Power tube loses bias at full output.

[leadfootdriver]

If anyone would know, it would be the big brains on this board!

A fellow Trainwrecker was monitoring the bias on his Express clone and noticed that the 1st power tube swings high on the bias but the 2nd power tube loses bias. Is this normal?

Here's a vid of the situation and thanks!

[g1]

Bias is for idle current. Normally we don't look at the current under signal conditions unless there is a problem we are trying to find.
Since he's looking anyway, swap the tubes and see if the problem follows the tube or stays with the socket/probe.

[leadfootdriver]

Thanks.. He switched them and the result was the same. It must be the way that amp responds cuz it's in full power clip to get it's distortion.

[Chuck H]

Thanks.. He switched them and the result was the same. It must be the way that amp responds cuz it's in full power clip to get it's distortion.

If by "the same" you mean that the bias shift remained with the socket and didn't follow the tube I'm actually surprised. I could reconcile the tubes actual dissipation being grossly different in heavy clipping. I've seen this a lot in modern tubes. They're matched at idle and that's it. What they do when they're actually amplifying, when it matters, is all over the map even with "matched" tubes now. I don't think there's anything about the circuit that should cause that phenomenon.

[pdf64]

It would be interesting to scope the wave along the amp's signal chain.
I guess that when overdriven it gets asymmetrical, and the output mark space ratio would be rather uneven.
I think it was the Express that had its overdrive characteristics exhaustively examined and analysed in a thread over on TAG, maybe by katopan.
It turns out that it's one of the very few amps where, under sustained overdrive, the LTP clips, rather than the power tube grid, on one of the p-p 'sides' at least.

[Chuck H]

Pete has it. I woke up with the answer. Must have been mulling it over n my sleep

The offset begins with the PI. The LTP PI clips asymmetrically. 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. I suppose I'd have to call this behavior "normal" then. But I'd look on a scope just to see if the duty cycle looks the "normal" amount of uneven. Indeed, different PI tubes and different power tubes could balance things a little, but never completely because the circuit IS imbalanced WRT it's intended operation. That is, clipping hard most of the time.

In this light it might be a good idea with those amps to swap the power tubes occasionally for more even wear. I'd certainly qualify the clip as sounding right for the design

I linked a site that has the scope shots:

Trainwreck Express Clone Oscilloscope Photos

[J M Fahey]

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).

Performance as seen is horrible, but since a tube guitar amp is not an amplifier but an effects box, anything is possible.

Sound is good, who cares the amp emits cold neutrons and sterilizes anybody within 20 yards?

FWIW: current rising a lot is normal in an AB class amplifier; staying the same is not and indicates tons of grid rectification.
Signal out must be quite assymmetrical, we *deserve* a scope capture, showing just a couple needles is teasing followed by leg crossing.

[Chuck H]

IMHE the LTP PI can become asymmetric under heavy clipping. I can't site the mechanisms on a technical level, but I expect it's for the same reasons you usually see different plate loads on the circuit as an attempt to pad output from the inverted side. 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 DO know that I've done experiments to balance LTP circuits, and therefor the drive to the power tube grids. With the circuit I kludged up I was able to adjust symmetry on the fly. I wouldn't say balancing did the actual tone of the amp any justice. Different maybe. Not better or worse. Since the asymmetry is part of the TW sound any such attempts at balancing the currents would likely ruin the tone.

[nickb]

It would be interesting to scope the wave along the amp's signal chain.
I guess that when overdriven it gets asymmetrical, and the output mark space ratio would be rather uneven.
I think it was the Express that had its overdrive characteristics exhaustively examined and analysed in a thread over on TAG, maybe by katopan.
It turns out that it's one of the very few amps where, under sustained overdrive, the LTP clips, rather than the power tube grid, on one of the p-p 'sides' at least.

All true but I find it hard to imagine that a tube producing useful output power has a lower average current than when idle. That is what the meter in the OP is saying. Given the peak current is likely a few hundred mA and the waveform looks like a pulse, the mark-space ratio would have to be <1:10 to make this happen. I'm not buying that is normal.

Do what Pete says a put a scope on both grids on the power tubes and let's see what's really going on. Anything else is just speculation.

[g1]

Also flip the bias probes around to make sure there is not some issue with the probes themselves.

[leadfootdriver]

Thanks for all the replies! I had my guy switch the bias probes, and the outcome was the same. So it's unique to this amp which is a Ceriatone build.

[nickb]

Here is a set of (somewhat idealized) waveforms I'd expect you see as the input is raised from just clipping to full blown hellfire level drive. The top trace shows an approx 2:1 ratio in average currents in the two EL34s. The asymmetry happens due to grid current flowing in the LTPP input grid. You'll notice that both of them go UP, not one up and the other down as the drive is increased.



PS (edit)

.and here is what happens if you change the PI grid resistor from 1meg to 180K. You completely change the character of the amp to be much more symmetrical.

[g1]

In the absence of a scope, something to try.
Use a sinewave at the input. Set the amp up for a basic clean sound.
Slowly bring up the level of the sinewave, the current shown on the meters should increase on both sides.

Have you checked the screen resistors?

[J M Fahey]

Loudthud was kind enough to link a Trainwreck schematic for me, and no, itīs not a cathodyne but a very conventional LTP inverter followed by conventionally coupled power tubes, so whatever it is, itīs not coming from there.
There *is* a cold biased gain stage before it which does clip assymmetrically, but itīs way too early in the chain to have such a big influence on the power tubes
Have we checked that the difference stays in the socket or follows the tubes?

[Pedro Vecino]

That behavior in vumeters seems completely abnormal. The impact of standby on one of them, too. 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 dīont know that model, but I would try the following:
- Make sure the preamp output is connected to the appropriate input in the PI.
- Check their PI resistors values again (with multimeter).
- Disconnect the feedback.
- Retest inverting output transformer primary to identify abnormal operation in the transformer (if so)