I don't get the expected power output.

[Alan0354]

Now that you have all your old test gear out, you can do some real testing. One thing wasn't clear in the results posted was the actual grid drive voltage at the power tubes. They should be pretty close to the bias voltage you use, i.e., if the bias voltage is -30V, then Eg1 = 30Vpeak, but if they are far from it, and the output is already visibly clipped on the scope, then something is still amiss. BTW, is the amp cathode biased or fixed biased?

I just look at the grid voltage on the Weber 6V6. I confirm the grid voltage reach 0V right when the output signal starts to clip. The grid bias is about -37V or so, I saw the sine wave at the grid goes from about 0V to -80V center at the idle grid bias. So the drive is plenty.

My amp is fixed bias using a HV opamp to track the +B for power scaling.

[g1]

What is idle grid voltage of JJ versus Weber/Ruby ?

[jazbo8]

Ok, that's great. Next question, does the B+ and screen voltage sag a lot at full power? If so, what are their readings at full power?

[Alan0354]

Ok, that's great. Next question, does the B+ and screen voltage sag a lot at full power? If so, what are their readings at full power?

+B sag about 30V for both tubes.

I have a 1K screen resistor. When driven to clipping, the screen of the Weber had a 50Vpp clipped wave. The JJ had a 40Vpp clipped wave.

[jazbo8]

I see, the drop in the B+ & Eg2 would mostly explain why the full output power were lower than the estimates. But why were the JJ's is pulling less current at full power than the Weber's?

[Alan0354]

I see, the drop in the B+ & Eg2 would mostly explain why the full output power were lower than the estimates. But why were the JJ's is pulling less current at full power than the Weber's?

Actually at this point, I have over 20W with the Weber and Ruby tubes, only the JJ is below par.

I don't understand, the drive is the same, the plate of the JJ just swing less. From the former post, JJ only reach down to 220V and bottom out. At this point, I think I had probe all the points that are relevant already, JJ just swing lower.

I am starting a new thread about something that is funny. The amp sounds really good until the power amp clips. Then it sounds cracking up.

[jazbo8]

Actually at this point, I have over 20W with the Weber and Ruby tubes, only the JJ is below par.

I don't understand, the drive is the same, the plate of the JJ just swing less. From the former post, JJ only reach down to 220V and bottom out. At this point, I think I had probe all the points that are relevant already, JJ just swing lower.

Mystry solved, I think... Martin Manning posted a comparison of the actual traced 6V6S vs. 6V6GT specsheet on TAG awhile back, it can be seen that the 6V6S is quite a bit less sensitive than the 6V6GT. But I am still puzzled by your measurements, how come it only swing down to 220V? Ditto for the 6V6 which was only a bit better - neither was anywhere near the saturation region of tube...

[Alan0354]

Mystry solved, I think... Martin Manning posted a comparison of the actual traced 6V6S vs. 6V6GT specsheet on TAG awhile back, it can be seen that the 6V6S is quite a bit less sensitive than the 6V6GT. But I am still puzzled by your measurements, how come it only swing down to 220V? (ditto for the 6V6 which was only a bit better) That's no where near the saturation region of the tubes...

[ATTACH=CONFIG]29885[/ATTACH]

The input hits 0V, it's clipped after that. The tubes are being driven hard. I got plenty of drive!!! Further increase the drive does not make the plate go any lower.

[Alan0354]

Since I responded, I went back and think why I did not managed to drive the plate down further. I read back post #21 where I posted the +220V bottom. It was done with the 8 ohm load connected to 4 ohm tap of the OT to give 8K primary. But since, I have been connecting the load to 8 ohm tap that make the primary and Get 24W out of the Weber tubes. So I decided to measure the clipping voltage at the plate again.............

Surprise surprise, I drove the plate down to 80V with the Weber. The JJ is still higher, at about 130V.

I even question whether I was seeing things this afternoon. I plugged the load back to the 4 ohm tap, The JJ can only go down to 220V.

So the tap is the one that make the difference. At lower primary impedance, ( from 8K to 4K) the tube managed to swing lower......WHICH DOES NOT MAKE SENSE!!! Can anyone explain this?

[jazbo8]

So the tap is the one that make the difference. At lower primary impedance, ( from 8K to 4K) the tube managed to swing lower......WHICH DOES NOT MAKE SENSE!!! Can anyone explain this?

Nope, it does not make sense to me either, need to think about this one...

[frus]

me neither, unless you got the taps backwards...?

[J M Fahey]

Think about this: IF plate were able to swing *more* amd into half the impedance to boot, you would have over double power available ....... yet you show 10/20% variations ....
How´s that possible?

That said, that JJ swing less than others under same load means that JJ either have less transconductance, or lower emission (so they run short of electrons needed to carry current) or both.

Personally (and since you are inclined to experiment) I´d kludge some variable resistance load box , sweep its value upwards and downwards and find what´s the best load impedance for each tube brand.

Easier done than said.

FWIW I found out LONG ago that optimum load impedance for generic 6L6 tubes, any brand, old or new was 5500 ohms.
I got easily over 40W RMS from any of them, so I wind my transformers for that value.

You can pull a little more out of selected ones, say extra 8 or even 10W ... not a significant increase (around 1 dB) and my choice makes it easier to source or replace tubes (generic Russian/Chinese, "any" shaped glass perform well) and they last a lot.

Being behind Customs walls in Argentina means that I can´t always choose the brand and type I´d like, I just walk to the shop counter and ask : "which 6L6 do you have today?" ... and I´d better use that



In this particular one I used EL34 ... same thing.

Designing for average performers make life easier.

[Alan0354]

Damn!!! I read the schematic WRONG. I read from bottom up and GREEN was the next tap and I thought that was the 4 ohm tap. I did got the black wire correct.

YES...........I reverse the tap!!! I had it on the 8 ohm tap!!!! Now it makes the world of sense!!!!

Sorry to take up all your time for my stupid mistake. Why they put the common on top of the diagram? Hoping to play an April fool on someone one day?

Ha ha, the only face saving for me is I did tried the "8ohm"(which is 4 ohm) tap yesterday when it did not make sense!!! Ha ha!!!

[Alan0354]

Last question to you guys, given the PT I use https://taweber.powweb.com/store/025130eusch.jpg, Is it safe for me to push and put a pair of 6L6 in it? The PT is rated 150mA but it's only rated for 25W.

I use SS rectifier, so I don't use the power on the 5V winding to power the filament of the rectifier tube, that save some power. I don't have reverb and tremole, so I save 3 dual triodes. Does that help to make up the extra power used by the 6L6?

Or should I go down in +B to 380V for the 6L6. I am sure I can get more than 24W with the 6L6 at 380V. I'll be happy to get close to 30W with this amp....And yes, I will put the load to the "real" 8 ohm tap!!!

Thanks.

[jazbo8]

I was under the impression that the 6V6S is a more rugged version of the 6V6GT with a higher Pda of 14W, so I guess you can push it a bit harder, but I doubt it would make much of difference SPL-wise.