Stromberg-Carlson 209 PA Amplifier

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Post by Greenmachine »

Hi Dudes,

This was a gift from my Grampa-in-law. Suggestions are welcome as to what to do with it. Think I'm gonna just try to get it functional.

Came with Marconi tubes. 6L6's that look like KT66's - coke bottle.
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Post by Greenmachine »

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Post by DrNomis »

I reckon that would be a great platform for building a cool looking guitar amp, you pretty much have everything you need right there mate, I really wish manufacturers would follow Stromberg Carlson's lead here and include a schematic..... :D


Nice score by the way..... :thumbsup
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Post by Greenmachine »

Thanks Dr. N.

What's the inductor-looking thing that's attached to the output jacks and power line? It's not in the schem.

Also, if you or anyone has a suggestion as to which parts to alter for guitar amp conversion. Please feel free to chime in. I wouldn't know where to begin.
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Post by DrNomis »

Greenmachine wrote:Thanks Dr. N.

What's the inductor-looking thing that's attached to the output jacks and power line? It's not in the schem.

Also, if you or anyone has a suggestion as to which parts to alter for guitar amp conversion. Please feel free to chime in. I wouldn't know where to begin.


That inductor sounds like it's part of the supply filtering for a +HT supply for a pre-amplifier that get's plugged into that socket, it was pretty common practice to include that at the socket in those days, you could re-use that as part of the HT filtering system... :hmmm:


Back when that amplifier was designed, it was standard practice to use an inductor and two electrolytic capacitors to form what's called a "Pi Filter" to filter the raw rectified DC into a more or less steady DC..... :D


To convert the amp into a guitar amp, you probably wouldn't have to do too much, since it was designed for use with mics, so it's probably got enough gain, maybe add a tonestack of some sort, make sure you give it a good checkout before powering it up though.... :thumbsup


Just noticed that the two preamp valves are small signal pentodes, which means that it would deffinitely have heaps of gain.... :D
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Post by RnFR »

if it works, it'll probably sound great with a guitar plugged straight in, no mods. these old PA's usually do. you probably just have to update the input jack to a modern quarter inch type.
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Post by Hides-His-Eyes »

Also, I'd replace the electrolytics before powering up just to be safe.
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Post by Greenmachine »

Right on; thanks for the advice.
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Post by Greenmachine »

What about removing that strange B+ filter completely and just installing a regular rotary impedance switch? The OT has like 5 secondary taps or something. I don't think I've ever seen output jacks like those before - they look like the sort of power plug for North American washer and dryers.
Last edited by Greenmachine on 23 Jul 2011, 15:15, edited 1 time in total.
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Post by JOHNO »

I think you could add an extra input jack to the inputs (one for each input) and then you could use a patch cable to jumper Mic1 and Mic2 inputs. I would use 68k resistors on input 1 also instead of 1m as shown in the schem. Then you have high and low inputs. Check out a fender bassman schem for the wiring.

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Post by Greenmachine »

Why do people jumper channels? (Sounds like a silly question ...)
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Post by JOHNO »

If you look at the schem you can see one input uses one half of a tube and the other input uses the other half. When you jumper them you use both halves of the tube so you get more gain into the next stage. Not to mention you now have in effect two volume controls on the input to control the amount of gain you want. You can dial in some nice tones.
Edit; You actually have two tubes on the input (one on each input) but the same principal applies.

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Post by DrNomis »

Greenmachine wrote:What about removing that strange B+ filter completely and just installing a regular rotary impedance switch? The OT has like 5 secondary taps or something. I don't think I've ever seen output jacks like those before - they look like the sort of power plug for North American washer and dryers.

Yep, very do-able, in my amp head, I've set things up so that it uses a 3-throw rotary switch to select the 4, 8, or 16 Ohm tap, and it works fine..... :thumbsup
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Post by DrNomis »

Greenmachine wrote:Why do people jumper channels? (Sounds like a silly question ...)

I seem to remember it being done with the input sockets of a Marshall Super Lead Plexi, the idea was to blend the tones of the bass and treble inputs together to get a wider variation in tonal quality, I think Jimi Hendrix used to do that, and he used to use the other sockets to chain his Marshall Stacks together..... :hmmm:
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Post by Greenmachine »

I notice on the OT there's a tap titled 3A - does this mean 3 Amps? Can I just tape it up?

I can tape up all the other odd taps too right? 250 Ohms, 500 Ohms. They don't have to go to ground or anything like that, eh?
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Post by DeanM »

Greenmachine wrote:I notice on the OT there's a tap titled 3A - does this mean 3 Amps? Can I just tape it up?

I can tape up all the other odd taps too right? 250 Ohms, 500 Ohms. They don't have to go to ground or anything like that, eh?
For unused OT secondaries like the 250 and 500 ohm taps you mentioned yes you can just tape them and not connect them. dont know about that 3A tap tho!

Also another reason for jumpering channels that JOHNO didn't mention is the frequency responses. Old marshalls had bright and normal channels so people would jumper them and then use the volumes to get the best mix. In this schematic the preamps have the same frequency responses. both jumpered would fatten up the tone, but what you could do is change the frequency response on one of the tubes. If you look at the scheme for the marshall vintage modern thats what that does. one half of a tube is used to control the gain for the lower fundamental frequencies and another is used for the harmonics and upper frequencies! the result is that you can control the overall gain and harmonics and body of the sound, its really versatile and it even does most of the EQing!

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Post by Greenmachine »

Well, this thing has been a pain in the ass, but I haven't lost complete hope yet.

It's passing signal and aside from a lot of hum, sounds decent. Unfortunately, there's a high pitched oscillation that's present when you turn up the treble control. It's unaffected by the other controls and disappears when I pull the 6FS5 tube. When I move my hand around the the 6N7 tube with the treble set to almost oscillation, the oscillation appears. Kind of like playing a Theremin.

I've tried replacing all the tubes, but the problem persists. I've star grounded. Reflowed every joint, disengaged the phono input. Replaced every capacitor and a few resistors (measuring resistors in the circuit - which I know isn't accurate - seems to show most resistors off by a bit, the most drastic being say 200k where it should be 150k). Three prong ac cord. Replaced all pots except the Bass. Retensioned and cleaned all sockets.

Tapping on power tubes is fine - no "woof-ing" there. Tapping on preamp tubes can be heard through speaker on each of them, the 6SJ7's the loudest I think. Think this is sorta normal.

The only thing I can think of doing now is replacing all the resistors and trying some new tubes (which blows, because I just bought some NOS ones and that'd be lame if they were all microphonic).

I guess I'll rewire the heaters if the oscillation goes away - not too concerned about the hum at this point.

:scratch:
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Post by DrNomis »

Greenmachine wrote:Well, this thing has been a pain in the ass, but I haven't lost complete hope yet.

It's passing signal and aside from a lot of hum, sounds decent. Unfortunately, there's a high pitched oscillation that's present when you turn up the treble control. It's unaffected by the other controls and disappears when I pull the 6FS5 tube. When I move my hand around the the 6N7 tube with the treble set to almost oscillation, the oscillation appears. Kind of like playing a Theremin.

I've tried replacing all the tubes, but the problem persists. I've star grounded. Reflowed every joint, disengaged the phono input. Replaced every capacitor and a few resistors (measuring resistors in the circuit - which I know isn't accurate - seems to show most resistors off by a bit, the most drastic being say 200k where it should be 150k). Three prong ac cord. Replaced all pots except the Bass. Retensioned and cleaned all sockets.

Tapping on power tubes is fine - no "woof-ing" there. Tapping on preamp tubes can be heard through speaker on each of them, the 6SJ7's the loudest I think. Think this is sorta normal.

The only thing I can think of doing now is replacing all the resistors and trying some new tubes (which blows, because I just bought some NOS ones and that'd be lame if they were all microphonic).

I guess I'll rewire the heaters if the oscillation goes away - not too concerned about the hum at this point.

:scratch:

It could be that the 6SJ7 may be microphonic, and this tendency to microphonic-ness, might be being exasperated by turning up the treble control, the high-frequency oscillation may be cured by using sheilded-audio wire for the control-grid connection for the 6SJ7 if there's just a wire, usually, high-frequency oscillation in Tube Amps is caused by signals from the output-section getting into the control-grids of the preamp-tubes, since they are at high-impedances, and the preamp-tubes are operating at reasonably high gains, also, the signal from the output could be getting into the preamp tubes via the HT supply if the bypass caps are getting a bit old too.... :hmmm:


Hope that helps... :thumbsup
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Post by Greenmachine »

C15 and C16 are decoupling caps right? They've been replaced. There's shielded wire on the two preamp grids; but perhaps I should stick some new stuff in there. I'll try playing around with the 6SJ7's a bit more.

Thanks!
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Post by DrNomis »

Greenmachine wrote:C15 and C16 are decoupling caps right? They've been replaced. There's shielded wire on the two preamp grids; but perhaps I should stick some new stuff in there. I'll try playing around with the 6SJ7's a bit more.

Thanks!

Yes, that's correct, those two caps(C15 and C16) are coupling the control grids of the power tubes to their respective halves of the 6N7 tube, the 6N7 tube is the "phase-splitter" tube and it's job is to generate the the two anti-phase signals to drive the power tubes in push-pull mode, what you could do is insert "grid-stopper" resistors from the junction of C15/R 29 to pin 5 of their respective power tube, and likewise for the junction of C16/R27, this stops something called "parasitic oscillations" from happening, a suggested value for the grid-stopper resistors can be 1k, or 2k2.... :thumbsup

Note, make sure that whatever value you use for the grid stopper resistors, that they are the same value, here's a diagram to show where they are placed in the circuit:
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Grid Stopper Placement Diag
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