18 Watt TMB Build issues

[Spunky99]

Hi,

I hope someone can steer me in the right direction. I have a TMB that is pretty much the same as a Ceritone TMB 2010.
I get a weak sound, it crackles and if I turn the master vol up past 2/3rds it starts a quiet squeal. I've been desoldering and resoldering to see if I can get anything to change but no luck so far. I took some voltage measurements that are in a spreadsheet.

I started this a few years back and life got into the way and it sat in the garage until today. I have 2 of these 2x12 amps to finish and then a 50 watt Plexi head with NOS 1968 Drake trannys I scavenged about 15 years ago. I decided to finish this up hopefully before they cut my elbow and wrist for carpal and cubital tunnel next week. My rehab will consist of playing a brand new RG655 hopefully through this amp.


Any help would really be appreciated.
Thanks in advance.

[10thTx]

the master vol up past 2/3rds it starts a quiet squeal

I am wondering if you have the OT primary wires hooked up backwards? IF you disconnect the negative feedback, does the squealing stop?

IF so, try reversing the OT primary wires.

With respect, 10thtx

[Spunky99]

Thanks for the suggestion. I looked at the layout and did see the OT red and brown wires were reversed. I swapped them and the sound is the same.
I looked around to see where the NFB is to disconnect it but haven't found out where that is yet. Do you know what I am supposed to desolder?

Thanks

[10thTx]

IF you have swapped the OT primary wires around and it made NO difference, then it is unnecessary to disconnect the NFB.

Disconnecting the NFB was simply one way of possibly determining if the OT wires needed to be swapped.

With respect, 10thtx

[Spunky99]

Thank you Sir!

[sluckey]

That amp doesn't have a NFB circuit. Unless you have an unusual variant.

[Richie]

I would make sure you have a good 12AX7 in the preamp section, and also good to have or use shielded cable on the inputs, and also on the grid of the TMB channel, move the resistor to the tube pin. It could also be a lead dress issue.

[Spunky99]

Thanks for the reply Richie,

I did use shielded leads on the inputs. Tubes are new Tungsol, I'll swap them out as I have a ton of them all new.

I don't understand the :move the resistor to the tube pin". Can you be more specific please. I'm a bit of a dunce.

[Colossal]

Solder the grid resistor directly to the tube socket pin.

[Spunky99]

Thanks Colossal,

I understood that part but I don't know what or where the grid resistor is.

[Spunky99]

I swapped all 3 ECC83 and EL84 tubes. Same thing, no difference.

[sluckey]

I took some voltage measurements that are in a spreadsheet.

When did you take the measurements? Which sheet are they on?

You have a bunch of totally unrelated voltage charts but I can't find one for an 18W TMB.

[Spunky99]

I am so sorry about that. I thought I saved just one sheet.

Here are the measurements on a simple Excel spreadsheet...

I am pretty much lost at this point so any help is really appreciated.

[Littlewyan]

Those voltages seem very high. Have you checked the bias of the EL84s to ensure that they are getting enough current?

[Spunky99]

Thanks for the reply Littlewyan,

I don't have a clue as how to check the bias current.
Can you point me in the right direction to test it?