can you substitue 12At or AU7s in a Trainwreck?

[jjman]

Many people say that a 12at7 should not be used in preamp positions of guitar amps. Only in the PI. I think because of the low plate resistance. I've never seen one in a preamp position on a schematic. At least not a common brand. Other tubes can be used for lower gain like 12ay7. You'll see those on schematics of old Fenders and other amps all the time.

[paulster]

Many people say that a 12at7 should not be used in preamp positions of guitar amps. Only in the PI. I think because of the low plate resistance.

In reality you shouldn't just dump a 12AT7 in place of any12AX7 as it biases very differently because of the low plate resistance. This would also include the PI of an amp designed for 12AX7s.

However, people do, and it often sounds okay but you may be pushing the tube beyond its maximum ratings.

The reason 12AT7s are specified for PI positions in some amps is because they have plenty of current drive available, so are happy to really push the grids of a bank of power tubes, so they are quite a different tube from the 12AX7 really.

[sliberty]

Paulster,
So, which "limits" might be exceeded when subbing a 12at7 in for a 12ax7, and how I can measure to determine if I am in fact exceeding them?

[paulster]

You need to look at the power dissipation in your plate resistor, the power dissipation in the plate, and the overall plate dissipation for the glass envelope (as the compound figure can be lower than the limit for the two individual triodes in some cases).

In the case of the Express it can safely be substituted in because of the reasonably high value plate resistor and low voltages.

If you measure your preamp B+, say 280V and the drop across your plate resistor (100V or so for the first stage) you can calculate the current flowing through it, and therefore the power dissipated in your resistor (100mW). Using the same current figure you can calculate the power dissipated in the plate as you know the plate voltage (280 - 100V = 180V) (180mW).

Substituting in a 12AT7 you'd have a voltage drop of about 130V across the plate resistor (170mW) and a plate voltage of around 150V (for a plate dissipation of 195mW). This would still keep it within dissipation limits.

You can calculate this from the tube curves themselves (Merlin's book has a really good section on this) or cheat and use something like Tube Cad or a SPICE simulation to help you.

[sliberty]

Very cool. Thanks so much for those details.

[tubetek]

Another option to try would be the 5751; I like the GE "5-STAR" version.
It's actually a 12AX7 type but much lower max gain. I use them often in several situations... They're getting harder to find NOS but still much cheaper than the typical "premium" 12AX7. Also, maybe I've just been lucky, but I've NEVER had a noisy one... The real ones have extra micas and supports. Hope this helps,
Tom

[paulster]

Absolutely. The GE 5-star tube is a fantastic tube I keep trying to find more of these since I was a bit late to the party discovering them!

[redshark]

Very cool info guys!! I wonder if someone has use 5751 in V1 and V2 of the express with 12ax7 in V3. Maybe it tames the beast a little and makes it more "bluesy". Anybody has tried that?

[argonaut]

I use a 5751 in V1 and the rest 12ax7s in my Express build. I find the 12ax7 in V1 a little too 'hairy' on the edges for my tastes...and that's coming from a high-gain nut. Like the GE's already mentioned, Sylvania 5751s, IMO, sound great in the Express as well.

[Mont]

I use the groove tube 5751m in the V1 on all my amps. It will drop the gain about 30% but it will have ALOT more tone...

[jaysg]

Trying to recall...

Tube Gain
------------------
12ax7 100
5751 70
12at7 60
12ay7 44
12au7 17

Since the really massive gain in a TW is V2, I'm going to play with that one a bit.

Has anyone deleted the bypass caps? I know that can add a bit of noise, but it does drop the gain.

[ampdoc1]

The first Wreck circuit I built, I changed the tube compliment to 12AU7, 12AT7, 12AX7. This really kills the "hiss", but the gain is down some.

a'doc