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Author Topic: Old guitar Amp power polarity and live chassis  (Read 14800 times)

Kevin Maxwell

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Re: Old guitar Amp power polarity and live chassis
« Reply #50 on: July 02, 2016, 10:06:46 am »

I didnít read all of the posts in this thread so this may have already been covered. I did an American Cancer Society Relay for Life once where the guitar player in one of the bands had an interesting problem with his amp. He had it modified to have a 3 prong grounded cord. But he still had the switch wired to flip the hot and neutral. The only power we had had a GFCI breaker on it and it took us a little bit to realize that he was the one that popped the main circuit feeding everything with his amp. I think he did it twice as we were setting the mics for them. I donít know if this amp had the cap that is being mentioned or if this was just the weird way it was wired.     
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #51 on: July 02, 2016, 02:17:15 pm »

I didnít read all of the posts in this thread so this may have already been covered. I did an American Cancer Society Relay for Life once where the guitar player in one of the bands had an interesting problem with his amp. He had it modified to have a 3 prong grounded cord. But he still had the switch wired to flip the hot and neutral. The only power we had had a GFCI breaker on it and it took us a little bit to realize that he was the one that popped the main circuit feeding everything with his amp. I think he did it twice as we were setting the mics for them. I donít know if this amp had the cap that is being mentioned or if this was just the weird way it was wired.   

Most likely he still had the stinger cap installed, so flipping the switch one way would be fine since the cap was connected between neutral and ground, while flipping it the other way would place the cap between hot and ground. Those old stinger caps tend to get leaky, so if it exceeded 6 mA of current it would certainly trip any GFCI upstream of it. He just needs to cut the stinger cap out of the circuit and it should work fine.

BTW: This is why I think that there should be a separate GFCI for each backline amp. If one stage amp looses power, then it's not that big a deal. However, taking down the entire PA system due to one GFCI far upstream tripping is what gives GFCI's a bad name.
« Last Edit: July 02, 2016, 03:44:20 pm by Mike Sokol »
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Jeff Bankston

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Re: Old guitar Amp power polarity and live chassis
« Reply #52 on: July 03, 2016, 03:33:40 am »

I will be picking up a 1972 Fender Twin Reverb in a few weeks that I'm buying from a friend. It has a 3 wire cord someone installed but he doesnt know anything about the cap so I will be removing it if it's still in place. I'll also replace the PS caps if they are old. I'm also going to remove the master volume circut. Now I will be able to do pig snorts again through a mic while taping on the reverb tank like use to do on the other geetar players Fender Twin Reverbs amps.
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Bob Leonard

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Re: Old guitar Amp power polarity and live chassis
« Reply #53 on: July 06, 2016, 08:10:29 am »

Questions of the day.

Q: What parts of what circuit(s) will be replaced, and what components added, after you remove the master volume control and it's associated components.

A: Trade secret, find out yourself.

Q: When removing the death cap do you, A) bypass the switch and leave it in the chassis to fill the hole, or B) Just remove the death cap?

Answer - A. Bypass the switch and leave it in the chassis to fill the hole and give the appearance the amp is still unmodified.
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BOSTON STRONG........
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Geoff Doane

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Re: Old guitar Amp power polarity and live chassis
« Reply #54 on: July 06, 2016, 10:43:30 am »


Answer - A. Bypass the switch and leave it in the chassis to fill the hole and give the appearance the amp is still unmodified.

It now becomes a placebo, and the guitar player can place it in whichever position he feels gives him better tone (or less noise, or more gain, or....).  :)

GTD
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Mike Sokol

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Re: Old guitar Amp power polarity and live chassis
« Reply #55 on: July 06, 2016, 02:03:14 pm »

It now becomes a placebo, and the guitar player can place it in whichever position he feels gives him better tone (or less noise, or more gain, or....).  :)

GTD

I like it. One of my studio engineering buddies used to place a 1/3 octave equalizer back at the producer's desk so that the "producer" could tweak the EQ. But the equalizer wasn't actually patched into the mixing path. This made the producer feel like he had ears and was providing real input, and let the engineer do his job without the producer yelling at him for more jellybeans or whatever toher wacky nonsense words that producers like to use.
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Stephen Kirby

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Re: Old guitar Amp power polarity and live chassis
« Reply #56 on: July 06, 2016, 09:00:46 pm »

When I rebuilt my DR I took the switch out and snapped a feed through grommet in the hole to plug it.  Although any guitar player worth their salt can reach around the top of a Fender amp and manage the power and standby switches without looking, I didn't see any need for an extra switch back there.  Because I'd replaced the bad OT with an Allen part, any pretense of originality was gone anyway.

As far as cutting out the master volume, it's easy enough to get an AB763 schematic and follow it.  What may be more involved is fixing the wire dress so that the amp is stable.  My SF to BF conversion went well and tweaking the dress from some on-line pics came out fine.  I've heard that some amps can be real bears though.
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Jeff Bankston

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Re: Old guitar Amp power polarity and live chassis
« Reply #57 on: July 07, 2016, 05:00:45 am »

When I rebuilt my DR I took the switch out and snapped a feed through grommet in the hole to plug it.  Although any guitar player worth their salt can reach around the top of a Fender amp and manage the power and standby switches without looking, I didn't see any need for an extra switch back there.  Because I'd replaced the bad OT with an Allen part, any pretense of originality was gone anyway.

As far as cutting out the master volume, it's easy enough to get an AB763 schematic and follow it.  What may be more involved is fixing the wire dress so that the amp is stable.  My SF to BF conversion went well and tweaking the dress from some on-line pics came out fine.  I've heard that some amps can be real bears though.
The only schematic I could find with the MV removed mod is this one. Is this the same circuit as the 1972 Fender twin reverb ? I plan to replace the MV pot with a rotoray switch and use it for the standby switch. It will have the V knob on it. The faceplate is well worn as is the grill cloth.

                   
« Last Edit: July 07, 2016, 05:03:48 am by Jeff Bankston »
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Bob Leonard

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Re: Old guitar Amp power polarity and live chassis
« Reply #58 on: July 07, 2016, 06:37:16 pm »

Jeff,
I'll suggest that you find a silver face layout and schematic that doesn't include the master volume circuit. Follow the schematic/layout closely and you'll find any number of components that will have to be changed. The fun will start when you find that after CBS components were substituted on a whim depending on what was in the bin that day.

The change you show above is typical and not actual. You will need to treat the amp as a one of a kind working from the power supply to the speaker. And if the amp is a high output version the transformers will need to be changed. I've done more than 50 and maybe as many as 100 MV conversions, and every one was a new experience. Go forth and be careful my son.

Steve,
Reverting back to AB763 is more than just a few components, but I'll bet you knew that. The PI, BIAS balance (if there is one), pre amp tube negative feedback voltages and power supply all have to be changed, removed or rebuilt.
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BOSTON STRONG........
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Stephen Kirby

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Re: Old guitar Amp power polarity and live chassis
« Reply #59 on: July 07, 2016, 09:10:23 pm »

Steve,
Reverting back to AB763 is more than just a few components, but I'll bet you knew that. The PI, BIAS balance (if there is one), pre amp tube negative feedback voltages and power supply all have to be changed, removed or rebuilt.
Yeah, I pretty much gutted the amp and rebuilt the main portion from the ground up.  Spent some time on TGP with Aiken and Allen and added several tweaks like selected carbon film in the signal path with metal film elsewhere, orange drops, poly and silver mica caps.  Was a labor of love but I'm not really interested in doing it again.  Lot of work.  Voltages are slightly different because of the Allen/Vibrolux PT in it as well.  Because of the better OT it's about 28W at clip but the new PT is wound for modern 120 input instead of 110 so B+ is where it originally should be and the amp is slightly softer than an unmolested or perfectly restored one.  It's a player, not a museum piece.  I only have 3 amps.  The Fuchs ODS, this, and a MiniBrute.  Pretty much covers any flavors that I like since I don't really play rock any more and have any need for authentic Marshall/Hiwatt/Orange types of sounds.  Stomp boxes work fine for the times I need it.  There are cool things out there like a JMI AC30 or Watkins Dominator that get noises you can't get any other way but if you can't work with a good black face Fender, more practice is in order.
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