455 Plate Voltage too high for RCA Black Plates?

[Timinator]

I'm measuring with a Weber Bias Rite. The plate voltage is 455 and the tubes are both at 41ma. Is this plate voltage OK or is it too high for RCA's. I've been told that it's too hight the tubes won't last long. The amp is a '67 Super Reverb. Can I lower the voltage, is so how is that done? Thanks, Tim

[Old Tele man]

...I'm running 460Vdc on the "old" blackplate 6L6GC RCA's that are STILL in my '69 Super Reverb (3rd set since original purchase).

[solitaire]

Depends on the rating of the valve. A C-rating should hold up to 600V

[schmidlin]

Yup: GC's = you're fine. All others = pushing it.

[jcs]

Make sure the screen resistors are up to snuff, you should have zero issues unless you bias too hot, 455 is not high for 6L6GC tubes.

I lower the wall volts with a variac usually set to around 110, this will pull down the volts and current draw, meaning once you lower the plate volts you can then up the current draw to get your dissipation back to normal levels.

I have lots of old Fender amps, have collected RCA Blackplates for years and i normally bias them at 55-65%,there is no need to bias these hot imo, they are excellent tubes that love od pedals too!

[Timinator]

I bought a pair of tubes from what seems like a pretty knowledgeable amp guy. The "matched" pair don't match very well at all, at 456volts one tube is at 37ma and the other is at 17.8. He's says the tubes are matched but at a much lower voltage and I have to lower the voltage to around 400 or so (but be sure the the ma don't go above 48). Does this sound right? He's willing to replace the pair and has been pretty helpful but he's sure the tubes were matched correctly.

I have 7 other NOS RCA's and have matched a nice pair in there at 455volts and 41ma and the amp sounds great.

[Frostbite Slim]

Well if you do the math..

455V x .041 = 18.6 watts static dissipation which is well within the range of a 6L6GC.

So how does the amp sound?

[Timinator]

Here's what I got back from him. Nice guy and obviously pretty well versed.

Here (below) are the max ratings for a 6L6GC manufactured by Sylvania and
"everyone" should know that Sylvania specs were about 10% higher than RCA's
specs (these are maximum values, normally not reached in real life).

Sylvania 6L6GC Maximum Ratings
Plate Voltage ................................. 500 V
Grid No. 2 Voltage ............................ 450 V
Plate Dissipation ............................. 30 W
Grid No. 2 Dissipation ........................ 5 W

Fender was well noted for pushing the envelope on plate voltages and had a
LOT of trouble with RCA tubes because they often had erratic behavior when
pushed (this is why Fender switched to Sylvania). RCA eventually stopped
making 6L6GC tubes and had GE and Sylvania manufacture them with the RCA
logo in the end. We've had more trouble with biasing RCA tubes in Fender
amps than any all other amps combined.


The battle goes on.....

[jcs]

If you lower the wall volts with a variac which thus lowers the plate volts to say 420 you then have to ADJUST the current draw UPWARD to get full power dissipation.

When you lower your supply volts everything drops, volts AND current.

Take a look at Ted Webers chart at his forum to see what i mean.

Keep in mind, brand new tubes that have not had some hours of burn in, can and will drift in current draw!

I much prefer my tubes to be say 4-7 ma apart as the harmonics are better, but 15-20 ma apart is a stretch to say the least.

The most important thing is you dont want 1 tube redplating and/or exceeding 70% EVER!

[Timinator]

I think I"m getting it finally. He's told me ship them back and he'll provide with another pair. Thanks for the help.

[Baxtercat]

...just a random RCA 6L6 anecdote:
My wife found this early '60s Bell PA head with ancient blackplates in it in the basement of the old folks home she worked at. It was originally in the office, running 24/7 every day [announcing meals, bingo, etc.] for who knows how many decades.

I made a box for it to make this fake 'Marshall' 1-10 combo [loud!]. The plates are still at 484 volts and the RCAs are still running strong.

[Timinator]

Don't tell me this you're going to screw me all up and I'll have to start over!

[Marble]

baxtercat, that is bad ASS! How does it sound, and what pre's do you have in it?

[Baxtercat]

Thanks.
The Bell-35 PA came with some old Bugle Boys and an odd little 7-pin 12av6 as a first stage. Stock it was a great lead sound but I bypassed that and now just run thru the 3 12A*7 preamp tubes [I need cleaner sounds sometimes]. I also swapped in a Russian 12ax7WB for the 1st preamp [my fave smooth and quiet 12x'er!]
The input jack is where one vol. pot was. I added a green glowing eye-tube near there for kicks.

Amps are fun.
This site has some more 'Guitar Bob' Baxter amp hobby stuff near the bottom.
http://www.surfrock.org/
http://www.youtube.com/watch?v=_S7f4Xq7iSo

[JlMMY]

Originally Posted by Timinator

I think I"m getting it finally. He's told me ship them back and he'll provide with another pair. Thanks for the help.

Was it this guy?

Andrews
GTFBS - Good Tubes For Better Sounds... since 1956!