5C5 Tweed Pro low preamp plate voltages

[jvm]

I have a 5C5 Pro (tube chart shows 5A5 but circuit is 5C5) with a low preamp plate voltage issue. Power supply nodes measure 412v, 348v and 314v. Both 6SC7 preamps show 68V at the plates and one side of the phase inverter shows 60V plate and 237V on the other plate. Coupling caps are new, plate resistors all within spec. I have 359V at all four plates with all three 6SC7's out of the amp. New power transformer. Classictone 40-18029. Original PT had a shorted secondary. Any ideas as to what is pulling my plates down?

[Enzo]

Your plate loads are 250k. Your B+ is 314v. Plate voltage is 60. so 250v dropped across the resistor. That comes out to 1ma tube current, which is my rule of thumb reading. Sorry 68v not 60. Doesn't change much, still 1ma. I don't see it as a problem.

Look at 5E5, they went to 12AY7 and 100k plate loads, with a noted plate voltage of 100. That is still right on the 1ma current. With 250k resistors, I see 60v as reasonable.

237v on one plate? That B+ node is still 314v, so we are only dropping 77v across the 250k plate resistor. I=V/R=0.3ma, pretty low. I would wonder why that triode is conducting so lightly. Swap your 6SC7s around, is that one pin still real high like that?

I suspect your meter is influencing things. 359v at all the empty plate pins means you are still dropping 53v. With all tubes out, your B+ ought to be the same at all nodes. Measure directly from the 412v node to the 314v node and see how much drop there is. Do not read each one to ground and calculate. I bet you get less than 50v. Unless your board is leaky or some such.

[Randall]

Which side of the PI has the 237v plate, the side being fed directly from the pots, or the other side? I suspect that is where your problem .... wait, what is the problem? Does the amp sound right, or not?

I have never really looked at this circuit before. It's really, I dunno, primative? No grid stops or cathode bypass on the preamp tubes. Volumes and tone pots mix directly to first side of PI. And I'm not sure what is going on with the second side of the PI with the 6800 ohm resistor. Interesting.

[Enzo]

Look at the 5C5 schematic again. There are no cathode bypass caps in the preamp because the cathodes are grounded. See the 5meg grid resistors and input caps? These tubes are grid-leak biased.

The PI is a basic paraphase circuit:
http://www.valvewizard.co.uk/paraphase.html

yes, it is a basic circuit, this is a basic amp, nothing fancy about it.

The PI is not hard to understand. The signal enters the top triode at the grid, and the inverted plate signal feeds the upper power tube grid. Note the signal at that power tube grid is inverted from the signal at the PI input. SO they sample the signal at that grid and feed it to the lower PI triode grid, and that signal feeds the lower power tube.

Don't worry about that 6800 ohm. The two power tubes have 250k grid return resistors, adding 6.8k to one is less than a 3% difference, so the amp won't care. But it is enough to voltage divide the grid signal for the other side.

[Dave H]

And I'm not sure what is going on with the second side of the PI with the 6800 ohm resistor. Interesting.

They are assuming the gain of the bottom triode of the PI is 38. The 250k and 6k8 divide the signal from the top triode down by 38 i.e. (250+6.8)/6.8 and apply it to the grid of the bottom triode which inverts it and amplifies it by 38 to make the PI outputs equal amplitude but out of phase.

[Randall]

AH. That is pretty cool stuff.

[jvm]

So my preamp plate voltages are ok. I am not too familiar with the tweed Fenders and wasn't sure about them. The original issue with the amp was a shorted PT. I replaced it but the amp has very low headroom. With the low plate volts on one side of the PI, I looked at the wiring on the PI and noticed that it was wired incorrectly ( I assume for a different triode?). Pins 1 and 3 were reversed. I wired it like the factory layout and we have a proper sounding Pro! One other question, the field coil speaker has been replaced with a C15R. No choke or resistor has been added to the amp. Is it ok as is?

[g1]

There was a field coil speaker in it? Seems odd for a 5C5.

[jvm]

The tube chart reads 5A5 but I've been using a 5C5 schematic and layout. Sorry for the confusion, g1! The owner says the field coil speaker was removed about thirty years ago.

[g1]

One other question, the field coil speaker has been replaced with a C15R. No choke or resistor has been added to the amp. Is it ok as is?

The field coil speaker in the tweed Pro's were wired up the "other" way. Not in series as a choke but from B+ to ground. They were a much higher resistance, around 7K versus the few hundred ohms of the series type.
So if you just remove the field coil, it still works with no re-wiring or changes required.

The only consideration is that you have now removed a load from the power supply so the voltages will rise a bit. I guess you could compensate with a 7K resistor rated for around 25watts. But now you are adding substantial current to a valuable old transformer in an amp that has been doing fine without the extra load for the last 30years since he replaced the speaker.
I think I'd check that the voltages are not exceeding the ratings on the filter caps and leave it be. Or maybe look into a rectifier tube type that would give more voltage drop than the 5U4.