help with parallel preamp tubes

[hesamadman]

I remember touching base on this a while ago but I never got to try it. I have a quick design on my breadboard right now and I am trying to experiment with some tubes in parallel. I want volume pots before each tube. Right now everything works great when the volume pots are all the way up, but any adjustments between the two pots creates motor boating and all sorts of craziness. Can anyone give me some insight on tubes in parallel?

[hesamadman]

I've been pondering on this since I called it a night. I just realized that I did not put shielded wire on the the grids of my paralleled tubes. The pots distance from the grids are pretty lengthy. This could possibly be part of the motor boating I'm thinking.

[Fresh_Start]

Do the parallel triodes share cathode R-C network and plate resistor?

Not all of us have the jsch program installed...

Respectfully,
Chip

[kagliostro]

Ciao Chip

Here the jpg of the schematic



Franco

[Willabe]

The 1M power tube volume is too much, should only be 250K max. 

And it wouldn't hurt to use some grid stoppers, 1k5, on the power tubes.

[hesamadman]

Thanks for going that K

Willabe, I'm sorry I'm a little confused when you mention power tube volume?

[SILVERGUN]

Just an incorrect approach...
Look at your parallel triodes as one single tube, but with 2 grids.
In a typical parallel stage you would also have the grids tied together, so again, it should be thought of as one single tube stage.
By (you) feeding each grid with a different signal amplitude you are creating your own problem.
 
Here's a quick (not great) visual...the problem is occurring where you are joining the different signal amplitudes back together
 
 

[hesamadman]

This makes sense. Looking at it this way it seems that separate volumes are not possible?

[terminalgs]

the problem, I suspect, is when vol is turned down on v2a but left up on v1b (as you have them named), you are creating a LTPI.,  forget about half-position volume settings...  turn v2a all the way down: that's a LTPI.  except you share a plate resistor (which must wonk up the current flow of both....)


maybe if you put a big cathode bypass cap on there, so no AC can survive to make it to the cathode (and thus the plate) of the other triode., maybe it'd work. with the small cap on there, only some freqs are grounded, other freq's make it to the plate of the other triode (the one with the grounded grid) at opposite phase to the first triode...  bad stuff.,

[SILVERGUN]

This makes sense. Looking at it this way it seems that separate volumes are not possible?

Some of the "coolness" factor that might be in a parallel stage might come from a slight mismatch of triodes...
Since you are experimenting, here's a less extreme way to 'purposefully' unbalance the signal, and manufacture your own mismatch.
 
Not a volume control, but a balance control for that single stage...you'll have to experiment with pot values, and please remember this is just an experiment

[Willabe]

Willabe, I'm sorry I'm a little confused when you mention power tube volume?

Sorry, had just woke up, 'saw' power tubes not preamp tubes. 

[terminalgs]

I said:

maybe if you put a big cathode bypass cap on there, so no AC can survive to make it to the cathode ..

the more I think about it,  just divide the cathodes and give each its own resistor and bypass cap...


Then, no out-of-phase LTPI to worry about...

[hesamadman]

the more I think about it,  just divide the cathodes and give each its own resistor and bypass cap...


Then, no out-of-phase LTPI to worry about...

What is the abbreviation LTPI?

If I separate the cathodes, do I need to do anything the plates?

[SILVERGUN]

Sorry, had just woke up, 'saw' power tubes not preamp tubes.

Get some coffee and then re-read my last post...
I think there might be something there for YOU to try on YOUR breadboard
 
If hesamadman doesn't try it, it could just disappear into the abyss of mediocre ideas that never fulfill their destiny (and that would be sad)

[terminalgs]

the more I think about it,  just divide the cathodes and give each its own resistor and bypass cap...


Then, no out-of-phase LTPI to worry about...

What is the abbreviation LTPI?

If I separate the cathodes, do I need to do anything the plates?

Long tail phase inverter, or "long tail pair" PI: see  http://www.valvewizard.co.uk/dcltp.html


for what you are doing, trying to coax different sounds out of the two different triodes, I'd divide up the plate resistor as well.


I don't know if you'll really see a difference that noticeable without adding some tone shaping to one path or the other.   parallel triodes are usually used because there is extra triode leftover.  it can be used to increase gain slightly,  and when compared to a single triode, I think people unanimously prefer the paralleled triode,  so I suspect that without adding any tone shaping to either side, if you build this into an amp, those controls will be set once, and never tampered with..


If you decide to add tone shaping, you are going down the active tone control path. (See Sound City 120 for example).  the idea is 2-3 triodes, each with a specific tone shaping network and the pots  are the 'treble', 'middle', and 'bass' knobs.. the SC120 puts the pot on the other side of the triode, and the tone shaping network is out front, but its the same general concept.  Not to steer you away from it, but there aren't many amps with active tone stacks that get much praise (are there any?)

[hesamadman]

I see a lot of abbreviations on here. I have to really think about them sometime. That one just threw me for a loop for some reason. 


Actually what I was going to try was use a 12aY7 in v1 and a 12aX7 in V2. So a 12aY and 12aX would be in parallel. I wanted to do different cathode values on each and have some sort of attenuation between the 2. Right now i was just trying to get something functioning on the board before I started to really experiment.  I definitely planned to ad a tone stack and a MV after the paralleled stages. The schematic I posted is very dry and very rough. Just a starting point.

[hesamadman]

 
Not a volume control, but a balance control for that single stage...you'll have to experiment with pot values, and please remember this is just an experiment

I like this idea. Its basically an adjustable grid stopper right? What do you suspect this would do to the tone?

[HotBluePlates]

Actually what I was going to try was use a 12aY7 in v1 and a 12aX7 in V2. So a 12aY and 12aX would be in parallel. ...

The traditional way is give each triode its own plate resistor and its own cathode resistor, have a coupling cap from each plate, then a series resistor from each coupling cap to a common mixing point. Each triode is then 100% independent of the other.

If you share the plate load, you can mix each triode's output together at that shared plate load, but the triodes are semi-dependent on each other: plate current variation of one alters plate voltage of the other, and the load must be the same for both (can't have different-load for working a different part of each triode's curves).

Shared cathode resistor, as noted before, causes signal-input to one triode to create a signal input to the other triode due to the shared cathode connection. r you add a bypass cap to eliminate that, but now both triodes share the exact same bias & voicing. That might reduce the value of having 2 triodes, and it seems like you should just strap the grids together while you're at it.

I don't know if your original setup is the cause for oscillation (never tried that particular connection), but it seems logical that the triodes were fighting you trying to make them amplify differently (with volume controls) when they were tied tightly together in every other way.

Perhaps you might try completely separate triodes first with the mixing resistors (see the 220kΩ channel mix resistors in a vintage Fender 2-channel amp, right before the channels go into the phase inverter). Get your voicing, etc sorted such that you like it. Then perhaps move to more & more shared parts, if feasible. At that point you can figure out where it ceases being workable to combine circuit elements.