Design of output section of rack tube preamp?

[ACHIEVEIT]

Hello,

I am interested in building a rack tube preamp for guitar and I am trying to grasp better the design aspects.

Let's take the JCM800 2204 schematic as a basis for simplicity's sake (i have isolated the preamp section - it is shaded in red).
If I were to make a preamp out of that and connect it directly to a power amp, say my Mesa Boogie 50/50, as far as I understand I would need a low impedance output section after the preamp output (after the 22n connected to the Master Volume in this case), is that correct?
If yes, isn't that the same function as the "Send" portion of an FX loop? In other words, would I be able to simply use the Send portion of any LND150 based FX loop (like Tube-Town, Metro Zero-less, Mojo and so many others) to accomplish that?
Is there any other tried and tested sub-circuit that is recommended for this specific purpose? (I would stick to a tube-less design to avoid having to add another tube).

Many thanks, happy holidays and a happy new year to everybody!

[Chuck H]

Using an effects loop send type circuit is the typical way to terminate a "preamp only" type circuit. So that would work fine. But I don't think it's necessary in your case. I've had good success simply padding for a sufficiently low impedance. Which may be entirely possible because a "preamp only" will need to have a much lower output voltage than the output of the tone stack in the actual amp. Only about a volt of AC. Also, because that tone stack is already fed by a low impedance cathode follower circuit you can reduce the impedance of the stack circuit itself before padding allowing you to use relatively low resistances in a divider before the output. I estimate you could get down to maybe 1k or 2k for a shunt resistor and that would roughly represent your output impedance. This should be sufficiently low for anything but very long cords for hookup.

[Chuck H]

Ok...

I put something together that should serve you. I like taking on design ideas and working them out.

First I tried just padding the tone stack output, but with the existing tone stack values the pad divider resistances were either too high to achieve a low enough output impedance or too low to allow good tone stack performance. But like I mentioned above, the tone stack in that preamp is fed by a low impedance driver. This allowed me to reduce the impedance of the tone stack by it's component values and that allowed me to reduce pad divider resistances to get both good tone stack performance and a low enough output impedance. I used Duncan TSC for some preliminary work and then finalized it on LTSpice. Below is the TSC circuit I modified on mspaint to post here.

Tada! No need for any additional drivers or amplifiers in the build Just the Marshall preamp with the tone stack values shown in the diagram and a 15k resistor to a 5k pot as a divider to the output. The tone stack will operate just like the stock amp with any tiny differences being unnoticeable. It will provide a bit over line level and is adjustable with the 5k output pot. Simulations showed less than 1dB of loss at 10kHz using a 20 foot cable.

You may need to use an unusual pot for the mid control since I've never seen the 1.5k value for the usual pots you find in amps.

[ACHIEVEIT]

Hello Chuck,

thank you so much for the detailed replies and for taking the time to do the analysis!

I had some free time today and a few components laying around, so I experimented a bit. Simply padding the tonestack output with a 1M resistor in series with a A50k pot seemed to do the trick.
I will definitely keep in mind your suggestion of modifying the tonestack itself, I had not thought of that!
I see in your second post that you were not really happy with the padding. If it is not too much to ask (you've already done more than enough!), would you mind sharing with me why such a solution is not good enough? (I understand the A50k is not ideal but from a practical standpoint I didn't notice anything too dramatic when hooking it up to my Mesa 50/50)

Thanks again for your help and I wish you a happy new year!

[Chuck H]

Hello Chuck,

thank you so much for the detailed replies and for taking the time to do the analysis!

I had some free time today and a few components laying around, so I experimented a bit. Simply padding the tonestack output with a 1M resistor in series with a A50k pot seemed to do the trick.
I will definitely keep in mind your suggestion of modifying the tonestack itself, I had not thought of that!
I see in your second post that you were not really happy with the padding. If it is not too much to ask (you've already done more than enough!), would you mind sharing with me why such a solution is not good enough? (I understand the A50k is not ideal but from a practical standpoint I didn't notice anything too dramatic when hooking it up to my Mesa 50/50)

Thanks again for your help and I wish you a happy new year!

If the tone is good (for you) and you're only going to use short cords (10' or less) I think padding is probably alright. A higher impedance output loses HF due to cable capacitance loading. That's why most outputs on devices that will connect with cables have a low impedance buffer. The circuit you're using will drop about 2.5dB at 10k using a 10' cord to connect. You're probably very close to line level at high or maximum pot setting with the 50k pot. The HF roll off may actually be a good thing since Marshall preamps lean bright anyway. What you have now isn't "bad" but for the most consistent performance regardless of cord used or output pot setting and to preserve the HF of the original signal you would want a lower output impedance. But like I said, it's all about what sounds good.

BTW, if you DON'T want to change all the values in the tone stack you would still do much better to use a 470k resistor and a 25k pot. You'll get the same output, better HF preservation (about 1dB loss @10k with a 10' cable) and the tone stack will still adjust and perform just fine.

[Chuck H]

Just a follow up...

I worked that circuit out 'just cuz'. The idea came to me some years ago and I figured there was a way to take advantage of the low impedance driver (cathode follower) feeding the tone stack for Bassman/Marshall topography amps if I ever wanted to build a preamp for myself. So I was going to do this some day anyhow I just think it's cool that it can be done to professional standards without the need to add a buffer stage by simply changing the preamp component values

[pdf64]

Although the typical 12AX7 CF has a small signal low impedance output, with large signals its capability to drive low impedances isn’t very good. It just can’t provide the same signal swing as impedance falls. Even trying to drive 2 regular tone stacks in parallel, an arrangement used in some switching amps, quite noticeably changes the waveform.

[Chuck H]

Although the typical 12AX7 CF has a small signal low impedance output, with large signals its capability to drive low impedances isn’t very good. It just can’t provide the same signal swing as impedance falls. Even trying to drive 2 regular tone stacks in parallel, an arrangement used in some switching amps, quite noticeably changes the waveform.

I didn't use a tube model for my simulations. Just an AC voltage with the series resistance approximated at 1K. Do you expect that the performance of an actual tube model would be different in the circuit I proposed? I can't simulate it because I've never been able to upload tube models to my LTSpice successfully.

[Dave H]

I can't simulate it because I've never been able to upload tube models to my LTSpice successfully.

Try this. Extract the attached .zip, double click on the EL84.asc file and it should open LTSpice, click on the running man to start the sim. When the sim has finished click on the output (Load) say to see the waveform.

EL84pp.zip

[Chuck H]

Thanks Dave. Your helpful as always. I've tried to find the error in "my" particular circumstances and followed some tutorials in an effort to expand my spice but I guess I don't already have the scholarly insight or haven't put in enough time on the matter. Here's what I get following your post above.

[Dave H]

Was the dhTubes.inc file in the same directory as the EL84pp.asc file when you clicked on it?

[Chuck H]

That's the thing. I've tried doing that but I may not have the poot fu to be doing it correctly. I just now made some headway. I simply placed my LTSpice on the desktop and then dragged a copy of your files onto the desktop and IT WORKED! So this is the fist time I've been able to get a tube model to run on my spice. I think I just don't understand all of both the computer and spice allocations and parameters well enough.

[Chuck H]

Thanks Dave!!! I have to go do a day job thing now but I'm looking forward to playing with this some more later

[Mick Bailey]

Thinking out loud here, but there's the potential for ground loop issues in building a rack preamp. One way to sidestep this is to use an output matching transformer which not only provides the required impedance matching but offers isolation, switchable ground lift and also if centre-tapped, a balanced output which is a useful feature. Without over-complicating things, a simple cab simulator could be included (a direct lift maybe off a Marshall design) and then you have the ability to go straight into the desk in the event of an amp failure.

[Dave H]

I get that error when I try to run it from inside a zip file without extracting. As long as the two files are in the same folder or on the desktop it should work OK.