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

[Jazz P Bass]

Gibson Medalist

I like the Phono Input
Kinda presaged the CD insert on some Marshalls.

[davohilts]

my question on the reverb transformer is how do I know the correct impedance to match the pan?

[Bluwail]

Dave: I figured out the super hum. It is coming from a 1000mf 50V cap in the power amplifier. The most interesting thing is that I cannot find it on the schematic. If anyone can point out it would be appreciated. RS sells a speaker cap at the same ratring for less than three dollars. I will tackle the trebble issue since the amp is in working order, perhaps a pot could be placed the area (the RC network) where the P&S 2 pin one enters the recovery amp.

Regarding the other post regarding the impedance to match the pan; a lot of these reverb units are very similar of that era. I would call Acutronics, I am sure they could help.

Jerry

[davohilts]

I'm glad you found the problem. I suspect that someone put the wrong value cap in and that is why you can't find it. You are probably talking about the 100uf 50V cap, that's the negative bias voltage supply. It feeds the grid bias voltages to V3 P2, V4 P5, and V7/8 P6. The cap is installed with the positive to ground which is of course backwards and you should have -15V. You can verify if this is in fact the cap by following it on the schematic, it is located off the D3 diode and a close look at the schematic shows the positive lead going to ground. If this is what you are talking about you should replace it with a 100uf 50V cap.
If you wouldn't mind could you take a couple of pics of the inside of your power supply chassis and email them to me. I'm curious to see how the wiring in your amp compares to mine with respect to the green/white wire coming off the power transformer.
Let me know how things go. I can help you with the treble issue. I just have to find the info so I can hook you up. It worked like a charm with my amp.
send pics to davo@clear.net
Thanks,
Dave

[Bluwail]

Older Eyes

Thanks again Dave:
I misread the Cap which was much easier to read when I placed on the bench and moved the two resistors. I replaced the cap with an old Sprauge "Atomic" which has a yellowish orange paper cover with blue lettering. The cap is a 500uf at 50V. Sorry it was the only 50V I had in the box near 100.

The amp sounds good , a little noisy,however cranked no one notices. I tried to get the tremolo working, however the schematic shows that Pin 4 needs to be set to ground. It appears that pin 4 and one are tied internally and it looks original.

For the person that wants more boost out of this amp, use the old Gerald Weber trick of jumping the Normal 1 to the Reverb 2 and crank all the normal channel controls full on, it will bust your eardrums. This is a professional opinion! Reverse the procedure if you are not a reverb freak like me

Dave I will try to get you pics, however I may have to do it by email as I don't know how to do it here unless it is just an attachment

[Bluwail]

Acoustic AMP

P.S to the last post:

I used the amp with an ME-70 and a Tak Nashville with the tube preamp and it sounded fine and is my original reason for using this amp. It seemed to lower the noise floor when plugged in to the Boss. I am thinking of rewiring the plug with a grounded med grade connector. Has anyone converted to grounded plug and if so any differences noted?

[davohilts]

Thanks again Dave:
I misread the Cap which was much easier to read when I placed on the bench and moved the two resistors. I replaced the cap with an old Sprauge "Atomic" which has a yellowish orange paper cover with blue lettering. The cap is a 500uf at 50V. Sorry it was the only 50V I had in the box near 100.

The amp sounds good , a little noisy,however cranked no one notices. I tried to get the tremolo working, however the schematic shows that Pin 4 needs to be set to ground. It appears that pin 4 and one are tied internally and it looks original.

For the person that wants more boost out of this amp, use the old Gerald Weber trick of jumping the Normal 1 to the Reverb 2 and crank all the normal channel controls full on, it will bust your eardrums. This is a professional opinion! Reverse the procedure if you are not a reverb freak like me

Dave I will try to get you pics, however I may have to do it by email as I don't know how to do it here unless it is just an attachment

Thanks just shoot them to me via Email at davo@clear.net

The tremelo is very sensitive it seems to me to voltage so if some of the resisters are out of tolerance you might have voltage drop and the tremelo won't work. Do some testing and replace any that don't meet spec in the circuit. Also check the solder connections on the 'tremelo bug' to be sure they are good. I have a Gibson Hawk and there was a bad cap in the tremelo circuit and when I replace it the tremelo works great.

My amp sounds fantastic and I'm in the process of replacing the original 12" speaker with an old utah 15. This should really improve the tone and bottom end of the amp but I haven't removed the baffle board yet and I'm hoping there is enough room to slip in the 15. If not I'll build a separate cab and play it as an extension speaker.

I think I've narrowed the reverb issue down to the transformer so now I just have to find one that will work.

Thanks,
Dave

[davohilts]

Back to work on the Duo Medalist

I've got some time and I'm back to work on my Duo Medalist.
I have everything working but the reverb. The reverb transformer was suspect so I have replaced the transformer with a fender transformer from Weber and installed a fender tank but I still get no sound. I get a nice strong buzz when I touch the jack into the tank but no sound leaving the tank!
Does anyone have any idea how to approach this problem.
Thanks.
Dave

[52 Bill]

The buzz should be on the cable to the output jack of the tank, the cable from the new driver transformer goes to the input jack.

Have you tested the tank itself?
You could try hooking up a small speaker to the output of the new driver transformer to see if there is any signal there.

[davohilts]

thanks, i'll try the speaker trick to see if I get any sound and verify that I have the tank connected properly.

[TheezGoTo11]

I removed the tone network from both channels of my duo. It did increase the gain considerably but now both channels are extremely bassy. Almost not treble what so ever. One extreme to the other...any ideas?

[davohilts]

I would put the tone circuit back but replace the resisters and install new tone caps. Use silver mica 100pf 500V caps instead of the stock caps and if still too much bass use 250pf. Good luck

[twangmon]

I'm editing a column for the next issue of Premier Guitar magazine, and I'm searching for images (exterior and interior) of a '70s Gibson Duo Medalist. If you have any you'd be willing to share, we'd credit you with a special thanks at the end of the column and also run an appropriate photo credit. You can reach me at: andy@premierguitar.com. Thanks!

[drewl]

I'm editing a column for the next issue of Premier Guitar magazine, and I'm searching for images (exterior and interior) of a '70s Gibson Duo Medalist. If you have any you'd be willing to share, we'd credit you with a special thanks at the end of the column and also run an appropriate photo credit. You can reach me at: andy@premierguitar.com. Thanks!

I'll have to check, I think I have some pics.

I got one of these from my bandmate who had one rotting away in his basement.
never really liked the sound, so I gutted it and used the parts to make a great sounding Marshall 18w clone.