Resistance Check [solved]

[mSummers]

I just completed assembling my Crack and did the resistance check.  Here's the results:

1- 0.68M
2- 0.54M
3- 0.3
4- 630k
5- 13.3M
6- 0.4
7- 2.94k
8- 0.5
9- 2.93k
10- 0.5
12- 0.5
13- 0.8M
14- 0.6
20- 0.8
22- 0.7
B3- 2.94k
B6- 2.94k
RCA Ground- 0.7
RCA Center- 91.3k

I did the startup voltage mod, which I think explains the low reading on 6 & 10.  13 started well above the 270k mark and stabilized around 0.8m.  Is that too high?  Also, everything that is supposed to read zero is a couple of 10ths above zero.  Is that a problem?

In case it matters, I built the amp stock except for Teflon sockets and a TDK volume pot.

-Michael

[Paul Birkeland]

Your resistances look good. 0.3 Ohms may just be the resistance of the probes coming out of your meter.

The 270K value not reading 270K is just a particularity to your meter, generally you just don't want to be missing any 0's on the list (missing ground connections) or additional 0's on the list (shorts).

-PB

[mSummers]

Thanks Paul!  I'll work on the voltage check tonight. 

Quick question about attaching the negative probe of the meter to the ground.  The manual says that terminal 12 is a good location to do that, but terminal 12 is rather full and somewhat difficult to attach an alligator clip to.  Would it be fine to clip the negative probe to terminal 3 instead?

[Paul Birkeland]

Yeah, the easier option might be any of the attachment points of the black wire on the headphone jack. 

[mSummers]

Just completed the voltage check and I have a couple that aren't right.  Here's the results with anything more than 15% out in red

1- 23.1
2- 172.8
3- 0
4- 173.3
5- 23.5
6- 0
7- 102
8- 0
9- 98.3
10- 0
11- 0
12- 0
13- 172.5
14- 0
15- 195.7
20- 0
21- 219

A1- 24
A2- 0
A3- 1.5
A4- 0
A5- 0
A6- 22.7
A7- 0
A8- 1.5
A9- 6

B1- 22.5
B2- 173.1
B3- 102.8
B4- 23.7
B5- 173.4
B6- 98.6
B7- 6
B8- 0

The diodes on the 9 pin socket both lit up and both tubes glow.  Thoughts?

[mSummers]

Update - after looking the amp over, I found that I forgot to check the volume and it wasn't turned down.  I'll check the voltages again tomorrow.  Would that have caused the voltage issues above, or is there anything else I should check before doing the voltage check again?

[Paul Birkeland]

Can you triple check that you have a 12AU7 (or one of its close variants), and not something else.

Do the voltages at T1/T2 change if you unplug the 6080 and run the amp?

[mSummers]

The 9 pin is a 12AU7A

Measurements without 6080
T1- 28.4 - LED connected to A8 turns off when probe is connected
T2- 229.5
T4- 230
T5- 30.5 - LED connected to A3 turns off when probe is connected

[Paul Birkeland]

Can you carefully measure A2 and A7?  It may be easier to just measure the center wire on each level of the volume control, as they connect to these pins.

Something sure isn't adding up, a glowing LED means 1.57V on the cathode, you have a nice amount of voltage at T1/T5, and there aren't enough parts in the kit to be replacing the 22K resistors with something else (they'd have to be higher, and there aren't two higher value parts in the kit).

Essentially, this means that the voltages at pins A2/7 can't be correct, which would indicate an issue with the grounding on the volume pot, or with the connections of the wires leaving the pot and going to those pins on the 9 pin socket.

[mSummers]

Paul,  do you want those voltages with or without the 6080 installed? 

[Doc B.]

That would be with both tubes in.

[mSummers]

Here's the latest checks:
A2- 0
Pot Left Channel, Center Pin, 0
A7- 0
Pot Right Channel, Center Pin, 0

I double checked the following resistances:
T1-T2- 22.22k ohm
T4-T5- 22.23k ohm

Volume Pot:
Left Channel:
Outboard to Center - 91.4k ohm
Outboard to Ground - 91.4k ohm
Center to Ground - 12 ohm

Right Channel:
Outboard to Center - 91.1k ohm
Outboard to Ground - 91.1k ohm
Center to Ground - 8.9 ohm

[mSummers]

The 6v reading at A9 & B7 in the original voltage check is incorrect.  I had my meter set to Auto-V.  I checked them again tonight and I noticed that I missed the AC designation on those readings the first time I did it.  With the meter set to DC, both of those read 0v, so I re-checked all of the voltages with the meter set correctly and got the following:

1- 78
2- 177.4
3- 0
4- 177.3
5- 79.2
6- 0
7- 107.1
8- 0
9- 103.3
10- 0
11- 0
12- 0
13- 176.7
14- 0
15- 198.5
20- 0
21- 220.5

A1- 78.9
A2- 0
A3- 1.55
A4- 0
A5- 0
A6- 77.6
A7- 0
A8- 1.55
A9- 0

B1- 77.6
B2- 176.7
B3- 106.8
B4- 78.8
B5- 176.8
B6- 102.9
B7- 0
B8- 0

All of those are within the 15% margin.  Since I did the startup voltage mod, the voltage stays at 0v, so it's time to try it out.

Sorry for the wild goose chase.  I'll chalk that mistake up as experience.

[mSummers]

Just tried it out and it sounds great!  Thanks again for your help!