Well...it started out good....

[big mac daddy]

I finished a build of a 5e8a kit today - everything went well...powered up good, little bit of hum, but I could live with that (no more than some OEM amps I have. The about ten minutes in, the sound volume cut by about 90%. 12 sounds like 2 should. All circuits (tone bass presence) work ok, both the bright and normal inputs work just at the reduced volumes...even though it is quiet, the amp breaks up nice right around 9 or so. I have checked all the connections and they seem fine...any thoughts on what to try next would be greatly appreciated...I am a bit of a novice. Thanks.

[menger66]

With some nice detailed pix im sure we could figure it out, no smoke or fire so im sure your very close. mark

[martin manning]

Same result on both channels? Then you know the problem is after the volume controls. A quick thing to try is swapping a known good tube in at the third and fourth preamp sockets (V3 and V4). Hum and low volume makes me wonder if V3 has had a heater-cathode insulation failure.

If you have no luck swapping tubes, then get voltages (carefully!) at all of the tube pins and we can see where you are. Pictures are a good idea.

[TUBEDUDE]

You tried a different speaker I suppose.

[big mac daddy]

Thanks for the swift replies all -

same on both channels ill get a hold of some tubes today and try swapping them out.

no smoke or fire or abnormal smells

I did try a different speaker - thanks also different guitars and different cords

One thing I did find after I posted was a bad solder joint at a ground point...found it by touching it with a chopstick while on - made some crackles...powered down and resoldered and a bit (very small bit) of the volume came back when dimed. There is no inconsistency Is it possible that when firing up the first time that the loose ground took out a preamp tube?

[Phil_S]

Is it possible that when firing up the first time that the loose ground took out a preamp tube?

I don't think so. Try simply swapping equivalent tubes (keep track) to see if the problem moves around. That's a decent diagnostic for the tubes, but not always.
Pictures and voltage readings provide a pile of useful information.

[TUBEDUDE]

so the amp still sounds normal until 10 minutes or so of operation? And after the volume loss, the controls work the same, and the overdrive characteristics are unchanged but just at the lowered volume?

[big mac daddy]

100% correct tubedude.
The only thing I did in between the correct operation test run and when the volume decreased was mount it in the cabinet. after mounting, it worked correct for a few mins then cut out completely. After finding and resoldering the loose ground, the volume went to its current state (there on both channels but dramatically reduced) all controls and overdrive characteristics work "normal"

[Structo]

Check the power tubes.

If you are using a speaker cable from the amp to speaker be sure to check that out as well.
I had one that was intermittent and I thought it was something different.

Does your amp have two 5U4GB rectifier tubes?

[big mac daddy]

I'll get voltages now with some pics.

It does have 2 5U4GB rectifier tubes.
I've attached a schematic and layout for the build (if that is not allowed let me know I will edit it away and sorry in advance)

There were 2 changes made

1 - took the "ground switch/death cap" out of the picture entirely. basically it is a useless switch on the chassis now

2 - the PT had a heater centertap to ground so the two 100 OHM resistors off the pilot were omitted.

[Structo]

I assume you are using 6L6GC power tubes?

How are the grounds from the power tubes?

Do you just have pin 8 to ground?

Good idea to go around and check all grounds.

Do you know how to chop stick an amp?

With the amp on and connected to a speaker, take a wood dowel/chop stick or similar (don't use a pencil as the lead is conductive) and poke around at connections, solder joint, wires.
Listen for crackles, loud, soft changes.

You may have a loose connection or bad ground.

[big mac daddy]

yes 2 6L6GC power tubes
pins 1 & 8 to ground

I checked the grounds yesterday (continuity from all ground points to chassis), but will do it again today

chopstick is how I found the initial bad ground. It was the grounding point for the AC in, all pots and caps from pots to ground, standby switch cap, and the two ground leads from the filter caps (left top two on the layout on the fiberboard)

[big mac daddy]

OK...fiberboard pics attached. more posts for the pots and tube sockets ( hope the wiring isnt too bad! - please give me feedback/constructive criticism - its how we learn!)

here are the voltage readings:

Rectifier 1 & 2 No tubes installed:
2: 5.38AC
4: 353.7AC
6: 350AC
8: .001AC

Rectifier 1 & 2 Tubes Installed:
2: 452DC
4: 348AC
6: 349AC
8: 453DC

Power Tubes Tubes Installed:

V6
1: .001DC
2: 3.31AC
3: 439DC
4: 441DC
5: -52.8DC
6: -52.9DC
7: 3.307AC
8: .001DC

V5
1: .0DC
2: 3.307AC
3: 445DC
4: 447DC
5: -53.6DC
6: -53.6DC
7: 3.322AC
8: .0DC

Preamp Tubes Installed:

V4
1: 228DC (Note - loud ground sounding pop when i touched this pin...repeated several times)
2: 1.5DC
3: 1.67DC
4: 3.297AC
5: 3.297AC
6: 169.1DC
7: 167.2DC
8: 166.1DC
9: 3.294AC

V3
1: 124.3DC
2: .9DC
3: 1.958DC
4: 3.26AC
5: 3.26AC
6: 249.4DC
7: 122.8DC
8: 125.5DC
9: 3.25AC

V2
1: 124.6DC
2: .0DC
3: 1.999DC
4: 3.263AC
5: 3.263AC
6: 254.8DC
7: 0.3DC
8: 2.34DC
9: 3.31AC

V1
1: 137.9DC
2: .3DC
3: 2.336DC
4: 3.278AC
5: 3.278AC
6: 130.6DC
7: 0DC
8: 2.333DC
9: 3.304AC

More pics and posts to come

[big mac daddy]

Pics of the pot side of the chassis

[big mac daddy]

and last set of pics