![]() ![]() |
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
97.84.194.114
In Reply to: RE: Fisher Amplifier Voltages Questions posted by Qman on September 16, 2013 at 20:01:02
if you isolate C1 you will drop your B+ a lot..Remember,C1 takes the left over AC off the rectified DC.Also.look and see if there is an extra lead for setting the primary..Some units had that for the different AC voltage ratings across the country.
Some people like second order distortion in their tube amps,some people like honesty in their tube amps. I like honest!
Edits: 09/16/13Follow Ups:
Ok, did some cleaning up of the rectifier socket and now I'm getting 6.8VAC across the filaments of all the tubes. Did some monitoring upon power-up and I'm getting 405VDC intially but starts to drop down to 369VDC and holds steady at this voltage. Getting red plating now on one of the EL84's. I'll check the grounds to the chassis next. This is a fun one...not really. There is some lamination splitting on the power tranny too. Is there an easy way to test it and fix it. I have some options here. Keep for complete rebuild or return it for a full refund.
As was said below, you may have a bad coupling cap. You really need to be certain all power supply and bias supply caps are good and replacing the output caps is pretty cheap. K40 caps are good, or Illinois, or any good film caps. Check the grid resistors, and of course any cathode resistors. I am not looking at the schematic so don't know if fixed or cathode biased. Really, it is a simple amp, just rebuild and enjoy it. I always rebuild the power supply and do the coupling caps on 50 year old amps, unless I see a great coupler that I know is still good. There is this mystique about old caps that is pretty much hogwash though. I have seen folks selling old bumblebee caps on ebay. They were decent in their day, but there are a number of modern caps for a few dollars or less that sound much better. Also, Fisher was notorious for putting two different types of caps in the same channel. They were incredibly cheap and would do anything to save $1 in parts costs. I have seen countless fisher amps with 400v rated caps on one tube of a pair and a completely different 200v cap on the other tube. How much would it have cost them to use identical 400v caps? So have a look at that amp and see how it was built. For a few dollars you can probably improve it and not waste too much time troubleshooting. Just rebuild it..... Rant over, my 2 cents
good luck
Don
![]()
Parts are on order. Pretty much rebuilding it like the 30-A I did a few months ago. The Russians are coming and will closely mach the the phase inverter resistors. Got the seller to refund me a couple of hundred for the grief. I'm still worried about some of the split laminations on the power transformer.
What Jay said.....if it runs cool and all voltages are within 20% of spec I wouldn't worry about it. You might try pulling the bolts, cleaning them up or sourcing new ones, and then slowly re-torquing them to seem if you can pull the laminations closed if it really bothers you. It isn't hurting anything though if the bolts are tight.
Again, if it is built with rivets then redo all the riveted ground points with screws and lock washers that will bite into the chassis to provide good grounds. Don't mess with their grounding scheme until you have it running with no hum. Then if you want to modify it to a more modern star gounding you can try it. Sometimes it improves old amps, and sometimes they hum!
One of the coupling caps and the cathode bypass where bad. Replaced the coupling caps with Russian PIO's, replaced the pf caps with silver mica's, matched inverter resistors added and dropped in a 200ohm cathode resistor in place of the 120ohm.
Voltages are close to spec and it sounds very smooth with a tighter and fuller bottom end than my 30-A. Power transformer runs cooler too. P/S filter caps are next.
Thanks for all the the tips!
Good to hear that all is well. Yes, go through and completely rebuild the power supply. Diodes if there are any too. Try and get all the old sand resistors out of there and replace with good modern 1% or 5% non-inductive ones like Mills or Dales. They are really not very expensive. You will find that the bass will tighten up with the stiffer power supply. Have fun!
If the power transformer is producing correct voltages under load and does not feel burning hot when touched (or passes the dim light bulb test), I would not worry about it.
Jay
(Tube Amplifier Repair)
The red plating tube is bringing the voltage down due to excessive load. It's possible that the other tubes are drawing excessive current due to bad coupling caps.
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: