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Champ 5f1 rectifier/gain question

Discussion in 'Shock Brother's DIY Amps' started by choosebronze, Apr 26, 2018.

  1. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Hey all. I'm a long-time guitar tinkerer and pedal-builder. I finally tried my hand at a tube amp, and I got it working! I built the BYOC 5f1 clone kit. And I managed not to kill myself with all that scary voltage!

    My question is about the amount of gain in the amp. I fired it up with a new Sovtek 5Y3GT, a new JJ 6V6GT, and an old Groove Tubes 12AX7 I had lying around. From the get-go it had a TON of gain. That's what I was expecting from a Champ. I mean around Vol 6, the distortion was going strong. The problem was high end. With single coils high notes were total icepick. I read a couple things online about how the Sovtek 5Y3 doesn't meet 5Y3 specs. IIRC the gist was the Sovket puts out too much voltage, so the tubes are running hotter than they want to be.

    I got a NOS G.E. 5Y3 on eBay for $9. Not much financial risk. Popped that in and the icepick thing is gone. The tone is smooth and spongey, just like it should be. BUT, I have way less gain with the GE. Now I start to get crunch around 8 or 9 if I strum hard. Have to turn it up even more to get a driven sound with soft playing. My understanding is the Sovtek level of gain is more like what I should be expecting

    I'm just wondering if anyone has any thoughts? Is this going to be a trade-off and I'm just going to have to pick one? Is there some tangible measurement in a 5Y3 I should be looking for? Or is there some brand of new/old 5Y3 that's going to be reliably at the correct spec?

    Thanks in advance. I might be addicted to amp-building now. Already looking for my next build...
     
  2. Old Tele man

    Old Tele man Friend of Leo's

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    Rectifiers do NOT directly affect gain (how much bigger the output signal is versus the input signal); that's done predominately by the PREAMP tube(s) and (slightly) by the output tube(s). However, the rectifier DOES indirectly affect gain when it changes the B+ plate voltages to all the tubes -- more voltage yields more output power.
     
    Last edited: Apr 26, 2018
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  3. Finck

    Finck Tele-Afflicted

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    Exactly what I was started to write, but Old Tele Man was faster...

    Probably the NOS 5y3 has introduced a different voltage drop, which has affected the tone the way you have liked and at same time has reduced the power.

    You could try use the 5y3 that provides the lowest voltage drop and play with the resistor on the PSU filter, to get an intermediate voltage that fits your desires...
     
  4. jimgchord

    jimgchord Tele-Afflicted

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    My experience is that the sovtek runs around 35v higher than a new JJ 5y3.
     
  5. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Interesting, so maybe a new JJ falls somewhere between the GE and the Sovtek...
     
  6. Old Tele man

    Old Tele man Friend of Leo's

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    Sounds like that Sovtek is NOT what it's labelled as, but rather a "close enough" re-labelled tube. The 'specified' forward plate voltage drop for rectifiers are well published, but stuff being sold today seldom adheres to those specs.
     
  7. jimgchord

    jimgchord Tele-Afflicted

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    I have an old ge and it's within a few volts of the jj
     
  8. jimgchord

    jimgchord Tele-Afflicted

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    Definitely
     
  9. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Maybe this is what I should ask: The way I described the amp with the GE -- is that what I should be getting? Am I way off thinking I should be getting good crunch around 6?
     
  10. dan40

    dan40 Friend of Leo's

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    Can you take some voltage readings for us? That would help to determine what is actually happening. What DC voltage do you read at the first filter cap with each rectifier? You could also add a tone control very easily to help tame the high end.
     
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  11. clintj

    clintj Friend of Leo's

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    The original 5F1 should run about 340/325/250V or so at each node. BYOC has designed their kit to run at a blistering 425/375/325V. That's just ridiculous for a 6V6, especially since they didn't adjust the bias resistor from the original 470r.

    I'd love to see some real world filter cap voltage numbers, and a full set of power tube readings would also be great. I can get a ballpark bias number with that info.

    The 5F1 amps I've built will start to crunch lightly around 6 or so with a Tele, and get to full hog wild distortion at around 10.
     
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  12. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Sure. With the GE: 369v. With the Sovtek: 401v. So, I guess jimgchord was pretty spot on.

    Happy to do that as soon as I know what to do... This tube stuff is all new to me. You just want readings on every pin of the 6V6?
     
  13. clintj

    clintj Friend of Leo's

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    Yes. Voltage at each pin will be perfect.

    That supply voltage is much closer to vintage than their schematic says. Wonder if they had a run of red plating amps and tweaked it....
     
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  14. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Many new builders focus on the tube socket pins and neglect to give the power supply voltages when requesting troubleshooting help. It's good to always include those.
    Fender-Champ-5F1-Schematic.png
     
  15. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    A couple of these numbers fluttered a bit. Like 2-3 tenths. Using the naming on the schematic Snfoilhat posted, with the GE I get:
    B+1: 368 (1v lower than before?)
    B+2: 314.5
    B+3: 272.8
    Pin 3: 343.6
    Pin 4: 313.1
    Pin 5: 12.1 mV
    Pin 8: 19.23

    Pins 2/7 read 0v. The amp works - so I assume that's normal...
     
  16. clintj

    clintj Friend of Leo's

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    With the GE rectifier, assuming that's a normal 12W 6V6 (not a 14W JJ) I get 104%. Just a wee bit hot, and about on par with what I build. Voltages look very good to me, too. I think I'd probably leave it be.

    The point of distortion will depend greatly on your pickups, but some crunch at around 6 or so is reasonable. You can expect clean further up the dial with something like vintage Fender single coils or Gretsch pickups. The 2 input should be quite a bit weaker than the 1 pickup as well.
     
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  17. choosebronze

    choosebronze TDPRI Member Ad Free Member

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    Alright, thanks Clint. Maybe I'm making something out of nothing. It's an EH 6V6, so I would assume it's normal. Maybe I just need to play around with different guitars and pickups I've got, to see which sounds most like I want it to.

    Thanks for all your help guys. This was a really fun project. I made the mistake of following some steps just by doing what the picture showed, instead of stopping to think about the connections I was actually making. I hope to build another, more complicated amp soon, and with that one I'll make a point to stop and learn a little more along the way so hopefully I can do some of this troubleshooting myself next time.
     
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  18. jimgchord

    jimgchord Tele-Afflicted

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    Holy cow , 425 is hot!!! I like to run mine at around 320
     
  19. Snfoilhat

    Snfoilhat Tele-Afflicted

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    In case you didn't follow along when clintj did this calculation off-screen, he took your power tube's cathode resistor's value (nominally 470 ohms but worth measuring and writing down) and the voltage drop across that resistor. That is voltage at pin 8 - voltage at ground, 19.2 - 0 = 19.2VDC

    He then used Ohm's law, V=IR, or voltage = current * resistance, to calculate the current through the cathode resistor. All the current through the tube comes 'up' through the cathode as it flies toward the anode (or plate), and so knowing the total current through that resistor is valuable.
    19.2VDC = I A * 470 Ω
    I = 0.041 A, or 41 mA

    Ik (cathode current) = Ia (anode current) + Ig2 (screen grid current). In a 6V6GT and similar tubes, about 95% of cathode current flows to the anode, and about 5% to the screen. So estimate anode current as Ia = 0.95*Ik
    Ia =0.95*0.041 A = 0.039 A

    Anode voltage, Va, is anode supply voltage (pin 3) - cathode voltage (pin 8) = 344-19.2 = 324.8 VDC

    Idle anode dissipation = anode voltage * anode current
    = Va * Ia
    = 324.8 VDC * 0.039 A = 12.7 W

    12.7 W as a percentage of the 12 W maximum anode dissipation given on the 6V6GT data sheet is 12.7 W / 12.0 W = 106% I believe clintj and I differ by a couple percent here because there is some estimation in the calculation. No big deal.

    Were this one of the 6V6-type tubes with a maximum dissipation of 14W, then 12.7 W / 14.0W = 91%

    Robrob has a great tool on his web page to do these calculations a little quicker: https://robrobinette.com/Tube_Bias_Calculator.htm
    But it still helps to think through all the numbers even when plugging them into the calculator.

    Most of this same math and the thought process behind it can be applied to the resistors in your power supply and the preamp tube's plate load resistors, and so in a future build (especially a more complex one) you will very likely be employing it a lot, if you choose. Cheers
     
    Last edited: Apr 27, 2018
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  20. clintj

    clintj Friend of Leo's

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    That pretty much sums it up. I've got a full plate this week, so I didn't have the free time to type all that out to explain. Thanks for adding that. :)
     
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