Any help from fellow TAGers who have built from this schematic is greatly appreciated!
Symptoms:
1. A substantial amount of hum when increasing MV and Rev Receiver. With Rev Receiver at 0 and the Master is all the way up there is slight oscillation, some blue glow appears in the tubes. Turn Rev Receiver up just a bit and the noise / glow intensifies- The signal immediately starts to fizzle.
Original changes from original schematic to note:
1. Bias Resistors changed from 3.3k to 1.5k and 27k to 15k to get me in 6L6 Bias range.
2. No NFB on V1b (I decided to add this on last night- makes the over all output much quieter- hum still there)
3a. No Mid switch - wired .002/.001 in series ~670pf. 3b. dropped .001 down to 330pF.
4. No Deep Switch
5. No Effects loop .02 to master wiper
*6. Used all non polar Mundorf caps for cathode bypass on V1 through V4. I have done this before in a few amps with no issues.
7. 4.7k GNF resistor to 4 ohm tap. 16 ohm tap is un needed and tied off. Seems to work fine in my #183.
8. B+1 changed from 110uF (220/220) to 50uf (100/100). Typical for 50w builds. Added 220k 3x MO Resistors across the 100uf caps + 150k 3w MO from B+5 to ground. Decreases a few values in the drop string to get V1 in a good spot.
9. Changed Screen Filter B+2 from 22uF to 47uf.
10. Added 22uf cap to B+2A for reverb transformer.
11. Removed Contour circuit as it seemed to be contributing to the noise. (this did not seem to be the problem)
12. Added .0047 cap from receiver wiper to ground. This took away the oscillation with Master at 10.
Specs and Readings:
PT - Edcore
305-0-50-305 @300ma
6.3V @ 7A
5V @ 4A
The PT is center
OT - Edcore
45W
5K - 4/8/16
Choke- Hammond 4H 90ma
Rev Transformer - Mojotone Fender style
V1a -
1 - 189
2
3 - 1.6
V1b
6 - 188
7
8 1.6
V2a
1 - 279
2
3 - 2.1
V2b
6 - 279
7
8 - 2.1
V3a/b
1/6 -410
2/7
3/8- 4.1
V4a
1 -209
2
3 -1.68
V b
6 -203
7 -
8 -1.63
V a
1 - 301
2 -
3 - 59
V b
6 -296
7 -
8 - 59
V6
1 - Tied to 8
2 -Heater
3. 430
4. 431
5. -44
6 -NC/Grid resistor harness
7 -Heater
8 - 31.9mA
V7
1 - Tied to 8
2 -Heater
3 -430
4-431
5-
6 -NC/Grid resistor harness
7 -Heater
8 - 31.6mA
B+
1- 436
2- 433
2b - 419
3- 413
4-300
5-200
Assumptions:
The hum is drastically minimized by removing V5, the PI tube. My voltages look to be ok. My lead dress isn't sloppy. I have reflown every joint on the boards, pots, and pins twice (tubes out of course). I have replaced all coax wires. Measured every capacitor. Tried 3 separate matched pairs of 6L6GCs and nearly 2 dozed preamp tubes (voltages above are from whatever was in there last)
My next moves were going to be:
1. Desolder CT/tie off and add 100ohm resistors to the heaters as an artificial center tap.
2. Ask amp garage for some insight
