New Express build

[bigbeck]

I copied the schematic with UR12's revisions. All B+ voltages are within 3V of the schematic. Bias voltage is -33V . I measure 24.5 MV at each 1 ohm resistor on the EL34 cathodes.

I also installed a 500K MV pot and .022 coupling cap between V2a and the PI and eliminated the presence control with a 2.2k fixed resistor to ground.

My problem is that the amp farts out when the volume is turned past 2:00. It sounds great below that. What to do?

[FYL]

Swap tubes
Check PS caps
Check the caps in the PS
Check coupling and bypass caps
Check plate R's

What kind of MV are you using ?

[bigbeck]

What kind of MV are you using ?

A .022 poly cap from the V2a plate to the MV and back to the .1uf cap. Works good. But it still farts out if I remove it from the circuit.

I'll check the PS caps and plate resistors later.


Thanks

[Richie]

the OT mounting placement might not be a good choice. Down over the preamp section.

[bigbeck]

Hmmm....... Only about half the OT is over the board. I've mounted the OT like that on other builds without a problem. I know,this is not like other builds. I have an 1/8" thick sheet of lead. Maybe I'll unbolt the tranny and slide it under just to see what happens.

[Cliff Schecht]

Is it a motorboating type sound (loud thumps)? Could be inadequate decoupling on the stages or the decoupling caps are too far away from their intended sources (i.e. the wires from your electrolytics are too long).

[bigbeck]

I had the motor boating problem but solved it by rerouting the OT wires. This farting out,or blocking distortion, or whatever, is like the amp cuts out at high volume. When I turn the volume control past 2:00 I can hear the amp get more quiet - less hum & hiss. If I strum the strings very lightly,I get no sound. If I bang them hard, I get very loud harsh distortion that only lasts for a second - no sustain at all and not very musical.

However, the amp sounds very good at 2:00 volume setting and below.

Another thing, I used a shorting input jack and I get a lot of hash or white noise with the guitar unplugged. When the guitar is plugged in, the hash goes away and is replaced by varying amounts of hum,depending on how close the guitar is to the amp.

[bigbeck]

Swap tubes
Check PS caps
Check the caps in the PS
Check coupling and bypass caps
Check plate R's

What kind of MV are you using ?

I replaced all coupling and bypass caps,removed the MV, put the presence control back in and moved the OT wires a hair. I ended up with a JJ 12ax7 for V1,a Sovtek 5751 for V2 and a EH 12ax7 for the PI. I still can't believe that swapping tubes makes such a huge difference! On most of my other amps,a tube swap usually only makes a very small difference.
It's very quiet now. Sounds like any other amp - until you play it. Still too bright with a Tele but really kicks a$$ with an SG.

Thanks to everyone for their advice on this beast.

[paulster]

I still can't believe that swapping tubes makes such a huge difference! On most of my other amps,a tube swap usually only makes a very small difference.

You're starting to see the light! Next you'll be saying that changing caps makes a difference too!

[bigbeck]

You're starting to see the light! Next you'll be saying that changing caps makes a difference too!

I have a fur lined cap that I use to get a little extra warmth. Some people can't tell a difference, but WTF do they know?