Modifications to run e80cc/12bh7a/5687 for Crack+Speedball

[Paul Birkeland]

It definitely be nice to be able to dial in each side of the triode!;-)

You really want to set each half to have the same current running through it, not whatever current is required to produce the same plate voltage as the other side.

-PB

[NightFlight]

You really want to set each half to have the same current running through it, not whatever current is required to produce the same plate voltage as the other side.

-PB

Okay. Can you expand on that?

[Paul Birkeland]

Okay. Can you expand on that?

If you have a really dead triode and you adjust the current strongly to counteract that, then plug in a good tube, your voltages will be way off.  There's also the general philosophy that if you don't need to provide an adjustment for a particular aspect of an amplifier, you probably shouldn't. 

The Crack is reasonably tolerant of wandering driver stage voltage, so that's not as much of a concern as it might be in a circuit like the Paramount 2A3.  Or, for another example of matching plate voltage, the Mainline offers an adjustment to set the plate voltage to the ideal value, which allows the amplifier to produce the most power. 

[Jimb0]

How do I set up the 12BH7's using one DPDT? It's hard to tell from the picture (from Snaari's mod) where the wires go and where to put the resistors. I should have went the easier route by installing two SPDT's instead but I thought I could make it easier by just having one switch for the 12BH7's. Oh well. If you guys can help me install this or show me some clearer pictures to how the wires are connected I would really appreciate it. Thanks!

Jim

[Paul Birkeland]

You really want to put one switch on each board. 

[Jimb0]

http://www.4shared.com/download/3SMRnnbkce/Crack-1.jpg?lgfp=3000

This is the picture of Snarri's mod where he was able to use one DPDT. If I could get a better angle of the DPDT it would be very easy.

I don't really want to buy a pair of SPDT's if I don't need one.

[Paul Birkeland]

If you need to pop a board off, it will be pretty hairy to get all that apart. 

[Jimb0]

If you need to pop a board off, it will be pretty hairy to get all that apart.

That's very true. I guess I can work on the 7119 switch in the meantime if I order some SPDT's. The thing is I probably won't need it after installing the 7119's.

7119 here I come.

[Jimb0]

Great news. Just installed the switches for the 7119's and 12AU7's!

Now to test the voltages... What should I be testing and what readings should I be getting between the two tubes?

[Jimb0]

After a week of modding I finally got my SPST's and DPDT's installed to run the 12BH7/7119's!

For the resistance mod, I added a 330 ohm instead of a 350 ohm because I did not have it in my resistor pack. Voltages came out a bit higher than the range mentioned.

T1/T5:

7119: 88/88
12BH7: 81/78
12AU7: 100/100

Are these voltages okay?

[Paul Birkeland]

The 100V kind of sucks, are you getting that with the stock resistors, or the 330 Ohm resitors? (The 330's are not to be used with the 12AU7)

[Jimb0]

The 100V kind of sucks, are you getting that with the stock resistors, or the 330 Ohm resitors? (The 330's are not to be used with the 12AU7)

I have the resistance mod turned off for the 12AU7's so it would be with the stock 235 ohm resistors.

[Jimb0]

So I tried using a higher ohm resistor to try to get my voltages down. First I tried to add a 22 ohm in series with my 330 ohm but my voltage ratings only went down slightly. (Btw, before I added the 22 ohm in series with the 330ohm my 12AU7's were around 85/85. In my last post I think I may have forgotten to turn the switch off XD.)

T1/T5

7119: 86/86
12BH7: 76/73
12AU7: 84:84

I replaced the 22 ohm with a 47 ohm resistor but got near ratings. Then I replaced the two resistors in series with a single 470 ohm resistor

T1/T5

7119: 83/83
12BH7: 75/72
12AU7: 82/82

Should I keep replacing it with higher resistors till I get between 70 - 80?

[Paul Birkeland]

The high voltage just indicates a 12AU7 that isn't super fresh.

-PB

[Jimb0]

Thanks PB.