Another 'ultimate marshall' build

[Thiez]

Thanks for the reply, i'm gonna try that.

Only the last two PS things i dont get what you mean, can you explain it a bit?

[catalin gramada]

think I made a mistake considering you use those 8+8uF for screen and PI, now I saw those cap is used for bias circuit. I will redraw as quick. my appologise.

later edit: I corected the drawing. relative to your question:1" negative pole " of you rectifier- in your case PT center tap should be conected directly to the "-"terminal of the main cap to keep high ripple pulse into tight loop.this wire should be keep away from any signal wire to not interfere.2. "common "wire output of OT should go first to the speaker jack , and then from the speaker jack to the ground to avoid cross oscilations.
Catalin

[Thiez]

Allright thanks! I understand it now.

And in your drawing i can see a good example of the so called starground. I've read about starground but coudn't "see" it, now i understand it.

[catalin gramada]

Allright thanks! I understand it now.

And in your drawing i can see a good example of the so called starground. I've read about starground but coudn't "see" it, now i understand it.

well.. it is not quite a good one, just a optimisation .
you can use regular wire over all, exception for the segment between main cap and star lug- you can put a litlle bit thick wire there, but try to keep everything as short as you can.
Catalin

[Thiez]

Allright, so the preamp to starground is a bit long?

[catalin gramada]

Allright, so the preamp to starground is a bit long?

No, keep all ground wires as short as you can. The return currents are small so it is not necesary to use thick wire over all but is more important to be short cause you must try to keep path impedances as low as possible. But use a thick wire between main cap and star lug (big red point in you pic) this segment must be "seen" more like a point so can use even better a thick solid core here.
I'm not sure I understood precisely you question. the wire from preamp cap to star lug is a bit longer from design. In your drawing I saw you have a point to ground in upper/right side so I supposed you allready have a lug screwed to chassis there. If is not like that a better place for the star lug will be somewhere arround main filter cap and screen/pi filter cap. those wires carry the heaviest currents from you amp so is better to keep the wiring from those caps to star lug as short.
sorry please , my english is pretty relative.when I said "as short" I mean to not leave excesive wire into chassis from aesthetical consideration or whatever....pose it on the bottom of chassis straight to further conection. keep in mind each cm. more means +noise.Thanks
Catalin

[Thiez]

My question was more for clearing the grounding thing to me. YOu said to keep the wires as short as possible. On my amp the preamp ground wire is pretty long going trough almost the whole amp. So it would be better if that was shorter. For this build no possibility but maybe things to consider on my next build.

The groundlug is now verry close to the main caps, as a matter of fact the lug is the same bold as the bracket that holds the main cap.

Thanks a lot for your information!!

[catalin gramada]

You're welcome.
try to put the PT center tap at the main filter cap terminal though.
cheers
Catalin

[Thiez]

Yeah it's there now. Bussy to redoing my groundings.
Where do i go with my heater CT? To main ground or to the filter caps?

[catalin gramada]

well...depends...you have not too many option there
if you grounding circuit works without any issues solder directly to the star (but honestly I have doubt it is the best place in your case) you can also tie directly to the nearest point to chassis- it is the same thing-but please avoid the power transformer bolts
so, the next (and last) alternative is to tie at the preamp cap terminal- you will inject a little less noise into signal path doing like that.
but, if could be by me ,with your permission, I will do like that: isolate the heater center tap(do not use it) / create a virtual center tap using two 100 ohm/1w resistors / tie this virtual center tap at preamp cap terminal. you still will hear cycling buzz in you speaker but I think will be minimal noise you can achieve in your case,
hope this help
Catalin

PS: man, we not solve nothing like this, grounding as complex seems to be in fact is a pretty simple philosohy: Aiken, Merlin Blencowe, R.G.Keen,and even here,but not the least-Larry Grohmann have a lott of explicit ,almost didactical materials regard this. Thank you all.It is better to try to understand the concept to choose the best compromise from begining rather than try to fix.
Take a look there:http://www.valvewizard.co.uk/Grounding.pdf
It is my sincerly honest sugestion.
Best
Catalin

[Thiez]

Thanks for the info Catalin! I appreciatie it.
I will change my grounding and see how it turns out in the end.

[Thiez]

It's been awhile but verry busy with my new house.

The grounding is done different, the amp is pretty quiet now. The hum was caused by a leakage in the V1 tube. The hiss is now verry minimal, only at high gain settings.

Now next up. I hate it when the gain settings have influence on the freq respons. So i cut of the brightcap of the bright channel. Only this results in a bit too much low end (same as with bright cap and gain dimed).

What's the best way to get rid of this?
- Lowering coupling cap even further? from 2.2nf to 1nf or something.
- Add a resistors in parallel with the gain control, so i get 500K gain control (allmost same freq shift as the 1nF cap).
- Other option?

[Thiez]

nobody?