Just finished a 6G5-A Pro copy -- inconsistency between layout and schematic --

[mort]

The layout shows B+ connected to p5 of v8 (first power tube). The schematic does not show this connection. Can someone please clarify which is correct?

http://www.thevintagesound.com/ffg/s...5-a_layout.gif

http://www.thevintagesound.com/ffg/s...g5-a_schem.gif

I'm still double checking my circuit but I'm pretty sure I have it wired exactly like the schematic. The problem I'm having is really low output regardless of which input or channel I'm plugged into. I can here the vibrato effect i the vibe channel so that seems to be working. I'm taking a closer look at the output section and not coming up with anything. I tried swapping the OT secondaries but then it just went BBBBRRRRRRRRMMMMMMPPPPPPPTTTTTTT, so the polarity was correct to begin with.

output tube voltages are identical to each other:

p1- 14mV across 1 ohm resistor to ground
p3- 459 vdc
p4- 460vdc
p5- -56.2 vdc (both fluctuate by less then 1V)
p6- 462vdc

[mort]

I'm thinking that on the layout, the 448v B+ node should be landing on P6 instead of P5, and that P5 should be getting unfettered negative bias voltage. Which is how I have it wired.

[Justin Thomas]

I'm pretty sure the layout is wrong, as I'm pretty sure there should NOT be +450VDC on the input grid! Does seem to be biased a bit cold, though... 6.5W? It's a nice trem circuit - don't give up! Before you tear anything apart,

Check component values - resistors. 1M for 100K, that kinda thing. With a meter, not by color code. Especially if you have any 5-banders - I read em backwards all the time.

Justin

[mort]

Yeah I can kinda hear the shape and dynamic of the vibe and it is sweet and gets pretty slow.

[mort]

here are a few pics. It's a big circuit and I tried to keep it as neat as possible. I'm sure there's just one silly mistake in there somewhere...


http://i.imgur.com/NYHkVzN.jpg

http://i.imgur.com/Xla6n5u.jpg

http://i.imgur.com/bDf7mZV.jpg

http://i.imgur.com/smtiY2z.jpg

[g1]

I'm thinking that on the layout, the 448v B+ node should be landing on P6 instead of P5, and that P5 should be getting unfettered negative bias voltage. Which is how I have it wired.

That is correct. The error in the layout is that it shows the supply voltage A connected to pin5 when it should go to pin6.

Does seem to be biased a bit cold, though... 6.5W?

Agree, bias is very cold. Not sure if it would cause your "really low" output, it would certainly affect it.

[frus]

why would they put -55V on a 50V rated capacitor?
anyway, somethig like -45V would be more in the ballpark

[mort]

What could I check for the cause of low bias?

The PT is putting out 51.9 vac on the bias voltage tap, after passing through the 1K resistor there is 49.3vac left over.

Afgter the diode it's -56.8vdc, and after each 220k resistor there is -55.4vdc left on each, going directly to the tube sockets. It all seems reasonable to me. I'm not sure why I'm not pulling more plate current ....

[g1]

Like Frus mentioned, you will want a higher voltage cap for that bias filter, not sure what voltage you used but 100V would be good.
You need to reduce the bias voltage to increase the plate currents. You could increase the 1K resistor feeding the diode, or decrease the 27K across the bias filter, or you could make adjustable bias by putting a 25K pot in series with a 10K resistor where the 27K is.

[mort]

ok I replaced the 1K resistor to a 1500 and it brought the plate current up to 22mA but made no difference in the output.

I double checked the inputs to make sure they were all wired correctly and they are. Gonna go pull voltages from all of the 12ax7's and see if there might be something not getting voltage that would effect both channels.

[g1]

Also check that all the heaters are good.

[mort]

All heaters are working.

Here is a complete list of DC voltages on V1-V6

V1 -- 1-212, 2-0, 3-1.63, 6-209, 7-0, 8-1.68

v2 -- 1-208, 2-0, 3-1.7, 6-202, 7-0, 8-1.68

v3 -- 1-165, 2-.005, 3-1.82, 6-357, 7-164.5, 8-170.6

v4 -- 1-na, 2-na, 3-na, 6-307, 7-24.3, 8-68.8

v5 -- 1-336, 2-.031, 3-3.64, 6-339, 7-.082, 8-3.65

v6 -- 1-350, 2-117.4, 3-176, 6-349, 7-129, 8-176

[mort]

I'll attempt to describe the "low output".

There is no audible output up to 50% channel volume. Past the half way mark, there is very quiet guitar sound with some unwanted noise coming through the speakers. When I reach 75% volume, the guitar sound is only slightly louder (still quieter than TV volume) and if I hit the low strings pretty hard there suddenly comes much louder guitar volume but it is not very pleasant.

None of this is intermittent, and there is ZERO constant noise/hum.

[mort]

In my searching I found an unsoldered connection and an untrimmed resistor leg making contact with the body of a pot. Corrected them and there was zero effect against my problem.

The primary of the OT reads resistance as would be expected, ~250 ohms between the two outside taps and ~125 between each of them and the CT. But when I read the secondary I get about an ohm(that was with the black lead lifted from the output jack). Do I have a DOA OT??

I'm using the 4 ohm tap on this transformer: http://www.classictone.net/40-18006.pdf

[frus]

All heaters are working.
v6 -- 1-350, 2-117.4, 3-176, 6-349, 7-129, 8-176

if V6 is the phase inverter, it's WAY off!
compare with schematic above

check your cathode and tail resistors (820 and 6800) in the PI