Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: JamieMcC on February 10, 2014, 11:41:11 PM
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First the disclaimer I am new to diy the bhc is my first and only build so far, however its sparked an interest that I am enjoying and which my other half now refers to as my new hobby!
Moving on.
I have noticed several posts which touch on swapping the two led's used for biasing on the input tube for resistors in order to achieve more optimal operating parameters (plate loads, I think) for different input tubes like the e80cc and 12bh7.
While swapping the leds for resistors is straight forward enough and an inexpensive thing to try it is not something I would want to do on a regular basis. I was thinking more along the lines of something switchable with set bias which might be top plate operated while listening (if practical)to reference effects .
I have seen mention of "bias pots" but have a feeling this might not be the same thing.
Might this be a practical thing to try or more detrimental to the sound quality with the added componentry.
Thoughts, pointers, links for further reading or associated parts welcome.
Cheers
Jamie
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It's totally doable.
If you have the Speedball, it's a little more plausible to use resistors instead of LED's, but it won't sound quite as good.
IIRC, going up in bias voltage a little bit from 1.5V for the 12BH7 will nudge the plate voltage back into the desirable range.
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ok thanks Paul that's interesting, the implementation looks like something I could tackle with some thought and further reading on the subject. This would also help me to have a better understanding of what I am actually doing and I might just start learning a bit about how the amp works rather than just swapping out one part for another.
Looks like I have plenty to read up on for starter.
Jamie
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ok thanks Paul that's interesting, the implementation looks like something I could tackle with some thought and further reading on the subject. This would also help me to have a better understanding of what I am actually doing and I might just start learning a bit about how the amp works rather than just swapping out one part for another.
Looks like I have plenty to read up on for starters.
Jamie
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It's totally doable.
If you have the Speedball, it's a little more plausible to use resistors instead of LED's, but it won't sound quite as good.
IIRC, going up in bias voltage a little bit from 1.5V for the 12BH7 will nudge the plate voltage back into the desirable range.
What bias voltage should those with 12BH7s be shooting for? Would series HLMP-6000s work or maybe another LED?
Also, when using resistors would you recommend a bypass cap? (and value?)
Thanks
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Something higher than 1.5V, but not by much. 1.7-1.8V ought to be enough.
I don't think the bypass cap is necessary if you're going to use resistors.