Machine Gun Amps
Notes on the tweed Deluxe.

This is a very popular circuit for hobbyists to build. It's simple, breaks up at a low volume, and sounds GREAT. It's also a lot cheaper to build one of these than a bigger amp.

If you have an original tweed Deluxe, DON'T MOD IT! They are classic amps and only deserve to be restored.

1. A lot of people like the tone of a 12AX7 in the first position. This tube gives more gain and compression than the stock 12AY7. If you like dynamics, you'll find that the 12AY7 will probably be better suited to your tastes. If you are somwhere in between, then the 5751 is also a good choice. I carry all these tubes on my tubes page.

2. When restoring an original Deluxe, there are usually some things that need to get done that are unfortunate. These amps originally came with yellow or red colored Astron coupling (also called blocking) caps. These caps almost universally leak DC voltage after this many years of use. Any DC leakage past a blocking cap will cause the next tube stage to do strange things. In extreme cases, the amp will motorboat and burn up tubes in short order. I ALWAYS check for DC leakage in amps, but tweeds especially. This amp only has a few caps that do DC blocking, so it's not expensive to recap it this way. If the caps are leaking DC, you have to change them for the amp to work properly. If the caps are not leaking yet, they probably will. It's up to you do decide if they need to be changed.

3. When changing electrolytic caps (if they haven't been done, then DO IT!), be sure to use a higher voltage rating on the cathode bypass cap on the power tubes. I use the 25uf-50v Sprague Atom. It's also a good idea to mount the cap away from the cathode resistor to get the full voltage handling capacity of the cap. Another problem with electrolytic caps in these (well actually, the power supply), is that the amp sometimes tends to motorboat when all three controls are full up with a 12AX7 in the first position. One way to cure this is to simply not use a 12AX7 or back down on the volume of the channel you are not using. If you must have this setting and the amp motorboats, then there is something that can be done. Increasing the value of the last filter cap in line (the one furthest to the right in the layout) to over 20uf will sometimes cure the problem. I've found that paralleling te 16uf cap with an 8uf-450v will usually fix it. Another way to fix it is to seperate the plates that get fed off the last cap and put them on their own filter cap. I've seen Doug Hoffman do this in a tweed super that was oscillating. I think he seperated the caps with a 1k resistor and used an 8uf at the end. This is only for extreme circumstances though.

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Last updated 11/17/00