Channel Sudden out (A Side) [resolved]

[Paul Birkeland]

Is A3 really 12V? (maybe 12mV?).  If you actually have 12V there, that could possibly point to part of what's going wrong with your kit.

[HeyDerAudioBob]

Paul,

Made me think I made a mistake on A3, so I rechecked as I do have an auto ranging meter - but my reading was accurate.

What starts at near zero rises within 3 mins or so to 12v but also continues to rise with time topping at 14.88

I made following other voltage checks on the terminal strips #'s marked as follows

1 = 365.4  2 = 384.2  4 = 14.87  6 = 0       16 = 69.8  17 = 399.4

Others I had listed in prior post(s) are still fundamental the same.

Paul, sorry progress is slow - I have reviewed and tested what I believe I know the check. I do appreciate your working with me on this.

Cheers

Bob S

[Paul Birkeland]

I would swap 2A3s and double check that you don't just have one with a lot of grid leakage. If the 12V moves to the other side, then this is just related to the tube itself.

[HeyDerAudioBob]

Good evening Paul,

Ok for the requested next test, I swapped the 2A3 tubes and the voltage problem on Pin 3 (A3) moved to C3.

C3 now starts at near zero rises within 3 mins or so to 11.95v but also continues to rise with time topping at 14.85. A3 voltage is pretty steady at 0.64v - 0.71v.

For further reference, the following voltage measurements were unchanged regardless of which tube was loaded in Side A or Side B.

IB = 398     OB =175,    IA=398   OA=382 - 384.0    through multiple measurements across 4 minutes.

So now I assume I do have a problem with my 2A3 tube (RCA Grey plate, circa 1958 purchased as NOS). This current pair has 6 years of regular use, estimating around 300 or so hours per year.

I do have several other pairs but have not loaded those, wanted to make sure this was the right step considering this problem started suddenly and we still have not identified the source of the C4S board voltage issue on the A side.

Ready for what ever next test or actions, large or small you think I should take. I feel that it is a failed component in the C4S board, although I understand this boards purpose, i am no where near a complete understanding of what can cause the 0A voltage problem short of a bad 12AT7 which I think we have ruled out with multiple tubes delivering the same results on the C4S.

Thank you again, look forward to hearing back.

Cheers

Bob S

[Paul Birkeland]

That 2A3 is getting gassy, so probably time to retire it. 

A while back in this thread, I mentioned that you could remove the C4S board, rotate it 180 degrees, then reinstall it. 

This would be a more definitive test to understand what exactly is going on in your amp, and if rotating the board 180 degrees still provides a consistent voltage problem (where OA still produces high voltage, even when flipped to the other side), then a replacement C4S board and parts would most likely resolve that issue.

If you rotate the board and OB then reads high voltage, the C4S board is not the issue.

[HeyDerAudioBob]

Paul,

Thank for the directions. I will do that this weekend and report back when completed with the results.

Good Weekend !

Cheers

Bob S

[HeyDerAudioBob]

Good evening Paul,

So I have reversed the C4S 180 degrees (See Picture, makes a mess of the board !)- Little harder than I thought this would be.



Following are the key measurements on the C4S board - (basically the same)

IB = 399.8  OB = 178.4  IA = 399.9  OA = 382.1   Both Kregs = 2.498.

A1-4, C1-4 fundamentally the same.

I hope this helps with the next steps or answers.

Thank you and I await the next instructions.

Cheers

Bob S

[Paul Birkeland]

I would go ahead and replace the C4S board.

[HeyDerAudioBob]

Paul,

Just to follow up here -- I have ordered the replacement parts from Eileen. Below is what she asked to forward back to her. Looking forward to her getting those out to me and getting this completed. Will update you when completed and tested. Thank you again for the assistance and help here on getting me to this to this stage. This exercise has, as you might expect, has given me a further insight into the design and workings of these amps/preamps. Appreciate it !!!

1 - C4S PCB
4 - HLMP-6000 LED
2 - 237 ohm 1/8w metal film resistor
2 - 300K ohm 3w resistor
2 - 2N2907A transistor
2 - MJE5731A transistor
2 - LM431 regulator

[Paul Birkeland]

We don't stock the 2N2907 anymore (I think).  If you end up with PN2907 transistors, be aware that they look about identical to the LM431A regulator, so be careful not to mix them up.  The PN2907 goes into the 2N2907 holes with the rounded side of the transistor facing the little flag tab on the board outline. I've attached a picture of what this should look like, but if you have any lingering uncertainty just pop on here and let us know.

[Doc B.]

Yup that is correct, PN2907 is the current stock.

[HeyDerAudioBob]

Paul,

I received my new C4S board/parts on Wednesday. I got time today to put the new board in place. It was quick and easy. Checked the key voltages and all is well.  Put AMP back in service and is playing as sweet as it always has. Have had playing a good part of the afternoon and evening.

I want to thank you and all of the Bottlehead team for continuing to support your products whether new or 10 or more years old,  it is a testament to your customer care and what makes Bottlehead a great company.

I ordered a Crack+Speedball this past September and look forward to finally getting to work on this kit early next week.

I still am holding out for the MonAmour, hoping it comes back into availability. I want to take this Stereomour amp and move it to a 45 but love the 2A3 tube such that I don't want to not have one in my collection of amps.

Again Thanks and Happy Holidays

Cheers

Bob S