Is there any hope for this as a guitar amp? - diyAudio
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Old 7th January 2007, 09:45 PM   #1
Gunny is offline Gunny  United States
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Default Is there any hope for this as a guitar amp?

We found a RCA Schematic that is very close to the chassis my friend found. He has a Model The only Rub is it appears to be a Stereo amp and he has one half of the system that had two twelves and tweeters and evidently had an other speaker cab. It has two output tranformers and the 6v6's are wired completely different than my mono amp (Zenith). Here is the schematic, can he save any of this to make a guitar amp out of or does he have to gut the whole thing and start over from scratch just using the tubes and one of the output transformers.

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Old 8th January 2007, 12:51 AM   #2
Gunny is offline Gunny  United States
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What I forgot to add was he has a chassis model RS-171 and this one in the schematic is a 171 A not sure if there was much if any difference in the schematics but I think this one is close enough to get an idea what he found and if it has any possibilities?
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Old 5th December 2007, 03:40 AM   #3
iconpsy is offline iconpsy  United States
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hey, did you ever have any luck converting your RCA amp for guitar?...i have almost the same one i was considering...with the two 6V6s i thought it might be sort of like an old deluxe....
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Old 5th December 2007, 05:37 AM   #4
Gunny is offline Gunny  United States
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Amp belongs to a friend I do not think he has done anything too busy working. I doubt you could make a delux with his as it is stereo so I think each side is single ended but I could be wrong I would think you would need a new output transformer for push pull and then completely rewire. I have a Zenith that is a push pull and it does not have the power as the preamp section is not enough I need to completely rewire and add a couple more stages to the preamp. I think it can be done but it may be trial and error to get it just right.
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Old 26th December 2007, 11:39 PM   #5
Brion55 is offline Brion55  United States
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Actually it could make a decent guitar amp as is, somwhat similar to the "Champ" amp (single end 6V6). If two speakers were used, the inputs of both channels could be tied together for twice the output power. It was hard to read the input resistors going to ground, but for guitar, they should be around 1m.
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Old 27th December 2007, 12:05 AM   #6
Gunny is offline Gunny  United States
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Thanks Brion I will let him know that all he should do is hook up some speakers and change the input resistors and give it a try.
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Old 27th December 2007, 01:07 PM   #7
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It should have the makings of a very nice guitar amp. As pointed out, single sided 6V6 gets you a Fender Champ. Actually you seems to have potentially gain to burn. As is, it will probably not sound too brilliant, the art in guitar amps is largely all about the tuning of the the stages to provide the right frequency shaping and distortion so that the overall sound has the nice rich harmonic content, good feel, and all sorts of other intangibles. But this amp is a great place to start. (One part of the tweaking is to ensure that the bass does not dominate the distortion and lead to a sound filled with mud. So inter-stage coupling is critical to tweak right.)

A plug for the right forum to ask about this. The AX84 forum. www.ax84.com is a great place, and the original P1 amp that started the forum was a single ended EL84, and more than a few 6V6 versions exist as well. You won't find a more knowledgeable and friendly bunch of guys and gals that live building guitar amps.

(Annoyingly the site seems to have failed just as I write this. It was up a few hours ago. No doubt it will be back soon.)
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Old 27th December 2007, 05:19 PM   #8
Brion55 is offline Brion55  United States
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A few minor changes here and there could improve the tone shaping of this amp too. If we could see the part values better I'm sure we could make some simple suggestions.

My very first amp was an old tube tape recorder. It was really bottom heavy, but worked. Was real easy to overdrive too!
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Old 28th December 2007, 12:25 AM   #9
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Hmmm, having a deeper look at the amp is interesting. There are a few weird things going on. The totally whacky messing with the feedback and the "front" switch wafer has me scratching my head a bit.

But, the amp shows every sign of having been an afterthought modification to a PP mono amp chassis to make a cheap SE stereo amp. The location of the second OT on the back, shoe horning the additional phono gain stage into one corner, that plus the layout of the tubes (which is perfect for mono and far from optimal for a stereo amp) all points this way.

So, depends upon how much time and money could be invested in it. But, it has the makings of a very satisfactory 18 watter. As pointed out above this would mean a new output transformer. But that would be the only big ticket item. In addition assume that you will need to replace the electrolytic capacitors - you will anyway. The standard OT would be the Heyboer which will cost about $70 US. Say a bit over $100 in parts all up and you could have an amp that is arguably better than the $2000 Marshall equivalent.

The place to go - www.18watt.com I see Graydon visits here occasionally, he might have some opinions too (It won't be a perfect 18Watt clone - that uses EL84s, but there is a lot of experience with 6V6 versions, and they are pretty close.) The limited front panel space and general size might mean you go for the 18 Watt lite.

One non-negotiable change to the chassis. Remove the power cord and add an IEC power socket and ground the chassis to the ground pin on the socket. At least I can't see the "death cap" often seen on amps of such vintage. But a proper safety ground should be done before any other changes or messing with the amp. Back when this amp was built life was cheaper.
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Old 28th December 2007, 03:55 AM   #10
Gunny is offline Gunny  United States
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My Mono Zenith chassis is what got him interested in this in the first place. Mine has two 6v6 and a 5y3 and a octal preamp tube and a 12ax7 split for half the preamp and half the PI. I did some tweaking but still was not satisfied as is. So I am buying a Loaded 5e3 board and making a delux out of mine and changing the 5y3 tube for a SS rectifier for higher voltage if it does not work well with the tube I may try the tube first though I think the voltage could stand to be higher. I told him about the renewed interest in his chassis as a amp and I told him to try to post a better version of it for more ideas here. Not sure if he wants to spend 70 bucks on a new tranny. If he could use the existing ones as a champ I think he would prefer that route. I am hopeing my OT will work as is it is a 3.2 ohm so a 4 ohm speaker load should work fine and I am planning on a head version so putting it into a 2x12 cab should work good. If the OT does not work well then I guess I will buy one since I have cash in the board but it will be one step at a time I will try the board with what I have for now.
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