watt finally builds an amp (results TBD)

Well.

Doesn't work yet.

Tried bringing it up on variac + DBT, with meter monitoring B+. When I get to around 90V, the DBT bulb (100 W) begins to glow sort of faintly, gets brighter as I increase the voltage. Not full intensity, but enough to make me stop and post here.

After powering it down, I started to check for errors in the wiring. Here's something that I noticed that confused me at first, but the more I think about it, it might be OK. I hope that you'll tell me.

I have continuity between the heater circuit and the ground bus. Also continuity between the HT winding and the ground bus. Is that explained by the fact that both have center taps that I connected to ground? Or does it point to a problem that I need to correct?
 
B+ is 325V

EL84 plates: 305 and 326V
EL84 grids: 304 and 304V

12AT7 plates: 324 and 320V (too high? isn't plate max 300V?)

When I measure the filaments, should I be measuring between each pin and ground or between both pins?
 
When I measure the filaments, should I be measuring between each pin and ground or between both pins?

Google is my friend. Between both pins.

About 5.4 VAC all the way down the line.
 
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I made the voltage measurements above while the amp was connected to a dim bulb tester. That's wrong, right?

I'm not doing that anymore, and I measured voltages again. They're pretty high.

B+ is 370 V. The schematic specifies 315V. As I read it, the power supply design assumes a 300-0-300V power transformer. (Quoting from the schematic note: For use as above with 5R4, HT secondary must be 300-0-300V 120ma (lowest acceptable value).) I think the one that I've used here is 315-0-315V.

EL84 plates: 352 and 370 V
EL84 grids: 350 and 350 V

12AT7 plates: 364 and 368 V

I also have an intermittent filament connection between the first and second EL84. I'm now getting around 6.5 VAC at the filament of the first tube and virtually nothing at the second. I re-soldered the connections last night and, after that, everything seemed to work. I was able to hear a signal through the amp for the first time, which was exciting. But today, nothing. Second EL84 and the 12AT7 after that are cold.

I used solid core wire for the filament connections, and I'm thinking that I may have broken one by getting a little overzealous with the twisting. I don't see anything that looks like a break in the wire, but I don't know what else it might be.

I don't have stranded wire of a small enough gauge to work with these socket terminals, so I need to order some of that. But it seems that I'm probably gonna need some other parts to bring down the B+.

So I'm gonna sit here and think about it. Advice welcome!
 

Redboy

Knobophobe
How confident are you in the health of your EL84 tubes? Your plate voltage readings are pretty far apart in both cases, so I wonder about 'em.

Would you post a couple of clear underside shots so we can proof your work? I don't see the choke in your last photo, but I assume it made it into the finished amp... If so, we could tinker with your power supply to tune the voltages down a bit, but it would be best to confirm that things are wired properly, first.

You got signal! That's exciting!
 

Wntrmute2

Not So Mediocre Member
Can you measure the continuity of your heater wires? I used 18g solid cor mil-spec stuff and twisted the shite out of it without breaking the wire. I doubt it is that. You can see how tight I twisted mine just below and on the right upper tube socket where it pokes out of the shielding.
IMG_20200215_113234 (2).jpg

IMG_20200215_173931 (1).jpg
 
How confident are you in the health of your EL84 tubes? Your plate voltage readings are pretty far apart in both cases, so I wonder about 'em.

Mostly confident. They're new Sovteks, bought when I realized that I only have one pair of EL84s, and they're Amperexes. Didn't want to power up with those in place.

I'll get those pics up this afternoon.
 
Can you measure the continuity of your heater wires? I used 18g solid cor mil-spec stuff and twisted the shite out of it without breaking the wire. I doubt it is that. You can see how tight I twisted mine just below and on the right upper tube socket where it pokes out of the shielding.

Good point! I do have continuity.

<At this point, I was pretty stumped. So I just went down to where I was working on the amp and reseated the offending EL84. Moved the heater wire a bit, too. And then this happened . . .>
 
Don't know if you all have ever had this experience on an audio discussion board. But let's say there's some poster asking for help, and he posts voltages to try to give some kind of information to the people who are really nicely trying to help him.

Then he comes back in a later post and says, wait, those voltages I gave you were wrong. And, reading it, you're like c'mon man, learn to take a measurement . . .

Well, that's me today. After re-seating the tube and wiggling the wire, here's what I get (should be a video here, but I'm getting an error at the end of the upload).

Anyway, all tubes lighted. Bill Evans playing through some test speakers. And voltages look a lot better:

B+: 325V

EL84 plates: 306 and 306V

12AT7 plates: 159 and 149V

While I am, on the one hand, elated, I am also afraid that next time I go down to power it on, I'll find the same problems.

So, those wiring pics are coming up . . .
 
Can you post a closer pic of the terminal strip by the power transformer? It's hard to tell, but the solder joints for the heater wires look bare'ish. May just be light and angle, hard for me to tell.
 

Wntrmute2

Not So Mediocre Member
I don't have anything meaningful to contribute other than your build seems straightforward and since it works the problem has to be a faulty part or a poor connection. I'd like my builds to come out so neatly and simply laid out.
 

Wntrmute2

Not So Mediocre Member
Can you post a closer pic of the terminal strip by the power transformer? It's hard to tell, but the solder joints for the heater wires look bare'ish. May just be light and angle, hard for me to tell.
I see what you are. Actually all three outermost connections look like they need a closer look.
 
Can you post a closer pic of the terminal strip by the power transformer? It's hard to tell, but the solder joints for the heater wires look bare'ish. May just be light and angle, hard for me to tell.

Thanks for looking, gable.

I wouldn't say I'm 100% confident in any joint soldered to those strips (I found that these didn't take the solder very well, and the holes are really big to fill), but I just shone a light on those, and I think you're seeing reflection. They're full, and the wire's in there pretty good. Still, that is the 6.3V winding, so it couldn't hurt to touch it up. I'll do that.
 
Also, I can't seem to find where your recitifier's filament is tied to one leg of the anode. I could just be missing it in there.
 
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