Help with Mark III Rebuilds

This is a note I sent to Triode:
Hello-
I hope you can solve a confusing problem with my Mark III rebuilds.  Your records will show I bought two SDS cap boards from you, and I bought two Poseidon boards direct from DIYtube (their forum has not solved my problem).  I built one Mark III with original Mark II transformers, the output being the A-430 with 8 and 16 ohm outputs, with your SDS and a Poseidon board.  It works fine.  The second amp I used the A-431 transformer I bought from you, and a new P-782 that I bought from Dynakit Parts, using the same boards.  On startup, as soon as the tubes have warmed, I get a very loud motorboating.  I have checked and rechecked my wiring and both amps are exactly the same.  The first one is dead quiet with no hum, so I don't think the problem is grounding.  One answer from the DIYtube forum was that the SDS board is at fault (this person seems to be very much against cap boards).
Any help and/or suggestions will be very welcome!
Thank you,
 
Frank Dodson

Please help if you can!

did you check the polarity of the transformer leads on the OPT?   If you've got them backwards you'll get positive feedback, loud and bad sounding.
It's easier to swap the two output leads on the board then to swap the four wires on the output tube sockets.

Roy, you are a genius! That was the problem and now all is well. Thank you!

Roy, you are a genius

I second that.........

OK, Roy (or anyone else)
I received tubes today and fired them up.  I had used only cheap "test" speakers with inputs shorted.  Now, the original good amp sounds great, but the one that I had the OPT reversed has very low, distorted volume.  I swapped tubes from one amp to the other with no change.  The problem amp has all new parts, whereas the good one has the original transformers.
Please give me some clues/solution.  I am at wits end and do not understand what is wrong.  I have built many amps over the years, starting with Dynacos in the late '50s and never had trouble until recently.  Maybe at 75, I need to give it up, but I get a lot of enjoyment from building amps.

Well, I am a little red faced, but the solution is so bizarre, that I will share.  One tab on the slide power switch had too much solder and created a tiny gap between the tab and the metal body of the switch which is of course grounded.  Now, all really is well and sounding great!

KYG - at least you could sit back and find the problem.  
I get WAY too many customers who do something similar . . . built a kit, something wrong, with no thought at all, throw it in a box and mail to me for $50 shipping.
Turns out their problem is a solder blob shorting something out.   So I fix it in 5 minutes, then have to charge them another $50 to ship it back.
I know teenagers don't have the brains we did back in the 60s, but when 30 or 40 YOs do this kind of stuff, it bugs me! 

Well, I am doubly red faced now!  I have NOT cured the problem with the second Mark III.  I guess my ears are going along with everything else!  The amp with all new transformers (not suggesting anything - just for identification) still has very low, distorted output with no high frequencies.  I swapped tubes again with no effect.  Roy, and anyone else, please come to my aid.  Any and all suggestions gratefully and eagerly accepted!

Grab the schematic pick a wire and follow it.  Start with the power supply.  Follow each wire and make sure it goes where it's supposed to.  You shouldn't have solder blobs anywhere.  Remember to heat the joint before applying solder.  Resolder every joint and if there's excess solder, suck it out and do it over.

Take a break.  A day or 2 or a week.

Then go back and check every wire and solder joint again.

take a day off, a step back, then look at both amps again.   One works perfect, the other doesn't.   Find the difference, and fix it.

You both give good advice!  Yes, I am to the point of seeing only the trees, not the forest.  Thank you!

Using the VTA MKIII 9 pin board, and just so I don't get my wires criss-crossed:
C6 output should go to the power tube that has the OPT's solid green and blue wiring, and C8 output should go to the power tube that has the OPT's striped white/green and white/blue wiring.
Please correct me if I have it backwards and I appreciate the check.

Hi -- I am looking at one of the pictures that Roy sent me for the wiring of the VTA Mark III 12AU7 Boards.

The tab marked V3 OUT which is next to R30 goes to Pin 6 of the output tube next to the quad cap.

The tab marked V4 OUT which is next to R32 goes to Pin 6 of the output tube next to the GZ34/5AR4 tube.

So the V3-V4 outputs on the PCB are criss-crossed.

Since you reference C6 and C8 ---
(1) C8 shares a trace and solder pad with V4 OUT (on the bottom of the PCB)
(2) C6 shares a trace and solder pad with V3 OUT (also, on the bottom of the PCB)

The Output Tube next to the quad cap has the striped white-blue and striped white-green wires at pins 3 and 4.

The Output Tube next to the GZ34 Tube has the solid blue and solid green wires at pins 3 and 4.

Hope this helps.

If not, let me know, Stew

I am not rebuilding a MKIII, I am scratch building something that is a cross between a MKIII and a M70 that is laid out completely differently than a MKIII.

All I need to know is which power tube the output of C6 goes to; the power tube with the OPTs solid green and blue wiring pair, or the power tube with the OPTs striped green/white and blue/white .

I'm looking for someone to say "the ouput of C6 goes to the power tube with the OPT's ________ colored wiring.

Thank You.

Hi Again --- Sorry, I was unaware of what you are building.

All that I can say is that on a Mark III with the VTA board the wires are criss-crossed. 

When they are not criss-crossed, you get a "motorboarding sound" thru your speakers when the system is playing.  I had this happen with with the 12AU7 version on a pair of Mark IIIs and also on a pair with 6SN7 tubes. 

Maybe asking a direct question to Roy will get you the answer??

To get back to my original inquiry, I now have the final solution, and although embarrassing to me, perhaps it will help someone else with a similar problem.
Roy's advice cured the motorboating, and while the solder "blob" was worrisome, it had no direct effect.  As it turns out, the amp was working
fine after making Roy's change.  The problem was that a tube in my preamp went bad and that was the source of the poor sound!  Finally, I compared the two amps, and had them in a temporary setup.  I needed a sound source. so I used a spare FM-5 tuner and the test speakers.  Both amps sounded fine, so I reinstalled them in my system and got the bad channel again.  Yes, I am slow, but it finally dawned on me to switch the two outputs from my preamp, and lo and behold! the trouble changed channels!  Pulled the preamp and sure enough, a 12AX7 had gone bad!  Hopefully, someone else can learn from my experience and save some time.  It was a rarity for the preamp to go bad at the time I was trying new amps, but it does happen!
Thanks again to Roy - this just shows how valuable this forum is.  Nowhere else have I consistently received such quick, knowledgeable help.

the output labeled V3-OUT goes to the blue-green tube, the output labeled V4-OUT goes to the blue/white-green/white tube
(same as gener8r said in post 12)

Stew and Roy:  Thanks to both for help and assistance rendered. It is greatly appreciated!

kygeezer:  you're only a geezer when you're too old to learn!