Vibrochamp trem question

[SoundmasterG]

I've got a quandry here in a build. The amp in question is loosely based on a Fender Deluxe Reverb....it is single channel, 6L6's, about 420v B+, single 12, fixed bias, about 28 watts. It has power scaling on it too...and everything works fine....the reverb and bias scale with the power scaling control etc. The problem is the trem. I have bias vary trem on the power tubes, but it isn't happy with power scaling. Before adding power scaling the bias vary trem worked great with lots of intensity etc. When you add power scaling, the intensity drops way down when at max power scale settings, and there isn't much of a way to increase it. As you turn the power scaling control down, the intensity goes up..but if you play around to get more intensity at max, then at lower settings it thumps badly, and there doesn't seem to be a happy medium. If you scale the trem oscillator, then it stops working as the scaled voltage goes down....I'm using a tube oscillator and a mosfet source follower in the trem circuit. I've played around with the values after the source follower to increase the intensity and just can't find a happy medium.

I was reading about some people using the vibrochamp circuit....that would solve the problem for me in some ways since the preamp is not scaled and the trem effect wouldn't change then as the power scale control is moved. I also read however that the vibrochamp circuit thumps and has too much bass response and doesn't work as well with larger amps?

Can any of you guys who have played around with this circuit offer any suggestions on how it might work with my application? Before you ask I do not have a schematic I can upload since it is hand drawn and I don't have a scanner, but I can tell you what values are used in the trem circuit if needed.

Thanks!

greg

P.S. Are you guys using the 50k-RA pot that Hoffman sells for the Vibrochamp circuit? I could only find a Fender 25k- RA pot at CE/AES.

[SoundmasterG]

Well I guess no one has experimented with this trem setup....cause no answers. I changed around to the Vibrochamp circuit and it works ok and sounds fine though not as good as the bias modulating trem. The one problem is that if you leave the intensity up when you switch the trem off, the volume of the amp goes away until you turn the intensity pot down.....not a killer but a problem.....unfortunately if you change the footswitch around to leave the oscillator on all the time, you have to switch out the 25k pot and the DC on it and that creates a pop every time. Have to figure out how to get around that problem but otherwise it should work I think.

Greg

[HotBluePlates]

... The amp in question ... fixed bias ... It has power scaling ...I have bias vary trem on the power tubes, but it isn't happy with power scaling. ...

Well I guess no one has experimented with this trem setup....cause no answers.

I haven't even done power scaling yet, though I've had a kit for it for years.

My guess is this: You're amp is fixed bias, and the power scale circuit also has control circuitry in the bias supply so it tracks the scaled B+. I'm thinking the added circuitry doesn't play well with common-mode a.c. of the trem.

You may be stuck with the switching transient on the VibroChamp circuit, unless you switch to another style circuit.

I haven't tried JFET switching elements, but it seems if you could use a JFET between the 68kΩ resistor and the 25k pot, you might be able to get a fast ramp-up/ramp-down in element resistance rather than an abrupt change from short to infinity ohms. That might cure the pop, but would require extra circuitry to build a power supply for switching voltages as well as the control circuitry for the JFET switches. Maybe too much hassle.

[Willabe]

In 1 of the TUT books Kevin said you have to PS the LFO and the driver on bias trem.

I only trust KOC's bias tracking circuit for fixed bias PS. The other 1's don't track as well.  

            
              Brad      

[HotBluePlates]

In 1 of the TUT books Kevin said you have to PS the LFO and the driver on bias trem.

... When you add power scaling, the intensity drops way down when at max power scale settings, and there isn't much of a way to increase it. ... If you scale the trem oscillator, then it stops working as the scaled voltage goes down. ...

Make me think Greg tried Sluckey's trick of using a LED to bias the oscillator to get maximum output. But that is the same as a fixed bias voltage for the oscillator, so if you scale the supply voltage down it biases the oscillator into cutoff and it can't get started (at some low B+ voltage).

Because everything worked great until the power scaling was added, and it was only problematic at full voltage, it seems like the scaling circuit (probably with the bias) is creating a problem.

... if you play around to get more intensity at max, then at lower settings it thumps badly, and there doesn't seem to be a happy medium.

The problem here is that as you scale the voltage down, the bias voltage gets smaller too, which makes the trem signal look bigger. You have to find a way to get the trem signal to be made smaller in step with the reduction of B+ and bias voltage.

Hell, maybe that's the real answer: adapt the control circuitry that forces bias voltage to track the B+ reduction; add a 2nd control circuit that reduces trem signal output in sync with the scaling. Optimize the trem for maximum B+, and don't scale any of the trem circuit (If Brad can find the reference in TUT, maybe there's an easier way).

How to do this? I dunno. Would have to review the power scale circuit and figure out how the bias tacking works to begin with. But you'll pobably wind up with a series JFET or optoisolator between the trem output and the bias/220kΩ resistors.

[Willabe]

The problem here is that as you scale the voltage down, the bias voltage gets smaller too, which makes the trem signal look bigger. You have to find a way to get the trem signal to be made smaller in step with the reduction of B+ and bias voltage.

I'll find it but that's almost exactly what he said.

Be back in a little bit.


              Brad     

[Willabe]

Since the PS forum is no longer, and this probable only would directly apply to a KOC PS kit, I'll post this. I'll say it again I wouldn't trust any other PS/VVR type tracking circuit to track as well as Kevin's.

It's a little more than I remembered, I was thinking K bias.  

"tie in the our tremolo circuit, which we do through the tracking bias regulator. The tracking bias regulator has a FB loop to keep the bias voltage in a fixed proportion to the screen supply. The regulator is essentially a virtual-earth stage, so we can treat it as if it were a
v-e  mixer. The summing node, or the v-e node labeled "O", is where we add another input R from the tremolo circuit.

Ideally, the trem modulator drives a pot that is used to set the amplitude or intensity of the modulation signal. The wiper of this control feeds the weighting R tied into the tracking regulator summing node. We might have to experiment with the weighting R's value to keep the tremolo intensity within a useful range at full PS setting.

It should be obvious that the trem oscillator stage should be PS along with the output stage, regardless of whether any other circuitry is PS. If we don't do this, then the trem modulation will effectively increase as the output stage is made to look smaller."

I'll make a quick block drawing of what he shows in the book, TUT 4. I'll be right back.


                 Brad          

[Willabe]

Ok, here it is.


             Brad     

[SoundmasterG]

Willabe,

Thanks for posting that. It is the same thing that is in TUT5 (I think)...one of the TUT books anyway. I ended up calling Kevin last week about that whole setup...and pin 1 on the bias tracking board on the SB1 kit is for adding the trem signal in.....but it just doesn't work very well. Kevin didn't say this, but to really work right the oscillator needs to run full up all the time and you would need a tracking setup just for the oscillator output to track itself down as the voltage to the power section is reduced. It can be done but not with the stock circuitry that comes with an SB1 kit. I might play around with it in future amps as the bias modulating trem sounds great...but for now the amp is sticking with the Vibrochamp circuit.

HBP,

Yes I did bias the oscillator with a red LED and it works well...but if you scale that and the follower, then it goes into cutoff and stops oscillating. JFET switching might work better but no time to figure it out right now. Another approach would be to use an oscillator/follower to interface with the signal though a mixer stage....the AC signal goes into one side of the mixer and the trem output, also an AC signal would go into the other side of the mixer stage. It could work very well and would work best in the preamp, but is a bit complicated to set up when most people just want the minimum number of tube stages in an amp. I'll play around with the idea more in the future I think....getting something to behave while running power scaling in an amp, without other complications, would be nice.

For now the build is done and it works fine as long as the customer will remember to turn the trem intensity down to 0 if he is not going to use trem. So its out the door and on to another project now for me...probably my senior project for school...building a studio quality mic pre.....

Thanks for the suggestions!

Greg

[HotBluePlates]

... Kevin didn't say this, but to really work right the oscillator needs to run full up all the time and you would need a tracking setup just for the oscillator output to track itself down as the voltage to the power section is reduced. ...

Yes, this is what I was suggesting above. It's not a stock feature of the kit, but rather a need to duplicate/adapt the bias tracking circuitry to reduce the trem signal as B+ is reduced.

[jjasilli]

An imperfect but maybe good enough work around:  keep the trem circuit separate form the power scaling circuit & use a 2 or 3 position SW to control fixed resistors which attenuate (or divide) trem voltage strength, in keeping with the amount of power scaling selected.  (Hope that's clear). 

[SoundmasterG]

... Kevin didn't say this, but to really work right the oscillator needs to run full up all the time and you would need a tracking setup just for the oscillator output to track itself down as the voltage to the power section is reduced. ...

Yes, this is what I was suggesting above. It's not a stock feature of the kit, but rather a need to duplicate/adapt the bias tracking circuitry to reduce the trem signal as B+ is reduced.

I think if I was going to mess around with it some more, then just adding another tracking network in to track what the bias or screen voltage was doing and work on the trem signal would be the ticket....I may mess with it at a later date on a different amp...it would be easy enough to do as almost all the circuitry would be duplicated from the bias tracking network with some minor changes.

Greg

[tubenit]

Physconoodler has used VibroChamp type vibrato on some of his amps.  I'd contact him for some info.

with respect, Tubenit

[SoundmasterG]

Physconoodler has used VibroChamp type vibrato on some of his amps.  I'd contact him for some info.

with respect, Tubenit

Thanks Tubenit!

I have the issue solved for this particular amp but it would be nice to hear Phychonoodler's opinion and experiences with that trem circuit...

Greg