Hoffman Princeton Reverb no reverb or tremolo

[Stevewdewitt]

I have converted a pignose GV40 into a princeton reverb clone, with no reverb or tremolo, and used the Hoffman schematic and layout. I took a few shortcuts with the layout because I didn't have the room in the chassis. I used the transformers from the GV40. This amp was working fine before I gutted it. It doesn't have a center tapped HV winding so it has a full wave bridge rectifier. I followed the schematic and just took out the reverb section. I added a MV that I have used in a prior princeton reverb build. The voltages all look good until I put in the power tubes. I have 450 on the plates without tubes and 250 with tubes. The tubes get hot. The volume is pretty low and the current limiter light comes on pretty bright. My other princeton with the same circuit and reverb the light barely lights up. I have tried 3 different pairs of new JJ 6v6s

I think there is a short but I don't know where.

I added the schematic I used. I adjusted the preamp section and took out the reverb. I didn't change the pwr section of the schematic but my amp has a solid state full wave bridge rectifier.

I will attach photos

[Stevewdewitt]

better pics

[Stevewdewitt]

more pics

[Stevewdewitt]

final pics

[PRR]

Never fire-up a fixed-bias amplifier until you check for ample -negative- voltage at the power tube grids.

I don't think that bias rectifier works with that main rectifier. I suspect you have no grid bias at all. The power tubes try to suck "infinite" current, pulling down B+ and burning the tubes fast.

[Stevewdewitt]

you are correct there is only a few millivolts on the bias circuit. Here is the pignose fixed bias section.

[sluckey]

Use the pignose bias circuit in the PR. Copy it  ***EXACTLY***.

[Stevewdewitt]

Will do. Thanks. I will give an update and update the schematic

[sluckey]

Play it safe!

Don't plug the output tubes in until you measure approx. -40VDC on pin 5 of each 6V6 socket.

There's a good chance those 6V6s are already damaged.

[Stevewdewitt]

I have -35 VDC on pin 5 of the power tubes. I measured before I put the tubes in.

I also have a new set of tubes in and no output. I get noise if I touch the plates of the tubes with a probe to measure voltage. I also get a really loud hum that won't stop if I touch the grid of V2 pin 2 or pin 3. oscillation? The second half of V2 is the Phase Inverter. I removed the master volume and replaced it with the 1M resistor.

voltages
V1 P1: 174vdc, P3: 1.5vdc P6: 175vdc, P8: 1.5vdc
V2 P1: 268vdc, P6: 212vdc, P7: 21vdc, P8:59vdc (the second half of V2 is the PI)
V3 P3: 382vdc, P4: 379vdc, P5: -35vdc, P8: 28mvdc (with 1 Ohm resistor to ground)
V4 P3: 382vdc, P4: 379vdc, P5: -35vdc, P8: 29mvdc (with 1 Ohm resistor to ground)

I attached the adjusted power section schematic

[sluckey]

Voltages look OK. But what is the voltage on V2 pin 3?

Probably gonna be a wiring error. Double check, especially input/output jacks.

[Stevewdewitt]

Voltages look OK. But what is the voltage on V2 pin 3?

Probably gonna be a wiring error. Double check, especially input/output jacks.

If I try to measure the voltage on pin 2 or pin 3 of V2 there is a loud hum. Even with the volume down. Scared me the first time it happened.

[sluckey]

If I try to measure the voltage on pin 2 or pin 3 of V2 there is a loud hum. Even with the volume down. Scared me the first time it happened.

You need a 1M resistor from pin 2 to ground. Your schematic shows none.

[Stevewdewitt]

I don't think I should add a 1M resistor to V2 pin 2. I have attached a schematic with the correct tube socket numbers. Sorry for the confusion.

You can see that V2-A is the stage prior to the PI and V2-B is the PI

[Stevewdewitt]

Found the issue. I need the 10pf capacitor between the plate of V1-B and the grid of V2-A.

Attached the preamp section as it now is in the amp.

Thanks very much for all the help

[sluckey]

I don't think I should add a 1M resistor to V2 pin 2. I have attached a schematic with the correct tube socket numbers. Sorry for the confusion.

You can see that V2-A is the stage prior to the PI and V2-B is the PI

There was no confusion. I knew the tubes were labeled differently. However V2-2 MUST HAVE A RESISTOR TO GROUND! Without it, V2 cannot bias properly.

[SILVERGUN]

A wise man once said :
"You need a 1M resistor from pin 2 to ground. Your schematic shows none."


That is way more fact than opinion.


And, if you leave that 10p there it is the frequency equivalent of running a fire hose through a silly straw.

[Stevewdewitt]

Ok I will add the 1M resistor

thanks very much

[SILVERGUN]

And, if you leave that 10p there it is the frequency equivalent of running a fire hose through a silly straw.

Subtract the hyperbole and there is a critical point there.
I'm assuming that you carried the 10p over from the original circuit schematic. It appears there in the context of that specific circuit and does not fit the context of yours unless your intention is to cut everything under 15Khz.
I'm also assuming that was not your intention and even though the amp works you might want to hear all of those mistakenly attenuated frequencies.

[Stevewdewitt]

I added the 1M to ground on pin 2 of V2. I left the 10pf cap in but now think I should take it out.

[PRR]

sigh

[Stevewdewitt]

Thank you

[Stevewdewitt]

sigh

well I put in a short wire and if I turn up the master volume at all there is a very loud "honk" on the speaker.

If I have the 10pf cap and the 1M resistor to ground it is fine (it is attenuated as stated before but it isn't making a loud honking buzz). See attached.

The short wire from C13 to pin 2 of V2 is just not working

The voltage V2 P1: 268vdc, P3: 1.4vdc, P6: 212vdc, P7: 21vdc, P8:59vdc

[SILVERGUN]

sigh

well I put in a short wire and if I turn up the master volume at all there is a very loud "honk" on the speaker.

If I have the 10pf cap and the 1M resistor to ground it is fine (it is attenuated as stated before but it isn't making a loud honking buzz). See attached.

The short wire from C13 to pin 2 of V2 is just not working

The voltage V2 P1: 268vdc, P3: 1.4vdc, P6: 212vdc, P7: 21vdc, P8:59vdc

Steve, if there is one thing that we all agree on it is that the 10p cap does not belong there.

It would be in your best interest to remove it and then try to resolve where the "honk" is coming from.

I don't know how else to say this.

This is not a test.

[Stevewdewitt]

The cap is out. The noise happens when I turn up the master volume. Even if the volume is all the way down. I used the MV as described above.
I will keep at it
Thanks again

[sluckey]

well I put in a short wire and if I turn up the master volume at all there is a very loud "honk" on the speaker.

Wire has nothing to do with the honk. Let's see your master volume. Maybe you just need to swap the OT plate leads.

[Stevewdewitt]

The MV I used looks like the one from Rob Robinette except it has a jumper from the middle lug to the .022 cap

Is that correct because his site doesn’t have it that way

The first post in this thread has the MV I used. Attached is what is on the robinette site.

I can swap the OT leads. I never removed them when I gutted this amp.

Gives me something to try.

Thanks

[SILVERGUN]

The MV I used looks like the one from Rob Robinette except it has a jumper from the middle lug to the .022 cap

Why?

[sluckey]

Why indeed?

Remove that jumper.

[Stevewdewitt]

Ok, thanks all. I removed the jumper on the MV. I swapped the OT plate leads. Problem solved. 

It sounds really great without that 10pf cap on pin 2 of V2

I will upload a schematic Thursday.

thanks again!

[Stevewdewitt]

I added a 470k resistor on the grid of the PI. This is based on mods suggested by Rob Robinette for cathodyne phase inverters. I have also attached the schematic.

Thanks again. It sounds good. Only issue I have now is that the volume control squeals very slightly at full volume.