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In Reply to: RE: tube reverb driver in Guitar amp - impedance matching question posted by bdgregory on March 03, 2015 at 08:56:50
I once ran a test on a E6400 reverb drive xfmr from a Gibson, and found the turns ratio to be 2.6:1.
Gibson always used a 4FBxxxx tank, which has a 1,457 Ohm input impedence (it will measure around 200 Ohms DCR).
You can calculate the plate load from this info. In practice, I have found it to be not very critical. I ended up using a 70V speaker line stepdown transformer and selecting a secondary tap that gave good drive to the tank without distortion.
Follow Ups:
so that says the impedance shown to the 12AU7 would be 9971 = ~ 10000 ohms? with the 8 ohm fender tank and the OT I'm using that's ~ 5000, not as bad of a mismatch as I thought, but still 2x.
Actually, 10K is probably close. Note that it only uses 1/2 of the tube for the driver, unlike Fender, where both halves of the tube are in parallel. (Fender uses a 12AT7)
Which tube did you use as the driver? 12AT7, 12AU7, or some other one?
Thanks!
This was a Gibson GA-35RVT. It (and most other Gibsons) use 1/2 of a 12AU7 for reverb drive.
the amp I'm working on is a 65 Gibson GA-20RVT. It uses a 12AU7 for the send driver.
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