1962 98RT and Reverb 20 Schematics and recap

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Hi, I'm new to this forum-- but think I have some valuable info for the list.
Had an amp in for repair-- single owner prior to my client's purchase. I found not just a legible 98RT schematic, but the bonus Reverb Converter schematic (on a fold).
Guild98RT_1962.jpg


Please note that this schematic seems to have some errors
1) The plate resistor for the reverb driver was 2.2k on the amp.
2) Missing some filter caps. Compare with a same vintage Ampeg R12R, it is missing 20uF filter caps on nodes S and T. Without these the reverb control will act as a gain knob since the resistor on the reverb driver isn't a high value. I also found that hum dropped when I added another filter cap to node U. As the amp had been worked on before me, I can't confirm the stock arrangement.

Since the input jacks are grounded to chassis-- care must be taken to avoid ground loops if you replace the can cap with individual caps. I converted to start ground. More specifically, what I did was remove the can cap, install a hole plug and mount a terminal strip on the screw from that cap. I removed the bus wire that is under the tag board (it was nice and rusty). All the power supply resistors that were cracked and failing on the amp were mounted on that creating room for 3 filter caps on the tag board. 2 more were fit on the board near the preamp stages by moving components around a bit. The final "bonus" cap was installed by with a zip tie to a ground wire bundle to avoid more extensive tag board changes. Grounds from each preamp stage (including pot) are run to the star point (ground of first filter cap. The backs of the pot are not grounded.

After that work and replacement of the usual suspects, the amp sounds great-- I wasn't happy returning it! It is interesting to note that amp arrived with a Amperex 5AR4 and some Sylvania 6L6GCs. The amp had been running that way for a least a few decades and that setup had a lot more clarity than a 5Y3/JJ 6V6s combo. In comparison, the later was compressed sounding which wasn't the voicing my client wanted.

I definitely prefer the reverb tone to a transformer coupled design. I was sad to give it up, but I have a late 50's 66-J. Alas, no octal preamp tubes...
 

capnjuan

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Hi benkuris and welcome to LTG. Another member recently recapped a 66J and had similar ground loop issues with the replacement can cap. I'm a little surprised that the amp survived the 6L6s ... that the original 6V6 bias point would work ok but stranger things have happened.

I bought an RC20 off eBay a while back (threaded here.) The seller forgot to mention that it was in near-death condition ... didn't have a set of interconnect cables as advertised ... and hammered like a steam engine. I didn't hang on to it. :evil: If you get the chance, please post the composite schematic at Prowess Amps. I've been slowly uploading Guild schematics there ... the 98RT with the RC20 will be a happy addition. CJ
 
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This is quite a coincidence. I'm currently working on a 98RT for a friend and just finished redrawing the schematic and layout. There were quite a few different components in mine compared to the Guild schematic. This amp looked closer to the Reverberocket schematic than to the 98RT schematic, and it looked like it had come from the factory this way. I populated my schematic with the component values that were installed in this amp.

So what was the problem with the amp? The electrolytics were getting old and cranky, but the real problem was that almost all the plate resistors were shot, with values from 5x the original value, all the way to dead open. The screen resistors had drifted, and the amp was way out of bias (hot, hot, hot), so I decided to do a complete overhaul, and I'm still in the process. I found over a dozen components that had drifted very far from spec. I'll let you all know how it goes. Thanks for the grounding tips!

I've uploaded the corrected schematics to Prowess (including one I drew for a 200-S stereo amp a few months ago):


http://www.prowessamplifiers.com/schematics/images/Guild_98RT_corrected_schematic.jpg

http://www.prowessamplifiers.com/schematics/images/Guild_98RT_layout.jpg

http://www.prowessamplifiers.com/schematics/images/Guild_200S.jpg

-Chris
 

capnjuan

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Hi Chris and welcome to LTG. Can I ask what application you used to generate the schematic and layout? Your layout work is visually similar to some of the Weber kit schematics ... I mean it just looks the same and I thank you for putting the schematics up at Prowess; I've got a few more to go but it's the only place I found on the web where they could be stored. Maybe if we can get enough Guild schematics there, the webmaster will give them there own section.

I can't say that I'm surprised about the lack of match between the amp and schematic; everybody expects that Fender's level of engineering and fabrication discipline extends to other makes and models ... it just isn't so and thanks also for lining out the differences; your work might save somebody else a lot of trouble down the road.

A lot of people comment about the similarities between the Guild 66J and the Ampeg Jet; I always thought the resemblance between the 98RT and the early Ampeg RRs was even stronger. If you get the chance, post some pics of the 98RT; they are soon going to be on the endangered species list. We have another member who was going to refurb his 98RT but the last time I inquired, he said he'd gotten bogged down with other stuff.

Thanks again for the schematics, welcome, and some pics if you find the time. John
 
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I used free software from Express PCB for the schematic:

http://www.expresspcb.com/

They are a prototype/fab house for PCB boards, and they offer free software for both schematic drawing and PCB layout.

I should note that the differences noted in my schematics are the differences between my actual Guild amp and the Ampeg schematic. I didn't actually have a Reverberocket to compare to. Just last week I was talking to Ampeg old-timer Dennis Kager about my Echotwin ET-1 and how a couple of components differed from the Ampeg schematic. Dennis basically said that the schematics wouldn't keep up with the engineering revisions, so it was pretty common to see differences between factory builds and the official schematic. I noticed that the earlier 98RTs even came with 12" speakers (15" in mine). That's not a subtle difference!

I'll try to post some pictures pretty soon, but I'm going out of town for a couple of days, back for a day, and then out for a week.
 

capnjuan

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Hi Chris; your drawings of the 98RT are much appreciated. I am a little embarrassed to say that I have the expresspcb software on my computer but don't have enough skill to make any use of it. I also have the jschem ... same thing. As I mentioned, there's a lot of similarities .. physical and electronic ... between some Guild and some Ampeg models. I made up these slides using your schematic and clipping stuff out of the Reverberrocket R12R (Piazza) schematic. With the exception of the reverb, these are remarkably similar amps:

inputpreamp.jpg



tremolo.jpg



pioutput.jpg



Interestingly, the comparison falls apart at the reverb ... I've thrown in a clip from a Sano schematic and notice that the 98RT, like the Sano, uses two stages and capacitor coupling to drive the reverb tank and two stages to recover. By comparison, the R12R using single stages to drive and recover as well as capacitor coupling. Later R12s used one stage of a 6U10 to drive with.

reverb.jpg



Anyway, I thought it interesting that there'd be so many similarities and one big deviation. Sano had been around for awhile (McNally) and was another NJ musical instrument company. As for the speculation that Ampeg built a few Guild amp models, it's sort of hard to imagine Guild going to Ampeg and asking Ampeg to dupe its RR but, while they were at it, scotch-tape in a Sano reverb section.

Finally, this is a pic of a 98RT owned (formerly?) by matsickma, one of our members; as pristine as they come.

98rt1.jpg



Thanks again for documenting the 'endangered' 98RT.
 
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Check the Reverberocket schematic again, there are two reverb drivers and two recovery stages, the schematic sort of conceals the true signal flow, same as the original Guild and original Ampeg ones. The 98RT and the RR differ by only 5 resistors and 5 capacitors. The designs are otherwise identical. Here's the RR schematic rearranged to show the signal flow better. One interesting note: If you remove the whole reverb section, including both drivers and both recovery stages, you are left with the earlier Rocket amp, or one half of the EchoTwin-1. Notice that I took the R and C numbering scheme for my redrawn 98RT schematic straight from the Ampeg schematic. Everything lines up perfectly.

reverberocket2.jpg
 

capnjuan

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Hi Chris; you're right ... I pulled the reverb section out of its context ... mis-reading V2's relationship to V3 :oops: Unusual to hang on to the second recovery stage ... both Fender and Gibson dispensed with it ... I wonder if it had to do with just having another unspoken-for tube stage left over? If you find the time, a side-by-side comparison of the Guild 66J and the Ampeg Jet would be interesting to see. I have a Guild 100J schematic but not a 99J ... and am not sure there are 'match-up' Ampeg models anyway. Thanks again. J
 
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