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Jaime 17th November 2006 08:07 PM

Cheap spud amp
I am looking for a small and cheap amp.
The 6BX7-GT tube is a nice dual triode.
The power dissipation is 12w (both triodes) and the amplification is 10. I think will be a good option in an spud amp.

The input transformer an Edcor WSM 600/10k can supply x 4 amplification.
The output transformer a toroidal (see Steve Bench web).

This idea will work?
Best regards Jaime

kevinkr 17th November 2006 08:17 PM

Seems like a viable approach, the important thing to remember is that the input transformer needs to be driven by a low impedance source, the lower the better for good performance.

mwiebe 18th November 2006 03:18 AM

A 6BX7's low mu, 10, will probably need a a bit of gain in your preamp to make it all work, even with the four times stepup.
If you cascade the two 6BX7 halves, then you should be okay. A dissimilar triode--6EM7, 6FD7, 6GF7--or triode pentode like a 6LR8 may work easier if your idea is just one tube per channel. These all have plenty of gain without a preamp.

If your idea of spud is one triode sectioin per side then a higher mu tube may work easier, like the 6C45, 5842, 8233, 7722. If your speakers are very efficient you may not need a preamp with these either.

I've never had good luck using torroids for output, but they do work. While I've no experience with Edcor's cheap single-end outputs, something like the XSE10-16-8K may not cost more then a torroid and a good blocking capacitor. But cheap usually means what you have at hand so your idea will work if that's what you have for parts.

Shoog 18th November 2006 08:58 AM

Toroidals do work well, but getting the right ratios is tricky. Also you are stuck with the added complexity of a valve CCS or a SS CCS. If you are going to use a valve CCS then the spud idea is lost. Another cheap alternative is to use Microwave oven tranaformers as choke loads - big and heavy though.
All in all I think gapped EI's are a better bet for a spud amp.

Consider the highly respected ECL86, better than the ECL82.


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