DG-SE1 Tube Amp Build, Part 1: An Awesome Single-Ended Circuit 

Thanks!

That was really interesting! I might have to look at making a switch between ultralinear and triode operation in my A3600 but have it inside the chassis so I don't get tempted to switch it while it is on!

It's so hard! When you flip a switch with music playing, the difference is obvious, but not so much after powering down and waiting a respectable time for high-voltage capacitors to bleed down.

@@Lancaster_Hi-Fi Yeah - my interest in such a switch is not only to hear how different it might sound, but also how the amplifier and circuit adapts to triode mode (how class A and lower output changes heat dissipation). The Luxman A3600 is designed to be able to handle 50W but my 4x KT120s might be putting out much more than that in ultralinear configuration. The casing of the power transformer heats up over time and gets quite hot after a couple of hours of listening.

I've used the little DG-SE1 in my office as a hand warmer. Tube amps don't make good space heaters, but they do function as bad ones!

I didn't mean to imply that you would. Nor would you want to put a high AC voltage across speaker terminals. Without a transformer, you'd block the DC with a capacitor.

I re-watched the video because I was puzzled by your comment, but now I'm even more puzzled. Where did you get the idea that I was connecting a high DC voltage across the speaker terminals?

@@Lancaster_Hi-Fi Do a freeze frame at 9:55. At the output transformer's speaker winding, note that one terminal (green wire) is connected to ground. Now trace the other speaker winding (black wire) backwards and note that there's a 10k resistor and a 150k resistor in series, which in turn is connected to a 255 volt DC source. That's 255 volts DC being fed to the speaker terminals.

@@1834RestorationHouse The 150K resistor is the upper leg of a voltage divider, where the lower leg is 820 ohms. That divider sets the cathode bias voltage to 1.46 V. Ok, that 820 ohms is actually in parallel with the 10K resistor and the speaker coil in series, so the effective resistance is a little less than 820 ohms. In any case, the 10K resistor and speaker coil form another voltage divider. The coil has a very low DC resistance (i.e., much less than the 8 ohms impedance seen by AC signal voltages), so the DC voltage drops to nearly zero across the 10K resistor, and the DC voltage applied to the speaker terminal is negligible. The 10K resistor in parallel with the 300 pF capacitor is the feedback network.

I don't know. If the tubes are grid, or fixed, bias, then you can put a 10 ohm resistor between the cathode and ground. The resistor will allow you to measure the bias current without significantly affecting the bias and also give you a sort of fuse to prevent runaway current and redplating. If the tubes are cathode biased, as in the circuit in the video, the bias current is determined by the value of the cathode resistor. If you want to be able to fine tune the bias current, then you can use a variable resistor in combination with a fixed- value resistor. You need the fixed value to determine the bias current and the variable resistor to change that current. Often, though, cathode- biased circuits eliminate the variable resistor. Sometimes these are called auto- biased, but it really just means cathode bias that you can't adjust.

I got single ended kt 66 cathode bias @@Lancaster_Hi-Fi

I have not. I have built similar push-pull amps with 6BQ5 and 6V6. The 6V6 can maybe deliver a bit more power, but it has lower gain and therefore requires greater amplitude at the grid for full power at the plate. To achieve similar sensitivity from similar input and phase-splitter stages requires decreasing the negative feedback, and so the resulting amp has greater distortion, and the difference is visible in the scope at lower frequencies. That is, for the same amp, decreasing the feedback to change the sensitivity from 3 Vrms to 1.5 Vrms significantly degrades the signal below 100 Hz.

That resistor forms a voltage divide with the cathode resistor and sinks much more current than the tube draws by itself, so that the cathode voltage is stable. It's a simple constant current source. This is a power amp only. No preamp, so no tone control.

@@Lancaster_Hi-Fi Right - it could be - such configuration allows to decrease output resistance keeping plate current at very low. Good driver for end stage

Of course, it does use potentially precious milliamps, and sometimes every milliamp matters!

If the capacitor adjacent to the rectifier is too big, the rectifier will fry. Tube rectifiers are less tolerant of big current spikes than solid state. That doesn't explain small values after the choke.

@@Lancaster_Hi-Fi no it would Not fry the tube rectifier. i routinely run a pair of 100uF/500V caps off the EZ-81 , for many years now,,,,, with no issues. You don't know the real reason , do you ?