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BF Vibro Champ issues

Discussion in 'Glowing Bottle Tube Amp Forum' started by Stan Martin, Sep 24, 2019.

  1. Stan Martin

    Stan Martin Tele-Afflicted Silver Supporter

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    I'm working on a "64 VC that sounded terrible when it came in, so I replaced all the leaky electrolytic caps, the bias cap has been changed to a 25uf 50V and the 470ohm bias resistor has been changed to a 1K as the 6V6 was drawing 49mA and according to my math 19w dissipation (ouch), now it's closer to 9w dissipation. I replaced the .02 coupling cap as that was not even close to spec as well as a few resistors that have drifted upwards more than 30%. So now that the amp is electrically stable and it still sounds like junk. Very raspy and no volume between 2-4. The amp does have a Weber 8SigF 4 ohm speaker, which is reading 5.1 ohms. I would think it should read lower than 4 ohms. Tubes have been sub'd with zero effect. I do not have another 8" speaker to sub. Is there something I may have missed?

    I appreciate any input.
     
  2. Whatizitman

    Whatizitman Friend of Leo's

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    Define raspy and no volume.

    Are all the pin voltages/amps within spec? Plenty of folks here will tell you to list voltages first thing.

    I'm certainly no expert. But I think it would hard to tell where to begin checking since it reads like you changed a lot of components toward spec. I would be inclined to first change the cap can and adjust bias, and then see if how it works/sounds. Then go back and check other components for drift, etc... An old tube amp can sound pretty good even with some drift on coupling caps and resisters. Those should be the last thing to change, if at all.

    5ohms shouldn't matter much. But it may be time to check voltages/impedance on the PT and OT. That's when things start to get weird. Another good reason to do, uh, "test driven development". I'd want to rule out everything else before having to deal with transformers. My .02.
     
  3. Stan Martin

    Stan Martin Tele-Afflicted Silver Supporter

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    I agree old amps sound good with component drift, however the amp needed leaky bypass caps to be replaced as well as the bypass cap resistors that are reading 3.1k instead of 1.5k. A 68k resistor reading 91k is considerable drift ,also a .02 coupling cap reading <.01 should go.

    all voltages are within spec compared to VC '64 schematic.
    no volume between 2-4 then volume kicks in about 5. The sound between 5-10 gets progressively raspy as you turn up.

    I'm not one for component swapping,but this amp was terrible to begin with,so replacing a few components that are >30% out of tolerance seemed like a pragmatic step.
     
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  4. Whatizitman

    Whatizitman Friend of Leo's

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    Raspy as in distorted? Folks will pay quite a bit for VC that breaks up between 5-10. Have you tried it with another speaker? Maybe the cone is bad. Is it buzzy/fuzzy? Any other weird noises?

    No volume as in no sound between 2-4? Have you cleaned or changed the pots, checked the solder/resistor joints at the pots?

    Everything spec on the pre amp tubes/pins?
     
  5. Commodore 64

    Commodore 64 Friend of Leo's

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  6. peteb

    peteb Friend of Leo's

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    those are all DC voltages. It looks like the DC power supply is good and the problem is AC, the signal, and the problem should be in the signal path.



    Is it a bad volume pot? or is it a weak, distorted signal? Bad connection in the signal path?
     
  7. Stan Martin

    Stan Martin Tele-Afflicted Silver Supporter

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    volume pot is good. okay I can take a hint:oops: so here are the tube voltages:

    V1:12AX7
    1)242V
    2) 0V
    3)1.9V
    4+5)6.84
    6)238
    7)0
    8)1.9V
    9) GROUND

    V2:12AX7 VIBRATO FS PLUGGED IN
    1)178V
    2) 0V
    3)1.9V
    4+5)6.84
    6)409V
    7)179V
    8)184V
    9) GROUND

    V3:6V6
    1)NC
    2)6.84V
    3)419V
    4)411V
    5)0V
    6)0V
    7)GROUND
    8)30V

    V4:5Y3
    2)419V
    4)356V AC
    5)356V AC
    8)420V
     
  8. Stan Martin

    Stan Martin Tele-Afflicted Silver Supporter

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    looking at the schematic there is a 330(?)pf cap missing between pin5 and pi8 for the 6V6
     
  9. peteb

    peteb Friend of Leo's

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    that is a later addition to the circuit in the silverface era.




    the voltages look fine
     
  10. Commodore 64

    Commodore 64 Friend of Leo's

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    How are you measuring cathode current?

    I like the output transformer resistance method: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm


    30V across 1k ohms cathode resistor would suggest 30mA in the cathode. At 390 plate voltage, that would be about 11.7W in the cathode. So about 10.7 w plate dissipation. Should put you round 90% bias with a 12W tube, you didn't say what tube you were using.

    Doubt its the speaker, but you could plug any 4 ohm or even 8 ohm speaker in there to test it. Yes, I know it's nominally supposed to be 3.2 ohms load for the OT in there.
     
    Last edited: Sep 24, 2019
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  11. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I may have missed it but did you replace the cap can? Also how do we know the pot or pots are good?


    Sent from my iPad using Tapatalk
     
  12. Wally

    Wally Telefied Ad Free Member

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    Put the 470 ohm resistor back in the bias circuit. A single ended amp is not designed to run at that 64% of max plate dissipation. In fact, it is common to find these Fender single ended amps running at much above 100%....and that is where they sound good. Ime, and I have done the experimenting, anything below 16 watts of plate dissipation starts to sound cold, raspy...unpleasant to my ears.
     
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  13. schmee

    schmee Doctor of Teleocity

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    define "no volume from 2-4"...? If really no volume you have a bad volume pot.
     
  14. Stan Martin

    Stan Martin Tele-Afflicted Silver Supporter

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    no sound between 2-4 then about 5 it starts to kick in. I replaced the original 1M volume pot and the new pot works the same as the old one. On 10 the amp sounds like it probably should on 4 or 5. My assumption is something is loading down the signal,however I do not have a scope to determine where that would be happening. The raspyness is gone as I put back the 470ohm bias resistor (Thanks Wally)
     
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  15. Dacious

    Dacious Poster Extraordinaire

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    +1. My two SF VCs ran the power tubes very hard, well over 100% dissipation and both were 30-40 volts over spec for B+. A class A amp should be running 100% dissipation at idle. That is the definition.

    68 and 78 both almost identical B+ on 110 volt stepdown with new cap can on 68 (in fact most board components replaced as it was gutted when I got it).
     
  16. Wally

    Wally Telefied Ad Free Member

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    One cannot discount the possibility that the OT is shot. If you had a signal tracer you might find where the signal is ‘disappearing’.
     
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  17. muchxs

    muchxs Doctor of Teleocity

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    Typical Champ / Vibro Champ dumbness is to run them without the speaker connected. There is no transformer protection like there is on larger Fender amps.

    It's a '64. What are the chances it hasn't been subjected to inconceivably über stupid treatment at some point during its long life?

    It's a rhetorical question.


    If it spent 20- 30 years sitting in a closet the coupling caps have become resistors. Replace 'em.
     
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  18. peteb

    peteb Friend of Leo's

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    You can still do some signal checking without a scope. Connect your meter to the grid of the power tube, set it to AC, and play guitar. A large signal here should be around 10-12 VAC, and I am assuming your bias is now between 20 and 25 volts.



    If you can’t get the signal here up to 10 VAC, it’s low.




    It would take about 5 minutes.






    I’ve done the 1K cathode resistor mod on a champ and got 11W, 29 VDC and 29 mA very similar to the numbers above. It either sounded the same as the stock bias or similar but it did not ruin the sound and make it unplayable. That’s a common mod, I don’t think it’s the problem, but maybe in conjunction with another problem like low signal it could make a difference. I thought the vibrio champ in question sounded bad with the stock bias and then with the cool bias. I did the mod but I changed it back to stock mostly because I didn’t want to miss out on anything, even though I wasn’t exactly sure what I was hearing or not hearing. I think they should run anywhere between 40-50 mA and 16-20 W depending on the rectifier tube and the power tube.
     
    Last edited: Sep 25, 2019
  19. muchxs

    muchxs Doctor of Teleocity

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    Seriously, your coupling caps are probably spanked.
     
  20. Stan Martin

    Stan Martin Tele-Afflicted Silver Supporter

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    With the rectifier and power tube pulled
    OT Primary 23 ohms measured between the primary leads (red & blue), secondary measures 5 ohm, measured between yellow & black . What should the OT be reading ?
     
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