st70 clone with 6L6GC tubes

[kheper]

Finally, I am gathering the parts for the new
clone. I have decided to use 6L6GC tubes,
but am concerned about the voltage,
hitting the screen grids as supplied by the
PA60 power supply. To wit, the maximum
voltage for the JJ 6L6GC tubes is specified
as 450V. I am pushing 450V on the screens
in my rebuilt st70. Would a 350-0-350 power
supply be more desirable, than the PA60?
The value of the dropping resistors at C-->B
and B-->A on the multi-cap would need to
be adjusted to get the correct voltage to the
driver board with the 350-0-350 supply, as
well. Many of the 350-0-350 power supplies
I've seen are more expensive than the
PA60, and some lack 55V bias taps.

[Shannon Parks]

I wouldn't worry about it. You could put a resistor in there for safety (check do a search on Mesherm + screen + diode), but those tubes will be fine.

[Uncle Ned]

People put JJ 6L6's in big Ampegs (they also make a 7027, which is identical but with two extra pins) regularly, I've heard of folks putting them in Dynaco Mk3's as well (presumably with the bias toned down a bit), both applications have them running at around or over 500V on the plate and the screen. I've put the JJ 6L6GC in ST70's myself and did not note any ill effects.

Dynaco themselves noted in the manual that 6L6GC may be used to sub for EL34.

Personally, I like KT66 and 350B in a Dynaco ST70, I'd give my left.... incisor for another set of 350B's (but not money, dental work is cheaper than real 350B's!)

[mesherm]

If you want to tone down the B+ a bit for 6L6s just use a reectifier with a higher voltage drop like a 5U4 or a 5R4GY. Thats what I do on my universal tube amp. That way you can always change back just by popping in the 5AR4 again.

[Sal Brisindi]

If you are using the standard ST-70 power transformer you might over heat it with a 5U4 as the filament is 3 amps vs. 1.9 amps for the 5AR4.

Regards,
Sal Brisindi

[mesherm]

If you are using 6L6s, they use 0.9 amp each compared to EL34s which use 1.5 amps each. So you add 5.5 watts using a 5U4 but subtract 15.1 watts using 6L6s. The power transformer should run cooler.

[EWBrown]

The new JJ KT-77s also play very nicely in an ST-70. The bias may need a slight upward re-tweeking, I found that when swiching from JJ EL34s to 77s. I kept them set at 50 mA.

6L6GCs, just pop-em right in (set the bias all the way down first, then adjust as needed).

/ed B in NH

[Uncle Ned]

If you are using the standard ST-70 power transformer you might over heat it with a 5U4 as the filament is 3 amps vs. 1.9 amps for the 5AR4. Regards, Sal Brisindi

Sal:

The original transformers had 3 amp rectifier tube windings (and so do all the replacements), so unless the transformer is in poor condition to begin with, it's really not a problem.

There's really no reason to back off voltages using 6L6's in an ST70, anyway, Fender amps treat them worse in terms of voltages and in guitar amps they're often driven into massive clipping, something you'd be unlikely to do with an ST70 (albeit, ST70's have been used as guitar amps in various ways). I guess if I had a set of 50's black plate RCA or Ken-Rad 6L6-G's or GEC KT66's, I'd be a bit more careful. Sure, 6L6's crap out once in a while, but so do other tubes. Guitar strings break and wear out,. too.

[Sal Brisindi]

Ned,
Thanks for the info on the transformer, I didn't know it had a 3 amp filament rating for the rectifier tube.

Regards,
Sal Brisindi