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Mixing two channels in the preamp or at the phase splitter?
Mixing two channels in the preamp or at the phase splitter?
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Old 26th April 2009, 07:41 PM   #1
flysig is offline flysig  United States
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Join Date: Apr 2009
Default Mixing two channels in the preamp or at the phase splitter?

This seems like such a simple question, and all the stuff I want to do would be cookie cutter simple in op amps! But, what is a good fidelity way to mix two channels in tubes?

Specifically, this is an acoustic guitar amp, so it will be more hi-fi than an electric guitar amp. So I want good frequency response, not the mid heavy typical guitar amp.

This amp will have two channels which will be used simultaneously, one for guitar and the other for a mic. Both channels will have to be summed together at some point after the tone stack.

The RCA Receiving Tube manual page 279 shows a very simple arrangement using 1/2 of a triode for each channel, with a common cathode resistor. The plates have their own resistors. There are two resistors in series between the plates, with the output being taken from the point between these two resistors. Thus the inputs are completely isolated, which will be necessary for my application to keep the tone stacks isolated.

Is this a good implementation? So many schematics online are very complicated using many tubes. Simple is better in my opinion.

Am I on the right track here?

The bottom half of the image below is the mixer. Ignore the top half, I can't get the image edited.

Click the image to open in full size.
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Old 26th April 2009, 10:36 PM   #2
jjman is offline jjman  United States
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As far as the mixing aspect of that schematic, the key difference I see (from what I’m used to) is the lack of dedicated DC blocking caps on the plates. The 2 resistors (R4,R5 ?) that are doing the mixing are each attached to one plate. Normally I see them attached to a cap on each plate. They look to be 1meg each in the posted file. I think I normally see 75k-220k for each.

Fender’s often use:

Plate A > cap A> resistor A > “output junction” < resistor B < cap B < Plate B

Of course you also need a plate resistor directly on each plate from the power supply.

I have read that using separate cathode caps/resistors is good to reduce crosstalk. A voltage gain of 15 seems pretty lame and I’m thinking that the 1meg value for each mixer is part of that story.

Have you decided on the preamp circuitry before the mixing? Is it based on any particular amp?
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Old 26th April 2009, 11:27 PM   #3
flysig is offline flysig  United States
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Join Date: Apr 2009
jjman, thanks for that input. The preamp is still a bit undefined, but the plan is to have an input tube, then a Baxadall tone stack, and then another tube for gain. That last tube would be the one that feeds into a long tail phase inverter. The second tube would likely be a 12AY7. The first either a 12AX7 or an EF86. This is for a guitar amp.

Though there is an idea that I may use an optoisolator for some feedback so as to include a compressor into the preamp. That isn't yet figured out yet, though. Perhaps the LED of the optocoupler would be used for cathode bias. Where the photoresistor goes is the big question right now. It seems that the specs call for a minimum of 1mA current through the resistor for a consistent low resistance value. So maybe the resistor will go in the cathode bias chain and the LED driven from the plate circuit.

More info than you wanted probably...
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Old 27th April 2009, 10:15 PM   #4
costis_n is offline costis_n  Greece
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Hmmmmmm, that's what is been my issue with the guitar amplifier i am building right now, and design as i go. So far the cleanest option is to have the 2 channels on each input of the phase inverter. BUT then you have no place to insert the feedback (which i am sure you want to apply, too) so , in my case , one channel is going to be "dirty", but i have no idea how i am going to parallel (or series) connect the 2 clean channels to have decent distortion. For now, it goes like this:

1/2ECC83 -> Tone&Volume -> 1/2ECC83 -> ECC81 (PI) -> 2xKT66

So how would i go bout adding an ECF82 (small triode/pentode) ?

1. second input ( with a switch to parallel the two inputs)?
2. after the first section of ECC83 (which also drives the Baxandall tone, and volume) Looks bad.

If i Plug the ECF82 in the second input of the phase inverter, all i get is 2 clean channels, and nowhere to insert the feedback.

Any suggestions welcome. I can also post a (so far) schematic, if it helps.
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Old 27th April 2009, 11:17 PM   #5
flysig is offline flysig  United States
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If you want a Baxandall tone stack, it must be driven by a low impedance stage. So that implies a cathode follower, which is low gain. So, I think the best thing to do is to buffer the tone stack.

input > 1/2 ecc83 (normal high gain circuit) > 1/2 ecc83 cathode follower > tone stack > 1/2 ecc83 > ecc81 PI > power tubes

If you want two channels you can sum them using the circuit in the picture I put in the first post. Do that going into the ecc81, at least that looks to me to be the best place to do it.

I'm not sure I understand the rest of what you said (I'm new at tubes). A schematic would help.
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Old 28th April 2009, 03:57 AM   #6
gingertube is offline gingertube  Australia
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Location: Adelaide South Oz
An ancient Oz guitar amp showing a Baxandall tone stack. Also shows use of a ECF80 (6BL8) except it uses the pentode section as the tone recovery stage and the triode section as a concertina splitter for the power amp stage.

For some reason 95% of guitar amps in the last 15 years have used the "stacked schmidt phase splitter" (The differential amp sitting on top of the feedback). My own experience is that the concertina splitter sounds better for a guitar amp and that is what I now (for the last 2 guitar amp builds) use.

As far as the mixer function goes, your posted RCA schematic is a (more complicated) variation on what is known in guitar amp circles as a "Tweed Mixer".
The two channels feed the grids (1 each) of a twin triode. The 2 triodes use a common anode load resistor (of about 1/2 the usual value for 1 triode) - that is, the individual triode signal currents mix in the common anode load. . The cathode circuits can use a common cathode resistor and bypass cap OR can use separate resistors and bypass caps which allows you to tailor the sound of the 2 channels differently (by messing with idle currents and bypass caps). Look up "Tweed Mixer" or simply look for some of the Fender "Tweed" circuits (which is where the commonly used name came from) - you will find examples of this mixer. While it takes an additional tube, it does provide some additional gain and it provides full isolation between the 2 channels so that their volume and/or tone controls do not interact. The simple mixing circuits in guitar amps which utilize separate high value resistors into the grid of just one triode will always suffer some interaction of controls.
"Schematic Heaven" website is a good place to go perusing guitar amp schematics.

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Old 28th April 2009, 07:06 AM   #7
revintage is offline revintage
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The "tweed mixer" is good enough and sounds right for electric guitar but not for your application as it has quite high distortion.

If wanting something more appropriate I suggest you add a anode follower as this can be used as a virtual earth mixer. But this one doesn´t sound right for electric guitar as it stiffens the sound......
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Old 29th April 2009, 07:59 AM   #8
costis_n is offline costis_n  Greece
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Gingertube , thank you very much. I think that this idea will work best, though not necessarily with the tubes in this order. What i am thinking is to have the ECC83's set for a bit higher gain, and switch the pentode part of the ECF82 in the circuit when distortion is needed. The last stage before th KT77's will still be the ECC81, for no other reason that i have JAN ones from my daddy's shop. This thing will be able to also work with EL34's also (have two nice German ones from aforementioned shop)

Will post a schematic.
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