What do the tubes do in each position?

Wow, good question. I'd love the answer to that as learning experience, too. I thought the general recomendation was to replace the tubes all at once, no matter what you are replacing them with.

Here is a very informative article that talks about some of the more famous amps, and how they have different pre-amp stages.

http://www.harmony-central.com/Guitar/guitar-amp-evolution.html

As far as substituting tubes, you are already doing what many do, just trial and error.

Here is some very interesting info at Eurotubes.

http://www.eurotubes.com/euro-y.htm

And here are some of the popular tube substitutions for Peavy Classic 20s at Eurotubes.

https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=8&sub_category_id=77

:D

Wes: +1,000 instant karma points for that Eurotubes find!!! That guy seems to know his stuff, even if he only sells one brand of tubes.

P.S. Is "stages" the same as number of tubes?

I thought the general recomendation was to replace the tubes all at once, no matter what you are replacing them with.

I dunno, but I figured for $7 bucks it was pretty cheap to experiment. The amp's previous owner said the tubes had been replaced not too long ago and had less than 30 hours use, so it may be a while before I replace the whole set.

Mitch

I am no expert on amps, but I would assume you have a tube (at least) for each stage.

I don't think you have to replace all tubes at once. I bought preamp tubes for my HRD from Eurotubes, got the power tubes about 6 months later. Just didn't have the money at the time. I still have all the old tubes as spares just in case.

P.S. Is "stages" the same as number of tubes? No. It means amplifier stages. Tubes like the 12AX7 have two completely separate triodes inside one envelope. They can be hooked together in parallel to work as a single stage, but usually they're working independently as two stages of amplification in one tube.

In amplifiers "stages" refers to steps of amplification, or gain stages.

Often in schematics you'll see the stages labelled as (ferinstance) Pre-amp, Driver, Power Amp (and possibly an IPA - intermediate power amp- or 2).
Basically, the first stage is set up to expect a very low input level, and amplifies it by (say for arguement) 20 times. Then the signal may go thru a level control (the volume knob to most of us) and onto the next stage of amplification, and so on.

Each stage may be one or more tubes (at the lower levels it's usually one tube per stage, or even half a tube for some of the dual types), at the highest power levels it's often 2 tubes per stage. Some designs have 4 tubes in the final power amplifier stage.

There are a few reasons to do it this way, rather than in one big step. The main reason is that you get better stability at lower gain settings.
If you had a gain of (say) 100,000 in one stage, it would take a fair bit of design effort to keep it from having all sorts of noise and probably spontaneous oscillations going on.

It's not physically practical to build a tube with an amplification factor beyond about 100, which is what the 12AX7 has. The amplification factor is a static quantity, and a real amplifier can't achieve it. About 50-70% of that is achievable in practice. With more than 2 12AX7 stages in a row, you need to dump some gain to keep from way overloading the later stages. In fact, with 2 stages you can heavily overdrive the second stage, as anyone who's played with a Valve Junior knows.

What we're talking about here is voltage amplification. Power amplification is a different matter, and power amplifiers have a rather low amplification factor. All that's needed from the voltage amplifiers is enough voltage to drive the power amplifiers to full output.

We guitar players are weird, though. We want to be able to overdrive amplifier stages into distortion. None of the designers of the classic amps expected that, it was a happy accident that they sounded good that way.
:D

The First tube is likely hooked to the input jack it is the preamp
the second tube has the job of driving the power tubes (phase inverter)

You can replace either one with a 12at7 , there are some rules but generally it just "works"
tubes are forgiving..

however to be technically correct the plate resistance and amplification factor
is different, I have noticed on occasion there is a fatter (more bass) clean tone
with a 12at7, if you go down to 12ay7 or 12au7 you may get the amp to the point
where it actually loses power (but its finally clean!) el84 amps are not known for clean tones
they drive too easy
also a neato tube to try is the 12dw7 or the 7247 but only for V1 (preamp)

ok I will go back to my tubes..

el84 amps are not known for clean tones
they drive too easy

True, but if I can tame the preamp I'll be satisfied. Not looking for jazz clean, more of a power-pop chimey clean. With the 12ax7s I can get that if I keep the preamp gain under 3. With the 12AT7 in V1 I can goose it up to 5-1/2 before it gets a bit of hair. Very slight volume loss, but not significant.

Interesting about the 12dw7 - hadn't heard of that before. On Tube Depot they say "You can think of it as 1/2 a 12AU7 and 1/2 a 12AX7." Might be worth a shot! I'm also curious about 5751s and might give one of those a try.

Thanks for the info, and welcome to GN!

Oh, by the way Wes, the Eurotubes guy has completely ignored my emails, though he claims on the site that he encourages questions from potential buyers. He won't be getting my business.

Oh, by the way Wes, the Eurotubes guy has completely ignored my emails, though he claims on the site that he encourages questions from potential buyers. He won't be getting my business.

Wow, I talked to Bob several times via e-mail when I made my purchase. I replaced my pre-amp tubes on my HotRod Deluxe with the high gain package he offers. I'm very happy with them.

Try and give him another shot. Maybe he's on vacation or something.

Thanks Mike. I've been short on patience lately ... Maybe he spam-blocks messages from hotmail accounts or something. I'll give him another shot. Or a phone call. I'd like to deal with someone who seems to know my amp fairly well, rather than mixing and matching tubes at another dealer.

8)

Oh, by the way Wes, the Eurotubes guy has completely ignored my emails, though he claims on the site that he encourages questions from potential buyers. He won't be getting my business.

Wow, I talked to Bob several times via e-mail when I made my purchase. I replaced my pre-amp tubes on my HotRod Deluxe with the high gain package he offers. I'm very happy with them.

Try and give him another shot. Maybe he's on vacation or something.
Email's not 100% reliable either, there's any number of reasons why he might not be getting them...

https://www.guitarnoise.com/forums/viewtopic.php?t=25026
:D