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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Wed Apr 10, 2013 9:01 am
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So, if it came in say even at 15 off from the factory spec, how much difference would that make in where to set the bias at? How many milliamps off could one be if it was even 15 off? My thought was that if I put it in the middle of the recommended bias setting range that it would still be within the range, even with a say 15 point difference in voltage?


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Wed Apr 10, 2013 9:58 am
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I'd opine that it depends upon the type and brand of tubes you've installed.

Such variances in plate voltage are usually determined by the individual quirks of a power supply (which also feeds the bias-supply circuit as well).

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Wed Apr 10, 2013 11:02 am
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Retroverbial wrote:
I've noted plate-voltage variances of between 10 to 14 VDC against an "official" schematic spec of 442 VDC for the handful of reissues I've serviced over the years. My own re-issue TR (1999 model) measures 448 VDC, which is twelve volts shy of the 460 VDC specified for the original hand-wired AB763 circuit. It's risky IMO to set the idle bias without knowing that data, especially when installing expensive glass like the NOS blackplate RCA's I have in mine.

Arjay


+1

Biasing an amp results in a change to plate current. As plate current increases, plate voltage decreases. As plate current decreases, plate voltage increases. You cannot compare the bias settings and the resultant plate current and voltage of your amp to any other amp.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Wed Apr 10, 2013 7:51 pm
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Well I guess I am going to have to take the chassis back out again and check the plate voltage just to see. I take my advice on retubing amps from what I read at the Eurotubes website, from watching their biasing videos and from talking to the staff there. Everything they ever told me and everything I ever read and saw there has ended up being correct. I know each amp is a little different, but basically from my undderstanding these amp are coming in pretty similar with the plate voltage reading biased at the recommended bias settings and they recommend 38-44 mA per tube with most guys liking 38-40. In guess my point is even if the plate voltage varies a little that it will only make a point or 2 difference in where you set the bias at and there is a range they recommend and that range covers any differences in plate voltage. With that all in mind, I do not think the amp will be set running too hot or too cold and if I am a point or 2 off I will not be able to hear the difference anyway. I am am curious though and I do not want it set up wrong so I will get the readings when I can and post them.


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Wed Apr 10, 2013 9:41 pm
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DLW wrote:
Well I guess I am going to have to take the chassis back out again and check the plate voltage just to see. I take my advice on retubing amps from what I read at the Eurotubes website, from watching their biasing videos and from talking to the staff there. Everything they ever told me and everything I ever read and saw there has ended up being correct. I know each amp is a little different, but basically from my undderstanding these amp are coming in pretty similar with the plate voltage reading biased at the recommended bias settings and they recommend 38-44 mA per tube with most guys liking 38-40. In guess my point is even if the plate voltage varies a little that it will only make a point or 2 difference in where you set the bias at and there is a range they recommend and that range covers any differences in plate voltage. With that all in mind, I do not think the amp will be set running too hot or too cold and if I am a point or 2 off I will not be able to hear the difference anyway. I am am curious though and I do not want it set up wrong so I will get the readings when I can and post them.


I buy all my JJs for Eurotubes, but I take their bias information with a grain of salt.

According to the schematic, the factory setting of 30 mA cathode current (about 29 mA plate current)on my Blues Deluxe Reissue results in a plate voltage of about 430 VDC. That equals about 50% of the maximum plate dissipation of the stock Groove Tube Sovtek 5881WXT (assuming the tube is a 25 watt tube). Way cold in my opinion. My preferred setting for a JJ 6L6GC of 53 mA cathode current (about 50 mA plate current) results in a plate voltage of 403 VDC. Quite a difference. That is about 67% of the maximum plate dissipation for a 30 watt JJ 6L6GC. That is why you need to measure the plate voltage each time you adjust the bias seting.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 3:41 pm
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I checked the voltage from pin 3 to ground. I let the amp warm up and checked it in several sockets several times over an hour with different things running in the house. It came in at 449 volts when first turned on and after an hour was steady at 453 volts set at 39.5 to 40.5 mA per tube(average of 40 mA). I do not touch the bias setting since I did it the other day and basically the recomendations of Eurotubes were spot on as they recomended 38-44 mA per tube with most guys liking them at 38-40 mA. I set mine at 40 and the plate voltage came in at 453.

This gets the tubes out of crossover distortion and they sound good and they are not running so hot that they will burn out too soon.

Eurotubes recomendations......38-40 mA per tube

My amp 40 mA per tube and voltage of 453.

25(watts per 6L6 tube) divided by the voltage(453) = ).552 which is 55.2 mA x.7(70% to make sure its out of crossover distortion) = 38.64(38.6 mA).

So, 38.6 mA is 70%, 40.02 mA is 72.5% 41.4 is 75%. They recommended 38-40 without checking the plate voltage. I would say their math is right on. I get a good sounding, running amp out of crossover distortion without burning the tubes up too fast, without even having to take the chassis out and checking voltage.

For those who may say I got lucky and Eurotubes math is bad........

These amp will probaly not vary this much and I know that when you turn one thing up another goes down and vice versa, but even with an unlikely 20 point difference......433 it ends up 40.4 mA and at 473 its 36.99, so round that range to 37-40.5 mA and they recommend 38-40. Even if their is something I am not understanding or a slight variable, all that still only translates only a couple of mA difference from the center of their recommended range and it basically a'int gonna make a notable difference in any way.

I do not mean to sound arrogant. I honestly appreciate all help and advice. I am just a hack and even a fool at times. I have done at least 10 amps this way and it has always been sound accurate advice to the point of not needing to test the voltage and when I do its still ends up being sound accurate advice. From my limited experience, I just do not feel that there needs to be so much mystique and rocket science and precision in retubing-setting the bias of an amp and that an average Joe can do it with sound advice and recommendations.


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 4:26 pm
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25 watt 6L6?

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 8:04 pm
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Yes, the formula for biasing is the plate disipation of the tube (25 watts 6L6,EL34,KT66,KT77 or 14 watts for 6V6 and for KT88 use 35 watts) divided by the plate voltage(lets use 480 for example) times .7 just get you out of crossover distortion and you can go as high as .85 percent but you will wear out the tubes quicker.

25 divided by 480 = .0520 or 52 milliamps x .7 = .036 or 36 milliamps

So, yes, 6L6, 25 watt.


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 8:46 pm
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JJ 6L6 are 30 watts. In fact, most all new production 6L6 are 30 watts, and only a few of the old stock 6L6 were 25 watts. Most 5881 are 25 watts, but even some of them are 30 watts.

http://www.thetubestore.com/6L6-Tube-Review

I hope that you really don't believe that everything on the Eurotubes site is gospel, there is a lot of nonsense on there (they do sell only one brand :wink: ), but I think that even they know that their own tubes are 30 watts, even though they call them both a 6L6GC and (5881). :lol:

From the Eurotubes site
"JJ Electronic 6L6GC

Factory description: Special plate alloy material and gold plated control grid. Plate dissipation of up to 30 watts. Heavy gauge glass. Best sounding 6L6 ever made replacing all 6L6/5881 tubes in all applications.
"

Note their claim "Best sounding 6L6 ever made..." :lol: :lol:

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 9:30 pm
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I give up. I'm glad the OP is not working on my amps. :roll:

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 10:01 pm
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shimmilou wrote:
JJ 6L6 are 30 watts. In fact, most all new production 6L6 are 30 watts, and only a few of the old stock 6L6 were 25 watts. Most 5881 are 25 watts, but even some of them are 30 watts.

http://www.thetubestore.com/6L6-Tube-Review

I hope that you really don't believe that everything on the Eurotubes site is gospel, there is a lot of nonsense on there (they do sell only one brand :wink: ), but I think that even they know that their own tubes are 30 watts, even though they call them both a 6L6GC and (5881). :lol:

From the Eurotubes site
"JJ Electronic 6L6GC

Factory description: Special plate alloy material and gold plated control grid. Plate dissipation of up to 30 watts. Heavy gauge glass. Best sounding 6L6 ever made replacing all 6L6/5881 tubes in all applications.
"

Note their claim "Best sounding 6L6 ever made..." :lol: :lol:


I wonder how much trade-in value they'd allow me for my blackplate RCA's......?

:roll:

The only reference I'd ever refer to as "gospel" is my RCA Receiving Tube Manual.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 11, 2013 10:28 pm
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Retroverbial wrote:
...The only reference I'd ever refer to as "gospel" is my RCA Receiving Tube Manual...


Absolutely! And, if there ever was a "best sounding 6L6 ever made", it would be the RCA. A quick look at the RC30 reveals that it was a 30 watt tube (6L6GC)....even back then. :lol:

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 12:01 am
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Ok, to avoid anymore cornfusion :D .

JJ Data Sheet. Limiting Values are for Class AB1 and AB2 push-pull.

http://www.drtube.com/datasheets/6l6gc-jj2003.pdf

But, if your JJ sound ok to you, idling your calculated 25 watts dissipation --- go with it. True dissipation prolly is more-or-less, as no two tubes amps I know of, run exactly the same plate voltages with various makes or vintages of output tubes.

Truly, the only way to really know the output wattage is to measure the secondary AC voltage with a DVM or scope.


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 6:46 am
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OK, its sounds like most feel that I am drinking thre Kool-Aid from Eurotubes. Maybe I am. Maybe they are correct maybe not. I am not an expert, that is obvious. My intent was not to offend. My understanding was that in general those several types of tubes generally are used in 25 watt applications even though they can provide, handle 30 watts etc. Thus I assumed the 25 in the equation was generally correct common formula. If thats wrong information I would rather know. I do not have a scope etc. I have a multimeter and a single bias probe. If I am ignorant in all this I would rather eat some crow and be humble. So again my apologies if I am all wrong here.

What is the correct formula to correctly set the bias of this and other amps? Do I just plug in 30 instead of 25. Is there an entirey different formula. Is using a scope really vital? What is the drawback or risk of using the said formula stated from Eurotubes vs another. From what I can see using their recommendation would run the the tubes cooler, but I may not be understanding things well. Thanks


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 6:58 am
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Yes, just use the rated max Plate watt number for any tube in your formula and you're good. Frankly, using a scope to set bias is so inaccurate and unrepeatable that it isn't worth the effort. Using bias probe and multimeter will do just fine. If you are a little off on the Plate voltage, a little off on the tube wattage, a little off on the current, it can add up to way off overall. :wink:

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