Park 75 Schematic

[plumcrazyfx]

I got a Traynor YBA-1A chassis coming and I'm thinking of maybe taking another way.  Might want to go in the Park 75 direction but can't find a schematic.  I've heard it explained as a 1986 with solid state rectification but a schematic would help.

[Ritchie200]

I played one back in the 70's thinking it was a lower wattage Major.  It's not, but man did it sound good.  Better than the 50 watt IMHO, one of Marshall's best ever designs.  A couple of years ago I went on the same quest you are - looking for a schematic.  I found a hand drawn one and I have looked everywhere and can not find it again.  Mitch Colby is doing re-issues but with all sorts of "new" mods so who knows what has been changed and the schematic is not available for that one either.  There are Park schematics in the Marshall book, but no 75.  Info below showing 50 watt off the rack trannys for both output and power as well as choke.

http://marstran.com/69%20362.htm

More

http://marstran.com/Historic%20Data%20Base.htm

Hope this helps!
Jim

[Ed_Chambley]

Very odd, I had someone drop off a 71 Park 75 yesterday for servicing.  It was playable, but had the common plate resistor frying sound.  First time playing one with KT88 cranking.  I installed them, but he was in a hurry so I did not play it much at that time.  I must say this is a clean amp, loud and proud.  Got to get it up around 7 to begin hearing any breakup.


I do know this particular amp runs EL34's as well because I installed some for him a few years ago.  It is basically a 4 hole marshall with a volume knob in-between the holes.  It uses the same OT as a 50 Watt Marshall 1987, the 784-139 Drake.  Heybour makes a grand version of this OT.


This amp does (did not) have a split cathode and all the tone stack values are like a JTM45 using a 56K slope and .22 caps TMB.  In keeping with the JTM45, the coupling caps after the PI are.1.  I did notice it uses no choke.  Not sure of the resistor values in the B+.

The amp has slight ghost notes, so I will have it a couple days for recapping.  If I were to build one, I would simply get the iron.  The Drake 784-139 is 3k3, which changes the character from the JTM45 and so does the SS rectifier.  This amp does not have a bypass cap on V2, but I have seen later ones that do.  These usually have EL34 tubes too.

Instead of having the 2k7/.68 plexi stage, this amp just has a 1K resistor cathode to ground.

It has no bright cap, no mixing cap and a .1uf on the "Brightness" presence pot.  220K grid mix and had no grid stoppers originally.  Does now.

I am not absolutely sure yet, but the owner uses the amp a lot and I think he is going to use EL34's again.  This is an actual KT88 and will produce easily 50 clean watts.  I had it and a JCM 800 Head comparing.  For her volume there is no comparison.  This thing is punchy and will easily keep pace with a 100 watt, but it is much cleaner in stock configuration.

A while back tried the Plexi Bypass values, but ended up with a split cathode of 22uf and 3.3 uf, both with 1K.  It will scream and sustain forever, but it is doing it at well over 50 watts.  I have a cabinet isolation room which is very close to sound proof and I could easily hear this running 4 Greenbacks.

Finally, the filter caps are 2 x 32uf and 2 x 16 uf.  Voltages are very similar to a 50 watt Plexi as it uses the same Power Trans.  I question the filter cap values of 16uf and plan to try 32uf here tonight because I do not have a dual 16.  I believe after building and owning a few marshalls that all 50uf filter caps is my preference, but for originality sake, I will put his back stock.

I think these are all the differences.  As far as I know the first time I serviced the amp it was stock original.  Should have a silver plate, but has a Black one and is original.  Chassis is steel.

Let me know if there is anything you would like to know about the circuit.  The schematic is impossible to locate, but very simple to revise a JTM45 Schematic to a Park 75.

[Ritchie200]

The early 75 had a choke.  Check out the links I posted.  Like the JTM45 to the JTM50 it appears the 75 was also a work in progress!  The one I played was an early one with the choke, I do remember that.  A little push on the front end and that puppy sang!

Heck Ed, you could draw up a schemo when you get it back.  Post it a pay for play site and retire!

Jim

[Ed_Chambley]

The early 75 had a choke.  Check out the links I posted.  Like the JTM45 to the JTM50 it appears the 75 was also a work in progress!  The one I played was an early one with the choke, I do remember that.  A little push on the front end and that puppy sang!

Heck Ed, you could draw up a schemo when you get it back.  Post it a pay for play site and retire!

Jim

I looked at everything last night.  It is really a JTM45 using a Drake 3k3 multi-Tap 50 watt Output Trans.  On the 71, the choke was replaced with a 10ohm resistor, but I did look at the specs you posted and earlier and later ones had chokes, this is correct.  I believe for some reason when the changed from EL34 to KT88 sold new, the choke was removed.  It later reappeared.


I am sorry, but a drawn schematic wouldn't bring much.  Everyone who collects old Marshalls, me being one, knows Jim Marshall built Park amps and moved pots and changed very little so he could say "look here, no Marshall has this."  It is the Rose deal.


The amp belongs to a fellow collector and he says it will bring over $4000 as a head.  I have my doubts, but this is a rare bird so who knows.  I tried the tubes from the Park in my JTM45 last night, which has the Drake OT and was running EL34 and put i a SS rectifier plug.  Voltage actually remained about the same, but jumped up a little when I replaced the choke with a 10 ohm.


Just wanted to see if they were similar, but I had no way of really A/B them.  I am very familiar with my JTM45 tho, and it is spot cathode V1 with a Bright and Normal.  I could get the seemed the same tone, what is majorly different is the Park is less forgiving.  Both have the same iron, but I believe he was ear bias meaning I set it to 70% and lowered it until I hear it starting to just get cool and that is sort of where I leave an amp.


In mine, I could barely reach 60% and I did not want to change the resistor to raise the negative voltage as I was putting back the EL34.


The B+ dropping resistors values are the same as the original JTM45 except in this case no choke and a 10 ohm.  The first and second stage is a little different, but stock it was not.  I did notice later they began a split cathode from what you posted.  I would imaging George Metropolis has records on these Park amps, but from what I understand you might find a 500pf or 556pf and some even have a lower value treble cap.


I do not think drawing a schematic for his 71 would be of great benefit to anyone.  If you own an original and know Marshalls, it is sort of simple.  This guy looked for a 71 purposely because it is a weird one.  Like Fenders transitional, this is one.  Supposed to be orange tolex, but I believe it looks sort of pink.


I did try a set of XF2 EL34 Mullards and a Mullard 12ax7 in V1 in the Park that I took out of my amp.  It amazingly sounded a lot like a 1987, a little stronger in the lower mids, but responded to single coils great.