6bm8 Clipping question

[Tonewood]

In looking at a friends very low end looking SE 6BM8 amp that sounds bad, the sine wave coming out of pin 9 looks good. Looking at pin 3, the top of the wave is clipped way off. The resistors in the circuit measure ok and bypassing the coupling cap with a good one didn't help.
The attached schematic isn't the actual circuit but similar. There is no gird stopper on pin 3 in the circuit that I'm working on. Any ideas on what's likely to cause this? Thanks

[Jazz P Bass]

Sounds to me as if the coupling cap is leaking.

[mozz]

Bypassing the coupling cap is not gonna prove anything, unless the original was open, which isn't the usual failure mode. You have to take the old one out of the circuit, because it's probably leaky.

[Tonewood]

Thanks. I replaced the cap and it made no change. Any other ideas? Thanks again!

[g1]

Can you post the DC voltages for the second 6bm8 stage for the pins shown on your pic.

[Tonewood]

Here are some voltages- thanks:
edit: the pot isn't 22k. Also, the 22k Rk seems big so I tried a range of smaller values & it didn't help.

[Jazz P Bass]

I take it that you do not have another 6BM8 to try.

If the signal looks good coming out of pin 9 & it looks bad at the 6BM8 grid, then something internal to the output tube may be bad & it is affecting the signal.

You could try removing the cathode bypass capacitor.

[eschertron]

Is that "4v with the arrow" the bias voltage? Measure across the 180R cathode resistor. Also, what's the AC signal that you are applying?

If the signal coming off the triode's plate is OK, no need to look at that stage any further.

[Mick Bailey]

Does your output tube grid have a path to ground via the trem circuit?

[Jazz P Bass]

See if this Gibson schematic is close: GA5 T_ 6BM8.pdf

[eschertron]

The hand-drawn schematic shows 180R for the pentode's cathode resistor, but the two printed drawings show bigger values (680R and 750R). The valve data sheet I looked at suggested -16v bias for 200v plate-to-cathode. How close to these design values does this amp operate?

[Tonewood]

Jazz P Bass- I have tried 3 known good tubes with exactly the same results. I'll remove that bypass cap & give it a try.
eschertron - The 4 v is the grid to cathode voltage. The voltage across 180R is also 4 v. The AC input signal to the amp is 110Hz at about 1/2 volt. I vary the input to the 6bm8 via the volume control and it clips at almost any audible volume. I'll check exactly when it clips. I'll also check at higher Hz.
Mick Bailey: - I couldn't figure out the term ckt last night but I'm guessing it supplies a grid leak path to ground. I did clip a 1M to ground on the grid as a check and it made no change at all.
Jazz P Bass - thanks for the schem, that will help me figure out the tremolo ckt.
eschertron- The printed schematic isn't the actual circuit but similar. I used it as a pin reference for discussion. Sorry if it caused confusion. The hand drawn is the actual circuit.
-4v bias is quite small compared to -16. I will muck with the pentode bias & see what happens but I wasn't thinking that would clip the input to the pentode.

I don't understand blocking distortion & need to read up on it. Could that be involved?

Thanks everybody. After trying your ideas, I think my next move will be to disconnect the trem and supply a grid leak at pin 3.

[Jazz P Bass]

That 110Hz is a tad lowish.
Try 1K as an input frequency.

[eschertron]

+1 ^^^
the lower the frequency - and 110Hz is more than a tad lowish - then the more easily blocking happens. So the test freq could be a factor.

edit: I just saw "input 1/2v". What's the signal AC voltage on the plate of the driving triode? If it's more than about 2.5vac, you will be driving the pentode into grid limiting.

I looked again at the data sheet for the 6BM8. By your numbers it's dissipating about 3.3W, close to the design of 3.5W. So my suspicions of a 'wrong value' cathode resistor are probably unfounded.

[Tonewood]

Thanks for the help everyone. I tried the above suggestions & didn't find anything new. Distortion was the same At 1k Hz as 110 Hz. Removing the bypass cap didn't change it. Adding a 1M grid leak and removing the tremolo circuit let the amp get a little louder before sounding bad. I'm done with this one! It's useful for one overdriven sound and that's it. Thanks again.