SSS#002 low voltage in original drawing

[martin manning]

Looking good! I like the pin plugs for the preamp tube socket connections. Another possible solution would be to use sockets that mount from inside the chassis, and solder nuts to the bottom of the socket flanges so the mounting screws could be removed from the top.

[larsvictor]

Thanks, Martin. I was looking for a solution that allows me to remove the boards without having to many connections to desolder.

[CHIP]

Looks good. I like your ideas on how to pull those boards if need be.

[dorrisant]

What are the specs on those pins and plugs? The look to be silver coated...

[larsvictor]

Yes, the pins are silver-coated, the plugs are tin-coated. The connection is as tight as the one between the tube pins and socket plugs.

Lars

[lovetone]

What a great idea The one I repaired was very difficult to work on, that amp had the tube sockets hard wired to the PCB with single conductor wire.

There was an open circuit track on one PCB which I had to bridge out on the top side. I would liked to have removed the PCB to take a look at the track side but I would have been a lot of work.

Where did you pick the pin and sockets from?

Just for clarification I did not build this amplifier I had it in for repair.

[larsvictor]

Thanks lovetone, I bought them in a Conrad store. This is an electronics store in Germany like Mouser. They do have an online shop too.

[larsvictor]

Update. Soldered the tonestack, filter wiring and the other pots. Bias and power tubes cathodes. It´s time to order some tubes...

BTW this thread is becoming a typical "build thread" more and more. Is there someone who may move it to the discussion part?

Greetings
Lars

[larsvictor]

Hello guys,

Update. The amp is running. Sounds good exept for 2 problems:

Reverb: Almost no reverb with send and return pot turned fully up. If I hit the tank I can hear the typical noise quite loud. So it seems that the output transducer and recovery gain stage is working, but somehow the send circuit or the input transducer doesn´t. It´s an old accutronic tank that I bought on ebay for almost nothing and maybe it could be damaged. Voltages of the send and return tube are ok. I´m not experienced in reverb circuits at all. This is my firs amp with reverb, so help is appreciated.

FET: The FET input has less volume than the normal input and if I crank it, I have heavy ugly distortion. Maybe it´s just a voltage problem. Somehow the voltage is too low. B+7 is 361 VDC. The voltage divider is a 220k and 8.2k to ground. The voltage between the resistors is only 8 VDC. I´m afraid thats not enough for the FET, in my #102 it´s about 18VDC.


I´m posting the complete voltages and my schematics because this thread is still in the file section and maybe someone can use it for his project.

Voltages:
Wall: 232 VAC
HV: 345 VAC
Heaters: 6.79 VAC
Bias supply: -314 VDC (extra PT)

B+1 (PT PLates) 458 VDC
B+2 (Screen resist. & PT driver plates) 457 VDC
B+3 (Reverb driver) 381 VDC
B+4 (Reverb send and return) 428 VDC
B+5 (PI) 366 VDC
B+6 ("Mixer") 363 VDC
B+7 (V1 and FET) 361 VDC

V8-11
Grids: -48 VDC
Screens 457 VDC

V7 (PT Driver)
Grids: -49 VDC
Cathodes: -50 VDC

V6 (PI)
pin 1: 277 VDC (input)
pin 3/8: 47 VDC
pin 6: 285 VDC
tail: (4 resistors) 45 VDC

V5 (Reverb driver)
pin 1/6: 378 VDC
pin 3/8: 7.5 VDC

V4 (send and return)
pin 1: 240 VDC (send)
pin 3: 1.9 VDC (send)
pin 6: 244 VDC (return)
pin 8: 1.9 VDC (return)

V3 (Mixer Reverb)
pin 1/7: 189 VDC
pin 3: 1.57 VDC
pin 6: 363 VDC
pin 8: 209 VDC

V2 (Mixer Filter)
pin 1/7: 216 VDC
pin 3: 1.48 VDC
pin 6: 363 VDC
pin 8: 218 VDC

V1
pin 1: 249 VDC (1st gain stage)
pin 3: 1.79 VDC
pin 6 :271 VDC (2nd gain stage)
pin 8: 2.2 VDC

Thanks
Lars

[martin manning]

Definitely increase the supply voltage for the FET. Reduce the 220k and/or increase the 8k2. But first set the bias point on the FET. The best way to avoid chasing your tail is to disconnect the HV, apply 18-20VDC to the top of the drain resistor Rd, and adjust the source resistor Rs so that you drop half of that voltage across Rd. Having found the correct value for Rs, reconnect HV and adjust the divider to get 18-20VDC. There is a sticky in the files section on this.

[larsvictor]

Thanks Martin.

Yes, 220k/8k2 with B+7 361VDC was a bad calculation for the voltage divider. Of course I remember your instruction for biasing the FET circuit with the 2 9V batteries! I´ll do that first. Then I´ll stick a 470k in parallel to the 220k.

The reverb issue bugs me even more. I grounded the high/low filter circuit and there was no signal through the reverb part of the amp at all. When I patched the reverb send and return jacks there´s a signal, that can be altered by turning the send and return potentiometer. So I think, the send/return circuit in the amp is ok. Is there a way to test the transducer in the tank ? It´s an old 4AB3C1B Accutronic. Input DC-resistance is less than 1 Ohm. The return transducer seems to work - when I hit the tank I hear the typical noise.

Lars

[martin manning]

Only one ohm input resistance seems like it's shorted. Maybe give it a good look with magnification? Otherwise I'd try substituting a different reverb unit. I agree the output transducer seems ok since you can get it to produce a signal.

[larsvictor]

Input impedance of the unit is 8 Ohm - I thought ~1 Ohm DC resistance would be ok, but that seems to be wrong...

Unfortunately I haven´t a substitute. I have to order one.

[larsvictor]

Ok. Fixed all the problems.

Reverb problem was just a bad cable with missing ground connection to the input transducer. For the FET I followed the instructions from Martin Manning. Increased the voltages, fixed a bad solder joint at the source resistor an replaced the source bypass cap with a 22µF. Now everything is working. Beautiful amp, very brilliant with a "floating" crystal clear sound.

Next step is the cabinet. It´s going to be a 1x12" combo loaded with a EVM12L speaker. Ruby suede, Oxford blood grill cloth.
I´ll post some pictures and sound files.

[martin manning]

Great! Different models of reverb units have different arrangements re their ground connections, some carry through and some don't... Should have thought of that, but here you had a cable that was not providing a ground return?

Thanks for posting the schematics, by the way!