add-on: by "comparing voltages"  I meant comparing them in working and non-working channels.

    Quote:   Do you have a multimeter? If you don't then I definitely recommend buying one (HarborFreight had them for $3.50). With multimeter in hand I'd do the following: since nothing is burning you can test the  circuit as is. Draw a table, name the key points of the circuit...

most probably you either short-cut the mosfets or fried them resulting in a complete open mosfet. You can check if the filaments are still operable by testing the resistance between the corresponding pins (#3 and #4). For the future tests you can first use a powerful resistor instead of...

  Quote: There is a simple way to get rid of this problem: diode protection. The diodes should be switched in parallel to earphones and each channel will need either 2 of them (to kill both forward and reverse spikes) or 4 of them (to double the threshold above which the spikes/sounds...

if the filaments don't glow that means either burnt filaments (measure them with Ohmmeter when the tubes are out of the sockets) or burnt mosfets or closed mosfets.

all voltages are measured with respect to the ground.   The grid voltage in this connection should be = 0 (there's no voltage source at the potentiometer and the grid current must be negligible).

the correct way to test the circuit is to measure the voltages in key points. The cathode voltage should be >0 and less than a couple of volts, the grid voltage in this scheme must be equal to zero, and the anode voltage should be less than 48 V but greater than 40 (my guess, I don't have...

it's interesting... I wonder how it can be that 150K resistor switched in series with 50K pot doesn't help. It's 1:4 attenuation.

there are simpler ways to cope with high gain/9o'clock pot problem: add a resistor in series with your pot and you'll easily go beyond 9 o'clock

2 revolink24:   thermal material is __necessary__ to insulate your mosfets from the ground. Otherwise you'll have to insulate the radiators that might be tricky. Check the list of items provided by Peter Millet on the first page. I recall there were two mounting sets that included thin...

in principle, nothing bad should happen given that you've put the output caps. To be on a safe side you can start from the output: measure the DC voltage on the output jack. If there's no DC component, switch to AC measurement mode and check if the amp doesn't generate by itself when the...

First of all, do NOT measure the resistance when the system is powered. You can fry your multimeter.   Second, I'd suggested to use multimeter in voltage measuring mode to know for sure where is "+" and "-". Naturally, this measurement should be performed when the PSU works. Regardless...

check the resistance of the heater. The one that is working shines too bright for me - normally they should glow. I suspect the heater was burnt. If that's the case, look for the reason. Otherwise you'll fry the second tube.

I meant the resistance of the whole circuit when you unplug the power jack and put the ohmmeter leads to "+" and "-" of your circuit.

wait a second - you mean that the voltage was not equal to 48V when BOTH tubes were unplugged? And at the same time the unloaded PSU shows 48V? I'm a bit confused. Can you measure the resistance of the unpowered circuit without tubes? It might be that the PSU has gone crazy so it can't power...