36W-RP Rev 5 Ready for Santa

[TDuly]

Well I completed the 36 watt TMB Revision 5 amp head this morning. One thing I've noticed is that with everything dimed (channel volume, master volume, push pull tone boost boosting etc) and the guitar full out I get some unpleasent crackling distortion when I bang the guitar hard. Back off anything just a little and it goes away.

I had a half power switch lifting one pair of the power tubes cathode in the amp but the crackle distortion was even worse so I took it out. I used a hammond 17"x10"x3" steel chassis and had to sand the paint off every ground point but I believe all of the grounds are good as I ohmed them all to IEC green ground where I anchored it to the chassis and the most I got was 3 ohms on the farthest ground.

I had a microphonic Sozo coupling cap on V1 pin 1 and changed that which left chop sticking a quiet test process. The amp sounds great all the way up to full up everything and only crackles a little then.

Anyone else experience such a thing?

I used a Vox AC/30 PT, a Mercury Magnetics Vox AC/30 Woden 4,8,16 OT and a Vox AC/30 Choke.

[pezed]

3 ohms is a very bad ground connection.

[TDuly]

That's 3 ohms from one end of the chassis to the other through mulitple connections and it is typical with just about every amp I have worked on in 20 years. The meter leads show resistance as well. At each ground point you get zero just not from one end to the other of the chassis.

By the way, after an 8 hour burn in and the removal of a new issue Mullard 12AX7 the crackling has stopped and the amp sounds fine.

This thing is very loud. The recipient is very happy.

[pezed]

you must mean .03 ohms. if your getting 3ohms of resistance thats quite a lot! just for perspective im using .25 ohms to lower my filament voltage from 7 to 6.3volts. 3ohms would have lowered it to below 1volt i'd say.

[NewGuitar]

This thing is very loud. The recipient is very happy.

Awesome! I have some crackeling in mine. Maybe I just need to chang out the the abused 12AX7s JJs I've had in it for over a year...

[TDuly]

You know I forgot to update this crackling noise problem. Well the key was that it only happened when I hit heavy chords and the problem turned out to be a bad guitar cord. A Radio Shack gold plated ends unit. The amp was fine.