ST-70 rebuild concept?

[Blair]

Yes,

I did those mods on the amps to allow for safe operation of the EL34s in triode. Dang! The choke input seemed to be a good cure, but not looking very promising now:)

[kheper]

Hold on. What choke is it? Brand, DCR, inductance, etc.

A choke input design needs a low DCR. Cheap chokes will hum, but more expensive Hammonds are ok.

EDIT: scrolling back through the thread, it is a Hammond 193Q.

10 Henries, 500mA, 1000VDC, 53 Ohms

The specs are ok for this choke to be used.

[Blair]

Well,

It's the biggest choke hammond manufactures:) 193Q. 10H / 53 ohms / 1000v

Blair

[kheper]

Yes. I saw. The specs are ok.

[Blair]

The issue is that it still leaves me way too low on the voltage for B+, so I'm going to use the 5V winding to drop the B+ a little and see what that gets me.

[kheper]

For EL34s and 6L6GCs, 442 is fine. (I'd still get a 550V cap.) ~400Vdc will be good for 6L6s.

[Blair]

Yeah,

I was saying for choke input. If all I get is 315V, then it is useless.

I'll get some HV caps and use those instead.

Thanks!

Blair

[battradio]

You might try something like this , but a low esr cap would be needed .
http://angela.com/angelametalizedpolypr ... vdc-2.aspx

[Blair]

I'll give the poly cap a try. I've done that before.

Anything wrong with this bias circuit? I was thinking about using it to fire things up and get the voltages checked.

http://webpages.charter.net/dawill/tmor ... L6_PPP.gif

If all goes well, I should have tubes in tomorrow at some time possibly. I think I might just wait for the 6L6GCs though for now. I only have a quad of RFT EL34s on hand and I'm not killing $200 in tubes because of a mistake.

[Geek]

That's an olllld schematic of Tim's. If you use that configuration, use wirewound pots. If any wiper lifts, you loose that tube.

Cheers!

[Blair]

Got a better one:) my pots are 10K WW Clarostats.

What about 4 poseidon bias circuits? I can't find the bias circuit I liked the most right now.

Thanks!

Blair

[Blair]

For the sake of simplicity, would Shannon's ST-70 bias scheme work OK? Just use two? Anything need to be added? I think I read somewhere to add a 100K from the wiper to ground or something similar to keep the tube from going 100% negative if the pot fails?

[20to20]

For the sake of simplicity, would Shannon's ST-70 bias scheme work OK? Just use two? Anything need to be added? I think I read somewhere to add a 100K from the wiper to ground or something similar to keep the tube from going 100% negative if the pot fails?

Why wouldn't this work?

Pretend you are going to build a "fixed " divider, but one of the resistors is the strip of a pot. You design it so the pick off point for highest negative (the failsafe voltage) is the input to the pot. Tie the input to your grid line. Then you tie the input terminal to the wiper which then can bring the wiper and input terminal closer to ground along the strip, which gives a lower negative for more current. If the wiper lifts you get the failsafe voltage.

20

[20to20]

Which is basically saying design it so the input is at a good failsafe voltage and then tie the wiper to the input terminal which will bring that input voltage down to the proper operating point.

[Blair]

OK.

Playing now. Here are my voltages:

EL34:
Pin 3 - 433V
Pin 4 - 435V
Pin 5 - -37V

VA Stage 6H30:

Plate 184V plate resistor 28.8K
Cathode 11.7V with 1.5K resistor

PI Stage using 6CG7:

Plate 235V 33K plate resistors
6.82V cathode voltage using LM317 as CCS for 12.5mA

Everything look OK? Anything stand out?

Thanks!

Blair