2A3 Paramount Resistances don't check out

[Aural Robert]

I've configure my amps for 2A3 and checking the resistance on the right amp I get differences on:
T7 - I get 270.5
T9 - I get 270.2
T19 - I get 270
A3 - I get 270
B2 is 5.95
B4 and B6 are both 270
All the others check out ok.

I've double checked the manual against my build and don't see that I've missed any parts. Any thoughts and ideas are appreciated.

Aural Robert
... I'm sittin' in the fireplace burning up my time ...

 

[Aural Robert]

Well I've just checked out the left channel amp and I get almost identical readings. So whatever is happening, both amps are the same.

Ideas??? TIA.

Aural Robert
... One thing I can tell you is you got to be free ...

 

[Doc B]

Could you also post what the manual says each of those terminals should read? That makes it a little easier to dig into what is going on.

 

[Aural Robert]

NP Doc.

T7 - I get 270.5 - the manual says *
T9 - I get 270.2 - the manual says *
T19 - I get 270 - the manual says *
A3 - I get 270 - the manual says *
B2 is 5.95 - the manual says 8.2 (the other amp measures 6.45)
B4 and B6 are both 270 - the manual says *

Hope this helps.

Aural Robert
... this is a dangerous place ...

 

[Doc B]

If that is 270 ohms you have one end of your 270 ohm resistor going to ground rather than a positive terminal. If you are measuring 270K ohms, you are probably just measuring a 270k bleeder resistor because the filter caps aren't affecting the meter by charging.

 

[Aural Robert]

The 270K resistors are all on the power supply board. I touched up the pads with a bit more solder to make sure they are all good, and when I do the measurements again, no change.

I have a green resistor for the wirewound - it says 270R 5% so I think that's the right one - it came in the kit.

I am doing all my measurements with the tubes installed, if that helps.

Awaiting the next direction, Doctor. TIA.

Aural Robert
... She's tidied up, and I can't find anything! ...

 

[Doc B]

Are you really measuring 270 ohms? Or are the numbers you posted actually measuring 270K ohms and you are posting them as 270 ohms? It is a critical distinction. 270K ohms is fine, 270 ohms is not.

 

[Aural Robert]

Sorry about that. They are all K readings, every one.

So, can I continue to the voltage test then?

Aural Robert
... Show the color of your crimson suspenders ...

 

[Doc B]

270k ohms is fine.

 

[Aural Robert]

I finally got to running my voltage tests. Most check out ok, but a few are lower than they should be:

T5 reads 149vdc, should be 217vdc
A3 reads 157.1vdc, should be 217vdc
My KregA reads 2.99vdc, should be 4.1vdc

Any reason for the wide discrepancy? Am I good to go?

Aural Robert
... One little problem that confronts you ...

 

[Paul Joppa]

This looks suspiciously like you have wired the large cathode resistors for the 300B, not for the 2A3.

What is the resistance at T5? If it's 1000 ohms, then my guess is correct. If it's 4000 ohms then something else is wrong, and I think it would be most helpful to re-measure ALL the resistance checks and post them, along with the expected values.

Do not turn the amp on again until this is squared away; at present the voltages indicate a very bad situation for the 2A3. I doubt it's been damaged much in a few minutes, but better safe than sorry.

 

[Aural Robert]

The resistance at T5 is 3.986K ohms.

Here are all the measurements:

T1: Reads 0L, manual says *
T2: Reads 0L, manual says nil
T3: Reads 0.0, manual says nil
T4: Reads 0L, manual says nil
T5: Reads 3.986K, manual says 4K
T6: Reads 4.09K, manual says 4K
T7: Reads 270.7K, manual says *
T8: Reads 0.0, manual says nil
T9: reads 270K, manual says 124K
T10: Reads 0.0, manual says 0
T11: Reads 0L, manual says 3.0M
T12: Reads 0L, manual says 5.6M
T13: Reads 0.0, manual says 0
T14: Reads 0L, manual says 9.2M
T15: Reads 0L, manual says 15M
T16: Reads 0L, manual says *
T17: Reads 0.0, manual says 0
T18: Reads 0.0, manual says 0
T19: Reads 270K, manual says 128K
T20: Reads 248.2K, manual says 249K

A1: Reads 4.00K, manual says 4K
A2: Reads 0L, manual says *
A3: Reads 270.8K, manual says *
A4: Reads 3.99K, manual says 4K

B1: Reads 0.0, manual says 0
B2: Reads 5.95K, manual says 8.2K
B3: Reads 248.4K, manual says 249K
B4: Reads 270K, manual says 130K
B5: Reads 0, manual says 0
B6: Reads 270K, manual says 130K
B7: Reads 224.4K, manual says 220K
B8: Reads 0L, manual says *
B9: Reads 0.4ohms, manual says *

Thanks. Aural Robert.

 

[Paul Joppa]

OK, puzzle time.

B7 you say 220K, manual says 220 ohms.  Is this an error? Otherwise the resistances look OK.

My problem is with A3 and T5 voltages. T5 should be some 50 volts higher than A3, but you showed before "T5 reads 149vdc, ... A3 reads 157.1vdc". This is weird enough that I first suggest you make absolutely certain (probably for the 3rd time, sorry!) that the socket is oriented correctly, with pins 1 and 4 taking the fat tube pins, and 2 and 3 the thinner tube pins. If there is a problem here, fix it before going on!

Then, turn the amp on for 30 seconds, measure just the A socket voltages, and turn the amp off immediately after you get those voltages. If A3 is still greater than T5, swap the 2A3s and repeat - it's possible the first 2A3 is not functioning correctly.

If the second tube gives similar voltages, then measure T6,7,8,9,10 and T16,17,18,19, 20 voltages.

 

[Aural Robert]

I think my tubes are experiencing some difficulties. I'm going to pick up another set on the bay and will be back in a week or two. Thanks Paul.

Aural Robert

 

[Aural Robert]

I finally got time to plug a new tube in and check my numbers. The resistances all check out.

Voltages vary from the manual on these:
A1 reads 200.5, manual says *
A3 reads 149.3, manual says 217
A4 reads 197.9, manual says *
KregA reads 2.99, manual says 4.1
KregB reads 7.11, manual says 6.3

For reference,
A2 is dead on at 464
T5 reads 198.2, manual says 217
T16 reads 480, manual says 464

All the other readings are very close. I'm wondering if I'm good to go?

Aural Robert
... well my baby done done me wrong ...

 

[Paul Joppa]

I assume you have the current version 1.1 with the 5670? If so, then it looks close enough to me. The variances are almost certainly due to tube variations, not wiring errors.

 

[Aural Robert]

Excellent. Thank you Paul.

Stay tuned for the left channel.

Aural Robert
... Carry bloody baggage out ...

 

[Paul Birkeland]

Did you perform the bias adjustments on the driver tube?

 

[Aural Robert]

No I didn't. I just searched the pdf for driver, bias, and adjustment, and there isn't anything about adjusting the bias of the driver tube.

?????

Aural Robert
... Spent the last year ...

 

[Doc B]

It's described as adjusting the plate voltage in the manual. Adjusting the plate voltage of the 5670 is detailed at the bottom of page 66.