Scott 222C

[thefragger]

Well, my hand-draw schematic has the caps as 40-40-20-20, which actually is 40-20-20-20; my mistake!

Here are the top, bottom, and serial number photos of my amp;

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[tube-a-lou]

You did a great job job on it, I decided to get the JJ's Caps for the LK72 I'm redoing,
they look pretty good on there but I saw a thread were a fellow AKer used Epoxy
to attached them to the wafer boards. I'll just see how it goes. So how does it
Sound!

tube

 

[thefragger]

Tube-a-lou! No music yet, but soon!

Craig; measuring from pin2 on a loaded 7189 socket reads -16VDC at 110VAC and -18VDC at 120VAC. How does that sound?

 

[NOSValves]

Thanks Craig, I'll rewire up the can cap as you've noted and I'll get those photos up later this evening!

Rewiring that filter can is not a big deal I just included that so you know where the available filtering is best utilized. Changing those location will have no effect on these voltages. Forgot to mention this!

 

[NOSValves]

Tube-a-lou! No music yet, but soon!

Craig; measuring from pin2 on a loaded 7189 socket reads -16VDC at 110VAC and -18VDC at 120VAC. How does that sound?

2 questions.

What are the value of the 4 dropping resistors on the heater bias supply R/C network?

With the amp fully Loaded with all tubes and input voltage at 117 VAC what is the negative heater voltage after the 4 dropping resistors you installed (this is the connection where this voltage heads to the 12AX7's heaters and usually the output tube DC balance pots)

 

[thefragger]

2 questions.

What are the value of the 4 dropping resistors on the heater bias supply R/C network?

I was told to use this combination to get the correct voltage due to the higher output of the silicon rectifier: 33ohm/5W, 20ohm/2W, 20ohm/2W, 20ohm/2W, with four 100uF/100V caps.

With the amp fully Loaded with all tubes and input voltage at 117 VAC what is the negative heater voltage after the 4 dropping resistors you installed (this is the connection where this voltage heads to the 12AX7's heaters and usually the output tube DC balance pots)

Alrighty, I broke out my 'good' set of tubes (ones I want to run the amp with) and loaded 'er up; a 6P14P-EV set for the output, two 6GH8 tubes for the phase inverters, and four 12AX7A tubes. I've got a photo of the amp loaded and two measurements taken from after the dropping resistors and at pin 2 of one of the output tubes. 8 ohm speakers are hooked up, but no music or sound is playing through them.

I blew the 1A fast-blo fuse in my VARIAC when I started it up at 117V rather than ramping it up slowly. Lesson learned!

All Loaded Up

Current Draw, Supply Voltage, Filament Supply

Current Draw, Supply Voltage, Output Tube Pin 2

​

I think the current draws vary because of the warm-up time differences.

 

[NOSValves]

First off if the amp is fired up AT 117 VAC supplied a 1 amp fuse will blow every time. Tube amps have a huge inrush current when first fired up nearing 3 amps. That is why they spec a 2.5 amp slow blow. So the 1 amp blowing is of no concern.


I see you have a decent current meter! This will be helpful.

Step one.

Remove the 5AR4 rectifier. Fire up the amp at 117 VAC. Have a meter setup to read DC volts. Attach the red lead to one pin 2 of a 7189 and the black to the mating 7189 pins 2 and adjust the DC balance control for minimum or zero volts on each pair of tubes. This will insure both tubes are receiving identical negative grid voltage. Now measure all 4 pin 2's and report that back in the next post.

Step two

Shut the amp off and let the tubes cool down. Install the 5AR4 rectifier. Turn the amp on and let it warm up for about 5 minutes. Now set that meter up to read current. Attach the red probe to the center taps of the output transformers (red cloth leads) and then the black probe to the plate lead of an output tube (pin 7 with cloth lead from output transformer). Now go back and forth on each tube for that channels pin 7 and adjust the balance control until the current registered on the meter is identical and then write it down. Repeat above for the other channel and write it down. The above is easier and safer to accomplish if you have alligator clip attachments for your meter.

The meter will not be accurate this way for total current but the it will give me some idea where your at in respect to bias current and it will fairly closely balance the tubes in each channel. What the meter will be doing is shunting out the current be supplied to the tube via the output transformer but since the meter has some resistance built into it some current will still be flowing through the output transformer. So the actual number is not accurate in most cases adding about 10% is a safe bet.

Report back what all 4 tubes measure for current draw.

BE VERY CAREFUL SINCE YOUR DEALING WITH HIGH VOLTAGE! ALSO BE CAREFUL NOT TO TOUCH GROUND WITH THE BLACK LEAD YOU WILL GET A SPARK SHOW AND POSSIBLY DAMAGE YOUR METER

 

[thefragger]

Wow Craig, thank you for that post!!

Yea, that Motomaster meter's alright--I bought it to accompany my Fluke 87, but then I misplaced the Fluke. I might need to nab a couple more of those Motomaster ones, they can be got for $20 when they go on sale from $60+ regular. But I was reading in the Scott manual that I could use my oscilloscope for some sort of calibration... would that be a possibility for accuracies' sake?

Anyways, I zero'd the voltage between each pair of tubes and the pin2-relative-to-ground readings are as follows;

tube 1: -16.70VDC
tube 2: -16.86VDC
tube 3: -16.74VDC
tube 4: -16.85VDC

Looking at the front of the amp, tube one is on the far left and tube four in on the far right of the quartet.

Step two had me puzzled--not in the method, but in my results. All four output tubes registered 0.000A between pin 7 and the OPT C/T. It should be noted there were speakers tied to the 8ohm and ground taps on the other side of the OPT. Here's what I did;

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[NOSValves]

I bet you have the meter reading AC current. Set it up to read DC current. Because the best I can see you had it attached properly.

 

[NOSValves]

Be very careful not to short the probe to any of those naked cap and resistor leads near Pin 7!! This is very important you could fry the meter or a tube in short order since they will get slammed with maximum current from the B+ power supply.

 

[thefragger]

I bet you have the meter reading AC current. Set it up to read DC current. Because the best I can see you had it attached properly.

I had it set up for DC amps and thought the same thing when I was reading zero, so I tried it with AC amps and kept switching back and forth between the two and checking my connections. I'll make a pot of coffee and go back to it later. :scratch2:

Thank you so much Craig for all your help--it's really very very much appreciated!! :yes: :thmbsp:

 

[NOSValves]

It looks like the meter could be auto sensing in respect AC/DC current. I see in the picture above you have it set for 12 amp. Switch it down one notch to the mA setting. The reading should be in the 15 to 30 mA range so at the 12 Amp scale the meter possibly registers it at zero.

 

[NOSValves]

Misplaced a fluke meter.... That's it I'm done helping you :nono: :yes:

 

[thefragger]

Be very careful not to short the probe to any of those naked cap and resistor leads near Pin 7!! This is very important you could fry the meter or a tube in short order since they will get slammed with maximum current from the B+ power supply.

Yea, I made sure that the little covers on my alligator leads were covers as much as the clips as possible.

It looks like the meter could be auto sensing in respect AC/DC current. I see in the picture above you have it set for 12 amp. Switch it down one notch to the mA setting. The reading should be in the 15 to 30 mA range so at the 12 Amp scale the meter possibly registers it at zero.

I thought about that, too. I have one meter that autoranges and another that I notched from 200mA right down to 200uA, getting zeros across the board. :scratch2:

It should be noted I have a pair of 50ohm / 50W 1% Ohmite resistors I could use as loads if needed to perform tests anywhere.

How do the negative voltages for the tubes look?

Misplaced a fluke meter.... That's it I'm done helping you :nono: :yes:

I'm sorry! It'll turn up. I stopped using it because the 9V battery died and I had none on hand... then it disappeared... :thumbsdn:

 

[NOSValves]

Can you actually lock one of these meters into reading DC current in the mA range?

The meter above states auto at the top of the screen. Something is screwy with those cheap meters. No way are the tube conducting nothing. I suspect the current is low though and this is what is causing some of the raised voltages you are experiencing. Although the reading you had were not all that bad once the amp was loaded up with tubes.

 

[thefragger]

Right, so while I was diddling my meters' ranges, I completely forgot that I had to move the probe to a different socket since the 10A setting has its own plug. Oops! The reading I have with the balanced voltages from above are as follows;

tube 1: 13.10mA
tube 2: 15.80mA
tube 3: 7.14mA
tube 4: 13.23mA

And after balancing, the two pairs read in at;

tube 1: 14.27mA
tube 2: 14.24mA
tube 3: 9.55mA
tube 4: 9.52mA

How does all that look to you?


Ah, coffee.

 

[NOSValves]

Bingo! That is the problem at least with your high voltages!! You need to bring the bias voltage applied to the grids (pin 2) down (less negative) until you get in the mid 20 mA range.

First record the voltages at pins 2 of all for tubes for me now that you have them balanced. It looks like this quad of tubes are not very well matched in fact pretty poorly matched. But we may be able to swap them around and get the channels to be a bit closer...

The real way to fix this is to add a couple pots before the balance controls so you can bias them up and down not just balance adjustment. Also a 10 Ohm resistor from ground to each cathode would make measuring the bias much easier. But with a 222C it means moving many resistors that connect to the cathode pin on the tube sockets for the ground point.

 

[thefragger]

Bingo! That is the problem at least with your high voltages!! You need to bring the bias voltage applied to the grids (pin 2) down (less negative) until you get in the mid 20 mA range.

So those negative voltages I posted earlier (-16.7ishVDC) I want to make less negative (eg. -14VDC; closer to zero volts), in order to get the tubes to draw more current, up into the 20mA range?

First record the voltages at pins 2 of all for tubes for me now that you have them balanced. It looks like this quad of tubes are not very well matched in fact pretty poorly matched. But we may be able to swap them around and get the channels to be a bit closer...

Well, it's not an actual quad, they're just four of the strongest tubes I have from my 6P14P-EV stock. I'm working on modding my tube tester so I can read the plate current while testing in order to do some basic matching.

I'll get you those voltages in my next post.

The real way to fix this is to add a couple pots before the balance controls so you can bias them up and down not just balance adjustment. Also a 10 Ohm resistor from ground to each cathode would make measuring the bias much easier. But with a 222C it means moving many resistors that connect to the cathode pin on the tube sockets for the ground point.

I thought about adding multi-turn potentiometers (cermet?) in order to bias each tube independently, but had no idea how to implement it, or was told that the balance controls were 'good enough.' I'll see what I can do about putting in those resistors.

 

[NOSValves]

What tube tester are we talking about?


It's well worth either adding a control in front of the two balance controls or 4- one for each tube. Easy to implement. Scott built most of there higher end amps with the controls, 2 bias adjust and 2 balance adjust. But they never installed test points to measure the bias and balance. I believe it was because technicians were readily available and even made house calls back in those days.

It's also worth adding 10 Ohm cathode resistors so you can easily measure the bias.

Dig up the schematic on the 299B and you have the layout for the controls. You will have to fudge a few resistor values to get the range of adjustment you need. Basically you want -12 to -20. Do all the fudging with the 5AR4 out of the amp but all other tubes present.

 

[thefragger]

It's an Eico 667--I just made the modifications that let me read the plate current while the tube is under test (MERIT lever actuated), so I thought I'd measure the plate current while I was there. I'm not sure of the voltage applied to the plates (the manual states 'set PLATE to 95' on a 0-100 scale). Results are as follows;

tube 1: 72.9mA
tube 2: 72.0mA
tube 3: 72.9mA
tube 4: 69.3mA

I also re-measured the pin2-to-ground voltage as follows;

tube 1: -18.65VDC
tube 2: -19.29VDC
tube 3: -18.29VDC
tube 4: -19.87VDC

All measurements done with the 5AR4 out of socket and at 117V.