New CG build. Smoking PT

[rp]

Yea! Nice article. So basically, there is never a reason not to have a B+ fuse in any amp? Smaller transformers cost $$ too

There is a reason: fast blows in the B+s like to pop unexpectedly on boot even if all is OK, leaving you high and dry if you don't have a spare. So carry a spare / tell the client to carry a spare, and put the B+ fuse on the chassis. Wire it right, hot on the end not side. On ss rec amps, using a standby or not, I found a FB B+ fuse blows way too often, and a SB won't do much that the mains fuse won't cover. I can understand why large amp companies might forgo a B+ fuse. Chassis mount is kind of risky with pokey clients, internal would force too many service calls and unhappy customers who thinks the amp is wanky.

[DocJames]

Ok here's some pictures as promised. I'd love someone just to take a look over things to see if I'm missing a glaring mistake. Forgive some of the lead dress, as this was a bit of a tight squeeze into the chassis, I know it could be better.

[Blackburn]

Looking at your wiring is fine for now, but you should really get your power tubes checked along with the rectifier.

[DocJames]

Thought I'd update on my adventures with carmen ghia's. So I took the chassis and tubes over to a local Amp tech. This is purely a side hobby, so I don't have my own tube tester, etc. He checked the PT, and confirmed that it had gone the way of the dinosaurs. I got him to check the tubes, and they all tested within normal limits. SOooo, I rechecked all my solder joints, threw in the new PT that I had, and crossed my fingers. Bulb test was good, had a nice flare on the bulb and a soft glow from there, with all the tubes in. Perfect, nothing blew up!

But I have no sound..... I was getting sound before the PT blew and now I have nothing. My voltages are as follows:

5Y3 1 - nc
2 - 302
4 - 285 AC
6 - 286 AC
8 - 302

V4 2 - G
3 - 10.66
7 - 297.4
9 - 295

V3 2 - G
3 - 10.62
7 - 297
9 - 296.3

V2 1 - 278
2 - 24.56
3 - 26.78
6 - 179.7
7 - 23.55
8 - 26.8

V1 1 - 258.6
6 - 264

Somethings going on in V1 and V2, but I don't know what. Might be time in invest in a oscilloscope, whadda ya think?

[bal704]

In these cases I use an audio probe to figure out where I'm losing sound. Might be something simple like a bad solder joint that finally worked loose.

[martin manning]

If you are getting zero volts on V1 pins 3 and 8, it looks like there is no current flowing through either triode. Is the heater glowing? Are the cathode grounds good?

[Phil_S]

Are there some wires under the board to supply B+ for V1 and V2?

It is hard to see what wires go where because they are all black. I think I see the B+ supply going from the 8uf cap directly to the preamp plates. Counting from the right, I'm looking at turrets 4 and 5 (near sockets for V1 and V2.) On T4 I see a 100K (plate load?) with nothing obvious attached. On T5 I see a 100K + OD cap and nothing obvious attached. Can you clarify what's there?

Also, at T7 (8uf cap?) nothing obvious is attached.

I say "nothing obvious" because I expect to see wire emerging from under the board, but see none.

For your future builds....for the most part, experience tells me to avoid under board wires for this very reason. Now and then circumstances cause me to put a wire under the board and when I do that, I mark the top side of the board. Generally, I do my best to avoid under board wires between turrets. I also use a colored wiring scheme to help me to easily sort out the purpose of the wire, something like this: B+ =red, plate = blue, grid = green, cathode = yellow, ground = black or brown, screen = orange.

[DocJames]

Great advice Phil, thanks. Yeah there are a number of under board wires. I tried to get a look at them with a little mirror I have and they all seem to be in place. I had that happen on a rocket build, and what I've started to do is bend the wire at the very top of the turret so it can't "drop out." That being said, I may just do away with under wires that for the very reason, because it's a pain in the ass to get to at this point.

The audio probe sounds interesting. I'll google that a bit to see what it does, as I haven't used that before.

Grounding may be bad, I'll flow all those connections again. When I pulled the PT, I disconnected all the lines to lift up and check the OT, which seems to be good, so something has come loose somewhere, in the fiddling that I did.

Is the audio probe the only way to figure out where the connection is lost?

[bal704]

Here's a link I found to the audio probe:

http://diy-fever.com/misc/audio-probe/

After hooking up the audio probe end to another amp, basically you touch the free end of the capacitor to a point in your amp you're building. I hook up an ipod and play music into the amp under construction. When I touch the audio probe to the input jack, I should hear whatever is playing on the ipod coming from the other amp. Then just follow the audio through the amp under construction using the schematic. If you're getting no sound out of your amp, but you can hear audio going into the PI using the audio probe, then you've narrowed your search down a bit.

Obviously, you might need a bigger cap and be careful. I use a small battery powered amp as a slave amp.

[Firestorm]

When I want run wires under the board, I like to do it like the old perf-board Marshalls: bring the wire up in a hole next to the turret and then wrap it. Can't fall out and you can see the connections from the top.

[DocJames]

Just wanted to resurrect this post, and thank everyone who gave some input.

I talked with Ken at watts, and he stood behind that he hadn't seen a bad PT from heyboer in 10 years. I talked with Heyboer and they sent me a new one, at a reduced cost, which I was very thankful for. Blowing PT's can be an expensive hobby .

Anyway, as posted before, I had another PT sitting here for my next 18 watt build, so I swapped it in. Couldn't get any sound. floored me, so I ripped the the board out, took off all the components, re-tested everything, re-drilled a new board with no under-wires, re-soldered everything. Made myself a handy audio-probe, which was put to good use. I used an old mini-stereo cable, clipped one end off, and put a 0.01uF 650V cap on there. Other side went into an old set of computer speakers. I figured a cheap set of those getting blown is better then sending it through my acoustic amp.

Boom, she's working, and it's a beautiful sound. He's some pictures of it! I'm digging the sound, very low hum, lots of harmonics when dimed, and it can get mean. I think my nephew is going to like it, he better by the amount of grey hairs this one gave me!

[Blackburn]

Glad to see you got her working.

[rp]

Part of being a sophomore is when you really hit the wall you have no recourse but to rip it all out and start again. Pre internet and TAG in the late '80s a couple occurrences like yours caused me to hang up my soldering iron for 15 years. So far I've been mentored though may problems here, got all my old projects running perfect and every new amp has worked out great.

Glad you got it working, even if it was the hard way.

[mark enger]

Cool Docjames, looks real nice, great job. mark