Bias-vary trem with cathode bias pwr amp?

[Groover]

Can a bias-varying (ie 6G2&3, BFPR) tremolo work with a cathode bias output section?

[black_labb]

no, as the bias is set by the resistor, not a circuit

[tubeswell]

Heres a couple of e.g.s

Check out the Gibson GA17RVT, and other similar cathode-biased amps, which run a trem LFO to the output tube grid-load resistors. Its not exactly trem by varying the cathode bias resistor, but it does vary the bias and it might suffice for what you have in mind.

[Don Moose]

Apples, tomatoes - 'bias' is the DC voltage from grid to cathode. You can dink with the grid voltage (Princeton style) or the cathode voltage (in a cathode-biased amp) and you are dinking with the bias.

Firefox is barfing when I try to load the vibrochamp schematic, so I can't say whether it's a shunt trem or a bias-modulator.

[martin manning]

The GA17RVT's power tube grids are referenced to ground through a 220K from each to a shared 330K. The LFO output is applied to the junction of the 220's and the 330 via a DC blocking cap and 1M from the oscillator's plate. A bias varying trem for sure.

Don, I used to have problems opening attached files until I learned to always sign-in before trying to load the file. I'm on Safari, though, YMMV.

MPM

[tubeswell]

Yup - like I was sayin', the GA17RVT uses a bias-vary trem. (Nevertheless I thought it was kinder to be diplomatic about the previous poster's assertion - so there ).

[Groover]

So if one wanted to convert a push-pull Princeton circuit, 6G2 or Blackface, to cathode bias output stage, where would one inject the tremolo signal?

Would you emulate the GA17GRT circuit?

I see that the Vibro Champ injects it at the cathode of the V1B, but that is a two-stage LFO and I don't know if it could be done with a single stage - don't want to have to add a tube.

[martin manning]

The Vibro Champ has the same phase-shift LFO as the GA17, but it uses a cathode follower stage to produce a low impedance output and a better match to the impedance at the cathode of V1B (the injection point). This circuit modulates a preamp stage rather than the output stage as in the Gibson. Another example of this is the 18W Marshall, where the LFO signal is taken from the cathode of the oscillator directly to the cathode of the trem channel's first stage, both of these triodes being in the same envelope.

Looking at the 6G2 schematic, it looks like it would be pretty easy to make the Gibson type work since the injection point can be the same for cathode or fixed bias. You could probably even make this a "switched" option.

MPM

[6267]

You don't need the cathode follower, I've done it without. It worked, but you want to make sure the voltage you are injecting is not so much that the cathode is being pushed below the grid with signal. You might be able to replace a triode-triode with a triode-pentode and inject the signal on the preamp pentode screen if you're bored, too

[martin manning]

I was thinking something like this. Too easy?

MPM

[max]

Cathode Bias with Vibrato/Trem

Hey folks,

I just modified a 70's Princeton Reverb to have a Cathode bias PWR amp and modified the vibrato to resemble a pacemaker circuit and it came out great.

Attached is a JPG of the mods.

Thanks,
Max

[AcePepper]

I just stumbled across this thread and I've been thinking about the same thing (bias-trem w/cathode bias) Max, what value cathode resistor did you wind up with for the power tubes? I'm guessing you have pin 8 of the power tubes connected to a resistor somewhere, it's not shown in your drawing.

[max]

Bias-vary trem with cathode bias pwr amp - Cathode Resistor

Hi Ace,

I am using a 250 ohm 5 watt resistor with the 6v6's and a 25uf/25 v cathode cap. Attached is a drawing. I chose this because of the 5E3, Tweed deluxe. The tubes are running at 42 MA which is a little hot for 6v6's , but I like them like that.

Good luck and let me know if you have any questions.

Thanks,
Max

[hasserl]

Max, you got a schematic drawn of your mods? Much easier to see what you've done looking at a schematic vs a layout drawing.

Thanks
Hasse

[max]

Bias-vary trem with cathode bias pwr amp

Hi Hasse,

I don't have my own drawn up, but I used the Vox Cambridge Reverb Schematic as a reference. See attached.

Let me know if you have any questions.

Thanks,
Max