Looking for help setting the bias on my Scott 222B

[Onebean]

I just bought a Scott 222B and I would like to know how to check and set the bias on the power tubes. I found a post where Tinkerbell linked to the instructions on the Scott website. Here's what it says.

The Type 222B (also 6BQ5-based) featured an adjustable self-bias design.

To adjust a 222B's bias: place a DC voltmeter across resistor R-201, 1.2k-ohm, located across the preamp filament heater supply. (Note: Remove the bottom plate cover to gain access below the chassis). Adjust the Bias Control (located between the power transformer and electrolytic cap until the meter reads 38.5 volts DC.

So how do I locate R-201, and how do I connect my meter?

Is this telling me to clip my meter on the legs of the resistor? When I bias my Fender Brown Princeton clone, I'm pretty sure I clip my leads between a resistor and ground.

Is the bias control pot for each power tube accessed from the underside of the amp, or top side?

A picture would be worth a million words for me. Thanks for the help.

Onebean

 

[radioactive]

according to this picture theyre the ones ontop.

 

[Onebean]

Radioactive, is it the recessed pots? I see 2 of them, each one between 2 power tubes. Are the power tubed biased as a pair? Can you show me where to attach my meter?

 

[radioactive]

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Scott-222B-Service-Manual.pdf

 

[radioactive]

i scrached my head looking for r-201 but it dont exist .

 

[radioactive]

tje pots on the topside should be labled if the writing isnt gone.

 

[Onebean]

Yea, the writing is gone.

 

[drew_t]

http://akdatabase.com/AKview/displayimage.php?album=89&pos=14 (ignore the yellow mark to the right of the circle)

It’s a 7 watt resistor, so it shouldn’t be hard to spot. Post a photo of the guts if you’re still having trouble.

 

[Onebean]

Ok, on page one of the linked manual, it says to connect one lead of the DVM to point A, and the other to chassis ground. On page 4 ( the amp schematic) I found the "A" point, but it's very blurry, and I can't tell what components are around it. I'm not good at translating schematics to layouts either. Also, I only see one point "A". Should there be 4 places to set the bias? One for each tube???

 

[drew_t]

The PDF linked in my post above is a scan of official HH Scott documentation. I think what you’re calling the “linked manual” is a scan of a “SAMS” document. These were published by a third party company; they incorporate schematics that were drawn by their own personnel, often in a different style and using different component identification numbers, and sometimes not exactly correct. Here, R91 in the SAMS schematic is the same thing as R201 in the HH Scott schematic. (It’s shown as a 10 watt resistor in the SAMS, rather than seven.) Whether the one in your unit is a seven watt or a 10 watt, it shouldn’t be hard to find. It will be bigger than most of the other resistors.

The 222B does not have individual bias adjustment capacity for each power tube.

 

[Onebean]

Thanks Drew. I'll get the bottom plate off tonight and take a look. Does it matter which side of the resistor I clip onto for setting the bias?

 

[drew_t]

As you can see from the schematic, one side of the resistor goes to ground. You need to have one meter lead connected to the side that isn’t grounded, the other to either a convenient chassis ground point or the other side of the resistor.

I would suggest reading this before you start: http://www.aikenamps.com/index.php/safety-tips-for-working-on-tube-amplifiers

 

[Onebean]

Ok, here is a picture from the bottom side. I think I found the resistor I need to measure across to set the bias. Please take a look at the arrows I added to the picture, and let me know if I've got this right. I appears the adjustment pot is right next to the resistor too. My plan is to clip the leads of my meter onto either side of the resistor, power up the amp, set the bias, then power down and move to the other one. Then let the amp idle for 30 minutes and check it again. I'll have speakers hooked up while doing this. Let me know if I'm on the right track.

 

[drew_t]

I lose the detail if I try to zoom in too closely on the photo, but I think the resistor you’re looking for is the discolored rectangular cement one I have circled. And the adjustment pot is probably the one right above it.

 

[Onebean]

I'll take a close up shot of that area and post it.

 

[Dandy]

Check out this thread, for a link to the AK database including Scott manuals and explanation of the biasing procedure. http://audiokarma.org/forums/index.php?threads/scott-222b-topology-questions.564516/

See especially Dave Gillespie's explanation in post #14.

 

[Onebean]

Here are some close up pictures. Any help on where to clip my DVM leads, and which pots to adjust is greatly appreciated. I live in an area with no qualified techs that I know of. If I can't get this figured out and running correctly, it's going to have ot be shipped to someone. I'd prefer to do it myself, so I can service the amp down the road too.



Thanks for looking.

Onebean

 

[triode17]

Since this model uses a pot. to balance the cathode currents and unless you want to install 10 Ohm resistors in series with the cathodes, I would purchase two Bias Current Probes from eBay for EL84s (equiv to 6BQ5). Then, you simply use two meters set to milliamps and read each pair's current. Adjust the DC Balance control for equal current in each output tube, then adjust the main bias control R213 to set the output tube bias at 25 ma.each. This will provide a 70% Plate dissipation power at idle which will work well for this tube.

 

[drew_t]

Clip the leads where the red arrows point. Set meter to measure DC volts. Adjust the circled pot so that you get 38.5 volts on the meter.

 

[Onebean]

Thank you drew! So there's only one pot to adjust? The tubes are a mix of brands, so I doubt they are a matched set.