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5E3 Build w/ B+ Gone Awry

Discussion in 'Shock Brother's DIY Amps' started by Huddy, Nov 1, 2020.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I would set the 6.3 heaters to about 6VAC.

    Usually a JJ 5Y3 drops about the same voltage as a vintage 5Y3.

    Rob has a method for dropping voltage with a string of diodes. You might want to give that a look.

    Drop the voltage of the HT with resistors before the rectifier.

    Of course... another option... save this PT for a different build.;)
     
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  2. dan40

    dan40 Friend of Leo's

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  3. Huddy

    Huddy Tele-Holic

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    Very interesting! I've never seen anything like that. They don't look like the ones in the bias circuits of marshalls. Thanks!
     
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  4. dan40

    dan40 Friend of Leo's

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    They do make smaller units that look just like a standard diode. You can string a few of those together to achieve the desired voltage drop. Many folks use this method to achieve a lower voltage when the PT is just a bit to hot. I like these stud mount types because they are higher wattage (more reliable) and super easy to install on your chassis.

    https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage
     
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  5. tubegeek

    tubegeek Friend of Leo's

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    Remember, though, the Zener will have to dissipate all the same heat a dropping resistor would, to get the voltage down. That's the advantage of the bucking transformer method - the transformed voltage doesn't require wasting power via excess heat.

    It's not perfect in this case though - looks like that transformer won't give you both low enough B+ and high enough heater voltage at the same time.

    We can conclude that your power trans is not really ideal for this particular project.
     
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  6. dan40

    dan40 Friend of Leo's

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    Tubegeek is correct about the wasted heat but I have used both of these Zener arrangements in a 5e3 and the heat created never got excessive. The 50 watt zener is way overrated so it never seems to get excessively hot. I agree that a lower rated PT would be the best route if money allows.
     
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  7. Huddy

    Huddy Tele-Holic

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    Maybe I'm being optimistic or wishful thinking or whatever... just hear me out. The internet world typically says that a 5Y3 is 1.1 x your HT secondary - so 380 would be 418 VDC without tubes whereas a GZ34 or diodes are typicallly 1.4 x HT. 380 x 1.4 = 532 which is what I was getting with this TAD 5Y3 and a TAD GZ34 they were virtually identical. That doesn't seem right.

    I'm just crossing my fingers that one of the 3 5Y3s that I've got on the way will provide that unloaded B+ of around 420 VDC that will allow my tubes to plate voltages to be in that 350-370 range once the output tubes are installed.

    Again... maybe I'm off.
     
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  8. tubegeek

    tubegeek Friend of Leo's

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    It's not like it won't be an amp - it will. But it might not be as Champish as you hope. Wait & see. And as others have pointed out, there are some solutions.
     
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  9. Huddy

    Huddy Tele-Holic

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    Sometimes when you bake bread it's not the best bread... sometimes it's better as croutons... sometimes french toast.
     
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  10. tubegeek

    tubegeek Friend of Leo's

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    Exactly - if life gives you lemons, make a nice lemon merengue pie and save the leftover Graham crackers for s'mores. Nothing wrong with any of that!
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Crystal ball...

    ...I see a 420V B+ princeton reverb in your future...

    :D:D:D
     
  12. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Those multiples like "1.1 or 1.3 x whatever" are only good for specific scenarios. To predict your final B+, take a look at the characteristic chart. Find the curve closest to the RMS value per plate of your PT (about halfway between 350 and 400 for you.) The voltage all the way to the left is your unloaded voltage, regardless of the rectifier. Follow the curve to the right based on the approximate current draw of the amp:

    5y3_curves.png
    A 5E3 should draw about 85mA as designed, but I imagine a hot-running one could be up to 100mA or so. You can see that the unloaded voltage is not very meaningful, and how even a real 5Y3 may not sag enough with your PT to give appropriate voltages for a 5E3.

    (For extra credit, try measuring the DC resistance of one of your HT secondaries. It might be less than the recommended amount of plate-supply impedance, leading to even less sag!)
     
  13. tubegeek

    tubegeek Friend of Leo's

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    5E3 not Champ. I really need to read more carefully.

    A lot of what I've posted in this thread is still roughly true but some of it had the Champ in mind the whole time, apologies.
     
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  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    This thread is so full of good info. I just kinda wish I knew exactly what problem we're solving, and what target we're aiming at. :)

    Your variac experiment was inspired, but if I can say so, seems a bit extreme...

    These are JJ 6V6s? And you targeted <80% dissipation (JJ are at minimum 14W tubes)? And dropped your heater voltage by >> 10%, and your wall voltage ~ 10% lower than the 1950s? Basically: What made you choose those values? Theoretic plate voltage isn't the actual target; dissipation is.

    **But**, I'm glad (and impressed) you judged the results by *ear* -- really smart and sounds logical.

    Since you have a variac, when you get a 'real' 5Y3 that works, almost everyone would try dissipation in the 100% range (and absolutely everyone would calculate JJs as 14W, at least). I've seen one or two folks over the years who like cathode bias dissipation even below 100%, but tons of awesome sounding cathode-bias Fenders run in the 100-112% range (or higher, with JJs).

    A useful experiment is to set your variac to get 6.0-6.3V (as per @Lowerleftcoast ) and then check your bias and sound at that setting. Or try wall voltage 105-115V and check bias and sound.

    Finally, one more reason you may be liking really low dissipation: if you're running JJ 6V6s and you want sweeter, more Fender-y, 50s breakup and tone, try a 'real' 6V6. JJ are robust but don't sound 'authentic' or 'ideal' to many folks. The reissue Tung-Sol can be notoriously fragile. If you want new manufacture, the newer EH seem popular. Most of us end up with NOS Raytheon, Sylvania, or RCA (and others). NOS are robust but still sonically authentic and pleasant at 100-115% dissipation.
     
    Last edited: Nov 3, 2020
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  15. JamesAM

    JamesAM Tele-Meister

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    Hi there- everyone so far is much more knowledgeable than me on this subject, but I did want to throw my .02 in as I went through this exact problem this summer with the slightly less hot hammond 290bx. King fan and the rest of the old guard were very helpful in talking me off the ledge with my dissipation numbers. Link to post here in case it’s helpful: https://www.tdpri.com/threads/5e3-hot-pt-and-bias-sanity-check.1032949/

    The 290bx is rated 330-0-330, but was 342vac to the rectifier in reality- this gave me b+ of around 45ma (something like 114-117% dissipation) with a NOS 5y3 and a 270r cathode resistor If I remember correctly. Sounds like your 290bbx is hot as well.

    I also had tung sol 6v6s (which sounded fantastic) but red plated even after I fixed the circuit and got them to I think 104%. The JJs and old Marconis I have sounded and functioned fine at that level. The combination of a hot PT and my wanting to be gentler on less hardy power tubes was just enough to warrant a change to the circuit. I ended up at about 104% for standard new 12w production 6v6s (A bit below 100% for 6v6GTs) and the amp runs and sounds great.

    A 330r cathode resistor got me where I needed to be and didn’t raise the plate voltage too much at all, but my voltages were lower than yours. Given what your PT is putting to pin 8 of the rectifier, it likely will be easier and better in the long run to swap the PT out for the 290BX or a classictone (if you can find one). If that’s not an option, throw some JJs in there and enjoy.
     
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  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Most tubes need 6.3v +/- 10% so 5.7v would be my recommended minimum.
     
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  17. zenit

    zenit TDPRI Member

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    I would go with 290CAX for a 5e3 build instead of 290BX. CAX has two options for secondary voltage - 315-0-315 and 275-0-275.

    Although it might be too small if you have a pre-cut chassis. I make my own chassis, so I never had a problem with that.


     
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  18. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    To my ears, there isn't really a sonic penalty to undershooting the voltage by a little bit, either. Perhaps there's a tiny bit less overall gain and presence, and maybe you lose a watt or two in output, but my mutant 5B5 sounded beautiful at only 286V in low-voltage mode! :cool:
     
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  19. 5F6Animal

    5F6Animal Tele-Meister

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    The JJ 5Y3 produces similar voltage drop to my GE 6087/5Y3WGBT’s. I would go with a NOS 5Y3, they’re not that expensive and should last longer.
     
  20. Huddy

    Huddy Tele-Holic

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    Sorry I went ghost. I haven't been getting my thread notifications for some reason. 5Y3s didn't get shipped until Thursday and are still floating around New England before heading south for the winter.

    I could definitely be open ;-)

    The variac experiment wasn't really for sound but just wanted to check the bias at plate voltages I found listed on various layout diagrams. I plugged the guitar in just to check and see how it sounded and was amazed by the difference. I originally had the tung-sol reissues in there when I first started it up but switched to the JJs when I noticed the voltage and bias issue at wall voltage and just left them in for this test as well.
     
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