A newbie to tubes and to kit building.

Hello everybody.

A few weeks ago, I fulfilled a plan I hatched about ten years ago by ordering an ST70 from Bob Latino. Over the past decade I've revisited the idea of building a kit amp and did my research, and I am so happy to finally be doing this. I have very little experience with electronics (other than the ones I buy off the shelf) so I approached this with great care. I recapped a pair of Dynaco A25 speakers several years ago, but that was nothing.

Watched many hours of instructional videos, and read the sage advice by forumers here (like the advice in the other newbie thread a few posts back). This place has been such a great source for knowledge and I've learned so much already.

Got to say, Bob Latino puts together a great kit, and the instructions are super clear. Now I'm just a few steps away from the initial power up process, and I'm absolutely worried that I messed something up. While most of the soldering was easy, and I got progressively better as I put this together, so much so that I've redone some of the early work. And there are parts that are very tricky to reach (like those output tube resistors, after wiring up the tube sockets!)

I'm worried that I messed up somewhere along the way, and that some of my solders are cold. And I'm afraid that in some places, I may have touched a wire with my soldering iron and singed some insulation. (no exposed copper, but I still have worries). My fear is that something goes wrong and I have to diagnose it, and once I start doing that, I have to play with 400 volts as carefully as I can.

Anyway, that's all pre-startup anxiety. I do have concrete questions:

1). Upon Bob's advice I checked line voltage. I know that 122vac and higher is too high for these amps. My line voltage is 121v. It seems stable as I've checked it numerous times during the day from different outlets. Is that too close for comfort or should I invest in a voltage regulator or bucking transformer?

2) If I accidentally melted some insulation onto the board while soldering a wire onto the VTA PCB, is that a bad thing? (newbie question-- probably sounds like it's headed to disaster and brands all my other work as shoddy, but it's just this one instance, really. an accidental touch while navigating a tiny space).

Looking forward (with trepidation) to turning this amp on for the first time!! Hopefully it goes on without a hitch.

Welp. We have a problem. I finished assembling the amp tonight. And then before I started with the initial startup, I made sure all my solder joints were solid.

I got to the part where I start biasing the output tubes. The first three were fine, if a bit jumpy at first.

The last tube I attempted to bias was the rear right. And it is not responding to turning the pot. It shows a constant .037 at full CW and full counter clockwise.

What could be the potential source of this? I've turned the amp off, changed tubes to see if it's a tube problem, but still nothing has changed.

Thanks in advance for any advice.

What should I check here?

As always a good quality photograph speaks wonders.... Maybe a few more posts required first.

Thank you. I opened it back up and redid the soldering on the right rear pot and then restarted the initialization and lo and behold, it held a near stable bias!

A slightly "wandering bias" many times can be traced back to line voltage changes. An increase or decrease of even 1/4 of a volt on your line voltage will show up as a change in your bias. If all output tubes go UP in bias slightly, usually this is because your line voltage went UP just a little. Bias will also rise slightly as the amp warms up. Always set the bias after the amp is fully warmed up (30 to 45 minutes).

Bob

eatabagel wrote:Welp. We have a problem. I finished assembling the amp tonight. And then before I started with the initial startup, I made sure all my solder joints were solid.

I got to the part where I start biasing the output tubes. The first three were fine, if a bit jumpy at first.

The last tube I attempted to bias was the rear right. And it is not responding to turning the pot. It shows a constant .037 at full CW and full counter clockwise.

What could be the potential source of this? I've turned the amp off, changed tubes to see if it's a tube problem, but still nothing has changed.

Thanks in advance for any advice.

What should I check here?

Seems like the pot is either not soldered in properly, or the pot itself is defective, or there is a wiring error elsewhere in the bias circuit specific to that tube - make sure that the connections on the socket are correct (location) and complete (properly soldered).

With the tubes out, the amp unplugged, and upside down, take resistance readings on all the bias pots - the point is to make sure that they are all functioning correctly mechanically. So if moving pot A makes a change, that same behavior should happen the same way with the other three.

Troubleshooting is much easier if one may compare to a "Known-Good" part or piece. You have three of those.

Thanks, folks, for the advice. I have definitely reseated and resoldered that one pot and it works great now.

Now my only problem is that I think I'm addicted to the new amp smell. This can't be healthy, right? I'm sitting next to it right now and sniffing this thing. It is not the optimal listening position in relation to my speakers, but it is the optimal smelling position.

Then you need to build another, SP14 perhaps?