shelby1420
New member
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??
Low A2 reading......
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Dr. Beaker
New member
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
New member
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!!
shelby1420
New member
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
Moderator
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.
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
New member
Paul Joppa said:
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
Moderator
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
New member
So most likely not a tube issue then???
Paul Joppa
Moderator
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.shelby1420 said:
shelby1420
New member
Paul Joppa said:
Could I not just swap the WE from amp to the other amp and see if it follows the tube??
shelby1420
New member
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
New member
Paul Joppa said:
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
New member
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.
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.
shelby1420
New member
Dr. Beaker said:
No worries re suggestions or advice, I am open to all I can get! Really did like the WE, however, I have had the stock tubes that came with the soft start and to be perfectly frank I think I like them more!!
Paul Joppa
Moderator
I'm sorry, I don't know a good answer. You could just say that the tube won't bias up properly in your Paramount V1.1 (be sure to tell them what other tubes you have that do bias up properly. They have a good reputation and might exchange it on that basis. What we don't know is whether the transconductance is actually below spec, or within spec but on the low side.shelby1420 said:
I checked their web site, and they say "Our current stock of the Western Electric 396A tube is used. All test great and are guaranteed." That suggests they did test the tube before it was shipped, but of course used tubes may well have reduced transconductance.
If necessary we can reduce the tube current a bit to bring the voltage within the adjustment range - I'd hate to see you stuck with a tube that won't work.
As a matter of interest, the 396 was the very first 9-pin tube made. I have a pdf of the 1947 article in the Bell Labs magazine; it's just a hair too big to attach but if you ping me I'll email it.
shelby1420
New member
I just wanted to give a follow up to this. I called Jon at the The Store and advised him of the situation, without a blink he simply said send it back and I will send you another, so we swapped tubes and now I'm jammin with WE's in my amps, all is good in the audio world once again!!!!
Paul Joppa
Moderator
Thanks for the update - good to hear that The Tube Store's reputation is well deserved!
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