Old tube amp revived.

[JL Sargent]

Found an old console at the thriftstore for a paultry sum and brought it home. Inside was this sweet little tube amp from Stromberg Carlton. I installed new RCAs, Binding posts, switch, and ON indicator, only to find one channel was AOL. After some hunting I found a 33K ohm resister that measured a whopping 8 Meg. Replaced that and all is well.

This amp uses a compliment of four 7408 tubes which I understand is an uprated version of the 6V6GT along with two 7199s. Anyway just thought I would share this little project amp.

[Boxx]

What a great find and upgrade! How do you plan to use it?

[JL Sargent]

I've got just a little bit more work to do first. I'm replacing all the coupling caps (Eight .05uf and two .0047uf Orange Drops going in). Then I'll be using it with a HD tuner (bought from Jhoak here on the forum) for my office listening. Should work out real well for me as the tuner has a volume control and a remote!

[NOSValves]

If you're handy with a soldering iron I'd suggest swapping out those output transformers with something better and then doing some mild tweaking of the circuit. If you are interested and have a schematic of the circuit I could cook something up for you.

[JL Sargent]

Hey Craig, any and all input from you welcome! Here is the amp under the hood. I swapped that top cap hunting for my problem!

[NOSValves]

Nice picture! But I need an actual schematic to help tweak it. If your not sure what a schematic is then we may not want to mess with the amp. It's an electrical road map of the circuit listing the values of all the components in the circuit input to output. It may also have voltage specifications.

[JL Sargent]

Wow, found the correct one! Thanks for the help Craig. Oh, and well versed on schematics, 8 years electronics tech USAF.

[NOSValves]

Cool that makes helping you a bunch easier both with the scat and your experience!

Do a couple confirmation measurements for me.

Measure the wall voltage the amp is running at first.

measure pins 3, 4 and 8 of one of the 7408 output tubes.

this amp will be dead simple to modify for better output iron. If you look at the output transformer specs you should be able to see why it will be handicapped achieving full balanced frequency response across a broad range while driving a full range speaker like Klipsch.

[JL Sargent]

Here is what I have.

Pins 3 to 8 = 331v

4 to 8 = 311v

3 to 4 = 20.6v

[mike stehr]

The cathode biasing looks quite similar to what Magnavox did. Quasi-cathode bias, tied in with the heater winding through the two 47 ohm resistors.

Do you have feedback network connected?

There should be a 68K (with a .01 uF cap in series on some models) resistor from the cathode of the input section of the 7199, to the secondary of the output transformer. (green wire)

[JL Sargent]

Do you have feedback network connected?

There should be a 68K (with a .01 uF cap in series on some models) resistor from the cathode of the input section of the 7199, to the secondary of the output transformer. (green wire)

You know, there it is plain as day in the schematic but I can tell you that circuit was not in this amp when I pulled it. The Tranny outputs were on top of the amp and went to the speakers only. Any thoughts on the difference??

[mike stehr]

Do you have feedback network connected?

There should be a 68K (with a .01 uF cap in series on some models) resistor from the cathode of the input section of the 7199, to the secondary of the output transformer. (green wire)

You know, there it is plain as day in the schematic but I can tell you that circuit was not in this amp when I pulled it. The Tranny outputs were on top of the amp and went to the speakers only. Any thoughts on the difference??

Well, without feedback the amp could have a bit more noise, and a bit shouty with a more accentuated midrange response.

The turret board in the underside pic of the amplifier shows a couple 68K resistors, (blue/gray/orange) maybe that's the feedback resistor. See if if they are disconnected. If they are, maybe try it with the feedback connected, and disconnected to see which sounds best.

[jhoak]

What a GREAT find. Major kudos to you for bringing a piece of old techonolgy back to a useful state. I keep thinking that I'll stumble across something like that at some point.

About a bazillion years ago when the world was young I worked just up the street from a Stromberg Carlson R&D facility. At the time I managed a Radio Shack store. That particular store was like the number 2 or 3 in the country for "parts" only sales because of the engineers at S/C. They were in so often that I got to be pretty good friends with a few of them. I was always amazed at some of the S/C tubed audio kit that most of them listened to. They were all like "MEH" about their systems. At the time I would have KILLED for what they were listening to.

bought from Jhoak here on the forum

OK... I really don't want to wizz in your Cheerios but don't get too cranked up about that tuner. The internal DACs aren't what I would call "all that". Not horrid but just not very good. I always used it piped in by way of a coaxial connection to an Emotiva DAC. On the few occasions that I used the internal processing I wasn't very impressed. All of that is to say that when you get it all hooked up and you're disappointed with what you're hearing it is NOT the amp or the speakers it's the tuner. Pick up pretty much any low $$ DAC and you'll hear an immediate improvement.

[JL Sargent]

OK Jhoak, no big deal.

Mike, After lifting that board I can see that the feedback circuit is actually wired through those 68K resistors as you suspected. I had to think about it because these transformers have dual secondary wires. A pair out the top and a pair out the bottom.

[NOSValves]

Here is what I have.

Pins 3 to 8 = 331v

4 to 8 = 311v

3 to 4 = 20.6v

Measure again from circuit ground to pin's 3, 4 & 8

I also need to know what the AC wall voltage is at when making these measurements.

[JL Sargent]

OK,

Pin 3 - 348v

Pin 4 - 330v

Pin 8 - 21v

Power distribution block amp is plugged into measures 120.6v with amp working.

[JL Sargent]

For reference found this info on 7408 power tube:

ampage tube data		ampage tube data

7408 BEAM POWER TUBE
Pin Element 1 none 2 Heater (Filament) 3 Plate (Anode) 4 Screen Grid 5 Control Grid 6 none 7 Heater (Filament) 8 Cathode		Glass octal type used as output amplifier tube in high-quality sound systems. Tubes require octal socket and may be mounted in any position.

Heater Voltage (AC/DC)	6.3	volts
Heater Current	0.45	ampere
Heater Warm-up Time (Average)	11	seconds

Heater-Cathode Voltage	Average	Peak
Heater negative with respect to cathode	100 max	200 max	volts
Heater positive with respect to cathode	100 max	200 max

Direct Interelectrode Capacitance (Approx.)
Grid No.1 to Plate	0.7	pf
Grid No.1 to Cathode, Heater, Grid No.2, and Grid No.3	9
Plate to Cathode, Heater, Grid Ho.2, and Grid No.3	7.5

CLASS A1 AMPLIFIER

Maximum Ratings (Design-Maximum Values)
Plate Voltage	350 max	volts
Grid-No.2 (Screen-Grid) Voltage	315 max
Plate Dissipation	14 max	watts
Grid-No.2 Input	2.2 max

Typical Operation and Characteristics
Plate Voltage	60	250	volts
Grid-No.2 Voltage	250	250
Grid-No.1 (Control-Grid) Voltage	0	-12.5
Peak AF Grid-No.1 Voltage	-	12.5
Zero-Signal Plate Current	100	45	ma
Maximum-Signal Plate Current	-	47
Zero-Signal Grid-No.2 Current	22	4.5
Maximum-Signal Grid-No.2 Current	-	7
Plate Resistance (Approx.)	-	50000	ohms
Transconductance	-	4100	umhos
Load Resistance	-	5000	ohms
Total Harmonic Distortion	-	7	%
Maximum-Signal Power Output	-	4.5	watts

Maximum Circuit Values
Grid No. 1 Circuit Resistance
For fixed-bias operation	0.1 max	megohm
For cathode-bias operation	0.5 max

---

[Tractrix]

I would make sure that the following resistors were matched:

R47:R48

R45:R46

R55:R56

R53:R54

R61:R62

Also, because the line voltage has changed since this amp was built, going from 110 to 120VAC, you now have too much voltage on the output tube plates. These tubes are going to run hotter than intended. You may have to adjust R58 to bring the DC voltage down.

Otherwise, without changing the input and driver stages, there's not much to do that has not already been said.

[NOSValves]

Okay here are my thoughts but in the end we will have to tweak it after parts are installed.

I'd use a set of hammond 1650F output transformers (hopefully they can be shoehorned on the existing chassis). there are others that will work but with Hammond transformers or anything similar you are way better off going overboard then just up to spec if Sonics is your goal.

I'd change R63 to about 200 Ohms 10 watt. This will help cool down the current to the 4 7408 output tubes a bit. The 348 volts on the plates is within max spec and you also have to deduct the cathode voltage from the plate for effective plate voltage.

I would change R59 to correct the voltages for the 7199 driver/phase inverter. I bet a 5.6K or 6.8K 10 watt will do a fine job at that.

I'd replace C55, C56, C60 and C61 with .1 uf 400 V caps

1'd replace R47, R48, R55 and R56 with 120K 1/2 watt minimum rating.

Then since this amp is so simple and laid out to be easily worked on I'd replace every single component at stock value. For resistors I'd stick with Carbon types carbon film will do just fine. The matching to within 2% of the above mentioned resistors (by tractrix) is a real good idea for best performance. I'd absolutely replace C52 multi section filter cap and up the UF value best as possible on all 3 sections.

The feedback network will most likely need a bit of tweaking but the real problem with that is we need a function generator and a scope to tune that circuit to the new transformers. If need be you can ship it over I can perform that last step if we feel the need. Hey maybe you have a function generator and scope for all I know?

[JL Sargent]

Alright, I'm on it! Man, I'm gonna have something when this is done.

I notice that Parts Express has those transformersI Is there a better source?

Craig, are Orange Drops 600v good on the .1 caps or do you suggest another?

On the sectoral cap, how much higher on the UH can/should I go? Who is a source for those?

I do have a scope, maybe I can pick up a function gen.

Craig, thanks again for the help with this, I'm excited about it.

Tractrix, I appreciate your suggestions also.

I'll be working on my parts order together tonight.