EL34 to 6L6 impedance question

[rockon1]

If am amp has a OT optimised for EL34's and an 8 ohm output only what load would be optimal if I switched to 6L6's? Is it a minimal difference? Thanks Bob

[Chuck H]

You can get finite by looking at tube charts but in general, for guitar amps, treat them like the
same tube for the OT requirements. You will want to check the pin connections at the sockets to be sure the wiring is OK for 6L6's. And you'll need to re-bias the amp. Other than that EL34's often use 1k screen R's where 470R may be better for 6L6's. I don't see any reason to change the OT.

Chuck

[rockon1]

You can get finite by looking at tube charts but in general, for guitar amps, treat them like the
same tube for the OT requirements. You will want to check the pin connections at the sockets to be sure the wiring is OK for 6L6's. And you'll need to re-bias the amp. Other than that EL34's often use 1k screen R's where 470R may be better for 6L6's. I don't see any reason to change the OT.

Chuck

Thanks Chuck. I have already swapped in the 6L6GC's and rebiased. Im running the 8 ohm amp with a 16 ohm speaker and trying to figure out if Im creating more of a mismatch or less of one. If its negliable I'll stick with the 16 ohm. If its more of a mismatch I'll go back to an 8 ohm since I much prefer the tone with 6L6's. Bob

[Chuck H]

Most Marshall amps ran a pair of EL34's at into about 4K. Most Fender amps run a pair of 6L6's into about 4k. Some earlier Fenders (and at least one early Marshall) used the more tube chart inclined 6.6k primary. Many other builders use 6.6k for EL34's. And if you look at the off the shelf Hammond tranny specs you'll see that they offer the same OT primary Z most of the time for both EL34's and 6L6's.

Like I said, you can idealize the primary Z as it relates to plate voltage but this is rarely done. Especially true of guitar amps.

You mentioned that the OT was "idealized" for EL34's. Not sure if that means it's idealized to be for an EL34 GUITAR amp or idealized for the tubes themselves. If your using a 16 ohm speaker on the 8 ohm tap you effectively double your primary Z. So by doing this your presenting a primary Z between 8k and 13.2k to your 6L6 tubes. I would probably switch back to matching the speaker and impedance taps with the same ohm value.

Chuck

[rockon1]

Well its a Rivera Chubster guitar amp. Supposedly Rivera use different trannies for thier EL34 and 6L6 models but I hasvent confirmed that. Though its 8 ohm tap only the manual states its ok to run a 16 ohm load in the amp and nothing lower that 4 ohm. Unfortunately I dont know the specs on the OT. Since the OT is robust enough to handle a mismatch I figure its probaly OK. By the book I was wondering if I was going past a mismatch of more than one "step" though. Bob

[Chuck H]

No, your not going past the spec for the tranny. But you may be losing some efficiency. Even if there is a different tranny for 6L6 tubes I think running the "correct" impedance tap will be closer than what you are doing. If it sounds good (and has enough power and headroom), fine. You should at least try the amp into the "correct" load for it's impedance setting. I'm confident that it won't be further off than what your doing now and you may find you like it better.

Chuck

[rockon1]

Ive tried it at the correct impedance(8 ohms) with various speakers but my 16 ohm MIE G12H-30 sounds better. Some Rivera guys claim its due to the mismatch up but I think its that speaker or both. Wouldnt know unless I tried a broken-in 8 ohm version and considering it took probably 100 hours for that one to break in I doubt Im going to find out soon! lol! At any rate the mismatch sounds good with this speaker and gives me the option of running a 16 ohm ext. cab if I desire. Thanks much! Bob