Fifties Gibson Ga-6 Lancer Tweed

[loogie]

I'm working on what I'm 90% sure is a GA-6 Lancer, fifties.  This is the closest schematic I could find.  It was hacked up pretty good, but there are a couple of factory things I wonder about -- or at least I'm pretty sure they're factory.

It uses grid leak bias at V1, one leg of the heater is soldered to ground at V1 (cathodes, heater, tube shield base are all connected together).  There are other possible grounding issues I'm working through (very hummy).  I hate to mess with (alter) these old amps, but do you think I should convert V1 to cathode bias?  Do you think I should unground the heater?  I converted an old Bogen to cathode bias and it worked much better.  It was quieter and had more headroom.  The heaters are center-tapped, near as I can tell.  Why would one ground one heater leg at the end of the chain?

In other words I would make V1 look like the schematic.

[HotBluePlates]

There are other possible grounding issues I'm working through (very hummy). 

Can you be sure the issue is grounding? I mean, this amp sold to someone early in its life, so we might assume the hum was low originally.

It was hacked up pretty good ...

Then the hacks may be the source of the problem. Is the PT original?

... there are a couple of factory things I wonder about -- or at least I'm pretty sure they're factory.

... one leg of the heater is soldered to ground at V1 (cathodes, heater, tube shield base are all connected together).  There are other possible grounding issues I'm working through (very hummy). ...  Do you think I should unground the heater? ...  The heaters are center-tapped, near as I can tell.  Why would one ground one heater leg at the end of the chain?

The schematic you linked would have one side of the heater grounded at V1 as well, and at all the other tubes.

Look at the PT; one side of the heater winding is grounded there, too. It is probably soldered to the chassis right along with the HV secondary center-tap. Fender did it a different way, with both heater wires running to the pilot light socket, but one of the light lugs was spun around and soldered to the "arm" of the socket which attaches to the chassis. That provides the ground connection on one side of the heater circuit.

From there (either PT if grounded directly to the chassis, or pilot light if grounded there), only one wire would be run to each of the tube sockets, with the other side of the tube heater connected to ground. Depending on the type of socket, there may be a short wire running over to a lug that forms the metal shell of the socket.

This method won't have the lowest possible noise floor of all methods of wiring heaters, but most of the time, it's not a problem. My '55 Tremolux was wired this way, and wasn't any quieter when I carefully undid it and ran a 2-wire heater circuit.

So you might have filter caps that are failing, a leaking hum switch cap on the PT primary, poor grounding on the hacks you saw, or a phase inverter with some leakage.

I'd first look to anything new and be suspicious, as well as tack filter caps across the existing filters to see if the hum subsides.

[loogie]

There are two wires from the PT grounded at the chassis.  Red with yellow exiting very near the secondaries and yellow with red.  Is it possible the latter is the heater center tap?  I can't see any other place where the heaters are grounded.  They go to the pilot light, but nothing special happening there.  No heater grounding at the power tubes or V2.  Otherwise the heater wires are chained from lug to lug.

I changed the filter caps.  I probably should have mentioned that.  Someone tried to tack on a filter cap at the second 10k decoupling resistor.  Its a cc resistor, but it doesn't look original.  One of its legs was broken/burned.  I also replaced the power cord.  The PT looks original.  It has a newer speaker and the OT is mounted on the inside of the cab.  It looks original.

I'll keep looking for the things you mentioned.  I'm going to replace that broken resistor and follow the grounds.

What do you think about the grid leak bias?

[HotBluePlates]

The grid leak biasing shouldn't be an issue. I once had an early-50's Princeton that was grid leak biased, and it sounded great.

Now the absolute amount of bias is probably lower with that than you'd have with a cathode resistor, which means it will only need a small amount of signal before it makes its full output voltage and starts distorting. Really, this shouldn't be a big deal with guitar-level inputs, but is the reaosn you sometimes see advice against using boost pedals with it. Honestly, I never found that to be a problem.

Cathode bias will seem like it has more headroom, because it will have a larger bias voltage and need a bigger input signal to reach the same output voltage.

[loogie]

I'm glad you talked me out of messing with V1.  I cleaned up the grounds, replaced the bad resistor AND put in another set of 6V6s.  Much less hum.  Sounds great. 

[Redbird]

Just bought a 1958 ga-6 last week and it had the original instruction booklet with the schematic. This schematic seems to be rare because I haven't found it on any sites for Gibson amps. It's the missing link between the first version and the Lancer. Hope it helps and prevents people from modifying a great amp!

Here is the schematic I put on online:

https://flic.kr/p/rb85PV