The Amp Garage

… if the presence knob is not cranked to maximum (or slightly below maxium), I have some pretty obnoxious 120hz buzz. Additionally, switching in the additional gainstage in front of the plexi topology leads to some strange oscillations unless the presence knob is at MAX. How about if the NFB is dis...

palomarsound wrote: ↑Sat Jan 06, 2024 11:43 pm … I'm getting 0v across the primary. I do see continuity across the primary. …

It seems like the mains transformer is fine then.
With no voltage across the primary, you can’t expect any voltage out of it.

I suspect that the amp is oscillating; the hum is indicating that, you’ll probably find that the cathode voltage is increasing and HT voltages are reducing alongside it. It may be that the OT is very low spec, has too much phase shift to accommodate a feedback loop. But it also may be that the amp i...

People sometimes flip the signal polarity when fitting a LarMar, they don’t realise that the polarity has to be maintained.

If the power amp gain reduces when the feedback loop is opened, then the feedback is positive.

… To stabilize the bias voltage I used a Z-diode. https://ampgarage.com/forum/download/file.php?id=72380&mode=view … I can’t see a zener in the bias supply? Anyway, my thinking is that it’d be a bad idea to regulate the bias voltage. It seems better to allow the bias and HT supplies to track the ma...

Also Fender typically made more than a few soldered connections to the chassis. Those tend to not be not feasible for DIYers to do, unless they’ve got a big bertha iron and the skills to use it.

Maybe @wpaulvogel would be interested in taking this on?

leadfootdriver wrote: ↑Sun Nov 26, 2023 10:01 pm … I actually removed that very arrangement to add the 68K.
For me the extended highs are to much glaze on the sound...

Maybe use a higher value cap? eg 1nF.

leadfootdriver wrote: ↑Sun Nov 26, 2023 10:01 pm … Whatever hiss that's there is still good enough for nearly every Fender and Marshall worth hearing..

They work out to 34k.

68k grid stopper will introduce hiss.
Merlin suggests to use 10k and add a cap to obtain the desired high frequency response / RF rejection.
http://www.valvewizard.co.uk/gridstopper.html

My guess is that’s it’s unstable.
It’s to get the scope out, verify that, and identify what stages are included in the pfb loop.

… Does a so called "Bright Cap" on a volume control … actually boost highs, or does it really act as a HPF and rip out lows … The distinction between those 2 options might be seen as something of a matter of perspective really. What matters is that unless the volume is set really high, the amp soun...

… I'm wondering if I could get away with using a 5V4 rectifier considering the low to lower-medium volumes I play at in the small to medium size bars and restaurants I gig frequently at… I suggest to use a GZ34 with a sag resistor, eg 100R, in series with the rectifier output. By that you’ll get al...

Looks great!

Being as there’s people here with sim skills -
Doesn’t C13 0.1uF DC blocking cap feeding the tonestack render the 0.1uF C15 bass cap redundant?
ie change C13 to 0.05uF and link across C15; does that give the same response?

It looks a fun project. The HT supply seems a bit unusual, eg there’s no E node, reverb driver is usually fed from the B node. V4b circuit might better be described as a buffer, as it has voltage gain per se. Personally I’d put the reverb after the modulation, as deep reverb settings might sound a b...