5e3 Hot PT and Bias Sanity Check

Discussion in 'Amp Tech Center' started by JamesAM, Jun 25, 2020.

  1. JamesAM

    JamesAM Tele-Meister

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    Hi all,

    I rebuilt a 5e3 clone earlier this summer and it sounds amazing. However, the PT I used (Hammond 290BX) is running hot voltages across the board and is hot to the touch after about 30 minutes of operation. It’s not too hot, I can keep my fingers on it for a few seconds. I’m wondering if there’s the potential to do any real damage or god forbid start a fire.

    I’m alternating between tung sols and JJ 6v6. The Tung Sols sound better and have better B+ numbers, but are slightly red plating. The JJs are operating above spec, but are not red plating despite pulling much more current than the tung sols.

    Im hoping some smarter folks than me can either ease my worried mind and tell me to play it as is (some of you did say this in my build thread) or confirm a way ahead to remedy the higher voltages. Here are my specs and measurements for each set of tubes.

    PT: 290BX, rated to 138mA HV, 330-0-330
    Actual HV voltage: 342-0-342
    NOS Raytheon 5y3
    Fender Deluxe Reverb 8ohm OT
    270 ohm bias resistor

    Bias numbers with 12w tung sols (red plating):
    Voltage at rectifier (OT CT): 395
    V4 plate: 388
    V4 cathode: 20
    V3 plate: 388
    V3 cathode: 20
    P-c voltage for both tubes: 368

    V4R- 188.2 ohms
    V3R- 190.8 ohms

    V4 current: 7/188.2= .0371 37.1mA
    V3 current: 7/190.8= .0366 36.6mA

    .0371*368 = 13.65
    .0366*368 = 13.46
    Avg 13.5w dissipation

    Bias numbers with 14w JJs (not red plating, but much higher current draw):
    Voltage at rectifier: 390
    V4Plate: 381
    V3 plate: 382
    V4 Cathode: 22
    V3 cathode: 22
    P-c voltage v4: 359
    P-c voltage v3: 360

    V4 current = 9/188.2= .0451a
    V3 current = 8/190.8 = .0419a
    .0451*359 = 16.1W
    .0419*360 = 15.1w
    15.6w avg plate dissipation

    Ok, so after all of that, here are my questions:

    - Given these measurements, Is it worth putting in a 360 ohm cathode resistor so I can play with the tung sols, whose sound I like much better? Is there a way to compute how much this will raise the plate voltage?

    - is the high current the JJs are pulling going to hurt my PT (pulling about 90mA, PT rated to 138mA)

    - should I just play the tung sols with that high plate dissipation even though they’re red plating?

    - does anyone else have the Hammond 290BX, and does it run hot (temperature) in your experience?

    thanks for any help y’all can provide.
     
    Last edited: Jun 25, 2020
  2. King Fan

    King Fan Poster Extraordinaire

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    Before the upperclassmen show up, I'm gonna guess here:

    a) the hot-to-touch PT is probably fine
    b) yes, as with many modern 5E3s on modern wall current, your B+ is hotter than schematic / ideal voltage
    c) can we see a pic of the Tung-Sols operating in the dark? Is it really red plating? Modern TS are kinda delicate tubes, so it might be, but are we sure?

    I'd think any decent 6V6s should be able to tolerate 13.5W in cathode bias -- what is that, 112%, kinda the bullseye for a 5E3?

    *If* the Tung Sols die and you don't like the JJ sound (some here agree with you) any pair of decent NOS 6V6s should likely work and sound good.

    Finally, what are your preamp voltages? You can always up your cathode resistor, but that has downstream effects. TS are a special case, but the usual advice with those voltages in a 5E3 would be "play it and relax."
     
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  3. Nickfl

    Nickfl Tele-Afflicted

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    Unless you have separate cathode resistors for both of those power tubes, your math is wrong. 20 volts across that cathode resistor is from both tubes.

    go back and try your math again and see if you can get it on your own, but I'm getting around 9.5 watts per tube, so 70-80% diss, depending on who you believe about the actual maximum plate dissipation of a 6v6...
     
  4. JamesAM

    JamesAM Tele-Meister

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    Thanks! yeah, I was pretty happy with the plate dissipation, voltages, and sound until I played it for an hour one day and the chassis was noticeably hot. As it’s my first build, I figured this was likely my error as opposed to standard operating temperature and didn’t want to take a chance.

    here’s a pic of the tung sols. You can just barely see the red plates glowing on the sides- that’s not the filament.

    I never thought about checking the preamp voltages. The only one I really confirmed was my heater, which was almost dead nuts at 6.3v. What should I be looking for?

    3B7A677F-A4B9-4EB5-BD21-A45F9D196404.jpeg
     
  5. Nickfl

    Nickfl Tele-Afflicted

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    So, I just reread your post and see you were doing the OT resistance method. The cathode resistor method is giving a much lower number, which is interesting... The cathode resistor method is probably more reliable, so I'd assume you measured something wrong when you did the OT method.

    Strange that those tungsols are red plating, I can see see it pretty clearly in your picture. Have you tried them in another amp? Are those new or vintage tubes?
     
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  6. JamesAM

    JamesAM Tele-Meister

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    All, I apologize, that picture of the red plating 6v6es is with a 5v4 rectifier- however, they look nearly the same with the NOS 5y3. They are still slightly red plating, just a bit less than in the picture there. I’ll get another picture later tonight.
     
  7. JamesAM

    JamesAM Tele-Meister

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    Those are new Tung Sols. That picture is actually with a 5v4, but they’re only slightly less red with the 5y3. The difference is pretty negligible.

    I’ll confirm the voltage across the cathode resistor and take a new picture of the tubes tonight. I’ll also check on the preamp voltages.
     
  8. Nickfl

    Nickfl Tele-Afflicted

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    If your voltage readings are about the same when you recheck, I would assume the tung sols are just faulty. If you have another 6v6 amp you could check them in it and see if they red plate there too.

    As Kingfan mentioned, the new production tung sols seem to have a lot of reports of being temperamental/delicate. I've never tried any personally, but I've heard people say they had problems with premature failure. JJ 6v6 on the other hand are widely known to be able to handle voltages and dissipation well beyond normal 6v6 spec. So, you kind of have both extremes of the tube ruggedness spectrum here. That being said, red plating at <80% dissipation on a cathode biased amp is just pathetic. If that is actually what is happening you should ask the seller for a refund/exchange.
     
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  9. Nickfl

    Nickfl Tele-Afflicted

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    Ok, so your math wasn't wrong, mine was :oops:

    When I posted earlier, I calculated your bias on my phone calculator and for some reason multiplied the current by 268v instead of 368v.

    So yeah... from your cathode voltage, I'm getting a dissipation about 12.9W. So, about 92-108%, again depending on whether you call the 6v6 a 12w or a 14w tube. The JJs are definitely a 14+W tube (I've heard some people claim they are really more like an 18w tube). The tung sols, not so much. So, thats you issue.

    Sorry for the confusion.
     
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  10. King Fan

    King Fan Poster Extraordinaire

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    Heh, good stuff. @Nickfl is one of those smart upperclassmen who check the math on the board and then recheck their own calculations too.

    As he says, use the cathode resistor voltage method, and for grins, measure the actual resistance across the resistor (amp off) before you measure the voltage drop (amp on). And as the professors say, show your work on the exam and you'll get credit for using the right method even if your arithmetic fails. :D

    I'll look forward to your better tube photo but if it were me I'd check my measurements, and if you can't get a refund, play the Tung Sols until they die at which point I'd have a pair of *other* 6V6s. If you object to NOS prices, I've heard EH are maybe less delicate than TS and sound better than the JJs. Your amp *should* be running in the range of 100 to 115% dissipation (especially if you use 12W in your calculations). There's been a lot of discussion recently that the 6V6 after a certain date (some say ~1950, some say ~1960, some say to-mah-to) are in effect 14W tubes.
     
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  11. Nickfl

    Nickfl Tele-Afflicted

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    As far as cooling the bias or lowering the plate voltage, you have a few options.

    1. Do nothing. 368v plate to cathode isn't nuts, just use the JJs or a more robust tube, probably anything other than new production tung sols would be fine. Vintage radio pull 6v6s are good and can be had on ebay cheaper than new production.

    2. Bigger cathode resistor. This will cool the bias, but also push the plate voltage up a bit (how much, I don't know). I'd go up to 300R or maybe 330 if you want to use those tung sols. They probably won't sound as good though, they sound good now because they are being pushed so hard...

    3. A string of zener diodes on the PT center tap. See robrob's website for details. This is the best way to actually lower the plate voltage. It will also cool your bias. If you want "vintage" voltages this is the ticket. OTOH, nobody seems to definitively know what correct "vintage" voltages for a 5e3 are so...
     
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  12. JamesAM

    JamesAM Tele-Meister

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    no worries! Yeah I figured the tung sols wouldn’t be able to handle above 100% plate dissipation but I knew the JJs would. I just wanted to confirm that if I leave the JJs in, the extra current draw wouldn’t burn out my PT, which runs hot to the touch with either set of power tubes in. Since it’s rated to 138mA, I think I’m fine- the JJs pull about 90mA. Just wanted to check with the group to see what they thought.
     
  13. JamesAM

    JamesAM Tele-Meister

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    Thanks! One more question for you guys while we’re on the subject of incorrect math- should I be accounting for screen current in those measurements above? As in, should I be taking about 5.5% off of the current measurements to account for grid loss, or is that already factored into the current calculations since I measured from the tube pins rather than the cathode resistor?
     
    Last edited: Jun 25, 2020
  14. Nickfl

    Nickfl Tele-Afflicted

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    If I am thinking about it correctly, you should be getting just the plate current with the OT resistance method since the screen grids are fed through a different power supply node and that current isn't passing through the OT. However I notice that your voltages work out to give the same numbers as the cathode resistor method with the tung sols and actually more current than the cathode with the JJs. I'm guessing that your actual cathode resistor value is a bit lower than 270R. This is why when using the cathode resistor to calculate dissipation it is important to measure the actual resistance.

    Also, did you measure the OT resistance when the amp was warm? This makes a difference. Another tip is to measure the OT voltage drop directly instead of measuring to ground and then subtracting, if you clip the meter lead to the OT center tap and then measure to each plate, you will get a more accurate reading. This is more important when you are dealing with a smaller voltage drop, but it will help accuracy in your case too.

    As for the PT getting hot, I wouldn't worry at all. If you can touch it for a few seconds its not really that hot and you are well withing the rated HT current anyway.
     
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  15. King Fan

    King Fan Poster Extraordinaire

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    Oof, I keep going back to your bias calcs and getting confused. As above, I'd use Rob's cathode resistor voltage drop calculator (with measured cathode-resistor ohms). As Nick says, it's probably more precise (less measurement error) than the OT resistance method. It's certainly simpler and quicker. As a side benefit, Rob recently revised his 'standard' 6V6GT (glass tube) to 14W spec; **if** your resistor measures exactly 270, your plate voltage is 20, and your P-C voltage is 368, I get 113% dissipation at 12W max, and 97% dissipation at 14W. Both perfectly fine. I'm still curious to see those TungSols running on the 5Y3 -- I believe you, but it'd be informative. If you can, block any extraneous light. Here's a 'perfect' pic from (who else) Rob, tho note without a nice tripod/camera/lens, you're not looking to be this good!

    upload_2020-6-25_11-47-9.jpeg
     
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  16. JamesAM

    JamesAM Tele-Meister

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    Sounds good! Yeah for some reason I don’t think I wrote down my true cathode resistor value- I must have been too excited to get playing. I seem to recall the number 268, but I’ll check it later to be sure. I’ll get some validated numbers and post them later along with a pic of the tubes with the correct rectifier.

    I plugged the numbers into rob’s calculator as well and got exactly what you did- his results are similar to my calculations with the tung sols, but my math is way off from his on the JJs. I’ll recheck my measurements and see what’s going on- but I remember being super surprised when I saw a 9 volt drop through the OT with the JJs.

    thanks again for your guys’ time and attention. I’ll be back later with some updated numbers and pics.
     
  17. King Fan

    King Fan Poster Extraordinaire

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    LOL, I'm still not sure what method we're talking about. But when you re-measure, tell us your numbers and what method you use and we'll all be on the same page. At any rate, sounds like you have a well-working amp. :)
     
  18. JamesAM

    JamesAM Tele-Meister

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    Ahhh I’m sorry about that- all of my numbers above were taken with the OT resistance method. I measured voltage at the OT CT then measured voltage at the plate of each tube to find how much it dropped through the OT. The voltage drop from the rectifier to each plate was 9 and 8 volts with the JJs, which seemed very high to me.

    Here are updated numbers, using only the cathode resistor voltage drop method.

    Bias resistor = 266 ohms

    Tung Sol 6v6gt, 12w
    Cathode resistor voltage: 21.3vdc
    plate-to cathode: 372vdc
    Robs calculator lists this as about 37.9mA per tube for
    14.1w per tube dissipation
    117% max dissipation

    JJ 6v6s, 14w
    Cathode resistor voltage: 22.8vdc
    Plate-to-cathode: 362.1vdc
    Robs calculator lists this as
    40.5mA per tube for
    14.7w per tube dissipation or
    105% max dissipation

    Not worried about the JJs at all, just wish they sounded better. Is it worth running the Tung Sols? Should I be worried about a catastrophic failure and a short taking out half the board if they fail?

    I tried my best to get a picture of the red plating Tung Sols, but I couldn’t capture robs magic. Here’s the best one I got. You can see it ever so slightly burning red on both tubes.

    0636B3F6-09E6-4E84-BBCC-E0B0488E8F02.jpeg
     
    Last edited: Jun 25, 2020
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  19. King Fan

    King Fan Poster Extraordinaire

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    Good for you. Both 117% and 105% are "OK" numbers. FWIW I always write down B+, wall voltage, and voltage at the lamp for comparison to other days -- especially during air conditioner season.

    I'm no expert on tube failure -- try not to be -- but I didn't *think* tubes take out other stuff if they die. I'd be interested if experienced folks have a better idea. That's why I might just run the TS 'til they die, if they do, and keep the JJs as spares -- I like JJs for new startups and troubleshooting.

    Many folks now suggest that *all* modern 6V6s can be treated as if they were 14W max, and as you notice here, the JJs can be treated as if they're *more* than 14W. So in theory your Tung-Sols *should be* OK, but I agree that looks like a dull red glow on the plates. So, I can see where you'd be concerned. Did you buy the tubes from a reputable dealer? Refund/exchange isn't a bad idea, but...

    Also altho the TS have a rep for failure, the rep says it's early failure -- I had one of a pair die very shortly after first startup -- and supposedly their durability after that is 'OK.' You can roll the dice and keep 'em in there, you can try some EH (which don't have a rep as fragile and do sound nicer than JJs), you can hunt up some NOS or ANOS, or if you had a few different NOS 5Y3s, swap 'em thru there and run the one that drops the most voltage.

    All the other ways above to drop B+ a little (plus a bucking transformer, an elegant solution) could be tried, but the trouble / expense might not be worth it if we're just trying to keep a pair of somewhat defective output tubes going a while longer.
     
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  20. JamesAM

    JamesAM Tele-Meister

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    @King Fan , thanks so much for this- really makes me feel a lot better. I did buy the TS through mojotone, but it’s likely past the return window at this point.

    I like your logic about mods to cool the B+; it looks like sourcing some more hardy tubes (or just running the JJs) will get me where I need to be. Between the red plating, higher voltages, hot transformer, and my lack of experience, I figured I’d reach out to the board here to make sure I hadn’t built an obvious systemic problem into the amp.

    thanks again for your help!
     
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