A caution about individual power tube-biasing

Beware,
If you outsmarted yourself and ordered an individual tube-biasing option, beware that them great big output tubes bias at 0.6V apiece, not the 1.2V/pair we are used to using for the twin-biased amps we got from from Bob's kits.
Having recovered from all the hair-singing, and fights with the insurance company, I must compliment the 6550 design. It ran for four hours before nearly torching the house.
Current tubeage, should any one care:
Sylvania VT-231s in the front holes and RCAs in the back holes of the SP-14.
RCAs in the driver slots, and Jim McShane's GL KT-88s out front of the amp. Just a pair in each amp. Four just wastes electrons.
Still, I am amazed that that double-biased 6550 held up as long as it did ...

deepee99 wrote:Beware,
If you outsmarted yourself and ordered Holger's individual tube-biasing option, beware that them great big output tubes bias at 0.6V apiece, not the 1.2V/pair we are used to using for the twin-biased amps we got from from Bob's kits.
Having recovered from all the hair-singing, and fights with the insurance company, I must compliment the 6550 design. It ran for four hours before torching the house.
Current tubeage, should any one care:
Sylvania VT-231s in the front holes and RCAs in the back holes of the SP-14.
RCAs in the driver slots, and Jim McShane's GL KT-88s out front of the amp. Just a pair in each amp. Four just wastes electrons.
Still, am amazed that that double-biased 6550 held up as long as it did ...

All one can say is a great big "OOPSIE". Thank dog you live in a thinly-settled district. Now you'll remember to read the fine print....

What? There's fine print? I did not see no fine print.
I need glasses, apparently.

Deepee99, did you really torch your house?

No, Wintr, I didn't. But when that tube let go it sure smelled like it.

all I can say is D'OH!!!!!!!

Lessons to be learned:
1) TRIPLE-CHECK every step. That goes double for pre-action logic, i.e. what's the damned bias supposed to be?
2) DYNACO-BASED AMPS ARE PRETTY F'IN' SOLID. Can you imagine what such a blunder might have caused in, say, a nice $5000 Audio Research amp, with its serpentine and inscrutable power supply "regulation" ???

MontanaWay wrote:all I can say is D'OH!!!!!!!

Individual output tube bias pots are an option on Holger's (Montana Way)'s Big Bopper amplifier iterations of the M-125 mono-blocks. I highly recommend this option as you can monitor each tube that way, and if one tube gets out of sorts you don't have to guess which of the pair it is.
I had just gotten into the habit of shooting for 1.2VDC per pair in my original M-125s as opposed to 0.6V PER TUBE and when I last did an output swap I cranked one of the four pots up to 1.2V, which is twice the bias a single is designed to handle.
I owe Holger an apology for unintentionally implying this was a design flaw. Far from it. I ordered the individual bias option and am glad I have it. Montana Way's amps are well designed, sturdily built and flat-out gorgeous to the ears and eyeballs. We've been doing business for well over a year now and I'm also proud owner of his "Janis" phono preamp -- essentially a well souped-up PH-16. This lash-up, anchored by Don Sachs' SP-14-based line preamp, is as good as it gets.
There are several other good builders/modifiers of these splendid kits that Drs. Mottram and Latino have brought to our table. Troy Madden and Bill Gratix come to mind and there are no doubt others. Can't go wrong with any of 'em.
OR, you could sell the Cadillac, the Range Rover, your first-born and forget that swimming pool and get something almost that good.

BTW Burial Services for that post-war Tung-Sol 6550 commence at 1500 hours today. It will be interred in its original cardboard box in a shallow hole in the back-yard, along with $150 in Federal Reserve notes to remind future generations of its value and its ability to perform at twice its bias. It lived a good, long, and dutiful life. Flowers are not necessary. Regrets in its memory may be sent to the Macallan Scotch Co.

Thanks, David for the kudos. I do my best.

Bill Gratrix

audiobill wrote:Thanks, David for the kudos.  I do my best.

Bill Gratrix

Well, you missed a heckuva funeral! She went down peacefully.

Wow. 120mA into a 6550, and it lasted 4 hours? That's pretty good.

skriefal wrote:Wow.  120mA into a 6550, and it lasted 4 hours?  That's pretty good.

Yep, they don't make them like that any more.

Deeptee99,

You gave the Tung Sol a Viking funeral with all the trappings, I presume?

"Try to set the night on fire"

-Jim Morrison

Worth mentioning ... you also need to make a couple laps around the amp when setting individual bias for each tube. All that B+ is on the same line, and any one adjustment will affect all four tubes. I get mine in the neighborhood on the first pass, then fine tune again as needed. No sense getting too finicky on the first pass, as the first couple tubes will probably be off a good 5mv by the time you get the last one dialed in.

Just went through that here as a matter of fact after swapping out to the KT120's for the first time in a while. Surprisingly, those bias up much higher using the same settings as any KT88 in the stable, so the meters come in handy. I leave one on as the amp warms, and what was a 55mv reading on the KT88 will go up to around 80mv with the KT120. Turn them down to 60mv (the KT120's seem happiest there and give me a bit more punch), let the amp warm and stabilize, then fine tune with the VOM using the test points.

sKiZo wrote:Worth mentioning ... you also need to make a couple laps around the amp when setting individual bias for each tube. All that B+ is on the same line, and any one adjustment will affect all four tubes. I get mine in the neighborhood on the first pass, then fine tune again as needed. No sense getting too finicky on the first pass, as the first couple tubes will probably be off a good 5mv by the time you get the last one dialed in.

Just went through that here as a matter of fact after swapping out to the KT120's for the first time in a while. Surprisingly, those bias up much higher using the same settings as any KT88 in the stable, so the meters come in handy. I leave one on as the amp warms, and what was a 55mv reading on the KT88 will go up to around 80mv with the KT120. Turn them down to 60mv (the KT120's seem happiest there and give me a bit more punch), let the amp warm and stabilize, then fine tune with the VOM using the test points.

Yep, Skiz, every tweak you do to the bias on one tube will affect the other in the same Dynaco circuit. I think Bob L. wrote here or elsewhere is "Don't go chasing voltages." For the KT-120s, yes, they like 1.2VDC /pair or .6/tube. But if you're cold-starting a new rack, my best luck is about .53 or so VDC/tube. A half-hour later they'll be up to .55 or higher. So no harm done and you can start twiddling. After they've cooked for a coupla hours they will be close to .6/tube or 1.2 per pair and that's the time to pay attention.
They will also swing up or down depending on what your incoming line voltage is, and believe me, it does vary out in the sticks. It's worth the five minutes' call to the local power company to ask them when their peak and lowest demands are. Lowest demands give the highest voltages to your critters. Back 'em down and if you're not over the limits at low bottom, nobody dies.

To my ears....when the output tubes cathodes are tied to gather ... so they can talk to each other...after all it takes two for push-pull......then ran to ground though a one res.... they sound more like music...well too me...
hehe 40 years of..... playing with tubes.....
each cathodes with it own res.. too ground .... like trying too dancing with one foot nailed to the floor...an I get more output-drive it seems...
I done both ways.....minny times...with minny tube amps....after all.... as has been pointed out there only one power supply.......an your pulling more power with each tube res..
Hay look... if we were back in the day... an tubes were way out of match.....but today people like Jim McShane can give vary close match..... I never have a Problem with one bias pot.....I gess Dynaco did not eather.....I have 3 pr of MK3s here....some were made in the 50s.....with stock GE 6550s.........still sound great...an becase i run them at 110ac on a varic..too be at about 480-5 B+DCV....never lose tubes.....
have fun with tubes...........set them up anyway....great tone..
all just one mans o-pine....



thanks for the site Bob....................

deepee99 wrote:
They will also swing up or down depending on what your incoming line voltage is, and believe me, it does vary out in the sticks. It's worth the five minutes' call to the local power company to ask them when their peak and lowest demands are. Lowest demands give the highest voltages to your critters. Back 'em down and if you're not over the limits at low bottom, nobody dies.

I haven't noticed any real major fluctuations in bias voltage no matter what the wall socket is supplying. Long as the B+ reserve is high enough, the circuit seems to be smart enough to sort it all out and give the tubes what they need. Wonder how much the CCS tail on the newer VTA board has to do with that?  

I DO notice that the ST120 sounds a bit pinched running the typical 124vac or thereabouts we get here. Hence, my cute lil ol' bucker.

zx wrote:To my ears....when the output tubes cathodes are tied together ... so they can talk to each other...after all it takes two for push-pull......then ran to ground though a one res.... they sound more like music...well too me...

hehe 40 years of..... playing with tubes.....

thanks for the site Bob....................

David Hafler thought so, too.  He claimed better distortion performance when the two cathodes were tied together, as in
the BIASET circuit, rather than each having its own ground.  

It was suggested in an issue of Audio Amateur that one could get the benefits of both: if the two cathodes are separated
at DC so bias can be set individually, the two cathodes could still be tied together FOR AUDIO SIGNALS (AC) through a large capacitor.

Thanks for that input.....best cap I ever heard was.......No cap...less is more......it all about the sound..tone.....get there any way you can ....tubes..SS...with auto-fourmers......hehe....Mcintosh...
drakest sounding tube amps I have ever owned.....it know wonder people think the MC225 is the best souding.....tubes.....still making great sound....happy hoho to all





thanks for the site Bob...............

Well, what started as a little self-poked bit of humour -- and yes, Class, that 6550 really did last four hours at 1.2 before falling on its sword -- you've given me quite an education. (Perhaps our friends in New Jersey threw a little reinforced concrete onto the plates and screens and that's why the house is still standing.)
I am hearing a bit of consensus here that biasing tubes individually is not to be preferred over biasing them in pairs. I am no electrical wizard here, but is pairing them up for this adjustment a better way to go? I've always thought that singling each part for measurement was the better way to go, but am certainly willing to be corrected.
Thanks,
d

deepee99 wrote:Well, what started as a little self-poked bit of humour -- and yes, Class, that 6550 really did last four hours at 1.2 before falling on its sword -- you've given me quite an education. (Perhaps our friends in New Jersey threw a little reinforced concrete onto the plates and screens and that's why the house is still standing.)
I am hearing a bit of consensus here that biasing tubes individually is not to be preferred over biasing them in pairs. I am no electrical wizard here, but is pairing them up for this adjustment a better way to go? I've always thought that singling each part for measurement was the better way to go, but am certainly willing to be corrected.
Thanks,
d

Biasing individually makes it possible to balance the current exactly.
On the negative side ( as already mentioned ) with the "soft powersupplies" we have,
reducing bias on one tube will reduce the current in the same tube, and as a
result B+ voltage will rise and the other tubes will increase their current.

Adjusting bias will be a iterating process where adjusting each tube with
smaller and smaller increments.

The same goes for the original ST-70, the only difference is that it's half
the number of adjustment points.

So Peter, what might you suggest? (And BTW thank you for your quick reply).
I am probably not adept at anything more than waving a hair-dryer a few times at the under workings and see what pulls loose.
Bear in mind that I am at least aware than when you tweek one tube's bias of the other will respond in the opposite direction. It is a bit like herding cats to get everybody even. Is it worth the effort?
I have, by the way, had far more fun dealing with my preamp. Stuff does not explode and a gentle bit of tube-rolling takes all the inconvenience of the bother of tone controls.
Cheerio, and I believe your experiments and experience would greatly inform our humble board.
-d-

deepee99 wrote:So Peter, what might you suggest? (And BTW thank you for your quick reply).
I am probably not adept at anything more than waving a hair-dryer a few times at the under workings and see what pulls loose.
Bear in mind that I am at least aware than when you tweek one tube's bias of the other will respond in the opposite direction.  It is a bit like herding cats to get everybody even. Is it worth the effort?
I have, by the way, had far more fun dealing with my preamp. Stuff does not explode and a gentle bit of tube-rolling takes all the inconvenience of the bother of tone controls.
Cheerio, and I believe your experiments and experience would greatly inform our humble board.
-d-

My suggestion is : turn carefully and not to much, rotate among the tubes.
If any tube has large offset, don't adjust fully at once just a little bit and
do the rest next round.

Biasing individually allows you to set quiescent current exactly the same for both (or both pairs of) output tubes, even if they require different bias.  However this does not guarantee that under SIGNAL conditions, the drive required by the tubes will match.