The Amp Garage

I never really understood feedback until I started working with opamps. When I was learning tube theory they always talked about feedback as being "out of phase". Truth is that in any non-inverting amp like a guitar power amp, the feedback is actually in phase with the input (and naturally the outpu...

That would work except you might get a loud thump when you throw the switch. Leave one resistor connected all the time, then connect the second resistor for hotter bias.

I would simply add a resistor (or make it a resistor and a trim pot) to ground from the output of the bias filter network with a switch between the resistor and ground. That way the output tubes never see a condition with no bias. If you were really clever, you could make it so the hotter bias is ac...

You didn't mention which impedance tap you used to obtain the feedback. 27K seems too low unless you have a 2 Ohm tap.

If you have a two trace scope, compare the phase of the input signal at the PI to the phase at the secondary of the output transformer. They should be in-phase.

Try a bigger cap. Double the value of the cap and keep going larger doubling the cap every time until you find the value that works.

One little point. You can't just add the impedances of the secondary windings. If you had a tapped secondary with 16, 8 and 4 Ohm taps, the 4 Ohm tap would be the center tap. Do the math(s). Calculate the Voltage needed for say 25W and 4, 8 and 16 Ohms. 25W @ 4 Ohms is 10Vrms. 25W @ 8Ohms is 14.142V...

Isn't half of V2 not used ? I only see 5 triodes used plus the PI.

Link to schematic: https://el34world.com/charts/Schematics ... sman70.pdf

You need to observe where the "key" points on the center pin of the octal socket. Looking at the bottom of the socket inside the chassis, the "key" points between pins 1 and 8. Pin 1 is on the left and pin 8 is on the right when the key is pointing toward you. On a 6L6GC or 6V6GT there is no connect...

There should be a paper label inside the amp enclosure. Post a picture if you can or describe any hand written or stamped letters/numbers.

The 12AX7 cathode follower needs it's Plate Voltage to be at least 100V higher than the input. Otherwise, grid current will steal gain from the gain stage. You could drive the cathode follower with a cold clipper if you have a higher B+ node to hook to the cathode followers Plate. That could stress ...

I thought it was the amp discussed in this thread: https://ampgarage.com/forum/viewtopic.php?f=5&t=12873

Schematic at end of first page is 4x 6L6.

Is that the correct schematic ? Looks like a 15W amp with a Voltage Doubler power supply.

I always hated the loss caused by the 68Ks between the channels when you use a jumper cable. Here's what I came up with using Switchcraft jacks. If you are using cliff type jacks, you can arrange things so there are two Both jacks. You plug into one and run the other to another amp. Both_Input.GIF C...

The 6AV6 has an amplification factor of 100 like a 12AX7, The 6AT6 has an amplification factor of 70 much like a 6SL7. The pin arrangement is the same.

FYI: 12AT7 is 60, 12AY7 is 40.

I believe the schematic should look like this: