5010 conversion to 2204/1986 - #2

[StingRay85]

This thread is to keep track of the progress on amp #2.

Amp #1 is alive and kicking: https://www.marshallforum.com/threads/5010-conversion-to-2204.133155/

It's basically a bridged super bass with MV on the low input, and a 2204 with two parallel input stages on the high input. Seems a bit silly to have two times two stages in parallel, but it sounds great. Has more gain than a stock 2204, and the low input is a much better pedal platform than the low input of a 2204.

Amp #2 should be an exact copy of amp #1, but there cannot be a single compromise on the build quality. I decided to switch from a double 100 µF 500V reservoir cap to a double 220 µF 350V in series, as the HT is really close to 500V. Will also foresee 1 ohm resistors and bias sampling points, and an external trim pot for the bias.

 

[StingRay85]

Hooked up the reservoir caps, connected the secondaries, diodes, and fired her up for the first time. A solid 490V on the 250V setting. Enough for today

 

[Pete Farrington]

What transformers this time?

 

[StingRay85]

What transformers this time?

Same ones from Carlsbro TR60 from 1970 (S/N 30XX). No numbers on the bells though. The amp was already modified when I bought it a few years ago. They have this crappy brittle insulation on some of the leads. All copper wire is red lacquered, and doesn't conduct.

 

[StingRay85]

Heater elevation done. OT hooked up. Secured the unused secondary windings. Test fired it and everything checks out ok. "Open" wires left: the green wire of the choke, that needs to go the (to be installed) filter cap and screens. Yellow and black secondaries of the OT. Red wire on the fuse reserved for the negative bias supply, and red wire on the 4 ohm tap for the NFB.

 

[StingRay85]

Outputs and heaters done

 

[Pete Farrington]

It’s usually considered easier to leave Fender style up and over heater wiring until later in the build, so that it’s not in the way of fitting the other wiring.
Kinda opposite to the Marshall etc method, which is considered easier to do first.

 

[Marcomel79]

Can i ask why you positioned the preamp sockets that way? It seems that it leaves little room for the turret board

 

[StingRay85]

All the hardware is installed and ready to start wiring up the pre. I positioned a 47K Lin Philips pot, and a stereo socket for the 1 ohm sampling points.

Well, the heaters is surely the biggest pain in the a, and therefore prefer to do them first. The Teflon wire is not so easy to work with. When wiring the first amp, didn't really had problems there. It was very tight wiring at the pots, and the coupling cap selector switch. Since its a copy, I can use my previous build as a guide, and basically fly through this build. These industrial style solder strips have two wiring positions, that will make it a lot easier to hook everything up. A turret board is of course easier to work on, but it compromises on other aspects of the wiring.

Next up, the input sockets, and then pots and install the ground buss, which should become a single buss wire all the way up to the star ground.

 

[Ken Underwood]

The resistor across one of thew smoothing caps have legs far to long as the are very close to the chassis.

For what is worth your soldering is very iffy and i am not sure what type of solder you are using, i hope it is not lead free, if so you are asking for trouble

 

[StingRay85]

Thanks for chiming in Ken. The resistor is at least 1 cm from the chassis, surely safe. On the picture it looks indeed a bit misleading. Every solder joint is evaluated and tested. I re-use some of the old Carlsbro parts, that might look a bit worse than it is in reality. Its Pb solder, Sn60/Pb40 RA3%

 

[StingRay85]

 

[StingRay85]

V1 and V2 wiring completed. Next point to continue from in the middle lug of the preamp volume pot

 

[StingRay85]

Schematic of what has been wired so far.

 

[StingRay85]

Getting close. Next up, tone stack and PI

 

[StingRay85]

Finished up to the PI coupling caps. Nearly done

 

[StingRay85]

First sound produced. Used the scope to see which side of the primary of the OT is which. Confirmed it with a speaker. Yesterday during test firing I blow all three fuses, noticed the choke had failed open. Bypassed the choke, and all seems to be working now.

Have to pull the choke to see what happened, install a new one, wire the primaries, and then the proper testing can begin

 

[StingRay85]

Amp is fully wired now, and works great

 

[StingRay85]

 

[StingRay85]

Full schematic with voltage readings. Time to dig into the tube stash and put the good stuff in

Played around with it for a little while. Has lots of gain, sounds again really good. Noise floor is extremely low, even on tone controls like treble and presence. No hiss or hum whatsoever.