need 500c schematic for s/n 63420

[thevilla]

Hello All,

I am new to the forum, but have been dicking with Fisher tube gear for many years. After a 10 year hiatus from critical listening, I decided to break out a 500c I restored years ago. Performing voltage checks after a slow voltage power up to 115, I find that my grid voltage is about 400 with 375 being the target. Also, I am looking to target the bias to around -20 and am around -17 presently.

I have not been able to find a schematic that goes beyond s/n 49999 on the net. After looking at my brother 500c with a serial number below the 49999 and comparing it to mine, there are differences in resistor values.

Anyway, anyone have a schematic for these late models?

Thanks,
Rob

 

[audiodon]

Rob,
The voltages will run high if your AC is over 117. You can drop the voltages or live with it.
Some drop the voltages using a variac, or a Radio Shack bucking transformer, or an isolation transformer. i just live with it and make sure my components are robust.

Your grid bias probably needs to be pulled more negative though. My 500-C schematic that is not higher than 50k. I've done up at least a half dozen of these and have not worried about schematic number once as there are some, but not too many, changes and actually, there are variances even within the serial number ranges of Fisher's schematics.

That grid bias resistor is over near the AC cord.

 

[thevilla]

Thank you for the input. I changed r132 from 8.2k to 5.6k. This brought me to -19.7 on the bias @ 115V. The 1k carbon comp resistors at r126, r127, r129, and r130 measure a bit off spec. I have some 910 ohm Rikens on hand and am contemplating replacing those as well. Any opinions on the use of these at this juncture?

Also, what are the opinions on the .047 coupling caps? It has 715p orange drops at the moment, but I may try some Solen tin foils.

 

[Fisherdude]

I've been looking for a +50K SN service manual for five years, with no luck. I'm beginning to think they may never have published a newer one after production passed that point.

Oh, the 715p's will be just fine.

 

[thevilla]

I have a few quads of used 7591's that test good on the b&k 747. However, I would like to amp match these. How would I best accomplish this?

 

[Fisherdude]

This is not a simple effort.

"Matching" means that all four output tubes will draw the same plate current when installed in the same amplifier circuit. So, in order to match tubes, you need either a dedicated test setup or an amplifier circuit that will allow you to measure plate current for a specified plate voltage and bias. Then, you simply start swapping in different tubes until you find four that are very close to each other in plate current.

Or, you can use a "mutual conductance" tube tester, which (kinda) does the same thing. These tube testers must be restored and calibrated in order to be reliable, and will cost you several hundred dollars before you're done.

Or, you can find a tech who knows what he's doing.

Or, you can simply order a matched quad from your tube supplier, assuming you trust that he's trustworthy.

 

[audiodon]

What I do is put 10 ohm resistors from cathode to ground on each 7591 socket. Then I swap tubes until each pair is relatively close when read with a meter across those 10 ohm resistors. I like 'em to be match closely anywhere between about 25 and 34 ma. Beyond that, I don't obsess . . .

 

[Fisherdude]

What I do is put 10 ohm resistors from cathode to ground on each 7591 socket. Then I swap tubes until each pair is relatively close when read with a meter across those 10 ohm resistors. I like 'em to be match closely anywhere between about 25 and 34 ma. Beyond that, I don't obsess . . .

Works for me, too!:thmbsp:

 

[ccheath]

... Performing voltage checks after a slow voltage power up to 115, I find that my grid voltage is about 400 with 375 being the target....

I have a 500c serial number 39xxx. I was working on it last night setting individual bias for each tube. With trial and error plus with FalconEddy's explanation in looking at not just the bias but the voltages on bias and plates and screen, I found I was able to measure 430v the same as schematic as long as I run my 500c on 110v on the variac. I was also much closer to 375v but not quite there yet. If I plug directly to the wall, it can go as high as around 460v with screen being around 412v. Even bias voltages gone up. What I did find is the cathode current varies very small like a few milliamps going from 105v to 120v at the plug. If you want a target of 375, you may have to lower the line voltage to 110v. At least that's what I found on mine.