Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nateflanigan on July 21, 2017, 06:03:15 pm

Title: Ampeg m12 weak tremelo
Post by: nateflanigan on July 21, 2017, 06:03:15 pm

Hello,
I'm working on a very old Ampeg M12, generally speaking the amp is pretty clean and works fine.  It needs to be recapped and while I'm in there I'll replace the usual resistors as well. Two questions...


1) I've never replaced coupling caps before. On this amp I measured 10 VDC on the first lug of the volume pot (after the coupling cap) on the guitar channel and about 15 VDC in the same location on the accordion channel.  I also measured a small amount of DC, less than 1 volt just after the 0.02 coupling caps between the PI and power tubes.  I'm thinking these should all be replaced, right? Should I worry about the caps in the tone stack?


2) The tremolo is extremely weak, a web search shows that this seems to be common.  I plan on recapping the oscillator. and lowering with the 470k resistor that feeds intensity pot.  Another idea is to wire it up like the tremolo on a 5E9 Tremolux, the amps are very similar, is this a reasonable idea?


Thanks friends


http://el34world.com/charts/Schematics/files/ampeg/Ampeg_m12_6v6.pdf (http://el34world.com/charts/Schematics/files/ampeg/Ampeg_m12_6v6.pdf)
http://el34world.com/charts/Schematics/files/fender/Fender_tremolux_5e9a_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_tremolux_5e9a_schem.pdf)

Title: Re: Ampeg m12 weak tremelo
Post by: sluckey on July 21, 2017, 07:29:41 pm

Quote

1) I've never replaced coupling caps before. On this amp I measured 10 VDC on the first lug of the volume pot (after the coupling cap) on the guitar channel and about 15 VDC in the same location on the accordion channel.  I also measured a small amount of DC, less than 1 volt just after the 0.02 coupling caps between the PI and power tubes.  I'm thinking these should all be replaced, right? Should I worry about the caps in the tone stack?

Yes, replace the coupling caps. If the tone caps are old waxed paper I'd replace those as well.

Quote

2) The tremolo is extremely weak, a web search shows that this seems to be common.  I plan on recapping the oscillator. and lowering with the 470k resistor that feeds intensity pot.  Another idea is to wire it up like the tremolo on a 5E9 Tremolux, the amps are very similar, is this a reasonable idea?

First thing I'd do is change V2. You got three 6SL7s in the amp. The trem needs a strong tube to work well. Then replace caps and resistors in V2 circuit. DON'T TRY TO CHANGE TO THE 5E9A circuit. The oscillator may look very similar but the way the trem signal connects into the guitar signal circuit is totally different.

Title: Re: Ampeg m12 weak tremelo
Post by: nateflanigan on July 22, 2017, 09:06:12 am

Thanks Sluckey,


I forgot to mention, I've already replaced all the tubes.  The amp was purchased by a friend for $150, it's cosmetically very good, everything is original even the tubes, and the owner had the original set of spare tubes that came with the amp and were never installed.  All raytheon 6SL7s and and RCA 6V6s. 


All the caps in the amp are the original red astrons, are those are paper and wax?


Replacing the tubes perked up the amp a bit but did not effect the trem.  I do plan on recapping the oscillator circuit, I'll do the resistors as well.


Regarding the Tremolux idea, what I meant was moving the modulation signal from the grids of the 6V6s to the cathode of the PI.  I believe you, but can you explain why this is a bad idea, I'd just like to understand better.


Thanks