Stromberg-Carlson 209 PA Amplifier

Greenmachine · Post by **Greenmachine** » 29 Aug 2011, 20:45

Hi Dr. N,

Forgot to mention I've already installed grid-stoppers (1.5k on each power tube). Great diagram though! Much appreciated.

DrNomis · Post by **DrNomis** » 29 Aug 2011, 20:55

Greenmachine wrote:Hi Dr. N,

Forgot to mention I've already installed grid-stoppers (1.5k on each power tube). Great diagram though! Much appreciated.

Ah, no worries mate, ever since I've started scanning my hand-drawn diagrams into my PC, the resulting image clarity has been making me pretty happy indeed, so much better than photographing the diagrams with my digital camera....

Have you had a chance to try the amp out with a guitar and speaker cab yet?, if so, how does it sound?...

My little 5 Watt Legacy amp is working well after I did some work on it's power supply, replacing the original 22uF/450V filter caps with two 100uF/400V, and two 47uF/450V caps, result...the amp is almost hum-free, I have to turn the master volume and gain up full just to hear any hum at all....

Greenmachine · Post by **Greenmachine** » 29 Aug 2011, 21:44

Just measured the cathode voltages and they were way different. Stuck some other 6L6GC's in there and much better voltages. Not sure why I kept those old 6L6G's in there - brain fart I suppose. They're running hot, so I've got to get a larger cathode resistor in there; then I'll reassess the situation.

I've stuck larger value/voltage filter caps in every position. Usually 10uf's more than the first and a 100v's more too. I'll keep updating.

DrNomis · Post by **DrNomis** » 29 Aug 2011, 23:15

Greenmachine wrote:Just measured the cathode voltages and they were way different. Stuck some other 6L6GC's in there and much better voltages. Not sure why I kept those old 6L6G's in there - brain fart I suppose. They're running hot, so I've got to get a larger cathode resistor in there; then I'll reassess the situation.

I've stuck larger value/voltage filter caps in every position. Usually 10uf's more than the first and a 100v's more too. I'll keep updating.

Yeah, I usually replace old supply filter caps with either equal or greater than value caps, as long as the voltage rating is enough for the job,, I reckon the old 6L6 tubes are most likely worn out if they are showing lower cathode voltages, power tubes tend to draw the most current from the power supply, which is why they tend to wear out faster than preamp tubes, preamp tubes tend to draw far less current during operation, so they usually last quite a long time.....

Greenmachine · Post by **Greenmachine** » 31 Aug 2011, 23:44

Hey, what about larger value grid-stopper resistors? Would that stop the oscillation?

What about grid stoppers on the 6FS5? Would that help - that seems to be the problem tube.

DrNomis · Post by **DrNomis** » 01 Sep 2011, 00:34

Greenmachine wrote:Hey, what about larger value grid-stopper resistors? Would that stop the oscillation?

What about grid stoppers on the 6FS5? Would that help - that seems to be the problem tube.

You could try larger grid stopper resistors on the grids of the power tubes, maybe any value up to say 22k, a grid stopper resistor on the 6SF5 may work ,here are a couple of other things I could suggest trying that may also work to stop the oscillation:

1, Solder in a small value cap from the Grid (pin 3) of the 6SF5 to ground, maybe something like 47pF or 100pF.

2,Solder a small value cap from the Anode (Pin 5) of the 6SF5 to ground, say 100pF to 220pF.

Note: You may need to experiment with the cap values a bit till you find a value that works best in each case (make sure the voltage rating of the cap is adequate though)....

I just had a look at the datasheet for the 6SF5, and it says that the amplification factor is 100, which is the same as a 12AX7 tube....

Hope that helps....

Greenmachine · Post by **Greenmachine** » 01 Sep 2011, 02:43

Hey guess what? The oscillation is gone! I was chopsticking around and moved the plate wires from the OT and the whistling stopped.

The down side is that a mild sizzling noise has replaced it. I've been taught to keep these wires short, away from preamp wiring, and along the chassis. In this case, they're long because of the OT location, but I've done my best aside from relocating the OT, which isn't an option I think.

Between the hum and the treble sizzling, this amp is too noisy I think. I'm afraid to do the heater wiring because I've already broken two socket tabs. The last one I simply soldered to what was left of the tab rather than replace the whole socket. Can anyone recommend a GOOD brand of solder wick? All the stuff I've got blows - not absorbent at all. The solder sucker has too much kickback for these 60 year old lugs.

DrNomis · Post by **DrNomis** » 01 Sep 2011, 07:41

Greenmachine wrote:Hey guess what? The oscillation is gone! I was chopsticking around and moved the plate wires from the OT and the whistling stopped.

The down side is that a mild sizzling noise has replaced it. I've been taught to keep these wires short, away from preamp wiring, and along the chassis. In this case, they're long because of the OT location, but I've done my best aside from relocating the OT, which isn't an option I think.

Between the hum and the treble sizzling, this amp is too noisy I think. I'm afraid to do the heater wiring because I've already broken two socket tabs. The last one I simply soldered to what was left of the tab rather than replace the whole socket. Can anyone recommend a GOOD brand of solder wick? All the stuff I've got blows - not absorbent at all. The solder sucker has too much kickback for these 60 year old lugs.

If you can buy it from your local electronics store, see if you can buy a few rolls of 3mm X 1.5 Metre Goot Solderwick, I've had very good results with it, it works very well at soaking up the excess solder,here's a pic to show what it looks like:

Or if you want, I could buy some from my local Jaycar Electronics store and post them to you mate....

Greenmachine · Post by **Greenmachine** » 01 Sep 2011, 17:24

Thanks man, I'll look around for that brand. If I can't find any I'll take you up on your offer.