Maybe someone can understand why they left certain resistors out.
Makes sense, I'm going to remove this board and install the stock unmolested one I had laying around, after installing new capacitors in it.I think the lower two resistors are the 18k reistors that form part of the plate to ground HF filter network on the pentode. The larger pieces directly above it should be the 82pf caps that are the other half of that network.
guessing this is someone's misguided attempt at improving HF response. Yes it will extend it, but probably to the point of creating instability. I'd put those parts back in.
"unmolested" is the right word !
yeah, decided it was better for me to start with the unmolested board, not 100 percent sure all the other values are correct with the existing components. Obviously someone thought they were improving the amp, but to my ears on my test speakers, the highs were too bright and not natural sounding.
They are the exact same boards, but obviously someone thought they were improving things by changing components. Yanking that board out and starting with an unmolested one I have, replaced the main capacitors though.
It almost seems they had this running in triode or something, no wires going from pin4 to the board like stock shows.the parts that are missing won't prevent it from passing signal.
it would be the 82mmf and 18k resistor that run from the 7199 triode grid to ground. With that in place some of the highs will go to ground so the bandwidth of the amp isn't so large that it can turn itself into an oscillator.
fair bet if you ran square waves through it, there would be a lot of overshoot and ringing.
I did pretty much did that with that integrated. So many of the resistors and transistors were replaced with the wrong values I wound up just going over the schematic and starting over. I wound up replacing most everything on the amp board itself. The line stage wasn't too bad, mostly just missing bits.
I've certainly acquired some messy equipment over the years!
mods are the absolute WORST!!!!, I've spent hours stripping out hacked wiring and added components from many otherwise perfect tube amps, example: I bought a Heathkit EA3 amp at a hamfest for $40.00 it looked great but when I opened it there was no wiring on the tone controls, the selector switch had a network of resistors attached and wired to points that made no sense and the volume pot was incinerated. under the chassis was just as bad. I completely stripped it to just sockets, controls, terminal strips and transformers, bought the assembly manual and built it as a new kit with all new components, I had to make up a new volume pot out of parts. thankfully the transformers were not damaged, the amp sounds great and I'm still using it.It looks like I was correct, compared to a stock board I have hiding around, there are some resistors left out of this board, and most components look upgraded. This could be the main reason this amp sounds overly bright in the highs, more investigation is needed I guess. Ugh, hate running into these Hodge podge mods.
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Also part of the stability stuff in the feedback loop. Not related to triode mode, that would be at the el34 sockets
I've certainly acquired some messy equipment over the years!