New ST70 - initial startup problem

I really enjoyed the build. All seemed to go very well.

For the initial startup - following the steps listed in the instructions: first, the three driver tubes lit up as expected. Following the next instructions - connecting the speakers, and installing the rectifier tube and the LEFT side only EL34 tubes. When I applied power the V3 and V2 tubes both lit, but the V2 tube then began to glow intensely, significantly brighter than the V3 tube. After about 5-10 seconds - ever brightening, I shut down the power. The fuse did not blow. Each of the bias adjusting screws was set in the 12-6 o'clock position.

I have since reopened the bottom and have checked all of the wiring and soldier joints. All seems to be correct. I did not swap the tubes to see if maybe there was something amiss with the "bright" tube.

The second time around, I first turned the two left bias pots all the way counter-clockwise, to reduce voltage to a minimum. Still the V2 tube was significantly brighter than V3.

So, what now my friends?

Paul

Have you checked all the voltages? They are listed at the end of the instruction manual. Double check, if all resistors are at the right place. Additionally, swap the tubes. If the problem follows the tube, then you have a bad tube. If it does not, then there is a problem with your circuit.

I guess Bob will have more specific tips.

Michael

This time, put the second set of two tubes, new from box, on the left channel, 3-VTA tubes in, rectifier in and speakers connected. Same outcome - V2 got very bright and I detected a hum. Turned off and waited ten minutes.

Then, I went to the right channel. Same second set of tubes, 3-VTA tubes in, rectifier in and speakers connected. This time, V6 became very bright.

Photos to follow - they would not upload from my hard drive so I will host them somewhere.

sub'd

I had a similar situation - bent tube pin tabs out a little under chassis, fixed.

All your tube pins (especially bias) may not be seating properly.

so has this been solved SnugDynaco?

Problem not yet solved, but the pin seating is interesting. Is the thought that some of the pins are in contact with the chassis? I had to bend the bias pins to fit underneath the russian caps. The V2 - V7 power tubes do not seem to be in contact with anything else, and the power tubes fit snugly. What specifically should I be looking for in regard to the pins?

With too much voltage going into only two of the power tubes, I will check the voltages per the instructions.

I mean tube pins firmly making contact with tube pins. Bending them out a little (not making contact with chassis!) may help with good contact.

Or, just gently squeeze each one from top of tube socket.

https://www.flickr.com/photos/iiifrdst/

Build photographs to check the wiring.

Don't know about the amp, but that's a beautiful home!!

Many thanks to those who sought to help me. Bob got me straightened around by direct email, suggesting the following:

"I see from your photos it looks like the holes that the three driver board tube sockets are connected to are not filled with solder. Maybe it's just the photo but it looks like you can see through some of the holes on the pins of these tube sockets. Resolder them with enough solder so that you cannot see through the holes.

Check every solder connection on the driver board and resolder any others that don't have the holes completely filled with solder and have a connection. NOTE - there are extra holes on the board with NO connection of any kind. You don't have to fill those holes with solder.

Next we have to test the bias system to see if it is working. To do this ..
1. Remove all the tubes
2. Set the four bias pots at the middle of their rotation
3. Turn the amp on
4. Measure the negative DC voltage on both pins 5 and 6 of all four tube sockets to chassis ground. You should get between -30 an -45 volts DC. Make sure that you set your meter for DC voltage on some scale higher than 50 volts. The key thing I want to see here is if V2 and V7 are getting those voltages. If they are not - then there is an issue with the bias system.

Let me know what you find."

What I found was that two of the bias wires had extended through the VTA board, grounding to the underside of the chassis. Bending them down and out of the way resolved the "hot" tubes.

Good job! I was just about to mention the grounding issue if you leave the leads long on the VTA board. Good idea once you're sure everything's working to go around and snip any and all extra wire you don't need to complete the circuits. I'd also take the opportunity to poke around with an insulated hook probe and a magnifying glass. Most problems from here on in can more than likely be traced to bad solder joints. Maybe not an issue right now, but add some hot/cold cycles as you use the amp, and they tend to make their presence known.

(Maybe it's just me, but the can cap in particular looks like it might have some cold joints?)

I'd also double check the Russian filter caps for clearance. They look a bit tight to the board? Those metal cases can short out in some of the strangest places ...

Oh ... and I noticed a burned wire featured prominently near the can cap also. File that in the "it happens" category. I'd trace every wire and make sure you don't have any damaged insulation elsewhere that could cause a short.

Also, here's hoping you marked the tubes you red plated. That can be the kiss of death, and you'll want to keep a close eye on those for a while to make sure you didn't damage them. Especially check the bias regularly if you use them and make sure that's not erratic. Ya, I know, sucks, as tubes aren't exactly cheap, but hey ... just sayin' ...

PS ... long as you're in there, one fairly common complaint is that the power tube socket can wiggle around and make the tubes feel loose. A quick fix is to grab hold of each terminal, pull down a bit, and give them a very slight twist. This locks them in tight and shouldn't affect the tension on the pin grippers if you do it right.

don't you just love it when it all works out and you can enjoy some awesome music!!  

I don't think they make leads long enough for mine to short ... ;-}