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  1. #36
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    Quote Originally Posted by nosaj View Post
    Don't underestimate the resistance of a bad solder joint.

    nosaj

    Just thinking out loud...using a scope at the input then input of 12ax7 and so on. You could see the signal size increase and at some point see it choke out.
    The amp works fine and sounds fine. It was purposely built for 1 watt output but the new owner (my buddy) now wants full output.

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    Quote Originally Posted by nosaj View Post
    And the volume pot?
    nosaj
    The volume pot is 1 Meg audio taper, switched

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  3. #38
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Tone Meister View Post
    The amp works fine and sounds fine. It was purposely built for 1 watt output but the new owner (my buddy) now wants full output.
    Scope might make it faster and easier to see the signal changes throughout the amp. IT's being choked or throttled at 1 watt output somewhere
    nosaj

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    Quote Originally Posted by nosaj View Post
    Scope might make it faster and easier to see the signal changes throughout the amp. IT's being choked or throttled at 1 watt output somewhere
    nosaj
    https://music-electronics-forum.com/...ad.php?t=48333

    I don't have a scope at present. Mine was killed over the summer. Got one incoming, but not today.

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  5. #40
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Tone Meister View Post
    https://music-electronics-forum.com/...ad.php?t=48333

    I don't have a scope at present. Mine was killed over the summer. Got one incoming, but not today.
    And the value of the input resistors?
    nosaj

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    Supporting Member The Dude's Avatar
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    In the first picture in post #20, there looks to be a 100 ohm resistor (about middle of pic) going to a pin of the 5Y3. Can you tell us if it is indeed 100 ohm and where it actually goes.

    Edit: Or maybe it just looks like 100 ohm in the pic and it's actually a 10K?

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    Last edited by The Dude; 01-30-2019 at 03:05 AM.
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    Quote Originally Posted by The Dude View Post
    In the first picture in post #20, there looks to be a 100 ohm resistor (about middle of pic) going to a pin of the 5Y3. Can you tell us if it is indeed 100 ohm and where it actually goes.

    Edit: Or maybe it just looks like 100 ohm in the pic and it's actually the 10K?
    That is a 10K dropping resistor and it goes from the 5Y3 to the 6V6 screen

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  8. #43
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    Quote Originally Posted by nosaj View Post
    And the value of the input resistors?
    nosaj
    Both are 68K. Blue, Gray, Orange

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  9. #44
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Tone Meister View Post
    Both are 68K. Blue, Gray, Orange

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    Ok I'm counting 4 resistors out of 12 or so that have been measured. Almost halfway there, might as well start measuring them all and making your own schematic for this one.

    nosaj

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  10. #45
    Supporting Member mozz's Avatar
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    Leadfree solder + somebody who can't solder = mess.

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    Supporting Member The Dude's Avatar
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    I'm not as put off by the soldering as I am the wiring mess and "layout" (used loosely).

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  12. #47
    Don't forget the joker g1's Avatar
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    I see one resistor there that was almost buried in heatshrink, which leaves the possibility that there are others completely covered.
    I'm tending toward nosaj' suggestion of an attenuator there somewhere limiting the pre-amp signal prior to the 6V6 grid. Like a fixed PPIMV without the PI.
    But I haven't checked into the OT yet, has anyone? LT mentioned checking on the primary impedance. Could a massive mismatch drop the power this much?

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    Supporting Member The Dude's Avatar
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    The PT from post one is a Champ replacement, so no problem there. No part number given for the OT, so not sure. IMO, it would have to be an extremely gross mismatch to drop 5 watts to 1. It seems unlikely you would use a 6V6 and then some off the wall OT to drop wattage rather than just build a 1W amp with some smaller output tube and OT configuration. But, anything is possible.

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    The OT is a Mercury FTCHO-5 which is a direct 5 watt Tweed F51 replacement

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    Remove any lumpy heatshrink, there are resistors in there, who knows what else.

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    Quote Originally Posted by g1 View Post
    Remove any lumpy heatshrink, there are resistors in there, who knows what else.
    There it is, someone had to be THAT guy! Reckon that'll be my next move, just before I order an eyelet board.

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  17. #52
    Bent Member Chuck H's Avatar
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    Well it's a mess to try and diagnose by the photos for sure. But I can understand why you'd want to know what the wattage dropping mechanism is before rebuilding it. Because there's always that chance that your friend is going to say it sounds too different, he likes it less now and please make it the way it was

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    Quote Originally Posted by Chuck H View Post
    Because there's always that chance that your friend is going to say it sounds too different, he likes it less now and please make it the way it was
    Get a signed and notarized disclaimer that your customer *won't* want it the way it was. It'll likely sound 'different' even if the schematic matches exactly.

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  19. #54
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    Quote Originally Posted by eschertron View Post
    Get a signed and notarized disclaimer that your customer *won't* want it the way it was. It'll likely sound 'different' even if the schematic matches exactly.
    He wants it to sound different, even though it really does sound great at 1 watt of output. I have a Champ 5F1 build here with the same major parts as his putting out a little over 5 watts. He wants his to sound like mine. Ordering a board today and gonna look closer at the wiring before the 6V6 to see what I can find.

    Most definitely I want to determine what's dropping the wattage, for my own knowledge base if nothing else. And who knows, someone may want a 1 watt 5F1 one day and I'll know exactly what to do, except much more neatly than what we have here. I'm no tech, but I most certainly know wire management and how to solder with the best of them.

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  20. #55
    Supporting Member loudthud's Avatar
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    I think a blackface reverb transformer would drop the power somewhere near 1 W. Glad to see it got worked out.

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  21. #56
    Supporting Member eschertron's Avatar
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    Yes. Keep us posted. I, for one, think a one-watt pentode (er, sorry, beam tetrode) design might be nice for the home or coffee shop.

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  22. #57
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    Well, this amp has 14 resistors total, and looking at Rob Robinette's layout, if you add two 100R for the false center tap, that would equal 14 resistors total, right? I have verified all the values as correct, according to Rob's layout attached. I don't see any extra resistors anywhere in the signal path.

    It's a real mess, and as simple as it is, it's still difficult to suss out the actual layout.

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    The 6V6 voltages look reasonable. What is the AC signal voltage at the 6V6 grid at full output? It needs about 13Vp or 9.2Vrms for ~5W output.

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  24. #59
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Tone Meister View Post
    Well, this amp has 14 resistors total, and looking at Rob Robinette's layout, if you add two 100R for the false center tap, that would equal 14 resistors total, right? I have verified all the values as correct, according to Rob's layout attached. I don't see any extra resistors anywhere in the signal path.

    It's a real mess, and as simple as it is, it's still difficult to suss out the actual layout.

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    So is there any chance the 12ax7 is not producing the proper gain structure? What does a new 12ax7 do?

    nosaj

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    Quote Originally Posted by nosaj View Post
    So is there any chance the 12ax7 is not producing the proper gain structure? What does a new 12ax7 do?

    nosaj
    It seems to me that you believe I'm claiming there is a problem with the amp, there isn't. Or maybe I'm just interpreting your responses wrong. The amp was originally built to produce 1 watt output and that it what it produces. The "builder" made a note inside the cabinet and another next to the tube socket (1 watt). The amp sounds just fine, the work is beyond shoddy, but the amp works and sounds fine and produces exactly 1 watt rms, as advertised.

    The owner wants to find the intentional throttle-down point and restore it back to to 5 Wrms (4.5Vrms @ 4 ohms)

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  26. #61
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Tone Meister View Post
    It seems to me that you believe I'm claiming there is a problem with the amp, there isn't. Or maybe I'm just interpreting your responses wrong. The amp was originally built to produce 1 watt output and that it what it produces. The "builder" made a note inside the cabinet and another next to the tube socket (1 watt). The amp sounds just fine, the work is beyond shoddy, but the amp works and sounds fine and produces exactly 1 watt rms, as advertised.

    The owner wants to find the intentional throttle-down point and restore it back to to 5 Wrms (4.5Vrms @ 4 ohms)

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    With the data you've given...as far as component values versus the layout and stating they are the same. Something is throttling it, So when you state everything is measuring out correctly my suggestions are what's left to look over. Short of disconnecting the OT and doing a ratio test to find the impedance.....

    Something is the cause but what it is ... you;ve still to find out..
    nosaj

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    Quote Originally Posted by nosaj View Post
    With the data you've given...as far as component values versus the layout and stating they are the same. Something is throttling it, So when you state everything is measuring out correctly my suggestions are what's left to look over. Short of disconnecting the OT and doing a ratio test to find the impedance.....

    Something is the cause but what it is ... you;ve still to find out..
    nosaj
    True that. When I get back on it I'll measure the AC signal output at the grid of the 6V6 as Helmholtz suggested. Any other measurements you suggest while I'm in there?

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  28. #63
    Supporting Member The Dude's Avatar
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    I'm not sure you're willing to go there, but I found this:

    https://cigarboxnation.com/profile/M...les_memberList

    It's Michael Prout's page on Cigar Box Nation. I'll lay odds he built that amp. You could ask him what he did?

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  29. #64
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Tone Meister View Post
    True that. When I get back on it I'll measure the AC signal output at the grid of the 6V6 as Helmholtz suggested. Any other measurements you suggest while I'm in there?
    Sig gen in 100mv measure a grid V1A of 12ax7 ac signal, Then V1B grid, then 6v6 grid.. Same spots you;d use a scope, but using a DVM to get the data instead.

    nosaj

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    Bent Member Chuck H's Avatar
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    Ok, time for Spock to make a brief appearance...

    If the voltages and components measure in spec and there are no extra components then the amplifier makes around 5 watts like any other Champ. The amp only makes 1 watt, and is designated as such by the builder. You report that all the voltages and components spec as stock and that there are no other circuits added. There must a mistake in your observations so far.

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    Last edited by Chuck H; 01-31-2019 at 03:45 AM.
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    Supporting Member The Dude's Avatar
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    Just noticed the cap on the volume control. What value is that cap? Can't tell from the pic. Is it small enough to just be just for HF, or is it maybe shunting a bunch of signal? Maybe try disconnecting it temporarily.

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  32. #67
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    Quote Originally Posted by Chuck H View Post
    Ok, time for Spock to make a brief appearance...

    If the voltages and components measure in spec and there are no extra components then the amplifier makes around 5 watts like any other Champ. The amp only makes 1 watt, and is designated as such by the builder. You report that all the voltages and components spec as stock and that there are no other circuits added. There must a mistake in your observations so far.
    DISCLAIMER: "As best as I can tell." As noted, it's a mess in there. I won't say everything is within spec and there are no added circuits. What I will say is the observations I've confirmed are consistent with the schematic thus far. No odd values, no extra stuff detected ... yet.

    And most importantly, the amp produces 1 watt.

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    Quote Originally Posted by The Dude View Post
    Just noticed the cap on the volume control. What value is that cap? Can't tell from the pic. Is it small enough to just be just for HF, or is it maybe shunting a bunch of signal? Maybe try disconnecting it temporarily.
    It's silver mica so I'm guessing it's a bright cap. Will confirm tomorrow.

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  34. #69
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    The 6V6 voltages look reasonable. What is the AC signal voltage at the 6V6 grid at full output? It needs about 13Vp or 9.2Vrms for ~5W output.
    Just putting this back in view in the stream of discussion

    Indeed! If the signal is being padded before the power tube then this figure will be low. If the power is being damped after the power tube then this figure will be normal. This would narrow the search a lot.

    On the same note... The amp is reported to be making one watt. Ok, BUT, does it behave like a normal Champ circuit in all respects? That is, if you crank it with a guitar plugged in does it clip or does it stay clean right up to full output. This aspect by itself could indicate whether the signal into the power tube is being padded.

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    Right.

    The question to me is: Does the amp produce 1W just before clipping? In this case it would be really power limited (would require something like an extremely mismatched or defective OT- or power limiting resistors at the output).

    Or is it just 1W clean output with a 100mV input signal? This would indicate less than normal gain, leading to (insufficient) drive voltage >>voltage limiting.

    BTW, if the cathode bypass cap is defective/not connected, a much higher grid drive voltage would be necessary for 5W output.

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    Last edited by Helmholtz; 01-31-2019 at 08:31 PM.
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