5F6a Bassman troubleshooting.

[Anthon]

I posted a topic not so long ago, about making a 5F6a Bassman with 6V6 instead of 5881.
As a first step, I just made the original 5881 version - I will adapt it to 6V6 when it works properly with the original configuration.
I used Rob Robinette's layout, with half power switch and negative feedback resistor mods. I also grounded the power tubes thru a 1ohm resistor for easy plate dissipation measurements.

Unfortunately, it doesn't work as it should.
It does produce sound, but it also has very loud artifacts: high frequency screeching, ugly distortion at low levels. At some positions of the pot, the amp makes really loud distorted buzz sounds. So it's highly unstable.
The hum level is also higher than it should be, and the guitar sound thin in general.

I double checked all the connections - they seem to be correct (I will check again, because most of the time it is wrong connection, even if I quadruple check it).

Does these hints ring any bells to you? Where should I look?
What bugs me, is the strange color coding of the Hammond 1760L transformer. It uses black as 4 ohm, green as ground, yellow as 8 ohm, white as 16 ohm. I use 8 ohm output.

 

[moamps]

Maybe your amp oscillates.
Reverse blue and brown wires from opt and check it again.

 

[fazer]

+1 on reverse primary on output tran .  Sounds like positive rather than negative feedback.

 

[Anthon]

Thank you, that would explain a lot. It wouldn't be the first time color code of the Hammond OT didn't match the layouts I could find online.

Can I confirm this, by disconnecting the feedback resistor? I already installed the switch.

 

[CJ]

yes.

 

[Anthon]

I tried turning off the feedback loop switch, and it solved the instability issue.  ;D
So it must be the OT polarity. I will rewire it it first thing tomorrow.

There is still excessive amount of hum though. Not terrible, about as much as I would expect from an ungrounded amp. Grounding issue?
This is the layout I used. The only exception are the power tubes that are grounded  thru 1 0hm resistors to the screws for the tube mounting and not to the 'power amp ground' (I was curious if I could get away with it).

 

[walter]

Are all the grounds connected together? You can connect an Ohm meter to chassis and check each ground point, or if you like living on the edge, connect a ground wire to chassis and test each ground point while listening to the hum. Also, are the pots cases grounded? I usually extend the pots ground wire to it's case, and also have a ground buss across all the pots. The early fender tweed amps had a copper plate under the pots.

 

[Anthon]

Are all the grounds connected together? You can connect an Ohm meter to chassis and check each ground point, or if you like living on the edge, connect a ground wire to chassis and test each ground point while listening to the hum. Also, are the pots cases grounded? I usually extend the pots ground wire to it's case, and also have a ground buss across all the pots. The early fender tweed amps had a copper plate under the pots.

I checked with the continuity meter - everything that should be grounded is connected to the chassis.
Pot cases are grounded, by the mount (not wires).

I'm not even sure what to expect for a bassman, it is much louder that anything I've built so far. Maybe I'm hearing the normal amount of hum?
I've built champ, princeton and deluxe in the same manner, and they practically hum free even at max volume.
All of them have elevated virtual ground on the heater windings. This time, the PT had a center tap for the heaters, so I decided to use it. Maybe I should try the elevated virtual tap instead? If so, where would be a good point to connect the 2x 100 ohm resistors? Bassman is not cathode biased (with champ, princeton, deluxe cathode worked fine).

Also, it's worth mentioning that the volume pots affect the hum. With volumes on 1 you don't hear it - so I guess it comes somewhere from the preamp or the input.

 

[gridcurrent]

the power transformer high voltage winding center tap should connect directly  to the - of the 1st filter cap.

 

[Anthon]

the power transformer high voltage winding center tap should connect directly  to the - of the 1st filter cap.

It seems like all the layouts I find connect the center tap directly to the screws of the transformer. I've done it on all my tweed amps, and it doesn't produce any hum. This one however, does have a very long lead going from the power amp ground to the negatives of the caps.
What would be the reasoning behind it?  (I try not to resolder stuff, before I actually know what I'm doing).

I tried grabbing the preamp ground bus with pliers, and forcing it to the chassis - it does improve the hum levels quite noticeably  (still not good enough though).
Also, the hum is there with or without guitar plugged in.
Tried replacing preamp tubes - no noticeable effect.
When the volumes are turned down, the hum is also gone.

So I think it has something to do with the preamp ground buss. Maybe a ground loop somewhere?

 

[walter]

It could be lead dress. Try re-positioning the wires of the first tube, you can use a chopstick to move wires with the amp on.  You could also put shielded wire from the input jacks to tube, shield connected to one end only, either ground or plate. The back panel with foil should complete the shielding. Some amps hum when the back panel is off, you can glue some heavy aluminum foil to the back panel.

 

[Anthon]

It could be lead dress. Try re-positioning the wires of the first tube, you can use a chopstick to move wires with the amp on.  You could also put shielded wire from the input jacks to tube, shield connected to one end only, either ground or plate. The back panel with foil should complete the shielding. Some amps hum when the back panel is off, you can glue some heavy aluminum foil to the back panel.

I just ditched the layout completely, and went with Ceriatone layout - the hum is completely gone. 

Now it's just a matter of some details, and it's finally finished.
This is how it looks like. I made cabinets myself, from solid pine wood (dovetail joints).  8)

 

[opacheco]

I just ditched the layout completely, and went with Ceriatone layout - the hum is completely gone. 

Now it's just a matter of some details, and it's finally finished.
This is how it looks like. I made cabinets myself, from solid pine wood (dovetail joints).  8)

Anthem,

Man I am intrigued!!;

What was the problem then??......
What was the cause of your noises??

Opacheco

 

[Anthon]

Anthem,

Man I am intrigued!!;

What was the problem then??......
What was the cause of your noises??

Opacheco

Most certainly a ground loop somewhere, because after I changed the grounding, the hum was gone.
I looked hard, but couldn't see anything wrong - so decided to take more drastic measures, and went with a different grounding scheme.

The instability issue was caused by the wrong OT polarity - instead of negative feedback, it gave positive feedback, which created oscillation problems.

 

[CJ]

Awesome job!

here is the Fender chassis and the one you used for further refferal  ;D

 

[bluesman714]

A guitar playing friend of mine and myself build matching 5E3 tweed Deluxe amps. We decided to improve on Leo's design and used a star ground for all ground points. The result was a working amplifier that hummed like crazy. The hum went away completely when we changed to the old Fender grounding scheme. At least we can live and learn.