Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: seriesnuns on September 28, 2024, 01:17:49 pm

Title: Princeton Reverb noise
Post by: seriesnuns on September 28, 2024, 01:17:49 pm

Hi all,

Just finished a PR build with MM transformers and Jupiter caps. I also did an adjustable bias on the ground jack.

After my initial startup and I’m dialing in the bias setting, I was aiming for 19ma @ 400 plate voltage. I get a very loud womp womp that won’t lower with volume but does go away when I bias down below 15ma.

Anyone know what would cause this? My bias is a 25k pot with a 22k resistor to ground. I did not have a 10k.

Title: Re: Princeton Reverb noise
Post by: mresistor on September 28, 2024, 01:24:12 pm

disconect the feedback from the output jack.  If it helps then you have the OT out of phase and need to reverse the plate leads.


Also please tell us what PR you built. AA1164?   I assume that is it.   

Title: Re: Princeton Reverb noise
Post by: seriesnuns on September 28, 2024, 01:56:02 pm

Yes a pretty standard Aa1164 with a Rob R mods.


So you think swap the wires on the OT to the power tubes?

https://imgur.com/gallery/mIewOxn

Title: Re: Princeton Reverb noise
Post by: mresistor on September 28, 2024, 03:18:52 pm

first try disconnecting feedback    then if it cures the problem   yes  swap the plate leads from the OT     kinda what I already said friend..   


doing this to rule it out as a source of your problem.


What Rob R mods?

Title: Re: Princeton Reverb noise
Post by: seriesnuns on September 28, 2024, 08:48:30 pm

That was it! Output transformer wires out of phase. Biased up to 19.5ma no issues.

I will say the amp is a little less loud and the trem is not as deep as the other way.

Thanks a bunch! The Rob R mods I did were 0.001 coupling cap for 12” PI grid stop, resistor upgrades grid stop and screen resistors on the power tubes

Title: Re: Princeton Reverb noise
Post by: stratomaster on September 30, 2024, 09:27:35 am

Quote from: seriesnuns on September 28, 2024, 08:48:30 pm

That was it! Output transformer wires out of phase. Biased up to 19.5ma no issues.

I will say the amp is a little less loud and the trem is not as deep as the other way.

Thanks a bunch! The Rob R mods I did were 0.001 coupling cap for 12” PI grid stop, resistor upgrades grid stop and screen resistors on the power tubes

Looked at your photos.  I'd recommend turning the fasteners that are holding your circuit board around.  Keps nuts work by the teeth digging into the metal.  When you have them on the fiberboard side, there's nothing for them to bite into. They to loosen over time and bounce around in the chassis and can short things out.

Personally I'd recommend not using a 0.001μF coupling cap in this circuit.  I find decreasing the 2nd stage cathode cap to 2.2-3.3μF gets me to a tighter sound more directly without altering the phasing and impedance like a coupling cap would.