Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tony321owen on February 17, 2020, 02:41:34 am
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I love circuits that use bias-vary tremolo inserted on the grid of a power tubes in a push-pull amp! Is there any reason Not to use a similar circuit on a single ended amp?
Ive been looking at the Gibson GA-1 rvt. Anyone think of other examples of single ended amps with this type of tremolo?
Im looking for a one triode version for a current build using a 6v6 power tube.
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That method isn't used, as there's no way to avoid the modulation signal being amplified by the power tube and flapping the speaker like crazy.
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May be you'll like to read this stuff
http://el34world.com/Forum/index.php?topic=20727.msg218834#msg218834 (http://el34world.com/Forum/index.php?topic=20727.msg218834#msg218834)
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May be tremolo connected to G2 of the Power Tube will give an effect like you want ?
Franco
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Garnet amps has at least 1 amp model with SE power tube bias vary.
https://el34world.com/charts/Schematics/files/Garnet/Garnet_g15t_gnome.pdf
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I love circuits that use bias-vary tremolo inserted on the grid of a power tubes in a push-pull amp! Is there any reason Not to use a similar circuit on a single ended amp? ...
That method isn't used, as there's no way to avoid the modulation signal being amplified by the power tube and flapping the speaker like crazy.
There's no logical reason to inject bias-vary trem at the output tube of a single-ended amp; you should save yourself much trouble and just wiggle a preamp tube cathode like a Vibro Champ (https://el34world.com/charts/Schematics/files/Fender/Fender_vibrochamp_aa764_schematic.pdf) or a Vox (https://el34world.com/charts/Schematics/files/Vox/Vox_ac10_2.pdf) amp.
You can do it like the GA-1 RVT. Drawbacks are speaker-flap (and pulsating background noise), and injection at the output tube requires a big signal from the oscillator.
>May be tremolo connected to G2 of the Power Tube ...
This could be done, but there is less-gain from G2 to plate so the oscillator signal must be even bigger (nearly as big as the output tube plate signal.
The connection to the grids of push-pull output tubes to vary the bias was an idea Fender patented (https://patents.google.com/patent/US2973681A/en) and introduced in the tweed Tremolux amps. The point is to get rid of pulsating background noise by adding the trem at a point in the circuit that normally responds to push-pull signals. Those amps varied the bias of a shared cathode resistor in a phase inverter, but when Fender later changed phase inverter circuits the same idea could be applied to the output tubes. The downside is, as above, bigger signals are needed from the oscillator to vary the bias of output tubes (10's of volts) than in the bias of preamp tubes (~1 volt).
You're looking to forgo the push-pull noise-cancelling nature of the Tremolux, brown Deluxe or blackface/silverface Princeton amps by running single-ended. That being the case there's no reason I'm aware of the you should make the task harder by requiring your trem oscillator to deliver a larger output signal. Might as well make the trem circuit easier by varying the bias of a preamp tube.
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> any reason Not to use a similar circuit on a single ended amp?
It thumps like a trapped porcupine.
If you had two identical porcupines thumping oppositely in sync, the thump would cancel. There's one truly balanced stage in guitar amps: the Push Pull Power Stage. That's a natural.
Yes, we've seen trem applied to cheap SE amp. "Cheap" is key. They typically have a narrow range of trem rate, very limited bass response in OT, cheap stiff speaker which won't flap from the subsonic which does get through. And it gives cheap buyers a reason to trade-up.
I'm not keen on termming SE preamp stages but it is probably better than tremming the big stage.
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Thank you guys! I got some really good info out of this!
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Ignoramus's question: How does the Vibro Champ circuit work?
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At the cathode of V1b (the second triode on the signal path)
http://el34world.com/charts/Schematics/files/fender/Fender_champ_vibro_aa764_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_champ_vibro_aa764_schem.pdf)
Franco
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... question: How does the Vibro Champ circuit work?
The oscillator make a fairly-large voltage output but needs a bypassed cathode to have enough gain to wiggle, and its plate output cannot afford to be loaded.
There is a cathode follower after the oscillator to act as a buffer. The direct-coupled cathode follower has a high input impedance and looks like a very light load to the oscillator.
The cathode follower pulls cathode current through the 2nd preamp stage's cathode. The actual input to a tube (in most circuits) is grid-to-cathode voltage. The follower/buffer pulling current through that cathode resistor causes a voltage-drop that is effectively a changing grid-to-cathode voltage, and so an input signal.
The cathode bypass cap seems like it would be a problem, but a 25µF cap has ~1kΩ of reactance at 6Hz so the overall plan still works (especially if the bypass cap has dried up & fallen in value over time).
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> especially if the bypass cap has dried up & fallen in value over time
If the cap is crap, the resistor stabilizes the stage gain and trem effect is reduced. You want audio bypass. As you say, just-enough for guitar bass, not enough to suck-up LFO frequency much.