New build, dead amp, frazzled builder.

[sjcasas]

bmccowan: The 47Ohm resistor is spot on, the 1.5K is off by 23 ohms, still well within tolerance.


Platefire: I think that is exactly what I have to do... For now, I will leave it disconnected. When I am ready to dive in, I will set up two different guitars and experiment with a pot and measure the resistance when  find what sounds best, and set up a switchable NFB with the two equivalent resistors... sounds like a lot of work... Oh well.


acheld: Yes, I think it's the transformer... It's a Mercury FBFPO and they don't publish any specs. And it cost TWICE what the upgraded Hammond 1750H cost and I am pissed and about to order the Hammond because it will handle a SS recto and Stokes mod better. And I hate seeing unprotected paper windings under a chassis in a cabinet that might get cables and such crammed into. $130 for a transformer and they didn't put steel bells on the ends??!??! My fault for not checking, I guess.

[bmccowan]

Amp is weak with NFB connected, like VERY weak. When I disconnect NFB it sounds great. it has a 2.7k feedback resistor

I do not think that level of an issue will be solved by adjusting feedback loops for different pickups. Same thing for changing OTs if the OT is a PR spec (or close) OT. Unfortunately I think you have an issue (or two) that you have not yet found. Frustrating I know. But there are a lot of PRs out there, if the circuit is correct, it'll work.
Did you try swapping the OT-Power tube connections? If you haven't, I think you should really rule that out.
I sympathize because I've been there.

[AlNewman]

bmccowan: The 47Ohm resistor is spot on, the 1.5K is off by 23 ohms, still well within tolerance.

I read this as 470 Ohm.  I thought I had your problem figured out, then I read it again.

[AlNewman]

If you think it's just the different transformer, try doubling up on the 2.7k resistor, see if it helps.

[sjcasas]

Well, I found the problem... The turret lug that connects the 1.5K resistor to the speaker jack wire must have a solder blob under the board. It is almost a dead short to ground. When I pried the board up the other day I didn't see it. When my daughter was soldering all the components in months ago, I forgot to warn her about using too much solder. Hoffman even has a note about it in his instructions. When I did the first amp, I soldered all the components on the board first so I could do it more carefully and cleaner. On this one I had mounted it first so there was no way of noticing. Lesson learned.

Anyway, all works now. And once I got the feedback sorted out I did end up needing to swap the primary OT leads.

The only issue now is that, with the shorting plug in the trem footswitch jack, when the intensity is turned up past about two it causes a LOUD clicking vibration. Maybe 60 hz. Maybe a lead dress issue? Maybe I need to use shielded cable somewhere?

[bmccowan]

Cool. Sometimes those turrets want to just keep taking solder, eh? Although I seem to have landed on good technique, I keep a dental mirror handy to peak under boards. the drawback is that it reminds me of the dentist.
Info on trem ticking here: https://www.effectrode.com/knowledge-base/ticked-off-with-tremolo/ and many other places. I solved it in one Fender by moving wires, in another by changing to Sluckey's Tremonator.

[acheld]

It's a Mercury FBFPO and they don't publish any specs.

I understand your frustration.   MM transformers are generally very reliable, but expensive.  In fairness to the company, when I have had questions regarding specs I've emailed them and have gotten a fast and accurate response. 

But, I've had such good luck with pricing and service from HawkUSA.com with Hammond iron, I don't order much from MM now.   Even Edcor has upped their game recently -- I recently had a couple of OT's delivered to my door within a week of the order!

[tubeswell]

nothing to see here.

I left out an underboard wire.

Next time get yourself one of those tiny dentists hand held mirrors. Real handy for a quick look in hard to see areas.

[sjcasas]

Info on trem ticking here: https://www.effectrode.com/knowledge-base/ticked-off-with-tremolo/ and many other places. I solved it in one Fender by moving wires, in another by changing to Sluckey's Tremonator.

This is not that type of ticking at all. It's more of a buzz, maybe 60hz, that only happens when the shorting plug is in, which means the tremolo is OFF, and then you turn up the intensity past about 2. I know... why would someone turn the intensity up if the tremolo is off? I noticed it because I didn't have the knobs installed yet and was just tweaking pots to find out where they were at in the sweep. The buzzing scared the shit out of me and so I dug into it further. It is really loud and the Volume pot has no effect on it.

Also, I don't think Princetons (bias-wiggle tremolo) suffer from that kind of ticking (opto-isolator/roach tremolo).

[bmccowan]

Hmm - I think I'd check all those grounds, and resistance to ground in that V4A circuit. 

[sluckey]

This is not that type of ticking at all. It's more of a buzz, maybe 60hz, that only happens when the shorting plug is in, which means the tremolo is OFF, and then you turn up the intensity past about 2.

This issue is most likely a wiring error. Look closely. Maybe post some BIG hi-rez pics.

I know... why would someone turn the intensity up if the tremolo is off?

That's the way everybody wants to use tremolo. Set up the depth and speed then turn it completely off with the footswitch. Now you have the trem sitting on standby just waiting to instantly spring to life at a foot stomp.

[sjcasas]

Well, I finished the other amp. It works perfectly but has the same buzzing problem... I removed the 1Meg pot between pin 7 of the PI and the 1Meg grid leak resistor (sweet pot mod from Robinettes page) and the problem goes away. I have that same mod on the first amp (Miserlou) and it works fine. My two main guitars are a stock Stratocaster with 3 single coils, and a custom Strat with two humbuckers. I wanted to be able to dial in the sweet pot for both guitars with the pot in the rear ext speaker hole, and it seemed to work fine on the first amp. So I just figured I'd do it on the next two. Oh well.

Re: tremolo... I don't use it. And I think the three guys that do should just buy a stomp-box. I think I would MUCH rather use that V4A as a switchable gain stage, but maybe pre-reverb, and have a mid pot and presence (?) instead of Speed and Intensity. But that is a deep dive I am not about to do at my age. Anyone else willing?

[Willabe]

I left out an underboard wire. Yep. I can't even blame it on my daughter. I pulled the board up and STILL didn't see it. And I didn't check the underboard wires when I highlighted the circuit... TWICE!

Well, I found the problem... The turret lug that connects the 1.5K resistor to the speaker jack wire must have a solder blob under the board. It is almost a dead short to ground.

Glad you found them! 

And believe me, it happens to the best of them. 

When I pried the board up the other day I didn't see it. When my daughter was soldering all the components in months ago, I forgot to warn her about using too much solder. Hoffman even has a note about it in his instructions. When I did the first amp, I soldered all the components on the board first so I could do it more carefully and cleaner. On this one I had mounted it first so there was no way of noticing. Lesson learned.

Some like to fill the board 1st, I do. I think not only is it safer, because you can see both sides of the board, but it's much easier to work on the board without the sides of the chassis constraining you.   

Others fill it mounted in the chassis, which doesn't make as much sense to me. To each their own