Fisher 500c burned resistors
- Thread starter scombridae
- Start date
scombridae
New Member
Good News! I replaced the output tubes with the EH7591 and everything is playing super. The sound is clean and warm. I must have fried the original tubes or possibly they were damaged before I received the unit as it had plenty of signs of overheating.
I have one question regarding how hot my tube bias are running. The readings are taken off the cathode side with the 10ohm resistor installed to ground. I changed R132 to 3.3k to try and get the current down, but to be honest I don't know what I am doing. I have also replaced R121-R124 with 220k resistors in hopes of doing the same, getting the current down.
What do you think is the best way to get the bias down?
V8 60ma
V9 46ma
V10 60ma
V11 45ma
So close now I can't believe it. I'm trying not to run this amp for more than a minute or two until I get everything right. Thanks
I have one question regarding how hot my tube bias are running. The readings are taken off the cathode side with the 10ohm resistor installed to ground. I changed R132 to 3.3k to try and get the current down, but to be honest I don't know what I am doing. I have also replaced R121-R124 with 220k resistors in hopes of doing the same, getting the current down.
What do you think is the best way to get the bias down?
V8 60ma
V9 46ma
V10 60ma
V11 45ma
So close now I can't believe it. I'm trying not to run this amp for more than a minute or two until I get everything right. Thanks
Do yourself a favor and replace the 3.3k bias resistor with a 5k or 10k pot. It will be a piece of cake to adjust negative voltage from that point further. I just did it on my on-going 400 rebuild. I cannot imagine the PITA having to tweak that resistor to get the bias where I want each time.
Also, what is your negative voltage of the bias supply?
X2 on what Craig (NOSvalves) said. If the outputs are drawing that much current, your bias supply is not negative enough and you are running that new expensive quad at it's melting point. I would just pickup a pot and install it before worrying about powering up the 500c again. Set it around 2.7k prior to installation and have your multimeter ready. Adjust accordingly once you fire it up.
On my 800B -17.5volts puts all 4 of my old stock Sylvania 7591's right around 30ma which I prefer. Many will go cooler and run them 26-28ma.
-Kory
audiodon
Addicted Member
Negative bias supply on these units I believe is readable on pin 6 of each 7591. What's going on is that you have to envision the inside of the tube as having a rushing river of electrons and a gate. In this case, the gate is the negative bias supply on that pin 6. Basically it acts as a governor on electron flow so the tube won't run away or starve for electrons. Checking pin 6 (I think it's pin 6) will be mostly the same on every output tube. The value of that negative bias supply voltage is modified by that resistor that kpaxfaq is suggesting should be a potentiometer.
scombridae
New Member
Great ideas. What voltage/wattage should the 10k pot be? Looking at the schematic I see that it's -22v coming off CR6. I believe R122 is 1/4w resistor.
Also I noticed some other posts regarding building an adjustable bias supply that allows to adjust the bias on each tube. My question is Fisher designed this amp without separate bias controls. How much of this is a detriment to the overall sound and how does this effect the wear/life of the output tubes? Do you think the bias controls are a worthwhile upgrade?
Also I noticed some other posts regarding building an adjustable bias supply that allows to adjust the bias on each tube. My question is Fisher designed this amp without separate bias controls. How much of this is a detriment to the overall sound and how does this effect the wear/life of the output tubes? Do you think the bias controls are a worthwhile upgrade?
0.5 watt pot should be sufficient, my 2 watt selection I put in mine was known overkill.
While my limited knowledge has been growing extremely quickly lately, I am not comfortable explaining why you should or do not need to have individual bias on each tube.
I do not out of simplicity and fear of screwing something up, but as we speak it just hit me how ridiculous easy it would actually be to rig up a little board for it like another poster had recommended to me. I can picture what wires I'd be tapping into and everything in my head. Things have just been beginning to click like that lately out of no where for me after about 8 months banging my head against the wall when I got into tubes December 2009.
Look at it like this,
Here's a hypothetical scenario,
In my Fisher 400 lets say I adjust my sole bias pot to deliver -19 volts from the bias supply.
When I measure the current at the 10ohm cathode resistors lets say I'll find for a new matched quad
1. 29ma
2. 28ma
3. 27.5ma
4. 25.5ma
NOW
if you have individual bias control for each output tube you could get them all to draw say...
1. 27ma
2. 27ma
3. 27ma
4. 27ma
The advantages would be each tube would be drawing the same amount of current and running at nearly equal temperature. This means they should hypothetically age equally. (We never know how long each tube will last).
Here's a hypothetical scenario,
In my Fisher 400 lets say I adjust my sole bias pot to deliver -19 volts from the bias supply.
When I measure the current at the 10ohm cathode resistors lets say I'll find for a new matched quad
1. 29ma
2. 28ma
3. 27.5ma
4. 25.5ma
NOW
if you have individual bias control for each output tube you could get them all to draw say...
1. 27ma
2. 27ma
3. 27ma
4. 27ma
The advantages would be each tube would be drawing the same amount of current and running at nearly equal temperature. This means they should hypothetically age equally. (We never know how long each tube will last).
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scombridae
New Member
My understanding is that most potentiometers have three tabs. Which tab connects to which part of the schematic?
By the way, I liked Audiodon's description of pin 6 acting as a gate of electron flow the tube. Super analogy.
By the way, I liked Audiodon's description of pin 6 acting as a gate of electron flow the tube. Super analogy.
Basically you ideally want each tube pulling equally. With a single pot each tube is going to pull differently in that no two tubes are made exactly alike. They are close enough for the "masses" in that they give you an average range, but those of us who like to tweak need a bit more, Hence the individual pots. At that point you can adjust for a "hot tube" and bring it down to a point where YOU are comfortable with them being, that's within the manufacturers set points. or you can tweak a cold tube a bit hotter to match the others. Once all 4 are matched (more or less) the sound becomes clearer, and less distorted. The outputs are running smoother and cooler due to even voltage swings.
Make more NEGATIVE BIAS VOLTAGE to bring the bias current down. On the 500 I don't know which resistor you'd change but listen to Craig and DON.
BTW the 7591 MAX current draw is something like 35ma per tube. You're almost at double on one channel and 1.5 times on the other. If you leave it high, you can blow the tubes, get a runaway, and burn out the output transformers (very costly).
LArry
Make more NEGATIVE BIAS VOLTAGE to bring the bias current down. On the 500 I don't know which resistor you'd change but listen to Craig and DON.
BTW the 7591 MAX current draw is something like 35ma per tube. You're almost at double on one channel and 1.5 times on the other. If you leave it high, you can blow the tubes, get a runaway, and burn out the output transformers (very costly).
LArry
The pot I bought (potentiometer I mean
) I used the middle and one of the outside tabs. I remember NOSvalves stating you can just tie the middle to either outside tab if you'd like. An easy way to test is with a multi-meter because you want to set it to a safe range before installing so your outputs aren't running wide open when you fire her back up. I set mine to 2.7k when I installed and it was not far off where it needed to be when I fired my 400 up. -Kory
audiodon
Addicted Member
If scombridae can measure the tube bias, he has cathode resistors on pin 5. He'll be fine. If he gets a runaway tube, the resistor will burn, acting as a fuse and save the transformer.
Current draw is too high though. R132 isn't right yet. Just try adjusting that bias resistor for now to get all four of those tubes under 40ma and he can worry about the rest after he's got it under control. Once under control, he can listen to it while he decides whether to get one pot or four.
Current draw is too high though. R132 isn't right yet. Just try adjusting that bias resistor for now to get all four of those tubes under 40ma and he can worry about the rest after he's got it under control. Once under control, he can listen to it while he decides whether to get one pot or four.
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scombridae
New Member
I will pick up the potentiometer this week and see what happens. Thanks again for all the information. You guys are tremendous.
scombridae
New Member
Believe it or not, I have the Fisher running super perfect. I decided to learn a lot and build the separate bias board seen on another post here. It was well worth the effort as all the output tubes are running with the same 33ma bias and sound is quite sweet. I replaced both preamp tubes and four output tubes with EH equivalents. I think most of the distortion went away with the preamp tube replacement, but replacing all the tubes has cleaned up the sound significantly. The EH 7591 seem to naturally bias very high. There was a number 78 hand written on each box and seem to pull 60ma when using the same potentiometer setting as the old RCA 7591s which were biased at 40ma. But with the custom board, I can bring the EHs down to 33ma.
Below are pictures and the schematic I followed to assemble the board. I highly recommend doing it as the sound has a sweet sound that didn't exist before. THANK YOU EVERYONE and especially Drewbolce (for the bias plans), Fisherdude, audiodon, kpaxfax, and larryderouin. You guys rule! I can't say enough about how interesting and enriching this experience has been and I luckily have a working stereo now. It sounds wonderful. I just have to get some confidence and burn it in now that everything is running well. The longest I have played it for is between 5-10 minutes as I have had so many problems previous to this point.
Next step is to order the light bulbs and little details to get it perfect. I still have to have the phase inverters professionally set. Does anyone know of anyone in the Los Angeles area that would be able to do this? Thanks.
Below are pictures and the schematic I followed to assemble the board. I highly recommend doing it as the sound has a sweet sound that didn't exist before. THANK YOU EVERYONE and especially Drewbolce (for the bias plans), Fisherdude, audiodon, kpaxfax, and larryderouin. You guys rule! I can't say enough about how interesting and enriching this experience has been and I luckily have a working stereo now. It sounds wonderful. I just have to get some confidence and burn it in now that everything is running well. The longest I have played it for is between 5-10 minutes as I have had so many problems previous to this point.
Next step is to order the light bulbs and little details to get it perfect. I still have to have the phase inverters professionally set. Does anyone know of anyone in the Los Angeles area that would be able to do this? Thanks.
Attachments
drewbolce
Audio Analyist
Your welcome. The numbers on the tube boxes are a reading from their tube tester for matching purposes, they are not ma. 33ma is a good comprimise for sound and long tube life. The EH's come burned in (unlike the JJ's apparently) but watch them for the first month or so.
I found the bulbs were frosted in my virgin 500c. This explains why I could see the filament thru the dial faces in the dark on my other heavily modded unit. I could not find frosted bulbs, but I frosted them myself by dipping them in "Armour Etch" (Glass etching cream from art or hobby stores) for about 15 minutes.
The phase inverters are quite generous in there tolerance. Unless you are cranking the volume knob beyond the half way point it should not have much affect. I would center them and leave them.
I found the bulbs were frosted in my virgin 500c. This explains why I could see the filament thru the dial faces in the dark on my other heavily modded unit. I could not find frosted bulbs, but I frosted them myself by dipping them in "Armour Etch" (Glass etching cream from art or hobby stores) for about 15 minutes.
The phase inverters are quite generous in there tolerance. Unless you are cranking the volume knob beyond the half way point it should not have much affect. I would center them and leave them.
Excellent!! Congrats!! It's all about the journey, isn't it?
Very neat job on Drew's bias board, btw. You got skilz now!:yes:
Very neat job on Drew's bias board, btw. You got skilz now!:yes:
NOSValves
Super Member
Umm in my experience no the EH's are not burned in from the factory to an extent that stops major drift off the matched when new scores (unless some thing has changed at the factory in the last 5 years or so). If this fine gentlemen wants to confirm that over the next months or years he can easily do that.. record what the grid voltage is now and then monitor how much he has to change it over the next months/years to keep the tubes at 33ma. I bet it keeps rising more negative rather then dropping like most tubes behave as they age. My bet is it changes. This is the main reason I quit using EH years ago the bias drift was tremendous and very inconsistent tube to tube. It's possible they have remedied the problem but I doubt it.
I know if Jim McShane reads this next comment he will flip his lid But I absolutely believe the EH7591 is nothing more then a modified 6L6 tube with not so successful patches to try and make it work in the 7591 biasing setup. Makes perfect sense to me since a 6L6 require 100K max grid resistor value and they just couldn't seem to easily get it to live at the 300K spec of the 7591. Then factor in if you load the tube up in a tube tester brand new as a 7591 it will fail as a 7591 every time, Change the appropriate settings short of the pin out difference to 6L6 and the tube test just like a 6L6. The JJ has no issue with the 300K grid resistor but does have a quality control issue that is easily remedied by purchasing them from a tube dealer that really burns them in before testing.
That's my best guess theory on the EH7591
Craig
drewbolce
Audio Analyist
Compared to the JJ's which aparently have none the 24 hours EH claims is golden. Look at the video about 1:45 in.
http://www.boingboing.net/2009/01/27/bb-video-inside-elec.html
While the 7591xyz may have been a 6l6 the 7591a is not.
Craig,
This subject always tears me apart because I highly value your opinion along with Mr McShane's and to be 180 degrees apart is interesting.
I know another resident Fisher nut Sgmlaw agrees with you regarding the grid resistors in Fishers.
My comment is not meant to point out anyone as right or wrong.
It's just interesting to see the brains of the forum at opposite ends on a certain subject.
A simple google search would lead you to believe if you use JJ 7591's in your vintage amp that it will be destroyed immediately, burn your house down, and sacrifice your first born child
It would be interesting if we could start a thread specifically for people who has had problems with JJ 7591's and then require them to post WHERE THEY BOUGHT THEM FROM. I know you adamantly stand by Doug's tubes because he burns them in properly to do JJ's quality control for them and weed out the bad ones. :yes:
I am highly critical of internet forum info and have learned many times you need to sift through the BS and JJ 7591 seems to be one of those things I haven't taken a definitive side on yet.
