Is this typical of an Express?

[Robbie]

I just finished my Express clone and I'm not sure if it is behaving correctly. The amp has an incredible amount of gain. If I turn it up to 10:00 thats a lot! At this setting with all the tone knobs set at 12:00, the output sags asymetrically, indicating to me the output transformer is saturating quit a bit. I used a transformer from toneslut. The version I picked does not have the multiple taps. I'm running the amp into a 4x12" cab loaded with Celestion Greenbacks. It doesn't really sound like the other Express clips I have heard on the web. In general it seems to lack tightness and is a bit squishy in nature.

Any hints or suggestions would be greatly appreciated.

Robbie

[geetarpicker]

My original is pretty gainy on 10pm if the bright switch is in the full position which I usually run that way. That said, I find the bright switch too apparent unless the volume is up to at least 12pm. Once the amp is up that far the bright cap has less effect but it is still there. The bright caps does seem to add to the apparent tightness in my original. It also seems to add to the "clean to scream" range...

You might even check your pots. Maybe your bass pot taper is such that half way up is really like 3/4 or more in actual electrical value? My original Express does get pretty squishy with the bass up all the way or close to it.

I think the 5.2k amps are brighter and maybe tighter? That said, my original OT is a Pacific @ 6.6k.

Still, maybe something else is going on in your amp. Maybe some voltage checks can be posted for others to comment on?

Also what output tubes and bias are you running?

www.myspace.com/glenkuykendall

[Robbie]

I'm using 6V6's with the vintage toneslut output transformer (6.6K). This transformer does not have the optional 5k primary like the other tonslut version transformer. Maybe the 5K x-former doesn't saturate so easily? I did check the bias of the output tubes, and it was running a bit cold. I ran the tubes a little hotter and this adjustment increased the gain in a great way, eg the singing sustain effect came to life. The obvious thing that does occur is the more low end in the preamp the greater the x-former saturation. When you did your you tube clips how did you set your amp? Was the brite switch on and which setting was it?

Thanks for your support,

Robbie

[UR12]

I'm just finding it a little hard to believe that you are saturating Moose's big ole OT (especially running at 6.6k) with a pair of 6v6s. Are you sure one of your 6V6s isn't distorting one side of the wave form and you are seeing the same thing only out of phase at the sec of the OT? If you swap your 6V6s around, does the distortion move with the tube? Does your input waveform on the grid of the power tubes (P-P level) exceed the bias voltage? There are a lot of ways to get an asymetrical output without saturating the OT.

[geetarpicker]

For the last couple years I've been running GT EL34Ms biased at about 45-48ma per tube. This pic shows my typical knob settings. On my original Express the bright switch down position is the full bright, 500pf setting. I very rarely run the up bright position (which on mine is 50pf) nor the bypassed center position.

GK

[img172]http://img.villagephotos.com/p/2005-10/ ... loseUp.jpg[/img]

www.myspace.com/glenkuykendall

[geetarpicker]

Double post

[Robbie]

Thanks guys for the comments.

I too am surprised that such a large x-former would saturate / sag so much. I started this project using a princeton chassis / guts, and now everything has totally changed with the exception of the chassis, and fish paper and sockets. The princeton x-former really sagged at a much higher freq before changing to the beefy toneslut x-former.

This last weekend I swapped tubes and did not see a difference in symmetry of the output. I then swapped tubes form 6V6's to a "matched pair" of EL34s. Now I am dealing with a low freq oscillation that I need to track down. I also tried the 500pf cap across the vol control. This sounds very nice, very pretty and clean. I miss the heavy mids however when this is kicked in.

I've got more to chase down, but I'm having fun in the process.

Thanks again,

Robbie

[Fischerman]

Just a shot in the dark: are you 100% sure the PI plates are wired correctly? The reason I ask is because the typ Express layout is 'backwards' compared to say...a Marshall. Just make sure the 82k plate resistor is the main input triode (i.e. the top triode in a typ schematic) and the 100k plate resistor is the other triode (i.e. the bottom triode in a typ schematic). The PI would be very unbalanced if you had those plate resistor values reversed (it's still probably a little unbalanced even when wired right and with an 82k/100k plate combo).

EDIT: and if it is reversed make sure you swap the actual resistors...not the wires. The amp will squeal like a stuck pig if you swap the wires.

Are all the DC voltages correct? That would include all the plates/cathodes.

[Robbie]

I had some time to work on my Express clone this weekend and I'm getting really close! I swapped the output transformer leads which helped tremendously!! The amp is much more stable as you can imagine. To save my hearing I have been using a scholz power soak, which I'm afraid is killing part of the tone. Any suggestions here are welcome. I also have experimented with speakers using vintage 30's versus greenbacks. Right now the vintage 30's seem to have more body and tend to compensate for the stage 3 low cut coupling.

Thanks again for the help,

Robbie