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Scratchy noise problem fixed - next issues
Well I changed a few bits one at a time and when I was changing the presence pot, I noticed how there was excess flux residue around the contacts, and when I put a new pot in - bam! problem fixed. No more unwanted static. Phew! (many thanks to everyone involved so far)
Okay so now the amp functions as it should (i.e. there is no extraneous signal when the vol controls are cut, and everything seems 'normal) but the trem is not very intense, nor either is the reverb.
I have uploaded a schematic with the voltages described in red.
1) What can I do to get the reverb more intense?
a) Change the .01uF coupling cap from the mixer pot to the DC pair input grid to .1uF?
b) Use an ECC88 driver? - and if so would that require lowering the RT voltage? - the reverb driver plate voltage is current 435 VDC at idle - or is the JJE88CC maybe a more sturdy robust tube for this sort of application? - Actually can that - I just read that the ECC88 has a gain of 33 c/f the 60 of a 12AT7)
c) Or something else? (Maybe changing topology of V1 to take out the parallel triode input stage, and turn it into a single triode Voltage amplifier with vol after that and use the other triode to drive the 12AT7 - more like a BF?)
I notice when the tone control is trebled more, the reverb comes alive a bit more (so maybe the lack of reverb is frequency related - as well as drive related?)
2) Also w.r.t. the trem, would removing the .022uF cap from the plate to the cathode of the LFO stage (highlighted in green) intensify the trem?
Also there is a funny treble buzz/oscillation with the trem switched on when the presence pot is maxed out, which 'follows' the trem oscillation. It goes away as you back the presence off or the trem depth off. Wonder what would cause that? Probably need to tweak the trem circuit a bit more (or maybe the PA biasing?).
The 6L6s are biased at about 15.5W. I guess they could take a bit more, but they do heat up a bit with the trem intensity on maximum.
Also the cathode tail voltage is still high. In fact all the voltages are maybe a weeny bit on the high side(?). I guess I could put the GZ34 rectifier back in now, which would cut the B+ back about 30 VDC maybe. The voltages are as follows
V1 (12AX7)
Plate (bright channel) = 237VDC
Plate (normal channel) = 243 VDc
Cathode = 1.91 VDC
V2 (12AT7) Reverb Driver
Plate = 435 VDC
Biased with a 2k2 resistor bypassed with 22uF
V3 (12AX7)
reverb recovery stage
Plate = 210 VDC
Cathode = 1.6 VDC
Reverb bypass (CF) stage
Plate = 328 VDC
Grid = 153 VDC
Cathode = 166 VDC
V4 (12AX7) DC Pair
Driver stage
Plate = 186 VDC
Cathode = 1.14 VDC
CF Stage
Plate = 328 VDC
Grid = 186 VDC
Cathode = 189 VDC
V5 (12AX7) LFO Pair
LFO Stage
Plate = 210 VDC
Cathode = 2.28 VDC
CF Stage
Plate = 440 VDC
Grid = 210 VDC
Cathode = 217 VDC
V6 (12AX7) LTP
Tail = 47 VDC
Cathode = 48.7 VDC
Plate (82k side) = 251 VDC
Plate (100k side) = 239 VDC
V7 (6L6CG)
Plate = 444 VDC
Screen = 441 VDC
Tube current = 34.1mA
V8 (6L6CG)
Plate = 444 VDC
Screen = 442 VDC
Tube current = 35.2mA
(The DC resistance is slightly different on each side of the OT primary)
B+ = 446VDC
Filaments are all about 6.5VAC (so there is no problem with the PT running the extra 3 pre-amp tubes). Heater CT elevated to 40 VDC
Well I changed a few bits one at a time and when I was changing the presence pot, I noticed how there was excess flux residue around the contacts, and when I put a new pot in - bam! problem fixed. No more unwanted static. Phew! (many thanks to everyone involved so far)
Okay so now the amp functions as it should (i.e. there is no extraneous signal when the vol controls are cut, and everything seems 'normal) but the trem is not very intense, nor either is the reverb.
I have uploaded a schematic with the voltages described in red.
1) What can I do to get the reverb more intense?
a) Change the .01uF coupling cap from the mixer pot to the DC pair input grid to .1uF?
b) Use an ECC88 driver? - and if so would that require lowering the RT voltage? - the reverb driver plate voltage is current 435 VDC at idle - or is the JJE88CC maybe a more sturdy robust tube for this sort of application? - Actually can that - I just read that the ECC88 has a gain of 33 c/f the 60 of a 12AT7)
c) Or something else? (Maybe changing topology of V1 to take out the parallel triode input stage, and turn it into a single triode Voltage amplifier with vol after that and use the other triode to drive the 12AT7 - more like a BF?)
I notice when the tone control is trebled more, the reverb comes alive a bit more (so maybe the lack of reverb is frequency related - as well as drive related?)
2) Also w.r.t. the trem, would removing the .022uF cap from the plate to the cathode of the LFO stage (highlighted in green) intensify the trem?
Also there is a funny treble buzz/oscillation with the trem switched on when the presence pot is maxed out, which 'follows' the trem oscillation. It goes away as you back the presence off or the trem depth off. Wonder what would cause that? Probably need to tweak the trem circuit a bit more (or maybe the PA biasing?).
The 6L6s are biased at about 15.5W. I guess they could take a bit more, but they do heat up a bit with the trem intensity on maximum.
Also the cathode tail voltage is still high. In fact all the voltages are maybe a weeny bit on the high side(?). I guess I could put the GZ34 rectifier back in now, which would cut the B+ back about 30 VDC maybe. The voltages are as follows
V1 (12AX7)
Plate (bright channel) = 237VDC
Plate (normal channel) = 243 VDc
Cathode = 1.91 VDC
V2 (12AT7) Reverb Driver
Plate = 435 VDC
Biased with a 2k2 resistor bypassed with 22uF
V3 (12AX7)
reverb recovery stage
Plate = 210 VDC
Cathode = 1.6 VDC
Reverb bypass (CF) stage
Plate = 328 VDC
Grid = 153 VDC
Cathode = 166 VDC
V4 (12AX7) DC Pair
Driver stage
Plate = 186 VDC
Cathode = 1.14 VDC
CF Stage
Plate = 328 VDC
Grid = 186 VDC
Cathode = 189 VDC
V5 (12AX7) LFO Pair
LFO Stage
Plate = 210 VDC
Cathode = 2.28 VDC
CF Stage
Plate = 440 VDC
Grid = 210 VDC
Cathode = 217 VDC
V6 (12AX7) LTP
Tail = 47 VDC
Cathode = 48.7 VDC
Plate (82k side) = 251 VDC
Plate (100k side) = 239 VDC
V7 (6L6CG)
Plate = 444 VDC
Screen = 441 VDC
Tube current = 34.1mA
V8 (6L6CG)
Plate = 444 VDC
Screen = 442 VDC
Tube current = 35.2mA
(The DC resistance is slightly different on each side of the OT primary)
B+ = 446VDC
Filaments are all about 6.5VAC (so there is no problem with the PT running the extra 3 pre-amp tubes). Heater CT elevated to 40 VDC
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