DeArmond (Rowe) Guitar Amplifier Model R25T Restoration

[g1]

Agree with Mozz, I think he meant resistive voltage divider. With the 2 resistors rather than .02 cap and resistor.
At least thats how I think of it anyway (voltage divider), some will call it a charge pump.

[misterc57]

Speakers... What would you do?

I will have an opportunity to purchase the original speakers that were in there, P10Q, for the same cost of purchasing new P10Q.

[mozz]

Late 50s early 60s P10q/r go for good money. On the other hand, they could be dry rotted and blow up the first time it gets cranked. I would worry about getting it up and running first, measure your output power, try it through other speakers.

https://www.ebay.com/itm/25618252546...is&media=EMAIL

[misterc57]

Agree with Mozz, I think he meant resistive voltage divider. With the 2 resistors rather than .02 cap and resistor.
At least thats how I think of it anyway (voltage divider), some will call it a charge pump.

Here is the bias section after all components replaced per schematic. Cap is a 47/160V.

[misterc57]

Here are heaters connected to first 6L6 tube socket, pin 2 and ground. CT not connected to anything and taped off.

Next task review filtering section.

[mozz]

I know old fenders and such used the grounded filament scheme. If you end up with 60/ 50hz hum I would rewire it and even put in a humdinger.

[misterc57]

Filter section is good. I have sound from Channel 2 but no sound from Channel 1. V1 pin 8 is reading a low 0.25 VDC, I am going to replace all the 25/25V bias caps.

[khmassey]

Great work. Nice to bring one back from the dead. Especially one of the rarest out there.

[misterc57]

Replaced both V1 bias caps, still no output on Channel 1, replaced V1 tube and I now have output on both channels and better V readings. Funny that replaced tube measured okay on the tube tester but one side had no output at the plate.

Tremolo is funky, sometimes working and not working. Looking at this next.

Yesterday I purchased the original Jensen P10Q speakers and footswitch that belonged to this amp.

Measured 40 watts on the output tubes pair. Or is it half that because of paired tubes? See numbers below...

[Helmholtz]

Measured 40 watts on the output tubes pair.

So you measured 40W into 4R dummy load before clipping?

[misterc57]

So you measured 40W into 4R dummy load before clipping?

Measured at idle into 8R speaker. I can try again into a 4R dummy load.

[Helmholtz]

Measured at idle into 8R speaker. I can try again into a 4R dummy load.

Measuring at idle gives tube DC dissipation but no output power
To measure output power you need a feed a signal and measure RMS voltage across the load.
Scope will tell you if the ouput is clean or clipped.

There is no relation between tube dissipation and output power.

[misterc57]

Here are some scope pics, first is channel two volume at 6, second is volume at 7. Instead of seeing a clipping on the wave form, the wave begins to jump around past volume of 6.

[Helmholtz]

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

[misterc57]

Need advice on getting the Tremolo to work. Currently the Tremolo does not work, I thought I was getting some faint effect but was mistaken.

The Tremolo circuit uses V5. These are the readings at V5

Pin 1 = 145 V
Pin 3 = 1.7 V
Pin 6 = 333 V
Pin 7 = 145 V
Pin 8 = 148 V (this V varies with the Rate knob.