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Scott 222C


John Warren

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Another vintage classic for service.   Will document in the page below for those of you interested in this sort of thing.  

 

The interior wiring appears to be factory virgin, i.e. no one was ever in there.  It's a wee bit dirty and the multi caps, the selenium rectifier are shot so first thing is to get the power supply proper.  The power supply transformer is good.  

 

Scott 222C (ipage.com)

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The service is going along well! I found mine had a bad 8k ohm standee resistor. I was able to get a replacement brown devil inside, somewhat out of the way. Voltages are within spec now, although I think the output tubes took a pretty big hit. Mind posting your voltages?

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I changed the 1.2K series resistor (screen grid voltage, with the 8K/20w shunt resistor) for 3.3K, lowered my screens to ~315v.  The stock ~360v made the JJ EL84 have a 'harsh/busy' kind of sound, now they're smooth.

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Good to know. I added a CL-80 on the AC in and mine is still 10-15 volts DC over, in most places.  I am pretty sure my Telefunken output tubes are close to dead. One tube was red plating when I got it. After replacing all the coupling caps the amp still sounds pretty distorted. Ordered a quad of Russian 6p14p-ev from Ukraine. Not sure how long it's going to take to get here.

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10 hours ago, nbndtrain said:

Good to know. I added a CL-80 on the AC in and mine is still 10-15 volts DC over, in most places.  I am pretty sure my Telefunken output tubes are close to dead. One tube was red plating when I got it. After replacing all the coupling caps the amp still sounds pretty distorted. Ordered a quad of Russian 6p14p-ev from Ukraine. Not sure how long it's going to take to get here.

The first place to start, assuming the power supply transformer is ok. is to pull all the tubes and go after the three electrolytic multicaps, all three need to be replaced with either replacements (Hayseed Hamfest does a nice job) or discrete assembilies under the chassis.  Don't bother "reforming" or some other foolish thing.  One must first get all the DC rails up to snuff.  Replace all the carbons with ceramic compositions then get rid of the selenium rectifier.    These amplifiers are very straightforward to service but can also be a ticket to an early grave so ground your chassis and check the fuse first before you go in.

 

Updated link.

  

Scott 222C (ipage.com)

 

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^^^ This first.  Electrolytics, selenium rectifier, clean & tension sockets.   Don't worry too much about voltages being 'over' as long as the heaters are 7v or less. 

I'll look up what I did to make the bias voltage variable.  I think it was a couple of 10-turn precision resistors, will check the value.

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Thanks to both of you. I am just a hobbyist, so appreciate all your input. I've replaced the selenium rectifier, bumped up the first dropping resistor to 33uf and replaced all the coupling caps. Also cleaned all the switches and sockets with D5, and lubed the pots. It's running fine now, but sounds like crap. I have caps to do the power supply cans, but have not done them yet as voltages are within spec, just a bit high with current home AC voltage.

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On the rectifier replacement, the 12AX7 filament string should read ~ -72VDC without tubes and with all four plugged in drop to -55VDC.  If one tube socket is dodgy the whole string is out of commission and none will light, they're in series.  It's not uncommon to have the original 12AX7s in these amps, they last forever.  They were shipped with Telefunken.  

 

19 hours ago, nbndtrain said:

 It's running fine now, but sounds like crap. 

 

That got a chuckle out of me!

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On 5/26/2023 at 9:30 AM, John Warren said:

On the rectifier replacement, the 12AX7 filament string should read ~ -72VDC without tubes and with all four plugged in drop to -55VDC.

Where are you measuring? At the rectifier, or the first 12ax7? I am getting -48v after the first dropping resistor, which was swapped with a 33ohm resistor as you did.

 

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Found pix of my rectifier and bias board.  I don't remember what size the 10-turn pots are, but they work well.  somewhere between 10K and 50K, I'd guess.

the 10uF IC cap is on the screen grid line, which I lowered to ~315v or less by substituting the 1.2K series resistor with a 3.3K.    Every 6BQ5 or EL84 datasheet I've seen suggests 24W through an 8K P-P transformer with 400v on the plates and 300v on the screens; screen grid V is the more important of the two.

222C Bias.jpg

222C Bias2.jpg

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22 hours ago, John Warren said:

Try EL84M

Probably not a bad idea, but I had to order some more stuff from Eurotubes anyways.  The JJ's do sound great with those voltages.  They're not magic like the vintage RCA or Telefunken, but definitely livable for a broke-arse honky like me. 😀

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Original output tubes on mine were definitely completely toasted. While I waited for a matched quad of 6p14p-ev tubes, I replaced all the multi-section power supply caps with individual electrolytics under the chassis. Even after that, the amp sounded like crap. The new output tubes made all the difference and it now sounds fantastic! I had to play around with the phase inverter tubes, but found a set that really sounds great. All original Telefunken phono and preamp tubes sound great! I have always missed the HH Scott LK-72B I sold, and will be enjoying the 222c for a long time.

HH Scott 222c.jpg

Edited by nbndtrain
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5 minutes ago, nbndtrain said:

Original output tubes on mine were definitely completely toasted. While I waited for a matched quad of 6p14p-ev tubes, I replaced all the multi-section power supply caps with individual electrolytics under the chassis. Even after that, the amp sounded like crap. The new output tubes made all the difference and it now sounds fantastic! I had to play around with the phase inverter tubes, but found a set that really sounds great. All original Telefunken phono and preamp tubes sound great! I have always missed the HH Scott LK-72B I sold, and will be enjoying the 222c for a long time.

 

nice job

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