B+ voltage too high on 5f1 champ

Discussion in 'Amp Tech Center' started by frank.petrosino, May 13, 2021.

  1. frank.petrosino

    frank.petrosino TDPRI Member

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    Dear all,

    I'm building my first 5f1 champ tube amp and during the first voltage testing I performed too high B+ voltage. I used Tube Town tt-5e3-vpo power trasformer.

    In order I'm measuring these values with no tubes:

    242 Vac primary voltage;
    342 Vac HT secondary of PT;
    6.3 Vac across filaments and pilot light;

    After that I pulled in the rectifier tube TAD 5y3GT/6087 and I discovered:

    460 Vdc on B+

    Could you give me some suggestions? It is the wrong rectifier tube for this transformer? Do you thing I need to order the TAD 5Y3WGTB type that have more "sag" like NOS tubes?

    Thanks
     
  2. hamerfan

    hamerfan Tele-Meister

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    I think this TAD is an old russian tube similar to a 6087 with lower internal resistance and a higher voltage. The TAD 5y3 wgtb looks like a Phillips copy, which i see also as Mesa 5y3.
    I use currently a Sovtek in my 5F1, but use 1N4007 diodes to protect the amp. It also put off stress from the rectifier tube.
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    With no tubes installed the voltages will be high. If voltages are still high after the tubes are in place, try a vintage 5Y3 or JJ.
     
  4. J. Bonkosky

    J. Bonkosky Tele-Holic Silver Supporter

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    No tubes=no electrical load=high voltages from the transformer. Put your tubes in then check voltages.
     
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  5. 2L man

    2L man Tele-Meister

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    You can use that rectifier tube. Just install resistors between anodes and secondary outputs. They make almost the same effect what less efficient rectifier tube does.

    Two 100 ohm resistors decrease the voltage about 10V when current is 50mA etc...
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    What the guys said -- put in all tubes and then measure B+ again. Let us know what you get. :)

    Yes, a 'real' 5Y3 may be preferred to the versions that don't drop as many volts, and after that there are *many* ways to drop B+ if still too high. Also remember you actually *want* hot bias here -- values of 100-115% max dissipation are 'standard.' And max dissipation numbers also depend: what type of 6V6 are you using?

    *But* first let's see where B+ sits with the tubes loading it down.
     
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  7. frank.petrosino

    frank.petrosino TDPRI Member

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    Hi everyone,

    thank you for your advices. Now with tubes and speaker in I read these values:

    B+ 401 Vdc
    B+1 340 Vdc
    B+2 290 Vdc

    pin 3 (plate) of 6v6GT 388 Vdc

    All other check points as reported here https://www.flickr.com/photos/[email protected]/49595930948/in/photostream/
    stay into a range of +/- 15%

    I used these tubes:

    TAD 5Y3GT/6087
    Genalex Gold Lion 6V6GT
    Genalex Gold Lion 12ax7 balanced

    I played a bit with the amp without the cabinet (I'm building it) and the first impression is it sounds very good.

    What is your impression about these voltages? Do I really need to decrease them? If yes I could do the mod suggested by 2L-Man or I can order a TAD 5Y3WGTB.

    Thanks :)
     
  8. 2L man

    2L man Tele-Meister

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    What is output transformer impedance, carhode resistor value and voltage over it?

    Knowing also those it is possible to draw a loadline and see how bias point places to a loadline. I use this excellent calculator by Giuseppe Amato: On cathode bias power stage g1 voltage and V1 voltage (which is voltage between anode and cathode) together is amplifier B+ voltage so that needs some change after cathode current is changed to get it back right again. The rest is self explaining. I set A-class headroom so that 2nd and 3rd harmonics come about the same and then I can compare different setups.

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/
     
    Last edited: May 14, 2021
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If it sounds good, it is good.:)

    Ime a vintage 5Y3 will bring the voltage down a little more. A used 5Y3 is no more than 10 euro. It is always good to have a spare.

    Like 2L man, I am interested in your answers to his questions.
     
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  10. 2L man

    2L man Tele-Meister

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    Current limiter resistors are specified on rectifier tube datasheet usually line above where max capacitor is specified and they are needed to prevent max current limit exceed. They are not often used on instrument tube amps but I think they are essential when modern low impedance capacitors are used which can deliver high discharge current but they also draw high charge current. Also current machine wound, enamel wire, thin plastic insulated transformers have smaller impedances so output voltage does not drop much when current increase. Together this is big stress to rectifier tube.

    Also new production rectifiers might be weaker than NOS. But because NOS are expensive I like to protect them. Sag effect is only reason I use tube rectifier and series resistors increase sag so there is no reason not to use them. In practice resistors flatten sine wave peaks and then current flows longer time and it does not come as high.
     
  11. frank.petrosino

    frank.petrosino TDPRI Member

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    My OT is a TAD 125A3A with
    secondary impedance : 4 Ohms
    primary impedance: 8.2k Ohm

    I'm using a wirewound cathode resistor of 470 ohm 5W 5% and the voltage is 21.4 Vdc

    If I'm not wrong I have 45.5mA bias current on chatode and assuming 95% on the plate I have 43mA * (388 - 21.4) Vdc = 15.7 W that is 112%, considered as "standard" as King Fan said?

    Thanks to share the Giuseppe Amato calculator. I had a look and I think I'm a little hot bias. What do you think?
     
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  12. frank.petrosino

    frank.petrosino TDPRI Member

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    Thanks 2L Man, nice explanation. Can I put on resistors direct on the tube socket inside an heat-shrinkable sleeve? 2W wirewound resistors are good enough?
     
  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Perfetto! Brillante! You're in a great place; I see no need to drop your B+ much, but if you wanted to, the advice from LLC about an inexpensive NOS 5Y3 is exactly right. You're not necessarily looking for NOS tone or durability (tho they may exist), but NOS often tend to drop more voltage than modern. I don't know your rectifier. As far as I know, TAD tubes are 'selected' and relabeled from common modern factories. You want to avoid the 'Sovtek' type as they are actually a different design and famously don't drop as much voltage. JJ are 'real' 5Y3s, but NOS may still drop voltage better.

    Sounds like you've built an authentic, ideal 5F1. I can't comment on adding B+ resistors for the technical purposes discussed above, but as noted, I've never seen it done on an amp build here, and I don't think you need it to drop B+.
     
    Last edited: May 14, 2021
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  14. J. Bonkosky

    J. Bonkosky Tele-Holic Silver Supporter

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    Run it! Those voltages should be fine with those tubes.
     
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  15. 2L man

    2L man Tele-Meister

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    Using that data loadline looks like this. That B+ and 470 ohm cathode resistor makes idle power 16W which is few W over 6V6 rating. Red dot is idle point and it is about 4W over dotted red 12W curve.

    A-class power comes 8.5W which is one W more than what tube produce when g1 is driven to zero volt and full power needs g1 drive signal to go slightly positive. This together with quite high Screen voltage stress tube when amp is run full power when Screen current increase.

    You could increase carhode resistor value and cathode current will come down. I achieved 21,4V to g1 setting the Screen voltage to 333V but when tubes have tolerances your amp it might have different g2 voltage.

    image_72192707.JPG
     
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  16. 2L man

    2L man Tele-Meister

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    I install SS diodes to socket between 7-6 and 5-4 and resistors usually from 7 and 5 to turretts on board where I wire PT secondarys. Then there comes enough space for resistors so they can evaporate the heat.
     
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  17. frank.petrosino

    frank.petrosino TDPRI Member

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    Thank you King Fan, J. Bonkosky and 2L man for the very clear explanation.

    I think I will look for a NOS 5Y3 or its replica and I will order a 560ohm chatode resistor too. Any advice for a good NOS 5Y3 in Europe?

    So, summing up, I have 3 possibilities :
    1. Change the rectifier tube;
    2. Add the current limiter resistors;
    3. Increase the chatode resistor.

    As soon as I finish the cabinet I will play a bit more and I will try firstly with a different 5Y3 to monitor the voltages so I will let you know. After that, if still needed, I will do circuital modifications as 2L Man suggested.
     
  18. frank.petrosino

    frank.petrosino TDPRI Member

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    Thank a lot for that very clear explanation :)
     
  19. 2L man

    2L man Tele-Meister

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    NOS 5Y3 are expensive in Europe. I paid 50 euros for Philips and 35 for Sylvania.

    I have bought other NOS tubes from Italy so perhaps you can find them locally?

    I have used few russian 5C4S rectifiers on amps which call 5Y3. I recall 5C4S capacitor limit is max 10uF but it has not broke supplying 20uF but I usually install higher value resistors because I like sag-effect. I saw them on german shop for 10 euros but I bought four for 10 from ebay but shipping did double the cost.
     
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  20. frank.petrosino

    frank.petrosino TDPRI Member

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    Yes, I'm realizing that NOS are very expensive here. In Italy there are some GE 5y3gt but also expensive, more than 40 euros.
    Maybe I could try with a TAD 5Y3WGTB at 25 euros from Germany that is the unique affordable price tube with the sag NOS characteristics
     
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