high voltages with E80CC Mod

[mudvin]

Hello,

I recently built my latest crack with a few mods, aiming of running E80CC tubes as the input tube. Other than some cosmetic mods, I installed 470 Ohm resistors in the appropriate position and changed the transistors on the small speedball board from MJE350 to MJE5731 (why not).

The amp sounds great!, no noise at all. Unfortunately the voltages on the Big board are slightly high with the E80CC tube. I will mention that I am in Australia with a true 240 volt supply (so in practice it varies from 240 to 245 or so), and I think this may be related.

I have tried 4 different E80CC tubes from different sellers and date codes however the voltage only varies a little (10 volts up or down on the key measurements)

I have built a SEX plus another crack with no issues before, and I did try swapping in the speedball boards with the same result.

So given the figures below and wish to continue running e80CC , I'm just wondering if I should change the biasing resistors or if there is something else I need to look at.

Thanks,
Rob

Small Board (E80CC +470 Ohm resistor)

OA - 94
IA  - 178.2
B - 0
IB - 178.1
OB - 92.7

Small Board (12AU7 +237 Ohm resistor)

OA - 71.3
IA  - 184
B - 0
IB - 183.3
OB - 70.2

Large Board (E80CC +470 Ohm resistor)

OA - 121.5
OB - 120.3
G - 0
B+ - 190.3

Large Board (12AU7 +237 Ohm resistor)

OA - 103.8
OB - 105.6
G - 0
B+ - 182.3

Base Crack (12AU7 + 6080)

1 - 81
2 - 180
3 - 0
4 - 179.9
5 - 80

6 - 0
7 - 108.4
8 - 0
9 - 107.2
10 -0

[Paul Birkeland]

You could try a 510 ohm resistor to nudge the plate voltage down a little more.

[mudvin]

Thanks for your help Paul.

I swapped in 511 Ohm resistors that I had on hand. Im thinking it looks pretty good now, although the large board voltages are still high (due to wall voltage ?)

Regards, Rob

Voltages (E80CC  +511 Ohm  Resistor)

Small Board
OA - 84.6
IA - 193
B - 0
IB - 193
OB - 83.2

Big Board
OA - 115
OB - 116.9
G - 0
B+ -  192.5

[Paul Birkeland]

That looks good.  The output voltages on the large board will follow those of the small board a bit.

[mudvin]

Thanks for your help!

Rob

[Joe76]

I am planning to use a Philips E80CC in my standard Crack (without) Speedball. Would this be possible? I could replace the 22K1 Resistors to 33K's. Guess that might bring the plate voltages down to an optimum level?

Thanks

[Paul Joppa]

Based on the numbers above, I expect you'll need somethniing larger, mre like 47K or 56K.

[Joe76]

Ah, thank you Paul.

Will do some measurements to find out what my B+ voltage is, since here in Western Europe our wall AC is roughly 230V. Thought it was also in the range 175 - 180 volts.
Based on your comments in another thread where you mention to draw some lines on the E80CC Data sheet, 80 volts plate voltages could be indeed an optimal number in order to work correctly with an 6080. I just want the E80CC settle sweet without stressing the tube and reduce its lifespan.