Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Paulycoo on September 15, 2017, 12:31:57 pm

Title: Hoffman Princeton reverb build loud hum
Post by: Paulycoo on September 15, 2017, 12:31:57 pm

Hello again everyone,
I need your help!
Finished my Princeton reverb build and am overjoyed at how it looks.
But...
Has a loud buzz. Very loud. Not just hum.
I've pulled v4 phase inverter tube and the buzz stops...just low vol guitar signal.
Vol pot does not affect the buzz.
I've checked wiring twice using your highlighter method and the schematic.
Any ideas?
Thanks in advance! Regards from the Lake district UK Paul

Title: Re: Hoffman Princeton reverb build loud hum
Post by: mresistor on September 15, 2017, 12:58:16 pm

You might have the output transformer out of phase, try swapping the OT leads going to the 6V6 plates  - pin 3 on the power tubes.

Title: Re: Hoffman Princeton reverb build loud hum
Post by: 92Volts on September 15, 2017, 01:11:24 pm

I've had loud hum because of a heater-cathode short, which as a bonus can screw up the bias of the tube in question.

Buzz, instead of "hum", is usually poorly-filtered DC while hum is symmetrical AC. It could be a loose (or failed) filter capacitor. Alternately, something could be drawing so much current your power supply can't adequately filter. This could be caused by a loss of cathode bias (short to ground and/or heaters) OR it could be caused by DC hitting the grids.

So, check for leaky (or I guess missing) coupling capacitors.

The cap from the cathode of the cathodyne phase inverter to one of the power tubes seems like a prime suspect. This location will have 0 (ish) DC voltage with no tube installed, but more than enough with a tube to cause trouble.

If the (likely 1M) grid-reference resistor of the cathodyne PI is hooked to the cathode instead of the midpoint between the 2 cathode resistors, that would also bias the PI too hot.

Title: Re: Hoffman Princeton reverb build loud hum
Post by: Paulycoo on September 15, 2017, 01:42:09 pm

Thank you very much I'll try those things right away! Top dudes cheers Paul

Title: Re: Hoffman Princeton reverb build loud hum
Post by: jjasilli on September 15, 2017, 05:20:58 pm

You didn't specify what you overhauled.  Did you replace the filter caps?


If a few quick approaches don't work, use a flowchart.  They are posted here & on geofex.com

Title: Re: Hoffman Princeton reverb build loud hum
Post by: TIMBO on September 15, 2017, 05:26:07 pm

Could be an earth not grounded securely.

Title: Re: Hoffman Princeton reverb build loud hum
Post by: Paulycoo on September 16, 2017, 10:43:21 am

Thanks a million Mresistor!
I'd have your babies for you!!!!!
Just swapped blue and brown from the OT to the power tubes to!!!
Buzz gone!
Sounds immense.
Thank you all so much I'm well proud of myself.
Best regards from the UK Paul

Title: Re: Hoffman Princeton reverb build loud hum
Post by: mresistor on September 16, 2017, 04:37:30 pm

Glad you got her fixed ! 

Title: Re: Hoffman Princeton reverb build loud hum
Post by: purpletele on September 16, 2017, 09:39:41 pm

Quote from: Paulycoo on September 16, 2017, 10:43:21 am

Thanks a million Mresistor!
I'd have your babies for you!!!!!
Just swapped blue and brown from the OT to the power tubes to!!!
Buzz gone!
Sounds immense.
Thank you all so much I'm well proud of myself.
Best regards from the UK Paul

Pauly,

How about a photo?