Hamptone LCMP Almost Done

Hello- I have been building this, my first micpre for the past couple of months, and it's almost done. In fact it may be done, but I'm having a very hard time troubleshooting a high pitched sound coming from the tubes.
Here's the schematic if it's helpful:
http://www.hamptone.com/figure1.htm

Anyway, this is a 2 channel box I've made, and both channels are exhibiting the same behavior at the same point. when I turn the gain knob up, the amp is quiet from a starting point of 6 o'clock until about 2 o'clock. At this point one, or maybe both, of the tubes in the channel starts to emit a high pitched whining sound that lowers in pitch a little until about 3 o'clock and then starts what I think is called oscillating. It sounds like an ant's motorboat. The troubling sound occurs in the last 20% or so of the gain knob's range.
I have also found that corresponding to the behavior above, the stage 1 plate voltage on the 6072a remains steady at 106VDC until the noise appears, wavers a bit, and then increases to 161VDC. If the 20db pad is engaged, the sound is less present, and the voltage only increase to a maximum of 138VDC.

Weird?

Just so I'm being clear, the signal is actually coming from the tube(s). You can hear it in the signal path (both channels pass audio) but you can also hear it just sitting near the box with nothing plugged into it.
I have to say, I'm using crappy Ratshack pots right now. Audio taper. I'm ordering a pair of stepped attenuator kits from Marchand, but I figured I could find a use for the pre in the meantime even if the pots aren't great.

Could the low rent pots be the problem?
The pad switch?
I'm pretty sure it's grounded all around, but one never knows until one finds that little broken wire, eh?

Thanks in advance for any help any of you can offer.

Best,
Chris.

"Just so I'm being clear, the signal is actually coming from the tube(s)."

What I meant was is, the sound is actually coming from the tube(s).

Thanks.

have you tried replacing the tubes :?

Have you tried anything yet :?

I have not tried replacing the tubes, as they are NOS from Tubedepot. GE 6072a's and RCA clear top 12au7a's. I paid for testing and matching, and I've handled them very carefully. Now, I realize it could still be the tubes, but I put that very far down the list because I bought them from what I thought was a reputable company and I don't have others to replace them with. I'll check with a friend who builds some stuff, and see if he has some I could try.

Things I've tried:
Checking all the grounding points for continuity and checking the pin voltages against the ones listed on Scott Hampton's website. All this checks out. Resoldering all the tube socket pins, freeing one leg of resistors and checking that they're good, visual checking caps, checking for pass audio...I'm pretty much a novice, so I don't know what else to check for. I spent a great deal of time laying this out and making sure everything checks out against the schematic, time and again. As I mentioned in the original post, the preamps work fine until the upper end of the gain pots. In fact, they sound really, really good.
Do you have any other suggestions?

Thanks.

I apologize, but I have another correction to my original post. It was late at night after a few hours of trying to track down the problem and I wasn't as clear as I could have been.

When I wrote that the amp was quiet from the 6 o'clock position to about 2 o'clock, I meant that there was no undesirable noise. Otherwise the preamps sound great.

All apologies.

I shorted the input and the offending sound disappeared.

Glad you got it resolved...sounds like you need some sort of permanent termination at the input?

Thanks for the suggestion, but I think I'll be OK as long as I don't run either channel wide open with no input. It would be nice to have an unused channel quiet when it's gain knob is turned up full, but it's a convenience I'm willing to forego. Also, once I've got the stepped attenuators in, I won't need to worry about "re-finding" a setting. Scott Hampton confirmed that the problem may include the temporary, cheap pots as part of it's source. Most likely, noone needs to worry about this with this preamp if they're using good, quality components.

Rock like hell.

im about to build one of these myself. jensen inputs and altec output iron. what are you using for iron and power supply? any pictures? congrat on finding the problem.

Thanks. I used Jensen 115k's for the input xfo's and Edcor WSM15K/600's on the output. For power I used International Power PSU's housed in a separate box for isolation. One for phantom and heater power, and another for the B+. They were an expensive way to go maybe, but they seem to be dependable and easy to work with. I haven't taken any photos yet, and I'm not sure how to post them to this site, but I think I ran across a thread about how to do that recently, so I'll check into it. I think I saw you had a post about building this awhile back. I highly recommend it now that I've heard it.

Best.

yeah, ive had the iron for a while but the pultec, neve, 1176 got in the way. im going to go with international psu with external housing also. sounds good.

thanks

pants-

how is your audio coming into and going out of the box? i ask because if the wires are not well shielded, and they are near each other, you can get feedback from the output to the input, which may cause oscillation. try taking a chopstick or plastic pen and moving your wiring around while it is oscillating and see if that has any effect.

ed

OK- Here's a couple of photos, but I'm no digital photographer

As you can probably deduce, the Hamptone is the top box...



Here's a shot of the boards...




I got the epoxy glass board from a vintage guitar amp supply guy. I had a tough time sourcing turret boards for the project, so I bought this stuff, drilled holes in it, and put solder lugs on it with a pop rivet gun per a post I saw somewhere around here about making "cheap" turret boards. These wound up being a bit more expensive I'm afraid, but you could build a house with this stuff. It's really strong. A couple of days after I made these I received a catalog from Antique Electronics with turret boards on the cover. I clamped a vacuum cleaner nozzle to the table where I drilled the glass board to catch any dust. You don't want to be breathing that stuff.

A couple of notes:
I would probably make a way to put the boards in on their edge if I were doing this again. They would take up about the same amount of room, but the socket solder lugs would be much easier to get to for testing, resoldering, and troubleshooting.
Also, I used an oversized (6.8uF) Polypropylene for the output cap and put a 665ohm resistor on the output transformer.

Scott Hampton was very helpful with any questions I had.

Let me know if you run into any snags along the way and I'll try to help, but you should probably do it soon, as my mind is like steel sieve.

Best of luck.

I think the photos look fine. In fact I wonder if you could post some of the images you have on my website www.audiotransformers.net? I am looking for photos of how people use EDCOR products.

Looks cool! I've built a few guitar amps point-to-point and found lead dress is important. Ocillations can/will occur with these hi gain circuits. Must keep low level wiring away from higher level stuff and especially the PS.
Get a chopstick(wood) and move the wire around to see what effect rerouting may have.

Thanks Ray-
When you say "low level" and "high level" wiring, what do you mean exactly? Voltages? DC Supply vs. Audio? The PS is housed in a separate box connected by a 6 foot cord, so there's no 120AC near the amp. Despite appearances, most of the wires are as short as they can be in the context of this particular layout. I suppose I could make some longer if I were to find that they were the cause of the oscillation, I just don't know if I can move them enough to see any difference. I'll give it a try tonight though. Thanks again.