Selmer treble and bass 50 MK1 preamp voltage issue

[Alexanderspeer]

Hey guys, I own a croc skin version of a selmer treble and bass 50 and recently I used a sound City 50 plus chassis and transformers to kinda clone it. Of course, the differences are no tube rectifier and no physical choke, but the normal channel sounds really good, albeit,
distortion on the clone at lower volumes though quality sound.
The problem is, is that I'm getting about 20 volts on the bass channels treble potentiometer. It seems that it's coming from 10 to grid of the pi.. I can't figure it out and then I measured voltages on the original selmer that I'm cloning and it has the same thing! Is this a thing?

On the clone, there's a lot of 60 cycle hum, probably I need to rearrange some wires, the chassis is definitely laid out weird on the sound City. The preamp tubes are the directly in the center with turret rails on each side transformers on the opposite sides of the chassis. This kind of makes for some troubleshooting as far as layout..

The treble potentiometer with the 20dc voltage on it is attenuating the hum and also attenuating the channel in general. I'd appreciate any insight into this. It seems to be wired correctly, I'm using the original schematic.. maybe the original selmer that I'm cloning is wired wrong? According to the schematic it's not.

Also, I'm no expert in any sense of the word otherwise I would figure this out I'm sure. I'm a dabbler and a copycat. Mostly I play amps, but I do understand all of the dangers etc and I'm good about that. I've wired a lot of old Akai preamps into guitar amps using the rod mod.. and I've changed out a lot of power sections. Although that could be another issue in this amp.

Anyway, any input would be much appreciated as I don't have, locally many people to talk to you..

Alex

[maxkracht]

The problem is, is that I'm getting about 20 volts on the bass channels treble potentiometer. It seems that it's coming from 10 to grid of the pi.. I can't figure it out and then I measured voltages on the original selmer that I'm cloning and it has the same thing! Is this a thing?

20v on the PI grid could make sense, depending on the PI type. If there isn't a cap between it and the tone control you have 20v on the tone control. Could you post a schematic?

The treble potentiometer with the 20dc voltage on it is attenuating the hum and also attenuating the channel in general. I'd appreciate any insight into this. It seems to be wired correctly, I'm using the original schematic.. maybe the original selmer that I'm cloning is wired wrong? According to the schematic it's not.

If you have a known working amp, use that over a schematic. I'm guessing you just made a simple wiring mistake or copied a schematic error, easy to do.

[Alexanderspeer]

tnb4a (1).jpeg

[Alexanderspeer]

I appreciate you taking the time to check this out. So yeah the voltage seems to be coming from pin 2 of v4
There is no cap from pin 2 to what would be vr6, the trouble treble pot. Just a 500K resistor. On my clone version the trouble pot is amplifying any kind of noise and 60 cycle hum in the circuit. Even when I'm plugged into the normal channel it is audible if you turn that potentiometer which would be on the bass channel.

This doesn't happen on the actual selmer. Although I do think there's a bit of an issue with that one as well when you crank up volume on the bass channel there is some slight oscillation.. I suppose on this clone I'm making I really don't care so much about the bass channel. I probably won't use it that often maybe I should pull the whole section. But I really want to know what's up with the voltage and noise. It must be a mistake made on both amps? I've never messed with the original selmer..

[cdemike]

That design should have voltage on VR6: V4 Is a self-biased long tail phase inverter. Because the bias resistor does not connect to ground but rather a tail resistor, there will be positive voltage on the phase inverters grids relative to ground. 20V is low compared to the schematic's specified 34V, but that might just be differences between power transformers. In either event, there's no capacitor decoupling the DC voltage at the phase inverter's grid from the wiper of the treble control at VR6.

It sounds like the bass channel is amplifying the 60hz hum which could be a number of things. Is the filtering identical between the amps? Can you post a picture of the amp's internals? Often hum may relate to lead dress issues.

If it helps, I was able to find a schematic that I found easier to read: https://www.prowessamplifiers.com/schem ... assmki.pdf

I realized reading that schematic that the bass channel is a copy of the 6G6 Bassman, if it helps provide suggestions for lead dress: https://el34world.com/charts/Schematics ... _schem.pdf

[maxkracht]

To add to what cdemike was saying: your treble pot has dc on the wiper, but because of the other three caps attached to the pot there is no DC path to ground, so the pot doesn't notice. No scratchy scratchy or anything to worry about. If you got one of those cap values wrong, you could be cutting lower frequencies than intended.

[Alexanderspeer]

Okay here's where my real novice abilities come in. The clone has a double adjustable bias pots for the power tubes.. so I guess I built into the circuit more than it needs. So I need to adjust the circuit and take out the fixed bias circuit?

[Alexanderspeer]

Oh you mean the pi itself biased that doesn't have anything to do with the power to buy a section? Sorry guys, I'm learning or at least trying to. I will look at that basemann wiring scheme and figure out a better layout. Cuz I'm definitely getting a lot of 60 hertz hum.

[Alexanderspeer]

"It sounds like the bass channel is amplifying the 60hz hum which could be a number of things. Is the filtering identical between the amps? Can you post a picture of the amp's internals? Often hum may relate to lead dress issues."


I will post a picture later on when I can. The filtering itself is identical with the 47 UF bias cap with its own bridge rectifier and 5x32 UF with its own bridge. This is the difference between the two amps, the selmer has a tube rectifier. So perhaps the solid state rectification is noisier? Also I need to find tune the voltages on the power filter nodes as my voltages are a bit different than the schematic. It's a matter of using the right resistors. The ones on the schematic aren't really doing it, maybe because of the power transformer on this one?

[maxkracht]

Don't worry about the PI or poweramp. At least not in relation to your tone control issue. If the treble control turns down the hum, hum comes from before the treble control.

[Alexanderspeer]

Got it and many thanks!! I needed a bit of confirmation on a few things and got it, cheers!

One last thing, why do you think the distortion, overdrive is so much heavier at a lower volume on the clone? The selmer has a pretty clean headroom up until about 5 and 1/2 or so on the volume. Either way it sounds killer just looking for a little more headroom and ways to go about that.

[Alexanderspeer]

Ah, one more symptom that could be a clue as to the noise is, I've noticed that the bass channel, were the noises amplifying via the treble pot, if the treble pot is turned down and the volume and base are up to a good level there is a ton of sag to where the signal pretty much disappears when you hit the strings hard on the guitar. When the trouble pot is cranked up the channel sounds strong but of course I don't want to have to do that. That's another thing that's super weird that I don't get and I've rewired it twice..

[maxkracht]

if the treble pot is turned down and the volume and base are up to a good level there is a ton of sag to where the signal pretty much disappears when you hit the strings hard on the guitar.

Sounds like something could be oscillating, could be a lead dress issue. Pictures might help.

That's another thing that's super weird that I don't get and I've rewired it twice.

If the treble control is working (cutting highs), it's possible you have it wired right and the problem is coming from an earlier stage. Could also be making the same wiring mistake twice. (i've done it before)

One last thing, why do you think the distortion, overdrive is so much heavier at a lower volume on the clone? The selmer has a pretty clean headroom up until about 5 and 1/2 or so on the volume. Either way it sounds killer just looking for a little more headroom and ways to go about that.

If this is in both channels, it's probably a power supply issue or a poweramp issue. You mentioned the power supply voltages were off, lower voltage will give less headroom. Could also be a wiring error around where the channels meet (near that treble control). Accidental master volume before the PI could give you preamp distortion.