Author Topic: Help, I'm at the max of my bias range! (Read 4323 times)

hunter · « **on:** May 23, 2007, 05:18:49 AM »

Thought after 10 months of owning the Shiva now it'd be time to re-adjust the bias.

I was surprised to see it was set at 16mA for EL34s at 500V, so way too low. I should be at 30-35mA.

Now the problem is, I am at the max of the range of the bias pot at 19mA. How can I solve this issue? Different tubes? Change of a resistor somewhere?

I'm lost .... HELP!

:(

jpfamps · « **Reply #1 on:** May 23, 2007, 11:56:25 AM »

You have two choices.

You can get some new valves that draw more plate current. Many vendors have rating systems which will allow you to sellect for this, eg Groove Tubes numbering system. The advantage of doing this would be once you have found a suitably rated valve you wouldn't have to rebias when you change the valves.

A more expensive option would be to take the amp to someone who has a stock of valves and can do this for you.

Alternatively you could modifiy the amp to have a greater bias range. This might be a simple resistor swap, or you may need to fabricate a voltage doubler which can be tiresome on a PCB based amp. It depends on the circuit and as I don't have a schematic for this amp I can't really comment.

Kilby · « **Reply #2 on:** May 23, 2007, 01:37:38 PM »

I hesitate to say mail, Bogner and ask

There may be a reason they have a narrow range (can't think why though)

hunter · « **Reply #3 on:** May 23, 2007, 02:26:45 PM »

Thanks for the comments. I had a conversation with Doug from dougstubes, he'll try to find a pair of hot SED =C= EL34s that are reading in the 30-35 area at 500V. It will be trial & error then I guess.

Will call the Bogner guys later, they have no email adress on their website (ridiculous) so I have to wait for later to call them (California time).

hunter · « **Reply #4 on:** May 23, 2007, 02:31:55 PM »

Imagine Shiva sounds ridiculously good already now with this "wrong" setup, what kind of tones it could even produce with properly biased output tubes!!!!!

Twinfan · « **Reply #5 on:** May 23, 2007, 03:02:48 PM »

Quote from: hunter

what kind of tones it could even produce with properly biased output tubes!!!!!

It might sound worse - it could end up sounding like your Diezel ;)

hunter · « **Reply #6 on:** May 23, 2007, 03:36:31 PM »

Quote from: Twinfan

It might sound worse - it could end up sounding like your Diezel ;)

Bugger!!!

:D