Gain and harmonics problem

[Mr.Curwen]

I have been modifying an Epiphone Valve Junior now for some time, finally deciding to try out a TW Express preamp. From sound clips and reading this forum I was under the impression that the Express would carry an excessive amount of gain and plenty of harmonic content - to my surprise my own little Express mutant has very little of either.

With volume full on, the gain is in the ball park of AC/DC, not singing saturated lead at all. The power stage is an SE ECL82 pentode section (left there from previous mods) biased quite cold, but I still expect the preamp to overdrive much more than now. The amp sounds overall very cold and dead - I probably should up the voltages on the power section.

(I just noticed the schematic is lacking two resistors on the power tube input, grid resistor 22k and 50k to ground.)

The parallel resistors and caps around V2b are simply because I just used what I had at hand to get roughly the Express values - should really get more stockpiles of common components!

Any suggestions as to the cause of my amp sounding dead and lack of gain?

[soma_hero]

I remember reading about a similar problem with a full on Express build. The amp sounded good but had mild gain and no real harmonic content. I builder found one bad solder joint on a coupling cap and upon fixing it, the Express came to life.

Maybe re-check all solder joints, connections, the usual stuff...

[jjman]

In the Express, the output clips 1st on the dial which won't happen on yours if biased cold. Then the PI clips but you don't have one. Then the 3rd stage clips. This doesn't happen until about 75% volume on mine (which is not cloned voltages.) It's possible that your preamp is clipping exactly like an Express preamp should. (The PI is part of the output, technically speaking.)

[VacuumVoodoo]

The power stage is an SE ECL82 pentode section (left there from previous mods) biased quite cold, but I still expect the preamp to overdrive much more than now. The amp sounds overall very cold and dead - I probably should up the voltages on the power section.

(I just noticed the schematic is lacking two resistors on the power tube input, grid resistor 22k and 50k to ground.)

You definitely should have higher supply voltage to the output pentode.
Go up to 250V.
Make sure the tube sees reflected Ra ca 5kOhm
Cathode resistor should be in the range 680-820Ohm for plate current around 35mA., cathode voltage should then be ca 18V
Grid stopper 10k is enough, grid leak 220-270k.

SE power stage overdrive and clipping behavior is very sensitive to Ra i.e. OT primary to secondary ratio.

Pentode section of ECL82 will deliver up to 3.5W clean and overdrives nicely when properly designed.

http://www.youtube.com/louderandmore note that the sound is recorded with a crappy camcorder. No, there's nothing TW like in the circuit, the preamp is SS.