Screen Protection circuit done - now what?

thornev

Super Member
The posts since post 35 in the "Need help installing to a 500-C on-off switch a CL-60 current limiter" thread dealt with this subject, but I don't want to derail the thread anymore than I already have, so I started this thread.

I did the screen protector mod where I moved the C91D bias supply wire from pin 8 of V8 to V8 pin 1 and them wired all the pin 1's on the power tubes together. I also inserted a 100 ohm 1/4 watt resistor to tie each power tube's pins 1 and 4. After completing this mod it seems that the receiver puts out a little more power. I haven't measured the power tube outputs, but I will to see how the new numbers compare to the old numbers I took before the mod.

But now that the bias supply from C91D doesn't provide power to all 4 power tubes (well, not to the same pins on each tube), doesn't it mean that each tube will generate power independently from the other tubes which, in turn, means each tube should be "tuned"? That's why we have the IBBA and IBAM circuits, right? I guess I'll have to see if the tubes need re-biasing based on the new output numbers. Enough for one day !

Thank you All for providing all these fun mods and for all your support of my amateur adventures. I'm having a lot of fun with this stuff and of course the sound, oh the sound ! A 500-C with two AR-2Ax's and a subwoofer is going to be heaven on earth !

Thorne
 
You're confusing the SCREEN with the GRID. 2 different areas of the tube. The bias voltage on the GRID determines power output. The screen straightens the electrons from the cathode to the anode. The tubes aren't making any more power, but with the 100ohm resistor in the screen B+ line they are not susceptible to arcing over like they one were. And when they arc over, the game is over for that tube. Which is why the resistor is called a "Screen Stability Resistor". We're extending the life of OLD STOCK tubes and also making the slightly weaker Russian tubes more stable and preventing or minimizing the possibility of the tendency to Arc over, which can get expensive as you well know.
Here's some light reading for you on the Subject by DAVE G.
http://www.tronola.com/html/maximize_tube_life.html
 
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Thanks, Larry. I'm reading a lot and taking a lot of notes. Learning tube topology is one those topics about which I have only scratched the surface. I'll get there. Thorne
 
In your travels, if in a book store, try to locate an rca receiving tube manual, at least 1965 copy, or a similar Sylvania publication. Lots of good theory info on tubes in there! I've had at least a half dozen myself since the '70's.
 
Catman - Thanks for the tip. I downloaded about 20 PDF files of RCA Receiving Tube books and a lot of others. The one RCA book I've read so far does a great job of explaining how tubes are made and what the different tube parts do. Great stuff. But then they get into all the different builds of tubes and I get lost because I don't know that I'll ever need to know any of that stuff. I think I need a good community college or online course with hands-on projects to understand how and why the different components are put together. I know it all starts with electricity and farming out that power to the different parts of the circuit. I need a project that explains all that. With the great explanations from Larry and Dave I'm starting to understand what the modifications do, but I get lost in the terminology, another aspect of all this on which I need a book. I'll get there.

I have a few more projects for my 500-C and I'm also restoring a pair of AR-2Ax speaker cabs whose controls are shot and one woofer was blown (and is now replaced). A much simpler project those speakers.

Thorne
 
Thorne- you're right on that about speakers. have a stash of 'em in the attic, majority need repairs. Included in that stash is a set of ar2ax's, according to my inventory. I'll go up in there & assess their condition. If they look to be too much of a project & the pots are there & intact, I may part 'em out. I need to go in there anyway, I also have about a dozen or more turntables, most are with the speakers. There's a marantz 6100 that I want to pull out & assess. Stay tuned...
 
To answer your question about the power output, the screens are still supplying the same voltage within a volt or so even tho they are pushing thru a 100ohm resistor. The cathode originally was direct to ground which if a tube went Chernobyl, it would melt the tube, catch on fire, and/or ruin the output transformer. With the stock setup the tubes are running at or just out of the edge of the envelope for max dissipation on the plate which is 19W per tube. And there is no safety relief's built in as from the factory. The 10 ohm resistor is the safety valve. With the 10 ohm resistor from the cathode to ground, it acts as a fuse of sorts as when a tube starts to go Chernobyl, the extra current from the cathode will overwhelm the resistor on pin 5, blowing the resistor (it sounds like a Fire-Cracker)and shutting the tube down, minimizing the damage to the resistor and possibly the tube, and saving the transformer. POWER OUTPUT HAS NOT CHANGED with these mods, they've made the unit safer and more reliable. The IBAM/IBBA can change the power output from appox 50% plate dissipation up to about 100%-105% dissipation. In the interest of tube longevity, and reliability a recommended plate dissipation, based on data from a few of the members who are engineer's and have done research on the issues, of 70% to 80% of max plate dissipation is sufficient to get 24-25 Watts from each channel, maybe a watt or 2 higher. But with the NEWER Russian tubes they aren't as strong as the original 7591's, hence the70%-80% limitation. This limitation also helps NOS Tubes last a lot longer. I've taken EH7591's to 95%-100% max dissipation for a couple days and they DID NOT LIKE the extra load. They popped, scratched, wheezed, and generally made their displeasure heard. Back them off to 80% and they were Happy. 70% had even less distortion. From my Experience depending on the amp/receiver anywhere between 70 to 80% of maximum plate dissipation of the 7591A is fine for ANY 7591 be it a NOS Westinghouse or the Russian Re-issues. The IBAM / IBBA will allow you to individually match dissipation and balance the tubes in pairs. Why buy 7591's for $85. to $135. a quad yearly when you don't have to. This is what the IBAM and IBBA will prevent, and still let you enjoy the unit. You could conceiveably get 10 years from a quad of tubes when set to 70% max dissipation.
 
Thanks, Larry. I was just reading in the "RCA 1962 Receiving Tube Manual" (RC13) about plate dissipation and how, when adjusted too low, it can manifest in unsatisfactory performance, and too high, in limiting the life of the tube (and the symptoms you mention). I'm learning ! I like the concept of self-biasing. Have you ventured into that area? Thorne
 
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Self biasing is basically Fixed bias. As voltage goes up and down the bias will change slightly. Take and multiply Plate voltage of your 500c by the ma of the tubes you have running in it. covert to mv by multiplying the ma by 10ohms. Say for Example 430V plate voltage x .032v (32mv). The sum is 13.76W. Now change the value of the plate voltage to 480v and leave the ma at 32. So 480V x .032 = 15.36W (this would be a Sansui 1000A BTW) With a Higher Plate Voltage you have a lower ma range to get to the 70-80% range than you do with a lower Plate voltage. Note that the 430V equation is barely in the 70% range while the 480V equation is just over the 80% Range for the SAME mv. So you have to calculate for each individual unit.

The 7591 & 7591A & 7868 are all rated for a Maximum plate dissipation of 19Watts. 70% is 13.3W while 80% is 15.2W. As the plate voltage goes up the 70-80% Range goes down. And conversely as the plate voltage goes down the 70%-80% range goes up. An example of a lower Plate voltage of 360V X .032mv = 11.56W To get in the 70% we'd have to raise the ma up to 37ma, 360 x .037v=13.32w for 70% and 360 x .042mv=15.12W for 80% approx. At 42ma your really pushing the tube toward meltdown. Which is one of the reasons why most 7591/7868 units have a plate voltage in the 400's.
 
Larry... What are the symptoms of lowering the plate voltage in regards to performance? Does it lower the volume having the volume control at the same setting? The book was not clear on that point. Thorne
 
Tubes have an envelope that they operate under. Under voltage on the plate(anode), screen, grid 2 will cause increased distortion for various reasons I'm not equipped to answer, while over voltage can result in quite spectacular fireworks inside the bottle. As long as the plate, screen grid 2 and bias voltage is within the envelope for the type of amplifier they are used in they will not lower or increase the total volume of the unit with the volume set at a set point except as a function of the bias voltage or dissipation. And the difference in the envelope is minimal to the human ear.

The 360V example was more of an example to show you how the bias percentage range goes up and down based on a function of plate voltage and that you must adjust the bias voltage to compensate for the plate voltage. However the 430V example(500c/800c) and the 480V example(Sansui 1000a) are Hard repeatable examples of different plate voltages (I left out the screens as they don't come into play until the basic equation is done.) vs bias voltage to get into the 70% to 80% range. To get the final dissipation the Screen dissipation of 2 to 3 ma (use 3ma for safety)is subtracted from the plate dissipation and the final wattage is figured from there. So the interaction does involve the screen for total dissipation.

So 480V x .032v =15.36W now subtract 3ma which gives you 29ma So then 480V x .029v=13.92W (This is on the Sansui but the calculations are the same for the FISHER.) 430V x .032v = 13.76W subtract 3ma for the screens 430V x .029v = 12.47W About a watt and a 1/2 difference at the same ma. mainly due to plate voltage with the same ma draw on the cathode. So to get the FISHER to the level of the Sansui in the 70% range raise the ma up a couple and re-compute. Set for between 70% and 80%. I've checked all my units at various settings within that range and I don't hear any real difference in power although it show's up in the numbers and on a scope. Look up dbm to Watts converter. IT's not a linear rise, but logrythmic. 3db is about the lowest discernable change for a human ear.

Compute for the total end result with the screens subtracted to get the value between 70% and 80%. IDEALLY a range of 29ma to 34ma will give you the best sound with the least distortion with the tubes running from cold to warm in the realm of loading.
 
I just took measurements of the power tubes 7591A at pins 3, 4, 5, 6 and 8 with the receiver on. Are these values within spec, or do I need to do more work? If you'll point me to a book that states the range of OK values (never mind - I found them), I will look them up. The new values were taken after I moved the orange bias wire from pin 8 of tube V8 to pin 1.The previous values before I moved the orange wire.

Note the largest difference in pin 5 V9 from .337 to .357. Significant?

Or what about how much lower V11 value for pin 5 is so much lower than the other values for pin 5?

Or how much lower V8 and V11 pin 6 are from the other tubes' pin 6 values?

Thanks, Thorne

TUBE:....V8..........V9.........V10.........V11
PIN 3----- 429v ----430 ----430 ------- 430 .....previous values: 423 /423 /423 / 423
PIN 4----- 379v ----382 ----376 ------- 378 .....previous values: 377 / 378 / 377 / 377
PIN 5----- .346 ---- .357 --- .352 ----- .319 ....previous values: .344 / .337 / .364 / .312
PIN 6...... -17.81.. -17.63 . -17.56 .. -17.85 ...previous values: -17.61 / -17.56 / .17.44 / -17.60
PIN 8----- 377v --- 381v ----375v ---- 376v ...previous values not taken.
 
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They all look ok to me. Look at the schematic voltages. Add or subtract 10% to the nominal value for pins 3 & 4. That's your upper and lower limit. Pin 4 and 8 are going to vary a volt or two at any time. Pins 5 and 6 will vary as you adjust BIAS. Do you plan on IBAM or IBBA? This would even out the pin 5 values. And don't worry about the pin 6 voltages as they aren't really as important as the pin 5 voltages. You are checking the bias now by converting the mv across pin 5 to ma which is current (amps).

Schematic values
Pin 3.......430 Plate (Anode)
Pin 4/8....375 Screen (G2
Pin 5 ...... varies Cathode (G3 & K) Should be no greater than 4mv. Closer is better.
Pin 6....... varies Grid 1(G1) disregard as pin 5 takes precedence. in any case no closer to ZERO than -10V.

Are your tubes new or old stock? They could be better matched.
 
Thanks, Larry. The power tubes are the tubes that were installed in the receiver when I bought it. All I know about the unit's history is that the seller said he had the unit "professionally serviced about 6 years ago when I bought it." He said "All of the tubes are vintage tubes other than V16 & V17. Sylvania 7591A power tubes, Telefunken smooth plate ECC83's,RCA 7025 & 12AX7A tubes, etc." But he also said that the amp "works perfectly" and when I got it, I had all those FM reception issues about which I wrote when I first joined this forum.

The power tubes are all the same. "Sylvania 7591A-P MADE IN USA". They also have "HR" and "APA" written on them. They have "7591A USA" written on the top. There is a hole on the bottom between pins 1 and 7, and an O-ring inside the tube on top. The getter (that's the silver stuff painted on the inside top of the tube, right?) is slightly worn off i.e. it does not fully cover the complete top circle of the tube. I did order a set of "Matching Tung-Sol 7591A New Production Power Vacuum Tube" from TubeDepot. $31.98 each. The reviews I've read are mixed. I know when a seller says "matching" that there is no standard they necessarily follow. I intend to try them and if the receiver doesn't function any better (and I'll take measurements), I'll reinstall the old tubes.

I haven't cleaned or tightened the power tube pin slots yet so I might try that.

I recently replaced V16 and V17 with 2 of 12AX7 I had ordered for a headphone amp. One is a Tung-Sol and the other is a Sovtek 12AX7LPS. The only noticeable change was that the sound seemed more full and brighter. I like the change so I'm keeping them. I had replaced all three 12AX7 tubes on the MPX with new Russian Tung-Sol before Dave did his realignment magic. And I am waiting for two 12AX7 Telefunken smooth plate with "THE FISHER" written on them that I plan to try in V14 and V15 to see if it improves the sound. I also replaced V7 (6GK5) with a new Hitachi "with balanced triodes" tube but noticed no change so reinstalled the old tube.

I do think I'll attempt to build the IBBA. I'm still having a lot of fun trying modifications. I haven't decided yet on how to install it.

I wanted to replace R139 and R143, but those resistors are well-integrated into that SDS Labs "recap" board with which I don't want to screw.

Thorne

PS - I forgot to ask... Since I installed the CL-80 inrush limiter I notice the receiver takes longer to warm up to "ready to play" status. Does that make sense?

PSS - Some other mods I'm considering (to satisfy the vinyl junkie/audiophile side of me):
1) Improve bass response: change C35 and C36 from 1000pF (.001uF) to 2200pF (.0022uF) caps.
2) Bring receiver to RIAA spec: change three PC1 components. Process listed in my binder.
3) Reduce hum using phono inputs (not sure I need to fix this one by adding shunts): add 100K shunt resistors at input jacks 1 & 2. Change 6 caps. Process listed in my binder.
 
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On the CL-80. I notice about a 2 second change in most of my gear with CL-80's. Yes as it takesa few seconds for the varistor to heat up and reduce the resistance that it has when cold. Plus there is about a 2 volt drop when hot. So it may take a couple seconds more to warm up. Believe it or not, because of the way FISHER wired up the preamp tube heaters in my R-3 ('51 Coronet) it seems like forever and a day and a 1/2 for it to warm up to producing sound. I could revised the circuit with a separate transformer and cull out the heaters on 2 tubes and wire them separately, but I like mine stock so I live with 1 minute and 20 seconds warm up. But when it's running it'll shake windows in the house. P-P-P 6v6's for about 30W Mono pushing a 15"Jensen Coax from '56. Everything else is running in under 40-45 seconds avg.

#2. Use components with tight tolerance. 1% is good. 5% is fair and 10% is downright sloppy.

I'm not so sure #1 is a good idea, but that's just me. I like my gear mainly stock and any changes reversible add-on's that can be reversed by removing the circuit in question and leaving minimal remnants.
 
That's an interesting comment that you like your gear stock. There's a good philosophical debate there. I've been a collector of various items over the years (coins, baseball cards, guitars, vinyl, guitar gear) and after a few decades it is the stock items that seem to arouse the most interest among collectors especially with "vintage" guitars. Modifications reduce the value greatly. I'm not saying that's your MO because with the audio gear, it's not mine. I'm in it for the luscious sound and modifications are sometimes necessary just to get an item "back to stock !" I guess I'm saying that one aspect of the debate is did the designers achieve what they "might have" envisioned when they first designed audio gear given the state of technology back then versus the technology that is available now. OK, enough philosophy ! "Save it for a different thread, Thorne" :)
 
I installed 4 new power tubes today on my 500-C: "Matching Tung-Sol 7591A New Production Power Vacuum Tube". Just to see what difference they might make. I'll take measurements in the next few days, but just using my bionic ears, it seems to me the sound is punchier and more clear, and quieter when there's no music or only talking on radio stations. Don't know if tubes being replaced could logically make such a difference but that's my review for now.

Oh.. they seem to get hotter too.

Thorne
 
New tubes, I'd let 'em burn in for a couple days, but that's just weird ol' me. I'm burning in my formerly dormant x-101-d today. Checked bias voltage, was 28 instead of 39. A tweak later, tubes slightly cooler. What's your input voltage, straight from the socket, or thru variac or buck xfmr? Me curious!
 
Check the bias voltage across the cathode resistor (pin 5). Don't wait a couple days. Give it 15-20 minutes to stabilize and check all of them. You'll probably notice the cathode voltage is higher (close to 36ma-38ma) as the Tung-Sol's tend to run a bit "hotter" for a set bias voltage setting on pin 6. I know that all mine (3 quad's on 3 different units ) run "hotter". For a given bias setting (say for 32ma) the voltage on pin 6 is 2 to 3 volts lower (more negative) than a comparable EH7591 Mine average -21v to -22v for a setting of 32ma on both 800c's and the Sansui 1000A. Comparable EH7591's need -17 to -18 for the same ma setting. Just so you know before hand when you check yours, so you don't think something is wrong. It's just how they run.

Mod's. I'll put an upgrade/mod in if there is engineering to show that the mod is beneficial to the operation of the unit, in Safety and Longevity. Otherwise I don't do a mod. Values stay the same but voltages get bumped up to the next level to compensate for the increased B+ from the Higher Wall Voltage. You'd be surprised at how many caps are run within 5volts of the rated operating voltage of the cap. That's NOT accounting for the surge voltage. So now there is some more headroom for the surge voltage and operating voltages which as you know can vary during the day and night. I'd love to do the EFB mod to all of my receivers but I don't have the technical experience to be able to translate a strict schematic drawing to an actual unit, especially if it's Solid State like the EFB. So when I send each unit to Dave for an alignment I'll ask him to install EFB in each one. 2 400's, and 2 800c's, with one of the 800c's being a 10001-19999 series Executive VIII Model 950 unit. This series is specific to the 1963 Executive ONLY! No standalone 800c's in this series. It's slightly different than the 20001 and up 800c's (transformers are slightly smaller and NFB is different, as is the Driver/Phase Inverter Circuits). Which makes it even more of a limited series than the 1800, IMO. In 10 years here on AK I've only seen or heard of 2 10001 series 800c's. The other is owned by Omegaman.
 
OK, here are the measurements with the new Tung-Sol 7591A tubes after about 15 minutes turned on. Previous values are with the original Sylvania tubes. Thorne

TUBE:....V8..........V9.........V10.........V11
PIN 3----- 427v ----427 ----427 ------- 428 .....previous values: 429 /430 /430 / 430
PIN 4----- 382v ----385 ----380 ------- 382 .....previous values: 379 / 382 / 376 / 378
PIN 5----- .481 ---- .441 --- .482 ----- .432 ....previous values: .346 / .357 / .352 / .319
PIN 6...... -18.08.. -18.08 . -18.05 .. -18.03 ...previous values: -17.81 / -17.63 / .17.56 / -17.85
PIN 8----- 381v --- 384v ----379v ---- 381v ...previous values: 377 / 381 / 375 / 376

Look at how much lower the pin 6 values are and how much higher the pin 5 values are. That's a LOT hotter. Now I know where that extra "punch" comes from (that's too bad because I like it). Are the new values cause for concern? I guess this is a good case for building an IBAM, eh? Are the new values bad enough that I should put the Sylvania back until I have the IBAM installed?
 
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