Coke-Bottle 6L6A's

[barry robin]

i know that 6L6GS's will drop-in to the ST-35 with only an adapter and a bias pot change (to about 40Ma). the question is: can i also simply drop-in the older coke-bottle 6L6A's without diong anything more than the bias? should be fine but i thought i'd run it by the forum first.

-b

[nyazzip]

honestly i have no idea what i'm talking about, but, right off the bat most 6L6 types appear to require about 0.9amp filament current, as opposed to el84's 0.5 amp filament current. as designed, the power transformers on st-35 type amps seems already taxed to begin with: i would not want to ask it for 1.6 additional heater amps! but perhaps with a separate filament supply it would be ok
do please correct me if i am off base, i am here to learn as well

[barry robin]

honestly i have no idea what i'm talking about, but, right off the bat most 6L6 types appear to require about 0.9amp filament current, as opposed to el84's 0.5 amp filament current. as designed, the power transformers on st-35 type amps seems already taxed to begin with: i would not want to ask it for 1.6 additional heater amps! but perhaps with a separate filament supply it would be ok
do please correct me if i am off base, i am here to learn as well

on paper, so to speak, everything you say makes perfect sense. but if you look at some of the earlier threads on the board you'll note a preponderance of ST-35 builders all saying that the 6L6GS is more or less a drop in situation-as long as one makes up an adapter socket for it.

but as to the rearlier 6L6GA's....i dunno george...

-b

[Geek]

Hi,

All 6L6, even the little metal guys, are 900mA heater... poor ST-35 power tranny just canna do it cap'n

Cheers!

[DeathRex]

The EL84 is 0.76ma filament, the difference between all EL84 (3.06A) and all 6L6 (3.6) is little, but I still would want a extra 6.3 volt transformer.

But if the EL84s are biased at 40ma, and the 6L6 are biased at 40ma, you're not goona get any more power out. I do believe you'll get less audio out, because of the 6L6's lower transconductance. You might get the 6L6 sound, but EL84s are very nice. Kinda like putting a giant scoop on the hood of your civic.

[barry robin]

... Kinda like putting a giant scoop on the hood of your civic.

does that mean i should also forget about repainting the chassis with go-fast stripes?

-b

[DeathRex]

Speaking of hood scoops (or bonnet) and go fast stripes.
http://www.youtube.com/watch?v=QQh56geU0X8

[dhuebert]

I tried 6L6s in my ST35. It did not work. The split-load phase invertor in this design does not have enough swing to drive the 6L6s to full power. At anything above half volume it distorts badly. If you can figure out a way to get more swing from the PI and a fatter power transformer, it might work. Or just buy an ST-70 kit.

Don

[EWBrown]

The EL84s in the ST35 operate with approx 14VDC on the cathodes, ao the maximum drive voltage swing would be +/- 14V. 6L6 s operate in cathode-bias mode, between 25 and 36VDC, and the ST35's VA/PI circuit can't develop that much of a "swing" without going into some SERIOUS nasty distortion.

The ST35's cathodyne phase splitter has a gain of slightly less than 1, at both cathode and plate, regardless of what kind of tube is used. And the 12AX7 VA stage doesn't develop enough gain to make the phase splitter go much beyond +/- 14V peak.

The PA-774 would be insuffucient for both filament current and B+ current to properly feed four 6L6s of any kind.

The ST35's PSU would work well in a mono amp with a pair of 6L6s or 6P3S's feeding a 9K PP OPT and 360 to 380VDC plate voltage, for about 24 Watts output. Use one filament winding for the 6L6s, and the other for the VA and LTPI stages.

/ed B

[barry robin]

sounds about right. thanks EB.

-b

[EWBrown]

This is an application in which the original Dynaco ST70 or RCA tube manual 7199 driver circuit should work very well with 6L6s, and the SCA-35 driver board should also work, though a few resistor value changes may be necessary.

The 7199 pentode section has a LOT more voltage gain than does the 12AX7 half of a 7247 so it should easily be able to crank out 25 to 36 V of drive to the 6L6 control grids.

Of course, 7199 isn't octal, and would look kinda shrimpy next to some shapely ST enveloped 6L6s.

Maybe a 6SJ7 and 1/2 6SN7 for each channel's drive would have enough "soup" for the job requirements, but then the ST35 boards would not be usable in such a situation..

HTH

/ed B