The Fuxtor
Addicted Member
Wow, yes looking good!! Well I suppose your done being that there's no more mods to do on that one....I got an 800c I think is asking for some love like this.
Screen Protection circuit done - now what?
- Thread starter thornev
- Start date
thornev
Super Member
Hmm, Alberta, CA? Is that east or west of Saskatchewan? Now if you were in BC...! Thanks, Fuxtor. If you're ever in the neighborhood and feel like draggin' 40 pounds of Fisher, sure, c'mon over. Actually there is one more on my ToDo list and that's the "improve bass" mod although I've been advised not to do it. I like my bass punchy though. Also I can't stand those trimmers I put on my IBAM board. It's a hassle getting the mini screwdriver in the slots and keeping it there. I made a 2nd IBAM board which I'm considering. Oh that's right... you like pictures. Oh, and I have one more AR-2Ax to restore.
In a recent visit to an antiques store, I saw an old Emerson AM radio with a bakelite front and wood panels maybe from the 30's or 40's, tubes of course. I can't find that model anywhere online so I'm thinking it might be rare and hence collectible (though I can't imagine anything with only AM as being collectible). I was thinking that might be a relatively easy project to restore for this here amateur.
In a recent visit to an antiques store, I saw an old Emerson AM radio with a bakelite front and wood panels maybe from the 30's or 40's, tubes of course. I can't find that model anywhere online so I'm thinking it might be rare and hence collectible (though I can't imagine anything with only AM as being collectible). I was thinking that might be a relatively easy project to restore for this here amateur.
Attachments
The Fuxtor
Addicted Member
Calgary , Alberta.
West of the province Saskatchewan and east of B.C.
thornev
Super Member
I've looked at all my notes that I take when I read something relevant to my 500-C and also scoured the internet for an answer, and the problem is that I can't find a single post that is clear about how to measure and then set the bias of the power tubes. Here is what I think should happen. I'd appreciate corrections or concurrences. I have 7591A tubes that have a max plate dissipation of 19w. Thanks.
TO SET THE BIAS using --------------------------------------------------
1. Get the VDC at the PLATE, pin 3. It is 413vdc
2. Figure out the watts required to stay in compliance: 70% of 19 = 13.3w; 80% of 19 = 15.2w.
3. Determine the max current needed to stay in compliance with 19w:
current I = P / V
13.3 / 413 = .0317amps (31.7ma) for 70%
15.2 / 413 = .0362amps (36.2ma) for 80%.
4. Multiply current by 10 to account for the cathode resistor:
I = (13.3 / 413) * 10 = .317a for 70%
I = (15.2 / 413) * 10 = .362 for 80%.
5. Hence, when adjusting bias, the measurement at pin 6, CONTROL GRID, must be between .317 vdc and .362 vdc.
6. Do I need to do this step? Subtract 3ma to account for additional (screen) grid dissipation:
28.7ma for 70%
33.2 for 80%.
--------------------------------------------------
TO SET THE BIAS using I = P / V (measured result of bias setting = desired P in watts / pin 3 vdc ):
I = 13.3w / 413 = .0322 or 32.2ma for 70%
I = 15.2 / 413 = .0368 or 36.8ma for 80%
Set the bias so that pin 6 measures between .322vdc and .368vdc (I went from .0322 to .322 using the same logic as step 4 above.)
--------------------------------------------------
A method to VERIFY THE BIAS SETTING using P=IV (pin 3 VDC * pin 5 amps = PLATE dissipation in watts):
1. Substitute for I in P=IV which is I=P / V (after substitution we get P=(P/V)*V or ):
I = pin 5 vdc / 10 ohms resistor at pin 5, or 413 / 10 = 41.3a or .0413ma
2. Apply P=IV: 413 * .0413 = 17w which is well under the max 19w
TO SET THE BIAS using --------------------------------------------------
1. Get the VDC at the PLATE, pin 3. It is 413vdc
2. Figure out the watts required to stay in compliance: 70% of 19 = 13.3w; 80% of 19 = 15.2w.
3. Determine the max current needed to stay in compliance with 19w:
current I = P / V
13.3 / 413 = .0317amps (31.7ma) for 70%
15.2 / 413 = .0362amps (36.2ma) for 80%.
4. Multiply current by 10 to account for the cathode resistor:
I = (13.3 / 413) * 10 = .317a for 70%
I = (15.2 / 413) * 10 = .362 for 80%.
5. Hence, when adjusting bias, the measurement at pin 6, CONTROL GRID, must be between .317 vdc and .362 vdc.
6. Do I need to do this step? Subtract 3ma to account for additional (screen) grid dissipation:
28.7ma for 70%
33.2 for 80%.
--------------------------------------------------
TO SET THE BIAS using I = P / V (measured result of bias setting = desired P in watts / pin 3 vdc ):
I = 13.3w / 413 = .0322 or 32.2ma for 70%
I = 15.2 / 413 = .0368 or 36.8ma for 80%
Set the bias so that pin 6 measures between .322vdc and .368vdc (I went from .0322 to .322 using the same logic as step 4 above.)
--------------------------------------------------
A method to VERIFY THE BIAS SETTING using P=IV (pin 3 VDC * pin 5 amps = PLATE dissipation in watts):
1. Substitute for I in P=IV which is I=P / V (after substitution we get P=(P/V)*V or ):
I = pin 5 vdc / 10 ohms resistor at pin 5, or 413 / 10 = 41.3a or .0413ma
2. Apply P=IV: 413 * .0413 = 17w which is well under the max 19w
Your overthinking it. DELETE STEP 5. You've already determined what the BIAS CURRENT should be, and adjusted for it. This will be a constant hopefully across all 4 tubes and is what is wanted and needed. EQUAL current draw. The grid voltage will NOT be as spec'ed because instead of a constant voltage with varying current loads You now are testing for CURRENT DRAW, NOT VOLTAGE. Pin 6 is going to be in a range of -16v to -22v or more depending on the tubes you are using. With all the current draw at PIN 5 EQUAL (or as close to it as possible) the grid voltages will be all over the place. This is NORMAL. Set it for 70 to 80 %. That;s the sweet spot for good sound and longevity for most tubes including the 7591 family.
As for Step 6 you can compensate for the screen draw and adjust the bias slightly higher. I don't do it as I'm comofortable with 80% max dissipation. I can deduct it and say that while the BIAS is Set @ 34ma, it's actually @ 31ma after the screen is taken into effect
As for Step 6 you can compensate for the screen draw and adjust the bias slightly higher. I don't do it as I'm comofortable with 80% max dissipation. I can deduct it and say that while the BIAS is Set @ 34ma, it's actually @ 31ma after the screen is taken into effect
thornev
Super Member
Thanks, Larry.
One thing that baffles me is that you say pin 6 will be negative, yet when I measure it using the IBAM (a piece of info I forgot to include in my original post), I always get a 3-digit, decimal point number such as .NNN. The multimeter (MM) is on vdc, but I can't put it on measuring amps because then I have to insert the MM into the circuit and that's not how I have the IBAM connected. Should it be?
Anyway, if I wasn't clear, I'm looking for the steps that explains what using all the tube's pins can measure and set but especially how to use them to measure and set the bias. Obviously with the IBAM I have the "set bias" already working, but I still want to know how to do it manually.
So just steps 1-4 and 6. I'm using Tung-Sol 7591A tubes. The ones that draw more current than the Sylvanias.
Why am I getting .NNN when I measure pin 6 and not a negative number? I mean I know it's because I'm measuring volts. What should I be measuring? Did I connect the IBAM incorrectly? I followed Dave's directions...I thought: '+' lead to where the "filament out" from the power board (SDS board) is connected - tube V11 pin 6. '-' lead to chassis ground, and each 1K resistor from the IBAM to each power tube, pin 6. OH !!! I have to account for the 1K resistor, don't I ?
One thing that baffles me is that you say pin 6 will be negative, yet when I measure it using the IBAM (a piece of info I forgot to include in my original post), I always get a 3-digit, decimal point number such as .NNN. The multimeter (MM) is on vdc, but I can't put it on measuring amps because then I have to insert the MM into the circuit and that's not how I have the IBAM connected. Should it be?
Anyway, if I wasn't clear, I'm looking for the steps that explains what using all the tube's pins can measure and set but especially how to use them to measure and set the bias. Obviously with the IBAM I have the "set bias" already working, but I still want to know how to do it manually.
So just steps 1-4 and 6. I'm using Tung-Sol 7591A tubes. The ones that draw more current than the Sylvanias.
Why am I getting .NNN when I measure pin 6 and not a negative number? I mean I know it's because I'm measuring volts. What should I be measuring? Did I connect the IBAM incorrectly? I followed Dave's directions...I thought: '+' lead to where the "filament out" from the power board (SDS board) is connected - tube V11 pin 6. '-' lead to chassis ground, and each 1K resistor from the IBAM to each power tube, pin 6. OH !!! I have to account for the 1K resistor, don't I ?
Last edited:
Pin 6 to ground. DC Volts and it will be NEGATIVE. All voltages are referenced to chassis ground. The plus + lead from the SDS Board should go only to the IBAM. It's disconnected from the tubes and then the IBAM is installed between the + lead and the individual tubes. If you have the lead connected to both the IBAM and the grids, REMOVE ALL OLD ORIGINAL LEADS TO THE GRIDS at the Terminal strips where the IBAM tube leads go. Just read the voltage at the pin. You don't account for anything. As I said in my previous post the pin 6 voltage is not a part of the equation anymore. Just measure the pin 5 (cathode) CURRENT and MATCH THEM. If you match the pin 6 voltages you'll have cathode currents all over the map like you did BEFORE the IBAM. Don't complicate matters for yourself. PIN 6 is inconsequential now. Just use it as a reference as to health of the tube. When it gets down to about -10V and the cathode current is the same as it's always been, then it's time for new tubes.
thornev
Super Member
Dammit, my notes are all wrong ! I can't even document what I did correctly...Sigh. The tube IBAM leads are connected to pin 5 of each power tube, not 6. THAT'S why I'm getting .NNN. I should have known better.
The original 220K resistors are disconnected which i assume is what you meant by "REMOVE ALL OLD ORIGINAL LEADS...". I put the 200K resistors on the IBAM board, not in the chassis. Dave told me when one does that, the 220K (330K if stock Fisher) resistors have to be disconnected.
I don't know what you mean by "pin 6 to ground". My "tube V11 pin 6" comment is another documentation mistake by me as you'll see in a moment. Pin 6 on the power tubes are connected to a 1K resistor (R126, R127, R129, R130). But yes, if I measure at power tubes, pin 6, it is negative DC volts.
I assume "plus lead from the SDS board should only go to IBAM" means that the "filament out" from the SDS board is connected to the "+" lead of the IBAM because that's how it is connected, but the IBAM is connected to the 1st connection the filament out makes in the chassis which is at V14 pin 4 (again what I wrote in post 107 is wrong).
The original 220K resistors are disconnected which i assume is what you meant by "REMOVE ALL OLD ORIGINAL LEADS...". I put the 200K resistors on the IBAM board, not in the chassis. Dave told me when one does that, the 220K (330K if stock Fisher) resistors have to be disconnected.
I don't know what you mean by "pin 6 to ground". My "tube V11 pin 6" comment is another documentation mistake by me as you'll see in a moment. Pin 6 on the power tubes are connected to a 1K resistor (R126, R127, R129, R130). But yes, if I measure at power tubes, pin 6, it is negative DC volts.
I assume "plus lead from the SDS board should only go to IBAM" means that the "filament out" from the SDS board is connected to the "+" lead of the IBAM because that's how it is connected, but the IBAM is connected to the 1st connection the filament out makes in the chassis which is at V14 pin 4 (again what I wrote in post 107 is wrong).
The Fuxtor
Addicted Member
Nice looking numbers!
Numbers are good. When you measure Pin 6 METER PROBES AS FOLLOWS! RED to PIN 6, BLACK TO CHASSIS GROUND. Set to 200V DC or HIGHER.
When I said REMOVE ALL LEADS, I meant all the old BIAS LEADS across the terminal board to the grid return resistors. (should be all white wires to grid return resistors.)
then attach each IBAM lead to the 220k/330k resistors (as you had done that on the board, then attach each lead to the 1K resistor). There should be NO CONNECTION from one side to another except thru the IBAM.
When I said REMOVE ALL LEADS, I meant all the old BIAS LEADS across the terminal board to the grid return resistors. (should be all white wires to grid return resistors.)
then attach each IBAM lead to the 220k/330k resistors (as you had done that on the board, then attach each lead to the 1K resistor). There should be NO CONNECTION from one side to another except thru the IBAM.
thornev
Super Member
I just got my Marantz 2265B restored very nicely by Bob Speece in NY. He did a fabulous job. I brought it home and hooked it up to my AR-2Ax speakers and I gotta say that the Fisher 500-C sounds WAY BETTER with those speakers. Bob says that makes sense because they were both manufactured during the same era, the mid 60's. The Fisher frequency response is much smoother with those speakers. The AR's reproduce the entire frequency range from the Fisher much better but especially the low bass. I was surprised at how much different they sound with those speakers.
thornev
Super Member
I'm wondering if I have a tube problem. Of my four new Tung-Sol 7591A tubes, one of them is glowing red significantly less than the other 3 which are pretty much matched as far as glow goes. I measured the bias (via IBAM) and I have have all 4 running at 34mA. I noticed it when the unit was set to play PHONO STEREO, but I didn't think to test other sources. First of all, does it indicate a problem that one tube is glowing less brightly than the other 3? If so, would this be that this one tube is getting less juice than the others? What could be the source of the problem? Thanks, Thorne
Do you mean the one filament is less bright? The cathode current is the most important indicator and at 34 mA per tube the plates shouldn't be glowing at all. If it is the filament, that's probably not a worry, as sometimes the filaments protrude more or less from the cathode and this can affect the brightness to no other ill effect. You can check the AC voltage on each filament to confirm proper operating voltage. Changing the input source shouldn't affect what's going on with the output tubes per se.
bhamham
Super Member
You can get a blue glow too. Flourescence from the electrons bombarding the glass, usually recognized as a healthy tube. Blue good, red bad.
