How to Install Adjustable Bias on Hylight Era DR103

[fltundra]

Matthias,
If your going to use the amp regularly, your going to end up installing the bias board. Plus i like the 220k bypass resistor, so if the 25k pot fails you don't have an open bias circuit, which most no what that means!
Even if you spend $400 on a quad of nos Siemens, i wouldn't want them idling at 70% or above, there's no need to bias/run these old amps that hot.

[Hi Matt]

Hey Doc,
So Groove Tubes EL34M would be your recommendation if I finally decide not to mod my amp?
I've seen they are three types of GT EL34M: low, medium and high. Which ones would you recommend in a hiwatt?
If I try this route first, would you guys still advise me to check the tubes bias current with a probe to be sure I'm "safe" or is it pointless as I won't have any mean to adjust bias?
Thanks,
Matthias

[Dr.HI-TONE]

medium or high.

[Hi Matt]

What's the difference between both? (sound, reliability...)
And do you check your tubes bias current in your unmodded Hiwatts?

[Dr.HI-TONE]

I have only used the mediums #6 and #7. I would like to get some #10 to try, but my distributor only carries the mediums.

I rarely ever check the bias on my vintage HIWATTs. If they red plate with =C= or JJ's I put GT EL34M's in them.

I must admit I collect vintage HIWATTs, I don't get to play much anymore. I am an admirer of Dave Reeves work and I don't like to undo his work.

NOTE: My opinion is that of a collector, and is probably not the most practical view.

If I only had one or two that I was gigging with regularly, I would try Ted's nifty mod. If the bias was still too hot, I would use the voltage doubler.

FLtundra did a nice job with his!

[Hi Matt]

And definitely plan on using that amp. I don't have the money and guitar skills for a second Hiwatt. On the other hand, I also tend to appreciate keeping it the way it is. I'll try Ted's mod first and see what happens.
Thank you all for your input.
Matthias

[fltundra]

FLtundra did a nice job with his!

Thank you for the kind words! When i get some extra time this winter, I'm going to redo the bias board with mil spec high temp radial caps instead of axial, and maybe try to etch my own board also about a third smaller.
Gene

[Hi Matt]

Hello again,
By a 1/2 watt trim pot, do you mean half a watt or 1 or 2 watts? The higher the better?
Also, would you rather use a 1 Meg or 2 Meg pot?
Thank you in advance,
Matthias

[Ted B]

A half watt pot is sufficient. Not much current is conducted through the bias circuit.

A 1 Meg pot is what I use, is easier to find, and works well. A 2 Meg pot may give you 1V of additional range, so it's a small difference.

[Hi Matt]

Thanks Ted.
Matthias