MusicYo was selling these all-tube amps for $90. I was interested in running the output from a small amp into a dummy load and then tapping off a bit of this signal to run to run to a clean power amp. This is what Guytron amps do, and it allows you to get power tube distortion at any volume level.
In addition, there is the opportunity to put EQ and effects in as you would in a studio--after power tube distortion.The EQ is particularly interesting because placing it before distortion (like where it's found in guitar preamps) is supposed to affect the quality of the distortion, but placing it after distortion allows more sound-shaping. Same thing with reverb. In guitar amps it happens before power tube distortion, where here it can happen afterwards.
I liked the Tube 10 price, but also the fact that it was a single-ended amp (one 6L6), and should be class A. I didn't own an amp with these characteristics, so I thought I might learn something here as well.Since I'd be running the output into a 75W 4 ohm resistor, I didn't need a powerful amp because I'd be throwing away most of the power as heat anyway.
If the output of the dummy load is run into the effects loop of a guitar amp hooked to guitar speakers, I'll get the sound of speakers from the speakers themselves. But if the output is run into a PA, I need a speaker emulator or at least some EQ. Generally speaking, guitar speakers roll off everything above 6KHz, have a hump around 1KHz to 3KHz, and don't reproduce well below 100Hz.
I've just started making the amp sound good, so I haven't begun addressing these issues yet.
The first step was to make the amp sound as good as possible. Playing it right out of the box, it was limited in the tones it had. The speaker was a large part of this--running it through a 4-12 Marshall cabinet with greenbacks helped enormously. Second, it didn't really have any clean tones. I thought it only sounded good with the volume and gain set at 10--then you got a decent rock 'n' roll tone. Bass, middle and treble controls worked reasonably well.
Open it up and you find a reasonable looking power transformer, and a ridiculously small, open frame output transformer. The cathode-biased power tube is biased at 68mA with a plate voltage of 425v. There is minimal power supply filtering--4-22µf caps rated at 450v--not too much headroom on the voltage.
You can download a schematic from Musicyo. There are several mistakes on it (at least when I downloaded it there were).
The 115v-230v selector switch on the transformer is shown wired incorrectly
The screen is in the wrong place in the power tube
The screen resistor is R12, not R9, and its value is 470/1W, not 100K
Cathode resistor R3 is shown as 2K2. On my amp it is 3K3
My amp has a preamp out jack which isn't shown
Cathode resistor R3 is too large. I think this is what leads to the inability to get a clean tone from this amp. Changing it to 1K7 reduced the distortion.
The first coupling cap (C2) is too large (0.47µf), and passes all frequencies above 4Hz. R5 attenuates some of the signal, but with cathode resistor R3 being smaller, I didn't find attenuation was needed. I removed R5, and substituted a 0.01µf cap for C2. Now the turnover frequency is 173Hz. The lower octave of the guitar's range is somewhat attenuated, but this amp doesn't have the power for bass frequencies, and sounds better this way.
I couldn't hear much effect from the negative feedback circuit set up by R14 and C11, so I removed those two components since I generally prefer amps with no NFB.
I am now playing with a way to vary the bias using a method John MacIntyre used in his "Prince of Wails" mod to a Fender Princeton (Guitar Player, March 1996), which was suggested by Trainwreck's Ken Fischer.
As you lower the bias current, the plate voltage goes up, which is not good considering the filter caps are operating at almost at their maximum voltage. I put 600ohm, 10W resistor in after the standby switch which drops about 60v. I'm not done playing with this yet-more to come.
There's a DPDT switch used to set the operating voltage. I permanently wired the transformer for 120VAC operation, and planned on using this switch to switch to triode mode for the power tube. However, triode mode hums too much. At first I though it was hum from the heaters, but it was still there after biasing the heaters with a DC voltage. It turns out that the 22µf input filter capacitor is just too small. If I increased this, the hum went away. Eventually I may decide to spend a few bucks more on this amp and upgrade the amount of capacitance and the voltage rating of the filter capacitors.