Low A2 reading......

[shelby1420]

Hey all, one of my amps gives a low reading on A2 (440), this then became an issue when I went to adjust driver tube, I can only adjust it to 157v, have I buggered up a resistor somewhere or is this ok??

[Dr. Beaker]

You are running 2A3, correct?  What are the other (A1-A4) readings on the output tube socket?  Most likely you do have a problem.  Most frequently it's a bad solder joint somewhere. 

[shelby1420]

 Yup 2A3,  I will take all the readings when I get home and report back, thought all was ok as music sounds great through both amps!!

[Doc B.]

Did you try swapping 2A3s and measuring again?

[shelby1420]

No, could that do it?  Also would that limit how much I am able to increase or decrease by the trim pott (the one from the soft start), I lowered to 157v and it would go no further..................

[Paul Joppa]

440 volts is reasonable if your power line voltage is a bit under 120v. How is the other amp?

A typical fresh 5670 should be able to get down to about 135v; 157v is just at the outer edge of the normal range. My guess is that your 5670 is a bit low on transconductance. If the tube is new, the voltage may go down after some burn in time - say, 100 hours or so. These 5-star tubes have higher purity cathode material, and take longer to fully for the cathode.

You can swap the 5670 between amps, or just do the same measurement on the other amp, to see how the 5670s compare.

Even without burn in or tube swapping, you are not far off. At 440v on A2 and 157v on the driver, the "optimum" plate load is 2500, and you have 3000 ohms. This is a minor difference which will result in slightly less distortion and very slightly less power - around 4% less according to my calculation. It is very unlikely anyone would notice the difference.

[shelby1420]

440 volts is reasonable if your power line voltage is a bit under 120v. How is the other amp?

A typical fresh 5670 should be able to get down to about 135v; 157v is just at the outer edge of the normal range. My guess is that your 5670 is a bit low on transconductance. If the tube is new, the voltage may go down after some burn in time - say, 100 hours or so. These 5-star tubes have higher purity cathode material, and take longer to fully for the cathode.

You can swap the 5670 between amps, or just do the same measurement on the other amp, to see how the 5670s compare.

Even without burn in or tube swapping, you are not far off. At 440v on A2 and 157v on the driver, the "optimum" plate load is 2500, and you have 3000 ohms. This is a minor difference which will result in slightly less distortion and very slightly less power - around 4% less according to my calculation. It is very unlikely anyone would notice the difference.

Paul, other amp was 464v I think, spot on really. In terms of the tubes they are the WE 5670 from the Tube store, perhaps I should try the GE ones, i can tell you this, it sounds stunning, my sytem has never sounded so good, thumbs up to you mate, brilliant design!!!  I will do some extra sleuthing when I get home.

[Paul Joppa]

Both amps should have the same A2 voltage if measured under identical conditions (i.e. the same power line voltage) - at least, a lot closer than what you have measured. Power line voltage does fluctuate though, making it difficult to be sure you have the same unless you check. In some houses for instance, the voltage will sag when the refrigerator or heating system starts up.

[shelby1420]

So most likely not a tube issue then???

[Paul Joppa]

So most likely not a tube issue then???

Well, I think the 5670 that you measured has lower than average transconductance. IIRC, someone else had a similar problem with a WE tube. At that time I speculated their tube might be used with an exhausted cathode coating, but it might be that the WE does not match the GE 5670 whose curves I used in the design - similar but not identical parameters. The only way to tell would be a transconductance tube tester; until then we are just speculating.

[johnsonad]

That was me Paul.  Had to go through a few WE tubes to find a pair that would work. 

[shelby1420]

So most likely not a tube issue then???

Well, I think the 5670 that you measured has lower than average transconductance. IIRC, someone else had a similar problem with a WE tube. At that time I speculated their tube might be used with an exhausted cathode coating, but it might be that the WE does not match the GE 5670 whose curves I used in the design - similar but not identical parameters. The only way to tell would be a transconductance tube tester; until then we are just speculating.

Could I not just swap the WE from amp to the other amp and see if it follows the tube??

[shelby1420]

Well, you were right on Paul, I took out the WE tubes and put back the GE tubes that came with the upgrade and both came to 462v, thanks!!!

[shelby1420]

So most likely not a tube issue then???

Well, I think the 5670 that you measured has lower than average transconductance. IIRC, someone else had a similar problem with a WE tube. At that time I speculated their tube might be used with an exhausted cathode coating, but it might be that the WE does not match the GE 5670 whose curves I used in the design - similar but not identical parameters. The only way to tell would be a transconductance tube tester; until then we are just speculating.

Hey Paul, when I try and swap this tube at THETUBESTORE  what exactly do I need to make sure is matched??  Dos this mean one of the tubes is bad or just not matched to the other??

[Dr. Beaker]

Sorry to quickly offer the solder problem as a cause.  I confused you with another member that is making a lot of changes to his amps.  Glad you were able to quickly sort it out.

Aside from the problem with one of those WE tubes, how did you like them?  I have a pair that I want to try.  I have a nice tube tester so I think I will measure them first.