OK Then

[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)

[ATTACH=CONFIG]52304[/ATTACH]

[ATTACH=CONFIG]52305[/ATTACH]

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

[Tone Meister]

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?

[The Dude]

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?

[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?

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

[Chuck H]

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.

[The Dude]

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.

[Tone Meister]

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.

[Tone Meister]

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.

[Chuck H]

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.

[Helmholtz]

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.

[g1]

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).

I think you need to specify 'power tube clipping'. Otherwise, we see clipping at the 1W point yet it could be clipped by a preamp tube. In that case it's not really power limited. This is where the measurement you suggested at the power tube grid is important.

I say this because it has become common for manufacturers to 'fake' real power attenuation by limiting the drive and labelling the amp with so called 'power settings' like 1W,5W, etc. Often the clipping is in the PI stage, and what the '1W' label on the switch means is that when the power stage hits 1W output, some clipping happens somewhere in the amp. The '1W' then becomes more than that, actually whatever power a square wave at that same pk to pk voltage turns out to be.

[Helmholtz]

I think you need to specify 'power tube clipping'. Otherwise, we see clipping at the 1W point yet it could be clipped by a preamp tube. In that case it's not really power limited. This is where the measurement you suggested at the power tube grid is important.

You're right, I should have been more precise. I meant clipping above 1W though there is sufficient drive voltage.
If we have preamp clipping, this would be my "case B", namely drive voltage limiting.

[Tone Meister]

The amp produces 2Vrms with the volume control wide open. I can increase the input signal to 400mV and total power increases to a whopping 2.4Vrms @ 4 ohms. I don't have a scope yet, so I really can't tell what the wave is doing. I can guess if you want me to.

Parts on the way. I'm gonna gut it.

[eschertron]

I'm wondering if the original builder used a wrong-value part, or an under-spec'd part, or otherwise messed up the amp so that it only made "1W" and that the notation of that is an observation, not a design choice.

Do you have booster pedal or similar? I'm curious how hot you'd have to get the input before it starts to sound like a champ that's clipping. Do you have time for that experiment before dissecting the thing?

[Chuck H]

Could you please just plug a guitar into it? There's not much in a Champ to clip the preamp tube with a guitar signal (with the possible exception of peaks from a hot humbucker). So if the amp audibly clips generously I would suspect attenuation of the output signal. If it doesn't clip other than peaks (attack) I would suspect attenuation of the preamp before the power tube grid.