 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
173.73.15.78
I'm hoping someone can advise on potential risks of running too low, or too high primary impedance in the reverb transformer shown in the gibson amp attached.
The original transformer died, and I subbed in an O/T from a little SE amp. It works fine, but not ideal, but I want to run it with this installed for a little while. Depending on which tank I use, I believe the 12AU7 is seeing either 5000 ohms, or as much as 1Mohms. I think that the original transformer would run more like 15000 - 25000 ohms.
Will I damage the amp running it this way for a few months, so long as I don't drive the reverb too long or hard?
thanks in advance!
Follow Ups:
More impedance is not going to run anything too hard, what you would get is less output. With an output transformer, you would use a tank with a similar impedance as a speaker, like 8 ohms. The higher impedance tanks are used with transformers with less step down. The only damage will happen when you overly load down the tube.
That being said, if you want the reverb to function like it was designed, you have to either get the original transformer to go with the tanks higher impedance or get a lower impedance tank. An 8 ohm tank will set you back $20.
Almost all vintage Gibson amps use a 4FBxxxx tank. The input impedence is 1,475 Ohms. They read 200 Ohms DCR.
That original output transformer in the drive circuit is unobtanium. The OP mentioned needing one. The logical thing to do is to replace tank and transformer with 8 Ohm stuff.
thanks, understood. I have 3 tanks on hand for use - the original gibson (1475 input impedance), a Fender (8 ohms impedance), and one out of a Peavey amp (600 ohms). I just tried the Peavey tank and it seems to sound best, and be the most stable.
I'm trying to use this little OT for now while I tune up the amp to see if I can make it suck less ;-). I just don't want to damage anything in the process. If I can get the amp sounding to my satisfaction, I'll likely drop the $60 to get the Mercury Magnetics replacement transformer. If not, I'll buy a $8 Fender reverb transformer and put it in (or maybe just leave the OT in) there and use the Fender tank. Until then, I'm trying to avoid dropping to much in parts just to turn around and sell it at a loss.
thanks again to everyone.
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.
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.
Hammond & Accutronics tank information is available.
Lists two reverb trannies for Gibson E6400 replacement.
For the 4FB reverb (2 spring) 9.8K at 1475
For the 9FB reverb (3 spring) 13K at 1925
Post a Followup:
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
