Tube Amp Tremolo Thumping

Started by Paul Marossy, March 21, 2023, 04:57:25 PM

Previous topic - Next topic

RGP

It's on an Australian website, but I'm not sure which one. I can try to upload an image soon. What I did in my Mark V was wire the filter in after the tremelo tube's plate and the thumping stopped.

RGP

Paul. Here is the circuit I used.


RGP

Paul, you mentioned wanting to change the LFO rate. Check out Randall Aiken's site for info on that. It's very detailed and I used one of his circuits shown for a 2-6.5 hz LFO for a Wurlitzer Vibrato clone that still has to be built.

Rob Strand

When you modulate the power tubes there's no place to put a high order high-pass filter, it was mentioned earlier on,
https://www.diystompboxes.com/smfforum/index.php?topic=130317.msg1263939#msg1263939
Send:     . .- .-. - .... / - --- / --. --- .-. -
According to the water analogy of electricity, transistor leakage is caused by holes.

RGP

Rob. You're right. It won't work on the Gregory Mark X which is entirely different than my Mark V which is single ended, not push-pull. On the Mark V  the tremolo is modulating the 12AU6 preamp tube.     

amptramp

#25
If you want to modulate something, there are tubes specifically designed as mixers and converters that do that.  A 6BE6 can do a wonderful job if you use grid 1 (normally the oscillator grid) as your signal input and grid 3 as your LFO input.  Grid 3 is remote cutoff so you should factor that in when you set up your LFO waveform, but it should not matter all that much - the active range goes down to -30 volts and you should not need more than a few volts on the LFO input.

If you go the other way around with the LFO on grid 1 and the signal on grid 3, you can bias grid 3 from zero to -30 to get a gain control.  If you have a Hammond Solovox, the volume control is by biasing the remote cutoff 6SK7 output driver pentodes with more negative bias to reduce volume.  One advantage of not going down to zero output is that you never get a complete loss of signal that leaves you wondering if a plug came out, something didn't get turned on etc.  Remote control pentodes can also be used for LFO injection but you lose the isolation between LFO and signal and may be subject to pumping sounds at the output.  I maintain you should never be able to turn the volume on a signal chain to zero, even accidentally - the causes of zero gain are too confusing and not compatible with troubleshooting during a gig.

Hammond used an interesting method of eliminating modulation from the level input - there were two 6SK7's in push-pull driving the push-pull output stage so any volume modulation was common-mode and didn't make it to the output.  Important on this instrument as there were just a few discrete volume steps and they wanted to avoid switching noise going from one step to another.

Paul Marossy

Quote from: RGP on June 10, 2023, 01:10:19 PM
Paul, you mentioned wanting to change the LFO rate. Check out Randall Aiken's site for info on that. It's very detailed and I used one of his circuits shown for a 2-6.5 hz LFO for a Wurlitzer Vibrato clone that still has to be built.

I just doubled the value of two of the caps and installed a fast/slow switch on the back of the chassis. That works out pretty well!

RGP

Paul. I was looking at a tremolo schematic in a book last night & found one that has a 1 to 10 hz. LFO. It's op-amp based and runs on a mere 9 volts for a solid state tremolo. I re-drafted it and thougt I'd post it here for anyone looking to build a tremolo pedal.


PRR

Interesting. But 30k-5k variation in one leg of a 3-leg phase shift seems to be 1:1.8 rate, not 1:10.

Has it been tried?
  • SUPPORTER

Paul Marossy

Quote from: RGP on June 13, 2023, 02:53:10 AM
Paul. I was looking at a tremolo schematic in a book last night & found one that has a 1 to 10 hz. LFO. It's op-amp based and runs on a mere 9 volts for a solid state tremolo. I re-drafted it and thougt I'd post it here for anyone looking to build a tremolo pedal.


Where is the input supposed to be?

PRR

  • SUPPORTER

Paul Marossy

Quote from: PRR on June 14, 2023, 03:09:09 PM
Quote from: Paul Marossy on June 14, 2023, 01:23:10 PM
Where is the input supposed to be?

This is just the LFO.

I've tried to model it in LTSpice as drawn, and it's not really doing anything. I don't know if it's me or the circuit needs something more to work.

PRR

Ah, simulators often do not model oscillators without trickery.

In theory, an oscillator can sit on the razor edge of instability forever. In practice, universal hiss will start it "eventually". LOW frequency oscillators especially can take seconds to start; Fender had a kick-start trick.

Do you really need to model it?
  • SUPPORTER

Paul Marossy

Quote from: PRR on June 14, 2023, 08:01:03 PM
Ah, simulators often do not model oscillators without trickery.

In theory, an oscillator can sit on the razor edge of instability forever. In practice, universal hiss will start it "eventually". LOW frequency oscillators especially can take seconds to start; Fender had a kick-start trick.

Do you really need to model it?

Well I don't NEED to model it but I like to model stuff like this out of curiosity, partly to stay proficient with the program to some degree since I don't use it every day. Even if I have it go out to 10 seconds it still does nothing. I've tried to model several oscillator circuits. I have had about 50% success rate, and it could be mostly just my own ignorance with respect to how these simulator programs work.

PRR

#34
I don't know LTspice. Can you inject a 1V transient after 1 second?

There are real LTspice wizzes at DIYaudio.com but beware of ass-hats.

In PSpice I can inject a 4.5V transient at 25mS with a hack like this (time-delay switch):


Another, perhaps more elegant, trick is to inject a nearly-right very small sine, just to tickle it. Here I inject 5 milliVolts, and the oscillation is building toward 4V, so 5mV is like 0.1% of the output.

  • SUPPORTER

RGP

Paul I found that circuit in the Elektor 301 circuits pdf. It's the LFO for a tremolo. I haven't built it, so I can't verify that it works.

RGP

If I understand Randall Aiken's article correctly the oscillator should kick start when the switch breaks the gound connection. Is that correct?

PRR

Quote from: RGP on June 15, 2023, 11:11:09 PM....start when the switch breaks the gound connection.

I did not see that switch. Is an odd way to do it. (Not all opamps like having inputs nailed to V-, though most won't blow-up.)

Yes, that is actually a fairly smooth start.

  • SUPPORTER

RGP

PRR. Is there a better way to kick start the LFO, say by applying voltage via a footswitch? I'm working on a clone of the Wurlitzer 44 vibrato and want to use the opamp LFO I posted. The schematic is in the works, but there may be a few bugs to work out as I have it drafted now. Maybe I should start a thread on here to for anyone else who's be intertested in build a tube vibrato. Richard Dorf wrote a great article about the Wurlitzer 44's vibrato. Back when I was working on console organs that one really impressed me, but it was too fast for guitar, in my opinion.           

PRR

Quote from: RGP on June 16, 2023, 11:01:44 PM
PRR. Is there a better way to kick start the LFO,.....I'm working on a clone of the Wurlitzer 44 vibrato...

I will yield to Wurlizer's designer's expertise. Is it not working for you?
  • SUPPORTER