Scott 222C

I just went through and made sure my photos were in place. Stupid Photobucket!
 
Thanks for doing that Blast. It would be great if thefragger is able to restore his sometime.

I returned to this thread wondering about individually biasing the 7189s and dropping the line voltage a bit. Kegger mentions using CL80 thermistors. I wonder if you or others have used a bucking transformer instead?
 
I've done both.

I've made 3 bucking transformers for my two Scott 222C and 299A:

#1:

20100628 Bucking transformer #1a edit.jpg
#2:

20110122 Bucking transformer from Archer power strip and Hammond transfomer.JPG

#3:
20110604 Bucking transformer #3c from work edit.jpg
 
Thanks for the info and pics Blast. What is the rating or model number of that transformer?
 
You should take a look at this thread: http://audiokarma.org/forums/index.php?threads/bucking-transformer.294444/

To answer your question, for #1, above, I used a Radio Shack 120v to 6v (and, centertapped) filament transformer. The SPDT switch seen in the photo utilizes the center tap to provide either 6.3v or 3.15v drop depending which direction it is switched:

Bucking transformer schematic dual voltage.JPG

For #2 and #3, above, I used the Hammond 166N6 wired solely at 6.3v drop thusly:

Bucking transformer schematic edit.jpg

Two outlets on one side of the transformer are at full line voltage and the two on the OTHER side of the transformer are wired with the 6.3v buck.

Hope that helps!
 
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Thanks for doing that Blast. It would be great if thefragger is able to restore his sometime.

I returned to this thread wondering about individually biasing the 7189s and dropping the line voltage a bit. Kegger mentions using CL80 thermistors. I wonder if you or others have used a bucking transformer instead?
I had just rebuilt a 222C and was facing the problem of the too high B+ voltage. I decided to install a choke (6H, 150 ohm) in a CLC , power supply. I also completely rebuilt the power supply and replaced all the resistors in the voltage divider network. Since they were all carbon comp almost all were out of spec on the high side which meant that there was more resistance and higher overall voltages. I believe i also added a single cl80 which put the anode voltage and G2 voltage at only 10vdc over specs.
I did not decide to install individual bias adjustments for the power tubes mostly because of a lack of space. In my stock unit. even installing 10 ohm current measuring resistors would require a lot of work. You would need to install a mounting lug for the resistor and removing the the input grid to ground resistor to a new location since that resistor is installed on the cathode since the cathode is grounded. So all that to be sorted before working on the bias circuit.
 
Thanks for the input. My problem is that I'm dropping 240+v here down to either 104v (Japanese voltage) or to 120v. The former takes the filament voltage down to 5.6v and the latter results in vol ages that are way too high, esp for my EL84s (!) that are seeing 250-260v on the plates.

But I have a 120v + 120v to 6v + 6v transformer similar in size to your RadioShack one Blast, so I'm good to go. It's just a nuisance to have to add two trafos before the PT.

Edit: I've just added the bucking arrangement, and using both 6v secondaries In series it drops the mains voltage right into the zone from 120v. Wierdly, there's a tiny bit of hum that wasn't there before, but I'm much happier not to be over- or under-running it. Thanks for the reminder of how to do this.
 
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I may not have thought this question out very well but, here goes. What would happen if you buck your 240vac by 6 volts first? And, then use your 240 to 120vac?
 
Yeah, I wondered about that too. I went the way I did to keep the bucking arrangement with the amp, so that I can plug it into any 120v source. I've got just the right drop with 12v bucked, B+ 426v and 6BQ5 plates 420v. Screen grids are a bit high at 364v.
 
Just another update on my Scott 222C. After restoring it, I wondered why it worked and measured fine, but the volume was lower than I would expect. Once or twice when I turned the loudness or balance controls, it went much louder for a second, but I couldn't replicate that. Yesterday it coughed suddenly and has been LOUDER ever since. :) Any thoughts on the cause? Problem pot?
 
I just bought this and its on its way!

:banana:

It needs work, to say the least! I'm pretty excited!! I just have to sell a couple pieces before this one comes home and I'm golden, to keep the audio equilibrium happy in my house.


I'd like to start researching what needs to be done to this amp. Does anyone here have any good H.H. Scott resources that they'd like to share?


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I have it's brother the LK-48. Same mondo big output trannies for little EL84s. It's got balls! try the Russian 6P14P-EV for outputs. They warm up a bit slowly but work hard.
 
I was thinking of modding it to run EL84 and putting in some 6P14P-EV tubes. I haven't heard of the ECF80 tubes before tonight, but I've got my studies to tend to!
Can't use the EL84 because it won't handle the B+ voltage or screen voltage as they are too high, that's why Scott spec'd. in the 7189. Unless you are going to knock down the high voltage somehow and create a lower power amp. What mod. are you considering?
 
Wow Blast, your amp looks awesome underneath! I should have mine by Monday.

Aside from the links that Tinkerbelle posted earlier I haven't looked around that site much-- I'll be sure to join up there when I really get stumped, though!

I suggested the four bias pots as an alternative to having a matched quad of tubes, so that I could balance them out myself. (unless I'm completely wrong in my understanding how bias pots work, I still have some reading to do!)

I've got some MUR860 (8A, 600V ultrafast rectifiers from ONSemi) kicking around here that I could build a bridge out of, maybe overkill? I was thinking all-new metal film resistors as well. Advantages/disadvantages of metal film vs. carbon comp?
You don't need FOUR bias pots, only two. The amp. comes with DC Balance controls for each pair of output tubes. Set the Bias, Balance, check the bias, maybe reset it a bit, check the Balance until they are all the same. You will need to add four low value (I used 10 ohms) resistors to measure the cathode current of each tube. Plus, don't forget to subtract about 5% for the screen current to calculate the plate dissipation.
 
Can't use the EL84 because it won't handle the B+ voltage or screen voltage as they are too high, that's why Scott spec'd. in the 7189. Unless you are going to knock down the high voltage somehow and create a lower power amp. What mod. are you considering?
My EL84s handle the voltages okay, because the tubes are biased conservatively. Dave G explained this a while back when I first restored my 222C.
 
They are a mix of Russian tubes. Three are NOS Zaerix branded and marked 'Made in the USSR' and one is a 6P14P. None are -EV.

The amp came with some old Mullard EL84s that seemed to be still okay. (I don't have a tube tester, but tried known good tubes because of the lower volume issue I mentioned earlier.)
 
My EL84s handle the voltages okay, because the tubes are biased conservatively. Dave G explained this a while back when I first restored my 222C.

Love my 222c!

At what range do you bias your tubes at? I've been told two different things from two different techs. One said .220-.250. The other said 200. I bias mine at 200.

I use the Sovtek EL84M or the 7189 Preferred Tube. Both work great in my 222c.
 
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