Bandmaster tremolo into vibroverb reverb

[Paul P]

I'm planning to build a Fender style amp using the Bandmaster 6G7-A
tremolo circuit the output of which I'd like to send into the reverb
circuit from a Vibroverb 6G16 (the reverb will then come after the
vibrato instead of before as usually found in Fender amps).

It's not clear to me how to connect the two together. Could someone
tell me the best way to connect these circuits ? Here are the related
parts of the schematics (click on the thumbnail to enlarge) :



Thanks,

Paul P

[rathmann]

Hey Paul, not sure if you are "visually paraphrasing" here, but it looks like you've omitted significant chunks of both circuits.

The part of the tremolo you've shown is the mixer which combines the out-of-phase frequency split signals. this is the output of the tremolo. But you will definitely need the other tube-and-a-half which are the oscillator (utilizing a cathode follower to lower the oscillator's output impedance) and phase splitter. There's also no reason NOT to use the 6G7-A's vibrato input circuit. Acutally when I've used this tremolo I paralleled the first stage and used the "spare" half of the oscillator phase splitter tube as the second (post EQ) preamp stage.

The portions of the reverb circuit shown include the output stage of each channel of the 6G16 and the reverb recovery/mixer stages. The input to the reverb is omitted. Is there some reason why you need to used this reverb circuit? Unlike the blackface reverbs which are limited to reverb on one channel this provides reverb to both. That's adding to the complexity.

Anyway, the long-and-short of it is that the output of the trem (as you've indicated with the arrow) should replace either of the preamp output triodes.

[Paul P]

Thanks for the reply rathmann.

Yes, I was sort of paraphrasing the circuits. I've since worked on this a bit
and think I've come up with something that should be workable. In the
following diagram only the output parts of each circuit is shown in their
shaded boxes. What I'm interested in is the components that connect
the circuits together. The values of the various coupling capacitors are
up in the air right now, I have a feeling I'm going to have to go by trial
and error. Any suggestions for values would be appreciated.

I've changed to the AB763 reverb circuit. Note that I want the reverb
to come after the 6G7-A vibrato. I'd like to be able to use both channels
at the same time so they should be in phase. I'm not sure they are with
my current design but I have a spare triode in the vibrato circuit that
I could use to flip the signal from the normal preamp if necessary.

I'm planning on using a Champ-like single 6v6 output section.



Paul P

Edit : please disregard the component values in the normal preamp,
the piece of drawing was copied from another part.

[rathmann]

I think the values you've indicated should be a decent starting point, but yeah, some experimentation may be necessary.

A couple of notes on the trem circuit...I wound up moving the B+ supply to the oscillator essentially to the screen supply node. You really want to give it as much voltage as possible to keep it going at the lower rates. I also doubled the capacitance values of it to slow it down a bit more. You have to be careful though since increasing the values too much will cause it to crap out at the slowest settings. If you want to fiddle with the osc. some more check out http://www.aikenamps.com/PhaseShiftOscillators.html and/or http://www.geofex.com/Article_Folder.../sstremolo.htm.

One of these days I'll do a harmonic trem with a SS oscillator so I can get down to right around or just under 1Hz.

So were hoping to have each of the channels have more or less the same gain structure? Or were you considering having the Normal channel have more of a lead character?

[Paul P]

> So were hoping to have each of the channels have more or less the
> same gain structure? Or were you considering having the Normal channel
> have more of a lead character?

That's a good question :-) I think to start I'll go with the same preamps
but I've got this triode in the tremolo circuit doing nothing so I've also
been thinking about using it for extra gain in the normal channel. I'm
leaning in the direction of getting the amp up and running with only the
normal channel (so a Champ, basically) and then add in the tremolo and
reverb as separate steps. I'm going to put each circuit on separate boards
which will allow me to do this. This way I won't have to deal with
everything at once. Then I'll see about what to do with the triode.

Thanks again for your help and for supplying the links. I get the impression
that this tremolo circuit is not all that common, one reason that I'm drawn
towards it. It's good to know I'm not alone.

Paul P

[rathmann]

I get the impression
that this tremolo circuit is not all that common, one reason that I'm drawn
towards it. It's good to know I'm not alone.

Paul P

Yeah, I was never a tremolo guy. Then on the advice of Speedracer many years ago I built this circuit and have been hooked on it ever since. I'd love to build a true pitch shifting tube vibrato at some point, but this one still has a good bit of "goo" in it. (and I mean that in a good way.) Other trems still don't really do it for me. You might actually get a nice effect using the trem/verb channel with a "dry" channel. I've run my little combo with this trem along with my channel switcher head. The acoustic mix of the tremolo signal with the dry signal sounds really great. Not sure how it would work electronically though.

Note that the links provided are limited to just the oscillator which is not exactly the most unique or interesting part of this tremolo. Pretty sure this one's not that common because it takes three (ok, 2.5) twin triodes. I think Fender dumped the more innovative circuit in favor of a more mundane tremolo and addded reverb. Everyone wanted to be the Beach Boys and Dick Dale.

At some point I want to experiment with replacing the two 1M resistors (from either side of the phase splitter to ground) with a pair of 500Ks and a 1M pot between them (the wiper going to ground). I'm thinking this would allow for a kind of tone bias to the control.

Also if the preamps are identical, then with the mixer stage of the tremolo, yes, your channels will be out of phase. You could possibly try using the extra triode in the Normal channel, but instead of making a gain monster you could set up all the stages to be pretty low gain. Should create a very smooth transition into compression/overdrive.

[Paul P]

At some point I want to experiment with replacing the two 1M resistors (from either side of the phase splitter to ground) with a pair of 500Ks and a 1M pot between them (the wiper going to ground). I'm thinking this would allow for a kind of tone bias to the control.

Interesting. Since I'm still putting the parts together and haven't started
building anything yet I could introduce this idea easily enough, if you'll let
me steal your idea.

I must say I'm looking forward to seeing the tremolo waveform on my
'scope.

Paul P

[rathmann]

Well if you're feeling adventurous I'd suggest saving yourself a tube or two and swap the modulation part of the tremolo with MOSFETS a la RGs article. I might even use a MOSFET for the oscillator phase inverter, (http://www.geofex.com/Article_Folder...osfetfolly.htm) although the circuit for that is not quite there on the page. But if a MOSFET can essentially replace a triode as a cathode follower, I don't think it's too much of a stretch to assume the split-load PI wouldn't be all that much different. The MOSFET follies page has some pretty cool things to try out.

[Paul P]

Well if you're feeling adventurous I'd suggest saving yourself a tube or two and swap the modulation part of the tremolo with MOSFETS a la RGs article.

I don't think I'm feeling that adventurous. Tubes are enough to chew on
for the moment ! Interesting article though.

I just remembered something you might be able to help me with. I've
found two components in the "xxx-A" tremolo circuit whose values were
changed at some point and I don't know which I should use. They are
circled in the following piece of schematic from a Pro 6G5-A :



The components in question are the 4.7M resistor and the 2uf/25v
capacitor. In every other "-A" schematic I've looked at (Concert,
Bandmaster, Super...) the resistor has the value 3.2M and the
capacitor is 4uf/25v. But - the Concert amp shown in : http://www.timeelect.com/concert.htm
shows it has the values 4.7M and 2uf/25v, just like the Pro. Do you
remember which yours has ?

Thanks,

Paul P

[rathmann]

Sorry I don't remember. But the difference is that in the Pro/Concert (what you posted) the intensity and bass response of the tremolo is attenuated somewhat. I would guess was done to reduce "thumping". I get thumping in mine with the intesity control all the way up so maybe I'm using the earlier values. Then again, my setup is like yours where I go straight into a single ended output (parallel 6BQ5s). The driver stage of the Fender amps may serve to reduce the thump effect somewhat.

Interestingly, the non 'A' version of the Twin that used this tremolo had a 10M resistor with a 4uf cap, and changed to the 4.7M/2uf combo in the 'A' version. Sometimes I think the slight variations in the values of similar circuit topologies in those Fender amps had a lot to do with with the physical output. A 100 watt 2x12 output section has a difference output profile than a 50 2x10 (brown Super). Actually if you look at all the variations of the "brown" era amps with the harmonic trem the values are all over the place.

Just be aware that the larger value of resistor you use will reduce the overall amount of intenstity of the tremolo. The smaller value cap will reduce bass response from the mixer stage helping to reduce some "thumpiness", at the expense of overall bass response.

You can check out all the variations at http://www.ampwares.com/ffg/

[Paul P]

Just be aware that the larger value of resistor you use will reduce the overall amount of intenstity of the tremolo. The smaller value cap will reduce bass response from the mixer stage helping to reduce some "thumpiness", at the expense of overall bass response.

Thanks for this. I'm still too green to 'see' what the different
components do in a circuit. Now I can play with theses values
and know what I'm doing.

Something I thought of today regarding your idea of adding a pot
between the two 1M resistors : if I understand the tremolo circuit
correctly it works by super-imposing low and high frequency audio
on opposite sine waves out of the LFO and PI. These sine waves
are then removed from the signal in the final stage of the tremolo
circuit by simply mixing them together - they cancel out.

If you put a pot between the two halves of the circuit and adjust
it so that it is not centered, won't you cause the oscillator sine
waves to be assymetrical in which case they will not cancel out
in the final stage ? There will be something left over that will
remain in the signal. This something will ressemble a regular tremolo
output. The pot would not just affect the 'tone' of the tremolo
but would also add a beat to it, the depth of the beat being
dependent on how far you turn the pot off center.

Paul P

[rathmann]

Thanks for this. I'm still too green to 'see' what the different components do in a circuit. Now I can play with theses values
and know what I'm doing.

Something I thought of today regarding your idea of adding a pot
between the two 1M resistors : if I understand the tremolo circuit
correctly it works by super-imposing low and high frequency audio
on opposite sine waves out of the LFO and PI.

Exactly.

These sine waves are then removed from the signal in the final stage of the tremolo circuit by simply mixing them together - they cancel out.

Note quite. The oscillator affects the bias of each of the frequency dependent mixer stages modulating them out of phase - when the HF is boosted the LF is attenuated and vice versa back and forth.

The sine waves we're talking about here are from, say 2 or 3 to about 8 HERTZ! People can hear at best down to 20Hz and your amp probably won't be able to reproduce much lower than 100Hz. The oscillation from the tremolo is simply not in the audio realm and so doesn't really need to be "removed'. But occasionally there can be artifacts that get created at the peaks/valleys of the modulation and this is the thumping I was referring to. This would need to be removed through filtering or reducing the amplitude of the modulation.

If you put a pot between the two halves of the circuit and adjust
it so that it is not centered, won't you cause the oscillator sine
waves to be assymetrical in which case they will not cancel out
in the final stage ? There will be something left over that will
remain in the signal. This something will ressemble a regular tremolo
output. The pot would not just affect the 'tone' of the tremolo
but would also add a beat to it, the depth of the beat being
dependent on how far you turn the pot off center.

Paul P

You're right in assessing that the net effect of the modulation at the mixer would be asymmetrical. That's the point. The pot would affect the tone because it would effectively remove the bias modulation from one side of the frequency split (and to some degree accentuate the other).

All tremolos and vibratos have a "beat" to them. Unless you have a sinusoidal sweep effect with a rate of less than 1Hz (like a slow phaser or flanger effect) you're going to notice a pulse. That's really all that most tremolos are - pulsing variation of the signal amplitude.

[ktritty]

But occasionally there can be artifacts that get created at the peaks/valleys of the modulation and this is the thumping I was referring to. This would need to be removed through filtering or reducing the amplitude of the modulation.

How would you filter this? My guess would be a simple way could be using a small value capacitor across the input to the optocoupler, but what value?