Is my bias OK? + Tube questions

[vortex]

I recently got a Allesandro bias meter and have been checking out my amps.

I was surprised to see that my 59' 5E3 had a fair bit of mismatch on the 6V6's. 32ma & 41ma. The amp sounds great. I posted in the troubleshooting forum and the fine people responded that if the amp sounds great it was not a bad thing. One post warned that I was probably going to fry my output tubes.

So I went further and measured the plate and cathode voltages and did some calculations, do these look OK?

For the test I used some matched JJ tubes which the Bias meter speced out at 39ma each for both tubes.

a)cathode current = 19.7V / 250 (cathode resistor)= 0.08A /2= 40 ma/tube
b)plate V (357V) - Cathode V (19.7V) = 337V
c) Current(0.04A) x V (337V) = 13.48 Watts

I guess I am thinking that the 40ma/tube looks high but I don't know enough about how to interpret the data.

Another question I have is about tubes. I have about 20 old mixed brand 6V6's that a local tech "matched" for me. Some Phillips,GE, Sylvania etc. As I tried out the matched 6V6 combos I noticed most of them are biasing 6V6a)33- 6V6b)40ma? Is that OK? Additionally some pairs sound excellent but have a fair bit of hiss type noise. Is this a bad tube or a bias issue?

Lastly,a couple pairs of tubes biased 6V6a) 0ma- 6V6b) 61ma. Bad tube I suspect. The odd thing was I was playing guitar for a moment when this happened and the amp sounded really amazing.Of course I switched the amp off almost immediately.

So many questions... Any insights greatly appreciated.
Thanks!

[tubeswell]

a)cathode current = 19.7V / 250 (cathode resistor)= 0.08A /2= 40 ma/tube
b)plate V (357V) - Cathode V (19.7V) = 337V
c) Current(0.04A) x V (337V) = 13.48 Watts

I guess I am thinking that the 40ma/tube looks high but I don't know enough about how to interpret the data.

40mA would be high if it was plate current, but you have calculated tube current, which includes the screens (of about 2mA per tube). So taking that out of the equation, that would make the plate current around 38mA per tube, which = 12.8W - spot on.

Another question I have is about tubes. I have about 20 old mixed brand 6V6's that a local tech "matched" for me. Some Phillips,GE, Sylvania etc. As I tried out the matched 6V6 combos I noticed most of them are biasing 6V6a)33- 6V6b)40ma? Is that OK? Additionally some pairs sound excellent but have a fair bit of hiss type noise. Is this a bad tube or a bias issue?

I prefer to use the same type of tube made by the same manufacturer in PP amps. However (hypothetically speaking), a bit of mismatching will increase the hum a bit, but generally it won't hurt the amp. I'd be wary of older tubes crapping out tho'. M2CW

[vortex]

Thanks for the reply tubeswell. I have used this 5E3 for years and it continues to amaze me with it's ballsy responsive tone. The best amp I own out of several.
I find myself now wanting to understand it's operation and maintenance and am slowly exploring the nuances.

During my tube tests I found that mismatched tubes gave me the tone I was looking for. Harmonically rich and deep. But as a control test my only "matched "tubes were the JJ's which sounded flat and boring to my ears. It may be a case of NOS vs NEW tubes.I don't know for sure so I am putting this out to get more info.
Cheers!
I am looking for more info on the noisiness of some of the tube combinations. Old tubes can get noisy but if it's a bias thing I might be able to compensate.I am not above chucking a noisy NOS in the trash but I don't want to throw any "gold"in there either!

[Enzo]

How many times do we have to tell you it is OK?


I am suspicious of tubes that "bias" at anything in particular. Idle current is always a function of the bias voltage as well as the tube. You set the bias for the tube.

If you mean to say that supposedly matched pairs seem to always come up one side drawing more current than the other, then turn off the amp, unplug it, and discharge it. Now measure resistance from either power tube plate to the center tap. Are they different? If they are different, so will your current be, because that resistance is part of the deal.

And if one side checks as 33ma and the other 40, then swap places with the tubes. If the same tube in the new spot still reads 40 or 33, in other words the reading moves with the tube, then the tubes are off match by that much. But if the same socket measures one tube higher than the other regardless of which tube it is, then the amp has a differnce between the sockets. And start by checking that resistance.

And it won;t hurt the amp to run just one tube in it for these readings. SO in your 0 and 61 pair, try each one individually.

[vortex]

How many times do we have to tell you it is OK?

Hi Enzo, It might look like I asked the same question twice. When I posted the bias meter readings in the troubleshooting forum I just had the bias meters information.
I followed the advice given in the thread and gathered the plate and cathode voltages and followed the calculations.

Biasing is new to me so I wanted someone to "check my homework" . I figured that since we are talking about a 5E3 that this would be a good place to ask.

I will experiment with the tube position swap and measure the plate to centre tap resistance.

Thanks again for your help.
Cheers.

[Enzo]

Well, just keep one thing in mind: bias is not a critical consideration. It is important to be there, but anything in the ballpark is fine. It is like tire pressure on your car. The manual may say they are to be inflated to 34 pounds - or whatever the metric equivalent is - but if the tires are a pound or two more or less away from that, the car will continue to motor down the highway.

SO while it is not a bad idea to get to the bottom of a 34 to 410 difference, overall a few milliamps either way won;t matter. if you find that skew is part of the amp, then set the bias so the average between them is your target current. 37ma in this example.