Treble Control Killing Tonestack

[jordan86]

I have an issue with my tonestack that I'm not sure where to begin with. If anyone has ideas on what to check first (or second) I would welcome the advice.

I have an AX84 SEL (high gain marshall thing) that was built for me by a friend many years ago. It's sat for a while due to the terrible hum and this tonestack issue. I'd love to try and tackle it myself. 

A while back my bass started cutting out on me. I replaced the 500K bass pot thinking that may have gone faulty. The amp SORT OF works, but once I get the treble control past 11 o'clock to Noon territory it steals ALL my bass. Even with the bass MAX'D it sounds like a bright telephone tone. It's way beyond the typical "interactive" TMB tone stack. I created a video. The video doesn't really do it justice either but it's something to reference at least. It's quite drastic in person. Like a 10-20db drop in bass gain if you crank the treble. 

https://vimeo.com/318681841/bda5084b44

Attached a gut shot for fun and schematic.

BONUS: I have a TERRBILE hum. Even when the amp is on but still in standby. Any pointers on what to chase first would be appreciated. I assumed the noise was typical bc its a high gain amp, but upon further reading it sounds like these amps are designed to be pretty quiet.

The keen eye will notice that the two GAIN controls in the schematic have been put on a single dual gang pot. Not sure that really matters in this instance however.

[shooter]

here's the schematic to make it easier;

compare the TS values in yours to a  Marshall, nothing jumps out on the schematic, but I'm only 1/2 looking 

[shooter]

fwiw;

I like the after pre TS, here's one that got good reviews, maybe re-wire like mine n see if that heads in the right direction.

with standby OPEN, that about leaves filaments for your hum, pull tubes starting at V1 till it quits

[labb]

A couple of things: I have a SEL and it is very quite, just so you know....The original design of the SEL allowed one to use any of the 8 pin power tubes, ie 6V6, 6L6, KT88, etc. it had an external adjustment for the cathode resistance...Do you have this feature.....as to the tone stack problem, I would venture that there is a wiring error some where...Lift the ground of of the mid pot and see what the amp sounds like...That will take the TS completely out of the circuit. I could never make the heater elevation they show on the schematic (A) work. I had to have a CT for the heaters.

[jordan86]

 Very helpful ideas. I did confirm all the wiring was correct to the schematic but you never know. I guess I could redo it all myself and try again.

I will try pulling valves to see what/where the filament may be the culprit.

I do have an adjustment pot for bias. I prefer an EL34 over the KT88. Just seems a little less sterile when driven.

[ac427v]

Does one of the volume-gain pots have shielded wire that was grounded at both ends? I can't quite make out the details in the picture.

[ac427v]

I would measure the values of the tone control pots to verify the schematic.

[jordan86]

Does one of the volume-gain pots have shielded wire that was grounded at both ends? I can't quite make out the details in the picture.

It may. I will double check. That thought did cross my mind.

[jordan86]

I would measure the values of the tone control pots to verify the schematic.

Yeah I think I may gut the entire tonestack and measure values to verify everything is healthy and functioning per spec.

[shooter]

I could never make the heater elevation they show on the schematic (A) work. I had to have a CT for the heaters.

+1
I know some that like elevated heaters for SE, I never had much luck, the artificial CT, that's worked every time I use it.  (wire routing assumed good:)

[jordan86]

with standby OPEN, that about leaves filaments for your hum, pull tubes starting at V1 till it quits

Pulled all tubes in standby. Still getting 60hz hum. Guessing I will need to investigate ground loops or CT?

[nandrewjackson]

Hums with all tubes pulled???

[jordan86]

Hums with all tubes pulled???

Correct. With standby both open and closed.

[jordan86]

Looking more closely, i don’t see an CT anywhere. Definitely not one on the PT. I’ll pick up some 100R resistors and drop in an artificial center tap.

Thanks everyone. Will drop that in and report back.

Any other ideas on why the treble would respond that way would be greatly appreciated.

[Beezerboy]

you only ground one end of the shield, otherwise the shield becomes a path

[tubeswell]

Re: the hum


Some of your soldering connections look marginal - it's difficult to tell from the picture you posted, but some of the joints look like they don't have enough solder or that they could be potential dry/cold joints, e.g. some of the pot and switch tabs in particular, and perhaps some of the turrets. Try re-flowing these with a 40W iron and adding more solder to at least close up the holes. Not saying this is definitely your hum problem, but it may pay to reflow everything and add a bit more solder


(one of?) the ground connection(s) doesn't look that solid/secure. Better to attach/solder the ground wire securely to a ring terminal and then bolt that to the chassis with a locking washer/nut

[j_bruce]

$.02 from a beginner:

looking at Tubenit's red marks I notice the end of the lower diode does not look like a good solder joint so maybe check that as well. Seems like a great spot to cause some 60 hz hum.

My thinking is that now you have half wave so the filter caps can't really get it done, plus the first B+ usually goes right to the OT so sending a half wave would cause hum with no tubes perhaps? 

[sluckey]

I don't trust that "woven" ground buss. I much prefer smaller wire that can be wrapped completely around a turret rather than relying upon the small contact area given by your buss, which may or may not even actually be in contact with the turret. Some of those connections may be relying on the solder to provide contact and that can cause a lot of electrical problems.

[dude]

More than half of those solder points look weak, especially the Buss, as Sluckey mentioned. Look at Doug's wrapped turrets with small wire making a solid connection before soldering.  Looks like some connections have little to no solder. Mechanical connection should be made first, the solder just keeps that connection solid.   

[jordan86]

Thanks all. Will double check all connections. Again, I didn't build this. Just trying to revive it. I was very meticulous with my Princeton build and consider myself very fortunate that everything worked perfectly on first try. :)

[jordan86]

Interesting find today. I lifted the ground connection from the mid pot in my tonestack.  Lifted the connection at lug 1 of the mid pot so I could easily reposition it to A/B grounded vs not. Sounds a LITTLE different. Not much though. Just more background noise with tonestack lifted. One thing of note is that the treble control behaves the exact same way. As I turn it up, it still attenuates all the bass and makes things shrill as shrill can be. At max it cuts out the signal entirely. Going to gut the entire tonestack and start over.