Speedball Resistor Upgrade Question

Jimb0 · 351

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Offline Jimb0

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on: June 06, 2024, 06:53:24 AM
Could the 237-ohm resistors on the small circuit board be replaced with 220-ohm resistors, or does the value need to be closer to 237ohm?



Offline Mucker

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Reply #1 on: June 06, 2024, 10:33:10 AM
I'm assuming you have one of those resistor kits with values that are not the same as the Bottlehead ones. I have those kits too and what I usually do is use an additional resistor in series to get close until I can order the correct value. So say a 220 in series with a 22 will get you to 242.

The old Speedball big board used to use 2 resistors to do just that. (top photo)

Of course the 220 may work just fine. PB could answer that for sure.

You can also use resistors in parallel (bottom photo) to get the needed value. I did that with the small Speedball board in the past until I ordered the correct values. This was necessary for 12BH7 and E80CC usage.

If you used two 470 ohm resistors in parallel, the value would be 235. Pretty close to your 237.

Handy calculator for series and parallel determination: https://www.calculator.net/resistor-calculator.html



« Last Edit: June 06, 2024, 11:23:56 AM by Mucker »



Offline Jimb0

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Reply #2 on: June 06, 2024, 04:27:11 PM
Thank you for the info and calculator. Actually I have some spare 220R Shinkoh resistors that I've been wanting to try. Unfortunately I do not have 470 ohm Shinkoh's otherwise I would have used them in parallel.



Offline Paul Joppa

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Reply #3 on: June 06, 2024, 05:49:38 PM
It should be fine - it's only 7% different, and will decrease increase the plate voltage of the driver about 5%. Tube parameters alone typically vary 20% or so.
« Last Edit: June 06, 2024, 07:06:08 PM by Paul Joppa »

Paul Joppa


Offline Jimb0

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Reply #4 on: June 06, 2024, 06:06:28 PM
Thanks Paul!



Offline Paul Joppa

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Reply #5 on: June 06, 2024, 07:04:03 PM
Mistake - I'll edit my post above. It will increase the pate voltage, not decrease it!

Paul Joppa


Offline Jimb0

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Reply #6 on: June 07, 2024, 05:52:41 AM
No problem, thanks again!



Offline Paul Birkeland

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Reply #7 on: June 07, 2024, 06:56:31 AM
You could also use a 240 ohm resistor.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man