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FDP Forum / Amp Mods, Repairs, and Projects / Help with 1981 PR tremolo circuit!

Axe-victim

USA/Ohio

Feb 12th, 2019 12:41 PM   Edit   Profile  

I'm working on a 1981 PR w/boost pedal (not a PR II) and the tremolo doesn't work when the amp is warmed up and on. I can hear the tremolo when a chord is strummed and the amp turned off and the power is decaying. It comes through strong at that time. I've pulled every component in the circuit and they all check within tolerances. Checked continuity of all the connecting wires. Replaced V4 several times. Here's a table of voltages I've measured (Fluke 112) with the trem at 0 and 10.

Tr=0 Tr=10
BIAS -32.5v -31.5v
Plate 200-280(unstable) 240v
Cathode 1.25v 1.8v

The plate voltage varies greatly except on the other side of the 220k resistor where it's stable at 392v. The schematic calls for 250v on the plate and 2v on the cathode.

Is there anyone out there that can point me in the right direction?

pdf64

UK

Feb 12th, 2019 01:25 PM   Edit   Profile  

Re 'bias' do you mean that you're measuring -32Vdc on terminal 2 of V4?
If so the tube is massively into cut off.

Or do you mean the Vdc at the trem intensity control wiper? ie the bias applied to the power tubes.

When you refer to 'the trem at 0 and 10' do you mean the trem intensity or trem speed?

schematic

Axe-victim

USA/Ohio

Feb 12th, 2019 02:54 PM   Edit   Profile  

Bias refers to the power tubes. Yes, at the intensity wiper.

Yes, I was referring to the intensity and trem speed.

pdf64

UK

Feb 14th, 2019 04:50 PM   Edit   Profile  

What is the cathode or plate current at idle of the power tubes? eg mV across 1 ohm cathode resistors.

Axe-victim

USA/Ohio

Feb 18th, 2019 12:06 PM   Edit   Profile  

I finally got around to checking the plate current and it was way off! 39mA on both tubes. That’s 15.6 watts! I replaced the 22k resistor in the bias circuit with a 33k and got the current to 20mA with 440V plate voltage. Now down to 8.8W which is around 70% of the max wattage. Thanks for pointing me in that direction.

Still no tremolo, but I can’t hear it when the amp is powered down like before. Any more suggestions?

SoK66

USA

We had the hit but Van got the money
Feb 18th, 2019 03:15 PM   Edit   Profile  

Use the schematic pdf64 posted and check your voltages on V4. If they're ok at your new bias settings, it's possible one or more of the three ceramic trem caps (.02 & 2x .01) is leaking. Should have a nice, rich vibrato.

SoK66

USA

We had the hit but Van got the money
Feb 18th, 2019 03:15 PM   Edit   Profile  

Use the schematic pdf64 posted and check your voltages on V4. If they're ok at your new bias settings, it's possible one or more of the three ceramic trem caps (.02 & 2x .01) is leaking. Should have a nice, rich vibrato.

Axe-victim

USA/Ohio

Feb 19th, 2019 11:44 AM   Edit   Profile  

Fixed it!! Figured out that the tube socket wasn’t making good contact with pin 1 for the tremolo side of the tube. Replaced the socket and it works great. Thanks pdf64 and SoK66. Your questions and requests helped guide my to the solution.

Peace be with you both!

FDP Forum / Amp Mods, Repairs, and Projects / Help with 1981 PR tremolo circuit!




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