Idle plate dissipation for a D-Style 2x6L6 build?

[JazzGuitarGimp]

Hi All,

Did a search and came up empty handed. Is the idle bias / power dissipation for a 50W 2x6L6GC D-Style known? I went to the Svetlana website and looked up the Tung-Sol 6L6GC-STR reissue datasheet (the tube I will likely use), and they are recommending 450V @ 58mA for a 55W, 2x6L6 build. This equates to 26W for a 30W tube, or about 87%, which seems high. Though I admit I know very little about bias points vs. tube life vs. headroom vs. tone.

Many thanks,
Lou

[martin manning]

Hi Lou, if you havent run across it, Randall Aiken has some very good technical articles on his site, and biasing is a subject that gets a lot of discussion. See http://www.aikenamps.com/Biasing.html , and follow the link to "why 70%?." This is general information, of course. For some reason the D-amp camp seems to like their bias pretty cold, and I believe that goes for 50W as well as 100W. I think this is a "to taste within limits" thing myself, and it depends on the power tubes in use. For example, in listening tests SED ("Winged C") tubes are reported to sound best when biased a liitle hotter than others.

[212Mavguy]

Aiken's stuff is golden IMHO.

One of my interests within the amp thang is collecting vintage tubes. I have a couple hundred now, mostly 12ax7's, but also have a diverse selection of power and rec tubes.

I'm sure others have their preferences, but I tried biasing at different percentage of max settings in my 50w D-clones, #124-ish and HRM using a variety of tubes for the output. Whether I was using Sylvania or GE 6bg6ga/socket adapters, (personal faves) RFT el34's, Sylvania 6l6gb, Raytheon 6l6gb, or Svetlana 6l6gc, I noticed the same thing...a hotter bias causes the bloom as well as feedback to happen at lower stage volumes, sometimes the feedback became too difficult to control at higher master volumes with settings around and over 75% of a given tube's max plate dissipation.

Some of the tubes I tried were definitely undersized max wattage-wise for the circuit(s), but for some situations those lower power vintage tubes sounded wonderful, particularly in situations where a lot of sustaining bloom/feedback was desired at lower gain/volume situations, like clean+PAB, I used my OTS type in a large church running Syl 6l6gb's biased at an average of 38mv when hot, a very hot setting for them, but they were simply warm, wonderful, musically sweet and wanted to toss out octaves above fundamentals and sing for days with judicious use of a Keith Hilton (designed for pedal steel) volume pedal. At the end of six months, the tubes were still quite healthy when I pulled them and replaced with adapted Sylvania 6bg6ga, their 7581a's guts in a different pinout bottle.

In my experiences, bias setting is a global tone control as well as the amp's response to player touch. The pickup type/configuration of the instrument is also of consideration. I might be tempted to set bias a couple or three mv higher for Danelectro Lipsticks versus a hot, strong signal humbucker setup. I'm pretty happy running my Svetlana's at an average of 33mv in my SSS # Nik. They provide a sound pretty close to the Syl 6l6gc that they were modeled after. I'm thinking that is around 50-55%-ish max plate dissipation there.

YMMV...

[harris51]

is that per tube or for the pair your talking about

[JazzGuitarGimp]

Thanks Martin and 21Mavguy. I'll definitely be spending some time at the Aiken site.

Lou

[JazzGuitarGimp]

is that per tube or for the pair your talking about

Hi harris51,

It is per tube. The datasheet specifies 116mA for the pair. Here is the datasheet:

http://svetlana.com/pdf/tungsol/6l6gstr-tungsol.pdf

Many thanks,
Lou

[martin manning]

I see on the Svetlana data sheet they are using 5k6 load impedance, which will keep the max dissipation down compared to the 4k usually seen in guitar amps. Even so, that will put the max Pa on the Class A and AB load lines 10-20% over the 30W maximum. With a little sag in the power supply, maybe it's all good.

[talbany]

I personally use those spec sheets only as a guideline more so than a point of reference due to the different operating points most sheets have.. IMO follow the Aiken rules to set up your operating range and tune to taste!!

Tony

[JazzGuitarGimp]

Thanks everyone.

The amp in question belongs to a friend. It is allegedly a d-style clone. When I got to it, it was biased at 14.6mA @ 460V, or 6.7W (about 22%). I went ahead and adjusted it up to 40mA, B+ dropped to 444V, or 17.76W, or about 59%. Then I did a search for d-style bias and discovered that a lot of folks prefer a colder bias on these amps, but nothing was said about specific bias points.

My friend likes it better now that we've 'warmed it up' a bit. I have a feeling we'll push it up even further before we're done. But this is why I originally asked if anyone knows what the amp should be biased at. And I could (should) have been more specific in my query: What I was driving at is, what does HAD bias his 2x6L6GC, 50W amps at?

The Aiken site was quite informative, and I see that for a class AB amp, 70% seems to be a comfortable upper threshold for the bias adjustment. So we may shoot for that and see what we get.

Thanks again,
Lou

[martin manning]

Just one point of detail- are the mA numbers being quoted cathode current (plate + screen) or plate current alone? As a first cut, my usual strategy is to dial up 70% of max plate dissipation using cathode current (what is measured using current sensing resistors from cathode to ground) knowing that I am in reality ~3-4% below that. From there one can measure screen current and tweak to taste.

[JazzGuitarGimp]

Just one point of detail- are the mA numbers being quoted cathode current (plate + screen) or plate current alone? As a first cut, my usual strategy is to dial up 70% of max plate dissipation using cathode current (what is measured using current sensing resistors from cathode to ground) knowing that I am in reality ~3-4% below that. From there one can measure screen current and tweak to taste.

Datasheet says: "Zero-signal plate current" of 116mA for a pair of tubes - and at the specified B+ of 450V, that equates to 26.1 watts per tube. 26.1 / 30 = 87%. I thought this sounded high, based on reading here and there, over the years. After my visit to the Aiken site yesterday, I see that it is indeed high. Maybe I'm starting to get a feel for this tube stuff..... probably not!

[talbany]

I took a look at the old GE data for a 6L6GC. The characteristics and typical operation for a push pull ab1 amp says for a plate voltage of 450V, 400V screens, -37V bias, and 5.6K load resistance you will typically get 116ma of plate current for two tubes with zero signal. That's 58ma per tube and probably OK in a push pull amp where the tubs don't conduct all of the time .In fact, under the same conditions screen current would be 5.6ma so if you measured cathode current you would see 63.6ma at idle.


However, I wouldn't draw any conclusions about that data unless I had the exact operating conditions e.g. 5.6k effective load resistance and peak audio grid to grid voltage of 70v.( Quite low for guitar amplifier)


Just for grins I measured an amp on the bench - with the volume on 2, I measured 200v peak - 400v peak to peak. So, you would need to limit output of the PI to ensure you don't exceed design maximum ratings of 210ma plate current (105 per tube) and 22ma screen current (11 per tube).

As you know biasing output tubes is subjective. The less than 70% method is just a rule of thumb. I don't have time to run the load lines right now. That only works as a average anyway. This type of discussion is never ending.
As for me, I'll stick with the rule of thumb 70% (illustrated by Aiken). I don't like buying output tubes that much.


Hope this Helps

Tony
___________

[212Mavguy]

Both Talbany and Martin Manning ended posts in a very similar way...tweak or tune to taste...and either of them know WAY more about these amps than me.

Another one for their viewpoint here...my experiences go with them.

IMHO Formulae are a great way to get in the ball park, but the ears are the final decision maker for me. My preference is to start out a tad hot and cool things down a couple mv at a time until I hear a change in the treble and upper mids, it's at the point to where they become more pronounced, and that is audible at a certain point. 1-2mv adjustment change at that point yields a tonal difference.

I also noticed that my ears like a brand new amp set with the bias a bit hotter at first, sound of that amp after as little as 50-100 hours of breakin fattens and warms up a fair amount, like about 3mv/tube difference between the new and broken in settings in my D-clone experiences.

[Structo]

Also as I have read from numerous sources, power tubes lose their high frequency response as they age, in use.

So that corresponds to what 212Mavguy said.

It happens so slowly that many times we are not aware of the change.
Perhaps this is another part of breaking an amp in if the power tubes are new.

Of course different tubes, brands and operating hours of the tubes will also have telltale sonic differences.