diytube.com

I guess the good news is that I can retube an amp with a quad of EL34s for the same cost as refilling the gas tank in my truck. ~$60.

JT

Would I need to change out my bias resistors or any other components to run a KT66 tube? The 7027A tube sounds interesting but it is a bit more work than I'd like to go through changing out my tube sockets without knowing for sure how it would turn out (and thus being locked into tubes using that socket style). Additionally, I'm not a fan of JJ (just junk?) tubes. I had a set go pyrotechnic in no time on a different amp.

Thanks again to everyone who posted advice.

kingmatt wrote:Would I need to change out my bias resistors or any other components to run a KT66 tube?

Can't say for sure about the Triode ST-70. I did have to use the bias expansion mod on my Dynaco ST-70, below.

http://www.dynaco-doctor.com/ST7%20Bias ... %20Mod.pdf

What specs are the KT66 comfortable running at? I think I should be able to work backwards from there to come up with any modifications. Thanks!

Joe Stroud wrote:

kingmatt wrote:Would I need to change out my bias resistors or any other components to run a KT66 tube?

Can't say for sure about the Triode ST-70. I did have to use the bias expansion mod on my Dynaco ST-70, below.

http://www.dynaco-doctor.com/ST7%20Bias ... %20Mod.pdf

kingmatt wrote:What specs are the KT66 comfortable running at? I think I should be able to work backwards from there to come up with any modifications. Thanks!

The Triode ST-70 manual has 435V on the plates of the EL34s. So, at 1V on the bias test point, the plate dissipation will be:

435V x .050A = 21.75W

To me, that is a bit high, but that is what the manual specifies.

KT66s are 25W tubes like the EL34s. So, the voltage and idle bias can be set the same.

This looks like the manual for your driverboard. Here is how to mod the bias circuit, if necessary.

http://site.triodestore.com/New_ST70_Driver_Board.pdf

"3.1 Bias circuit modifications

The bias circuit has been designed to provide the proper bias voltage levels to EL34 tubes. This is not an appropriate range for some other types of output tubes. The range can be extended by the following changes:

*Replace the original 10K bias potentiometers with 20K or 30K linear potentiometers. It is recommended to use a 2 or 3 watt unit (for robustness in the event of a tube failure), however a 1.2 watt unit will work fine.

*Replace the two 10K bias resistors on either side of the bias potentiometer and replace them with watt 4.7K parts. Turn controls full counterclockwise before turning amplifier on, and rebias output tubes."