Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on May 24, 2014, 05:27:23 pm
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Hi guys, I at a loss to which direction to go with this. The moody GA 40 uses a 6DX8 for the reverb circuit that I, at this stage don't want to change to another tube (12AX7) if I can utilize a reverb tank to suit.
The tank that is required has a input of about 1475 ohms and from what I have been told that this is probably a guess.
I would think that the original tank was a long TYPE 4 with an input of 1475 ohms.
I don't have room for a long tank and it looks like the short tanks only have an input of 1925 ohms.
Even the short tank may be a bit tight for a horizontal mount.
The best type would be a vertical mounted but these are not in existence, BUT I do have two short tanks that are vertical mounted but are 310 ohm inputs.
Either I strip down a horizontal tank to the bare workings.....
OR mount a horizontal tank in a vertical position......
Reduce the output resistance of the 6DX8 driver stage, I'm thinking that would have be done via a transformer....
Order a special tank for this build. Thanks
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> original tank ... 1475 ohms.
> short tanks ...input of 1925 ohms.
Same thing.
Literally the same coils, but measured at a different frequency.
Just put it in.
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If the tank on hand was 8ohm input, then yeah, you'd do best adding a little transformer, but since the one you have on hand is 310ohm input, you should be able to drive it with the pentode. If the load of 310ohm is too low, you can try to put 1K-2.2K in series.
Either the pentode or the triode also look like candidates to drive the tank with their cathodes too.
If you already have a reverb circuit in the amp, do what PRR suggests and try the tank in the circuit. also echoing PRR, the 310ohm rated tank, at some high freq., is 1400ohms, and the 1400ohm tank at some low freq. is 310ohm.
What does the Moody reverb circuit look like?
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What does the Moody reverb circuit look like?
(http://www.ozvalveamps.org/moody/moodyga40reverbcct.gif)
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that looks like it'd work with the 310ohm input tank, although I'd add a series load resistor so the driver sees more load than 310ohm, I'd try a 1K - 2.2K.
also,you could move the reverb on/off switch to a more typical setup, shorting signal path to ground on the recovery's grid. AS-IS, it seems like you'd get a pop when turned off and on.. I'm not sure what the pop sounds like going through the reverb tank.
regarding the output Z of the tank. The moody sch. seems to indicate either a 2K or 12K output tank, but the tank you have on hand could be as low as 500ohm..., if so you can lower the input Z of the recovery stage by tweaking the 470K. making it as low as 47K or 22K would work for a 2K-12K output, and if its a 500ohm out tank you might try something lower.
since reverb tanks in's and out's are inductors, you have a lot of wiggle room and leeway.