Restoring a 1974 Deluxe Reverb - Seeking Advice

[bcmatt]

OK, swapping the 27K in for the 10K, brought voltages generally down about 30 or 40 Volts for V1 and V2 plates, but that's only about halfway to the schematic and it is still overly clean. It didn't really change things for the power tubes.
I'll have to doublecheck, but I don't think it changed much at C either.
So theoretically that Resister could be increased more? What are the side affects?
Also, I'm trying to figure out the wattage it ought to be as well... For calculating the voltage part of the formula, it's how much the voltage drops across it? If it's the voltage above ground, I feel like it would need to be up around a 10 watt resistor...

[martin manning]

The D node supplies about 5 mA to the preamp (which you measured), so by P = I^2*R, a 27k would dissipate about 0.7W. A 2W would be fine. And yes, it’s the voltage and current across the resistor that matters.

Still don't understand why the preamp voltages are so high when the OT C voltage is pretty much spot-on. Is the value of the dropping resistor between B and C ok? C is also quite a bit high.

[didit]

Still don't understand why the preamp voltages are so high when the OT C voltage is pretty much spot-on. Is the value of the dropping resistor between B and C ok? C is also quite a bit high.

Exactly so ..

[bcmatt]

I took the 10K Dropping Resister that was between C and D and put it in the place of the 2.2K between B and C. 27K is between C and D. The plate voltages are much closer to the Schematic now... Cathode voltages are all rather low though:
Here's my readings in blue:

New Dropping Resister values.jpg

Another thing I noticed is that the Schematic shows a 25/25 Cathode bypass cap on V3 (Reverb driver) but my amp just has a resister. It looks to be a 470ohm instead of the 2.2K that is with the 25uF on the schematic (and every other Deluxe Reverb Schematic I looked at.
I think the reverb sounds fine though.

Everything is still too clean though. If bringing voltages down was any help... it was a very subtle difference. Still need to be above 8 with big chords on humbuckers to get any hint of breakup.

[bcmatt]

That sounds reasonable, but how the input stages can be 100 volts high is a mystery. I guess I’d go ahead and replace the cathode bypass caps, and check the cathode resistor values while the cathode caps are out.

Ya, I don't understand it. I tested all the plate and cathode resisters and they are all on spec. The dropping resisters as well.
I should just try some other 12ax7s. I already tried swapping the 2 7025s that were in V1 and V2 but perhaps they are both doing something weird because of their age.

[didit]

What is the ‘D’ node voltage with V1/V2 & V4 pulled? While sockets are empty can you add a static load temporarily to ‘D’ with 5W resistor at reasonable value (Ohm’s law) to hit a current target of ~10mA? What’s the voltage loaded? The goal being, 100% confirm "load" weakness somewhere in and around V1/V2 and/or V4.

Could be tubes yes, and a good place to start. However, could also be weak socket connections or some other troublesome part of the circuits.

..

[bcmatt]

What is the ‘D’ node voltage with V1/V2 & V4 pulled? While sockets are empty can you add a static load temporarily to ‘D’ with 5W resistor at reasonable value (Ohm’s law) to hit a current target of ~10mA? What’s the voltage loaded? The goal being, 100% confirm "load" weakness somewhere in and around V1/V2 and/or V4.

Could be tubes yes, and a good place to start. However, could also be weak socket connections or some other troublesome part of the circuits.

..

OK, I'll circle back around to this...

But tubes seem to be significant. I was using all the original RCA that it probably came with from the factory. I had swapped V1 and V2 previously to check if there was a difference, but it just so happened that those are the two most problematic tubes. I swapped v2 with v5 and all the gain came back and the plate voltages came down on V2. I'm not sure how many of these tubes have issues, but I have moved them around enough that it is believable that I I just moved problems around and with at least 2 bad tubes, it makes sense that it's been a more confusing process.
So, I'll put the dropping resisters back to stock for now and see if I can get close to schematic voltages through some tube swaps.

[didit]

So, I'll put the dropping resisters back to stock for now and see if I can get close to schematic voltages through some tube swaps.

Maybe hold off with your soldering iron until you’ve tested with a full set of known good tubes?

..

[martin manning]

It was looking like the preamp tubes were weak, but all of them being weak just didn't seem likely. A fresh set would be my recommendation.

[bcmatt]

I can't seem to find my box of spare preamp tubes at home. I probably left it at work. I'm hoping I have a couple 12at7s in it. When I swapped the the reverb driver with the phase inverter it gave some wacky voltages on the phase inverter plates - VERY uneven. because the reverb driver is wiring the triodes in parallel I guess the reverb still worked pretty normally.

I have other amps at home to steal 12ax7s from at least. It's quite possible that the only original tube still good on this amp is the 5u4.

So, for now, I have the dropping resisters as 5.1K and 27K. The 5.1K helps me get the correct 375V right at the C node. The phase inverter plates are still reading 13-20V high, but I would be willing to blame that on the tube, so I will wait till I can get a new on in there.
With new 12ax7s in V1 and V2, I am getting quite close to schematic voltages on their plates and cathodes ....maybe just slightly low, so I'll try a 22k instead.

So, the gain issue was tubes, not related to voltages, but I do have unexpectedly high preamp voltages with the stock dropping resisters. I do see different deluxe reverb schematics with a range of values.... most earlier ones using 10K and 10K. I saw one schematic with a 15K handwritten on... so perhaps there is a range of what was used as they kept changing their PT specs to use different rectifiers, etc.

But ya, I will concentrate on getting all good tubes in.