The Amp Garage

I wouldn't bother with making it selectable unless you really want to A/B the different sounds. I'd predict that you'd end up just leaving it in Pentode mode. This is just my opinion, but I think UL sounds very sterile. Others have described it as sounding like there was a blanket over the speaker c...

If your interested in trying it, you could add another power supply node for the screens and change to pentode operation. You might find you like the sound of KT66's better that way. I'm not a fan of UL for guitar amplifiers. I built one that had selectable UL / Pentode with KT66's into an 8K primar...

IMHO, ideal tube to OT impedance matching is relative to optimizing power/bandwidth/distortion...mostly for HIFI. For guitar amps (narrow bandwidth, coloration welcome) not as critical. With a 400v HT and an 8K primary OT, you'll have a 20 - 25watt PA that can use most of the typical power tubes and...

SiC Schottky diodes are super quiet. Schottky's reverse recovery time is nearly zero.

This might be sacrilege, but it's easy enough to try and undo, add grid stoppers to the PI grids. You can go reasonably big without a noticeable effect on treble - just use higher wattage, metal films to induce the least amount of noise. The Express has plenty of treble anyway. Merlin Blencowe discu...

There's a lot of ways to reach your goal.

Even for a "clean" pedal format, most want some color/sustain - setting the amp up for "edge of breakup". If you have a scope, it's useful to look at each stage, setting it up to stay as close to center-bias as you can throughout the useful gain range.

So, I built this: kuehnel-garage-amp.gif The voltages on my build are pretty close to the design. I had to increase the screen droppers to get the screen voltages down a little lower. The input pentode is working as expected. I can't get the second pentode to "center" bias. I put a variable resistor...

So far, redplating is just supposition, it hasn’t been observed. And we don’t truly know what load impedance has been used with it. Whereas tracking has occurred, and an intermittent open circuit on the secondary is likely to cause that. Just to clarify: The first failure mode is speculative, but l...

Running at 1/4 the nominal load would result in excessive current draw, leading to red plating and cooking the OT primary. Was this the configuration (selector set to 16Ω, actual load 4Ω) that caused it to land on your bench originally? In other words, Oops I Did It Again? I suspect that this was t...

xtian wrote: ↑Wed Sep 29, 2021 3:43 pm Also check screen and grid resistors.

I did check both the screen and grid resistors, when I had it, and all were good.

Socket.jpg When he originally brought the amp to me it had evidence of power tube socket arcing. The impedence selector was set @ 16 ohms - at the time I didn't know he was running (2) 8 ohms cabs in parallel... I put in new sockets, cap cans, bias cap, and new KT77's (burned in by Tube Depot) and ...

I repaired/updated a 2205 a couple of months ago, discussed in a previous thread - and it failed again... After talking with the owner, I found out that he had (2) 8 ohm cabs plugged in (parallel) and set the impedence selector for 16 ohms... While not as destructive as an open output circuit, a 4x ...

https://seasidechassisdesign.com/

I've purchased several chassis from this guy - good product and price.

He has 12g aluminum available, which is nice for holding the heavier iron.

So I played it for a couple of hours before the owner picked it up and all was good.

He had it at practice tonight, and sent me a few texts that it sounded great...after 30 mins of playing, he said the volume dropped off substantially on both the normal and boost channels.

Any ideas on this?