Using 6V6's in a 6L6 circuit

[mikepukmel]

HI All,
I found an old thread "Using 6L6's in a 6V6 circuit" where the guy was asking if in a pinch he could use 6L6's in his older Deluxe.
The discussion covered two main points:
- output transformer has different input impedance than the 6L6's are designed for
- power transformer might not have the current to drive the 6L6's.
- use different impedance speaker (

If I wanted to go the other way around, is the PT issue gone, i.e. the V's don't need as much current, so the PT will be safe to use.

I looked at a few schematics, and the voltage coming off the standby switch leading to the center tap in the OT is about the same on the Deluxe vs the Twin, 410 vs 405. (they were both GZ34 based circuits, I think)

So, this leaves two issues:
- the output transformer's input impedance. I don't mind swinging for a new OT.
- Re-biasing for the 6V6.

What I don't understand is if the circuit as is can support re-biasing 'enough' so that the 6V6's will run without blowing up, or sounding horrible.

Any help greatly appreciated.

***
Some background: I can pick up, in our area, a Fender Twin for, in many cases, way less than 1/2 the price of a lower powered Deluxe or similar. If I can swap in a OT, change a few components, re-bias, and be reasonably sure I won't burn the house down, or otherwise ruin the amp, and still get something decent sounding, its a go. I would like to do a nice enough job that I can put it back the way it was, if I need to sell (or my heirs if ...)

Thanks!!

[Enzo]

Are you talking about an old Fender Twin? Or about an old Fender Twin Reverb? I see Twin Reverbs all the time, I have a nice one and a tired old beater myself, but I very rarely see actual fender Twins.

[Dave H]

Are you talking about an old Fender Twin? Or about an old Fender Twin Reverb?

I think it must be an old Twin as it has a GZ34 rectifier. I'd just play it as it is but I suppose one speaker could be disconnected to make the impedance more 'correct' for 6V6s and the rectifier could be changed to reduce the B+ voltage. It shouldn't burn the house down like that and could easily be changed back to the stock circuit.

[Leo_Gnardo]

Yes you could do that, IF you re bias of course, also select super tough 6V6's, it would be a great idea to bring down the voltage to the screen grids, a lot. And sure, an OT with appropriate primary impedance would be a terrific addition.

Re scaling the bias isn't all that hard to do. You could increase the value of the series resistor in the raw bias supply, or decrease the resistor from the bias adjust pot to ground. Or a bit of both, all in all no big deal.

Larger value screen grid resistors also would be a good idea. Say 1000 to 2200 ohms.

Take a look at the Jim Kelley amp, he ran a quad of 6V6 at nearly 500V in a very Fender-like circuit. I believe these amps are back in production after a 30 year lapse. Of course not any old 6V6 will do. Jim suggested NOS RCA 6V6GT, also Sylvania, hold up best. Try Russian 6V6 tubes only if you want to see a fireworks display. I expect the JJ 6V6 would survive in Kelleys (I have used them) also in your 6V6 Twin.

Kitty Hawk made a sort of copy of the Kelley. A customer brought his in, said "I love the tone but I'd do anything to stop it eating 6V6's. I managed to find a power transformer that would supply a voltage the 6V6's would get along with much easier, in the mid 300's. The amp only produced about 30 watts after the PT change but sounded even better, smoother all around and its owner was extremely pleased though had to pay some $$$ for that PT.

I can pick up, in our area, a Fender Twin for, in many cases, way less than 1/2 the price of a lower powered Deluxe or similar.

That means, Twin Reverb, certainly not an early 60's white Twin or 50's tweed. Despite a price spike in the early 90's for BF Twins, now they're all a hard sell. One local store owner says "I can't even give Twin Reverbs away." Amp buyers are looking for lighter weight more portable amps that also won't cause club managers to get upset with guitarists on their stages. Deluxe, Princeton, Blues Junior have captured the market.

[mikepukmel]

Hi, Leo_Gnardo, Thanks!!! And thanks for all the tips and info.

Yes, these are not the 'real deal' pre-cbs era amps. Those all sell for a mint around here, probably everywhere. What I'm seeing are silverface later 60's through the 70's.

I tried out a few much later "circuit board" Fenders at the Guitar Center, and they did not sound very good. And as you mentioned, with the bazillion twin reverb re-issues all over Ebay, Criagslist and flea markets, the price of those amps is way down as well.

Oh yeah, I see the ads all the time: "Beautiful Fender Twin Reverb, works perfectly, can't stand lugging it around, too loud ...". But the used princetons, wow, wish I kept the ones I had back in the day.

I don't mind doing *some*work. Would not want to rebuild the whole amp.

[g1]

I don't think the Twin's you are seeing are the ones with rectifier tubes or 6V6 safe voltages. The ultra linear 135W versions are running closer to 500V B+ voltages.
You will need to reduce the voltages to the 6V6 plates.

[Leo_Gnardo]

I don't mind doing *some*work. Would not want to rebuild the whole amp.

Well we're thinking along similar lines. I'd like to do a similar project, lighten up a Twin, lower power, smaller transformers, neodymium speakers. Shave 20 - 30 pounds off at least, and get a tone & power level that won't blow out the clubs.

What I'm encountering now is resistance to carrying anything heavy or bulky. "Whaddya want, EVERYTHING???" An old friend is now squawking about needing a pokey PA amp/mixer. He found a good EV one, but it weighs 44 pounds. And that's too much! Crikeys.... the good stuff weighs a bit, get over it. What's wrong with some exercise? OTOH I've been offered a terrific pair of JBL Cabaret series 15 + horn cabs, must be 80 pounds each. I'd buy 'em in a heartbeat, only problem is the PA biz around here has gone to nothing.

[mikepukmel]

Yeah this thing has me bugged out. Im new to electronics, and tubes, but I got this schematic from a AB763 Deluxe that says it has 6V6's and a GZ34:

the voltages on the 6v6's there are 410v!

but when I looked up the data sheets I can find put the design center closer to a 285.




So, either they had some custom designed 6v6's or its a type'o, or those amps burned through tubes like crazy.

[mikepukmel]

Well we're thinking along similar lines. I'd like to do a similar project, lighten up a Twin, lower power, smaller transformers, neodymium speakers. Shave 20 - 30 pounds off at least, and get a tone & power level that won't blow out the clubs.

What I'm encountering now is resistance to carrying anything heavy or bulky. "Whaddya want, EVERYTHING???" An old friend is now squawking about needing a pokey PA amp/mixer. He found a good EV one, but it weighs 44 pounds. And that's too much! Crikeys.... the good stuff weighs a bit, get over it. What's wrong with some exercise? OTOH I've been offered a terrific pair of JBL Cabaret series 15 + horn cabs, must be 80 pounds each. I'd buy 'em in a heartbeat, only problem is the PA biz around here has gone to nothing.

Cool, I hope you have time to post some details. I don't mind a big bulky amp since I probably won't ever gig with it. Oh yeah, I hear that from friends all the time "ohh no that's to HEAVY!". Yeah, the good stuff was/is heavy.

For me, I don't mind the weight, its just the volume. I don't have a good practice space. And the 22 - 40 watt Fenders of he era are like 1,800.00 and up for a nice one. If I had lots of dough, Id buy a couple of those, but then lots of dough would get a better practice space as well!

[mikepukmel]

I don't think the Twin's you are seeing are the ones with rectifier tubes or 6V6 safe voltages. The ultra linear 135W versions are running closer to 500V B+ voltages.
You will need to reduce the voltages to the 6V6 plates.

Hi G1, oh yeah, these are late 60's up through very late 70's, tube rectifier, 4 6L6, nice solid beefy Fender. What threw me was the schematics for AB763 Deluxe that says they used a GZ34 with 2 6V6's. But that contrast with the era spec sheets for 6V6's.

Thanks for the tips!

[mikepukmel]

These all say the same:

Fender Blackface Deluxe Reverb | Ampwares

2 6V6 and a GZ34. maybe its a type'o. Or CBS bought stock in the companies who made 6V6's!

[Leo_Gnardo]

either they had some custom designed 6v6's or its a type'o, or those amps burned through tubes like crazy.

The tough RCA and Sylvania 6V6's of the time did put up with the high voltage. These days when I have to service Deluxe Reverbs, I use JJ, and avoid EH and fake so-called "reissue" Tung-Sol which look - exactly like EH to me!

An in between solution you may want to think about, Fender Pro Reverb. They're a little bit rare though not so much a collector item. Sometimes I see a used one offered in the affordable $500-750 range. Much the same amp as a Vibrolux Reverb, in a box with a pair of 12's, with a tube rectifier. Keep eyes open, maybe one will fall your way, then you're halfway to your goal without having to even light up a soldering iron.

If concerned about OT impedance matching, yes removing one speaker will - sort of - compensate for the primary impedance. Or you could go for a pair of 16 ohm speakers. Going the other direction, using 6L6 where an amp normally takes 6V6, I've found it's not all that much of a concern. The Fender amp seem to be fairly forgiving of impedance mismatches.