JMP 6550 - EL34 Conversion Worse.

[JES1680]

I had my 78 JMP 50 watt converted from 6550s and now I'm running the GT 6CA7s.
Since the conversion, the amp has lost its tight low end, and has a piercing upper end that can't be completely dialed out. I've tried many different preamp tubes to tone down the harshness, but to no avail.

Does the power tube conversion adversly affect the entire amp tone? I'm using the MV and playing at pretty low volume, so I wouldn't think that the power tubes are having that much affect (they're not working that hard).

I've removed the cap on the MV and that helped some, but there is still some harshness. I'm thinking of having it converted back to 6550s as this amp was incredible before the conversion.

One last thing, could new caps cause this?

Any advice would be appreciated.

Thanks.

[cap'n'crunch]

Some folks prefer the lowend of the 6550's. Sounds like you may be one of those folks however the 6CA7's are supposed to have some pretty good lowend too:confused: . Also, new filter caps would restore the lowend if the originals are/were going bad.

You say your not working the powertubes that much. That may be a key point too. Those amps like to be cranked up a bit.

All in all it sounds as though you were happier with the 6550's which can retain some lowend at lower volumes. Although I don't know where that harshness from the 6CA7's would come from. You may want to convert it back or get an attenuator so you can crank the amp up to big stage levels without blowing your eardrums out.

One more thing, Is the amp biased properly?

[Geetarpicker]

You might try some EH 6CA7s. I have some GT 6CA7s, some NOS Sylvania 6CA7s and some EHs 6CA7s. The new EH 6CA7s have the smoothest highend of the bunch IMHO, sounding more like a fat EL34.

[JES1680]

Thanks for the replies. I pulled the chassis and went over it. I checked the bias and one was fine. No reading on the other. Bad power tube.

Had a couple of old EL34's from a Chieftan laying around. Biaised it up and she's the old JMP I missed.
Odd that the tube glowed, got hot, no funny outward signs of it being bad, but did not pass any voltage.
I think I'll look for a good pair of 6550s and put it back to original specs.

Thanks everyone.

[hipfan]

Ah, yes. The old "bad Groove Tube 6CA7" routine. I've been down that road a couple of times. I think they're great sounding tubes ... when they're working. I've sung their praises. However, two out of the three pairs I've used have crapped out in one way or another, either by completely giving up the ghose or producing non-musical crackling with bass notes.

The one pair I had that worked? I sent it out with an amp I sold. Doh!!!

[brad347]

Groove tubes are evil!

I 100% AGREE!!!!

[Red Planet]

Plus a Million on GT. :Devil