Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Esquirefreak on November 18, 2022, 08:11:45 am

Title: Comparing bias trem circuits
Post by: Esquirefreak on November 18, 2022, 08:11:45 am

About to start sketching a board layout for a single channel build based around the bright channel of my '63 6G9-B but with some beefier transformers (Hammond 290EEX and Bassman OT).

I was comparing schematics on the bigger Fender amps using bias trem, especially 6G9-B and 6G16. They look alot similar - LFO+CF. The main differences I can see is an additional 20nF cap and higher voltages on the latter.

If I run into trouble with thumping and a diode across the INT pot doesn't fix it - I'll try and move the trem to the second gain stage K.

× Is one trem circuit superior compared to the other?

× Is the 20nF cap in the 6G16 schematic functioning as smoothing cap? Like the 50nF cap on the 6G9-B intensity pot? If not, what's its purpose?

Have a nice weekend, y'all.

/Max

Title: Re: Comparing bias trem circuits
Post by: Esquirefreak on November 18, 2022, 08:13:02 am

6G16

Title: Re: Comparing bias trem circuits
Post by: pdf64 on November 18, 2022, 09:09:12 am

6G9b Tremolux https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9b_schem.pdf

6G16 Vibroverb https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9b_schem.pdf

I think the caps mentioned may be rolling harmonics off the modulation waveform, in an effort to make it more sinusoidal.

I keep meaning to experiment with the trem arrangement of the
5E9a Tremolux https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9b_schem.pdf
As that might have a bit less thump, due to the coupling caps between phase splitter and output valves rolling it off.

Title: Re: Comparing bias trem circuits
Post by: sluckey on November 18, 2022, 09:25:21 am

I would not waste time on bias modulated tremolo for 6L6s (or bigger) tubes. You may get satisfactory results after tinkering a bit and/or compromising your optimum bias setting. As the tubes age, you'll probably find yourself tinkering again and when you change tubes you can just start all over again.

I think this is the reason Fender ditched this circuit in favor of the more reliable roach trem.

Title: Re: Comparing bias trem circuits
Post by: Esquirefreak on November 18, 2022, 11:08:01 am

Fair point. It might be that my Tremolux's trem sounds/works so great is due to the low voltages and cool bias.

Maybe I'll reconsider and move the trem insertion point to a preamp tube K - Vibrochamp style.

If I had any 7 pin sockets I would use a 6AV6 and your TON instead, but I only have noval sockets on hand. Don't wanna leave half a 12AX7 unused.

Although maybe it would be cool to add a 3rd (possibly parallell gainstage) prior to the PI.

Thanks for chiming in.

/Max

Title: Re: Comparing bias trem circuits
Post by: tubeswell on November 18, 2022, 03:22:18 pm

This works (and still works stunningly after 10 years - same tubes)