Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Sonny ReVerb on October 19, 2023, 10:09:52 am

Title: Princeton 6G2 trem distortion
Post by: Sonny ReVerb on October 19, 2023, 10:09:52 am

Hey all,

New build is a Princeton 6G2 variant. I added one preamp tube. Channel 1 is brown Vibrolux. Channel 2 is brown Princeton. The amps clean tones are great. Distortion at upper volume levels is fine. I had to dial down the Vibrolux side. I went from a 12AX7 to a 5751 and split the load on V1-B.

That all went fine. The problem is the tremolo. It's not deep and lush like it should be. It has quite a bit of distortion as the intensity is increased. I went through the circuit several times. It seems correct. I messed around with my o-scope some. The LFO is putting out around 10VAC, which seems appropriate. The waveform is not a well shaped sine wave. It's similar to the rough sketch below.

The problem could be lead dress. All the 6G2 pics I've seen show twisted wires around the trem controls. Suggestions?

Title: Re: Princeton 6G2 trem distortion
Post by: tubeswell on October 19, 2023, 11:36:47 am

It’s severely clipped on a one side. Where are you scoping that waveform? The LFO signal insertion point (at the preamp V1 cathode)?, or the LFO plate?

Title: Re: Princeton 6G2 trem distortion
Post by: sluckey on October 19, 2023, 11:41:23 am

The LFO signal is being injected at the grids of the output tubes. 10V is not enough.

Title: Re: Princeton 6G2 trem distortion
Post by: Sonny ReVerb on October 19, 2023, 11:43:31 am

The scope was connected between the 100nF and the Intensity pot at the LFO output.

Title: Re: Princeton 6G2 trem distortion
Post by: dogburn on October 19, 2023, 06:48:22 pm

My first amp build was a 6G2 (with Framus Mid pot and NFB mods), and I love the sound. I didn't have any problem with the tremolo, so fortunately I didn't have to troubleshoot that. But I wonder about your heater wires - they go overhead, but there doesn't seem to be much clearance, so I wonder if they might be interfering with the tremolo signal.

Title: Re: Princeton 6G2 trem distortion
Post by: PRR on October 20, 2023, 12:54:15 am

So there are no measured DC voltages in there? Because it sure looks woefully mis-biased. Perhaps a wrong part?

'Scope the LFO grid in DC-coupled mode. If you have an X10 probe, use that. It "should" be distorted but how does it sit around zero DC?

Title: Re: Princeton 6G2 trem distortion
Post by: Sonny ReVerb on October 20, 2023, 10:45:28 am

Thanks for the replies. PRR - please treat me like a scope neophyte. I think I did what you asked, but the trace is slow and I can't really get a photo of it. I have a Tektronix 2235 and a 10x probe.

I've attached a voltage chart, if that helps. The DC readings for the trem triode V3-A were blinking on and off, so I read AC there as well.

Title: Re: Princeton 6G2 trem distortion
Post by: Sonny ReVerb on October 20, 2023, 10:51:28 am

Two more pics of the trem area.

Title: Re: Princeton 6G2 trem distortion
Post by: tubeswell on October 20, 2023, 11:33:52 am

Quote from: Sonny ReVerb on October 19, 2023, 11:43:31 am

The scope was connected between the 100nF and the Intensity pot at the LFO output.

Double check the cathode resistor value for the LFO stage

Title: Re: Princeton 6G2 trem distortion
Post by: Sonny ReVerb on October 20, 2023, 12:04:18 pm

It measures 3.26k in circuit. It's Orange-Orange-Red...

Title: Re: Princeton 6G2 trem distortion
Post by: PRR on October 20, 2023, 06:07:38 pm

Replace the cathode cap. Bigger may be better.