unexpected capability discovery in my amp

[glassman]

Sorry about the rant here...but I gotta tell somebody!

As many of you know, I am building my own amps now and am currently putting the finishing touches on my website before it goes live (by Jan 1). So, just to get all of the information I can out there about it's capabilities/limitations, I decided to test the amp to see what level of output tube mismatch it would handle before it starts to hum and act weird.

As I've said before, it can take 6L6s, KT66s, EL34s or 6550s and any of thier variants. All of my previous testing has been with matched pairs. So I grab a pair of 6L6s that are 10mA apart from one another...no hum and great tone. Next, I grab a set of EL34s that are around 10mA apart and they also sound great with no hum. All I have left at this point to throw at it are a set of EL34s that are 20mA apart...once again no hum and great tone.

Wanting to find the point of failure is an obsession of mine so I decided to take the experiment to what I felt was an extreme...one 6L6 and one EL34. They biased a full 30mA apart from one another but the amp didn't hum and the tone was great...and different...a mix of EL34 and 6L6 character that I hadn't heard before. Almost impossible to describe but I will include a clip of it on my website.

Meanwhile...still not satisfied that I had found the edge of capability, I set it up with one EL34 and one 6550...a full 60mA difference between the two at idle. I got a little bit of low level hum but not enough to pi$$ off a recording engineer. This combination sounded like nothing I've ever heard before. Warmth and clarity in the cleans and heavy crunch with a warm clean tone in the background at the same time...sounded like I was playing two amps at once; one clean, one dirty. This will be on a clip on the website too.

I apologize about tooting my own horn about this...its got me pretty excited and I obviously didn't design this capability into the amp. And to think, I was contemplating adding a bias balance or separate bias adjustments for each power tube sometime in the future so I could explore this...guess I can cross that off the list!

Is anyone aware of an amp out there that can do this?

[drpietrzak]

Re: unexpected capability discovery in my amp

THD Bivalve does this. ...

http://www.thdelectronics.com/pdf%20stuff/BiValve%20Lit%20text%207_10_02.pdf

I didn't think the issue was so much tube matching (that was not something early amp builders worried about - at least what I have read) for the amps. It was the differing voltage levels that killed or the other of tubes that has been the issue I think. But I just read - not build.

[some_dude]

Re: unexpected capability discovery in my amp

How long will your amps be able to withstand that kind of mis-match?

It seems advisable to test that before you list it as an 'builder endorsed option' lest it turn into a warranty nightmare.

[glassman]

Re: unexpected capability discovery in my amp

THD Bivalve does this. ...

http://www.thdelectronics.com/pdf%20stuff/BiValve%20Lit%20text%207_10_02.pdf

I didn't think the issue was so much tube matching (that was not something early amp builders worried about - at least what I have read) for the amps. It was the differing voltage levels that killed or the other of tubes that has been the issue I think. But I just read - not build.

That's VERY interesting! I see that the THD BiValve is a single ended Class A amp. Mines a push-pull AB amp that can be biased to Class A. Two different animals so I'm safe for now!

Keep em coming guys!

[glassman]

Re: unexpected capability discovery in my amp

How long will your amps be able to withstand that kind of mis-match?

Indefinitely. The mismatch shouldn't present a problem. My torture testing in the initial stages involved running a pair of 6550s flat out (110mA each) for 24 hours...not even a hiccup. In fact, I tested the first 5 amps in the same way. I continue to torture #1 still. This was just my latest arcane experiment.

It seems advisable to test that before you list it as an 'builder endorsed option' lest it turn into a warranty nightmare.

Rest assured that I will do this. A 24 hour mismatch marathon is in order!

[drpietrzak]

Re: unexpected capability discovery in my amp

Here is what I was referring to ...

"Now a word about matched or matching tubes.

Back in the 50's people were plugging in whatever was on hand and because of this some otherwise reliable tubes met their maker. In a basic tube amp, the cathode current of both tubes is measured at the bias test point. For example, say our reading is 70mA. That could roughly be 35mA each, or with an unmatched set 50mA through V4 and 20mA through V5. The latter would appear to be operating safely, even after calculating static plate dissipation, but V4 would obviously be running much hotter than V5. Increasing the total bias could destroy the tube in V4, and such a large mismatch could cause long term damage to the output transformer. This is an extreme example, though possible.

One argument against matching tubes is that it is "unnecessary" because the phase inverter and output transformer are "unbalanced." This is true, though we're not trying to perfectly balance our poweramp; it's damn near impossible, not to mention counterproductive. Almost all production tube guitar amps comes with an adjustable bias control, like a DSL50. However, one bias pot controls the current flow for both power tubes. Here, balancing the power amp really isn't even an issue.
...

(Okay Cuda) Originally matching was a "Hi-Fi" thing, along with "balanced" phase inverters and the such. In truth the phase inverters used in guitar amps will never be perfectly balanced because both split load and long-tail phase inverters are unbalanced by design. (Ignore the "balanced" phase inverter hype.) The idea of matching was to get the signal as clean and hum-free as possible. In push-pull amps the power tubes are run 180° out of phase of each other by the aptly named phase inverter, sometimes called the power inverter. The power amp is set up like a humbucker, which cancels hum and noise but not the guitar's signal. A Class AB push-pull power amp is ideal for HI-FI because it's eliminates most of its second order harmonics, which affect the clarity of a audio recording. (Remember that Fender's goal was very loud and clean, an idea that wasn't fully reached until the Blackface Twin.)

What most people don't know is that asymmetrical (uneven) output produced by mismatched tubes may actually be desirable in guitar amps. Classic vintage Fender amps did not use matched power tubes for economic purposes. Vintage gear has literally reached a religious status, and as a result some vintage enthusiasts assume "if Fender did it in the 50s and 60s it's right!" This is simply poorly reinforced conjecture. They know what works, but they don't know why it works. What most guitarists don't know is that mismatched tubes actually increase second harmonic distortion, which many find beneficial for tone creation but not tone recreation. In my opinion, the importance of matching "for a better tone" has been overemphasized by audiophiles and those who read vintage Hi-Fi! Great for your stereo, but not as important in your guitar amp unless you're trying to achieve the "cleanest" sound possible.

If a power amp is more "balanced" than not, even-order harmonics are cancelled out, which lowers THD (total harmonic distortion). It was, and is, important that vintage Hi-Fi receivers have less than 1% THD for the clearest sound reproduction possible; but, as I explained earlier, many guitarists prefer lower even order harmonics (2nd and 4th) as they add "life" to the tone. It's not at all unusual for a tube guitar amp to be routinely driven passed 10% THD.
Mismatched tubes may increase desirable second harmonic distortion, though at the expensive of increased 60-cycle hum. (I've heard some people describe mismatched tubes as sounding "fatter.") Also, in theory a large enough mismatch could damage the output transformer, though it probably won't happen. It's safe to say that tubes drawing within 8mA of each other should be fine. Anything greater could cause long term damage. I see nothing wrong with a little "controlled" mismatching if biased at 50-60% plate dissipation (to make up for unbalanced current draw), though I'm against plugging in random tubes just because "Fender did it," or because "that's how we used to do it back in the day." All tubes should be burned in to help control drifting, but they do not need to be matched "perfectly" unless they're going inside your stereo.
Matched, mismatched? It's subjective. Which ever you choose just be sure not to exceed maximum plate dissipation. Some find that slightly mismatched power tubes that are properly biased yield the greatest tonal results." http://www.marshallforum.com/workben...istortion.html

Also ...

http://www.tdpri.com/forum/amp-central-station/173651-unmatched-power-tubes.html

http://forum.grailtone.com/viewtopic.php?f=4&t=1517

http://www.audiokarma.org/forums/sho...d.php?t=289499 (last comment is funny)

http://tone-lizard.com/tubes.html

[glassman]

Re: unexpected capability discovery in my amp

Excellent post drpietrzak.

The hum issue was my only concern when it came to mismatching. I totally agree that a degree of mismatch can produce a more musical output in a guitar amp...as long as the design is capable of withstanding it. Many, if not most guitar amps are not; the power supply determines that. The output transformer only recieves as much stress as the power supply allows and most guitar amps have a power supply that has more muscle than the tubes and the OT...that's where I differ.

[DankStar]

Re: unexpected capability discovery in my amp

sounds cool - so one wouldn't need to even bias the amp with a tube swap out, and you could just grab whatever is on sale and drop in it? that's nice!

[joelap]

Re: unexpected capability discovery in my amp

Show some pics of the prototype! Interesting discovery on the output tube front, and thanks for sharing. Too many builders go into secrecy once they start deciding to sell their amps/pedals/etc.

I'm in the same boat as you, planned to have my amp/pedal/pro audio thing going by Jan 1st. Then I took a 2nd job and things got outta hand. Keep me in the loop of your progress.

[firebirdV]

Re: unexpected capability discovery in my amp

Cool, can't wait to hear clips.

A couple of years ago I came up with the idea of putting some sort of crossover circuit in the preamp which would allow you to have separate distortion controls for a couple of frequency ranges (mainly so one could reduce the bottom end mush in many high gain amps). This sounds like it kind of has a similar effect, although through a different means.

Keep us updated, these posts are some of the most interesting on the forum.

[drew_half_empty]

Re: unexpected capability discovery in my amp

i think the randall with the little module things has tubes you can bias individually, but that's the only one i've seen other than the THD

[Brown Note]

Re: unexpected capability discovery in my amp

When you're fiddling around with your amp, the only things you really need to be wary of are RMS current and power of tubes and transformers; even gross over voltage does not destroy the tubes right away and 110% voltage will work just as reliably as 90%. Marshalls had their "high voltage" amps that were running with 600V+ on both anode and screens. EL34 screens are only rated for 500V operation. Scoping the cathode current is a good way of measurement as it is inheritly "safe" due to cathode current also flowing through the screens. Screen current is roughly 10% of the anode current hence cathode current is a bit more than the anode current.

Any mismatch does not hurt the amp, unless some of the tubes are overdissipating. A regular 50W amp that has four power tubes could use any mix of tubes, including 6v6! The output transformer does not care how it is driven as long as the max RMS currents are in order. You can have one side amplify 80% of the signal cycle and 20% on the other and it does not matter(as long as the transformer can handle the current). If you have 4 bias controls, you can use a pair of big tubes(6550 for examble) that are biased to almost class A and a pair of 6V6 tubes that are class C and the amp would work fine. Hifi amps commonly have a push pull transformer output transformer with two sided voltage feed with the centre tap grounded and power tubes feeding the transformer as cathode followers.

So keep good look on the dissipation of the tubes, and don't measure with just full power with the tubes driven into saturation or plain sine wave. The worst dissipation does not happen at max power, but when the signal amplitude is half way between the saturation voltage and max anode voltage. Pushing pulse signal at half of the max amplitude is the worst case scenario if the tube dissipation is conserned. By the way, if you're conserned about the splitter being unbalanced, you can easily balance the output by just changing the anode resistors. They need to be mis mached as the schmitt splitter has one input as return point for the feedback.

[SlyFoxx]

Re: unexpected capability discovery in my amp

It just sucks when a plan comes together....oh wait. I think we all need to crank one up!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[glassman]

Re: unexpected capability discovery in my amp

Show some pics of the prototype! Interesting discovery on the output tube front, and thanks for sharing. Too many builders go into secrecy once they start deciding to sell their amps/pedals/etc.

I'm in the same boat as you, planned to have my amp/pedal/pro audio thing going by Jan 1st. Then I took a 2nd job and things got outta hand. Keep me in the loop of your progress.

Check out the thread I started in thr Sound Room called "Letting it all hang out". That has all the progress and pics.

Cool, can't wait to hear clips.

A couple of years ago I came up with the idea of putting some sort of crossover circuit in the preamp which would allow you to have separate distortion controls for a couple of frequency ranges (mainly so one could reduce the bottom end mush in many high gain amps). This sounds like it kind of has a similar effect, although through a different means.

Keep us updated, these posts are some of the most interesting on the forum.

Sweet...experiment away...you may find something really cool in that idea.

i think the randall with the little module things has tubes you can bias individually, but that's the only one i've seen other than the THD

Good info...and I'm still safe!

So keep good look on the dissipation of the tubes, and don't measure with just full power with the tubes driven into saturation or plain sine wave. The worst dissipation does not happen at max power, but when the signal amplitude is half way between the saturation voltage and max anode voltage. Pushing pulse signal at half of the max amplitude is the worst case scenario if the tube dissipation is conserned.

Power tubes are safe...I really can't comment too much beyond that without giving out too much information.

It just sucks when a plan comes together....oh wait. I think we all need to crank one up!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

If I play my cards right, you will be able to do just that some day.

[Gearjoneser]

Re: unexpected capability discovery in my amp

And to think, I was contemplating adding a bias balance or separate bias adjustments for each power tube sometime in the future so I could explore this...guess I can cross that off the list!

Is anyone aware of an amp out there that can do this?

I'm thinking that adding two independent bias pots would be a great selling point since then you wouldn't have to run the bigger tube so cold, and the ability to tell people they can throw any octal tubes in there they wish.

I know you're pushing the circuit as much as you can, in order to get failure, but does this stress the transformer or change the speaker impedence at all?