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Vintage threads from the first ten years

Re: Can I interchange EL34's with...

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4/22/2000 10:07 PM
andyfuchs	Re: Can I interchange EL34's with... Lets not forget heater current. I'd be careful about the 6550 pulling more than an EL34 does....

4/22/2000 10:23 PM

andy,

you may have just answered a question i've always had about the jcm 800. i had an early one with 6550's. i biased it for 34's and changed the tap on the impedence switch as was mentioned, and the 34's sounded great. then i later bought on of the models that came with 34's stock, and i converted it to 6550 to see how it sounded, and it didn't sound as good as the 6550 amp sounded with them. so when i saw your post i figured the heater current must have been the reason, eh?

dale

4/22/2000 11:59 PM

Thanks Guys!!

I just picked up a 81 JCM 800 MDL 2204 at a pawn shop for $80.00(it had broken 6550's in it). I put in new 6550's, set the bias at 39ma and it rocks!! I was looking and just about to ask about the EL34 conversion and there it was. BTW if any one uses the Swamp probe in this amp don't use two at once, because one tube will run away, I shut it down at 100ma!!. If you use one probe at a time every thing works fine.

Thanks,

Dave K

4/24/2000 12:06 AM

Marshall's (as they use either tube in many models), may not have this problem. I would guess they likely set the transformers up for the higher current tubes to be safe. My worry would be a Music Man or Fender amp (Yes, I've seen 6550's plunked into Fenders), where the 6L6 (which is much less heater current than EL-34 or 6550), and the tranny's get HOT ! I would doubt the change in sound was due to heater current issues. It might have been feedback not being changed to match 6550's or something else.

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