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Another 5e3 voltage question

Discussion in 'Shock Brother's DIY Amps' started by mgrossm, Nov 24, 2020.

  1. mgrossm

    mgrossm TDPRI Member

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    Trying to figure out what to do here...new build...first 5ee..first cathode biased amp. Classictone pt and MMOT

    Measjring 252ohm across 250 ohm 8 watt resistor and getting a 20.59V drop when powered up (isnt that kind of high?). Plate voltages with A NOS 5y3wgt is 372. Not good. Have a set of nos rca 6v6gta i want to use but i dont want to put them in now and have them get burnt to hell.

    Have a regular nos 5y3 on order that shohld drop plate voltages more than the wgt which is a slow warm up indirectly heated rectifier thay tends to NOT drop voltages like most other older 5y3s. Aslo havr some 270, 300, and 330 ohm resjsters in the mail. But sbould i just eat it Nd buy one of those $100 pt 5e3 from mojo that puts out lkwer voltages i.e., 330.
    Signed,

    Advice needed. Please help
     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    How does it sound? Heater voltage? Preamp voltages? Any redplating? What output tubes are you running now? Which PT now?

    If I’m plugging your numbers in right, that’s 120% (tad high?) dissipation if you figure 12W tubes, but 104% (excellent) if you figure 14W. FWIW, based on data sheets and lots of folks' experience, Rob (and many others) treat GTAs as 14W.

    For sure I’d try the most “real” 5Y3 you can get. If that doesn’t help, I’m not a general fan of bigger bias resistors; they just push more B+ to the preamp. But modest increases may be safe — if you really need 'em. Better solutions may be zeners (many threads here) or a simple 6-7% bucking transformer as per Rob.
     
    Last edited: Nov 24, 2020
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  3. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Last edited: Nov 24, 2020
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  4. Wally

    Wally Telefied Ad Free Member

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    GTAs are 14 watt tubes. I see nothing wrong there. What alarms you? I have figures from the last 5e3 I serviced....368vdc on the plates and right at 100% of max plate dissipation. With a vintage P12Q reconed by WEber, it was a magnificent sounding 5E3.

    That amp was a 1957...original transformers.
     
    Last edited: Nov 24, 2020
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  5. NTC

    NTC Tele-Meister

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    Those voltages are also close to mine. I used a 270 ohm resistor, but King Fan has a good point about the higher preamp coltage. The sound could change. I do like mine as is, however.
     
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  6. Wyatt

    Wyatt Tele-Afflicted

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    • Is that 372VDC B+ voltage off the rectifier tube cathode?
    • Or 372VDC measured plate-to-cathode on the power tubes ("actual plate voltage")?
    For cathode-biased amps, we use the latter measurement (for fixed bias, these two points will measure pretty much the same).

    Everyone has their personal sweetspot, but a B+ of 360-370VDC is considered "vintage spec" for a 5E3.

    Typically, we shoot for 90-100% dissipation at idle for cathode-biased amps. With cathode-bias, that'll drop some once input votlage is applied.
     
  7. Nickfl

    Nickfl Friend of Leo's

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    Is your 372 volts on the plates measured from the plate to ground or the plate to cathode? Remember that plate voltage means plate to cathode, and with a cathode biased amp that means you need to subtract the cathode voltage if you were measuring plate to ground. This gets overlooked by a lot of people who are used to fixed bias.

    As mentioned, a full set of voltages would give a clearer picture.
     
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  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Facepalm, @Wyatt and @Nickfl had their coffee, and I apparently didn't. :)Yeah, we need plate-to-cathode measurement. *If* 372 is just plate, your P-to-C will be around 372–20.6=~352, and if my coffee is working, that's ~114% dissipation even at 12W, which is in the range many stock (good) 5E3s run... which would be why your voltages are near the typical values the guys mention above.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    And... different 6V6 tubes will have a different bias resistor drop voltage. Put in the RCA and measure, then swap the positions of the RCA's and measure again.

    370 B+ in a 5E3 is in the ballpark.
     
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  10. mgrossm

    mgrossm TDPRI Member

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    That 370 was plate to ground. Forgot to subtract the 20 dropped across the cathode resistor. And was also freaking ojt bexause i forgot to divide by 2 and was thinking i was at over 20 something watts. So, yeah...its right at 14 watts uskng tung sol 6v6s....i have a set of nos rca 6v6gt vt-1074a blackplate grey glass from kca i want to try and ive read 12 watts but that 14 watts is ok as well. So maybe upping the cathode resistor to 270 or 300 ohm from the now 250? See what happens with a plain old nos 5y3 in terms of voltage?


    Sounds like exchanging the PT ( using a classictone pt and mm ot) for a lower b+ is overkill. Why does mojo and mm sell the lower b+ pt's then? I have a 5e3 pt rated at 330 lower b+ i could exchange but i wont and will jist otberwise save it for anotber build but kt begs the question...why did i get it befpre in the first place...? Where will having the lower b+ vet me and will it be too low.

    In terms of filter caps i have in place 30-20-20. Is that a problem. From what ive read...the 5y3 should be able to take it.
     
  11. mgrossm

    mgrossm TDPRI Member

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    sorry for all the typos...thumbs are big and keyboards small
     
  12. mgrossm

    mgrossm TDPRI Member

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    i realized i had already bought the 120 pt lower b+ after writing the post....and also that the tubes are not gtas but vt-1074as...which i havent put in yet...the ones in now are tung sol new production 6v6s
     
  13. mgrossm

    mgrossm TDPRI Member

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    PT $120
     
  14. mgrossm

    mgrossm TDPRI Member

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    i like the sound but those tubes and the back get friggin hot and got me worried
    ...had never played a 5e3 so no real live comparisons....put screen resistors in..., put in pi grid resistor to v2b, reduced 1st 2 coupling caps to .022 and .047, increased filtering caps to 30/20/20, elevated heater ct, those are only real mods. Speaker is weber alnico 12a125. Grounded shielded signal wires on v1. Ot is mm slightly larger size. Have a ge 12ay7 v2 and rca12ax7a in v2.
     
  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Some of the new TS are reported to have trouble with redplating at 12-14W so look closely at the plates in a dark room — only the filaments should glow. They’re said to sound good if they work tho.

    Amps get hot near the output tubes. Not a test of function.

    Lower HT PTs are handy in many other small tweed and brown amps. Lower than 330, like down to 275...

    I still don’t see a problem where you'd need a bigger bias resistor or smaller PT, tho now I’m confused about which exact PT you have. But for sure any 5Y3 can handle a 30uF reservoir cap.
     
    Last edited: Nov 25, 2020
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I think you are good to go.
    The 30/20/20 filter caps are of no concern. The 5Y3 datasheet shows a 32uF cap and many here have used 40uF.
    MM is pretty tight lipped about their products but I have read the MM FTDO-59 has a 8K primary, so that is correct for a 5E3.
    The other mods you have mentioned are of no consequence to the bias.

    The vt-1074as (is that correct? Maybe vt107 ?) will most likely change the bias and B+ numbers. There is no use contemplating changes when the actual tubes you will be using have not been tried.

    I think you may have a little more room with the bias number because I don't think you have subtracted the screen current.
    As King Fan said check the TungSol in the dark for red plating. Also swapping the position of the power tubes V3 to V4 can give different bias numbers.

    Pop in the different 5Y3 you have coming and throw the vt tubes in.
     
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  17. JamesAM

    JamesAM Tele-Meister

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    I’ll agree with the experts on here and say you’re probably good. Your voltages are close to the ones in my 5e3, and I had my own thread on here about six months ago where all of these smart folks talked me off the ledge. I’ll also third what king fan and others have said and check for red plating in a dark room. If you’re not visibly cooking the tubes and it sounds good, you’re probably fine.

    The new rectifier will probably get you where you want to be. I wouldn’t worry about the heat- that chassis is tight and the heat from the tubes rises into the chassis and the PT. Mine gets toasty after about an hour of playing, but can still be touched.

    I’d keep the RCAs in there and skip those tung sols. I had a set in mine biased to about 104% dissipation (assuming 12w tubes) and they still red plated. Sounded good as heck though. My solution? Stick some jjs in there and play it. I believe muchxs has said in one non-scientific experiment he remembered from some poster on the gear page that they didn’t redplate until 18w or something crazy. I’d take that with a grain of salt though. Your RCAs are probably pretty hardy and can likely handle the voltages you’ve got now.

    I need to get back to someone about some NOS GEs now that I mention it, actually.

    have fun and enjoy your new amp!
     
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