5E3 PROBLEMS: oscillation and hum issues... HELP!!! 1961 GIBSON GA-6 LANCER

[doldrumdesign]

Hey all! So I'm currently redecorating the interior of a 1961 Gibson GA-6 Lancer, giving it new caps (new board for the filters too), and a less "rats nesty" wiring job. Since I first got the amp, it's had odd problems with a high pitch squelch on vol. 2 around 8ish, slight oscillation at high volume and hum throughout. I decided to take the almost 5D3 style circuit even more towards the Fender 5E3 schematic with .1uf coupling caps and 16uf filter caps, 1500k grid stoppers and added the V2 and V3/4 cathode bypass capacitors. The re-wiring was completely needed. I can't believe how bad Gibson's 50's and 60's amps were made! Half the parts not on the board are obviously picked up off the shop floor and reused. Anyhow...

So I just finished the rewiring and was sooooo stoked to find my voltage just about the exact same as after the cap change and before I started the rewire. On turning up the volume pots (nothing plugged in) I noticed a lot more hum and thumping oscillation that would come in at 9 or so on each channel. Channel 2 still has the squelch only it now starts EARLIER at about 6 on the pot!

Any ideas on where the hell I went wrong???

[Chuck H]

The thumping oscillation is called motor boating. It's most often a problem with the power supply. If any of the nodes aren't properly filtered or sometimes even if a filter ground is poorly located this can happen.

The reason the squeal got worse is because you raised the gain.

Just because an amp has untidy "looking" wiring does not mean it's more likely to be unstable. And just because an amp has tidy, aligned components and wiring does not mean it's less likely to oscillate. It's all about location. Where are things routed, where are things mounted, where are things grounded?

You should triple check the power supply and all it's functions. If anything has been changed as to node placement or the rail circuit arrangement, please provide a schematic.

Can you supply high res full chassis shots? It's much easier to trace leads and recognize proximity that way. A 5E3ish amp shouldn't be too tough to make stable unless some mounting locations, routing or grounding is grossly wrong.

[Steve Conner]

Even a 5E3 type circuit can oscillate if it has non-shorting input jacks that leave the grid of the first stage floating when nothing is plugged in. Does the oscillation go away when you plug in a guitar?

[doldrumdesign]

The thumping oscillation is called motor boating. It's most often a problem with the power supply. If any of the nodes aren't properly filtered or sometimes even if a filter ground is poorly located this can happen.

The reason the squeal got worse is because you raised the gain.

Just because an amp has untidy "looking" wiring does not mean it's more likely to be unstable. And just because an amp has tidy, aligned components and wiring does not mean it's less likely to oscillate. It's all about location. Where are things routed, where are things mounted, where are things grounded?

You should triple check the power supply and all it's functions. If anything has been changed as to node placement or the rail circuit arrangement, please provide a schematic.

Can you supply high res full chassis shots? It's much easier to trace leads and recognize proximity that way. A 5E3ish amp shouldn't be too tough to make stable unless some mounting locations, routing or grounding is grossly wrong.

Well I fixed the "motor boating" soon after I posted this thread last night. I was getting too excited I guess when starting the rewire. See, the resistors that go between the filter caps (5k & 22k // 10k & 10k for this amp) are usually soldered between them on the board. With the GA-6 Lancer, one of them was soldered to pins 5 & 7 of V4 and the other is already on the small original board (last one on the right). When I rewired it, I relocated the resistor on V4 to between the last 2 filter caps, and, in my excitement, bridged the other spot where the resistor would have gone. But there already is a wire from one end of the resistor to the middle 16uf, and the other end goes to the left 16uf. So I took out that wire between the two and all of the hum and motorboating went away and it sounds better than ever!!! Barely any noise at all!!!

BUT the very high pitch squeal on Ch.2 is still there when turning up past 8 or so. What's the deal with that? This has been an issue since I got the amp. I've tried swapping out 12ax7's and a different set of 6v6's, but that odd squeal is still there.

And, FYI, I rewired the rats nest not purely on it's terrible looks, but it had tons of noise (and that damn squeal...) due to all of the grounds returning to V3!!! There are no grid stoppers in the Gibson schematic so I wanted to add them to cut a bit of the noise, but the previously mentioned 10k resistor across pins 5 & 7 of V4 was hindering that. The preamp section was not changed at all, except for the fils wiring.




-Nick

[doldrumdesign]

So I have just noticed that this squeal only appears on Ch.2 input 1. Does not appear on any other inputs. Hmm...

[doldrumdesign]

Any ideas? I'm just not getting why that one single input would squeal with volume but all the other inputs are ok.

[Chuck H]

Does the squeal respond to the tone control?

Is channel two "brighter" than channel one? (it looks to be on the schem)

I can't see in the pics where the OT is mounted or enters and exits the amp. I can see a grommet near the power tubes, but I can't see the wiring.

[doldrumdesign]

It does yeah. If the tone is full the squeal comes in sooner on the volume control. The 2nd channel, or "instrument" as Gibson has it, is not necessarily brighter I don't think, just responds to the tone control differently.

The OT is mounted to the top of the chassis. The grommet near the power tubes is where it comes in then it's wired to the speaker. No speaker jack in the GA-6.

[woodyc]

This isn't much help, since you're problem is specific to one input, but... having the output transformer in such close proximity to the preamp tube is an invitation for squealing, even in a low gain amp. (I'm assuming its on the chassis right behind the preamp?) Different tubes have different sensitivity to that kind of thing.

[Chuck H]

I was thinking the same thing. Having raised the gain similar to a 5e3 also raises the sensitivity of the effect. It could be that since channel two likely uses the triode nearest the OT OR since channel two is more affected by the tone control (which is wired and mounted right over the OT) that the layout is inhearently problematic. I might try lifting the tone control circuit (at both ends). If the problem stops then some shielded cable may solve the problem. I might also try a tube shield for V1. I know niether of these things is keeping with the vintage look of the wiring, but I think functional is better than pretty.

You might try sielded leads for the V1 grid inputs as well. Every little bit helps.

[doldrumdesign]

Well the bonus of having a semi-5E3 amp at $300 is I can move anything as long as it will get me to the best sound. Now I do see the OT vs. PRE issue a thing, but would it make only 1 input squeal and all others quiet??

[Chuck H]

If the "one input" is the one with more lead length and sensitivity (having more signal through the tone wiring), as well as being amplified by the triode nearest the OT, then sure. That input may squeal while the other behaves.

[woodyc]

Another thing that sometimes helps when you have parasitic feedback is to change the polarity. You can do this in a Lancer (since it has no global feedback) by swapping the plate or grid leads on the power tubes or at the splitter output, whichever is easiest.

[Jecarroll7]

You've got some smart guys with more experience than I have helping here, so take the below with a grain of salt...

Assuming you've covered all the suggestions received already, if you have access to an o'scope I like to walk the tone path when I have any noise that shouldn't be there. Put a continuous signal at the input (I use an ipod playing guitar tuning pitches) and start at the jack and work your way back. Set the o'scope to trigger to display a single audio sign wave of a given note, so the wave stays in place on the screen. For the most part this should be a smooth wave that reshapes for the duration of the note through decay. If you see anything sharp/jagged or out of the ordinary mixed in the sign wave you're close to an issue. The good news is one channel seems to work fine so you can compare between the two.

[doldrumdesign]

Hmm. So I tried playing through the amp and I think the squeal and oscillation is definitely in both channels, just more pronounced through channel 2. Tried putting a pedal in front of it, thought it was going to blow up. Could the polarity be the issue? Should I flip the wires from the 3 prong grounded plug? Should I try moving the OT over near the PT? I don't think I'm going to have any hair left after this...