AC30 clone redplating in outer sockets only?

[fdesalvo]

Hello, all. Have any of you seen this before? Newish hayseed 30 build no leaky caps resistors test good. Very haphazardly assembled! Known good tubes redplate there as well. Also he connected the inner pairs plates with a cap and 3.3k resistor in series. Is this a usual practice with these? Ty!

[Helmholtz]

Please post a drawing of actual wiring.

A cap between plate and OT would render the tube inoperative.

[fdesalvo]

I’ll see what I can do. Looks like a 60’s to boost with a master volume, switchable bias resistor (47ohms/80ohms). That plate cap is literally connecting the plates of the inner el84 pair. I’ve never seen anything like it outside of a local feedback circuit in a preamp.

[g1]

If everything else on power tubes has usual connections, a resistor & cap in series across OT primary is sometimes used. Sometimes called 'conjunctive filter'. Sounds like what you are describing.

[Helmholtz]

If everything else on power tubes has usual connections, a resistor & cap in series across OT primary is sometimes used. Sometimes called 'conjunctive filter'. Sounds like what you are describing.

Yes, that's what I think from post #3.
But it's typically used to improve stability with global NFB. An AC 30 clone shouldn't have global NFB.

[fdesalvo]

Good call on the conjunctive filter. I’ve seen one in a 1942 National 400 I serviced last year. Still puzzled why only the outer tube sockets are redplating tubes.

[Helmholtz]

Measure tube socket voltages with tubes in (quickly) and out.

[The Dude]

^^^^^^Yes, that. Also, a schematic drawing would help (as Helmholtz suggested in post #2). Is this amp cathode biased? Is there maybe an incorrectly configured half power circuit/switch?

[g1]

If everything else checks out, maybe oscillation that is only affecting the outer pair?

[Mick Bailey]

The RCA manual describes the filter as "A corrective filter can be used to correct the frequency characteristic of an output stage using a beam power tube or pentode when inverse feedback is not applicable. The filter consists of a resistor and a capacitor placed in series across the primary of the output transformer. Connected in this way, the filter is in parallel with the plate load impedance reflected from the voice-coil by the output transformer. The magnitude of this reflected impedance increases with increasing frequency in the middle and upper audio range. The impedance of the filter, however, decreases with increasing frequency. It follows that by use of the proper values for the resistance and the capacitance in the filter, the effective load impedance on the output tubes can be made practically constant for all the frequencies in the middle and upper audio range."

The Dr. Z Carmen Ghia (2xEL84) uses this type of filter and I have an SE EL84 amp with the same type of filter. I originally had this switchable but liked it much better switched in so hard-wired it and used the switch for another purpose. I've not come across it in a 4xEL84 build.

It would be more usual to wire the filter directly across the OT rather than at the tubes, though right now I can't think of any adverse effect of this. If the amp is prone to HF oscillation could the filter be damping this on the inner pair? I would consider removing the filter for testing purposes.

[pdf64]

Some Watkins / WEM models used that filter eg https://el34world.com/charts/Schemat...nator_1965.pdf
Here's one with 4 x EL84
https://el34world.com/charts/Schemat...Joker_30w.jpeg

[fdesalvo]

I sorted it out gentleman. Dying battery in my meter was masking the nearly 400v on the plates lol. I swapped in an old 5R4 rectifier tube and she’s down to 306 V. No issues. Thank you.

[Helmholtz]

Confused.

1) High plate voltage doesn't explain why only the outer tubes redplated if all tubes connect to a common cathode resistor like with an AC30.

2) No good 5R4 should drop something like 100VDC. What was the original rectifier?

[fdesalvo]

Confused.

1) High plate voltage doesn't explain why only the outer tubes redplated if all tubes connect to a common cathode resistor like with an AC30.

2) No good 5R4 should drop something like 100VDC. What was the original rectifier?

Plate voltage was 385 sorry. I can’t explain it either, but the power tubes are all measuring within a few percent of each other.

[Helmholtz]

Again, what was the original rectifier?

So you measured tube idle currents?

Does this amp have a Hi/Lo power switch?