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Replace SS with vintage tube amps...
Replace SS with vintage tube amps...
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Old 21st September 2019, 05:59 PM   #1
knockbill is offline knockbill  United States
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Default Replace SS with vintage tube amps...

I was at a big Radio/audio show yesterday and was looking for some vintage git amp speakers...
Found a couple of SS guitar amps, in not working condition,,, A Montgomery Ward Bass amp with a nice 15" Rola,,, and a newer Marshall with a 10" MG series Marshall speaker...

Also found a couple of other loose speakers, and thought rebuilding the non working amps with tube amps could make winter projects... So,,, anyone have a idea/schem for a vintage Marshall or MG/Airline tube amp I could build for either box??

I have some experience with vintage Fender circuits,,, but Marshall is new to me, and has its own unique tone, that I would like to keep with this project,,, The Marshall amp PCB appears to have been damaged(but there is no physical damage on the outside of the cab) as 6 of the[ 9 pots are sheared off the PCB, which I read was a design problem with them,,, There doesn't seem to be a schematic available for it, so the tube amp chassis seems like a good "fix"...
Thanks
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File Type: jpg P4220020.JPG (79.1 KB, 194 views)
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Old 21st September 2019, 06:56 PM   #2
Enzo is offline Enzo  United States
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Schematic not hard to find. Here.

As ti building a tube amp in either solid state amp, you will have a metal chassis and the cab with speaker. None of the rest of the parts will be of use.

As to design flaws, some people gripe that pots ar mounted right on the board, instead of flying wires. But really, the ONLY way all the pots broke off is physical abuse.
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Old 21st September 2019, 07:23 PM   #3
knockbill is offline knockbill  United States
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Thank for the reply and schematic,,, I was looking last night, and came up empty... however I did read that the pots usually broke off their mounting to the PCB,,, the board is fastened to front plate by the pots and two thin plastic sticks hold the rear of the board to the chassis... horrible mounting system, which is prone to failure by bumping or even the amp tipping over... Since this seems to be a cheaply built chip amp, with broken parts,,, I would rather find a vintage Marshall tube amp schem, gut the metal chassis, and try to use the speaker (which sounds very good with my Champ project), and rebuild the nice cabinet, with its speaker,, and a Champ sized Marshall tube amp clone...

The Wards amp turns out to be made by Marlboro USA, and I found some examples of a simalar amp with their name on it,,, no schem turned up yet tho... This Bass amp has a nice 15" Rola,,,, compared to the 12" amps I found reference to,, so far... I can see using an old Airline Bass tube amp schem for this one....

Thanks again for the input....
P4230021.JPG

Here's the broken pots,,, lugs are all snapped off clean,,, not sure where they ended up,, they aren't in the amp...
P4230022.JPG
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Old 22nd September 2019, 04:16 AM   #4
Enzo is offline Enzo  United States
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Seen it many times, and again, the only way that happens is abuse - abuse includes tipping the amp over etc.
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Old 22nd September 2019, 05:26 AM   #5
JMFahey is offline JMFahey  Argentina
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Build one of the 20W Marshalls.

Although they are labelled Guitar/Bass/PA they are basically the same.

This is the "PA" one:

Click the image to open in full size.

the only other Marshall amp which can fullly reproduce all sounds achieved with a 100W "Plexi" Marshall head.

YouTube
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Old 22nd September 2019, 10:58 AM   #6
knockbill is offline knockbill  United States
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Thanks!!! This is exactly the info I needed,,, looks like EL84s,,, so they will be easier to source than EL34s or the KTxx that I saw on most of their schems... The British OPTs may be a bit different sounding, but I have some vintage PTs that should provide the right specs...

I'll experiment a bit with the speaker, see how it handles 20W of tube power,,, it sounds good with a couple Champ ckts I built so far... This SS amp also has a small reverb tank,,, not sure it can be used here tho...

I also cleaned up the SS amp in the Monty Ward bass amp,,, fixed some wiring, cleaned the pots, and it sounds good, like to find a schematic for it, to check bias but its OK for now...

I'll clean up the cab and Tolex, tighten up the grill cloth and concentrate on the Marshall!!! as soon as I finish the 5F1 Champ project!

Thanks again for the replies...

Quote:
Originally Posted by JMFahey View Post
Build one of the 20W Marshalls.

Although they are labelled Guitar/Bass/PA they are basically the same.

This is the "PA" one: the only other Marshall amp which can fully reproduce all sounds achieved with a 100W "Plexi" Marshall head.
This circuit looks fine for my project,,, I found two other schems that are very similar, as you said...Only question I have so far, is what are the OPT specs,,, they aren't on the schems... Looked at replacement tx at ClassicTone,, Their replacement OPT is 9.2K at 8R, which is the speaker I have to work with... 6BQ5/EL34 Specs 8K PP... Think this is a good match, or should I look for a closer output tx?
Thanks...
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Old 26th September 2019, 01:37 AM   #7
knockbill is offline knockbill  United States
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Cleaned up the Bass amp a bit,, re glued Tolex,, new 3 wire cord,, stretched grill cloth... Sounds pretty good for a SS amp... 15" Rola speaker is full range...
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File Type: jpg P4260022.JPG (79.6 KB, 112 views)
File Type: jpg P4260023.JPG (80.7 KB, 112 views)
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Old 27th September 2019, 01:29 PM   #8
shanx is offline shanx  Canada
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Quote:
Build one of the 20W Marshalls.
Yeah they do sound great and a very good fit for that size combo cabinet. 20 "more than enough"watts to get "that sound" without the earsplitting volume of a 100W stack.
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Old 27th September 2019, 05:09 PM   #9
knockbill is offline knockbill  United States
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That's the plan,,, I'll bend a new chassis and reuse the original face plate with pots/jacks mounted to match the 20W schem, probably be some unused holes..

Are the 18W Classictone OPTs the right ones for the 20W schematic?
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Old 27th September 2019, 08:36 PM   #10
shanx is offline shanx  Canada
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Classictone is a good OPT choice, yeah "18Watts or so" is what they say on the web page. Bandmate has them in his amp and has no issues with the sound and quality. Good prices as well.
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