Hello DIYer's, I enjoy building amps as a hobby, but i'm daunted by the maths involved in straying from the tried and tested so have only made amps with well detailed schematics. I would like to do a Park 75 but no schematic is available. So far i have gathered snippets of info and i'm trying to glue them together. I have the PT but now need to find the right OT. Tranny will put out 530v . The 2 KT88's ( from snippets ) require 6000 to 12000 ohm anode to anode ? Does anyone know how to work out the true value? Or better still have a schematic. Cheers Longscale
The circuit is basically a Plexi-era Marshall model 1986 Bass 50 but with KT88's instead of the EL34's, so the primary impedance should be around 3.4k.
Thanks jazbo8, Thats one number confirmed, have you ever seen a schematic ? In other forums i have read they say "just a Plexi with a hot preamp and beefed up OT for the KT88's". I wonder if by that they mean a cascaded input stage and maybe used the 100w OT. I'll keep searching. Cheers
No, never came across a Park schematic... I think the Plexi preamp should have enough grunt to drive the KT88's, so little or no modification would be necessary. In any case, you can breadboard it and see what happens. Good luck with your project!
You will find 100W Park schematics using 4 x EL34/KT77 .
Lacking the original 75W one I would build the 100W one in full, except using 2 x KT88 or whatever you like and proper supply/biasing/OT fo them.
You would very probably get 75W out of a pair and have full Park flavour.
Lacking the original 75W one I would build the 100W one in full, except using 2 x KT88 or whatever you like and proper supply/biasing/OT fo them.
You would very probably get 75W out of a pair and have full Park flavour.
According to the KT88 datasheet by G.E.C. (http://www.mif.pg.gda.pl/homepages/frank/sheets/084/k/KT88_GEC.pdf) and with respect to your 530 V coming from the PSU, I'd recommend a plate to plate load of about 4k5.
Best regards!
Best regards!
AFAIK, all Park amplifiers used stock Marshall transformers, which for the 50W Plexi's, it's 3.4k, even if it is not optimum per the datasheet.
WHat KP said is optimum but after some searching, j8 is correct. Marshall used their std. 50 watt OTs for the Park 75. Other sites have said that the PT used is from a 100 watt Plexi, but with 530 volts, they probably made some up especially for this amp.
The original Marshall 200 (PIG) that Ronson used had the 2.25K primary OT using 4 - KT88s, which is right in line with 4.5K for a pair of KT88s.
Here's a post that may help you:
Vintage Amps Bulletin Board • View topic - Park 75 vs Marshall 50
Here's another post that says both transformers on the Park 75 are the same as a Marshall 50 watter but 420vdc isn't enough voltage to make 75 watts from 2 KT88s:
Park 75. | MarshallForum.com
The original Marshall 200 (PIG) that Ronson used had the 2.25K primary OT using 4 - KT88s, which is right in line with 4.5K for a pair of KT88s.
Here's a post that may help you:
Vintage Amps Bulletin Board • View topic - Park 75 vs Marshall 50
Here's another post that says both transformers on the Park 75 are the same as a Marshall 50 watter but 420vdc isn't enough voltage to make 75 watts from 2 KT88s:
Park 75. | MarshallForum.com
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That's what I gathered as well - "you can have whatever color you want as long as it's black" - for the Marshall/Park amps, you always get 50W - 3.4k, and 100W - 1.7k. At least, it kept the parts inventory down. 🙂
Thanks for the feedback, i am still finding bits of information , all conflicting. You would think there would be a schematic somewhere but have found others have been searching since 2004 with no luck
Thanks for the feedback, i am still finding bits of information , all conflicting. You would think there would be a schematic somewhere but have found others have been searching since 2004 with no luck
Strictly friendly curiosity 🙂 .... why do you want to build just *that* amplifier? 😕
Absolute worst case, why not straight the 100W (or 50W) versions which seem to be available?
Or if you want KT88 in the mix, all evidence and anecdote points to it being a "boosted 50W one", as mentioned above "a 50W amp, including standard Marshall 50W OT, with KT88 and a custom wound higher voltage PT" .... which is achievable 🙂
It started by reading beaming reports about how good that amp was, i thought that will be my next project. I didn't think getting info would be as hard as it is. I might take up tennis instead
Strictly friendly curiosity 🙂 .... why do you want to build just *that* amplifier? 😕
He could build this one... (Insert shameless self promotion) LOL!
Marshall 200... I Mean Marshall 100 Build (pig 0.5) | MarshallForum.com
Comes complete with schematic and will give you 100 watts from 2 KT88s; and it does use a 4.5K OT.
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