Switicable bias for Crack+Speedball?

[JamieMcC]

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

[Paul Birkeland]

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. 

[JamieMcC]

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

[JamieMcC]

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

[bdr529]

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

[Paul Birkeland]

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.