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5G9 Trouble Shooting

Discussion in 'Shock Brother's DIY Amps' started by jmp81sc, Sep 11, 2020.

  1. Wally

    Wally Telefied Ad Free Member

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    jmp81sc, it appears to me that you have more than one problem....scratchy pots, channel interaction question, bad signal in some situationsand the trem function. The fact that Re-routing a wire or two as you report in an earlier post had a positive result might indicate a lead dress issue...or more than one. Because you have built this circuit in a chassis that is totally different from the original, you have given yourself the task of dressing leads without a known guide.
    I would lift the connection the trem from the biasing circuit....jump the bias voltage directly past that Depth control after disconnecting the trem circuit from that lug on the Depth control. This would allow you to see if that circuit is having a negative effect on the signal. If not, then resolve the ‘straight’ signal issues before bring the trem circuit back into play. I might even life the plate voltage supply to V3 so as to get the voltages completely out of that circuit’s wiring.
     
  2. jmp81sc

    jmp81sc Tele-Meister

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    Thanks Wally, will do today.
    John
     
  3. jmp81sc

    jmp81sc Tele-Meister

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    Ok per Wally's advice I took the Trem circuit out of the loop and do not have power going to the trem tube.

    The scratchy pot went away. With no guitar plugged in the amp is whisper quiet at full volume.

    Good strong guitar sounds out of both channels, however using the opposite channel volume has no effect until at full volume then it reduces the volume. This shows up on the wave form with the scope.

    If I didn't know about the tweed volume control interaction I would not think any thing is wrong with the amp.

    I have a attached a schematic with my voltage readings. I am concerned about the high plate voltage on V1. I also find it strange that V2A plate is 210 and V2B plate is 217. I do not have a very expensive meter so that could be the issue.

    thanks
    John

    tremolux_5g9_schem with voltage.gif
     
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  4. Wally

    Wally Telefied Ad Free Member

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    The triodes in V2 might not be matched. That might account for the difference in those PI plate voltages.
    Channel interaction. I find the most usable setting for my purposes is to have the off channel set anywhere from 6-8 on the dial. It is normal for the signal to weaken as the off channel volume is increased from there all the way to max.
    Can you measure the current draw? That is a factor that can be informative, ime.
    Do you have a shorting plug for the tremolo function? one can easily be made...just short the tip to the ring of a plug. This will allow you to take the trem out of the circuit. I would be curious to know if the scratchy volume pot comes back when the trem circuit is brought back to the circuit and if the scratchiness goes away when the trem is grounded out?
    Do you have a 12AY7 in V1?
    What power transformer is in use here? What are the specs? What kind of 5Y3 is in use?
     
  5. jmp81sc

    jmp81sc Tele-Meister

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    Hi Wally,
    How would I measure the current draw?

    I did make a shorting jack, so I will rewire the trem circuit into the amp.

    I usually have the unused channel volume about 1/2 way up also, but I do not hear any change in tone when doing this with this build.

    I have a new 12AX7 in V1.

    I am using a new Sovtek 5Y3.

    PT is a Classictone Spec sheet below. they describe as for a Deluxe amp but also for a Tweed Tremolux

    Thanks for your help
    John

    Screenshot_2020-09-15 40-18016 pdf.png
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Thanks for that PT sheet. That explains the higher B+.
    As for measuring current draw, I believe in some reading. https://www.aikenamps.com/the-last-word-on-biasing

    R
    ob Robinette (robrob) has some excellent guidelines on his site. For your amp, I would suggest installing some 1% tolerance 1 ohm resistors between the power tube cathodes and ground. This allows you to read the voltage across that resistor in millivolts, and this directly translates to milliamperes of current draw. This makes things very simple. This current draw may or may not have anything to do with your present situation, but in a bias vary trem that applies its variance to the power tubes, the power tube biasing can have an effect on the trem function. If the current draw is high, the trem effect can be weakened even to the point of not functioning at all.
    I do think that anyone building or working an amp should be able to determine current draw in one manner or another.
     
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  7. tubegeek

    tubegeek Friend of Leo's

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    Just to expand (amplify?) what @Wally suggested: the 1 Ohm resistor is just a special case of Ohm's law. If the R value = 1, then Ohm's Law, Voltage = Current x Resistance simplifies to Voltage = Current, which is convenient and easy.

    But you can find the current, through any resistance you care to examine, the same way - you need to know the Ohms measurement of the resistor first. Then under test conditions, measure the Voltage from end to end of the resistor.

    Finally, we can rearrange Ohm's law to an equivalent formula:

    Current = Voltage/Resistance

    So take the measured Voltage, divide by the Ohms you measured just a minute ago, and the answer is Current.
    ---
    We usually find currents inside guitar amplifiers in the range of **milli**Amps (mA) and this is nice because then we can express resistance in **KILO** Ohms and not have to play with decimal places. So for a 33KiloOhm (33KΩ) resistor, divide Volts by 33 and you will have the answer directly in milliAmps. (For an 890 Ohm resistor, say, you'd express that as .89 and divide Volts by .89 to get a milliAmps answer - that's the only time you'd need to be careful with the decimal point, if you are under 1KΩ.)
    ---
     
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  8. jmp81sc

    jmp81sc Tele-Meister

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    I adjusted the bias on the output tubes to around 60% dissipation. 20.17mA and 21.63mA. These are JJ6V6S tubes.

    I am sorry I misunderstood your current draw question, I thought you meant total current draw of the entire circuit.
     
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  9. tubegeek

    tubegeek Friend of Leo's

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    That's actually a trickier proposition because you don't usually have any resistance to measure that covers the whole amp's current draw.

    You could measure the drop across the first power supply dropper, and add on the draw of the output tubes that you have on the rectifier side of that, and get a total that way, but in any case that wasn't the question. The only other thing that would need to be accounted for would be any current idling in the fixed bias circuit (if there was one) and that can also be found by Ohm's Law. The assumption that there is no grid current being drawn from the bias circuit is OK, so just the current through the resistors present in the bias network itself.
     
  10. Wally

    Wally Telefied Ad Free Member

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    So, for 21.65ma of current draw at 410Pv, which I am taking from your schematic above, the plate dissipation is 63.4%.
    That may or may not affect the tremolo functions. Once you get the entire circuit functioning without thumps, scratchiness, and all other problems, then you can adjust that bias up and down from that point to judge how that particular circuit with those particular power tubes woks. I have seen a Custom Vibrolux Reverb in which the bias vary trem ceased to function above 62% of MOD, and I have seen another CVR in which the trem would function until that plate dissipation reached the high 60s in plate dissipation.
     
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  11. tubegeek

    tubegeek Friend of Leo's

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    That range is much tighter than I ever would have expected!
     
  12. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    You're probably a math teacher. :p:oops:
     
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  13. jmp81sc

    jmp81sc Tele-Meister

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    Thanks again for all of your help. The trem was working fine before I took it out of the circuit.

    The problem with the amp appears to me to be related to the interaction of the volume controls. With the trem circuit removed I am not getting the typical volume control interaction typical of the tweed amps with 4 inputs.

    It is possible that the volume or tone pots were damaged. I isolated the volume pots and measured the total resistance and the sweep through the pot range they seem fine.

    I will add the trem back in to the circuit and see how it reacts now.

    I will keep looking and testing.

    John
     
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  14. tubegeek

    tubegeek Friend of Leo's

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    Is it possible that you have one or another of the shorting jacks/grid stoppers etc., in the "4 hole" input circuit mis-wired somehow? In a way that would affect the "off" channel's interaction with the "on" channel? There are a lot of connections to get right and mistakes in that section aren't unheard of. Also see if the two tone caps .0005 and .005 measure close to spec.

    These are just guesses based on the schematic, I have no experience with this problem myself.
     
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  15. D'tar

    D'tar Friend of Leo's

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    .
     
  16. Wally

    Wally Telefied Ad Free Member

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    Jmp81sc, what are you expecting from the interaction of those two channels?
     
  17. jmp81sc

    jmp81sc Tele-Meister

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    I just played my 5F6a Boothill kit I put together some time ago.

    I hear the volume go up slightly and some of the character of the other channel come into to mix. So if i'm in the bright channel and turn up the normal channel it takes some of the brightness out, If i'm in the normal channel the bright volume adds some treble in.

    The 5G9 build drops volume slightly when the opposite channel is turned up and the overall signal gets muddier and I don't really hear much tone difference.

    Input jack wiring appears to be ok, I have cleaned up some of the wiring and checked it carefully, all inputs work as they should, and if I jumper the channel inputs I get as expected results. The tone caps were replaced with new caps that I tested with a capacitor checker, they are good.

    I have wired the trem circuit back in. The crackling is much better, the trem is very good, if the unused channel volume is at 0. If I turn up the unused volume the trem gets very week.

    The amp is very usable as is, it just bugs me that the volume controls aren't working as they should. When I had the amp in the old chassis the volume controls worked as they should, and I could definitely hear the interaction between the volume controls.

    Could it be heat damage to the volume pot wipers when I soldered them up? I just watched the Dr. Z video on soldering technique and he states that the pot should be full up or down when soldering. I am not sure if I did that or not.

    thanks
    John
     
  18. D'tar

    D'tar Friend of Leo's

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    Does tone pot work as expected. The unused "inputs" should shunt to ground. The interaction comes from the volume/tone loop created as you move the wipers changing the load on the used triode. Check your tone and bright caps perhaps.
     
  19. jmp81sc

    jmp81sc Tele-Meister

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    Ok I think I found the problem. I did not have a footswitch installed during testing. I was playing the amp and just turning the trem on and off with speed and depth pots.

    I plugged in a foot pedal switch and the trem stays strong with the both volumes turned up. Trem turns on and off with footswitch like it should.

    I am still not hearing any interaction between the channel volumes. I'm going to listen to some 5G9 and 5F6a demos to see if my hearing is just shot, or if I still have something going on.

    Thanks Everyone for your help.
    John
     
  20. jmp81sc

    jmp81sc Tele-Meister

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    I don't understand why plugging a foot switch in would change anything. Can anyone explain this to me?

    With the switch open no trem signal goes to ground, which is what happens without a plug inserted.

    With the switch closed the signal goes to ground and the trem is turned "off", same as a shorting jack.

    The only difference I see is whatever resistance occurs due to the foot switch cable length.
     
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