Peavey Valve King 100 head

[jondoe]

I've built a small audio signal tracer to try and help me locate the problem and so far I can hear that the humming noise is coming straight out of pin 7 of U101, but it isn't coming out of 1, 2 or 3 so I don't think the humming is coming before the circuit. I swapped the IC out for good measure, but it made no difference.

[g1]

If the hum is not at pin1 but at pin7, it is either getting picked up by the tank (which will act like an antenna), or there is a ground fault.
Someone asked early 'how bad is the hum' but you did not answer. Later, I see you mentioned seeing others complaining about it on youtube.
They all have a certain amount of hum as you turn up the reverb (tank acting as antenna). If you have the normal amount that others are complaining about, that is probably the best it's going to be. Is there significantly more reverb hum than other of the same model?

[jondoe]

Hey g1, thanks for the reply. The hum becomes intrusive above about 3 on the knob. I found the following video on youtube which shows a different Peavey model, but with a very similar symptom https://www.youtube.com/watch?v=Znx9y5JpAzs I've never had the pleasure of trying one of these heads out so I can't compare it, but I'm assuming that isn't correct operation. With only 8 parts around the output I suppose I could shotgun replace the lot.

[g1]

The user in that video makes a critical error which means the video is useless. He does not plug anything in to the amp and play it.
We have no idea what kind of volume he is setting the amp to. Many amps will hum if you don't plug anything in and turn everything full up. For all we know, when we hear the hum in the video, he has the volume set so loud you could not sit in the room if you played guitar at that volume setting.

Plug a guitar into your amp. Turn it up to a fairly loud volume that you actually play it at. Slowly bring up the reverb level. Does the hum seem loud compared to the guitar sound?
Sorry to go through all this, but it seems you may not be familiar with the 'usual' level of hum from reverb.
We can't make any assumptions from a video that has no reference to the guitar sound at playing levels.

[nickb]

The user in that video makes a critical error which means the video is useless. He does not plug anything in to the amp and play it.
We have no idea what kind of volume he is setting the amp to. Many amps will hum if you don't plug anything in and turn everything full up. For all we know, when we hear the hum in the video, he has the volume set so loud you could not sit in the room if you played guitar at that volume setting.

Plug a guitar into your amp. Turn it up to a fairly loud volume that you actually play it at. Slowly bring up the reverb level. Does the hum seem loud compared to the guitar sound?
Sorry to go through all this, but it seems you may not be familiar with the 'usual' level of hum from reverb.
We can't make any assumptions from a video that has no reference to the guitar sound at playing levels.

...and it might help to do this:

Put your meter on ACV and connect across the speaker. Unplug your guitar. Report the readings for low volume w/o revverb, high volume w/o reverb and high volume with reverb.
.

[jondoe]

The user in that video makes a critical error which means the video is useless. He does not plug anything in to the amp and play it.
We have no idea what kind of volume he is setting the amp to. Many amps will hum if you don't plug anything in and turn everything full up. For all we know, when we hear the hum in the video, he has the volume set so loud you could not sit in the room if you played guitar at that volume setting.

Plug a guitar into your amp. Turn it up to a fairly loud volume that you actually play it at. Slowly bring up the reverb level. Does the hum seem loud compared to the guitar sound?
Sorry to go through all this, but it seems you may not be familiar with the 'usual' level of hum from reverb.
We can't make any assumptions from a video that has no reference to the guitar sound at playing levels.

Please don't be sorry about going through it, I have much to learn and really appreciate the feedback. I've not been using a guitar for testing, I've been using a 400hz test signal when tracing the audio and a cheap scope to look at the waveform, I've also been using a ticking clock sample to get an idea on how well the delay on the reverb is, a little unorthodox . The hum relative to the test signal has always been considerably louder when the volume of the amp is low, which is why I thought it was a problem, but I do lack experience so it might be a case of me being a worry guts.

...and it might help to do this:

Put your meter on ACV and connect across the speaker. Unplug your guitar. Report the readings for low volume w/o revverb, high volume w/o reverb and high volume with reverb.
.

I'll get on that later and report back, I take it that's a reading without any form of signal?

[nickb]

I'll get on that later and report back, I take it that's a reading without any form of signal?

No signal, just hum (and noise).

[jondoe]

No signal, just hum (and noise).

It read as 0.10 ACV at all positions of the volume/reverb as you requested.

Afterwards I feed a metronome signal through the amp after and listened to the reverb output on pin 7 of the opamp, I've realised I can't actually hear the reverb under the humming noise, although the source audio signal is there. If I tap the reverb tank I get a strong and clear spring clanging sound.

[nickb]

It read as 0.10 ACV at all positions of the volume/reverb as you requested.

Afterwards I feed a metronome signal through the amp after and listened to the reverb output on pin 7 of the opamp, I've realised I can't actually hear the reverb under the humming noise, although the source audio signal is there. If I tap the reverb tank I get a strong and clear spring clanging sound.

With just 0.1V hum and noise with all turned up I don't think you have a hum problem at all.

So, the only question now is does the reverb sound normal otherwise?

[jondoe]

With just 0.1V hum and noise with all turned up I don't think you have a hum problem at all.

So, the only question now is does the reverb sound normal otherwise?

Hmmm, well there is no reverb effect on the output signal that I can make out as cranking the reverb past 5 way causes the humming to drown out the input signal entirely, so if there was a slight reverb I can't hear it. I can't hear any reverb effect on pin 7 of the opamp and the only reverb I can hear clearly is the tank springs if I give it a whack, they sound loud and clear! I've had the tank off again and checked the contacts and resistance readings and they all appear good.

I really don't think this is operating as it should.

[nickb]

Looking over the responses in this thread it seems that it's all got very confusing. For example the hum with reverb up was just 0.1V but it then stated that it swamps the input signal, which at 100W, is tens of volts, even just 1 watt is almost 3 volts. Certainly there is a complication that this is intermittent and that means pain.

What I would do is apply a 1khz 100mV signal to the input, plug something into the return to kill the main output (or use a dummy load) and look at the signal on the cable on the input to the tank. If that was missing I'd check U101 pin 3 and then U101 pin 1.


Are you within driving distance of West Devon? I'd be happy to put it on the bench and take a look.

[jondoe]

Looking over the responses in this thread it seems that it's all got very confusing. For example the hum with reverb up was just 0.1V but it then stated that it swamps the input signal, which at 100W, is tens of volts, even just 1 watt is almost 3 volts. Certainly there is a complication that this is intermittent and that means pain.

What I would do is apply a 1khz 100mV signal to the input, plug something into the return to kill the main output (or use a dummy load) and look at the signal on the cable on the input to the tank. f that was missing I'd check U101 pin 3 and then U101 pin 1.


Are you within driving distance of West Devon? I'd be happy to put it on the bench and take a look.

I think I'm confusing myself at this point I think it's best I start from the beginning, I'm going to order up some new 12AX7's as I'm using my known good spares, but I shouldn't trust them 100%. I do have a home made 50w dummy load clamped to my box steel workbench frame for a heat sink, I'll feed in a signal and see how it looks.

Unfortunately I'm several hours away from West Devon so I can't take you up on that generous offer, but thank you

[nickb]

I think I'm confusing myself at this point I think it's best I start from the beginning, I'm going to order up some new 12AX7's as I'm using my known good spares, but I shouldn't trust them 100%. I do have a home made 50w dummy load clamped to my box steel workbench frame for a heat sink, I'll feed in a signal and see how it looks.

Unfortunately I'm several hours away from West Devon so I can't take you up on that generous offer, but thank you

I doubt you need more valves as there is no valve section that is unique to the reverb and further it just complicates things with more unknown changes. Please try the suggested tests and see what we get. It's important to realise that the signal level you get at those points does depend on the volume setting so make sure they are not at zero.

[jondoe]

I doubt you need more valves as there is no valve section that is unique to the reverb and further it just complicates things with more unknown changes. Please try the suggested tests and see what we get. It's important to realise that the signal level you get at those points does depend on the volume setting so make sure they are not at zero.

Understood, I'll hold off valve swapping. I've also put some new batteries in my multi-meter, they died earlier on today unexpectedly, that probably doesn't help accuracy. I fed in a 1khz 109mV (closest i can get to 100) and used my DIY tracer to follow the tone all the way to pin 3 on U101, but it's not on pin 1. Should I be able to hear the tone on pin 1?

[nickb]

Yes it should be there on pin 1. Measure the DCV on pin 1,2,3,4 and 8 wrt ground please.