6j5 dc coupled to 12b4a as 2a3 driver

[alexg]

Funny you should say this - I was just building a PP2a3 amp with 6S4A drivers, because I too use 6S4A outputs in my main amp. Unfortunately I ran out of space and have substituted a ECC99 as driver. I wanted to transformer couple to the 2a3s with a LL1660 but again, no space so it will have to be cap coupled. I'm going to try using a 1J6G as input tube. It's a nice neutral sounding small DHT. I suspect this should sound pretty good. Should know in a few days.

Andy

Hi Andy,

Would you mind sharing your schema of your 6s4a driven 2a3 PP. I have some LL1660S lying around, I might as well put it into good use. Thanks.

 

[andyjevans]

I never wrote out the schematic because I was trying to direct couple the input stage to the driver stage, and I wasn't getting the right voltages so I abandoned it. I think I was having some trouble with the 1J6G as input - I wasn't using a CCS under it, again through lack of space, and the voltages weren't as equal as I wanted. So right now I'm cap coupling both stages - will feed back the results.

 

[notleon]

AndyQ and Alexjevens:

Thanks for the replies. Especially the one about how "12b4s go all over the place...". That helps a lot to restore my confidence in myself. I've built other tube pres, but I'm kinda like one of those guys who can't read but still get by. That pretty much describes my electronic skills, so whenever something bad happens, I automatically assume I did something wrong.

Andy, the heater voltage is always 6.3 volts, AC. A funny thing is this though: one pair of tubes failed to light up, and the heater voltage was zero. The PS that was supposed to put out 340V was up to 450V. When I pulled the (unlit) tubes and put in another pair, the second pair lit up. The PS was at 350V. Of course, after getting a bias of about 12 volts, an hour or so later, the bias started dropping towards minus 5 volts with the corresponding loss of gain. The PS also climebed back up to +400V.
Hum increased as bias dropped also. Throughout this "going bad" process, the filaments remained lit. Anyway, that's what I mean when I say the tubes go bad. They usually start off good, then go bad, or right from the start, I can't get bias above 5 volts, so right away I know the tube is bad.

I just thought of something though: the grid connections are at pins 2 and 7. Since it's a single triode, I always just connect to the grid at pin 2. If I don't connect the grid at both pins 2 and 7, could this be the source of my problem?

Once again, thanks very much for the response and input from all.

 

[andyjevans]

"12b4s go all over the place...".

I didn't say they GO all over the place. I said they TEST all over the place - there's a big range of test values, more than you expect with most tubes. I didn't have problems once they were in the equipment.

Andy