Power tube loses bias at full output.

[nickb]

The impact of standby on one of them, too.

+1
That's a great observation. That has to be a big clue.

[loudthud]

TrainWrecks are prone to oscillation. Even with the "approved" layout, lead dress can be a problem. Without a scope, you might never find it. Any odd behavior makes me suspect an ultrasonic oscillation. It's possible it only oscillates when a signal is present.

[Mike Sulzer]

What kind of PI does it have?
LTP tends tobe quite symmetrical; cathodine is horribly unsymmetrical as far as current driving capability, since one output comes from a plate (high impedance), the other from a cathode (very low impedance).

Output impedance and current driving capability are not the same thing. In general, you cannot get more AC current out of a cathode follower than a common cathode circuit. The significance of the low impedance is that the voltage follows accurately and then clips suddenly when the limit is reached. This is why I do not like cathode followers in a guitar amp except in situations where they cannot overload: the clipping onset is too rapid.

The situation with a cathodyne is even more complicated since the two outputs have what some people call "identical differential output impedance" (if I have the terminology right).

[Merlinb]

Using this layout: http://www.ceriatone.com/ceriatone/w...nCeriatone.jpg
Looking from the back, the left tube would be the one driven by the LTP's inverting output. With a warm-biased LTP the duty cycle from the inverting output will increase under overdrive, i.e. per cycle it spends more time positive than negative. This will drive the corresponding power tube harder, and vice versa for the other tube. This is indeed the trend we're seeing in the video. However, the effect appears to be quite extreme in this case, perhaps compounded by the GNFB loop whose influence I can't figure out in my head just now!

[DRH1958]

On the other hand, I am the only one who hears the sound of ripped paper? With long, sustained notes/chords would be better appreciated.

I heard this as well. Listen at 1:19. I wanted to post this yesterday but my computer crashed.

[Chuck H]

... The impact of standby on one of them, too...

I think we need to revisit this. So far the explanations have been excellent. But they continue to focus on the signal conducting behavior of the tubes. Here there is evidence of a problem that isn't signal dependent.

[Malcolm Irving]

Have you checked the screen resistors?

I like this suggestion. Was it followed up? If the screen is 'floating' on one of the tubes all bets are off.

[nickb]

I like this suggestion. Was it followed up? If the screen is 'floating' on one of the tubes all bets are off.

It's a great idea but got lost in the noise. I think we need some measurements from the OP now...

[Merlinb]

I think we need to revisit this. So far the explanations have been excellent. But they continue to focus on the signal conducting behavior of the tubes. Here there is evidence of a problem that isn't signal dependent.

It doesn't strike me as unusual. When you power on, the inverting triode will be turned on hard as the input coupling cap charges up, but the non-inverting triode will be held cold until the cap on the 'unused' grid charges up. This in turn means the output coupling cap from the non-inverting side will charge up faster, turning the corresponding power tube on harder and quicker, while the other side takes a little longer, as we see in the video.

[Chuck H]

So it's like I said in post #6 ?

There's also an impedance difference between the inverted and non inverted PI outputs under heavy clipping that makes the time constant unequal for coupling cap discharge so one power tube grid DOES end up biased colder. This is probably exacerbated by the third stage cold clipper which only clips on the same cold swing of the PI and the colder side power tube grid while the other swing is free to keep driving harder.

How about this from post #8 ?

Local NFB in the circuit is always trying to rebalance things, but maybe since the inverting side already has an advantage the circuit can become more asymmetric when the clipped signal no longer matches the input signal. But I'm just guessing on that.

I'm not being snarky. I actually just want to see if I got this right.

In any case... It's been reasoned so far that the behavior is probably normal? If that's the case I'll reiterate what I said before. It's probably a good idea to swap the power tubes every now and then in these amps since they pretty much live at a full grind.

My logical brain is siding with Nick in post #12. That is, I wouldn't expect the current to actually decrease in the weaker tube. But unless the amp is bench tested there isn't likely to be any definitive answers.

[Mike Sulzer]

It's a great idea but got lost in the noise. I think we need some measurements from the OP now...

"I like this suggestion. Was it followed up? If the screen is 'floating' on one of the tubes all bets are off."

If one screen was not supplied with the correct voltage, I think the idle current would not be right.

My interpretation of the video is that as soon as the output is large enough to move the operating point out of class A into B the right hand tube goes bananas. As suggested, oscillation is a real possibility. But there are some simple things to try . New tubes? Swap the output tubes?

Then lots of measurements.

[g1]

If one screen was not supplied with the correct voltage, I think the idle current would not be right.

Agree but I was thinking of a screen resistor (or connection) that was opening up beyond idle, not completely open. Also like the oscillation idea, but not sure how it could cause the current in one tube to decrease.
In any case, the test with a sine wave gradually increasing in volume would be instructive.

[HilltopExplosion2]

Hi guys! Leadfootdriver has pointed me this direction for help, and instead of aggravating him, I joined. I do not have a vast knowledge of electronics, just the basics, so please bear with me.

My friend brought this amp over for me to try out, and I liked it, so a deal was struck. Shortly after, I lost a power tube. The one that is going negative on the bias meter. I asked if there had been any problems before and he said no, but the tubes had been in it for a while. I replaced the set with a new set and all was ok, no hum, sounds fine. When I measured across pins 8 and 3 of the power tubes, it was 400 VDC on both. When I checked the right one ( the tube that goes negative) my voltmeter started clicking. I then figured 40 ma would be a safe place to start and see how the amp sounds. That is when I noticed the right tube go in the negative on the bias. I'm new to this amp, and this occurrence. The same tubes in a 100 watt amp, along with the same PI, did not have the same effect. I pulled 2 tubes and turned the amp to 8 ohms to rule out the tubes. They were very close all the way up to 80ma.

To answer a few questions, I checked the resistors, and the showed 989, and 996 ohms.

The reason for the bad recording is I just wanted to show what the amp was doing, so I just used my phone..

I do not have an oscilloscope, but if I can't figure this out, I think will take this to a repair shop to get it checked.

When the amp's volume is at 9 o'clock, and the guitar volume rolled back, it is very clean. I can then play a chord lightly with not much movement of the bias needle. When I hit the string hard, or chord hard, the needles will move in opposite directions. As of today, the amp has developed a hum, and sounds funny. With the amps volume at 9 o'clock (which is where it was in my vid) and the guitar volume maxed, there is a PEEEOWW to the attack of the string, or chord. It almost sounds like you hit a tight cable with a wrench.. also my voltmeter still clicks when I check pins 8 and 3 on the right power tube..

Any and all help are appreciated! With my limited tools, any suggestions of where and what to check would be appreciated. Thank you!

[Chuck H]

Well I think this confirms that something is amiss. Even if someone wanted to find flaw in the design, there are too many out there working fine to be that glib. So, yeah, take it to someone with bench gear to test and check things at a professional level. I won't be surprised if there's a blown screen resistor (as has been surmised), probably taken out with the tube failure. I hope the new symptom are nothing serious.

[nickb]

It really sounds like instability might be the issue. Tough to be sure about it and fix without a scope.


Just sort of throwing ideas out here:

If is has always done it then it could be a layout or component value issue. You could try upping the EL34's grid stoppers, no more than 47K. It's quite common to see a small 630V ceramic capacitor between the plates of the phase inverter - try 100pF.

If it just started then something has changed, wires got moved, solder joint, bad grounds esp around the filter caps - you did say the hum had increased recently too.