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Help with High Voltage in Blackface Vibro Champ

Discussion in 'Amp Central Station' started by Josey, Jun 27, 2018.

  1. Josey

    Josey Tele-Meister

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    Wally I thought 12 Watts was 100% max dissipation for a 6V6GT and 14 Watts was 100% for the 6V6GTA?? I've never really understood if there's actually a difference between the two or not? I think I remember someone saying all NOS 6V6s are rated for 14 watts max and the only reason there's a 2 watt difference had something to do with a change in the way they started labeling the design specifications for tubes back in the day. I decided to go with 12 Watts Max for the NOS RCA 6V6GT I'm using just to play it safe, but from what you're saying it sounds like I could at least bump that up to 14 Watts Max dissipation.

    I did go through last night and measured every 5Y3 rectifier tube that I have. Ended up that it was a Magnavox labeled NOS Sylvania 5Y3GT that brought down the 6V6 readings the most resulting in a Plate Voltage of 376 volts and a Cathode Current of 30.7 mA which is right at 11.5 Watts Max. I do have a 470 ohm 5 Watt wire wound resistor. I'll swap out the 1K cathode resistor with it and report back what that does to things.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Josey, all I know is what I have read. There is a design maximum rating and there is an absolute max rating. Most tube cognoscenti consider 6v6’s from the late ‘50’s on to be 14 watt tubes.
    At any rate, it takes something way beyond 14 watts of plate dissipation to redplate one of them in a cathode biased circuit, ime.....way beyond.
    Fwiw, in a cathode biased amp, the tube is working its hardest at idle. When signal is applied to the control grid, the plate dissipation figure decreases. It is just the opposite in fixed biasing.....applied signal increases the current draw above the idling situation.
    Again....I have never seen a Fender Champ type amp that was operating as low 100% of MPD. It is always more..usually above 16 watts. The tube sets that biasing...unless one changes the resistor. Only in the last couple of years have I seen mention of people running higher resistances to try to get that dissipation down. I already know what that sounds like because I did the experiment....out of curiosity when I observed my SF Champ sound better than any .champ I had ever heard...including the BF Champ my poor buddy had beside it. I measured the two amps. His amp, which sounded very good but not as lively as mine, was dissipating right at 18 watts. My Sf was about 20 watts. As I noted....it had been running that way for about forty years and was going strong the last time I talked to the young fellow who received it as an engagement present from his fiancé.
    Based on what I heard from that SF .champ when I put an 820 ohm bias resistor in it and read 14.7 watts of plate dissipation, I would hazard a guess that IFLeo Fender put the Champ out with such biasing, we would all be playing little solid state amps and saving our money by not needing a higher dollar tube amp. (;^)
    Leo’s little Champ used that 470ohm resistor from the late ‘40’s until the end of the production in 1982....and surely the reissues use the same resistance???? That said, if a person wants a little tube amp that sounds like a sterile solid state Fender Frontman, neuter the little amp with a big bias resistance number. For my money, I want a Champ!!!! Or actually, I want my 1964 Rickenbacker B9A...solid copper chassis and an OEM Jensen C12Q!!! Single 6V6 and trem....best amp of its type I have owned. A C12Q ina 5 watt amp. Leo didn’t even use a Q in the 25 watt DR. Rickenbacker put some money into those amps.
     
    Last edited: Jun 28, 2018
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  3. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    Duh, I wasn't even thinking about the 5y3 tube... I was just thinking of changing rectifier tubes to drop the B+.

    I wouldn't run a 5u4 either Wally. But, as you describe on solid state rectifiers, a copper cap doesn't draw the current like a rectifier tube. The nice thing about the CC rectifiers, is they are designed to react like a tube rectifier... A little sag here and there. As you know, Fender used something similar in it's first reissue Bassman and built it into the circuit of the Vibroverb you and I have had so many conversations about.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Axis, I don't see anything except solid state rectification in those two amps. Where is the 'emulation' of a tube rectifier?
     
  5. tweedman2001

    tweedman2001 Tele-Afflicted

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    I had a first year reissue bassman which had a sst rectifier which plugged into the tube rectifier socket. I don't think it had anything other than diodes in it. If you wanted sag you could plug in a tube rectifier. I think the Weber copper caps have other components beside diodes to simulate sag. I never had the Vibroverb RI but I do have the CVR first year in white which is based on the VVRI. One of these days I will do the Moyer mods and either install a Weber CC or replace the transformer and install a tube rectifier. But I don't think stock it has any components for sag either unless I am missing something here.
     
  6. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    @Wally - I probably fell for the marketing hype! LOL I was under the impression they had done more than just SS rectification in the VVRI. Not the first time I was wrong. The Bassman has a tube socket with a plug in SS rectifier (I so wish they had done this with the VVRI as well). I also thought it had more than just a few diodes. Oh well....

    @tweedman2001 - The CC's need a tube socket, which neither of our amps (VVRI and CVR) have. So, a Copper Cap won't help.
     
  7. Wally

    Wally Telefied Ad Free Member

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    I don’t know, Axis, what they did other than I see diodes and no resistors in the rectifier positions when I look at the schematics. However, if there is a socket with a SS plug in rectifier, one can run any solid state emulator that one wants as long as voltages and bias are understood. One could run the GZ34 emulator..?WZ34?...or a 5U4 or even a 5R4 emulator...whatever the designation for those might be. The GZ34 would drop voltage from the ‘straight’ solid state and a 5u4 emulator would drop even more voltage.
     
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  8. tweedman2001

    tweedman2001 Tele-Afflicted

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    @tweedman2001 - The CC's need a tube socket, which neither of our amps (VVRI and CVR) have. So, a Copper Cap won't help.[/QUOTE]

    There is a copper cap module that does not require a socket. Also it would be easy to install a socket without changing the transformer as the cc tube version does not require the 5 volt filament wiring. Of course you would not be able to install a real tube rectifier without changing the transformer too. :)

    https://www.tedweber.com/wbr
     
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  9. Josey

    Josey Tele-Meister

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    Wally, last night I pulled the 1K cathode resistor and temporarily installed a 470 with some jumper clips and the results were just as you said. Plate dissipation is now right at 17 Watts (Plate Voltage = 33.7mV & Cathode Current = 50.2 mA) and the B+ voltages are roughly 20 Volts from schematic values (B+1 = 378 Vdc, B+2 = 367 Vdc, & B+3 = 340 Vdc). Even the voltages in the pre-amp stage of V1 as well as V2 were right in line with the spec. This was also using the modern AC wall supply voltage which like I mentioned earlier is usually about 121 Vac. I'd be willing to bet if I used my Brown Box to drop the AC supply to 110 Vac or so, I'd be right on top of the schematic values. This also gave the amp a much fuller sound and a sweeter distortion as the signal was increased. I'm thinking I might go back permanently with a 470 ohm 2 Watt Carbon Comp similar to the dropping resistors that I installed. Is there a resistor material that is favored in this portion of the circuit more so than any another?

    Also while I was in there last night I checked out all of the 1/2 Watt resistors and found some that have drifted outside the tolerance range. One I found was nowhere near the spec value. The 2700 ohm resistor that is last in line leading to the speaker output was reading in the 50 ohm range. Kinda wondering if that one was just color coded incorrectly back in the day and no one ever caught it. What kind of change might I expect if I replace it with the proper value? I'm also getting some readings that might possibly be the sign of leaking tone and bypass caps. I'd like to replace these drifted resistors first and take some new readings before I bring those questions to the table. Stay tuned and thanks for all the help so far.:)

    Also I've read were others have added a screen-grid resistor across pins 4 & 6 of the 6V6 in their Vibro Champ, the same as Fender did in their larger amps. Is their actually any value in doing this or is it unnecessary for this build?
     
    Last edited: Jun 29, 2018
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  10. Wally

    Wally Telefied Ad Free Member

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    Leo knew what he was doing, didn't he???
    RE: out of spec resistors..... I would replace any and all resistors that were seriously out of the range that was spec'd. One would want to make sure that one was reading the resistance of just one particular resistor. That might necessitate lifting one end form the circuit. That 2700ohm resistor is the negative feedback loop resistance. IF it really is only a 50 ohm resistor now, it would cause the amp to be loosing a fair amount of the signal due to phase cancellation. You may be reading something other than just that resistor if you are reading it in circuit. A 2 watt bias resistor should suffice.
    IF you still have the original electrolytics on the board, imho those need to be replaced. I have found that fresh e-caps plus good general service solve a lot of problems.
     
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  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    On the resistor, I've never seen a resistor drift low - they always go high, so there is likely something else going on with that too.
     
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  12. Josey

    Josey Tele-Meister

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    I was shocked as well. I will lift a leg as Wally suggested and see what it measures but I don’t know what I could be doing any different from the others I measured?
     
  13. peteb

    peteb Friend of Leo's

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    At this point, a full voltage chart would be usefull.

    Voltages at each pin for all three tubes.
     
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  14. Josey

    Josey Tele-Meister

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    Wall Supply Voltage = 121 Vac

    Cathode Resistor = 465 ohm
    Voltage Drop Across = 22.3 Vdc

    B+1 = 376 Vdc
    B+2 = 366 Vdc
    B+3 = 338 Vdc

    NOS Sylvania 5Y3GT
    Pins 2 & 8 = 4.9 Vac
    Pin 4 = 354 Vac
    Pin 6 = 358 Vac

    NOS RCA 6V6GT
    Pin 2 = 6.5 Vac
    Pin 3 = 364 Vdc
    Pin 4 = 366 Vdc
    Pin 8 = 22 Vdc

    V2 NOS 12AX7
    Pin 1 & 7 = 165 Vdc
    Pin 3 = 1.78 Vdc
    Pin 4 & 5 = 6.5 Vac
    Pin 6 = 366 Vdc
    Pin 8 = 183 Vdc

    V1 NOS 12AX7
    Pin 1 = 211 Vdc
    Pin 3 = 1.91 Vdc
    Pin 4 & 5 = 6.5 Vac
    Pin 6 = 210 Vdc
    Pin 8 = 1.80 Vdc
     
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  15. Josey

    Josey Tele-Meister

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    Here you go King Fan. Let me know if anything jumps out at you that I may need to address.


    I went ahead and used the 5 Watt 470 Ohm Wire Wound resistor I had on hand for the Cathode Resistor. I may go back later and swap this for a 2 Watt Carbon Comp someday but for now I just want to get this amp back up and running. Wally I pulled a leg of that feedback resistor as you suggested and it reads within 3% of 2700 Ohms. Wired it back into the circuit and it reads in the 50’s. I’m no EE so I won’t pretend to know why it does that. Why can the other resistors be measured in place? After double checking all the others again the only one I found out of spec was 1500 ohm on the far right side of the board that leads to ground off of pin 3 of V1. It was reading in the 1700’s. I may just go ahead and replace it along with the other 1500 ohm off of pin 8 of V1 just for safe measure. It was on the high side of being within tolerance as well.

    So what do you guys think? Now that you can see all my readings in my previous post is there anything of concern that I need to address? Anything else I need to measure for you guys?
     
    Last edited: Jun 29, 2018
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  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    1700 vs 1500 on a cathode resistor wouldn't bother me at all unless you really wanted to replace it.

    See if you can scoot over the bypass cap away from that power resistor a bit. That sucker will generate heat and many Champs have cooked bypass capacitors because of that.

    No need to bother going with the 2W - that 5W you put in will last forever.

    From a quick visual inspection everything looks good.

    As far as the resistor, look at it in the circuit. That negative feedback resistor goes across the entire output stage, so there is going to be other resistances in play. Sometimes those just don't measure what you expect in circuit.
     
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  17. peteb

    peteb Friend of Leo's

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    Voltages look good


    Thanks
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks for the pics. Love the vintage caps and resistors -- looks good.

    You asked above if there's a resistor type for the power resistor that 'is favored.' Although I love CC resistors for their authenticity in this simple circuit, 'authenticity' will include several downsides in the power resistor. I agree the wirewound resistor is just about ideal; they're quiet, stable, and extremely robust. I have CC dropping resistors in mine -- but I don't have a CC power resistor. If you can move its bypass cap further right, as Corliss1 suggested, your amp oughta be about perfect.

    BTW, I'm glad you went with Wally's advice and went back to stock on that 470 (instead of a 1M?). Increasing the 6V6 cathode resistance in cathode bias amps is a popular way to try to cool bias, but it has the unintended consequence of actually increasing effective B+ -- especially not a good thing in a VibroChamp. Your description of the sonic changes you got is great.
     
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  19. Josey

    Josey Tele-Meister

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    Thanks for all the help guys! It’s encouraging to know I’m on the right track. I’ll leave that 1500 ohm cathode resistor alone for now since it doesn’t seem to be that much of a concern. Which brings me to my last question on components...

    I don’t have the proper equipment to test whether or not my capacitors are are actually good or bad. I read somewhere else that a way to check for a bad or leaking cap would be to take a voltage reading on the negative side and see if it’s allowing anything to pass through. I don’t know if this applies to bypass caps as well as tone caps or not. I’ve taken some readings and posting the results hoping you guys might could tell me if I need to replace something here.

    Moving from right to left:

    25uF/25V Bypass Cap = 0 Vdc
    10uF/25V Bypass Cap = 55 mVdc
    25uF/25V Bypass Cap = 0 Vdc
    50uF/50V Bias Cap = 0 Vdc

    .1uF Bass Tone Cap = 3 mVdc (up to 30 mVdc when rolling up bass control)
    .047uF Mids Tone Cap = 3 mVdc
    250pF Treble Tone Cap = 30 mVdc
    .02uF Ceramic Disk Cap = 1.1 Vdc

    I couldn’t get an accurate reading on the tremolo caps. As you see the readings are very small but are any of these a concern? Just trying to learn as much as I can.
     
  20. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    You'd have to lift the caps from ground in order to take that measurement. Anything that small wouldn't worry me at all.
     
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