Scott 222C

Hiya Lou, they're connected to 10ohm resistors that I installed a few posts back so that I can measure the current and adjust the bias of the tubes without opening the amp up!

I got the tip jacks on eBay--fantastic build quality, all the nuts are machined! Bought some silver-plated banana jacks for $cheap from the same buyer, all made in USA!
 
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This is an interesting thread. I'm working on an early model 299D and a 7189 model 299 now. I could use some help locating a source of supply for those (radial leaded?) ceramic power resistors that stick up through the chassis. The Manufacturer was Standee and they are no longer available. The one I need is the 1200R_10W. Any help would be greatly appreciated.
 
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The Manufacturer was Standee and they are no longer available. The one I need is the 1200R_10W

Here is a better question:

What is really odd about this resistor is how greatly oversized it appears to be for the application. The voltage drop per the drawing is about 16.4 volts for a current of 14 ma (16.4/1200). Thats less than a quarter-watt. I wonder why Scott saw the need to design for a 10 Watt resistor above the chassis in this light duty application? It must be for some upset condition. Any thoughts?
 
Here is a better question:

What is really odd about this resistor is how greatly oversized it appears to be for the application. The voltage drop per the drawing is about 16.4 volts for a current of 14 ma (16.4/1200). Thats less than a quarter-watt. I wonder why Scott saw the need to design for a 10 Watt resistor above the chassis in this light duty application? It must be for some upset condition. Any thoughts?

This is a big guess...But IIRC, the standee resistor is between the 5AR4 rectifier tube and the first capacitor. My guess for the resistor's use is for the pulsing of either DC or residual AC from the rectifier tube.
These pulses can amount to quite of bit of wattage.
The value for the resistor is usually low, and doesn't drop much voltage.
These pulsing peaks may be the reason why the resistor is high wattage, and they do dissipate some heat. The R soaks up the pulsing peaks.

These are total guesses on my part...I may be part right, or all wrong...
 
That's an interesting idea. Are you saying that more heat is evolved than predicted by ohms law? It's common to add a 100R resistor to older amps between the rectifier and the first cap of the CRC to account for the higher mains voltages on vintage EL-84s which can run close to the plate voltage limit. I can't recall any of those resistors operating at excessively warmer temps than expected.

Another thought that occurs to me is that when a ceramic resistor operates at it's full rated capacity, it's probably hundreds of degrees above 140F (where burns begin to occur). By de-rating the resistors 20:1 they have much greater surface area and can therefore operate at cooler temps while dissipating the required heat; Scott may have intended to keep temperatures down to a safe temperature for exposed components. (those resistors may feel hot to the touch but they can have much greater max operating temps)

The resistor I happen to need is actually the third resistor. The first you speak of is 20R, the second 270R and the 1200R. The same theory applies to all three exposed resistors though.
 
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I would thing those are practically unobtainium...

You could replace with an equivalent ceramic resistor with leads going through the chassis (with heat shrink insulation obviously).
 
I was told at one time that those stand-up resistors are bleeder resistors to bleed off the voltage stored in the caps after the unit has been turned off. Don't know if that's the whole idea of them being there, but that's what I was told.

Larry
 
I was told at one time that those stand-up resistors are bleeder resistors to bleed off the voltage stored in the caps after the unit has been turned off. Don't know if that's the whole idea of them being there, but that's what I was told.

Larry

Wouldn't a bleeder resistor go to ground to drain the capacitor it's hooked up to?

I just gazed at the schematic for the LK48B. And the first resistor,(20 ohms, 10W) comes off pin 8 from the rectifier to the first cap. The rest are dropping resistors for decoupling of other sections of the circuit...none go to ground.


That's an interesting idea. Are you saying that more heat is evolved than predicted by ohms law? It's common to add a 100R resistor to older amps between the rectifier and the first cap of the CRC to account for the higher mains voltages on vintage EL-84s which can run close to the plate voltage limit. I can't recall any of those resistors operating at excessively warmer temps than expected.

Well, I can't defy Ohms law...like I said, I'm guessing.

I rebuilt a Altec 345A PP EL34 stereo amp, and was dealing with trying to beat down the PS DC voltage like most vintage tube amps.
The Altec amp uses a pair 5U4s for rectifiers, with a 20 ohm 20W Standee type resistor between the rectifiers and the first capacitor.

I figured I could drop some voltage with the first resistor, and had a Standee type 70 ohm 25 watt resistor. The 20 ohm R dropped about 3-5 volts...not much. The 70 ohm only dropped 10 volts. So it isn't a voltage dropping resistor.

It was still a lot less voltage drop than I wanted.

I emailed a friend and ask about using a higher value resistor, and guessed to what it's actual purpose is.

My guess: The 20 ohm 20W resistor sops up some of the inrush current from the directly heated rectifier tubes to help the first capacitor.

His reply: Or more likely to limit the repetitive peak charging current to within the tube's rating. And went to mention using a higher value resistor will lower voltage and decrease the PS regulation.

Also to make sure when using a larger value resistor, that it is well within it's dissipation limits. If below the dissipation rating, it could be a fire hazard.

Other than that, I'm over my head...
 
I would thing those are practically unobtainium...

You could replace with an equivalent ceramic resistor with leads going through the chassis (with heat shrink insulation obviously).

I think you are correct about unobtainium. But I just can't see putting the replacement resistor above the chassis, it's just too high a voltage. That stuff belongs tucked away in the chassis IMO. Who's to say how long the spaghetti or heat shrink will hold up? I'd be afraid of a cat fire or something of that ilk.

So, I mounted a tag strip to the same hole in which the resistor clamp mounting screw was, but under the chassis, and placed a 3 watt power resistor in the tag strip (cause that's all I had handy). When I finish with the bias rectification and coupling caps, I'll fire it up and see how hot the resistor gets and check to see if my less than half-a-watt calculation holds true in practice.
 
This seems to be the only current HH Scott thread going now, but I don't want to threadjack it any more than I already have.

So, if any of you HH Scott rebuilders need bottom panels for your amps, look for my thread on the prospect for a group buy on some repro bottom panels.
 
I have one going as well it's the " Scott LK72 Rebuild" Thread, and I just got off
ebay a bottom panel for $35.00. which doesn't seem to bad a price.
 
I have one going as well it's the " Scott LK72 Rebuild" Thread, and I just got off
ebay a bottom panel for $35.00. which doesn't seem to bad a price.

Nice score. Can you post or send me a photo? I'd love to see what the original looked like!
 
Have any of you connected a speaker to the center channel taps of your Scott gear? You should check it out.

You probably understand how it works. It's pretty damn cleaver.

They obviously wanted a center channel output, like in HT today. Except, in what 1958? Because there was no source material available with center channel programming, they created a center channel by lifting the ground from the zero taps and placing it on the 4R tap. Then they take a lead from the right channel zero tap and the left channel 16R tap (both of these taps are live now). If programming is present on both channels, then it is "seen" across those taps and 3db higher than the rest of the programming (because it's across 16Rs played on 8R speakers I assume here). If the programming is only on one channel, then it's down 3 dbs. The result is obvious, that programming recorded on both channels, like vocals, is presented boldly on the center channel and programming like horns or guitars recorded only on one channel is not really heard on the center. It's a pretty damn clever idea and the effect is really not bad. I can't recall ever seeing one of these in use with the center channel, then or now.
 
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Using the handy-dandy tip jacks my bias is still where I set it, aside from one tube being down by 2mA; 23/21/23/23. I'll have to fix that! I love how easy this is now, compared to before. :) I owe you big, Craig!

Very interesting, I'll have to hook u pa centre channel and see what I get!
 
Well shoot, the number of tubes that will work in place of the 6U8 position (phase inverter) is huge! I nabbed a sleeve of Raytheon Japan 6GH8A tubes, a pair of Amperex Holland Orange Globe 6BL8 tubes, and four Admiral France (+2 USA) E802CC tubes all for less than the price of my fave 12 pack!

Rolling around in this position definately has an effect. My ears are in love with the Amperex tubes right now, there's more dimesion than the Raytheons and are quieter than the black plate RCAs. The Admiral tubes are in the mail.

My ears are really loving these Amperex tubes! Sounding better the warmer they get!!

I also have eight NOS 6P14P-EV + eight NOS 6P14P tubes in the mail.

This is madness.
 
Amperex 6BL8 is defiantly one of my favorites, probably best of the bunch of
that particular pin-out tube that I have used, there are quite a few European
brands of the 6BL8, to me they do tend to be better sounding then the 6GH8.

That tube is not only the phase splitter here but the voltage amp as well, an
will make a pretty good impact on the sound of the amp.

To see how well they match in circuit, measure plate voltages of the pentode
section on each one, as that sets the bias for the triode section as well there.
(tube tester won't tell yu much, need to measure within amp an while running)
EDIT: I still use the tube tester first and weed out any truly bad ones though.
 
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