Clean Preamp Configuration?

[JJH0906]

Hey all,

I'm working on building my own "pedal platform" amplifier and I'm having a little trouble figuring out the right arrangement for the preamp tubes to provide the cleanest sound possible with very little distortion. I've seen arrangements where there is a gain/volume potentiometer right after the input jack, in between the two triodes and then even after the triodes, so I'm really unsure as to which configuration will give the best results.

Note that my intent is to run a number of pedals in the front end of the amp, including a compressor, distortion and booster so there's a potential that the input signal will be literally amplified as it comes into the amp but I can dial in the level of those pedals to make it only slightly higher than a typical clean signal from the guitar.

If anyone has any thoughts, experience, schematics, whatever... please share!

Thanks!

[wpaulvogel]

Volume control after the first triode between the first and second stage is best. The first triode will usually be able to handle an input signal ten times greater than a guitar output level and maintain clean output and placing the attenuator after this stage produces good regulation of volume.

[JJH0906]

Very well, thanks! I have been looking at replicating Fender preamp circuits which for the most part show the preamps split with a gain control, so that's good to know I'm on the right track.

Any comments about where the tone control should be placed? I've seen it placed after the first stage, then on to the game, then on to the second stage of the preamp. I've also seen it after both stages of the preamp.

[wpaulvogel]

It depends on the number of gain stages and their configuration and the style of tone adjustment used. Tone controls can really load the previous stage. A three knob FMV style is a heavy load. If driven by a cathode follower, there’s no need for a recovery stage afterwards. If driven by a typical triode stage, a recovery stage is needed. For what you’re after with a pedal platform, I’d use a fender preamp architecture. I’ve used the Marshall jtm45 values with the volume control following with good success.

[Stevem]

To me also a going for pure clean means the least amount of plate decoupling caps as possible since each one used adds a given amount of phase shift, and this in turn means the fewest amount ofgain stages as possible before it gets out to your ear!

I would be looking into a build , or coming up with a build with a pentode for the first gain stage to get the signal up a large amount right off the bat and then as simple a tone stack as you can live with to get the tonal variations your looking for, and then a Triode stage to get the signal level back up to what's needed to drive the outout stage.

Since you are taking max clean then my pick would be a single ended outout stage using a tube type and OT that could get you the kind of wattage you might be looking for.

[JJH0906]

Hmm.. I hadn't heard of using a pentode as a preamp. Interesting! I've attached the current architecture I'm starting with, which is essentially a Fender front-end using a 12AX7 that's split with a modified FMV tone circuit that includes a shifter to move the frequency notch around, if desired.

If anyone has comments on the architecture itself, good or bad, please comment!

[pdf64]

To optimise signal to noise, input stage cathodes should be fully bypassed, decoupled to well below mains freq if using AC for its heaters.

[TUBEDUDE]

Hey all,

I'm working on building my own "pedal platform" amplifier and I'm having a little trouble figuring out the right arrangement for the preamp tubes to provide the cleanest sound possible with very little distortion. I've seen arrangements where there is a gain/volume potentiometer right after the input jack, in between the two triodes and then even after the triodes, so I'm really unsure as to which configuration will give the best results.

Note that my intent is to run a number of pedals in the front end of the amp, including a compressor, distortion and booster so there's a potential that the input signal will be literally amplified as it comes into the amp but I can dial in the level of those pedals to make it only slightly higher than a typical clean signal from the guitar.

If anyone has any thoughts, experience, schematics, whatever... please share!

Thanks!

This might be of interest. The feedback loop in the first tube allows for adjustment for a variety of input levels. This circuit has a clean and quiet output with great tone. You can sub the 6SN7's for noval tubes if necessary.

20211105_085452.jpg

[JJH0906]

Thanks TUBEDUDE!

One question: I thought the 6SN was sort of noisy? I had seen some circuits with that and had read the break up pretty easy. Please educate me!

[stephenl]

There's a lot of ways to reach your goal.

Even for a "clean" pedal format, most want some color/sustain - setting the amp up for "edge of breakup". If you have a scope, it's useful to look at each stage, setting it up to stay as close to center-bias as you can throughout the useful gain range.

[JJH0906]

Thanks Steve!

Scope is warmed up and ready toeasurw, I just gotta build the darn thing first, haha!

[TUBEDUDE]

Thanks TUBEDUDE!

One question: I thought the 6SN was sort of noisy? I had seen some circuits with that and had read the break up pretty easy. Please educate me!

I've heard that too.
That's not been my experience.
Just as a safeguard, I use sorbothane isolation washers under the sockets and screws. But only in combos, not in heads and have not had microphonic vibrations.
I prefer them to the ubiquitous 12AX7. Clean and tonefull, even the newer ones.
The early breakup may have had more to do with the circuit than the tube. I've used variations of this circuit for 20 years with good results. You can adapt it to 12**7 tubes if you like.

[pdf64]

…
This might be of interest…

The high value, unbypassed cathode resistor on an input stage in that design will tend to introduce noise into the signal path. I suppose that the high degree of NFB at low gain settings may prevent it being noticeable. But at high gain settings, a different design may provide improved performance.

[Colossal]

This might be of interest. The feedback loop in the first tube allows for adjustment for a variety of input levels. This circuit has a clean and quiet output with great tone. You can sub the 6SN7's for noval tubes if necessary.

Please forgive the slight derailment. Tubedude, you have had good results with running that midrange control plate-loaded? I would imagine it would be a bit lossy there, and maybe that was a design goal. I like having it all on its own like that, after another stage.

[TUBEDUDE]

…
This might be of interest…

The high value, unbypassed cathode resistor on an input stage in that design will tend to introduce noise into the signal path. I suppose that the high degree of NFB at low gain settings may prevent it being noticeable. But at high gain settings, a different design may provide improved performance.

Greetings pdf64,
I hear ya on that, but the 4k7 isn't a giant resistor, and bypassing it will effectively ground the feedback signal from the second triode. There's no detectable noise. Using 3W metal films here likely helps also. I use them most everywhere.