Restoring a 1974 Deluxe Reverb - Seeking Advice

[bcmatt]

Ok, I have another question... and I realize it may come off as rather stupid.
Exactly how clean is this amp supposed to be? I can't even seem to get it to break up even slightly unless I use humbuckers and hit full chords while the volume is at about 8.5.
The standard tricks only very subtley change that... Pulled V1. Swapped V2 for another 12ax7. Switched off NFB. Pulled off the grounded RCA jack for the Tremolo (that allows it to stay on without a pedal).
I'm pretty sure I could get a little breakup with single-coils around 5 on the volume before I started "fixing" the voltages.
It's not like this thing is quiet either. It's definitely in the "hurting the ears" volume range while it's still spanky clean... and I'm not a wuss. I like my loud music and loud guitar.
Is this normal for a silverface Deluxe? Where do most people expect a stock deluxe reverb to start breaking up on the dial? Am I just not used to Fenders? Is this why some people just have always-on boosters or compressors.... or should I be digging back into this thing and checking all my voltages?
I never did swap out all the little white cathode bypass caps because I heard that they usually hold up.

[martin manning]

I think you should be getting more distortion than you describe when it's turned up. Maybe a good idea to verify the preamp tube operating points. Perhaps you have some resistors that have drifted. What about the cathode bypass caps? have you replaced those?

[bcmatt]

I think you should be getting more distortion than you describe when it's turned up. Maybe a good idea to verify the preamp tube operating points. Perhaps you have some resistors that have drifted. What about the cathode bypass caps? have you replaced those?

Sorry, I was distracted when I was writing my last post. I meant to say that I hadn't replaced the little white cathode bypass caps. I had bought replacements, but had heard they seldom drift so hadn't yet replaced them.

[didit]

Hello —

Depending on guitar pickups somewhere round 4 to 5 is that obvious edge threshold. This was consistent within one notch across half dozen examples I had here at one time. Martin, as expected, has the guidance for you. Check preamp plate voltages too, if you haven’t already.

..

[bcmatt]

OK, I took some current voltages before changing out the old Mallory Cathode Bypass caps.

old cathode caps Deluxe.pdf

Voltages with old Preamp Mallories.jpg

The preamp plate voltages are all quite high. V1 and V2 are all between 80-120V over what the schematic says they should be while my power tubes are slightly lower than the schematic.

I guess I'll change the old Mallories out and check again.

Would it more likely be plate resisters drifting causing these high voltages?

[martin manning]

B and C voltages are not too far out but D is way high. Is the 10k dropping resistor from C to D really 10k?

[bcmatt]

B and C voltages are not too far out but D is way high. Is the 10k dropping resistor from C to D really 10k?

Yup! I just had to double-check, to make sure:

20240311_162223.jpg

[bcmatt]

Do you think I should try swapping it for something like a 27K resistor before replacing the cathode bypass caps?

[martin manning]

You could give that a try. Hard to believe that those early stages can be that much higher than the schematic when the PI is only 15-20V high, and the power stage plates are only a few volts high. It's as if there is some leakage somewhere. Just for grins, pull the 10k and see if there is any voltage on the D node.

PS Measure the voltage across the 10k and see how much current is flowing through it.

[bcmatt]

You could give that a try. Hard to believe that those early stages can be that much higher than the schematic when the PI is only 15-20V high, and the power stage plates are only a few volts high. It's as if there is some leakage somewhere. Just for grins, pull the 10k and see if there is any voltage on the D node.

PS Measure the voltage across the 10k and see how much current is flowing through it.

I'll check the current in a moment when I re-attach it.
With it pulled I get -0.85V on the D node when there is a bout +405V at the C node.

[bcmatt]

You could give that a try. Hard to believe that those early stages can be that much higher than the schematic when the PI is only 15-20V high, and the power stage plates are only a few volts high. It's as if there is some leakage somewhere. Just for grins, pull the 10k and see if there is any voltage on the D node.

PS Measure the voltage across the 10k and see how much current is flowing through it.

It's about 5.1mA across the 10k Resistor

[martin manning]

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.

[didit]

What’s the story between B & C? Seems the elevated potential begins there ..

[bcmatt]

So I finished the cathode bypass caps swap. I managed to check a few of the cathode resistors and they all seem to be quite reasonably within spec.
I checked voltages again and they are all about the same... Most have climbed about 4-10 volts because my wall voltage is up a few volts to about 125VAC since I took the earlier voltages when the wall was about 122.5VAC.
So obviously the sound is as spanky clean as before.

Any other thoughts?
Interestingly, I do think that I remember the amp would break up closer to 4-5 on the volume knob back before I began "fixing" my high B+ voltage. (I only had it like that for less than a week before I was trying to bring voltages down).

So, if I try increasing that 10K dropping resistor, a 3 Watt resistor should be fine, right? Because I do have a 3W 27K that I'm thinking I will try.
I'm guessing that would also end up pushing up the voltages before it at C and B? So I'd probably end up trying to increase the 2.2K resister in the process of balancing things...
Am I approaching this the right way?

[bcmatt]

Just to clarify the present situation, the amp is now stock aside from 2 thermistors on the wall side of the PT, and 2 68 ohm resistors on the Rectifier Tube socket on the PT (AC) side.