The presence control has a big effect, so check all your wiring. I even copied the exact hookup and values and put it in a Fender conversion and it has a big effect.
Here is a picture of the back of my presence control that may be of some help.
In terms of 50 to 100w conversion, anything is possible, but it would likely involve a new output transformer and possibly a new power transformer, but the other changes would be small.
i just measure the bias again using the DC 200mV setting and i am now getting a voltage of 96mV! seems a bit far off the 26mA i was getting. Something wrong perhaps?
You do want to use a 'voltage' setting on your meter. You want the voltage potential difference on each end of the 1 ohm resistor. Then you calculate the amperage by multiplying by one. (even I can do it in my head ).
At 96mV, maybe you are measuring both tubes? A single tube with that actual reading would likely be red!
keep the 270pf/220k network on the i/o jacks, have it wired so it is only in place when you put a patch cord between In and Out. It makes a cool addition to the BRIGHT, MID BOOST and JAZZ/ROCK switches. (I guess if you still wanted to use the effects loop, and liked the 270pf/220k effect, you could use an extra hole on the back of the Overtone for a little DPDT switch)
Those are the main changes. It really moves the amp away from the "Modded Overtone"' schematic, but my amp sounds great with a Strat and Telecaster like this. BTW: I did not come up with any of this, all these changes are exact values chosen by Mr. Dumble.
If I measure from one of the testpoints (the tipjacks) to to the center testpoint between the output tubes, I get 1,2 ohms, which means I have to deduct 20% from my mV readings.
Hereof, a measurement om 25mA (mV) actually means 21 mA - a pretty severe error margin don't ya think??
Whats the moral?? use your freaakin' ears to bias an amp
Here we go....
The additional 0.2 ohm resistance of your test leads would only come into play if these leads were in SERIES with the one ohm resistor. This is not the case. When you take a VOLTAGE reading with the multimeter, the total resistance across the leads, INCLUDING THE BOX, is probably on the order of a thousand ohms. So the additional 0.2 ohms of the leads would make it one thousand point 2 ohms. This will not change your readings.
What you do need to be aware of, is not all the tubes current comes through the cathode. Some leaks through the grid, and this is what you will NOT be measuring with the cathode bias method. The leak is usually small. So when you measure 32mv you have 32ma coming through the cathode. The total tube plate current (including that going through the grid) would be like 35ma or so. Its OK to make this estimation, because actually measuring the plate current can be dangerous, but is easy to do if you are so inclined
I am in 100% agreement to use your ears, but I was getting a smoother sounding tone up past 42ma on one set of tubes. That's fine, but one arced on me when they were that high. So, you want the best sounding bias setting that is up to or below the recommended heat dissipation of the tube in question.
#124 may have been built for a single-coil guitar!
My #124 really sounds killer. But with a double coil guitar it is too harsh. If you put a patch cord in the effects send/return, it activates the 220k/270pf network (if you wire it like #124) and this turns the double coil guitar sound into butter smooth tone!
I need to get my notes in order but where this is heading is a second overtone mod for folks to try. This one will be for single-coil players, and basically it will involve changing the Overtone components to #124 Dumble values.
To make a useful post on the mod, I need to go through all the changes and pick out the 'main ones' because there are about 50 differences and only a few would be needed.
I recently spent some time with 4 sets of 6L6 tubes in my amp and I found there was a little 'grainy' character with the bias low (25ma) and this got better for each tube up to about 35ma. One set even sounded best at 42ma, but another old, old set arced (internal spark) at 42ma. The sound at 25ma was still very acceptable for all the tubes, though, just depends on how critical you are. It is a trade-off between smoothness of tone and heat. I wound up liking from 35ma to 38ma, but this is a home recording amp. For gigging, less heat and more reliability might dictate around 30ma.
In terms of changing your bias circuit, either raise that 1.2k resistor with experimentation, or e-mail Nik to see what he recommends.
So, my thoughts from reading this board and the AMP GARAGE board, and after my experimentation with #124 circuit that someone with an Overtone that plays primarily SINGLE-COIL guitars, might want to try going to 100k/1.5k/10uf on V1 and 100k/1.5k/4.7uf on V2 and compare it with the 220k/150k mod. on the "Modded Overtone" layout.
Or even a combination of resistor values can sound good. I just saw this today. A Zion (Single-coil Strat style guitar), plus a Red Plate Dumble style mod with 100k on V1 and 220k/150k on V2 amp. In a live setting this can yield great tone.
It does seem to "rock" more with this 'older' circuit. I can't help but thinking old Jackson Brown etc and those silver face ODS amps. From what I gather #124 was near the point where HAD moved away from the 100k amps. I heard #123 had 220k and 150k on the plates. So, he intentionally made #124 more like those older amps (which had 100k on the plates).
Another 80s amp I was thinking about when I did the parts swap was this amp, owned and played by Gregor Hilden. I suspect it also has 100k plates:
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