Ampeg vt40 El34 conversion output/headroom

[Alex R]

A few months ago I had this amp in for a pretty thorough rebuild. It runs 2 x 7027A, and one was weak, the other shorted. The owner is aware that there are new-manufacture 7027A out there, but since this amp puts 625v on the plates and made a few 6L6 types I tried spark and pop, I felt the best way forward was an EL34 conversion. So I put in 1K plate resistors instead of the 470s, and rebuilt the bias supply with all new parts plus a trimmer and resistor to ground. With a few value changes it settled down and gave a good range on the trimmer. It puts out 20vAC RMS into 8 ohms before clipping - 50 watts.

With over 600v on the plates I might have expected a bit more, but 50w doesn't seem too bad to me!

The guy felt on playing it that there was not enough clean headroom, and brought it back. It still puts out those 50watts. The speakers seem ok, no rub or noises. It is a little compressed-sounding, I guess, but ok, though it isn't sharp and chimey really. Sounds like an EL34 amp to me.

Anything strike you that I might do to sharpen it up? Or tell him he's trying to get a quart out of a pint pot?

[Chuck H]

A lot depends on the tubes. I would have expected a little more power too with 600V. Have you tried biasing it cooler. How about a pair of 6550A's.

Even with the 7027's the amps volume would have been similar though he may have gotten a few more watts out of it. The fact that it came in in neglected condition tells me that the customer may have an erroneous memory of how loud the amp WAS once upon a time. I'd say he's trying to get a quart job from a pint amp (More like a half gallon job from a quart amp, my little 20 watter would be a pint)

Chuck

[Enzo]

Loudness isn;t about the watts. If an amp gained 10 watts, that is less than a decibel louder. perceived loudness is more about gain and sensitivity, as well as dynamics.

[Diablo]

Maybe get a pair of NOS 7027A tubes and stick those in. I've bought 'em on ebay for $25 each.

[Alex R]

Yeah maybe I should have done that in the first place (NOS 7027s), then I could have said, look, it's stock .

The thing is, it does sound a little soft and compressed. Along the lines of what Enzo said, it might have the watts but it doesn't have the dynamics. Maybe the preamp does that. I was surprised by the way it sounded with the EL34s, I must say. I wasn't thinking headroom would be a problem.

I didn't change the big filter caps, could be them also I guess. It would be such a pain replacing those big chassis caps, normally I put some modern ones on a tag board inside the chassis but there's no damn room in there. I know this is a bad reason!

Well I suggested NOS 7027s and screen resistors back to stock, or else 6550s, I'll see what he says and report back when it's done. The heater supply is ok with the EL34s by the way, holds up well above 6.3v.

Thanks for the help guys.

[rf7]

I've had a pair of JJ 7027's put out 68 Wrms in a B-25B. And that's with the standard bias set-up.

[Diablo]

So I put in 1K plate resistors instead of the 470s, and rebuilt the bias supply with all new parts plus a trimmer and resistor to ground. With a few value changes it settled down and gave a good range on the trimmer. It puts out 20vAC RMS into 8 ohms before clipping - 50 watts.

With over 600v on the plates I might have expected a bit more, but 50w doesn't seem too bad to me!

I think you meant 1K for the screen resistors. I swapped out the 470 ohm resistors for 1K-3 watts in my V4 amp. It didn't make a noticeable difference on the sound. So, I think you can leave those new resistors in and just swap in new 7027A tubes and see what happens. The VT40 should put out 60 watts RMS.

[Alex R]

I think you meant 1K for the screen resistors. I swapped out the 470 ohm resistors for 1K-3 watts in my V4 amp. It didn't make a noticeable difference on the sound. So, I think you can leave those new resistors in and just swap in new 7027A tubes and see what happens. The VT40 should put out 60 watts RMS.

Yes I did mean screen resistors, CRAFT syndrome again. This is good news, just plug n play (n twiddle the bias trimmer). But I'm thinking about those filter caps (sigh).

[Guitarist]

You could try KT88 tubes! Close to 6550s but physically bigger so they must be louder! I had converted mine to 6550s and this is likely the issue, el34s overdrive easily right?

Does that one require a 12dw7? Try some 12au7s or other lower gainers, so that perceived volume aside, the preamp wont breakup as much. You could tap from the preamp/line out to isolate what's going on prior to the PI.

Is it a combo? The Ampegs I had had long since pulverized the speakers into pulp.

By the way elevating the heaters REALLY quiets down those Ampegs and the reverb hum you get nearly vanishes.