Stew Mac 5F1 Kit Voltages

Discussion in 'Shock Brother's DIY Amps' started by freddarl82, Apr 18, 2019.

  1. freddarl82

    freddarl82 TDPRI Member

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    Long time member, rare poster here. I've gained a lot of great information here. Recently completed my first ever full build of a Tweed Champ 5F1 using the Stew Mac kit. Overall great kit, but also some great tips I found on here, too (like standards to properly ground the main power cord ground to the chassis).

    The amp sounds great. Beautiful cleans at lower volumes and that special tweed Champ distortion when cranked. Really quiet, too, which I wasn't fully expecting on my first build.

    These forums and robrob's site have really driven my interest. I've already done the first few mods: treble bleed on volume control, 4.7uf bypass cap in V1 (didn't like 25uf), and swapped out the .022 Orange Drop coupling caps with Sozos. Curiosity next pointed me to digging in and measuring values to check tube bias. That's where things got weird...

    I've tried both the "Tube Dissipation Using Cathode Resistor Drop" and "Tube Dissipation Using Plate Current" methods, both with manual calculations and robrob's calculator. The numbers all agree and come out REALLY high: like 42 DC milliamps plate current and 125% plate dissipation. Here's some specific numbers:

    B+1 388 (this is the OT center tap voltage, right?)
    B+2 334
    B+3 289

    Plate voltage 375 (pin 3 on a 6V6, right?)
    This would mean I am getting a 13 volt drop (388-375=13)

    Transformer resistance (pin 3 to OT center tap): my meter reads .309K ohms, so this should be 309 ohms. Seems high, Rob's 5E3 resistance was 70-80 ohms.

    plate current = voltage drop/OT resistance, 13/309=.04207 amps, or 42.07 milliamps

    ===========================================================
    Moving over to the Cathode Resistor Drop method, I come up with the following:

    Cathode Resistor 463 ohms (as measured, rated as a 475 ohm resistor)
    Voltage drop across resistor 19.7 VDC
    Plate to cathode voltage 356 VDC (pin 3 to pin 8)

    I plug these numbers into rob's calculator and it yields 42.5 milliamp cathode current and 40.2 milliamps plate current, pretty consistent with the calculations above. Problem is, the plate dissipation per tube (=plate current * plate voltage) works out to .04207 * 356 = 14.97 watts. For a 12-watt rated 6V6, this would be a plate dissipation percentage of 125%!

    BTW, I also have a bias meter -- the type with an in-line octal plug that plugs into the amp's socket, then the tube plugs into the meter's socket. It reads about 42 milliamps.

    ===============================================================


    So, am I misusing the numbers in the formulas, or, wholly crap, this thing is running really high? I have not observed any red-plating on the tubes. One reason I am concerned (along with others) is I have some nice vintage tubes in there (a 1960's Phillips 5Y3 and a 1940's Raytheon 6V6). I don't want to fry those things after 20 hours. I have plugged other tubes in (the kit-included JJ's, a vintage Tung Sol 5Y3, modern production Tung Sol 6V6) and the voltages appear in the same ballpark (withing 10-15 volts), so no major differences.

    I have read in other threads one can run a resistor between the rectifier and the board to drop the voltage down (i.e. pin 8 of rectifier to first filter cap B+1). Is this an option? Or desireable?

    I'm really wondering what I should do here. Past threads confuse me...in one, posters are concerned a B+1 voltage of 367 is a concern, in another robrob said B+1 in a Tweed Champ should be about 390.

    What say the amp wizards who reside here?

    Thanks so much for any assistance!
     
  2. jimgchord

    jimgchord Tele-Holic

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    Definitely high, but that's what you get these days from kits, throw a newJJ 6v6 in and call it a day.
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Seems fine to me. Check the dissipation on a black/silverface Champ someday for a real eye opener.

    Really though, if it sounds good and the tube you are using doesn't redplate in the application, the amp is running properly.
     
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  4. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    It is probably okay. But you could try like a 510 ohm cathode resistor and see what sounds better to you. I’m no expert but I think what you are seeing isn’t far from typical.
     
  5. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I use 500R cathode resistor on mine. If you are concerned, you can get them for next to nothing on Amazon or eBay. Mine uses lower voltage PT at 340v B+. 75% with a 5y3 and 95% with a gz34. Perfect for me either way.

    Sent from my XT1585 using Tapatalk
     
  6. gusfinley

    gusfinley Tele-Holic

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    What is your line voltage?

    I've seen EL84 plate voltage vary from 390 to 425 within the same day due to variations in Line Voltage.

    A few volts over the intended line voltage will make a large change in plate voltage.

    Is Stew Mac using Mojotone ( Heyboer ) transformers?

    Some transformer manufacturers are stuck in yesteryear making transformers for 110 and 115 line voltages. Throw 125 VAC into those and you get some really high voltages!

    If you're a few volts too high, you could build yourself a Voltage "Browner"
     
  7. D'tar

    D'tar Tele-Afflicted

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  8. Linkjr

    Linkjr Tele-Meister

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    Mine was also running high and not hearing a real 5f1 live I didn't think much of it until I also got red plating, i recommend diode method to drop voltage it improved my tone immensely
    Make sure they are the right way around and long leads.
     
  9. freddarl82

    freddarl82 TDPRI Member

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    Thanks to all for the replies. So, it sounds to me like a number of things conspiring to lead to this situation: the original design was a bit “out on the edge” to begin with, higher wall voltages (mine consistently measures 121.7), and the “vintage spec” Heyboer transformer not designed to deal with it. As far as my vintage tubes go, I bought them from reputable eBay tube sellers and they are all “Tests as NOS,” not true NOS, and in most cases, I didn’t pay much more than the cost of new production tubes. So, for now I think I will leave it as is and enjoy it. If it starts eating tubes at an unacceptable rate I will revisit some of the solutions offered here.

    BTW, I do own another champ. Bought it as a completed chassis from a DIY’er. It’s a Blackface circuit in a Silverface chassis and includes a 3-position switch to select various gain levels. I may have to check that one out of curiosity. I have had no tube issues with that one.

    Thanks again for the replies! I am still open to further input/suggestions in case I need to revisit this in the future.
     
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  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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    If you end up changing the power transformer at any point, Hammond has one with a 275v tap on it that nails the 340v vintage schematic and every point across the board.

    Sent from my XT1585 using Tapatalk
     
  11. monkeybanana

    monkeybanana Tele-Meister

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    I had the same issue with my first Mojo kit which I think is the same one with a Heyboer PT (I think it was meant for a Blackface kit). A quick search reveals we're not the only ones. I rebiased the amp first but I didn't like how hot the chassis was getting. I burnt out two pilot lights with the heat. I ended up buying a new transformer. Both Hammond and Mercury have ones with lower windings and keep the the much cooler even after hours.
     
  12. freddarl82

    freddarl82 TDPRI Member

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    I am very intrigued by this solution, especially because of the “Cheap, easy, effective” part. I am trying really hard not to be dense here, but I have a question when referring to robrob’s instructions. The first part I get, solder the diode’s striped end to an eyelet and bolt it to the chassis ground. The other end is attached to the OT’s center tap, but how? I am guessing it would attach at the board eyelet where the B+1 16 uf filter capacitor and OT center tap are attached? Is that how it is most easily/efficiently accomplished? So the diode(s) are taking off “some” of the B+ voltage and draining it to ground?

    Also, I found some zener diodes that are 12V/5W. I assume they will work fine but perhaps give a slightly different drop than the 8V drop per diode in rob’s article?
     
  13. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    The center tap of the PT high voltage winding is likely going to ground, correct? Usually a red/yellow wire in Fender amp land.

    So, you'd disconnect that from ground and connect that to the non-striped end of the diode. This effectively "raises" the ground potential 12V (with your example part), lowering the high voltage supply by that much. You can run a bunch of zeners in series to lower the voltage even more.
     
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  14. freddarl82

    freddarl82 TDPRI Member

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    Duh, my bad...for some reason, I kept reading the instructions as the output transformer center tap. Thanks to your input, I went back to re-read it and finally noticed it’s the POWER TRANSFORMER center tap. Big difference. That makes sense. Thanks so much for straightening me out.
     
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  15. freddarl82

    freddarl82 TDPRI Member

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    To follow-up with the “solution...”

    I used two of the 12V/5W diodes in series. It dropped my B+1 from 388V down to 361.5V. I now have a plate voltage drop of 11.1V, and I am now running at about 99% plate dissipation. Better than 125%!

    I plan to stick with this for awhile and see how it goes.
     
  16. gusfinley

    gusfinley Tele-Holic

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    Glad you got it under control!
     
  17. jhundt

    jhundt Doctor of Teleocity

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    another case of worrying about the numbers. Why do some people get so hung up on numbers? It the amp works and sounds good, what difference do those numbers make? There are millions of single-end cathode-bias amps out there running quite happily at 125%. They were designed to run this way, and to sound great. Since the 1950's. And now you're gonna come along and solve the whole problem and fix Mr. Fender's faulty design?
     
    corliss1 likes this.
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