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I'm measuring 5.4VAC (supposed to be 6.3VAC) cross my filaments of the EZ80 rectifier and 354VDC out. The schematic shows that I'm supose to see 405VDC. Is this lower filament voltage causing the lower VDC output? The VAC into the EZ80 is measuring correctly.
Follow Ups:
Another thing that can bring down B+ is tube leakage.
Test all tubes for leakage. I replace all tubes that have leakage as at some point they will fail.
I have tested brand new Russian tubes, and they have serious leakage issues.....
Jay
(Tube Amplifier Repair)
Have you replaced all filter caps? They must be replaced.
I agree that a leaky coupling cap will bring down B+. This can be verified by measuring voltage on the grid of the EL84. Should be 0V according to the schematic.
Are you measuring around 10VDC on the EL84 cathodes? If the cathode bypass filter cap were shorted, it may bring down B+. That cap needs to be replaced also.
B+ will be high when amp is turned on, and as the EL84 tubes begin to conduct, voltage will drop significantly. That is normal.
Jay
(Tube Amplifier Repair)
Hi
Possible the downstream cap(s) are bad, but also it appears there is a center tap on that winding(red/yellow). If the ground is bad then it will raise the resistance and therefore lower the output voltage. I have rebuilt more old Fisher gear than I can count and most had rivets for everything. Many rivets make poor chassis contact for the grounds after 50 years. If your meter doesn't go straight to zero and stay there for the resistance from that center tap ground to the chassis then knock out the rivet, put an internal tooth lock washer between the ground terminal and the chassis and put a good old screw in there to replace the rivet. Then measure resistance to ground. A good chance this will solve the problem. You should check all the ground points in the amp. Your hum will go down if you have any....
Rebuild the entire power supply if you haven't already and check the signal path caps, especially those .02 on the output tubes.
Good Luck!
Don
I think you just explained a grounding issue I was having with my Dynaco SCA-35. It was not a kit so it has rivets.
Dave
Hi Dave,
Don't ask me how I know this :-). Too many hours troubleshooting stuff only to find bad grounds. Now it is the first thing I do when I rebuild something. Check the key ground points. If it doesn't take my good meter to 0 ohms and stay there then I pop the rivet, install an internal tooth lock washer between the ground terminal and the chassis for a good bite, and then put a good old fashioned screw and nut in there. That is the way the darn gear should have been built at the factory. Rivets are nonsense in my opinion. Gear should be made to be fixed someday..... A good tube amp should outlive the builder or restorer.
cheers,
Don
I here you!
Dave
There are a couple things that may cause this..The first thing I would look at is to make sure your volt meter is working properly.
Then I would pull out the EZ80 and measure your voltage at the 6.3vac winding.The biggest culprit of low B+ is capacitor C1..You said your 6.3vax is down? Check it on the other tubes as well.I'm looking at something else as well..At the octal socket,it is suppoose to read 355v and you are reading 354 I wonder if a couple wires got switched. I have a Fisher 100 that measured 53vdc lower than my other Fisher 100 and it turns out someone changed the power transformer to a lover voltage one..They just pounded it into the potting can..It worked fine but it wasn't right.
Some people like second order distortion in their tube amps,some people like honesty in their tube amps. I like honest!
Edits: 09/16/13
Yes, the 6.3VAC is down plus I'm not getting the 405VDC out at pin 3. I'm getting 354VDC. If I isolate C1 shouldn't I get the 405VDC from the EZ80? My meter is good. I'll pull the tube in the morning and check 6.3 the winding.
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: 09/16/13
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.
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