tube power for Modeler

[studiodunn]

I have a donor amp that I am thinking about turning into a Power unit for an incoming AX8. Yes, I know....Blasphemy!!!
I love my tube amps, but they just don't work for my rehearsal situation and I cannot get them to work on Sunday in the PB. I also cannot stand the sterility of an all digital signal. I had an Atomic AA3 and ran it through a Fryette PS that sounding great, but sold it all off for other "needs" and now I'm back at it.

So I'm thinking of building a 50w 2)6l6 combo to see if the modeler can hang in typical tube situations. Is this just as simple as building a line level input to a typical fender power section?

[pompeiisneaks]

Pretty much yes. Most outputs of an effects loop come out about line level just before the PI. The input to the PI is also returning from effects at about line level. Effectively, in practice, you can just build a power amp from the PI, Power tubes, to the output transformer as a standalone "Power Amp" for any full preamp type unit. It may not work perfectly in all cases, and adding a 'level' pot at the input isn't a bad idea if some preamps are a bit hotter than others so you can stop them from slamming the PI too hard, but that's just a decision on the specific use case. Some amps even intentionally have an effects return pot to compensate for overly hot preamp outputs.

~Phil

[studiodunn]

Pretty much yes. Most outputs of an effects loop come out about line level just before the PI. The input to the PI is also returning from effects at about line level. Effectively, in practice, you can just build a power amp from the PI, Power tubes, to the output transformer as a standalone "Power Amp" for any full preamp type unit. It may not work perfectly in all cases, and adding a 'level' pot at the input isn't a bad idea if some preamps are a bit hotter than others so you can stop them from slamming the PI too hard, but that's just a decision on the specific use case. Some amps even intentionally have an effects return pot to compensate for overly hot preamp outputs.

~Phil

Thank you Phil.

I agree and think a level pot is a must. I don't want to induce any clipping from the PI. I would think the idea here is to be as neutral as possible. I really need the feel of the tubes more than the volume. And as much as it goes against my typical desires from a tube amp, I don't want to color the modelers tone. I may try and dig up a schematic of the Fryette PS and try and emulate it. Or should I be looking into HiFi designs as my template?

I've also read allot of users(modelers)saying the presets become very boomy through guitar cab as opposed to the sound through FR or headphones. I was toying with the idea of putting a bass cut switch/pot in as well. Something that would help tune the resonance freq. into the cab.

[pompeiisneaks]

I've yet to try it, but I've got my kemper and am thinking of putting something similar into an existing power section of one of my amps. The poitn there is that you turn off any cabinet simulations, as your power section and speaker cab ARE that. If not, you need to use an FRFR cab and a solid state or very hifi tube power amp section to NOT color the sound more than the modeller/profiler does.

~Phil

[studiodunn]

Yeah, I found the same thing with the Atomic. You have to turn off the IR, but surprisingly I preferd the power amp simulation left on even when going into another power amp. Obviously the power amp simulation is critical to the modelers ability to mimic an amp, but I don’t think they were really designed to go through a guitar rig. As usual, if it sounds and feels good, then it is.

[strelok]

You may want to consider having one additional gain stage. I don't know what level your modeler is putting out, but standard line level is 1.4v. A standard 12ax7 gain stage will have a gain of about 60, a LTP PI will have less than that. If you're using NFB you'll have even less, which you'll want to do if you want to keep things pretty clean. Depending on your plate voltage, output tube choice and how much NFB you run you might find the PI itself isn't quite enough.

So an important question comes up here. Just how clean do you want it to be? You might be surprised how clean you can get tubes to run, there is such a thing as too clean. You've decided you want to run tubes for a reason but you don't want to add color to the sound. I think its fair to say you must want *some* character added, but obviously not any outright clipping. The gain stage might be helpful here too, you won't run it into distortion, but it will introduce some asymmetry in the signal which will result in some even order harmonics and give just a bit of the tube warmth you might be looking for.

First things first though, err second, or third, wherever we're at. I'd first get a speaker cabinet that would be suitable for this application. Then I'd take a tube amp you already own and just run directly from the PI through a cap to the output of your dsp. See how you like it. That'll give you a starting point. Here's a basic rundown on P-P outputs from dirtiest to clean:

- Pentode Mode, No NFB, Cathode biased. Ex, TW Rocket, Tweed Deluxe.
- Pentode Mode, No NFB, Fixed Bias.
- Pentode Mode, Low to medium levels of NFB, Fixed Bias. This is were most push-pull guitar circuits 25w or more lie.
- Pentode Mode, High Levels of NFB, Fixed bias. Silver face era Fenders are one example.
- Ultra-Linear Mode, Medium to High levels of NFB, Fixed Bias. This is most tube Hifi gear from late 50's-60's onwards, including the Dynaco Stereo 70. Also the infamous Ultra-Linear Twin Reverb.
- Triode Mode, anywhere from no NFB to lots. Fixed Bias, Class A. A lot of modern audiophile gear runs this way. There's a discussion to be had over whether NFB helps more than it hurts but I think this is probably beyond the scope of this thread.

This is not meant to be comprehensive and the order is not set in stone. Just want to give you an idea. Additionally if you're using a long tailed pair PI or differential input, its quite common in HiFI to bias it with a constant current source. This increases the balance between the two halves by increasing the differential effect of the stage, as well as lowering distortion, especially with a single ended input.

Also two 6l6's might not cut it depending on how clean you want your 50 watts to be. Especially if you decide to go ultra-linear you may want to consider a pair of KT-88's or 6550's.

-

[studiodunn]

Thanks for all the help. The more I think about this, why wouldn’t I just build an amp that has a great loop. It seems a waste of an amp when bypassing the preamp is so easy and really wouldn’t change my model needs.

[pompeiisneaks]

Thanks for all the help. The more I think about this, why wouldn’t I just build an amp that has a great loop. It seems a waste of an amp when bypassing the preamp is so easy and really wouldn’t change my model needs.

This is exactly what I'm planning, either upgrade the loop on my dumble clone, or add a high quality one tube loop to some other amp I have here. Not sure which yet. Either that or just add a 'return' with a pot so I can trim it up/down a bit.

~Phil