Bias switch?

[NathanCahill]

I have a guy that is in the band at church that makes single 6V6 amps. My question is would it be feasible to add a 3 or 4 position switch before the bias pot to choose different value resistors to change the bias enough to use 6L6, EL34, and KT88 tubes or would the filter caps etc need swapped as well? I know basic electronic theory but not much specificaly related to tube amps. I know all of these tubes use the same socket and I believe pin outs. Thanks in advance. :scratchch

 

[marvar]

Re: Bias switch?

something to think about is voltages- and if your transformers and circuits are up to task.
There's more to changing power tubes than just bias.

 

[NathanCahill]

Re: Bias switch?

I was going to run it by him after I got some info to see if it was feasible. If he needs to bump the transformers or the wiring he can do that no problem as long as I have a direction. He is planning on making a short run as he ordered a batch of boards, cabinets and chassis from his suppliers.

 

[NathanCahill]

Re: Bias switch?

If the the circuts/transformers can handle the voltage would the mod i mentioned be simple enough or is there just too much more to do?

 

[marvar]

Re: Bias switch?

yes, it's feasible, just look at the THD univalve, bivalve,and the Flex series etc.
http://www.thdelectronics.com/product_page_UniValve.html

 

[Fresh_Start]

Re: Bias switch?

Yes, it can be done. No, it is not simple as it may seem.

Different power tubes like to "see" different primary impedances from the output transformer. You could pick an impedance which is a good compromise for several power tubes, or you could do some fancy switching with the OT.

Also, one tube might be happiest with a plate voltage of 350 while another might be better with 450 on the plate. That you can compromise (except for 6V6s maybe).

Also, different power tubes draw different amounts of current. That will have an effect on voltages "downstream" in the preamp - unless you have a regulated power supply.

Not necessarily bad, just pretty complex.

Chip

 

[NathanCahill]

Re: Bias switch?

Thanks for the info guys. I'll just run it by him and see if he's game.