Voltage problems, help please

[Hank S]

So I'm not a newbie but I am stuck.

First startup was fine, heaters 3.36 vac and 4.9 vac between pin 2 and 8 on rectifier
Second with GZ34 in and power on stby off, 325 vac to one side of the stby switch and pin 5 on V4 V5 reads -45 vdc
So hear comes the problem:

I put in the 12ax7 and everything is wonky
All these are VDC
V1 1) -0.868 3) 0.017 6) -0.892 -8) 0.017
V2 1) -0.593 3) 0.004 6) -0.915 -8) 0.008
V3 1) 0.914 3) 116 6) 0.002 -8) 116

I, of course, have looked over the instructions and have checked the connections.
The only question mark in my head is the OT wiring because it has just a few different color wires so I have checked and compared Marstran to Metro to correctly wire it up. I think I did it correctly.

Any suggestions as to where to look or ideas as to what these numbers could mean would be greatly appreciated.

Thanks

[Reeltarded]

Oh no! I have no clue, but you know I am rooting for you, and a simple fix.

I am going to pretend to know anything and predict a wrong value resistor in the dropping string?

Yes? No?

[Hank S]

So I was hoping too that it was the single resistor issue. Need to look longer at layout and amp. There is one issue that folks have which is a the 470k between the two 1 meg resistors. Is it supposed to be the 470 or the 470k? I'll look that one up again.

Hey how's the JMP doing? So many great amps out there and that one has such awesome tone.

[hired hand]

Are you saying you have no B+ on the plates of your preamp.?
Follow it from the start or the finish, take your pick and find out where you loose it.
Check all your cathode connections and cap polarities.
Best

[Hank S]

Since my first post I figured out that I had switched the 16ohm tap and the 32uf cap wire which are next to each other on the board. Fixed it and all voltages were right on.

Next I had a giant hum and assumed that leads from Marstran OT were wrong internally and switched them now I have just a high pitched squeal. Now maybe I was wrong with my fix for loud hum and it was something else. Anyway it a serious high frequency squeal and it goes away when I take out V3.

Next suggestion? Am I wrong to think I fixed one thing if something else is not right?

I have some 2.2k and 8.2k 2 or 3 watt resistors, can I put these as the screen grids to try and stop squeal?

Oh one last thing is I did not have a 5K pot for the Presence so I put in a 10K. Is that a problem?

[Reeltarded]

Last first, strap a 5k across the outside terminals of the 5k pot. Poof! 5k!

Umm.. Ok, I am trading hum for squeal. I think you may have been righter with hum. Does it scream at any volume? Just from turning it on?

[Hank S]

I thought that was the trick with the pot. Thanks.

So just flipping the standby does it. No volume change moving the pots. I read a thread about some of the Chinese KT66 squealing in this amp and the fix was a 5.6K screen grid.

[Reeltarded]

Can someone comfirm the flipped primary result? I have been lucky never to have got that one backwards, yet. I think you might have been right when it hummed, just something else upstream causing the hum.

Hey, post down in the repairs forum for a little more traffic, let's get this thing running!

Oh... NEIL! Help! Roe! Hayulp!

(it's like a bat signal)

[neikeel]

Quick way to diagnose reversed primaries is to lift one limb of the NFB resistor (the squeal is +ve feedback - ie in phase, you want negative feedback, ie out of phase). Lifting the resistor breaks the NFB loop. If it stops the squeal you have reversed primaries and you will need to correct this and then need to track down the cause of the hum (usually a ground loop )

[Hank S]

Alright I lifted the NF and it was quiet. So I fixed the primaries and now I'm on to the ground loop problem.

BAMM! I found it. I never grounded the output jack! With grounded everything is working fine. Read the instructions is the moral of the story here folks.

So right now I have JJs everywhere but the KT66 which are Chinese . So i'll start checking into some of my other 12ax7 tubes and I have a Sovtek 5AR4 which I might try.

Thanks for the encouragement and help.

[neikeel]

Alright I lifted the NF and it was quiet. So I fixed the primaries and now I'm on to the ground loop problem. I have bus wire along the back of the pots, but these pots don't have the lock washers on them. Should I run it to the PT transformer bolt? How do I find the ground loop?

Good.
I prefer to use lock washers between the pots and the chassis - it helps to roughen up the plating too to get a really good contact. Similarly do the same where the other grounds are on the chassis. I generally use SDMs scheme http://home.comcast.net/~jbjdav26/1992/ ... ubbers.JPGon my builds but my originals have the Marshall set up and do not hum .
The Larry grounding is very popular, but you must make sure that you have good contact with your star washers. Yours should be an aluminium chassis so the star lugs should dig in nicely.

Places that often get missed are the bias circuit and creating loops around the inputs, oh and around the presence pot (I cut my bus wire so the bass and presence are not on the bus and the presence is grounded to the PI cap star ground - see above).

[Hank S]

BAMM! I found it. I never grounded the output jack! With grounded everything is working fine. Read the instructions is the moral of the story here folks.

So right now I have JJs everywhere but the KT66 which are Chinese . So i'll start checking into some of my other 12ax7 tubes and I have a Sovtek 5AR4 which I might try.

Thanks for the encouragement and help.

[Reeltarded]

And that is good news! Congrats!