 4 tubes.


Magnetised OTs are vary rare things.




......... Phil

[Engineer[_2_]]

On Oct 11, 6:49 pm, "Phil Allison" wrote:
"Sander deWaal"



"One 6BQ5 grid had been shorted to ground since the thing was built,
so the sound must have always been off."

This leaves open a possibility that's not yet mentioned: a magnetized
core due to a possibly large DC current.

** The amp uses cathode bias.

A 100 ohm power resistor serves all 4 tubes.

Magnetised OTs are vary rare things.

........ Phil

I'm not a restoration purist, but I would immediately modify that
single 100 ohms to 4 separate 400 ohm (but see below)resistors (say, 2
watts each) with 4 separate bypass caps of at least 100 MFD each.
Single, shared Rk's are not good as the tubes begin to deteriorate
unevenly.
Then I'd check the exact ratios of the OPT's (P-P to speaker winding),
viz:
Use a 12 VAC, or so, "wall-wart" P/S from a variac, to the OPT
secondary and measure the OPT voltage ratios - keep the "ww" voltage
below about 6 VAC. Then reflect that to the P-P secondary impedance,
e.g. a ratio of 30:1 with 8 ohm secondary gives 7200 ohms P-P load.
While at it, check the screen tap ratio.
Then, while listening to it playing Mozart piano concertos into my
classic Ditton 44 speakers, I'd figure out a good working point for
the 6BQ5/EL84's given the B+ available (360 VDC, I think, draw load
lines on the tube plate curves - details beyond scope of this short
note) and (after switching it off ! g) consider if I needed to
change the Rk's a bit. Or quicker, just take a design from another
proven schematic - I'd use the Mullard 5-10 O/P design, see
http://www.bonavolta.ch/hobby/en/audio/el84_7.htm )
Good luck!
Cheers,
Roger
Disclaimer: potentially lethal voltages under the chassis - don't do
any of the above unless you're experienced in fixing tube gear.