200 s Phase Mod

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I recently bought a 200 s on ebay. I love my 66j and thought it might be in the same vein as that. It's a sweet sounding amp-but not as dirty. I brought it to my amp guy and noticed that the amp's stereo jack was wired for an out of phase pickup. I think this corresponded with the s 200 guitar? Hence, when I plugged a normal guitar in the stereo jack it gave me an out of phase tone. So we reversed the speaker connection and that corrected the problem. Now the thing is pushing some air. Is this how these amps were wired? Any info would help.
 

capnjuan

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cray2008 said:
I recently bought a 200 s on ebay. I love my 66j and thought it might be in the same vein as that. It's a sweet sounding amp-but not as dirty. I brought it to my amp guy and noticed that the amp's stereo jack was wired for an out of phase pickup. I think this corresponded with the s 200 guitar? Hence, when I plugged a normal guitar in the stereo jack it gave me an out of phase tone. So we reversed the speaker connection and that corrected the problem. Now the thing is pushing some air. Is this how these amps were wired? Any info would help.
Hi cray2008 and welcome to LTG. You can find schematics for your amp at Prowess Amplifiers / Miscellaneous. The fragment below is from the original schematic showing the front ends of both channels, the stereo jack, and the stereo switch in magenta.

Guild amp dates can get a little murky but my understanding is that the 200S amplifier would have pre-dated the Thunderbird/S200 guitar by a few years; that is, I don't think the 200S amp was built with the S200 guitar in mind. Also, I believe the 200S amp was sold with an adapter cable; 1/4" jack on one end and with a male stereo plug on the other and the signal split between the two hot connections in the plug.

Assuming the S200 guitar had a phase switch - to put the pickups' signals out of phase with respect to each other - the 200S amplifier was designed for true stereo input; two signals ... a left and a right or a single mono signal being split by the factory adapter cable. Whether the p/u outputs were out of phase with each other, that wouldn't matter to the 200S stereo jack.

200Sfragstereo.jpg



If your amp was wired like the schematic above and if you jacked in a guitar with conventional mono output (one channel only ... regardless of pickup phasing) to the stereo input, if the stereo switch were in the null position, the signal wouldn't appear in either channel. If the switch were in the stereo position, the signal would appear in one channel but not the other ... and not both. The stereo switch only connects each channel's input to it's corresponding hot post on the stereo jack ... it doesn't bridge them so that the same signal is sent to each amp channel. Many (all?) of the high-line Gibson stereo amps GAs 78/79/83/90 have a separate bridging switch that puts the mono input into both channels producing a stereophonic effect (two signal sources) although it's really just dual-mono.

Don't know what your tech meant by tinkering with the stereo switch with respect to phase but if a downstream buyer didn't have the adapter cable and didn't want to mess with making one up, the easy answer would have been to convert the stereo switch to a bridging switch so that when a standard guitar were plugged into the stereo jack, both channels would receive and amplify the same signal.

As far as speaker wiring goes re/ phase ... who knows but the number of gain stages in both channels is identical. Each gain stage inverts the phase of the signal but if the number of gain stages were identical, then the signal out of each channel would be identical with respect to phase. Maybe somebody was just screwing around with the speaker feeds or took out / put back a speaker and just got the + and - connections wrong.

In any event, welcome to LTG and congratulations on your 200S Guild amp. I have a copy of the original schematic, PM with your email address ... I'll send it to you.
 
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Thanks for the info. I got the manual from that guy Noah who posted it prior.

I had another question about this amp, if you know. Do you know if this amp was designed to be a cleaner sounding amp? I can only get minimal overdrive from it. I know it's a different beast than my 66j but I was hoping for a little more gain. You know if there's any simple tweaks I could do? A tube or something?
 

capnjuan

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Don't know what the amp can or can't do and don't know of any published tweaks or mods for it. If yours is in reasonable condition, what you're getting out of it is what it does.
 
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don't get me wrong, the amp sounds great. It is in great shape-all original except a few caps. I don't want to do any serious mods to it. I'll keep running a pedal in front of it. Thanks for the info.
 
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