Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on July 03, 2015, 08:16:15 pm

Title: been there done that but.....princeton reverb trem wont oscillate
Post by: phsyconoodler on July 03, 2015, 08:16:15 pm

yeah i know,ive been here before but this once in a blue moon oscillator issue is weird. I built another princeton reverb and cant get it to oscillate....again!
 replaced every component. Tried sluckeys idea to decrease the 1 meg resistor to 220k.Nadda.
Tried all my band-aid fixes that have worked in the past.....nadda.
   I hate oscillators!

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: sluckey on July 03, 2015, 08:28:44 pm

Surely you don't have a wiring error.  :icon_biggrin:   That oscillator needs a strong tube. Get a bucketful and start rolling. Could be that simply. Probably not.

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: tubeswell on July 03, 2015, 08:33:12 pm

Yeah - check you haven't cross-wired the plate and cathode pins etc or something like that

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: clyde on July 03, 2015, 08:38:29 pm

Bigger cathode cap?

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: Deric on July 03, 2015, 08:46:15 pm

Just to be clear...


"won't oscillate" or "can't hear it" ?  If it's "can't hear it" make sure the power tubes are biased cold - at least to start with.  If that works and it just needs "more" try an LED in place of the cathode resistor/cap.


Good luck!

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: Willabe on July 03, 2015, 09:30:26 pm

IIRC, on the 20w Marshall(?) Mojo kit you upped the oscillator plate R to 470K(?) and put a LED in for the K R and then it kicked the oscillator in gear. Then you backed down the K R.

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: lego4040 on July 04, 2015, 07:48:57 am

Is this a kit? If so,who's. I'd like to build one(after listiening to Jim Compilongo's Orange Album)eventually and if I run into this issue I'll have the answers. Good luck with the build

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: phsyconoodler on July 07, 2015, 12:41:18 am

No its not a kit.I built two identical ones with everything the same.The other one works just fine.
Changed every component associated with the oscillator now including the pots. Upped the plate resistor to 470k from 220k and still nothing.I have a bias pot on this amp and tried every bias setting from 10ma and up.Nothing.
  tried ten different tubes. the only thing that is remotely different is the plate voltage is at 355v and the schematic calls for 260v.
Im at a loss here.  i have been here before with the princeton reverb and once with a marshall kit amp and it makes no sense.


 

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: AZJimC on July 07, 2015, 02:03:25 am

Quote from: phsyconoodler on July 07, 2015, 12:41:18 am

the only thing that is remotely different is the plate voltage is at 355v and the schematic calls for 260v.

Could it be the cathode R is wrong value or not grounded? Seems like if that tube was idling, it would pull the plate voltage down, especially through a 470k.

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: sluckey on July 07, 2015, 06:31:08 am

Quote

Could it be the cathode R is wrong value or not grounded?

That seems likely. The high plate voltage indicates the tube is not conducting. What voltage do you measure on V4 pin 3? What resistance do you measure from pin 3 (directly on the socket) to chassis?

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: lego4040 on July 07, 2015, 10:18:01 am

Maybe not same problem but on my Rene I just got working I had a similar problem where all my voltages where high and I had almost no volume. I found one of my heater wires off of v4. Once I rewired and got that powered up I had my head running again

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: phsyconoodler on July 07, 2015, 11:06:51 pm

The voltage on pin 3 is 5.4v.
   I haven't measured the resistance.  I will do that tonight.
I will revisit that socket for sure.

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: sluckey on July 08, 2015, 07:09:40 am

I'm betting you have a 33K instead of 3.3K on the cathode (pin 3).

If you have a 3.3K and 5.4v across it, that means there is 1.6mA flowing thru the tube. That same 1.6mA would flow thru that 220K plate resistor causing a 360v drop across the plate resistor and leaving the plate voltage at only 40V. That's assuming that Node B is 400v like the schematic says. Well, this can't be!

But if the cathode resistor were really 33000 with 5.4v across it, there would only be 0.16mA flowing. This would cause only 36V to be dropped across that 220K, leaving 400-36=364v on the plate. This sounds like your situation.

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: phsyconoodler on July 09, 2015, 03:04:54 pm

I shall check that forthwith

Title: Re: been there done that but.....princeton reverb trem wont oscillate
Post by: phsyconoodler on July 09, 2015, 03:20:32 pm

Sluckey I will need your mailing address or an address where I can send you a big fat bonus cheque!
  Not only was it not a 3.3k it was a 270k. How that happened is beyond me but it just goes to show that I should get back into the practice of measuring every resistor before installing it. I know better!
   The resistor was damaged somehow cause if you look at it under a magnifying glass it has a chunk missing.
  The voltages are the clue. I was walking around with blinders.
 Thank you for this and countless other help you have given us over the years! £ :worthy1: