Noise out of a Princepton amp kit

[jamviva]

Hi!

I'm building a Doctor Amp princeton kit and after building it completely I can't make it to work properly. The only sound I get out of the amp is a loud noise.

I've checked if I had positive feedback but the noise is still there after reversing primaries on the output transformer or removing the feedback wire.
This is the noise I'm getting Amp Noise

Any advice of what should I checked? any mistake I should have made? Any bad component?

Thanks!!!

 

[rayma]

Post the schematic, and some photos.

 

[jjasniew]

Is this it? Sounds like a problem with the power supply. Diode in backward? How do the voltages measure across the B+ cap / resistor string? Be careful with your meter probe; maybe power cycle off before connecting / disconnecting to the 3 nodes A, B, C with the design voltage values shown.

 

[6A3sUMMER]

Lots, and Lots, and Lots of gain . . . plus a very small amount of feedback from the wiring dress creates an oscillator.

3 stages of 12AX7 gain is a lot (the concertina does not count, its gain is slightly less than unity).

I am guessing that another possible cause, may be a ground loop that is creating feedback.

But now I listened to your sound link. It sounds more like hum.
The curious thing, is it starts soft, quits, than comes back strong.
Ground loop?

Come on Guitar Amp enthusiasts, do you have a Princepton amp?
What say you?

 

[avtech]

Is this it?

Could also be the more simple 5F2 Princeton, so need to be specific.

The audio clip sounds like a tig welder!

Post the schematic (or clearly identify which amp) and chuck in some photos and maybe someone will see something that might be out of place.

 

[dubadub]

Do you know which version of the Princeton you are working with? Here's a schematic of the 5F2A. Check the lower drawing with all the wiring and compare it to yours.

The hum sounds like it's tracking the rising HV after power on, like a lot of Ripple current is passing thru the power supply to the audio circuit. Double check all the wiring. In a point-to-point build, one wire out of place can allow all sorts of noise into the audio path. Neatness counts.

Don't electrocute yourself. Unplug the thing before you go in there, make sure the caps in the power supply are drained before you work on it. Every. Time.

 

[jjasniew]

The hum sounds like it's tracking the rising HV after power on, like a lot of Ripple current is passing thru the power supply to the audio circuit.

If the bias diode was reversed, killing the output tubes as power comes up...

 

[jamviva]

The schematic I'm following is this:

Thanks for your responses. I've managed to find that if I remove V4 (vibrato and phase inverter) the noise is gone but I still can't find out where it comes from.

Any idea?

Thanks again!

 

[tubelectron]

Thanks for your responses. I've managed to find that if I remove V4 (vibrato and phase inverter) the noise is gone but I still can't find out where it comes from.

Any idea?

Yes : it comes from the preamp section, not from the power stage and its supply.

Reinstall V4, put your amp on normally, and successively ground the grids of V1A, V1B, V3B, V4B, through a 1µF capacitor if needed for V4B. When the noise ceases, then you know which stage is the culprit, and so where to start the troubleshooting search...

T

Come on Guitar Amp enthusiasts, do you have a Princepton amp?
What say you?

Said above !

Yes. Below my mid-70s Princeton Reverb, in mint condition :

T

 

[jjasniew]

Below my mid-70s Princeton Reverb, in mint condition :

My roommate in sophomore year of university had one of those, along with an off brand Les Paul style guitar. I had a Gibson EBO bass then.