Peavey Encore 65 Misc Parts Question

[Helmholtz]

I was thinking more about the possibility of a partial short in the transformer, or a leaky parallel cap.

Both effects would reduce or even kill reverb drive but neither of them could noticeably change tube current as the tube acts as a constant current source with an equivalent internal plate source DCR above 40k.

[g1]

There seems to be a misconception. A DCR difference at the plate of a few k won't change tube current. As the internal plate resistance of the tube is much higher than the plate load DCR, it's the tube that dictates the current (acting as a constant current source).

I was thinking more about the possibility of a partial short in the transformer, or a leaky parallel cap.

[Helmholtz]

As you can see, the 6L6 tubes are running "cool." I don't see that altering R67 or R58 buy me much. The problem must be that the transformer is loaded such that we can't get the desired -57 vdc at the junction of CR12 and R67. So as we lose voltage across the divider and 220K Control Grid resistor(s), I can only get -52.2v at the Grids. I don't know if the difference of 3 volts would make a substantial difference, in raising the plate current, dissipation, or overall sound of the amp.

If you want hotter bias (higher plate current) you need less (negative) bias volts. That's easy to achieve by adjusting resistors. Just increase R67 value.

BTW, the voltage drop across R56/R57 is most probably caused by your meter (indicating 10M meter resistance) and not real as grids don't draw current.

[TomCarlos]

And a shout out to Enzo who did mention "DC Resistance of the winding" in #19. I didn't connect the dots on that. So thank you.

Last topic on this amp, I wanted to post some notes on the Bias Control for this amp. I guess we refer to this as the non-adjustable fixed bias circuit? Note: I am using JJ 6L6GC 30 Watt tubes.

I made some notes in the picture below. As you can see, the 6L6 tubes are running "cool." I don't see that altering R67 or R58 buy me much. The problem must be that the transformer is loaded such that we can't get the desired -57 vdc at the junction of CR12 and R67. So as we lose voltage across the divider and 220K Control Grid resistor(s), I can only get -52.2v at the Grids. I don't know if the difference of 3 volts would make a substantial difference, in raising the plate current, dissipation, or overall sound of the amp.

I have seen other threads where it was suggested a redesign of this circuitry (some type of full wave negative rectifier) could get you there. After reading your comments on my other thread (Pre Planning For Modifying A Fender Super Reverb Amp), it is clear that "current" in the secondary winding must also be considered. So I don't think this is an option.

Unless someone has a rebuttle, I think this is as good as it gets.

Reference - Rob Robinett Tube Bias Calculator

[TomCarlos]

Will do!! Thanks!!!

[Helmholtz]

Just did a graphical construction of the operating point with Rk = 470R. My results perfectly match your measurements (4.7V/10mA) There is some extrapolation involved, so the error might be up to +/- 20%.

I tend to think that the values in the schematic are wrong. As a cathode resistor of 470R will result in a plate current of around 10mA, the max plate dissipation (2.5W) of the 12AT7 is exceeded by around 66%!

For safe operation the cathode resistor would need to be increased to 1k or more.

[TomCarlos]

Thank you Helmholtz. In fact, if I used different 12AT7 tubes, I will get a different voltage at the Cathode.

I just thought it was odd that the other tube voltages came close to the schematic but this one was off.

[Helmholtz]

There seems to be a misconception. A DCR difference at the plate of a few k won't change tube current. As the internal plate resistance of the tube is much higher than the plate load DCR, it's the tube that dictates the current (acting as a constant current source).

Does grounding of V3A, pin 7 change cathode voltage?

[TomCarlos]

G1 -

R31 measures 475 ohm in circuit.

I removed C21 and no change on Pin 8, still at 4.6v.

I think you figured it out. I was pondering the notion of swapping out one transformer for another, that it would (perhaps) behave differently. Your analysis makes sense. I see how you came up with the 700 ohms DC Resistance for the Fender Transformer. And following the same logic, the Peavey transformer would be approx 3700 ohms.

Per the schematic....

Current Calculation: 2.5v / 470 ohms = 5.32ma

Transformer DC Resistance = (465 - 445) / 5.32ma = 3753 ohms

That is a big difference.

So let me see if this is correct. The Fender's lower primary resistance allows for more current to flow through the tube. As a result, without changing the Cathode resistor, a higher voltage will appear. No matter what the Cathode resistor, we will have 10ma flowing through the tube. So it wouldn't make a difference if R31 were lowered to ~250 ohms. We would still have 10ma through the tube. And this would be ok because the max current rating through a 12AT7 is 15ma.

[g1]

Another thing Tom, your voltages written on the schematic indicate a 700ohm resistance for the transformer. Try disconnecting C21.
Fender numbers are usually around 1K7 here I think. The peavey voltages indicate the peavey transformer primary resistance would be around 4K.

[g1]

Do you get that same 475ohm reading for R31 when it's connected in circuit?

[TomCarlos]

The junction of R29 and R40 have .001v.

Pin 7 has .024 volts.

I removed R31 to check it. When I reinstalled the resistor, I scraped away at the trace to make sure the resistor lead had a good solder connection to the trace.

[Helmholtz]

Thank you Helmholtz... I will take a look at C19 as well.

Just took a second look at the schematic and I meanwhile doubt that a leaky C19 could be the culprit as the voltage at the junction of R29 and R40 should be zero.
Does R31 have a good ground connection?

[TomCarlos]

Thank you Helmholtz... I will take a look at C19 as well.

[Helmholtz]

So it's not clear to me why I am getting close to 10ma when I would be expecting to see around 5ma.

Tube current is mainly determined by the bias voltage (grid voltage minus cathode voltage). The small difference in plate voltage cannot explain the much higher current (actually it would take a higher plate voltage to increase current).
You may have a leaky C19 and the the actual gid voltage might be higher than .05V because your meter resistance loads down the voltage.