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HD-130 <-> HD-65 switch?


MArk P.
Posted on Friday, August 26, 2005 - 07:05 pm:   

I was looking at the schematics for the HD-130 and HD-65 (chassis 2275 and 2475), and noticed that apart from two extra output tubes, the circuits were almost identical. Which got me thinking about the feasibility on adding some switching to effectively take two of the output tubes out of the circuit to make my HD-130 into a HD-65 for smaller amplification needs.

Are there any subtle issues I'm missing? I assume I'd have to have two bias settings that would also need to be switch in and out, as I would expect to have to bias seperately for two tubes in circuit and four tubes in circuit. This looks pretty easy to figure out, however.

What about the output transformer? On the 2275-130 circuit this is described as "4-130 XFMR" and on the 2275-65 circuit it is decribed as "3-65 XFMR". Would there be any issues there?

Any other thoughts about what such a mod might entail? I quite like the idea of the flexibility of being able to have four power settings (HD-130 hi/lo; HD-65 hi/lo).

Thanks for your advice.

mike kaus
Posted on Friday, February 17, 2006 - 08:55 am:   

Not really sure why I never saw this one but, hey-I'm old. I think it could be done-check out how Fender did it wit their 1/4 power switch. I think it's roughly the same principal but never bothered to look to see how they did it. 65 high and 130 low would rougly be the same though. It might be interesting but a lot of work to go through to hog an already functioning amp. Maybe if you had one that was smoked, you could build it up that way as you were going. I don't really think the OT would be affected TOO much but as you remove two tubes from the ouput stage, the OT wants to SEE twice as much impedance so that might be something you SHOULD address.