DeArmond (Rowe) Guitar Amplifier Model R25T Restoration

[g1]

How much swing are you seeing at the pin8 voltage?
If it's substantial, most of the trem is working and the varistor would be suspect.

[misterc57]

Yes, that happens when the plate signal hits the rail and there's (too) little filter capacitance.

Up these two 10@500 to 22@500? I was thinking to do this if I had an issue with filtering.

[Helmholtz]

First make sure power tubes are balanced, i.e. idle currents are close.
Then check if filter caps are in spec (capacitance and ESR).

I don't recommend to unnecessarily increase filter caps because that changes amp dynamics and feel.
Increasing the first 10µ to 22µ, though, shouldn't hurt.

[misterc57]

How much swing are you seeing at the pin8 voltage?
If it's substantial, most of the trem is working and the varistor would be suspect.

145V to 200V. It is not linear, should it be? Numbers bounce around all over and it is not linear with the rate pot, should it be?

What I am seeing now is that there are certain finite points along the rotation of the rate pot that produces a tremolo effect. Wondering about the rate pot.

[khmassey]

You could try jumping in a fixed resistor in place of the pot.

[misterc57]

Both rate and depth pot track perfect on the DVM.

Not familiar with that varistor. What can I replace it with?

[g1]

What I am seeing now is that there are certain finite points along the rotation of the rate pot that produces a tremolo effect.

Can you be more specific? If it is good sounding trem at those points, the varistor is good and you don't need to consider it.

[misterc57]

Can you be more specific? If it is good sounding trem at those points, the varistor is good and you don't need to consider it.

I'm not sure. I rotate the rate pot very slowly and those "points" only happen occasionally, the point location are not consistent, and when I hit one the effect is only present for about one second and then disappears.

I tried using the footswitch and that made no difference. The footswitch jack is a "switched" jack but the gap has been set so that it is normally open = effect on. So I am testing without the footswitch connected.

Every resistor and pot in that network checks spot on. Every connection is solid. V5 voltages look close to schematic.

Pin 1 = 145 V, schem shows 175
Pin 3 = 1.7 V, schem shows 1.6
Pin 6 = 333 V, schem shows 320
Pin 7 = 145 V, schem shows 175
Pin 8 = 148 V (this V varies with the Rate knob., schem shows 175

I have worked on trem roaches before but not familiar with the varistor. Interesting that the effect seems to be placed on the V1 pin 8 cathode.

Thank you

[g1]

Not sure I understand what the varistor is doing there, but try shorting it out.
https://www.thegearpage.net/board/in.../post-12525397

[g1]

Further to above, the R5T has basically the same trem circuit, but no varistor and a 68K in place of the 22K at the cathode.

[misterc57]

Thanks for the advice on the varistor. I will be getting back to this project soon.

[misterc57]

Jumping the varistor did not help.

I measured a variance of R across the varistor ranging from 700K to 2.5M

The depth pot is tracking very bad and needs replacement,, getting R all over the place including as much as 38M

[misterc57]

350K CCW seems hard to source. This is a reverse audio taper. What would be the better choice, 250K or 500K?

And is it possible to use a 500K audio taper and wire differently to get the "reverse audio taper" or "CCW"?

Example on amazon Set of Four (4X) Bourns 500K SHORT Split Shaft Audio Taper Low Friction Potentiometers

https://www.amazon.com/Bourns-SHORT-...MSJ9.5UIrb5epH KJzmeMqJIqpxkAu91JMSo9WkTuFk35zpHTZXrTnooBVKC_jjec10l0z2 SD-Y3O7eWtqsmQJnm65z-KpUT1rIPguCxl69fscW77G78oH8dPCJ3eBwKrtkCyVYuIlOEBGvuvwOS sXT0iEfyUN4psf9dZw41xMjkySvV0lwlYKkzMfbjLTLAaQysotNdTzU4 1Gejdgk5c03GwF321GqRdUMYu2UHYL1aAiqvazRHhaH_ENgRuvdnWppT neL2PhsnG0NXqnTz56hZEAicTyahgyFvYeZiTxibl0PHc.H64r1mAatP 0Pyu5MqBHXy0_ytoL8kRMr1HFJ30ocUbU&dib_tag=se&keywords=re verse+audio+taper+potentiometer+500k&qid=1717094565&spre fix=potentiometer+reverse+audio%2Caps%2C131&sr=8-5

Thank you

[misterc57]

I found that the 1M Rate pot is also bad. I have temporarily clipped in a replacement 1M pot for the Rate and a 500K pot for the depth.

I have lifted one end of the 22K and varistor and temp soldered a 68K in its place.

Still no joy, the trem effect is trying but barely audible.

[g1]

Have you checked that V1 pin8 voltage is good, and it's cathode cap?