Using Vari-Mu in a bass guitar amp

Ok so I was reading a little bit about the vari mu principle and an idea popped into my head.

For bass guitar, most players want a little bit of compression (using a pedal/effect or by driving it close to saturation).
So wouldn't it be possible to use the vari-mu principle in the amplifier.

The 12AT7 has been used in some vari mu compressors.
It is also used a lot as the LTP phase splitter in guitar amplifiers.

So if you change the current in the tube the gain changes.
So wouldn't it be possible to do this on the ltp phase splitter?

It needs to be balanced, otherwise there will be some feedtrough of the control signal.
Only problem would be that it shouldn't have to much voltage swing as this will result in distortion.
Could be solved using a gain stage after the LTP phase splitter I think
The control voltage can be made from rectifying the signal at the output transformer.
 
A rectified, smoothed output signal to control a CCS under the long tailed pair may do this a bit, but yould want to choose tubes with variable mu across their characteristics to get the most effect.

Interesting idea, I've wanted to find an elegant way to implement voltage controlled gain for a while now for some pro-logic style signal processing. Maybe look into automatic gain control circuits? An opto-isolator with LDR as a resistor divider?

Paging PRR! I bet he would have some good input here.
 
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6A3sUMMER

Member
2016-06-07 6:50 am
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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> possible to do this on the ltp phase splitter?

Badly.

The driver has to deliver BIG signal to flog the power tube grids.

With your plan, when signal is big, the AGC turns-down the current, the driver can not make big signal. This is the essential limit on a tube limiter. We can make-do by only expecting a few volts out of the VCA stage. Then we need another stage to make the dozens of volts for the power bottles.

That said... there was a bass-amp did it almost this way. Give me a few years and I may remember which.
 
FWIW *any* PP pentode output stage *is* a crude Vari-Mu compressor.

One BIG factor in Guitar and (a few) Bass players preference for Tube sound.

Power tube grids rectify drive signal, shift bias negative, pentode gain (actually transconductance) lowers and sound is compressed, somewhat clipped of course but way less than it should if this effect didn´t happen.

A bonus feature hidden in plain sight.

Of course, it requires little or no NFB which diminishes its effect.
 
> possible to do this on the ltp phase splitter?

Badly.

The driver has to deliver BIG signal to flog the power tube grids.

With your plan, when signal is big, the AGC turns-down the current, the driver can not make big signal. This is the essential limit on a tube limiter. We can make-do by only expecting a few volts out of the VCA stage. Then we need another stage to make the dozens of volts for the power bottles.

That said... there was a bass-amp did it almost this way. Give me a few years and I may remember which.

That's what I tought about the signal shouldn't be to large.
So adding gain stages after the LTP output can solve this I think.

For the tube the 6386 is a bit expensive.
I have seen the PCC85 (ECC85 with different heater?) and even the 12AT7 used for vari mu compression.
In simulation I've got around 30db of gain change just using the 12AT7 so this would be ok
 
A simple Vari mu Compressor is something like the altec 436 http://www.dvq.com/hifi/images/436c.pdf

The 6BC8 can be had at a normal price.

I think making the input stage into an LTP by adding a large resistor at the cathodes and tie it to a negative voltage (the bias supply for output tubes).
Adding a trimmer to one of the plate resistors to balance the LTP

Then adding gain stages after the first stage but with enough gain so the LTP shouldn't make big voltage swings.

Then there are the output tubes and then there is the OPT.

In the altec the control voltage is derived from the plates of the output stage.
So I would do the same.

Modulating the screens of the output tubes was also something I was thinking about.
I did see a compressor schematic somewhere where this was done using some ef86 pentodes.>

For the same power output this bass amp will probably sound a little bit louder. And the notes will have more sustain. Wich is good for bass guitar I think.
 

PRR

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Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> making the input stage into an LTP by adding a large resistor at the cathodes and tie it to a negative voltage

That spoils the variable gain. Pushing the grids around (how?) just makes the cathodes follow, so little change of tube current.

Also the THD of the Vari-Mu in hard GR rises if there is more than a few hundred Ohms AC impedance cathodes to ground.

We start to wonder if the Old Guys "knew what they were doing" when they mostly implemented limiters as stand-alone boxes.

While I like the "idea" of a tube limiter, today with $5 chips sold pre-built with jacks and knobs in $99 boxes at any Rock-Shop, it seems a poor use of time/energy. Your adoring fans won't know the difference, Fairchild 660 or a chip.
 
"shift bias negative". If you're going to derive a control signal from the OPT (or otherwise) why not just use it to "shift bias negative" on all the tubes that you can in the whole amp?

Build a voltage controlled fixed bias supply with multiple outputs. (I think someone has a patent on that already...) Does a tube "know" if its cathode is set above ground via a parallel RC, or any other Vsource?
 
"shift bias negative". If you're going to derive a control signal from the OPT (or otherwise) why not just use it to "shift bias negative" on all the tubes that you can in the whole amp?

Build a voltage controlled fixed bias supply with multiple outputs. (I think someone has a patent on that already...) Does a tube "know" if its cathode is set above ground via a parallel RC, or any other Vsource?

The only thing I can think of is the heater to cathode voltage limits on the tubes. Be sure to look at the specs for the tube you are actually using because the specs vary dramatically from one manufacturer to another . Too much voltage difference = dissipation of the cathode coating, eventually rendering it inop. Excessive heater to cathode voltage is the main reason cathode followers burn their cathodes, not the class of service. Probly not a problem in the instant matter, but something to consider... Always.
 

Gnobuddy

Member
2016-03-01 4:10 pm
If you're going to derive a control signal from the OPT (or otherwise) why not just use it to "shift bias negative" on all the tubes that you can in the whole amp?
Fred Nachbaur fed his (speaker derived) control signal back to the input pentode in his "Spunky" guitar amp design.

I would have thought this would create lots of thumping and bumping sounds through the speakers because of abrupt shifts in the anode voltage of the input stage. :confused:

I tend to agree with PRR about the practicality of designing a compressor using antique technology versus simply buying one that works (probably better). I like this one for a variety of jobs, including adding a little compression to bass guitar: Access to this page has been denied.

That little ART box is much more than a compressor - it's also a mic preamp, has phantom power, has reasonable signal level monitoring, has a plethora of balanced and unbalanced input and output jacks, and even switches output level to suit pro or instrument line levels. It is a very handy little box for live sound or the home studio.


-Gnobuddy
 

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PRR

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Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
I note that our forum software has turned the Sweetwater link in my previous post to read "Access to this page has been denied", but ...

URL /URL tags often work better (or longer) than bare urls.

ART Tube MP/C Tube at Sweetwater

I think Sweetwater et al webservers get freaked when a strange website calls just to get a page title. Give the link a title and the DIYA server doesn't do that.
 
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