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New to Tubes...800c score...help restore.

Okay guys this is how I see it...

Does it look right?

martin

IndBiasSupplydiag.jpg
 
Thanks Kory...thats kind of you to say. But I've had one big advantage...I didn't have any issuses to contend with from the start. I've had to diagnose nothing so far.

I hit a road bump a moment ago...I ordered 3.3uf electrolytics, not the 33uf specified...CRAP!
I have a PE order pending and they carry a 33uf 50V lytic.

Okay to use theirs over the Mouser Nichicons?

Speaking of 50V...does that indicate that the current flowing thru this circuitry is 50V or less... and 300v raterd wire ( sorry larry, I had to ask) is ok? The board is small and the cat5 plenum would work perfectly

Looks like the board is on hold till I get the new elecrtolytics.

martin
 
50V is the voltage limit for that particular cap. 300 Volt wire would work in this instance, depending on the current load(amps) running thru the wire you might want to go with 20ga or larger wire. I thought you were talking about the BIAS wire from the dual cap.

For the 33uf/50V caps you can run down to RadioShack. 9 of 10 times I get caps there they are Nichicons. But the ones from Parts Express won't hurt. They are good caps and mixing them up (manufacturer wise) won't hurt anything. I tend to be over cautious when it comes to electronics and wiring. "Use the highest voltage and go with the largest ga wire to keep the possibility of overloading the wire to a null point".

Basically it comes down to what is the highest voltage seen in the gear and add 50%. It usually boils down to 600V wire.


Larry
 
Hummmm....
I went ahead and installed the 220uf 350V doubling cap C98 and to be honest not much change from before. My tube current draw is still low.

Here are the measurements with a 4.42K resistor at R119 with 1/2 hour idle...

V10
pin 3 408
Pin 4 369
pin 5 21.15
pin 6 -16.95

V11
pin 3 408
Pin 4 369
pin 5 23.60
pin 6 -16.88

V13
pin 3 408
Pin 4 369
pin 5 24.60
pin 6 -16.89

V14
pin 3 408
Pin 4 369
pin 5 26.00
pin 6 -16.90

This is where I could really use some help.


Also went ahead and ran some tunes thru it later on. Plays clean but lacks definition and low end is flabby.

Would have installed the 10K pot, but to be honest I wasn't 100% sure what the wiper is suppose to be soldered to...?

Thanks

Martin
 
Your pin 6 negative voltage is not negative enough. Try getting it between -18 to -21 by changing that resistor you're playing with. That should firm it up unless there's an upstream in the signal chain problem.
 
My collection of resistors is limited. Would use the 10K pot if I could get the correct install instructions.
Martin
 
Just test the probes with a multimeter.
Turn the pot with the multimeter connected.
Valid leads, resistance will change.
Invalid leads, resistance will not change.
There are only three leads.
The answer will show itself.
 
Dawn breaks on the feeble mind!
Thanks for being kind don...don't know why my brain died on this one. Maybe just didn't want to think any more.
In on lead one (1), wiper (2) tied to out on lead three (3).
martin
 
Oh Martin,
I've put stuff aside for months because my own mind refused to work.
Sometimes it just takes a nudge.
 
Nudge's usually don't work for me. I need a "Gibb's slap" on the back of the head!
 
Okay...
10K pot in and hovering at -19.08 at pin 6.
As I bring my neg (-) current up (down) to to the -19/20 my tube draw gets less.
My 10K pot is in the 1.9K range.
What now?
Martin
 
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Okay, new numbers....

10k pot running at about 6.9K.

V10
pin 3 388
pin 4 348
pin 5 .304V
pin 6 -14.00

V11
pin 3 388
pin 4 348
pin 5 .334V
pin 6 -14.00

V13
pin 3 388
pin 4 348
pin 5 .337V
pin 6 -14.00

V14
pin 3 388
pin 4 348
pin 5 .347V
pin 6 -14.00

Do these numbers look right?

Note: All my basement outlets are only supplying approx 104V AC. Other outlets in the house show 124V.
I'm sure this will affect my bias when I make a move to the upper stories.

thanks
Martin
 
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Take the 800C upstairs and plug it in there. The voltages should go up some, and your bias voltages, and amp draw (current), should be different, if not higher (lower).

Right now you are taking all voltage readings. Current is defined as how much it takes to push thru the wire in amps. So you are "mixing metephors" as it is.

I'm thinking your B+ voltage(Pins 3 & 4) is too low still. According to the schematic pin 3 should be 430V or thereabouts, and pin4/8 375V. Your drawing between 30 and 34 ma on the 7591's which is a little hot. They'll work well @ 27-28ma and still sound good, plus run cooler.

Pin 6 is WAY TOO HIGH(low). This needs to be below(above) -17. Try to get it between -18 and -20. I run my 400 @ -18.00 and get between 24 and 26ma draw on 7868's. The principle is the same.

Basically you need to bring the - voltage into the -18 -20 volts range while getting the tubes to draw (current) between 27-29ma ideally.

Larry
 
Your absolutely right don. Going from 104V AC to 115V AC has made a world of difference. Glad I thought to look at my basement line voltage.

Thanks larry for your recomendations. Thought I read (more than once) that 30-35ma was the target for pin 6. My terminology is lacking...I'll try to do better.

There seems to be a bit of a juggling act going on here.
This about as ballanced as I could get.
So here are the new numbers

V10
pin 3 418
pin 4 379
pin 5 23.60
pin 6 -17.30

V11
pin 3 418
pin 4 379
pin 5 26.05
pin 6 -17.30

V13
pin 3 418
pin 4 379
pin 5 26.68
pin 6 -17.30

V14
pin 3 418
pin 4 379
pin 5 28.04
pin 6 -17.30

At about 5K on the pot.

Are these looking better?

martin
 
I realize every receiver is different. I found that 27ma-29ma was way too cold for both my 400 and 500C. I was really disappointed with my 400 when I first finished my rebuild. I understand that the 7868 and 7591 are the same tube in a different envelope, right? My 400 sounded really thin, tinny, honky. Going on advice from a couple of AK members I bias the tube at 32ma-35ma. The receiver came alive and sounds great. I set my 500C bias at the same 32ma-35ma. It sounds great and runs cool. I've logged a few hundred hours on the 500 C's vintage 7591's, they are holding up fine.

Larry: I'm curious if you are running vintage or current production tubes with the colder bias? Maybe this is the difference?

David
 
DSNDBLM; EH7868's. Current Production. 220K grid resistor and .1uf K40Y coupling caps. Pin 1 421 Pin 4 374 Pin 6 18.2 The tubes are drawing between 27 and 29ma right now. That's with a 1K resistor @ R35(48001 manual).

marloubow. On pin 5 are you reading Volts(V), Millivolts(mV), or Milliamps(mA). If you put in the 10 oHm resistor on pin 5, read it as mV, then slide the decimal over (left) 1 place and you will get mA. If it's mA you listed it's way too cold. What tubes do you have in there now. They may be worn out.
 
The best bias level, in milliamps, depends on the tubes in the outputs.

I've used original Fisher branded Sylvania 7868s (clear glass with yellow lettering and top getter) at 18 ma and I've been happy. Then again, they weren't the strongest. They were well ripened.
Typically I head for high 20s to low 30s with tubes of this vintage though, with both 7868 and 7591 output tube types.

With EHs and lowered grid bias resistors with .1uf caps upstream in a 500-C, 25ma will do it, mid 30s really makes it rock and brings it to life.
With Westinghouse 7591s, even low 20s is fine, but again, more life at a higher dissipation.
So the answer is . . . it depends.
 
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