Need Shamanistic Guidence From The Chart Guys

[rp]

Need Help Biasing A 6SL7

The two input tubes in my 5C8 are now 6SL7s instead of 6SC7s, much for the better, I even prefer the tone, the 6SL7s besides being less hummie and microphonic are a little bit smoother and more even, closer to the 12AX7 family. V2 is cathode biased in my amp, I just dropped the 6SL7 into the 5C8 schematic after rewiring the socket. But I’m not sure about the proper operating point, yes, it’s more or less the same but I curious to get it right.

I put in a 5K pot on the cathode and listened, higher voltage, lower current sounded best, hopefully one of the high priests can read the magic charts and confirm what I hear and recommend what looks most centered. 3.2K-4K cathode resistance sounded nicest, but 5.2K was pretty much the same. Below 2.2K sounded weak.

For various resistances I measured Kv, Pv, and current across the plate resistor. Note: the 6SL7 cathodes strapped, shared 220K plate resistor, the oddly low voltage is normal on the 5C8 pre-amp.

KR / KV / PV / I

1K / .66V / 56V / 1.9mA

2.2K / 1.2V / 87V / 1.1mA

3.2K / 1.5V / 102V / 88mA

4K / 1.7V / 117V/ 77mA

5.2K / 2V / 130V / .60mA

Much Thanks. I will provide tribute in jaguar pelts and quetzal feathers.

[martin manning]

What is the supply voltage at the top of the plate load resistor?

[sluckey]

I built an Ampeg J12B a few years ago. It uses two 6SL7s. Here's a link to the schematic with actual voltages. Not a tube chart but may be helpful as a comparison...

http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

[rp]

Martin, I re-read my post twice before posting wondering what I had forgotten: C4 node feeding the 220K is 218V.

Sluckey, first thing I did was go through the Ampeg schematics, knowing they liked octals so much they kept using them into the sixties But the Ampeg schematics all have different set ups one way or an other, usually different plate loads, for me to divine much from. You are close with 2.2K, it's what I initially tried. Sounds fine, but KV 1.2V was lower than seemed right.

Notice you used a LED on the second triode, did you have any trouble with that LED? Listened closely? I just went and posted this here on my experience with current draw and LEDs. Figured it was more useful there than adding it here:

https://tubeamparchive.com/viewtopic.ph ... 619#360619

edit: just noticed it's the vibrato oscillator, so it's prolly good. Sorry to question Sluckey

[martin manning]

6SL7 is pretty close to a 12AX7 as far as biasing goes. Using 6SN7 curves with 218V supply I confirmed that 12AX7-like values Ra and Rk will get it center-biased:

47k and 1k
68k and 1.2k
100k and 1k5
150k and 2k2
220k and 3k3

I'm assuming shared plate and cathode resistors, and I would try some of the lower-value combinations.

Do you still have two separate inputs for each channel, one to each grid with 0.1u input caps?

[rp]

How's this? Idle hands do the devil's work. The output stage voltages are a bit different as I'm using a 5AR4 at the moment, preamp voltages barely changed with the 5AR4.

Can't promise I drew (cut and pasted) it right, but I think it's ok.

Edit: updated schematic. Note it's for 5881 / 5U4GB, post info is for EL34 / 5AR4

[martin manning]

Very nice. For that configuration (220k and 2k2) with 218V supply I get 83V on the plate and 1.34V on the cathode, 0.3 mA.

[rp]

hmm...

I just re-checked and at 2200R I got 1.1mA and the same values posted originally. I flipped 6SL7s just to make sure it isn't an outlier or wanky tube but I got exactly the same values. The currents I listed seem to track nicely from 1K KR thru 5K KR on the pot, does that imply it's right?

I have two resistors on hand in range 2200R and 3900R. What do you think? Of course I could parallel something or other but that will look klugey ...or hyper attentive.

[Stevem]

Go on line and view page 642 and 645 from the RCA. rC-30 tube manual.

[Leo_Gnardo]

rp where do you get your SL's? or are you lucky to have a collection in stock?

I've tried NOS, expensive & no guarantee of getting a good one. Most tend to be noisy one way or another. The "cheap" Sovtek, same story. And I'm not about to hazard wasting triple the price for a Sovtek marked "TungSol" in hopes it's any better. So far I haven't been impressed (to say the least) with their TungSol line.

When customers need 6SL7's replaced, I make up adapter bases and they get 12AX7's. No complaints so far. Now JJ's making competent 5751 and 12AY7, there's a couple more choices.

Sure would be a wonderful thing to find a source of reliably good 6SL7's. There is something nice about the sound of good working octals.

[rp]

Sorry Leo, no source. I had 4 from my gear collecting days. Used to go down to Canal St. (Ramco?) and wade through piles of crap and pull the tubes you wanted, it's be liek $5 for 3 tubes! Needed a cap? They'd hand you a pair of snippers That was heaven - THAT was NYC! not Dubai-on-Hudson.

[martin manning]

rp, I am quoting current for each section, so if you are calculating using Vk, then divide yours by two.

[rp]

That would be .55 to your .3 Close enough?

Still, anything btwn 2.2K-5K KR sounds equally good, though I haven't tried hammering it. I looked through the RCA manual, but, how does one pick a happy value? I think Fender if they had had a cathode biased 6Sl7 in there they would have just used a 2200R. Never ever see anything like 3900R. From 1953-66 it's always the same dozen resistor values - it's like Leo preferred to eat ham & swiss on rye everyday

[rp]

I went back and looked again at what you posted Martin - sorry I didn't get it right away - so with 220K it's 3.3K. I'll see if the shop in town has a 3.3K. Gotta look through my scrap bin too.

[martin manning]

I would try some of those lower values too. Remember with 220k shared that's like 440k for each triode, putting the operating point way down in the curved portion of the plate curves. As far as the op point goes Leo liked to center it between 0V on the grid and cut-off. For 12AX7 with typical supply voltage that's the familiar 100k plate 1k5 cathode.