1970s bias specs... works with today's tubes?

[williestargell]

Hello, just finished biasing a 1972 Traynor YGL-3 Mark 3. Put in a new matched quad of JJ EL34's. The original tubes were Mullard EL34/6CA7 and apparently are the bee's knees and can take a lot of abuse.

I'm wondering if the schematic bias specs are going to be ok with these modern tubes. To bias this amp, you turn a trim pot to read 8v drop across a certain resistor (first voltage drop resistor). The individual legs of that resistor read around 455v give or take 8v.

Sound OK?

If it is wiser go a little lower, does that mean a greater drop or a lesser drop? Don't want to screw around too much, just want to extend the life of the tubes. It gets really hot back there.

 

[muchxs]

I generally don't participate in bias discussions. IMO it's not a user serviceable adjustment. Clearly you're in there measuring 450+ volts in a live amp. There are too many opportunities there to damage the amp or to damage you...

Having said that setting bias means setting plate dissipation at idle. The number to hit for Class AB1 is approximately 70% dissipation referenced back to your 455 volt plate voltage. That's around 39ma cathode current per tube.

There are those who will argue with that number, those who will argue with how to arrive at that number and those who will argue with how to achieve that number.

To open an additional can o' worms... some amps operate their tubes beyond the parameters their tubes were designed for. A decent tech will reference the tube data sheet for that particular tube and make a judgment call on whether or not new tubes will reliably hit the numbers.

We're fortunate EL34s are European tubes and the reproduction tubes are made in Europe. Or Russia. Close enough.

Perusing the old Mullard data sheets... they show a whopping 800 volts maximum plate voltage and 500 volts screens. You don't specify which repro tubes you're using although they're tough enough to take whatever you throw at them, even Traynor's inscrutable bias adjustment procedure.

I'd set bias the way I usually set bias just to make sure.

 

[williestargell]

Thanks for the reply. Everyone I know is a tire-kicker and they fill my head with gremlins and lore of tone lost to ghosts. It's hard to sift through all the crap and old wives' tales.

much appreciated!

 

[muchxs]

Considering some of my Traynors came with Sylvania 6CA7s and I swapped 'em back and forth with Mullard EL34s in the late '70s and most of the '80s without re- biasing...

Lotta guys will tell you,

"EL34s are a rock 'n' roll tube!" Urban Legend has it EL34s sound like a Marshall and 6L6s sound like a Fender. The legend goes you can stick EL34s in your Twin for "Marshall" tone.

Tell that to all the audiophiles who run EL34s in their hi-fi rigs.

6L6 hi-fi amps are less common. Sure, McIntosh ran KT66s but they're not 6L6s. Heathkit used 5881s in some amps... close but still not 6L6s.

Anyway... if your EL34s sound good and aren't redplating you're probably o.k.. Watch the lettering on the tubes. If you're running JJs and you cook the lettering with one gig you might be a little hot.

 

[Wyatt]

You're measuring screen current, and probing around a lot of high voltage to do so. Traynor's method not the best (or safest). Chances are the Traynor bias specs are pretty cold.

I would recommend using a bias probe, one that does plate voltage and bias current. And remember...dedicated NON-conductive trim pot tools only when turning any internal pots.