Question about 6L6 to EL34 mod in a Fender

[Regis]

Pretty much all the mod instructions for changing a Fender amp from 6L6's to EL34's say to change the screen resistors from 470 ohm to 1k, but I've never seen an explanation as to why.

Changing them to 1k does change the tone of the 6l6's if you reinstall them, so why can't the 470's stay when you use EL34's?

I'm not any good at theory or math, but if someone could give a plain answer as to why I would appreciate it.


Thanks,

Regis

[MWJB]

EL34's are typically rated with a lower max screen grid voltage than 6L6.

[Steve Conner]

Yes, the larger screen resistors are needed to keep the screen grids in EL34s from burning up when the amp is cranked. I have seen EL34 amps with 1.5k screen resistors that still put out 50 watts.

Unfortunately, the higher screen resistor then cripples power with 6L6s. You may only get 30 watts with 1.5k. What you probably want to do is have a choice of 470 ohm or 1.5k selectable by a switch.

[hasserl]

FWIW Carvin, in their Legacy and MTS amps use 350 ohm screen grid resistors. The Legacy comes with el34'S, The MTSw/ 5881'S. Both amps are fited with a bias switch for both tube types, so the user can use either type as they choose. Of course, the bias should also be checked and adjusted when swapping tubes, but with the switch it gets you closer to the corect bias voltage to start with. Anyway, I know of numerous different amps of both models using both tube types. MOst Legacy owners preer EL34's, but there are a few that like 6L6's. And most MTS owners prefer 6L6's, but a few like to use EL34's. The tubes don't seem to mind which way they are used, as long as they are biased correctly. Shortened tube life is not a problem, i.e. I do not see failed screen grids on EL34's used in these amps,

Plate voltage run in the 450 to 470 range, like most Fenders. I don't think you'll have a problem using the 470 ohm resistors.

[Enzo]

The math is beyond me, at least before my dinner. But as I understand it, the point with the EL34 is limiting the screen grid dissipation, and thus the current through it.

[Rob Mercure]

Well, well, how "common wisdom" seems to change with time. Around 6-8 years ago it was accepted as good technique on the Ampage board to increase the value of the screen grid resistors - where used - on Fender 6L6GC amps to 1 K to limit screen grid current -> heating -> "drooping"/misalignment -> thermal runaway. The variablitly of then current production Russian and Chinese tubes was one factor as well as the desire to extend the life of NOS tubes and those "heritage" tubes that were then in service. One of the most frequent repairs over the years to 1960s Fenders has been replacement of a burned up screen grid resistor and I still use 1K 1% 5W metal film to this day. Perhaps Dai has copies of some of these posts in his records.

During that time many of us experimented with different values of screen resistors and I really and truly don't believe that there is a sonic difference between 470 ohms and 1 K ohm that would pass any sort of a "blinded" test on Fender amps - one where the listener/player didn't know what the value of the resistor in the particular unit was and, even better, didn't even know that s/he was trying to determine any difference. Now I will admit in this beautifully complex and infinitely variable universe that there is some combination of amplifier/tube/player where the change may be sufficiently noticable to warrant the term "undesirable" but I also believe that variation between tubes and speaker and pickups, etc. is far greater. At least I know that I did an "A/B" comparison on my 1967 Vibrolux Reverb with a fellow musician who is much, much more sensitive - or at least purports to be <grin> - than I am and he really wasn't aware that I was doing anything other than tweaking the amp to match it up to some "new pickups" in my Strat - this same fellow drove me nigh onto crazy one evening auditioning 6SL7s in an Ampeg once and did a remarkable job of consistently noting differences that "carried" with the specific tube even when I intentionally attempted to fool him by stating I'd swapped the tube when I had not or making different swaps than he'd expected (as to the Vibrolux, to be fair, the change wasn't perzactly between 470 ohm and 1 K - I had some 510 ohm 1% 5W metal film milspec resistors that I temporarily soldered into my octal female to octal male output tube bias checkers so when these adapters were used it put the 510 ohm resistor in series with the existing Fender factory screen resistor. And while I'm quite sure that I measured the original resistors, and that they were not that closely matched, I can't find notes from that test). Anyhoo, this and other tests seemed to confirm what the other Ampagers were reporting YMMV -widely! Although the change to 1K didn't seem to make that much difference the Ampage consensus seemed to be that any higher value would/did although I cannot confirm that from my experience - reliability has always been of prime importance to me, especially with stage equipment.

As I mentioned above, I hope Dai or others can find the old series of posts cuz I seem to remember some of our theory "heavy hitters" offering their $0.02 on this one.

Rob

[Regis]

Guys, thanks for all the responses, I am much more informed now.

Regis