Gibson Duo-Medalist - too much treble, not enough volume!

[f13ndus]

Hi all, I just picked up a Gibson Duo-Medalist for something to fiddle around with. Price was right and I couldn't pass it up! It's all original, right down to the tubes.

3 x 6EU7, 2 x 12AU7A, (these 5 are the preamps/reverb driver/tremolo driver tubes) 1 x 12AX7, (I believe phase inverter) 2 x 7591. (power, of course!) SS rectifier.

First problem is volume... there's hardly any of it! It has two channels, one "Normal" and one "Reverb/Tremolo". If I run my guitar into BOTH channels, I can get a decent volume. Looks like each channel uses 1 of the 6EU7s for signal before the power amp, the other 3 (12AU7s and 6EU7) are for tremolo and reverb. Is this just a result of not enough gain stages in the preamp? I'm going to replace both of preamp 6EU7s and make sure the power tubes are good and biased correctly sometime this weekend. What else should I be looking at that could cause this lack of volume? I think this amp was designed to be clean, but some people (I'm talking about Harmony Central reviews here) say they get a great overdrive, others say they can only get clean - I seem to be in this second group!

Second problem is the TREBLE. This thing brings a new meaning to Ice Pick! The knobs go to 12, I keep the Bass on 12 and the Treble on 1-2 and I still have to roll off a decent amount on the guitar tone knob. Looking at the schematic, there's something I don't understand, after the first triode and tone stack and before the second triode that goes to the power amp - is this some kind of filter? I'm still learning about these things...

I've attached the schematic with the components I'm unsure about circled in black. Any ideas what this does in the circuit?

Anyway, hope you guys can help me out a bit, I've found plenty of info here and in the archives before, but this time I actually had to register to ask!

[52 Bill]

Those are tone shaping networks.

There should be more bass and power from that amp than you seem to describe. Besides the tubes, I'd check for off value coupling and bypass caps.

[f13ndus]

Those are tone shaping networks.

There should be more bass and power from that amp than you seem to describe. Besides the tubes, I'd check for off value coupling and bypass caps.

I figured they were, do those values indicate what frequencies they're affecting?

Also, how can I check capacitors w/ a multimeter? I've never tried to before. And any tips on where to start looking for problems in the circuit? Most likely area for things to go bad?

Since the lack of bass/too much treble affects both channels equally, am I correct to assume it's most likely not in their respective preamp circuits and I should focus on the power amp? (this is where it'll come in handy that the pre and power amps have their own respective chassis'!)

I've got experience with smaller SE tube amps (Epiphone Electar 10, Gibson GA-8), but this is the most complicated circuit I've worked with so far! That's half the reason I got it, to force myself to learn more about schematics, and tube circuitry in general...

[pdf64]

Like 52 Bill says, I would guess that a coupling cap had gone open circuit, so the only signal getting through was due to stray coupling. All you need is a multimeter and a bit of thought. Feed in a 400Hz test signal, if there's 10Vac one side of a coupling cap, and 0.1Vac the other, then try bridging the cap with one the same value. If the signal level shoots up to 10V, then you've found the fault. If it goes up to 0.2V then look elsewhere.
Caps can also fail by letting some dc through. Test for this by disconnecting the low voltage leg, powering up, and measure what voltage appears on that low voltage leg. Much more than nothing and the cap's bad.
For good info see Geofex
http://www.geofex.com/ampdbug/ampdebug.htm
From what you say, probably something bad around V6, assuming that the rest of the output stage (speakers?) is functional.
Those tone shaping networks create a scooped mid frequency response, like a fender tone stack with treb and bass full.
Peter.

[PRNDL]

That might be an RIAA tone circuit, which is not for guitar.

I found this quote on Harmony Central

I read a Ken Fisher article about modding Gibson amps, and I took the internal "tone network" out of the circuit. It now sounds absolutely awesome.

Should be a pretty simple mod to bypass it and test.

[Enzo]

No, Gibson didn;t put RIAA filters in their guitar amps. it is just tone shaping. In fact if it were an RIAA filter, then the resulting sound would lack treble and be boomy and bassy.

I agree with the open coupler idea, all you have left is crosstalk which will sound weak and tinny. Like either cap going to the grids on the PI tube.

One other odd possibility is a stuck on trem bug. If the bulb is stuck on inside or the photocell collapsed, the thing could be in a constant peak of trem cycle. Pull the trem tube V5 to see. Or lift the wires to either that .22uf cap or the trem photocell itself.

In that schenario, I would expect it to sap your volume considerably, but I wouldn;t expect it to make things trebly, but for want of pulling one tube for a moment, it's worth a check.

[f13ndus]

No, Gibson didn;t put RIAA filters in their guitar amps. it is just tone shaping. In fact if it were an RIAA filter, then the resulting sound would lack treble and be boomy and bassy.

I agree with the open coupler idea, all you have left is crosstalk which will sound weak and tinny. Like either cap going to the grids on the PI tube.

One other odd possibility is a stuck on trem bug. If the bulb is stuck on inside or the photocell collapsed, the thing could be in a constant peak of trem cycle. Pull the trem tube V5 to see. Or lift the wires to either that .22uf cap or the trem photocell itself.

In that schenario, I would expect it to sap your volume considerably, but I wouldn;t expect it to make things trebly, but for want of pulling one tube for a moment, it's worth a check.

I'll try it, but if that were the case, wouldn't the tremolo not be working? It functions fine, light and all... but you're right, it doesn't take long to pull a tube and try it!

I guess I'll start out replacing the caps to the PI tube, and the electrolytic caps in the power section. They're in a multi can, but there are only two caps in it. I'm assuming I can just install individual caps inside the chassis instead of trying to find the correct metal can to replace the original one? Should I just go ahead and replace all the caps in the power section so I don't have to worry about them afterwards? There are only... 7, including the metal can, so I think I will unless someone thinks otherwise...

Here are a few pictures of the amp: power chassis, pre chassis, control chassis, and the actual "box" (just for fun!).

More Questions:
*Is the black capacitor on the power switch the "death cap"? Should I just remove it when I install a 3-prong power cord?
*Is the trim-pot between the power tubes for setting bias, or something else? Bias is another thing I need to learn about...

[davohilts]

I just got one of these amps to work on. It came without tubes and it took me a while to get tubes so while I was waiting I replaced the filter caps. I got the tubes last night and put them in and fired it up and it hums loudly and volume controls don't effect the hum but tone controls do slightly. The clean imputs work and the amp sounds very ice pick like with any treble at all. The effects channel has very little volume and the tremelo light on the control panel won't go out. I can turn up the strength on the tremelo and the hum changes and speeds up and slows down so I must have a short somewhere? I have not biased the amp either. Did you ever get yours working? If you did what did you do? Thanks
Dave

[Gtr_tech]

Got the footswitch plugged in? If so, its possible to plug it in with improper orientation. That will cause a hum and strange behavior. The 4 pins are numbered both on the plug and on the socket. Clean them off so you can find those numbers and match them up. Paint a little stripe on the plug so you know which part faces out.

Anyone lookin' for one of these? Got one for sale....

[davohilts]

thanks for the tip, I just plugged it in figuring since the pins were not equally spaced it could only go in one way, i'll get a flashlight and check out the install to see it that is the problem. I did unplug the footswitch however and the amp still hummed and the tremelo light stayed on but not much volume at all on the effects channel.
Dave

[Bluwail]

Humming louderthan the amp ever produced

Turned on the Dou after many months of inactivity and it hums to beat the band. Replaced can on power amp and only found out yesterday the other cap is a 40-20-20. I in my zeal to test forgot to re plug the speaker so I most likely fried the OT. Does anyone have a handle on where the parts can be obtained and or specs on the OT. Any one have any similar experience of massive hum. If the input is disconnected (RCA jack) the hum is the same. Like to save the amp if not it goes to parts.

Thanks

[davohilts]

yes, I bought a duo medalist on ebay and it hummed really bad. someone had done a poor recap so I replaced the caps in both the preamp chassis and the power amp chassis and the amp still hummed badly. upon further investigation and a close look at the schematic I found that this amp power transformer had a center tap for the filament winding so i grounded that and the hum went away. i'm still trying to get the reverb to work and feel that I might have a bad reverb transformer and am trying to find out if there is a location to obtain one. I had a Falcon with a bad reverb transformer and ended up replacing the pan and transformer with a Fender model as I couldn't find a gibson model.
Take a good close look at the schematic and follow the layout carefully because if it still hums your output transformer might still be good. Check your power transformer, there should be 2 red wires, 1 red/yellow, 2 black, 2 green which is what shows up on the schematic. Mine had an additional green/white wire which I took to be the filament centertap. Let me know what you find out.
Dave

[Bluwail]

Thank you Dave:

First I replaced the Gibson unit with a reverb spring unit from Acutronics in Chicago. I did this in the mid Ninetys and they said that there would not be too much difference as long as you used the same number of springs. It is likely this is where Gibson purchased them from to begin with. I did find that the can which is paper wrapped in the preamp chassis is reading less than 2 mf in one of the 20 mf sections. I have not figured out a strategy to pull it without removing the rivet. I will try the filament supply and the xformer.

Thanks, Jerry

[davohilts]

Does your power transformer have 8 wires or 7. I suspect that since mine didn't match the schematic that is could have been replaced 30 years ago and I could not tell because it looks old. I left the cap cans in place on my amp and added new filter caps inside the chassis and rewired the connections. In the preamp section I used a snap tie to tie the three caps together and to the chassis. My problem is that I think the reverb transformer is dead but I don't know how to test it to be sure and if it is dead I'm not sure where to find one which will match the tank.
Good luck with your project. I removed the tone stacks on both the normal and effects channels and this got rid of the 'ice pick' treble issue I had. The amp sounds great now, just short the reverb. I'm thinking about putting in a 15 inch speaker to help fatten the tone up, there seems to be plenty of room in the cabinet.

[Gtr_tech]

The reverb driver xfmr is just a small output xfmr....the same rules apply.