5F1 Mojotone Voltages Too High and Dual Voltage Issue

Discussion in 'Shock Brother's DIY Amps' started by Tbeers, Aug 5, 2020.

  1. Tbeers

    Tbeers TDPRI Member

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    I am stuck in the USA because of Covid-19, I usually live in Zambia. (This is important due to voltages)

    I just built a Mojotone 5f1 Amp and I love it but the voltages are a bit high and it red plates the 6v6 some and misbehaves when I run it too long (an hour or two), it makes really loud buzzing and has low musical output when it misbehaves. When it is not misbehaving it is loud and has a lot of headroom (rather nice actually, I could do with some more grit).

    I am planning to move back to Africa as soon as Covid stops foiling my travel plans where I will convert the amp to work on 230-240 Volt operation. I will have to check my wall to see what power is actually supplied to my house since for most things this does not usually matter and I don't know it off the top of my head. I have taps on the transformer for both 230 and 240.

    AC voltages:

    Wall in USA: 118 VAC to Transformer
    Red to Ground: 350 VAC per side to 5Y3 (seems original spec is 325)
    Yellow to Ground: 5.4 VAC to 5Y3
    Green to Green: 6.75 VAC to pilot light

    DC voltages: (going left to right across Mojotone's wiring chart.

    Name: Measured Recommended
    B+1 417 VDC 370 VDC
    B+2 354 VDC 325 VDC
    B+3 310 VDC 275 VDC
    22 VDC 19 VDC
    1.46 VDC 1.65 VDC
    B+3 205 VDC 165 VDC
    1.54 VDC 1.65 VDC

    My bias Resistor is a 500 Ohm 10W power resistor
    5Y3 unknown brand (I lost the box)
    6V6 Electro-Harmonics
    12AX7 JJ

    (I got mismatched tube brands because I ordered it during lock down and I think they had to struggle to get all the parts.)

    I read here that JJ 6V6 Tubes can take more power dissipation than other brands and that perhaps their 5Y3 will provide lower voltages. So I ordered one of each which is coming in the mail in two days or so.

    So I have a bit of a conundrum will the tube swaps fix the problem? Should I re-bias the 6V6 by increasing my resistor value. Should I get new transformer that actually outputs 325 per side? Will everything I do now be immediately irrelevant the second I change the transformer wiring for 230-240 VAC? (I have no idea what kind of winding tolerances to expect.)

    I only get to the US once every 2-3 years because of my work and I'm sure vacuum tubes are not available in rural Zambia so making my tubes last a long time is important to me.

    Any suggestions or shared experience is greatly appreciated.

    Thanks,
    -Tony Beers
     
  2. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    If you wire it for 240 and the wall voltage is 230, all the secondary voltages should drop. The 230/240 taps may result in lower secondary voltages than the 120 tap. Until you know, you will not know.
    The JJ 6V6 might be able to take it. The character of your amp is different due to the high B+. I think you would be better served by reducing the B+.
    Some times the JJ 5Y3 results in a little lower B+. The consensus for 5Y3... use vintage tubes. They usually will outlast the new production and they are readily available at decent prices.

    You may want to have a look at reducing the B+ by another method. Have a look at *Rob Robinette amp mods* - zener diode stack.
     
  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I always use JJ 5y3 tubes and in my experience, if the PT is designed for it, the rectifier will deliver pretty dang close to vintage levels. So, that may help once you get the new one.

    Second, I am sure you aren't looking to buy a new PT already, especially if you can find a cheaper working solution, but if you happen down that road, I use the non-export version of this - Hammond 290AX - and the 275v tap hits right at vintage schematic level of 340v B+ with JJ tubes. Even the higher voltage option is still 325v. Mojo's is up there! It's a stout one though that also handles a 3A rectifier.

    https://www.hammfg.com/files/parts/pdf/291AEX.pdf

    https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/291aex
     
  4. Viejo

    Viejo Tele-Afflicted Silver Supporter

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    NOS 5Y3 will drop the B+ about 30 volts if you are running a new manufacture rectifier tube. They are still plentiful and reasonably priced
     
  5. Paul G.

    Paul G. Friend of Leo's

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    NOS 5Y3 or modern JJ 5Y3 will drop your voltages by 20-30 Volts, that should get you right in the ballpark. Most modern 5Y3 tubes, aren't. The only exception seems to be the JJ.
     
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  6. jman72

    jman72 Tele-Afflicted

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    I wholeheartedly stand by the JJ 6v6 tubes, especially when the voltage is a bit high. My 5e3 (with a vintage 5y3 tube) was running hot and red plating a pair of Tung Sols I put in it. Switched to a pair of JJs five years ago and they have been going strong ever since. Put one in my recent 5f1 build because I know they are built like tanks and I won't have to worry about them.
     
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  7. andrewRneumann

    andrewRneumann Tele-Holic

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    Do you have troubles with the grids rattling in the JJ's? I had that in my combo build and it drove me nuts. I put rubber rings on it, wrapped it in high temp tape, installed tube clamps... nothing worked. EH didn't rattle, but it's definitely a less aggressive sound.
     
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  8. jman72

    jman72 Tele-Afflicted

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    No, I've never had any problems with the JJs. Either in my 5e3 (a combo) or my 5f1 (a head cab).
     
  9. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Is there any chance you wired up the 100V primary instead of 120V? That's one way that voltages could be a few percent high across the board.

    The Mojotone 760 PT in that kit is rated for a lot more current than used by a 5F1 Champ, so that might also explain why voltages aren't dropping into the expected range.

    Why on Earth don't they use their 759 PT in the kit?? That would give much more appropriate voltages. I'd ask them if it's too late to make a swap!

    Kit page: https://www.mojotone.com/kits/TweedAmpKits_x/Test-Tweed-Champ-Amp-Kit
    760: https://www.mojotone.com/amp-parts/Power-Transformers/Blackface-Princeton-Champ-Power-Transformer-EX
    759:https://www.mojotone.com/amp-parts/Power-Transformers/Tweed-Champ-Power-Transformer-120V-240V
     
    Last edited: Aug 6, 2020
  10. Tbeers

    Tbeers TDPRI Member

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  11. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Zener diode is a good idea! Consider just subbing a 6L6GC for the power tube, too -- it would suck up that extra juice and be perfectly happy.
     
    Last edited: Aug 7, 2020
  12. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    A 6L6 would take more mA than the PT can supply. Unfortunately, not a good option.
     
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  13. Nickfl

    Nickfl Friend of Leo's

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    In my experience even the JJ 5y3s still run a bit higher voltage than a vintage rectifier tube.
     
  14. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    The Mojo 760 PT is rated for 75mA -- I wouldn't expect a single 6L6GC to draw more than 50-60mA in this circuit, even at 400V. :)
     
    Last edited: Aug 7, 2020
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  15. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Following that logic, the Mojo 761EX deluxe reverb PT would provide 300mA.

    The Mojo 760EX is rated for 75mA.

    Mojo's info states: "Mojotone transformers are manufactured on the same equipment and from the same materials as the original part. Our transformers look, smell, and perform like the original...". Mojo also gives a figure of 75mA on each *side* of the CT.

    For a PT with a CT, each side of the CT coil has equal voltage and current but they are out of phase. A rectifier tube (5Y3) is a full wave rectifier. It does not double the current. It only lets one *side* of the sine wave through at a time so the 75mA of current that is let through stops when the other *side* with 75mA starts to go through the rectifier. The current alternates. One *side* of the CT then the other. It does not add the two 75mA of current together.

    This circuit is class A so I figure around 100% Plate dissipation. A 6L6GC would use about 80mA according to Rob Robinette's bias calc. The whole amplifier would use a little more.
     
    Last edited: Aug 7, 2020
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  16. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Ah, you're right, I was double-counting the current rating! I'm used to other brands that spec the total current available across the whole secondary, like Weber, ClassicTone, Hammond, etc -- this must take the 50% duty cycle into account. 150mA would be a lot for a tiny Champ PT to deliver, come to think of it! :lol: I've fixed the other post.


    I do still think the OP's amp would be safe with a 6L6GC in it. Since I'm having a slow day working from home (shh!), I rewired my 5F2A to use the 660V secondary and plugged in a 5AR4 to jack up the B+ as much as possible. Then I stuck in a few 6L6GCs. The result was a B+ of 420V, plate about 410V, cathode 28V, for 56mA current draw. :cool:
     
    Last edited: Aug 7, 2020
  17. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Sometimes "the make it take it" crowd gets away with it. Too risky for my blood. These little amps seem to sound best at over 100% PD. I am afraid the temptation will take it too far.

    Now maybe a 5881 would be the 6L6 variant that would tax the circuit, as you suggest.
    The OP could consider looking at the other thread "5881 in a 5F2A" that is currently being discussed.
     
  18. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Well, I'm not in the "cathode bias must be at least 100%" crowd either... even if it might sound better, that mentality has harmed the reputation of cathode-biased amps, in my opinion. If that's a requirement, though, then a 23-watt tube like a 5881 would be a good way to go. My 6L6GC was almost there without any circuit changes!

    Ultimately, we're in agreement that the best solution would be to bring down the B+ one way or the other. I serviced a friend's BF Champ years ago that was red-plating badly. The B+ was 438V and the 6V6 was dissipating close to 17 watts! I used a wire-wound resistor to get the B+ down to 360V and the tubes were happy again. Sounded fine afterward, too. (The replacement filter cap can had an extra 20uF which I put in front of the wire-wound resistor. Bonus feature of this mod is that it killed ALL of the single-ended hum.)
     
    Last edited: Aug 7, 2020
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  19. Tbeers

    Tbeers TDPRI Member

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    Post Tube Swap Update:

    I put the JJ 5Y3S and JJ 6V6S into the amp and have not seen any red plating. It has definitely gotten my voltages closer to spec. The amp has more of the gritty tweed tone that I wanted and I has not made the terrible buzzing sounds since the swap. I played it pretty hard with dual inputs on Saturday.

    DC voltages: (Just the big ones)

    Name: Old tubes JJ tubes Recommended
    B+1 417 VDC 388VDC 370 VDC
    B+2 354 VDC 340VDC 325 VDC
    B+3 310 VDC 295 VDC 275 VDC
    B+3 205 VDC 197 VDC 165 VDC

    I think I'll run it like this for awhile and see how it goes. It seems to be behaving much better now. I am planing on bringing an extra set of tubes to Zambia and also my Vox MV50 as a backup if something goes wrong. If my house voltage is less than 240 in Zambia, then I will definitely experiment with the different transformer coil taps and report back.

    Thanks for all the advice. I was worried that this would be one of those "go digging through the archives posts" posts but it seems people here are friendly and interested in this. I also got some ideas like the diode mod that my deep diving did not uncover. I might pack a few diodes as well.
     
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  20. Tbeers

    Tbeers TDPRI Member

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    Sorry about the spaces, I've tried both tabs and spaces and get get my table to space out nice when I post
     
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