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Fisher 500C Rebuild Questions!

Ah, found the spacexpander jumpers gadget. Had no idea what those are for. Bet these get crude on them and the connection is lost.
 
yep, that. The SpaceXpander jacks can get crusty ,but the jumpers can also get shoved in too far and touch the grounded outer ring. If that happens it kills signal to that channel. Tape monitor can also break signal from all sources to one channel if its crusty.

The SpaceXpander is a reverb unit. Probably can find a lot more details about it on here somewhere but Fisher put connections for it on a lot of their gear, and when that isn't used the jumpers have to be in place in order to pass signal.
 
okay. I'll deoxit all of it see if that helps. These old slide switches can be a problem. My 400 was only playing one channel for a while. Forget what it was but I think it was a switch.
 
Hey Guys,
Working on upgrading my Fisher 500C and broke R124 trying to connect my IBAM board. These resistors are 330K but I am not sure about the wattage. Look like 1/2 watt but I don't know. I don't have any 1/2 watt resistors in that value but I do have 1/4 watt 330K. From what I can tell from the schematic it doesn't look as though there's must current (if any) going through these resistors. Does anyone know if I could use 1/4 watt replacement without wrecking anything? thanks.

Paul
 
Yes sir. Those are the grid resistors. Best I could calculate was a max of about 60 micro amps (or .06 milli amps) or about 1.3 millwatts but I do not know the details of what goes on inside these tubes that might increase those values. I don't think you can just through these voltage and resistance values into ohms law. So.... I ask. Don't want to blow anything up. Thanks for your reply. that's kind of what I thought.
 
330K is actually 10% over the maximum value specified by the tube manufacturers. Too high, and grid leakage can cause a tube to run away and red-plate, especially if it's run near max ratings. If you intend to use modern (Russian) 7591 tubes, it's been recommended that you drop the value to 180K-220K (and increase the output coupling caps in the same proportion (from .047 to .068 uF).

And yes, 1/4W will be fine.
 
Thanks for your reply. Yes Tom, I've read about that. What I'd like to know is what is the maximum current that can flow through the grid. My calculations show a maximum of 60 micro amps. However, it's more complicated than just throwing the voltage and resistance into ohms law. My old textbook from first semester electrical engineering from way way back in 1971 when we still used tubes shows a bit more complicated bits going on inside the tube and some rather messy calculations. I thought that since Fisher was using 1/2 watt resistors there must be a reason for that. So let me ask you, can you tell me what the maximum current that will flow through the grid with the tube at maximum output using adjustable cathode biasing? Or does that not even matter? I haven't done tubes in so long I forgot all that stuff. HAHAHA. I'm using old tubes at the moment but will be using J&J. The J&J spec sheet says .3 Meg ohms for FIXED BIAS. I do not have fixed bias. However, for cathode bias it says 1 meg ohm. If you are using one of the popular bias adjustment boards you are doing cathode biasing. As I recall there are many configurations for biasing so it gets messy.
 
No, the 500C is fixed bias, as are almost all 7591 / 7868 amplifiers (EICO ST40 is only exception I know of). Fixed bias uses a negative grid voltage to bias tubes, cathode bias has a cathode resistor to develop a positive cathode voltage (so grid is negative with respect to cathode). (Confusing, how we call adjustable bias "fixed" bias).

Grid leakage can occur from multiple causes, and amount is impossible to predict (though lower is better!) First is gas, leakage from seals or outgassing from overheated plates or mica - positive ions will be attracted to the grid. Second is conductive deposits on mica insulators, again probably from overheating. And there's grid emission - if it gives off electrons, it goes positive. Overheating the grid or deposition of cathode material on the grid (again likely from overheating) can cause this. So... if materials in modern tubes are not as good as the old ones, grid emission is likelier, especially if tubes are run near max ratings. Once it gets high enough to affect bias, it's the beginning of the end - tube gets hotter, grid emission increase, bias drops further, gets even hotter, etc. So back off the idle current a bit and hope your output tubes die of old age, not catastrophically.
 
As I said, I'm not using fixed bias I have modified the circuit and am using cathode bias using an IBAM board and a cathode resistor. Since the control grid (not to be confused with the two other grids) has a negative voltage relative to the input signal the resulting signal is always negative. Under these circumstances, the negative grid always repels electrons from the space charge thus the grid cannot draw current. In this configuration, with no grid current, the power absorbing ability of the resistor is mute. So there, I answered my own question. I'm not concerned with grid "leakage" at this juncture. I've run high powered RF tube amplifiers (1500-2000 watts) for my ham radio equipment for 4 decades. Never had any of these problems you refer to. Never overheating. I think it is grossly over stated.
 
Cathode resistor is what? 10 Ohms? Insignificant for cathode bias, as it only drops millivolts. Adjustable bias is fixed bias, as odd as that sounds.... defined as separate negative supply to control grid. Cathode bias would have the cathode at +20V.

Every tube data sheet has a maximum value for the grid resistor, and if it's exceeded, there may be problems with thermal stability. Guitar amplifiers often exceed this spec, but can get away with it since they're biased "cold", idling well below maximum power. Hi-fi makers had to minimize distortion and often biased tubes near the max rating.

The tube RF amplifiers I've worked on (150W-40 KW) all had very low DC grid resistance, so grid current has little effect on grid bias. This is for linear amplifiers - class C amplifiers are designed to self-bias from RF grid current, though some tubes require fixed bias at idle.

Good reading: https://archive.org/details/Getting_The_Most_Out_Of_Vacuum_Tubes - look at chapter 2. He doesn't like fixed bias but the fix is to to do what you're doing - individually bias each tube.
 
some of this also depends on the tube type. High GM tubes are more affected by small changes in bias, and the 7591 is definitely one of those.
 
Dave Gillespie and Steven Laffertty back in 2010-2017 wrote an article testing different brands of Old Stock vs. New Russian Stock. JJ's came in LAST! Dave Gillespie was involved in the testing and it wasn't just any old tube tester. Custom Built and tests them at actual plate and screen voltages and has a separate P.S. for Fixed voltage Bias. Does about everything but blow your nose and wipe your _____. Tung-Sol Reissues have tested out to be as close to the old westinghouse tubes as you could get coming out of Putinland. EH's came in a close 2nd to the Tung-Sols. JJ7591's in my opinion are only good for use with a .22LR or a 5.56/.223 for plinking. Too many cases over the years of Tube SIDS, cathode to heater shorts, and various other failure in the 1st 10-15 hours.

Here's the link to the Tube testing....... http://www.tronola.com/html/7591a_tubes.html

Dave's treatise on Maximizing output tube life (including the 7591)..................... http://www.tronola.com/Gillespie2544.pdf
 
I have one (JPG) that's 1.82MB, but AK won't let me upload it - too large. I tried decreasing the size, but quality was down to 20% and still too large. Then I tried converting to a different format (bmp, png and tif), but they ended up being much larger. Let's see how sending in a private message works. Nope, too large. If you provide an email, I'll email it to you. Thorne
 
@Paul Fischer

Providing your email on an open forum is generally considered a no-no as it could lead to being spammed etc. If you see this I would strongly suggest you edit your post to delete the email addy
 
I agree. I usually spell it out like "me at gmail dot you-know-the-rest". ON the other hand, there are plenty of ways a spammer can get email addresses. WE just shouldn't make it easier for them. Better to provide it in a private forum message instead of a publicly-displayed post.
 
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