• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tubes glowing pulsing. is this a problem?

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last night all 4 power tubes had started pulsing in the same fashion. i am going to try to replicate the issue tonight, but is there any possibility of damaging my speakers while this is happening? i dont want to cause any damage while testing.

and i guess i should be checking supply voltages? can i check those off of pin3 and 4?
 
As mentioned above, I've observed heavy woofers' cone motions while my amplifier was oscillating. But I didn't notice any cracking sounds or something like that. There's no warranty that your amp won't hurt your speakers, though.

Best check your supply voltages at the PSU's filter electrolytics, not at the power tubes' plates (pins 3). The voltages at the plates will go up and down by the oscillations' frequency, so this doesn't tell you anything about the PSU voltages' stability.

Best regards!
 
As mentioned above, I've observed heavy woofers' cone motions while my amplifier was oscillating. But I didn't notice any cracking sounds or something like that. There's no warranty that your amp won't hurt your speakers, though.

Best check your supply voltages at the PSU's filter electrolytics, not at the power tubes' plates (pins 3). The voltages at the plates will go up and down by the oscillations' frequency, so this doesn't tell you anything about the PSU voltages' stability.

Best regards!

i can probe around in the power supply section of the amp. what voltage should i be looking for back there?

according to the schematic (which i dont fully trust) should i be looking for 350?
 
I once had a low frequency oscillation like this in an amp, which turned out to be a problem in the HT.
Because of the current through the power tubes going up and down, the HT was going down and up a bit as well. This in turn was feeding the driver stage with a pulsating voltage which was send to the power tubes etc etc.
In the case of the OP, there are separate windings, rectifiers and caps for the different stages, so I don't think the PSU is the cause of the low frequency.
What is curious is the fact that the pulses are out of phase. This could indicate that it starts before the phase splitter.
 
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Joined 2011
What is curious is the fact that the pulses are out of phase. This could indicate
that it starts before the phase splitter.

This is to be expected. A large LF oscillation (around say 1Hz) will alternately drive one and then
the other of the output tube pair. It's possible that the zener regulator on the input stage is
not functioning. Then the power supply becomes a feedback oscillator network instead of a filter.
 
This is to be expected. A large LF oscillation (around say 1Hz) will alternately drive one and then
the other of the output tube pair. It's possible that the zener regulator on the input stage is
not functioning. Then the power supply becomes a feedback oscillator network instead of a filter.

regulator? is this a 3 pin deal? i need to know what to check out tonight
 
i just have a standard multimeter, but i attached a picture of the power section of the amp, i didnt have one i took myself so this is what i found online. ill have to get a better picture tonight of it all..
 

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i spent quite awhile trying to get the instability to show up with some different speakers tonight. it only seems to show itself when driving my most difficult speakers, which are av123 strata minis (3 way with midrange planar)

i was able to get the swing almost immediately after starting music with it.
i measured bias swinging from .6 to 2.0 volts, and the pin5 was between -20 and -34.

while this was happening i checked the back of the amp for power.

i saw somewhat steady values of..
187 volts
191 volts
390 volts

and on the other side i saw a pretty steady

6.15 volts
10.1 volts
and a 1.9 volt
also a pin showed -6.7 as well and it was steady compared to the wild swings of the bias voltage.

i will attach a couple pictures of the power sections, and rear half of the amp for inspection.
 

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ill tell ya what.

im not really sure.

i have had this amp for quite awhile, but i cant remember running these speakers on it.

it seems to start once everything gets warmed up on the amp. when it started right away it was because i moved it from one set of speakers to the other and it was still all pretty warm.

i took a close look and everything coupling looks to be .1. i will attach a few pictures for review

the one with sonicap shows the cap i pulled next to the sonicap.
 

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I am having a good time troubleshooting (learning). but since it has gone quiet, i will ask a couple questions...

when the tube amp is turned on, it will not oscillate. it only begins after playing music, and only on speakers that are a little more difficult to drive. once it begins to oscillate, it will not stop until i power the amp off.

i looked inside the chassis for a diode, i did not see one that i could probe to check before/after voltages.

i swapped the power tubes from side to side, but i did not swap the smaller tubes. is there a possibility that a bad/old tube could cause this? and is there any point in probing those tubes once the swing starts to look for trouble?

any other input would be helpful, i am having a good time learning how this thing works.
 
Just thought id let you guys know what i found;

I decided to start poking around, and i started in the first set of tubes that the audio signal went to, 12ax7. after some probing i found these tubes were exhibiting the same swings as the power tubes.

i decided top swap those tubes, and after that i could not replicate the crazy swings even after 2 hours of listening. so i guess it was a bad pair of tubes that caused it.
 
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