6G6-B Bassman clone project

dubtrub
Administrator
Posts: 3772
Joined: Sun May 04, 2008 10:12 am
Contact:

Re: 6G6-B Bassman clone project

Postby dubtrub » Sun Dec 18, 2016 1:10 pm

Mr. Bill, I have always experienced a slight audible hum in my 6G6 since I built it. Never enough to notice when playing live but it is annoying when sitting idle. Thinking back I'm wondering if it could be caused by grounding the two 100 ohm resisters from the pilot light directly to the lug on the transfomer? There was no center tap on the tranny is why you advised that I do that. Would it beneficial to move them over and ground them to that empty nearby chassis hole or is that still too close to the transformer? Also the ground from the negative feedback board (I think that's what it's called) is on the same grounding terminal.

Image
Danny Ellison

Mr. Bill
Top Producer
Posts: 678
Joined: Tue Aug 19, 2008 12:15 pm
Location: Chicago, IL
Contact:

Re: 6G6-B Bassman clone project

Postby Mr. Bill » Sun Dec 18, 2016 5:36 pm

The transformer mounting bolt is where Fender grounds them. I think the small board near the transformer is the bias power supply board. The small whole near the power transformer is for the head mounting bolt, isn't it?

As for the hum, you should try and troubleshoot the problem. Turn all of the controls to zero, any difference? Remove the first preamp tube any difference? Remove each preamp tube one at a time and listen for changes to the hum. That may help you figure out which stage the hum is in. Power supply grounding can make a lot of difference in these amps, how closely to the original Fender scheme did you go?

dubtrub
Administrator
Posts: 3772
Joined: Sun May 04, 2008 10:12 am
Contact:

Re: 6G6-B Bassman clone project

Postby dubtrub » Sun Dec 18, 2016 6:14 pm

I followed the Fender layout completely and I don't think there were any errors as I checked, rechecked then double checked each step before final assembly. I've pulled each pre-amp tubes one by one and it only quits humming when I pull the last one, V-4, next to the power tubes then it goes totally quiet. It doesn't change sound or volume wise pulling any of the other tubes. I've replaced V-4 with several different 12AX7 tubes and with some the hum gets louder but it never goes completely away. It's not a loud hum just enough to be noticeable.
Danny Ellison

Mr. Bill
Top Producer
Posts: 678
Joined: Tue Aug 19, 2008 12:15 pm
Location: Chicago, IL
Contact:

Re: 6G6-B Bassman clone project

Postby Mr. Bill » Sun Dec 18, 2016 8:15 pm

V4 is the driver/phase inverter, so pulling that one will leave only the two power tubes left in circuit. So the hum is being generated somewhere in the earlier stages. If none of the controls effect the hum, then it could be in the audio stage after the controls or in the power supply itself.

Changing the tubes is always a good place to start. I know that all of the parts were new, but it could be a weak filter cap. Do you have spare filter cap in your parts box?

And maybe post a photo of the open chassis as it is now.

Edit: I searched back through this thread and looked at the original photos that you posted. There are a couple of grounding things that you can try. You have the power transformer ground and the filter cap ground at two separate points. Try moving them to one point.

Also, try moving the preamp power supply ground to the same point as the first tube cathode ground at the brass grounding plate.

dubtrub
Administrator
Posts: 3772
Joined: Sun May 04, 2008 10:12 am
Contact:

Re: 6G6-B Bassman clone project

Postby dubtrub » Sun Dec 18, 2016 10:52 pm

I appreciate the observations and recommendations. I've now got some things to check out.
Danny Ellison


Return to “Amp Tech Q&A and Repair Shop”

Who is online

Users browsing this forum: No registered users and 2 guests