Super City DEA 200 troubleshooting

[StingRay85]

Got a cool Super City DEA 200 on the bench that's in need of revival. It's a 1974 made in Holland amp that's supposed to sound like a Marshall.

Johan did a cool demo on this model a year ago



I bought it as broken, without really knowing what was wrong. Fuses were good. One EL34 (=C=) measured nothing on my Maxi Matcher, no current passing through but also no short. Then it gets weird. This are the basic features of the power amp:

- 15K control grid resistors
- individual 47 ohm screen grid resistors, 330 ohm per pair (V1+V2 & V3+V4).
- 100K suppressor grid resistors connected to cathode (what the hell?)
- cathodes paired (V1+V3 & V2+V4), followed by two 12 ohm resistors parallel (6ohm) to a 2A fuse to ground. So two 2A fuses between cathode + 6 ohm and ground.
- anodes paired (V1+V3 & V2+V4) and 4x 22 nF in series between the two pairs. Wiring with coax cable, shields connected to pin 6, all 4 interconnected, and connected to a high voltage cap.
- heaters are connected with an unshielded buss wire

Now, what looks like the worst is the suppressor grid resistor of V2, looks pretty burned, although it still measures OK at 101K ohm. Burn marks on the common 330 ohm screen grid resistor of V1+V2. And then there's the 2x 12 ohm resistors of the cathodes of V1 and V3 that look damaged and measure 4.2 K ohm.

Still need to dig my way through the filtering and rectification.

Anyone has a clue what would be trigger for this failure? Why would there be a 100K resistor between g3 and cathode?

 

[StingRay85]

 

[StingRay85]

There's also the "Half power / full power" switch that took some of the heat as it is position under the 100K, which is basically switching a 330 ohm resistor parallel over a 4.7K resistor between the anode and screen filtering. It's still functional. There's a schematic of the diode and reservoir cap written on the wood, but since all negative sides of the caps are grounded, it seems unlikely this is (still) correct

Obviously replacing all bad components is an option, but I rather first find out what is the likely root cause of this, and the damage would cascade. And probably redo the entire power section...

 

[bizzybone94]

Hello StingRay85,

while digging through the internet for some more information on the Super City Amps I stumbled upon your post here on this forum and therefore had to sign up.
I happen to have the same amp as you do and just finished the repair of it.

How is the repair of your amp going so far?

Like you I bought the amp as broken and when I powered it up it would blew the main fuses almost immediately. I then switched out all of the rectifier diodes (at least one of them was burned) and the filtercaps for some new 50+50 1000V caps and hurray the amp stayed on. Second problem I encountered was the blown fuse from the power amp. If I turned on the plate voltage both of the fuses would blew. Turned out the 12 Ohm resistors from cathode to ground which u also happen to have were burnt through so I switched them out too. In my understanding the 12 Ohms resistors are in there to allow easy measuring of the bias voltage. Well, the amp is running fine now! I had to replace the 330 Ohms half-power resistor u mentioned because of a broken leg, but all in all it could've been way worse.

My amp is running a very high plate voltage of 840V under load (started out with up to 950V!, I now use a transformer in front of the amp to bring down the input voltage to 220V AC) which may be the case for its clean headroom. I turned it up al the way to 10 and there was almost no overdrive. It actually could be my speakers who distorted because the amp is super loud! It definitely did not happen to have that sweet overdrive that the amp in the posted video shows.

The amp uses a full wave bridge rectifier circuit which I am thinking switching out for a half wave rectifier to bring down the voltage and hopefully get back some of that sweet break up (if my logic is correct). But right now I am happy as it is.

I will upload my take on the schematic of the poweramp and rectifier circuit later today. I started drawing the preamp section but it's rather complicated since I did not pull out the pcb and just have taken fotos of it and right now the schematic is not complete nor correct.

Id love to hear back from you, maybe we can help each other out with some infos, drawings etc.
Greetings from Germany!

 

[StingRay85]

If I understand correctly, the amp has a double rail HT, where the ground part of the1st bridge rectifier is connected to the HT part of the 2nd rectifier. This way you get these extreme high voltages. In my amp, the ground side of the first rectifier was already switched to ground, therefore the plate voltage must have dropped already to reasonable levels. I would probably recommend you to do the same in case you want be in the 400 V ranges, it seems less invasive than altering the rectifier itself.

I didn't do anything else on the amp so far, but you motivated me to look into it again

 

[bizzybone94]

That sounds right, maybe I should try that. So I would just eliminate the first rectifier and therefore cutting the voltage in half, right?
Here is my take on the schematic of the rectifier and the poweramp circuit. Like you said, the ground part of the first rectifier goes to HT of the second rectifier.
I had to export the files because of the size...

If you do look into it again please keep me updated on what's going on inside your amp !

Edit: In my poweramp schematic platevoltage is described with (A) and grid voltage is described with (B+) which is probably misleading. Same for the rectifier schematic.

 

[StingRay85]

Finally rewired the EL34 power section. Looks like the secondary of the PT is broken. Great design where the PT isn't fused...

 

[StingRay85]

Soooo... the PT was fused inside the selector switch, and fuse was gone... also, the double stand-by switch was broken. All very confusing, but at least now I know how to fix it

 

[bizzybone94]

thats great news! Yes, on my amp the main fuse was in the selector for the input voltage too.

 

[StingRay85]

At least no PT swap needed, luckily. One of these days I'll get it up and running with proper fusing.

 

[StingRay85]

So one of the two PT secondary windings is now solely dedicated to the plate voltage. The ground path of the bridge rectifier is fused. Second winding is for screens, PI and pre, also fused at the ground path of the rectifier. The stand-by switch interrupts both windings. Survived the LBL test and voltage readouts, all grounds are checked, so its ready for tubes

 

[bizzybone94]

what does the plate voltage read with only one of the secondary windings? Im thinking of switching for full power mode for the first time after getting my amp back up running...

 

[StingRay85]

460 approximately. I fired it up two days ago, but have a very loud hum through the speakers. not responsive to the volume control, but stops when V2 is pulled. So it's somewhere between V1 and V3. More problematic, I have heard arcs or loud pops coming from the amp, need to proceed carefully to find this issue first

 

[StingRay85]

Finally, the amp seems to be fixed. Biased the tubes to 30 mA. Cleaned out the pots and contacts, no clue where the arcing noise or loud pop came from, but it didn't happen since. It seems the artificial center tap on the heaters was also a problem. Both 100 ohm resistors measured infinite on the DMM. I used some alligator clips to mock up heater elevation using 2 100 ohm resistors for the center tap, and then a 100K / 10 K voltage divider. This provides around 45V DC. The amp was also surprisingly silent, even with volume full blast. Only the treble pot has still a tiny scratch. Time for some soldering and then I consider it job done.

 

[StingRay85]

Sounds really decent with bass! Added all 4 EL34, retensioned the preamp tube sockets and cleaned and deoxit all contacts.

 

[bizzybone94]

Awesome! Happy for you that its up and running. I had those arcing noise and pops at the beginning too and i believe its coming from the EL34s. But it went away after a while of running and seems to stay away... Not to sure why or what it is exactly. The matching cabinet ist super cool. Did you test daisy chaining the two channels? I think thats where the magic really happens Only the noise level is a bit higher with this configuration.