Link to easy build switchable 1959 to 2203

[neikeel]

Done.

http://i832.photobucket.com/albums/zz24 ... 000334.jpg" onclick="window.open(this.href);return false;

Would the circuit be exactly like a 2203 circuit if I'd fellow your layout?

Fast work (where is your presence pot - fixed value?)!

Yes, you have to decide the NFB (I like 47k/8hm tap) but you can go metal panel 1959 and 2203 with 100k/4ohm, other than that yes.

[peterc52]

Done.

http://i832.photobucket.com/albums/zz24 ... 000334.jpg" onclick="window.open(this.href);return false;

Would the circuit be exactly like a 2203 circuit if I'd fellow your layout?

Fast work (where is your presence pot - fixed value?)!

Yes, you have to decide the NFB (I like 47k/8hm tap) but you can go metal panel 1959 and 2203 with 100k/4ohm, other than that yes.

The presence pot is going into one of the speaker outputs. As for the NFB I'm going to try both.

[sixpakldp]

I am thinking of employing the neil special mod to my metro 100 watt kitt. My amp currently has the dual voltage, zero loss effects loop and lar mar ppmiv. I ordered all my parts from weber today to do the mod, the one thing I want to make sure of is the neil special layout that is shown has filtering per a 68, whereas my amp is all 50/50 cans. If my board looks exactly like the neil special but with the 50/50 cans when I complete this mod, will it be correct?

I made a drawing and highlighted the components that will "go away" after this mod, going left to right across the circuit board: it looks like the 820K resistor and 250 on the left of the board get replaced with the 10 K resistor, the two 220k resistors near the bias section will be removed, the 47k will be removed and replaced with a lead between the two turrets and the two .22uf capacitors at the rightmost end of the board will be removed.

My amp works good right now, I'd like to try the mod but I don't want to damage anything performing it.

Please advise if this is what I want to do.

Also, anyone have any clips of the mod?

Thanks,

Paul

[neikeel]

I am thinking of employing the neil special mod to my metro 100 watt kitt. My amp currently has the dual voltage, zero loss effects loop and lar mar ppmiv. I ordered all my parts from weber today to do the mod, the one thing I want to make sure of is the neil special layout that is shown has filtering per a 68, whereas my amp is all 50/50 cans. If my board looks exactly like the neil special but with the 50/50 cans when I complete this mod, will it be correct?

Yes it will, just you will have a 69spec 1959 and a 2203

I made a drawing and highlighted the components that will "go away" after this mod, going left to right across the circuit board: it looks like the 820K resistor and 250 on the left of the board get replaced with the 10 K resistor, the two 220k resistors near the bias section will be removed, the 47k will be removed and replaced with a lead between the two turrets and the two .22uf capacitors at the rightmost end of the board will be removed.
My amp works good right now, I'd like to try the mod but I don't want to damage anything performing it.

The resistor you remove is an 820R and the 250uF cap (normal channel cathode cap and resistor) the 10k is 'colder' bias for the half of V1 triode that is cascaded - stops it all turning to mush.
The 220k on the bias section go because they are replaced by the dual 250k pot (the 2.2meg are safety resistors).
I personally leave the NFB resistor in its normal place and re-route the purple wires carefully.

The diagram Steve drew leaves the 68k input resistors in place you just unhook the green wires on both ends and cable tie them under the board/shrink wrap them. My mod lets you try out a 2203 in your Metro which you can remove if you want. I like it and find it very versatile. If you want to tweak for more gain (replace the 10k with a 4k7 or add a 470nF bypass cap then you will need to use the modified lead dress version and use shielded wire, but we can talk you through that if and when

[sixpakldp]

Thanks! I appreciate the info very much. Pretty much what I thought, but I always like to ask first.

[sixpakldp]

Quick question, is it neccessary for the 5000 uf bright cap to be installed for this mod to work best? I have the 500 uf that I originally installed in my kit and generally like the way the kit sounds, just wanted the added feature of 2203 gain structure.

[toner]

...

[neikeel]

The bright cap on that pot is only engaged when you select 1959 mode (you can choose its value to taste, we put only 5000pF on the layout as that is stock for most 1959 in lead spec.

You will see a 1000pF cap on the gain pot of 2203 whic is only engaged in that mode, again as that is stock for a 2203, you can omit it if you like.

The premise of the amp is to be able to have stock 1959 with the push-pull switch pressed in and the pre-pimv pot turned right up (then use the PPIMV to control overall volume, or not if you wish) and a stock 2203 with the push-pull switch out. In that mode you crank the Post PIMV ('Lar-Mar' dual pot) to get it out of the circuit and use the pre-mv and gain pot like a normal 2203.

Normally plug into the Hi channel 1 input for both modes but you can plug into lLo channel 1 if you want (for a strat?) or if you plug into Lo channel 2 and pull the gain pot to engage 2203 mode you will get 2203 Lo mode.

Pretty straightforwards really, the rest of the spec with loop, dual voltages and presence wiring are all personal taste

[sixpakldp]

. . . so I'm in Scranton, PA on business over the weekend and drove home to Pittsburgh yesterday very much looking forward to spending an evening modding my amp and a few beers to relax. Got my little workshop set-up, whip out the diagrams and I take a look at the pot that I ordered for this mod and find out it is a 1 meg linear DPST. Not sure what the hell I was thinking when I bought the pot, but it's obviously two lugs short to do the mod.

Guess I'll stop at radio shack today, but I doubt they will have one or anything of decent quality.

Bummer!

[sixpakldp]

So eventhough I didn't have the correct pot, I did some figuring and bought a dpdt switch and used a 1 meg pot I had laying around. I mounted the dpdt switch on the front where the push pull pot would go in SDM's layout and then installed the pot in the back of the amp where one of the speaker inputs would be. Truthfully, I kind of like the layout because the switch looks a little less out of place in the input jack hole than the pot does (imho).

In any case, I did the mod and sounds pretty good. Really like the 2203 sound. The only thing I can't figure out is it seems my Lar/Mar PPMIV cleans the amp up when you crank it, especially on the plexi channel. Haven't gone back and done a thorough check but if anyone has any ideas, chime in

[toner]

The only thing I can't figure out is it seems my Lar/Mar PPMIV cleans the amp up when you crank it, especially on the plexi channel.

The PPIMV increases (restores) negative feedback as you turn it up. This reduces gain in the PI and power section so it sounds cleaner. That's normal behavior.

[sixpakldp]

The plexi side of the amp is much cleaner than it used to be when it was just a plexi. When you crank everything with an attentuator, it's not really dirty at all. The 2203 side has a good amount of gain, the plexi is nearly a clean channel.

I was doing some experimenting the other day and just threw the 10K resistor on the left side of the board in lieu of the resistor and cap, I noticed just doing this made the amp quite clean. Haven't tried it yet, but my guess is if you vary that resistor it will adjust the gain to some extent.

I also tried the amp (when it was just a plexi) with and without the snubber caps. I was reading about snubbers and a lot of posts state that they are there to prevent popping sounds when turning on and off. The snubbers seemed to have a little bit of effect on the tone, the amp appeared to sag a little more and stiffened up without them. Maybe it was just my imagination, but I might try throwing them back in with the amp as it is now and see what it does.

[neikeel]

If your wiring is correct the plexi tones should be exactly the same as they were before. Have you wired the 10k cathode resistor into the plexi mode by mistake? (should be thw 0.68uF/2k7 part of V1 cathode). This will make the plexi channel sound like a 2203 lo input

Sounds like that is what you have done.

To hear the difference with snubbers does surprise me. I tend to use UF diodes nowadays which are very effective without snubbers.

[sixpakldp]

Neil,

Thanks for the tip, what you mentioned sounds (to my ears that is) like what I'm hearing. 2203 mode has good gain and plexi sounds damn near a clean channel. Plexi does not sound like my plexi used to. I figured I'd ask because everything else functions like it should with the mod in place.

I could be full of it on the snubbers FYI. I was experimenting with these on Saturday night and it was late, but who knows.

[Poxxie]

This looks really nice, since i've got a Super PA wreck that I'm planning to fill. Lots of extra holes!

I have two questions though.

Will the 2203-type MV be active in 1959-mode?
I assume that the PPMV will be active in both modes.

And could this be made keepin both pre-volumes (or at least the sound of both linked) for the 1959?