Choke orientation

[alerich]

This is my new build. It's a Marshall 2204 JCM800 but my donor chassis is from a Park G10 practice amp. I have had this chassis/cab sitting around for about 10 years. I was going to build a little Firefly low watt amp in it originally but now I have shifted gears to the 2204. I know it's not the optimum chassis platform for this build but that defeats the purpose of my project - to jam a Dodge Challenger into a VW Bug. I think it can be done because the Sovtek Mig50 Midget head is essentially a 2204 JCM800 (with 6L6 tubes and sans choke) in the same footprint.

The picture below shows my ideas on transformer and tube layout. Many of the amp layouts I have looked at have the PT and the OT in pretty close proximity to one another with the standard 90 degree alignment. Leo Fender typically aligned the OT and choke parallel to one another. At the same time I have seen some pretty wide variations in transformer placement and all seem to function ok.

I have my choke sitting in alignment with the OT and both are perpendicular to the PT. Reading online some people say choke position and alignment is not critical as it has little real AC signal flowing through it. Others say choke placement and orientation is critical. I know I should be using a bigger chassis but what are you thoughts about my proposed layout in this chassis?

[HotBluePlates]

Being that close, I'd probably choose to orient the choke 90 degrees to the OT.

Can you leave the choke wires very long for a test?

Build at the the B+ and bias supplies, hook up the OT and choke, install output tubes but don't bolt the choke down. Attach a pair of headphones to the OT secondary. Try moving the choke around and see if there is any position with more/less hum. Bolt down in the position with least hum and trim the wires to length.

[alerich]

I have read about the headphone trick. That sounds like a good idea. I think I'm ok with the PT and OT placement. I can leave the choke unmounted and tack it into the circuit last and experiment with different positions.

[eleventeen]

HBP = excellent suggestion.

[eleventeen]

I think I'd like to have a little more space between the EL34s. That's just my reaction.

[catnine]

Are you going to mount  a fan to keep this cool?

[Willabe]

I think I'd like to have a little more space between the EL34s.

I agree.


               Brad     

[DummyLoad]

hope you don't mind the suggestion. dimensions are not in proportion - just for reference. looking at the photo, it seems like it'd fit.

or, how i'd wedge the B-wedge in the PT cruiser...

--pete

[jeff]

Something's telling me to swap the OT and the choke. I think you'd want the OT as far away from the hum inducing PT as possible. If the choke picks up hum from the PT hopefully the caps will filter it out and the OT next to the PT has more potential for hum problems than the OT next to the choke.(I would think, better weight distribution too).
Also having the choke closer to the pt may be easier for wiring and running wires.
I assume the choke is the smallest one.

That's gonna be tight. I've built too big of amp in too small of a box before(I'm doing it right now actually) and it can be a pain. Make sure to lay out the board and pots(don't forget the pots taking up space, done that) first before you drill. If it won't work save it for a little SE amp and get a bigger box.

Another thing to try is do you have enough room to put the PT and choke width wise and move the OT to the opposite end? (PT lower left, Choke upper left, OT upper right, power tubes upper mid, pre tubes middle)

Good Luck!
    Jeff

[alerich]

Lots of good ideas - much appreciated.

I have considered the heat issue. This is going inside a smallish combo chassis (12" high x 13" wide x 7" deep) so there is room to mount a small fan inside the cabinet for circulation. That's a 'wait and see if necessary' thing but it's on my radar. I'm thinking the inverted chassis will likely necessitate a fan.

I can space the power tubes farther apart. That photo I did was more of an outline for transformer placement ideas. Copying the spacing of the preamp and power tubes in my Mig60 they will fit ok.

DummyLoad - I like your mock up. That's another option. It'll be tight since I can't mount the iron exactly up on the front edge of the chassis. I have to allow for the baffle unless I modify it somehow to move it which is another option.

After some more web surfing I have been considering swapping the OT and choke around but I can't move the OT too much more toward the right side of the photo since that's the input side of the amp. The closer the OT gets to that side of the circuit the more trouble I will be creating for myself.

I am beginning to consider (actually I have been considering it from day one) just dumping the choke altogether and using that space to move the OT a little further from the PT. The choke was a part I had on hand and had installed in a choke-less Peavey Triumph 120 combo after reading all the rave reviews about the improvement in tone. Maybe I'm just deaf from too much loud music but to me the benefits of a choke seem scant at best. On paper it makes sense but in practice I've never been an advocate. The main reason I wanted to try to use it it that it is part of the original design.

Thanks again - good food for thought. That's why I read these forums.

[DummyLoad]

maybe lose the choke or use a smaller one? el34 screens get hungry so maybe fender 90mA (super/twin reverb) choke? the fender choke may also fit inside the chassis, although it seem like the PS going to cramp you out of room quick.

--pete

[alerich]

The JCM 800 Mini lives. It sounds really good, actually. Powered right up with only a few minor glitches. I inadvertently omitted the coupling cap between the MV and the PI. Evidently that cap it a pretty crucial part. *snort*. The PI was not happy. Fixed that and powered up again. OT leads backwards (as usual for me). Fixed that and powered up again. Nasty hum coming from V1 input stage. Swapped preamp tube to fix that. There's a mild hum emanating from the V2 area (third gain stage / cathode follower). Amp is totally silent when I ground the wiper of the MV.  It's not really noticeable until you get up around 5 on the MV. I'll investigate that later. it could simply be a layout issue and when you cram this much amp into this small a space you have to be willing to make a few compromises. I built this amp to rock, not to play clean. I usually dime the preamp and set the master to taste. I will never get near 5 on the MV. At 3 on the MV you can't stand in the same room with this amp. At 2 it's loud as hell and sounds great. Loudest 50 watts I have ever heard.

The 4th pic is (obviously) a mid build pic.

[SILVERGUN]

Very cool lttle build you pulled off there,  kinda like sticking a half stack in a lunchbox 
Good luck getting it the rest of the way home....

I built this amp to rock, not to play clean.

Bravo! ...thats what A/B switches are for 

[smackoj]

Very nice 'tight' build there gato....Makes me want to try and build like that....settin' the bar high fo sho.

congrats     

[sluckey]

That is a nice looking point to point build. 

[jeff]

 I read somewhere that Marshall made amps under the name NARB for a while right before he made PARK amps. I guess it had something to do with exclusive licensing of the Marshall name. NARB was actually BRAN spelled backwards, named after Ken Bran, one of Marshalls designers.
 That's why it always makes me chuckle when I see a PARK amp.

[floyd]

You should probably go ahead and solder this cap to the lug.

[Willabe]

And this 1 too, if you haven't yet.


                Brad   

[HotBluePlates]

I think he said those pictures were taken while the build was in progress.

You'll notice almost none of the pots in the pic have any wiring. I'm sure he actually has those wired now.

[punkykatt]

Its always good to have hawk eyes on the forum. I wish my eyes were better.  Many times I look but don`t see.

[DummyLoad]

looks superb! very nice construction.

 

--pete

[alerich]

Thanks for the kind comments. Here's a link to short video.
Gain = 10
Master = 1
Treble = 2
Mid = 7
Bass = 8
Presence = 2

Getting a little nudge from a Marshall Guv/nor overdrive pedal.

http://www.alerich.com/goodsell/jcm800mini.wmv

[tubenit]

Thanks for sharing the video!  Amp sounds great.  Very cool conversion. I like the PTP wiriing.

With respect, Tubenit

[Ed_Chambley]

Love it.  Rock at its finest.  I have always wondered how 50 watts from a 800 seems so much louder than a 50 watt bassman.

[HotBluePlates]

Love it.  Rock at its finest.  I have always wondered how 50 watts from a 800 seems so much louder than a 50 watt bassman.

Distortion and bass reduction.

[jojokeo]

Amp sounds really good. Sometimes when trannies are placed so close things can get dicey when turned up & all the juices are flowing. Well done. You look like you were still in your PJs??? After the earlier AX84 reference and the time of your build, maybe you should call it "March"!? Or if you took a page outta Ken Fischer's naming system "Marge" would be close enough?

*edit - sorry I just read DL's SE KT88 thread prior to this one, my bad on the mixup.