The Amp Garage

I’ll go over everything one more time, but I’m fairly certain that I don’t. Is it safe to measure heater voltage with the leads off the socket and just hanging in the air? The only other thing I wonder about is the terminal strip where the LED connects. Could the phenolic part have absorbed flux and...

One thing I noticed was having the diode and resistor for the lamp going to the wrong pins - so I fixed that. Measuring as sluckey advises gets me 6.4vac and has always been right around that. It’s when I measure each pin to ground that I get the stated discrepancy. If measuring that way isn’t relia...

Understood and thank you for the advice

Currently retracing everything and will report back.

Ok, replaced the heater ACTs with 150R and fired it up. Now I have 6.2vac on pin 2 and 0 on pin 7.

Could the PT wires have been incorrectly marked? The clipped/wrapped heater CT has the white stripe on it and the two I’ve got going to V5 are solid green.

What else could it be?

Ugh! It appears that I do indeed. I guess that would explain it. I suppose it wouldn’t be one of my builds without a stupid mistake like this, lol. Hopefully I can limit it to just this one… I have a Mouser bag sitting here with 2 Koa 150R resistors in it that for the life of me I couldn’t remember ...

“ Don't understand this... If pin 2 to pin 7 is 6.2, I would think pins 2 and 7 should both be ~3.1 to ground. Is the only load you have on the heater circuit the panel lamp? What kind of panel lamp is it?” I thought that was odd… The panel LED strip is wired per the layout with the 1n4003 and a 511...

Ok; truck fixed - back to the build… Smoke test results: Plugged the limiter (40w bulb) into the variac and came up slowly. AC flowing as expected from power in to fuse to switch, stand by switch works as it should. Heater voltage was 2.9vac at pin 7 and 6.9 at pin 2 - but 6.2 measured across 2 &7; ...

Thanks and understood Martin! I’m shooting for Sunday hopefully.

Got to take the bed off my truck and weld two new frame cross members in tomorrow. Anyone with a Chevy or GMC built in the last 20 years can probably relate.

Rather be working on the amp…

Well, I’d rather do it your way - so that’s what I’ll do.

In other words - I trust your expertise more than my assumptions, lol

I’ll do some searching for a process, but any tips you have are very much appreciated Martin!

Franco,
I’ve read a lot of your posts and I’m confident you can figure this method out for certain

This adjustment is subtle, but IMO very important to the overall sound. This method gets me there every time and I usually don’t have a clue what I’m doing, lol…

Thanks Ian!
I agree with Martin and ynor! What else is there to say?

Really great

Plug the Variac into the limiter, amp into the Variac, then slowly bring the amp up Martin?

Today I was able to finish up the transformers and wired the PS board. Might be getting close to smoke test time. This time around I have an option for the first power-up: my trusty limiter or a recently acquired, economically priced variable transformer (which was more like buying a kit given the l...

Great, thanks Martin! I went back and re-read that whole exchange between you and Jelle and I and figured it applied here - but just wanted to be certain because there was some confusion surrounding which resistor values I was using which I think muddied the water early in that conversation. Thanks ...

Going over some details before doing anything else on the amp today and had a question: Given that I’m building a 50w BM, should I be using the 8ohm tap for my GNFB? I did this on my 102 because of the change from 100w to 50w - so I’m thinking this principle would apply here as well. Am I actually l...