Idea: parallel input tridoes with different cathode bias/bypass. Good? Bad?

[Koreth]

My understanding of parallel triodes is that they provide a reduction of noise relative to the signal through that stage because the signals will combine in phase and reinforce one another as they are correlated, but since the noise through each triode isn't correlated, the noise signals will cancel each other out partially. Sounds like a great idea for an input stage on an amp you want to be nice and quiet.

The parallel triode setups I've seen in schematics have shown both cathodes and plates tied together, sharing a common resistor at each point. What if you were to leave the plates tied together, but to separate the cathodes, and bias/bypass on each one different? Since the two signals would no longer be correlated, I'd think you'd loose the noise reduction effect, but what about tone shaping?

If you were put a larger bypass cap on one of the cathodes for fat bass and a small one on the other for bright highs while keeping the bias resistors the same, when the two triodes sum their signals together at the plate resistor, what would be the effect? Fatter bass than both cathodes having or sharing the bright cap but not as fat as it would be with both cathodes having or sharing a fat cap? Or would the difference in capacitors cause phase shift problems through the parallel triodes that made for a weird/bad phasey/flangey sound?

What about if bypass caps were held the same but the bias points were different. Let's take a setup with one triode set a little warmer than center bias and the other set a little colder Since each triode is running over a different range of it operating characteristic the two signals are going to be distorted slightly different, with different harmonic distortion. Since the harmonic distortion would be different through each stage, would this cause the distortion to be reduced when the signals combine back together, making for a cleaner, blander signal? or since each triode is compressing differently, would this increase the harmonic distortion through the stage?

[d95err]

It's quite common to use parallell triodes with a common plate but different cathodes. It should work fine. Check this thread out for some ideas:
http://music-electronics-forum.com/t19358/

[Koreth]

That is an interesting way of doing it. I'm wondering, am I asking for trouble tying the plates together and doing the blending at the plate resistor instead of leaving the plates independent and doing the blending with resistors or a pot after the coupling caps?

[R.G.]

I seem to recall from my copy of Ott's textbook that noise doesn't cancel, it adds in an RMS way.

However, paralleling devices does decrease impedances, and that may happen in a way that reduces the noise interaction with the electronics around the device(s) in question.

[Enzo]

Meanwhile this experiment amounts to removing a jumper wire between two cathodes and slapping in an extra resistor and cap. And to REALLY complicate life, we might have to change the old resistor. SO you could do the experiment in under 10 minutes and report back and tell us how it works.

[mooreamps]

My understanding of parallel triodes is that they provide a reduction of noise relative to the signal through that stage because the signals will combine in phase and reinforce one another as they are correlated, but since the noise through each triode isn't correlated, the noise signals will cancel each other out partially. Sounds like a great idea for an input stage on an amp you want to be nice and quiet.

If you look on a schematic, closely, you will find when the plates are tied together, and the cathodes share a common resistor, that cathode resistor value tends to be 820 ohms, or about half the value of a single cathode resistor. It is the lower value of cathode resistor which gives you the 3 db lower noise floor.

The parallel triode setups I've seen in schematics have shown both cathodes and plates tied together, sharing a common resistor at each point. What if you were to leave the plates tied together, but to separate the cathodes, and bias/bypass on each one different? Since the two signals would no longer be correlated, I'd think you'd loose the noise reduction effect, but what about tone shaping?

If you were put a larger bypass cap on one of the cathodes for fat bass and a small one on the other for bright highs while keeping the bias resistors the same, when the two triodes sum their signals together at the plate resistor, what would be the effect? Fatter bass than both cathodes having or sharing the bright cap but not as fat as it would be with both cathodes having or sharing a fat cap? Or would the difference in capacitors cause phase shift problems through the parallel triodes that made for a weird/bad phasey/flangey sound?

That maybe something you would have to experiment with. I did the same on the first 5 watt amp I built, but since I use fixed bias in the preamp, there were no by-pass caps.
It may give you a smoother tone, perhaps one reason why some of the Matchless amps were set-up that way.


-g

[soundmasterg]

Merlin has a whole section in his book devoted to the use of parallel triodes. Very excellent reading and highly recommended!

I incorporated the parallel input in a Bogen project with some small changes and I like the versatility of it. The key is in getting the cathode resistors and caps correctly sized so the amp with give good tonal changes between each option, yet still work well together and sound good.

Greg

[mooreamps]

Merlin has a whole section in his book devoted to the use of parallel triodes. Very excellent reading and highly recommended!

Ya, somebody tell "merlin" you don't need bypass caps when using fixed bias... :|

-g

[imaradiostar]

Ya, somebody tell "merlin" you don't need bypass caps when using fixed bias... :|

-g

I haven't posted to this forum in weeks and the first thread I click on has you being a jerk to another forum member. Seriously dude, you've gotta knock it off. I'm sure you have good thoughts to contribute. Please try to do so without insulting other users.

More on topic- I don't understand why the noise would be lower but I find the old plexi/metal face style 2.7k/.68uf blended with more fendery values in parallel makes for an excellent and simple tonal variation. I've wired a number of amps with way with two "gain" controls and a master volume at the end of the pre.

jamie

[R.G.]

Ya, somebody tell "merlin" you don't need bypass caps when using fixed bias...

It kinda depends on whether you bias them fixed voltage or fixed current.

Or, I guess, technically, fixed resistance.

Or what kind of gain and stability you're looking for.

I guess you really do have to know where you're going before any road can get you there.

[Russ]

If you look on a schematic, closely, you will find when the plates are tied together, and the cathodes share a common resistor, that cathode resistor value tends to be 820 ohms, or about half the value of a single cathode resistor. It is the lower value of cathode resistor which gives you the 3 db lower noise floor.

So if that's true then I guess you could lower the cathode resistor further to 470 ohms and get another 3db of noise reduction. And on down to 220 ohms and get another 3db, and so on and so on?

Russ

[Koreth]

Meanwhile this experiment amounts to removing a jumper wire between two cathodes and slapping in an extra resistor and cap. And to REALLY complicate life, we might have to change the old resistor. SO you could do the experiment in under 10 minutes and report back and tell us how it works.

Touché.

If I had an amp to hand with which I could perform this expirment, I would. My boogie is at the rehearsal space presently. I'm a little loathe to work on it as a) It's my main amp, gotta keep it functional for rehearsals and gigs, and b) not fully confident in my ability to *not* fry PCBs with my cheap soldering iron.

I need to get a kit or some kind of junker to expirment with instead of just asking questions all the time.

[Koreth]

It kinda depends on whether you bias them fixed voltage or fixed current.

Or, I guess, technically, fixed resistance.

Or what kind of gain and stability you're looking for.

I guess you really do have to know where you're going before any road can get you there.

Now this has me curious. I thought that using diodes on the cathode, be they zenners, switchers, rectifiers or light emitters makes for a constant voltage source. Isn't that how the p-n junction in the diode works? Once the threshhold forward or reverse voltage is reached, it stays fairly constant regardless of current through the junction?

I really need to read more about this stuff.

[mooreamps]

Now this has me curious. I thought that using diodes on the cathode, be they zenners, switchers, rectifiers or light emitters makes for a constant voltage source. Isn't that how the p-n junction in the diode works? Once the threshhold forward or reverse voltage is reached, it stays fairly constant regardless of current through the junction?

I really need to read more about this stuff.

It does clamp the cathode to a constant level, and there is very little you can read about it, since I have not published any of my research on the subject.

-g

[mooreamps]

So if that's true then I guess you could lower the cathode resistor further to 470 ohms and get another 3db of noise reduction. And on down to 220 ohms and get another 3db, and so on and so on?

Russ

True, but you get less gain. I just pop the resistor completely out of the circuit, and just be done with it.

-g