Need Help with a 6G16 63 RI Vibroverb problem

[Normybaby]

Happy New Year to all
A few weeks ago, I gutted the chassis on a Vibroverb Re-issue (printed circuit) and installed a 6G16 turret board with all new high quality components.
For the power supply I used the original PT with a Weber Chassis mount Bridge rectifier with minimal sag. The circuit is dead on 6G16, except for the SS rectifier and an adjustable bias. I also installed a new OP transformer from Allen Amplification; new speakers and tubes.
Here’s the problem I’m having, at low to moderate volume level, the amp sounds fine, but when pushed hard (it doesn’t take much to distort) the distortion has a raspy nonmusical sound especially when hitting low open strings. This occurs in both normal and bright channels.
The other problem is that this amp is a friend of a friend’s and I never tried it before I gutted it nor any other Vibroverb so I’m not sure if I’m chasing a ghost.
I’ve played with the lead dress, grounding wiring, swapped tubes, checked voltages, played with hot and cold bias settings; I’ve tried swapping OPT and the brown and blue leads, installed 1.5K grid stopper resisters on the OP tubes and 100pf across the PI output but nothing really totally eliminates it completely. The signal is clean; the amp is quiet, no hum or oscillation at idle
I don’t know where to look or what to do short of replacing everything; I’m not sure if that will change anything
Please help

[bnwitt]

Why don't you share your voltages on this chart. You say it is the same in either channel so that leads one to believe it is downstream from where the two channels sum. Have you tried pulling the reverb tubes V3 and V4?

[Normybaby]

Thanks, I will measure the voltages and attach tomorrow. Yes I have tried pulling the reverb tube, the trem tube, the tube from the normal channel and the tube from the bright channel and all but the preamp and the PI and no change.

[bnwitt]

Well it's problems like this that make a signal tracer a valuable piece of equipment. With it you can go from input to output and determine where the distortion is showing up. Here is a link to Doug Hoffman's page where he shows how to build a listening amplifier out of a 0.1uf cap, a 1meg audio pot, an old amp and a test lead:

http://www.el34world.com/Hoffman/tools.htm

or you can pick up a ready made unit like this:

http://cgi.ebay.com/Heathkit-T-3-Aur...lenotsupported

By the way, check all of you solder joints throughout the amp. I had an amp that did exactly what yours is doing once and the problem turned out to be a coupling cap I had forgotten to even solder on a turret.

[Normybaby]

Yeah Thanks, The Hoffman signal tracer is a good idea. I have a set of high impedance earphones that I will try.
I've reheated every joint and I just can’t seem to find anything wrong
Here are the voltages that measured

[tubeswell]

Or you can just use another amp as a signal tracer - rig up an alligator clip to a guitar lead and plug it into another amp on a clean setting, then clip your lead onto the various stages of your problem amp after each coupling cap until you find unwanted noise - your problem will be in a preceding stage to wherever the distortion shows up.

[Enzo]

and I never tried it before I gutted it

ASking for trouble. I ALWAYS make sure an amp works before I start any modifications.

[Normybaby]

Happy new year veryone!
Any comments on the Voltage readings....The B+ seems a little low..?
Thanks

[bnwitt]

ASking for trouble. I ALWAYS make sure an amp works before I start any modifications.

Enzo, this really isn't a mod, its a complete gut.

normy, unless your entry for V5 pin 6 is an error, there is something wrong there. both pins 1 and 6 are tied together at V5 and their voltages should be around 230VDC. You show pin 6 at 401VDC. That's the same as the plates of the power tubes. Is that a mistake?

Also V6 pin 6 is about 20 volts low. That's the phase inverter.

[Normybaby]

Hi bnwitt,
Sorry for responding so late but the Holidays were crazy. I double checked the readings and this is correct. Pin 6, V5 is connected to B+ and it should be around 400 Volts. My concern is that the 400Volts is too low. The schematic shows 430 Volts. I have a Weber Chassis mount Bridge rectifier which has a set of diodes, two 20 watt resisters and a Thermistor. It’s supposed to mimic the voltage drop of a tube rectifier when being pushed. I’m going to check that out tomorrow and report back.
Thanks again
Normy

[bnwitt]

Normy,
my mistake. I'm looking at really muddy drawings of the layout and schematic. It is pin 7 of V5 that is connected to pin 1. Is there a reason you didnt' record the VDC readings on pins 2 and 7 of the dual triodes V1 thru V5?

[Normybaby]

63 Vibroverb RI Update:
I found the problem. It was the low 400 volts B+. The Reissue has a solid-state rectifier on the main PC board. Remember, I replaced the PC board with a 6G16 Turret board and I had to come up with a rectifier for the power supply. My plan was to put the rectifier diodes in with the neg Bias power supply board. I thought it would be great if I was able to get the tube sag with a Weber Bridge rectifier module but apparently the thermistor and the resisters dropped the voltage too much. I gained 17 Volts when I by-passed the resistors and thermistor on the Weber module. It doesn’t seem like much of a voltage drop but it made a big difference in tone. Another improvement was to replace the GE 6L6 power tubes with a pair of 5881. It seems that these tubes like the lower voltage.
Thanks