Another Small Special build with a twist

[Synchu]

That was on the sw22 picture as far as I remember. I tried 100k and then 1.2k (and few values in between). 1.2k sounded best.
Niki

[Dr d]

Thanks for replying! This circuit interests me, particularly in its similarity to the 2nd gen design. Thinking of making a hybrid of the two to see what happens. Cheers.

[Synchu]

Sounds very interesting. Will wait (impatiently) to see/listen to the results

Niki

[SmallSpecial50]

100 watt/6L6's + reverb
it simply must be.

[tubeswell]

I know this thread is getting a bit old, but I 've got a Morgan SW22R in at the mo'. Took a peek inside - See gutshots.

Also reversed engineered a schematic - will post it later

[M Fowler]

Thanks for the Morgan 22 photos love amp porn.

[tubeswell]

Here's my schematic - any suggested corrections welcomed

Also attached is my schematic analysis of the 'Bass Emphasis' switch. (The 1M resistor in the 'Bass Emphasis' circuit is a noise snubber for the switch. With the switch on, this 1M is resistance is high enough that it is effectively out of the filter AFAICT).

[fred.violleau]

Thanks for the pics and schematics!
First time I see preamp heaters wired this way! No heater hum?
Merlin was very strict in his book about the tightness of this wiring. Anyway, always interesting to see how other builders work!
Cheers,

Fred.

[pompeiisneaks]

Heater hum is quite often overstated as a problem. I've seen a production amp (I can't recall which one off the top of my head) that had this design for all heaters, power and preamp. A straight line down low like that. You pretty much have to have a very sensitive stage AND significant hum to cause problems. If there's a center tap, virtual center tap, or hum balance pot, you can expect very little hum even in bad designs. I.e. you'd hear it at max volume, and only when not playing.

Most hum people hear is 100/120Hz hum due to poor filtering, or filter caps failing.

~Phil

[Mr. dB]

I've seen a production amp (I can't recall which one off the top of my head) that had this design for all heaters, power and preamp. A straight line down low like that.

Soldano did it.

And Marshall used zip cord on the heaters of some early 18W 1974s.

[memphis032]

Would anyone have suggestions of what reverb transformer and pan to use for that SW22R schematic? I've made a few amps before but never put in reverb...

[sluckey]

Use the Fender 022921 reverb transformer and a 4AB3C1B reverb tank.

[memphis032]

Also, the PI input cap - should that be 0.022 rather than 0.0022?

[tubeswell]

Also, the PI input cap - should that be 0.022 rather than 0.0022?

No, it is definitely 0.002. This is not unusual for a LTP input cap. The high input impedance from the grid bootstrapping means you only need very small amounts of input capacitor value to get decent bandwidth.