1969 studio pa head to pedal project

[neve1272]

this is something ive wanted to do for a long time. i got this head in the early 90s and its been my favorite
i have  been reading runoffgroove and wampler book to try to make this amp to a pedal version

using tayda j201's  have built on breadboard  to the wiper of the treble pot, from there it goes to the....( phase inverter?)  then to the power section

pin 9  of the PI i have no idea what to do with and am now stuck
the sound is very close to the amp
there is some weird high artifacts in the distortion but im hopeing fine tuneing the bias will help

the runoffgroove site has the PI in there builds i think this is so that signal can be sent split to power tubes without   phase canceling out?

  in this case is that even needed? and if so why do runoffgroove have ?
i would love some advice
here is the schematic
this is a Coliseum its the same circuit except it had 4 output tubes in stead of 2 in the studio
in 69 they went to  6550 instead of kt88 

[neve1272]

ok  those j201's suck they dont give readings any where near that the interwebs say i think i need to try something else any suggestions ?

[gordo]

Can't be of much help but I can shed some light on the signal path in a push-pull amp.  One most older Marshalls the first two preamp tubes supply the gain stages.  Usually only 3 of the available 4 "halves" are used.  Consider that the preamp distortion.  The Phase Inverter tube is the third tube and splits the signal sending the top of the signal to the "push" pair of power tubes and then the bottom of the signal to the "pull" pair of power tubes.  This is obviously over simplified, but to understand how the signal is distorted you need to think of the path in terms of preamp distortion...how hard that is hitting the PI...and then power tube distortion.

[neve1272]

thank you that actually helped me understand a little bit what my amp does

i always thought the volume was part of the out put section when its really the preamp...

[gordo]

Right.  With a master volume circuit the volume is controlled in front of the PI.  In a slightly more exotic version (some think it sounds better) a post PI master controls the two sides (and needs a dual pot) after the PI but before the power tubes.  Glad it helped!

[madbean]

I would ditch the input attenuator, make volume your gain control and then stick a volume control either right after the treble, or do a small recovery stage then the volume control. Should sound pretty good.

[neve1272]

thanks man thats exactly what ive been waiting to hear its still on the bread board and im afraid to touch it 
i ve learned a crap load on this thank you guys

[neve1272]

this is the way i have it on the bread board now
this is also my first time making a schematic and using eagle any pointers will be most welcome

*update i had to start over my schematic was sized wrong all capacitor from my madbean library didn't connect to pin
but boy have i learned alot...
this is the layout      i know it looks like hammered ass but i kept painting myself into a corner so to speak
please  feel free to offer harsh advice and copious brow beating 

*note new schematic preamp section works pretty well this version to the wiper

[alanp]

Did you try both 2n5457 and J201 in those positions? (Only asking since 2n5457 is a wee bit easier to source.)

This is one of the Sunn O))) amps that Sunn O))) the band use?

[neve1272]

yes im using 2n5457 in q1 and q2  j201 in q3 and q4 was only way to make the volume work like the amp does

with the j201 was too hot but (im not sure what im doing)  these are the j201's from tayda   

i used examples from wampler book and runoffgroove site to get source resistor values all other are from the old sunn schematic

not sure on sunn band gear ive used this as a guitar head and bass since early ninety's best 100 bucks i ever spent

[gordo]

If memory serves (and rarely does anymore), that was the original Leslie West/Mountain rig.  He ordered Marshalls but got 4 channel Sunn PA heads.

[neve1272]

https://www.youtube.com/watch?v=GdUSmSjipJA

this is the bread board so far
getting closer ill post a schem and layout as soon as i can redo the changes

[pickdropper]

Have you tried J201s sourced from places other than Tayda?  A lot of people are reporting that the have been getting fakes from there.  If you got them a while ago, they might be OK.  I got some a long time ago that were good.  Recent ones weren't.

[neve1272]

i went back and did it with all 2n5457 ive really been struggling with this

I've tried  so many output stages trying to get it to behave like my amp( loads more treble than the sunn)
ive done nothing but work on this and read jfet crap on the interweb

on 1 try i did the new fetzer valve sounded good but consumes lots of power when you get too many fets (walwort only)
the output in the video right now is the 1 from the fault line need to voltage divide between the treble wiper and it  to not overload

  i probably should quit while im ahead.....