Hoffman Princeton Reverb Build

[sclinchy]

Right, I haven’t wired the 2nd jack yet, I may in the future.  Will check the reverb wiring.

[sclinchy]

Here are the pots (yes, I haven't wired the second jack, I may in the future, but for now it's just the one).

[sclinchy]

And the back.

[pompeiisneaks]

Treble appears to be wired wrong.  If you look at the layout, it's supposed to have only three wires connected, one to each pot point, but I see four.  One red on the left side that's right from the board, center is from the volume, but third comes from board and jumps to bass, but that's backwards, it's supposed to connect to the bass first and then jump to the treble.  Looking for other issues.

[pompeiisneaks]

I also don't see grounding buss across the pots or at least near it.  You may not be getting good grounds on the pots from their body to the chassis.  You should at a minimum have a ground buss bar across them and connect grounds from them to that, and that to a solid ground near the input jack or at the jack itself.

~Phil

[pompeiisneaks]

You've also got the reverb hooked up wrong, it seems you're grounding the green wire, which, in the layout, should be connected to the center post of the reverb, and the black should be chassis grounded, is connected to the center post.  Also ground both of those side pins of the rca jacks themselves off the side tab. 

That may cause the motorboating you're hearing if the phase is reversed in the transformer via that hookup. 

~Phil

[bnwitt]

I think he just used the second jack to plug the hole on his store bought chassis/faceplate.

I love how you say "store bought"  like you and I are standing next to a moonshine still discussing it.

[sluckey]

One of us might be! 

[purpletele]

You've also got the reverb hooked up wrong, it seems you're grounding the green wire, which, in the layout, should be connected to the center post of the reverb, and the black should be chassis grounded, is connected to the center post.  Also ground both of those side pins of the rca jacks themselves off the side tab. 

That may cause the motorboating you're hearing if the phase is reversed in the transformer via that hookup. 

~Phil

Nice work Phil! 

[pompeiisneaks]

It remains to be seen if I got it right ;)

~Phil

[purpletele]

It remains to be seen if I got it right ;)

~Phil

It was pretty cool of you to spend the time tracing the circuit and its possible errors. 

Slinchey has a Correction Notice! 

[bnwitt]

Ok, I did a quick layout of the Hoffman PR with a Mojotone chassis.  I hope it helps

[sluckey]

I still don't see a 220K resistor on the reverb return (input) jack. You may never notice anything as long as the reverb tank is plugged in. But you will surely notice a problem if the tank is not plugged in.

[sclinchy]

Thanks, I reversed the reverb transformer secondaries.


I managed to keep it on long enough (without blowing my speaker to get some voltages)


V5 pin 3 - 326
V6 pin 3 - 346
V4 pin 6 - 30 (!)
V3 pin 6 - 180


Filter caps:
A - 345
B - 295
C - 244
D - 195


Hope some of this helps.

[bnwitt]

You really need to use your layout to run  through the amp and check for Wiring/ component errors.  Doug has and old saying, "If it were assembled correctly it would be working.

[pompeiisneaks]

Yea as bnwitt says, I am not seeing anything screaming at me other than what I noted (Did you fix the treble and bass hookup I mentioned?)  Also I mentioned a few techniques you could use to ensure all solder connections are giving you good quality continuity.  Did you do that? 

The fact that V4 pin 6 is so low is a hint that something's wrong there.  Also, where are voltages for pin 3, 2, 1, and pin 7 and 8 on the preamp tubes?  I'm guessing you just got worried?  If you don't want to roast a speaker and are building amps, it may be worth it to invest in a dummy load of some kind so you can plug it in, ensure the OT has a valid load, and then proceed with this kind of testing.

~Phil

[sluckey]

Pull V1. Does the loud hum go away? If not...

Pull V2. Does the loud hum go away? If not...

Pull V3. Does the loud hum go away? If not...

Pull V4. Does the loud hum go away?

Report your findings.

[sclinchy]

I've been away from this for a while, work and travel have been taking up my time.


First, thanks to all who have taken the time to contribute, this is really a remarkable group of people.


I've traced the layout twice now and I still don't see a mistake, although I know there must be one.  Thought I'd post some voltages, maybe that will give someone a clue to where I should start looking:


V5 pin 3 - 325 (low, should be 410)
V6 pin 3 - 450 (a little high)
V4 pin 6 - 125
V4 pin 8 - 40
V3 pin 6 - 125
V3 pin 8 - 0 (can't be right!)
V1 pin 6 - 130


Just to reiterate, when I turn the amp on, there is silence while the tubes warm up, then a 60 cycle (I think) hum.  No signal coming through at all.


Thanks again.

[sluckey]

Read reply #66.

[sclinchy]

Sorry, reply to #66 is that there is no change when any of the pre amp tubes are removed.


There was an earlier request for additional voltage readings, here they are:


V1
pin 1 - 160
pin 2 - 0
pin 3 - .9
pin 7 - 0
pin 8 - 9.78


V2
pin 1 - 299
pin 2 - 0
pin 3 - 0
pin 7 - 0
pin 8 - 5.87


V3
pin 1 - 119
pin 2 - 0
pin 3 - 1
pin 7 - 0
pin 8 - .983


Thanks!

[sluckey]

Sorry, reply to #66 is that there is no change when any of the pre amp tubes are removed.

Problem is likely in the output tubes or power supply.