Bias tremolo in cathode biased amp

[roseblood11]

Hello,

a) I would like to use the tremolo oscillator from the 6g16 Brownface Vibroverb in an amp with 2*6V6 and cathode bias. How would it be connected and what would have to be adjusted? Use a slightly larger value for the intensity pot, like 1M, and connect the right side to ground?

Tremolo schematic:
https://www.mojotone.com/Amp_Kits/Brownface/Vibroverb_6G16_SCH.pdf

The amp will get Deluxe Reverb transformers and voltages (GZ34 rectifier), and 6V6 JJ with 470R and 1k5 resistors.

https://www.mojotone.com/Amp_Kits/Brownface/Vibroverb_6G16_SCH.pdf


b) What would be a good starting point for the cathode resistor and capacitor?

[pdf64]

https://el34world.com/charts/Schematics/files/Epiphone/Epiphone_ea_35t.pdf

[roseblood11]

Yup, same concept. But that's an EL84 amp and a simpler oscillator with only one triode...

[sluckey]

This...

[Dave]

Sluckey, that schematic is too small for me to see and I suspect it is for most others as well.

Nevermind. It is fine I just didn[t know how to make it bigger.

Dave

[roseblood11]

This...

Have you built it like this? I wonder, if the cathode voltage changes with the tremolo, resulting in a low intensity effect?

[sluckey]

Have you built it like this? I wonder, if the cathode voltage changes with the tremolo, resulting in a low intensity effect?

I have not built that exact circuit but I have built an Ampeg J12B. I never looked at the cathode voltage but even with a single triode oscillator there is plenty of tremolo intensity.

One change I would make to the 6G16 trem circuit would be to replace the oscillator cathode RC with a red LED. This will give you a lot more tremolo signal.

[j-b-c]

I have not built that exact circuit but I have built an ...

Modulating half a paraphase inverter is quite a bit different than modulating the grid bias on the power tubes.

Don't mislead the guy.

With power tube grid modulation you have two issues that demand attention.

  . The time constant of the power section bypass capacitor must be significantly larger than the time constant of the coupling capacitor to the modulator circuit, otherwise the cathode voltage will chase the grid modulation.

  . Without care the modulation circuit can push the grids positive.

https://en.wikipedia.org/wiki/Lotka%E2%80%93Volterra_equations

[sluckey]

Modulating half a paraphase inverter is quite a bit different than modulating the grid bias on the power tubes.

Don't mislead the guy.

I don't know what you're talking about and I don't think you do either. The Ampeg circuit I proposed has nothing to do with the phase inverter. It modulates the power tube grids and does a good job at that.

Your wiki link is nonsense! I wonder if you may be a troll? Eyes are on you.

[j-b-c]

Your wiki link is nonsense! I wonder if you may be a troll? Eyes are on you.

The wiki link is exactly what happens when you modulate the grids of a cathode biased amp.  There becomes a dynamical relation between the time constant of the modulator and the time constant of the cathode bypass capacitor.

In your Ampeg the modulation is on the feed-forward of the paraphase inverter.  On the 6g16 the modulation is on the grids of the beam pentodes.  After the AC coupling, not before like on the Ampeg.

Part of the charm of cathode bias comes from the squishiness of the bypass capacitor.  If that capacitor is stiffened to the point where the cathode bias isn't chasing the DC grid modulation then that charm is lost.

If DC (meaning in phase) modulating of grid voltage on a cathode biased amp was a good idea Leo would have done it.

[pdf64]

Modulating half a paraphase inverter is quite a bit different than modulating the grid bias on the power tubes ...

In your Ampeg the modulation is on the feed-forward of the paraphase inverter.  On the 6g16 the modulation is on the grids of the beam pentodes.  After the AC coupling, not before like on the Ampeg. ...

I suspect you may be in a muddle.
With both circuits in question, it may be helpful for you provide your analysis of how the circuits operate. 

I can advise that the circuit I provided earlier works very well, no predator prey issues are apparent :)

I think Fender didn't used that type of applying the modulation to the output valve grids with cathode bias amps is because by the time it became a thing, they'd moved to fixed bias for push pull amps.

[sluckey]

In your Ampeg the modulation is on the feed-forward of the paraphase inverter.  On the 6g16 the modulation is on the grids of the beam pentodes.  After the AC coupling, not before like on the Ampeg.

You are misreading the schematic.

If DC (meaning in phase) modulating of grid voltage on a cathode biased amp was a good idea Leo would have done it.

DC does not mean in phase. Leo is not the ultimate guru of amp design. Leo had nothing to do with the design of the Ampeg J12B amp. There are plenty of well known amps that use this same circuit.

[j-b-c]

I suspect you may be in a muddle.

My eyes did indeed misread the Ampeg schematic, that does not correct however the troublesome nature of controlling two conflicting things at once.

I would not say bias modulating cathode biased amps works well.  The compromises are too severe.

You have to run a cold bias to accommodate the headroom of the wobble.  The headroom for bias wobble becomes more constrained as the music signal increases.  The bypass capacitor has to be quite stiff to compensate.

The implementation leads to a menu of unfortunate choices.

I've made very sad experiences trying to get a good tremolo by wobbling cathode biased amps.  In each case the solution was to implement fixed bias.

Maybe its just me, but Leo used the same solution.

Edit:  Implement fixed bias and wobble that.

[sluckey]

I've made very sad experiences trying to get a good tremolo by wobbling cathode biased amps.

Just because you can't figure it out doesn't mean that others can't. It's already been proven to work well. There are hundreds of top name amps that use tremolo on the grids of cathode biased output tubes. No need to reinvent. Just plagiarize.

Edit:  Implement fixed bias and wobble that.

Not an option. Did you even read the topic?

It seems to me that you have nothing useful to add to this thread. All you want to do is piss on others' comments. That's a hell of a way to introduce yourself. Good luck getting people to take you seriously!

[j-b-c]

Not an option. Did you even read the topic?

My impression is that OP is building a "green field" amplifier.  He doesn't seem to be constrained to repairing a customer's relic.

Rather than piss on others comments I'm trying to be helpful.  Its not my first rodeo.

Its 2024 and the world is our smorgasbord.

OP doesn't say why he wants to build a bias modulated tremolo, or indeed what is tremolo goal is.

There are lots of ways to do it.

In the amps most coveted by players bias modulation on top of cathode bias is seldom seen.

Players favorite cathode bias tremolo amps would be the Marshall 1974, the Gibson GA-40, and the Magnatone 214.  Not a bias wobble type amongst them.

[johnnyreece]

I've made very sad experiences trying to get a good tremolo by wobbling cathode biased amps.  In each case the solution was to implement fixed bias.

This seems like an odd blanket statement to make.  Several cathode biased amps have good tremolo.  Is there maybe some language/terminology barrier here? 

[sluckey]

Rather than piss on others comments I'm trying to be helpful.  Its not my first rodeo.

OK cowboy. Very poor attempt at being helpful.

The first thing roseblood11 said was "I would like to use the tremolo oscillator from the 6g16 Brownface Vibroverb in an amp with 2*6V6 and cathode bias. How would it be connected and what would have to be adjusted?"

The first thing I said was "This..." and then I attached a schematic that precisely answered his question.

The first thing you said was "Modulating half a paraphase inverter is quite a bit different than modulating the grid bias on the power tubes... Don't mislead the guy."

That clearly shows you missed the point and have nothing helpful to offer. Everything else you've said was simply to argue your point, which is totally useless to the OP.

I'm done. I have nothing else to say to such arrogance.

[Bieworm]

@J-B-C: Tone King Imperial!
Ok it’s a mosfet instead of a tube. But same deal. It wobbles the cathode biased 6V6 tubes tremendously. I use this circuit all the time, with great success.

eature=shared

[Bieworm]

Hello,

a) I would like to use the tremolo oscillator from the 6g16 Brownface Vibroverb in an amp with 2*6V6 and cathode bias. How would it be connected and what would have to be adjusted? Use a slightly larger value for the intensity pot, like 1M, and connect the right side to ground?

Tremolo schematic:
https://www.mojotone.com/Amp_Kits/Brownface/Vibroverb_6G16_SCH.pdf

The amp will get Deluxe Reverb transformers and voltages (GZ34 rectifier), and 6V6 JJ with 470R and 1k5 resistors.

https://www.mojotone.com/Amp_Kits/Brownface/Vibroverb_6G16_SCH.pdf


b) What would be a good starting point for the cathode resistor and capacitor?

Look at the Tone King Imperial schematics. It’s not a tube, but a single IRF840 mosfet. It works great! I simplified the schematic and I use it on nearly every build. Cathode or fixed bias 6V6 amps.

Here’s my Brutus. It’s a 6V6 powered tweed bassman circuit. It sports a great AB763 reverb too🤘

i=rLKZxdbsgLJc10yW

[HotBluePlates]

Have you built it like this? I wonder, if the cathode voltage changes with the tremolo, resulting in a low intensity effect?

Ignore the other noise.  Inject the tremolo at the output tube grid-leak resistors, as noted in the replies from Sluckey and pdf64.

Cathode-bias vs fixed-bias doesn't matter.  The trem is just as effective at wiggling the output tube grids as your guitar's signal is. (And you hear your guitar when playing through the amp, right?)
   - Fender's tweed Tremolux wiggles a cathode-biased stage (doesn't matter than it's a small dual-triode)
   - Fender's Vibro Champ wiggles the cathode of a cathode-biased stage.
   - Epiphone's EA-35T wiggles a cathode-biased power section.
   - Gibson's GA-20T wiggles the cathode of a cathode-biased stage.
   - Gibson's GA-25RVT wiggles the cathode of a cathode-biased stage.
   - Ampeg's J-12B Jet wiggles a cathode-biased output section.
   - Ampeg's R-12A Rocket wiggles a cathode-biased output section.
   - Ampeg's R-12-R Reverberocket wiggles a cathode-biased output section.

We could keep going...  There's literally 70 years of this being done, and creating amps whose tremolo seems to work just fine.


View the video at 1:30

[j-b-c]

Everybody knows better than me.

There is a little difference between wobbling the bias of a class A preamp or wobbling a push-pull amp as it approaches class AB2.

[tubeswell]

See the Fender Pawn Shop Excelsior circuit.

[roseblood11]

The Excelsior looks interesting. What exactly do the extra resistors do? Is there more isolation between the oscillator and the cathodes? Would that circuit work with an intensity pot to gnd? It's a bit strange that they don't use one...

I'm building a single channel amp, with a simple blackface preamp, 2*6v6 cathode biased power amp and a boost circuit (two triodes and a tone control) IN FRONT OF THE blackface preamp. Plus an FX loop and footswitches for boost and loop.
It's a best-of of all the things I liked in other amps that I built. It will be used for gigs, studio work and practising.
As there is a loop, I will mostly use a digital tremolo on stage. The tube tremolo is just added for recording, because I like it better. So, it's nice to have, but not that important.
It's still an option to modulate the cathode of the second triode of the AB763 preamp, like it's done in the Vibro Champ.


By the way: Big thanks to everyone, I think that everyone was trying to be helpful. And no, it wasn't my impression that j-p-c brought any aggression into the discussion. His concerns are important and I have to learn more at that point. A member in a german forum, where I asked the same question, had exactly the same thoughts.

[roseblood11]

@Bieworm
Can you please add your simplified version of the Imperial oscillator circuit?
________________________

Pawn Shop Excelsior schematic:
https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://docs.google.com/file/d/0B94M9utkFx4iVVgteEE0X3lYdlk/view&ved=2ahUKEwi5uuDPpPuFAxVj_7sIHTCSCrQQFnoECBYQAQ&usg=AOvVaw1xTVDW7O-t9otBGqCcmTrH

________________________
Tone King Imperial schematic is in this thread, reply #4. What I don't understand here, is the reference to -50V in the oscillator circuit.
https://el34world.com/Forum/index.php?topic=26462.0

[sluckey]

The Excelsior looks interesting. What exactly do the extra resistors do? Is there more isolation between the oscillator and the cathodes? Would that circuit work with an intensity pot to gnd? It's a bit strange that they don't use one...

I guess you're talking about R18, R19, and R32, R33? R18 and R19 provide the required grid ground reference to the 6V6s. R32 and R33 bring the tremolo signal to the grids. Notice this tremolo circuit is cap coupled and does not provide a ground path? That's why R18 and R19 are required. Could they have done it differently? Sure, but it is what it is.

Yes, you could add an intensity pot as you suggest. BTW, I added an intensity pot to my Ampeg J12B because the original did not have one.

[j-b-c]

Here is a PWM driven tremolo insert that runs good in SPICE.

. "PWM" is a TTL level signal from an Arduino that synchronizes to MIDI-clock.
. "0VDC_E" is the local TTL 0V rail at heater elevation.  About +50V.
. "D" is a well filtered B+ rail.
. "option master" connects like the Fender AB763 tremolo.
. "LFO VDC" is the reference bias voltage like on the 6G16.
. "LFO + bias" connects to the grid insertion point, like on the 6G16.
. adjust RV6 for approx 55VAC at R51.

Running the LFO on a microcontroller allows great flexibility in the oscillator frequency and waveform as well as the possibility to synchronize the modulation with a looper, drum machine or DAW.

 

[Bieworm]

@Bieworm
Can you please add your simplified version of the Imperial oscillator circuit?
________________________

Pawn Shop Excelsior schematic:
https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://docs.google.com/file/d/0B94M9utkFx4iVVgteEE0X3lYdlk/view&ved=2ahUKEwi5uuDPpPuFAxVj_7sIHTCSCrQQFnoECBYQAQ&usg=AOvVaw1xTVDW7O-t9otBGqCcmTrH

________________________
Tone King Imperial schematic is in this thread, reply #4. What I don't understand here, is the reference to -50V in the oscillator circuit.
https://el34world.com/Forum/index.php?topic=26462.0

the -50V is exactly what I left out. In one build it gave me hum and when I lifted it it worked just the same, without hum.  I have done this to dozens of amps since. Works great!