Metropoulos Forum

Hello all , I'm starting a new build of a Park 75 clone and I can use some advice. I have Ordered all the parts to build a 50w Plexi Marshall circuit But changed out to the needed 100w Power Transformer. Sozo vintage caps, Heyboyer Trannies, Mojo Chassis, and CTS pots. I plan on running GE 6550's instead of kt88's as I have them and I plan on this amp getting a lot of use. I figure the ge 6550's are better than the current kt88's? I have a 60's Park 75 already that is converted to el34's and has a Bradshaw master volume installed. No extra tubes or holes but this preamp couldnt be farther than stock... sounds amazing but very high gain. My plan is to make the new build a stock circuit and leave the real one modded. Then when I get the "tone" of the new park nailed I will switch out components returning the original to stock and modding the new one.

I am having trouble finding info on how park varied this circuit throughout the years. Is there anything that makes it sound like a "Park" other than the 100watt power tranny and the kt88's? Also how was the cathode of v1 configured stock on these amps? Shared or split? The output is just a stock 50watt transformer right?

thanks!

should be a standard jmp50 w/kt88s - nothing else, no 100w tranny

http://www.amparchives.com/album/Marsha ... index.html

Thank You, that link is very helpful! I never knew mine was actually a PA 75 from 1972... It has a Drake 1202 100 watt power transformer. Also Gerald Weber mentions the fact that Park used 100w pt's in the 75's in his book and that some builders still consider it a tone secret? Also I found mentions of it around the web from reliable sources. I think MM even makes a special 75 watt PT. ( that is probably actually just a plexi 100w?) I did notice the small box Park 75 in the archive had a 50 watt pt and a split cathode on v1 where the large box had a 100watt pt and a shared cathode... Seems like they switched it up over the years.

Thanks!
Bill

Marstran is selling 50W "OT's wound to the specifications of the Dagnall 100W OT" on their yard sale page.

Not Drake, but maybe something that may interest you for this build.

thanks, I will check that out.

I don't think this will have the correct impedance for two tubes. I think these are for putting four tubes into a 50 watt ala Kevin O'conner? (wasnt that one of his mods?)

Jingletone wrote:Thank You, that link is very helpful! I never knew mine was actually a PA 75 from 1972... It has a Drake 1202 100 watt power transformer. Also Gerald Weber mentions the fact that Park used 100w pt's in the 75's in his book and that some builders still consider it a tone secret? Also I found mentions of it around the web from reliable sources. I think MM even makes a special 75 watt PT. ( that is probably actually just a plexi 100w?) I did notice the small box Park 75 in the archive had a 50 watt pt and a split cathode on v1 where the large box had a 100watt pt and a shared cathode... Seems like they switched it up over the years.

Thanks!
Bill

to me it looks like a 1202-118 - a 50w PT

Everyone I've seen pictures of has straight up 50 watt JMP Iron.

Just biased up and loaded with KT88's.

David Grissom's been using one in the studio he's had. I want to build one at some point.
Let me know if you find a Park logos for the head box.

interesting... I will have to grab mine and double check it and take some pictures. here is the quote from Gerald weber in my All About Vacum Tube Amplifiers book. "installing a beefier, four output tube style, power transformer in a two output tube style amp is a neat trick to get that quicker punch and bigger botom. This type of voicing was used sucessfully on Park Amps which used 100 watt Marshall power Transformers on two tubes" -Gerald Weber

sooo, beats me seems like they did it both ways?? I will take pictures so we can put the debate to rest...

well the plot thickens... just pulled mine apart, and it is obviously changed out! I havent had it opened up in years and didnt realize it was changed. I just remembered bradshaw telling me it had a 100w PT. It is a very old transformer but not original. Maybe bradshaw put it in? (it was broken when I gave it to him to mod in the late 90's)

as soon as i figure out how to post pictures i will.

Hi i m working on ultrasound transmitter and it is base on simple circuit receiver circuit have one bc109 transistor and one bc107 resister and the comparator use for giving hight or low o/p which drive buzzer by switching transistor
but i stuck can any help me urgently
ibrar.chudher@gmail.com

Just finished wiring the amp up. It sounds freaking AMAZING! I changed up my direction a bit. I ended up building an idealized version of what I though an 80's Park 75 could have been. (Since I thought they used 100watt pt's) I used a 50w jcm 800 chassis (already had) Heyboyer 100w power tranny, two JJ KT88's biased at 58ma each for a 80 watt output (if i bump it up to 68ma each i can run it at 100w! and plate voltage only drops to 460v). For the power filtering I used all 6 50+50 caps to split up the load since I have almost 500v on the plates at 80w. I have a 50w Heyboyer output tranny in it now and it sounds amazing! If i am not over heating it do i need to worry about it? the impedance is matched so I'm ok if it doesn't get hot right? I have a hopped up 2203 pre amp in it so far with just a bit of added gain. I still have another preamp tube installed for either a one stage clean channel and an extra stage of gain for the dirty channel or possibly an ef86 clean channel. Not sure yet.

Basically, I'm super happy with the sound of the kt88's and stoked that the pair is working SO well with the 100w transformer. Sound wise it is more harmonic and rich in the mids than my el34 or 6l6 marshall circuits. I was going to experiment with wiring the screens up like a traynor or music man to simulate a dual rail power supply, 30ohm 10w resistor on screen supply to drop screens to 350v or so.( I am not sure that 492v is enough plate voltage though... Anyone know? I think I need at least 600w?) but I am loving the sound of the swamped screens I have now. I will get some pictures and sound clips as soon as i can.