What does a scratch built VTA ST-70 sound like when it clips?

Bob Latino wrote:

Downhome Upstate wrote:My scratch built ST-70 has all new parts (including the VTA noval driver board) except for the original cloth lead output iron. When I drive it hard, the signal begins to break up, producing static, cracks and pops. That doesn't seem like garden-variety clipping behavior. Any idea what this might be?

Thanks in advance for any suggestions and ideas.

Best regards,

Mike

Mike,

You said here in your first post (above) that this was a scratch built ST-70 that used a VTA noval driver board. Could you post a photo of the inside wiring and let us look at how the amp is wired ? One of us may be able to pick up on some issue in the wiring of your scratch built ST-70 that could cause the problems you are having with the amp.

Bob

Thanks, Bob. Absolutely. I'll do that when I get home tonight.

Best,

Mike

That's better

#3

Another view:

Pic #6

Another angle:

No. 8

Yet another,

No. 10 (a repeat?)

what brand/make of output transformers are you using? I don't recognize the wire colors used on yours.

Dynaco A 470's. The cloth leads are just really faded. Also, unfortunately, the iPhone pics came out blue-green, and don't help color id much. (I don't have another camera.)

I cannot see anything that is an obvious problem/concern in the wiring, although as Bob has indicated, color in the photo's is not the best, going by that blueish/green tint in all photo's, did you take those under fluorescent lights?
I have noticed what looks like some black sleeving or heat shrink on the speakers wires, were those wires extended? One area to maybe have a closer look at, if they are extensions, would be the solder quality, that of course means unsoldering the speaker wires from the tube sockets and removing the sleeving/heat shrink to inspect those solder joints.

one very obvious thing I see is that the speaker outputs aren't grounded, the black lead of each channel needs to be tied to chassis ground.
Check your wiring against the VTA70 diagram here:
 www.tubes4hifi.com/VTA70wiringdiagram3.GIF

tubes4hifi wrote:one very obvious thing I see is that the speaker outputs aren't grounded, the black lead of each channel needs to be tied to chassis ground.
Check your wiring against the VTA70 diagram here:
 www.tubes4hifi.com/VTA70wiringdiagram3.GIF

nicely spotted Roy...would not have picked up on that from the photo's.

Bob Latino wrote:Double check that the BLUE and BLUE/WHITE leads on those original cloth lead A-470 output transformers go to pin 3 of each output tube and that the GREEN and GREEN/WHITE leads go to pin 4 of each output tube. You did mention that the leads are faded and sometimes blues and greens can look the same. It's too bad that you don't have a better digital camera to post some better (larger) photos. It is hard to see the detail of the tube socket wiring on the output tubes ..

Bob

Thanks, Bob. Will do. And if I can take some better photos, I'll put 'em up & delete the blue-green tinted ones.

tubes4hifi wrote:one very obvious thing I see is that the speaker outputs aren't grounded, the black lead of each channel needs to be tied to chassis ground.
Check your wiring against the VTA70 diagram here:
 www.tubes4hifi.com/VTA70wiringdiagram3.GIF

Thanks, Roy. Damn. Don't know how I missed that. Will do.

No more blue/green, anyway.

much better!
As Roy has already suggested, first thing, run a wire from each of the -ve/black wire of the speaker terminals to ground, connect each wire to the existing ground star lug/s next to your multi cap, then fire it up and run it again and see if you still get the problem.

MontanaWay wrote:I cannot see anything that is an obvious problem/concern in the wiring, although as Bob has indicated, color in the photo's is not the best, going by that blueish/green tint in all photo's, did you take those under fluorescent lights?
I have noticed what looks like some black sleeving or heat shrink on the speakers wires, were those wires extended? One area to maybe have a closer look at, if they are extensions, would be the solder quality, that of course means unsoldering the speaker wires from the tube sockets and removing the sleeving/heat shrink to inspect those solder joints.

Good eyes (even if my photos were terrible).

That black stuff is heatshrink placed over a portion of the original (uninterrupted) output xformer leads where the insulation was cracking or frayed, and not a splice.

Thanks for looking.

And yeah, I'll ground the black speaker leads to the star ground and fire it back up!

Mike

Downhome Upstate wrote:

MontanaWay wrote:I cannot see anything that is an obvious problem/concern in the wiring, although as Bob has indicated, color in the photo's is not the best, going by that blueish/green tint in all photo's, did you take those under fluorescent lights?
I have noticed what looks like some black sleeving or heat shrink on the speakers wires, were those wires extended? One area to maybe have a closer look at, if they are extensions, would be the solder quality, that of course means unsoldering the speaker wires from the tube sockets and removing the sleeving/heat shrink to inspect those solder joints.

Good eyes (even if my photos were terrible).

That black stuff is heatshrink placed over a portion of the original (uninterrupted) output xformer leads where the insulation was cracking or frayed, and not a splice.

Thanks for looking.

And yeah, I'll ground the black speaker leads to the star ground and fire it back up!

Mike

oh good!
It will be interesting to see if you still get your problem once you've grounded your speakers -ve post!

PROBLEM SOLVED !!!!!!

Thank you Roy, Bob, Montana and everyone! This is just a terrific community of enthusiasts, and I'm proud to associate with all y'all (Groucho notwithstanding). Thanks again!!    

R2D3

Downhome Upstate wrote:PROBLEM SOLVED !!!!!!

Thank you Roy, Bob, Montana and everyone! This is just a terrific community of enthusiasts, and I'm proud to associate with all y'all (Groucho notwithstanding). Thanks again!!    

well that is AWESOME news! enjoy your 're born' ST70!