Does dual bias really work to 'Match' tubes?

[Ed_Chambley]

I have modded a few bias supplies for dual adjustment.  Problem I find is you never know if new tubes are matched unless you check the voltage and it is the same.  I have put 2 meters with the hold function while playing and even tho they are matched at idle, they are not matched at playing levels.  If you adjust for playing levels then the idle is not matched.

I received some KT-120's from Tube Depot which I think would have matched better if I would have purchased singles off Ebay and just took a chance.  I am reading where others are having this problem.  Everyone makes mistakes and we will see if they pick-up shipping both ways.  They should so I will hold off on bad mouthing, but I read where one member modded his Mig 60 to dual to get the idle current to match.  Same vendor.

There used to be a guy here in Atlanta where I got all my new tubes and he used a maxi-matcher and a burn in.  Great supplier, but he moved and I lost him.  I could go there and check the tubes at the voltage and current before taking them home.

Also, anyone ever used cryogenic treated tubes or is this just a selling BS? Where are you guys getting tubes?  I have had this problem for a while and I have tried a number of suppliers.

[sluckey]

Maybe we are too obsessive about bias and matching. Back in the day, a simple bias voltage pot was sufficient. Slap in some tubes and just set the negative bias voltage to manufacturer's specs and you're done. Millions of amps left the factory that way. And most of them sounded good.

  Are we really that much smarter than the guys that designed, built, and marketed this stuff?   

[Ed_Chambley]

Maybe we are too obsessive about bias and matching. Back in the day, a simple bias voltage pot was sufficient. Slap in some tubes and just set the negative bias voltage to manufacturer's specs and you're done. Millions of amps left the factory that way. And most of them sounded good.

  Are we really that much smarter than the guys that designed, built, and marketed this stuff?   

I do not think we are smarter, but I a quite sure tubes vary much more than they did back in the day.  Simply put, would you just set voltage, sock in some tubes and let her ride today?  Back in the day a lot of tubes came with test sheets.  I have some where in the box there is a sheet of paper with hand written test results.  They had a reason for doing this and I do not think it was because they thought they were smarter than amplifier designers.

Maybe today we focus too much on idle bias, but at what point does a mismatch cause problems?  This is the reason I posted this thread as I do not know.

[eleventeen]

Tube matching is a long and hairy topic. A few years ago I found an old fellow who sold me the contents of his WW2-radio packed garage which included about 1200 NOS tubes. Among them were maybe 22x Sylvania JAN 6L6G's....which I thought I could get more money for if I sold them as "matched". As you probably known, "6L6G" are overstressed in your typical Fender amp. So, I built a tube matcher which included 2 big-face meters that I could photograph to demonstrate that two particular tubes were indeed matched. Interesting project, used 3 qty transformers which I luckily had lying around. The filament and bias were OFF the variac, the B+ tranny was ON the variac. Anyway...the readings on those tubes (60 years old) were unbelievably all over the place (under constant op conditions) and I could find maybe 5 pairs out of 11 that could be matched. I am talking about 6L6Gs that idled at say 18 ma all the way up to about 37 ma under identical conditions, just to give an idea of what I mean by "all over the place".

I bought an old Radio Craftsman amp for $25 that had 2 qty KT66 Genalex. Original tubes. They matched within 1/2 ma across the entire range my matcher could produce. Unflaming believable.

I'm not sure exactly what I'm trying to say; some folks claim tubes are "matched" if they have the same base paint eg; are from the same mfr batch. I have a pair of Sylvania 6L6GC one yellow paint, one green paint, clearly made years apart, that match remarkably well. I have other pairs from the same mfr batch that are 8 mils apart (which is a lot over a total range of say 0-40) I have a set of RCA 6L6GCs that are at least 40 years old that match remarkably well...I of course don't even recall where I got them.

I don't think this matters a heck of a lot in a geetar amp as long as the outputs are within a few mils of each other. It's entirely possible that you or I could prefer the sound of an amp with mismatched tubes.

[6G6]

I agree that tubes used to be better matched.
Maybe it was that they cared more, but I think a
lot of had to do with making larger batches at a time, when demand was greater.

Not everyone realizes that matching idle current doesn't mean the tubes will match.

[6G6]

Sometimes mismatching can produce a sound you like.
Probably not a good thing if you are looking for maximum clean, though.

[sluckey]

Simply put, would you just set voltage, sock in some tubes and let her ride today?

Of course not! That would be boring. But then I'm an obsessive tinkerer.   

[Ed_Chambley]

Simply put, would you just set voltage, sock in some tubes and let her ride today?

Of course not! That would be boring. But then I'm an obsessive tinkerer.   

I resemble that remark.  All was fine until I learned to use a scope.  Used to I would simply switch tubes until they were matched best.  For instance, at any given time I have 10 to 50 new tubes according to the type.  I currently have 20 matched 6550 for Leslie amps and 6l6gc probably 30 sets.  I have been buying a lot of 6V6 as well as I am seeing them being used more and more.  They are all in pairs.  None are my private stash.  I got a little pissed when I fired up my recent build.  I put in new 6550 and Kt88's and the new Tungsol 6550's matched very well.  I have had these for a couple of years.  Installed the KT120 and at 50 volts I got 72ma on one and 39ma on the other from a $90 purchase.

What I really got bent over when I put in 2 random pulls of Tungsol USA.  I have a box with probably 40 pulls of 6550 marked GE and Tungsol.  Same tube.  So I pulled one and replaced it and they were still within 4 ma.

The story I was told is there is not enough profit in tubes these days, so they have to push volume and in doing so, they let out some mismatch.  To rephrase, they basically told me since they did not make enough money on them, they did not care so much.

The same color makes a match, now that is funny.  Makes me think of a Push Pull 6v6 amp I have.  One tube is marked delco and the other crosley.  Bases are different, one is smoked.  They are a mismatched match.  I have tested the mismatched idea and it is simply not true they sound good to me, but as most know I do prefer a clean tone.  Even in high gain amps you can hear crossover which can only be corrected with a colder bias.

Sorry for the rant, but I got mad at the response from the vendor.  I told them they would not have to worry about it much longer.  The chinese will begin selling them direct and when the get them right, they won't have to worry about small profits any longer.

[alerich]

I think biasing for play (instead of idle) may be a bit misleading unless you play pure sine waves. A guitar audio signal is a complex beast. Beyond that, if a seller advertises a matched set I expect them to be a fairly close match. If you're just picking two tubes and shipping them out as a pair then don't advertise them as having been matched. I won't complain about matching if I buy them. Can I really hear the difference? I dunno. Can I see the difference on my meter? I can. Does it bug me? It does.

[eleventeen]

"The same color makes a match, now that is funny.  Makes me think of a Push Pull 6v6 amp I have.  One tube is marked delco and the other crosley.  Bases are different, one is smoked.  They are a mismatched match.  I have tested the mismatched idea and it is simply not true they sound good to me, but as most know I do prefer a clean tone.  Even in high gain amps you can hear crossover which can only be corrected with a colder bias."

All over ebay....if they are the same paint, most people call them a "matched pair".

Wow, those were wildly different readings on your KT120's. What you said about those old Tung-Sol 650's....yeah, those were some very good quality tubes. Made in New Jersey, on Bloomfield Ave....near where I grew up! And the GE 6550's are very good too.

[jjasilli]

Tubes may match in test equipment but not in a guitar amp.  They should be tested in the same circuit to avoid circuit variations from skewing the readings.  E.g. the windings are different on each side of the typical amp's PT, so ea power tube is in a different circuit.  Also, new tubes must be burnt in before tested for matching.  Their current draw will vary at first; then stay the same for awhile; then drift with age. 

Anyway if tubes are matched you still need a bias balance circuit in your amp to rebalance them in the amps unbalanced circuitry - if that is your goal.

[PRR]

> Does dual bias really work to 'Match' tubes?

No.

> matching idle current doesn't mean the tubes will match.

True.

> Maybe we are too obsessive about bias and matching.

Yes.

> Are we really that much smarter than the guys that designed, built, and marketed this stuff?

No.

> Back in the day a lot of tubes came with test sheets.

No. (Define "a lot"?)

> I have some where in the box there is a sheet of paper with hand written test results.

Very odd. Not doubting you, but 1960s-1970s, I never saw such a thing.

Different amps respond different. I had no techical trouble (didn't do much ear-test) with a cathode-biased push-pull amp run mellowly so it could take "any" of the higher-power Octals. Even Sovtek 6550 on one side and 1939 metal 6L6 on the other side. Just no difference in traces, power, distortion, freq response compared to well-matched vintage 6550s or loose-matched Chinese EL34s. On the other hand a high-strung Bogen eight*8417 amp was shockingly sensitive to just one lazy tube.

[Davidg]

Correct me if I am wrong as I am recalling this from my not so great memory but the whole point of individual bias adjustment is so you can use mis=matched tubes.The theory being that say in a 2-tube push-pull power amp each tube can then be adjusted to pull the same current or even makes it posssible to use two different type tubes in one amp.I think Kevin O'Conner's TUT series is what brought alot of people around to this, although it is certainly not a new idea, he recommends this highly and is standard on all his London Power amps.

[PRR]

> each tube can then be adjusted to pull the same current

At idle.

When I'm playing, the amp is not idling.

[stratele52]

Ed  ,

What did you call "your receive tubes unmatched" ? How much milliamp at cathode ?

At idle difference of 3 to 8 ma between them is the best matched as write in Gerald Weber book (s) . Too close , perfect match , is not musical.

[Ed_Chambley]

Ed  ,

What did you call "your receive tubes unmatched" ? How much milliamp at cathode ?

At idle difference of 3 to 8 ma between them is the best matched as write in Gerald Weber book (s) . Too close , perfect match , is not musical.

Installed the KT120 and at 50 volts I got 72ma on one and 39ma on the other from a $90 purchase.

[jjasilli]

If not done already try swapping the tubes into the other socket and compare measurements.

[Ed_Chambley]

No. (Define "a lot"?)

> I have some where in the box there is a sheet of paper with hand written test results.

Very odd. Not doubting you, but 1960s-1970s, I never saw such a thing.

[/quote]
Very rare in any preamp tubes, but I do have a few.  I have 2 cases of Mullard FX2, el34.  Test sheets in every box.  I have never seen an GEC power tube new in box that did not have one.  The GEC's actually on their labeled test sheet.  The Mullard sheets state some other vendor doing the testing.  I am not sure of the purpose. American manufacturers did not with the exception of some vendors like https://www.upscaleaudio.com/pages/Testing-Power-Tubes.html
They still provide test sheets, just ask and they will include it.  I usually purchase from them, but I had an order with the other vendor I added these tubes to.  Did not expect such a bad mismatch or I would have gotten them from Upscale.

Here is a read on the topic
http://www.jacmusic.com/KT88/kt88.htm

I think the difference is when a tube vendor is searching the audiophile crowd.  Most every tube I own are used (pulls), so they would not apply.  For instance, I bought a box of pulls last month and finally got around to looking at this weekend.  I got lucky this time as all the tubes were audio.

I know this is a guitar amp forum, but I do have a few tube monoblocks and stereos.  A mismatch in these amps will cause you to lose the soundstage and raise distortion levels.  The only other amp I have that will do this is a Sony VFET.

Thanks for your response as I was hoping you would chime in.  That is the reason I asked and it is true different amps respond differently.  The problem is there is a limited amount of time you can return tubes and I usually buy for longer that 90 days.  In the past I could keep matched sets in stock, but I am having trouble with the thought of this.  It seems like it has gotten to the point where you need to install your tubes when you get them to test or you will end up with tubes not matched and you cannot return them.

That is why I am asking.  Do guitar amps need to be matched and what is considered matched?

[Ed_Chambley]

If not done already try swapping the tubes into the other socket and compare measurements.

Did this.  Follows tube.  I have others that match, 6550 and KT88, but not these KT120.  These are the first time I bought these.  They are on the way back for replacement.  No problem there.  Just have to pay shipping until proven defective.

[Ed_Chambley]

I was complaining, but really was not looking for a debate but rather a general consensus on what is considered matched.  I am doing many more repairs.  Normally, I would not let a amp go unless the power tubes were within 4 ma or less and usually at 60 to 65%.  If asked I will go to 70%, but usually 65% is fine.

I don't think adding a Dual Bias or Bias Balance to every amp is viable or necessary even though many of mine are that way.  Mine are that way because of Sluckey encouragement.  Not so much for the benefit (his words) but for the learning experience and the AH moment when it comes together.  Now I understand much about bias sections and can make dual bias or Bias with Balance.

Concern mostly as it is getting difficult to keep the 4 ma match which is the rule of thumb one of my teachers told me years ago.  Also, in my amps I change tubes a lot.  Always have.  I really do not know the drawbacks to long term mismatch.  Probably nothing, but I do not know.  Maybe after a little wear they will match better.

There are many opinions, but I am looking for experience here.  If repairing and re-tubing someone's amp would you let it go with say a 10ma mismatch?  Where is the line?  If too far apart will it cause early failure or any other problem.  Every fixed bias amp needs biasing.  How flexible is it?

[12AX7]

well, from a novie who's build a few amps and tweaked them forever but really not a tech at all, i can just tell of my experience from a laymans perspective. First, matched tubes mean little to nothing to be because 1-they always seem to drift apart often a lot after a short time, and 2-if one goes bad i save the good one and after years have lots of good tubes that aren't matched at all.

I 1st put dual bias in mine about 2 or 3 years ago and i'll never be w/o it again for several reasons. In any case, i prefer the tone when matched via dual bias regardless of how far off the tubes are from each other if used in a single bias amp. When i un-match the current on purpose It loses complexity and clarity and it just doesn't sound as good to me. It may sound great in a bright or harsh amp because mismatching them cuts brightness, but i feel thats putting a band aid on other design issues the amp has. That plus as i said i like to be able to use singles i have and match the flow for that sound i prefer. If you have a set of tubes that drift way off, and i've has some drift WAY off, then theres no way to get that sound. Plus i DO find that at some point when the bias is low that the tone suffers and if the difference is big one is either going to have to be in that range or the other will be taking a beating.

So while i'm no tech, i know what works for me tonally and logistically and thats dual bias.

[stratele52]

Ed  ,

What did you call "your receive tubes unmatched" ? How much milliamp at cathode ?

At idle difference of 3 to 8 ma between them is the best matched as write in Gerald Weber book (s) . Too close , perfect match , is not musical.

Installed the KT120 and at 50 volts I got 72ma on one and 39ma on the other from a $90 purchase.

Yes something wrong with tube if you swanp it an still same . By the way ,  50 volts , do  you mean at - 50 volts bias ( negative ) voltage ?

[Ed_Chambley]

Ed  ,

What did you call "your receive tubes unmatched" ? How much milliamp at cathode ?

At idle difference of 3 to 8 ma between them is the best matched as write in Gerald Weber book (s) . Too close , perfect match , is not musical.

Installed the KT120 and at 50 volts I got 72ma on one and 39ma on the other from a $90 purchase.

Yes something wrong with tube if you swanp it an still same . By the way ,  50 volts , do  you mean at - 50 volts bias ( negative ) voltage ?

Yes -50vDc.  Sometimes I a lazy when typing.  I have heard the 3 to 8ma suggestion, but I am seeing a lot of new matched sets much further apart.  That is why I was asking.  Since I have always gotten 4ma or better I was just checking with some of you to see if you are getting better results and if not what you were doing.

[Willabe]

Years ago I was doing some repair work at a local music store. A Sun bass amp came in and it had a quad of 6550's in it. It needed a cap job and he wanted a new set of tubes and I changed the screen R's to new MO, IIRC.

So I ordered a matched quad set from EH. The amp had ~550vdc loaded on the plates with the old tubes. After it passed the smoke test, I popped in the new matched quad set and went to set the bias to around 40mA's per tube. It was still sitting at ~550vdc. From the strongest to least current draw of the 4 tubes they were around 17mA's apart IIRC. And that was after I moved the tubes around in the 4 different sockets to get the least amount of difference.

So I called EH and told them what they were reading and they said send them back and they'll send a new set. So I did, got the new quad set and they were worse by a few more mA's.  

Called them back and asked them at what dcv were they matching them at? They said 350vdc.  

That dog won't hunt! They know perfectly well where most companies are running those tubes in PP, grid bias.

Now why would you test a tube and match them up at 350vdc when it's spec sheet says 600vdc and many amps that use that tube do run them at well over 500vdc on the plates? They said they were within 3 or 4 mA's when matched up as a quad set. I said maybe so at 350vdc but they're surely not when run at 550vdc.            



                    Brad            

[stratele52]

+1 Willabe , very interesting .

I think company don't want too much reject , so they test tubes at lower voltages than spec. Also IMO they know tubes manufacturer have poor ( ? ) quality test .

This like a tube tester. Most of them test tubes at voltage around 50 VDC or other low voltage , that is why you can't trust tube tester for testing any tubes need to work at more than 50 VDC . We know that there are no amp working at these low voltage.

So forget tubes tester to test your amplifier's tubes.
_____________________
Tube tester will be right if tube is completely dead , or no heater glow or dead short. Tube tester will say the truth.

 But all of these tests can be done easely when troubleshouting the amp.

[alerich]

Called them back and asked them at what dcv were they matching them at? They said 350vdc.
That dog won't hunt! They know perfectly well where most companies are running those tubes in PP, grid bias.
Now why would you test a tube and match them up at 350vdc when it's spec sheet says 600vdc and many amps that use that tube do run them at well over 500vdc on the plates? They said they were within 3 or 4 mA's when matched up as a quad set. I said maybe so at 350vdc but they're surely not when run at 550vdc.

I suspect that this is the problem I have been running into. The plate voltages in my amps are 485VDC and 640VDC. I haven't actually tried the Winged C EL34 tubes in the lower voltage amp. When I get some time I will swap the tubes around between the two amps and compare the mismatch effect at the two plate voltages. Logic seems to imply that the mismatch would be smaller in the lower voltage amp.

[PRR]

> at 50 volts

That's idle. Has some significane in hi-fi, little/none in guitar.

What's the current at ZERO Volts, when we WHANG the amp? That's more to the point.

Ah, but the tube will melt at high plate and screen voltage and zero (or small-negative) on grid. In the audio amp, it goes near zero less than half the time, the other half it is swinging to cut-off; also the plate voltage goes way-low (<100V) when current goes way high.

300V is a fine compromise for an "all uses" test.

For your amusement you might try a "sane" screen voltage, a 500V (or convenient) supply to a 2K 50W resistor to plate. As you get up near -10Vg1 the plate current may be 200mA, but plate voltage drops to (500V-(2K*200mA))= 100V, so dissipation is safe. Plot the Vg1 needed to swing 50mA to 200mA. Compare between tubes. If most of that range is within 20%, it aint bad. If the 200mA points are within 10%, you aint gonna get much more Power out (and this end can't trim much with idle adjustments).

[super&plexi]

hey ED, you got a couple mutt tubes?, send em my way, I'll send you some o mine.  I'll go first-what'cha want? pre, Pwr?, or?.                                                                                                                                         

With the amount of estate sales, family&friend  passings, etc. most all of us have encountered, we must have come across tons of puzzle pieces. trade/barter is just as good as cash.

And don't tell anybody,...they wouldn't believe you any way but, I've been known to give out tubes at Halloween, but it has to be a REALLY good costume. lt

[Davidg]

Sorry for a very unhelpful reply earlier but best supplier I have found for matching is eurotubes or actually a guy on ebay that runs a store called Yen Audio or Apex Jr.This is not near the volume you buy in Ed but a few quads here and there but all matched within a few milliamps and I think would be worth trying.I am not sure you could find a consensus on what is considered matched but I have had amps off 10 mil that sounded fine to me but I think it really depends on the amp.I say if it sounds good let it go and only your ear can tell what kind of mismatch any amp can live with.I am down as I just had back surgery and havent been doing anything last couple months and still got at least a month to go but I would like to do some experimenting with this and maybe others can as well and WE can have the final word on what is matched and what is deemed out of spec.

[Ed_Chambley]

hey ED, you got a couple mutt tubes?, send em my way, I'll send you some o mine.  I'll go first-what'cha want? pre, Pwr?, or?.                                                                                                                                         

With the amount of estate sales, family&friend  passings, etc. most all of us have encountered, we must have come across tons of puzzle pieces. trade/barter is just as good as cash.

And don't tell anybody,...they wouldn't believe you any way but, I've been known to give out tubes at Halloween, but it has to be a REALLY good costume. lt

Thanks for the offer S&P.  I got a plenty of tubes.  This was about New Production tubes from Tube Depot.  Till now I have not mentioned the company, but I thought I might as well.  They send out the wildly mismatched.

Actually, from what I see on Ebay I have enough to retire.

I buy them as well.  Bought a box of pulls recently and there were nothing but 6v6, 6550, 6an8, 4 RCA blackplates 6l6gb, my favorite tube next to GEC KT66.  There are others as well.  Most are Tung-sol or RCA.

I Have built an amp with 565 vDv on the plates and wanted to use the KT120 for durability.  I am waiting for some more caps.  The second set series of Illinois caps 100uf/450v exploded last night.  That is both sets now.  I ordered some F&T's last night as there is something wrong with a series 450V cap that will not handle this voltage in series.  I played the amp for well over an hour and the bam.

On a happy note, the amp sounds great.

[Ed_Chambley]

Sorry for a very unhelpful reply earlier but best supplier I have found for matching is eurotubes or actually a guy on ebay that runs a store called Yen Audio or Apex Jr.This is not near the volume you buy in Ed but a few quads here and there but all matched within a few milliamps and I think would be worth trying.I am not sure you could find a consensus on what is considered matched but I have had amps off 10 mil that sounded fine to me but I think it really depends on the amp.I say if it sounds good let it go and only your ear can tell what kind of mismatch any amp can live with.I am down as I just had back surgery and havent been doing anything last couple months and still got at least a month to go but I would like to do some experimenting with this and maybe others can as well and WE can have the final word on what is matched and what is deemed out of spec.

I know the 2 vendors had get lots of stuff from Steve at Apex.  Especially small pots, well smaller that PEC.  I do lots of radio repair and he had the Clerostat sp? long shaft pots.  He also has NOS Allen Bradley which have a log taper I prefer.
It is true we probably will not get a consensus, but everyone inherently knows when you are getting 10ma apart "it ain't right".
Here is the grind.  If you purchase matched tubes, you should get matched tubes.  Now define a matched tube.  Also, people say they don't really need to be matched in some amps, but I would bet the farm that if that same person had an option to get a better match, they would.  Last night a guy brought me a hombrew "special amp" which looked very similar to a 6G6A circuit.  He had his own tubes, Yay.  JJ 6l6gc.  Amp had a separate bias tap on the PT.  It  had a 10kl fender bias pot in it.  I could not get enough negative voltage to get below the bias running away.  Now there is a 1ohm resistor and a 1k on the pot lug.  The tubes are running fine now, but they have a mismatch of 12 or 13 ma.  The negative voltage is -62vDc.  Not sure where he got them and advised him to get some better tubes and he asked me if I would install them, meaning no charge.  I politely mentioned I had offered to supply tubes, but he wanted me to use his since he had already invested in them even after I called him and told him I was going to have to change his bias supply too much for my comfort.  He was fine with the sound of the amp, but it did not have very much punch as the circuit usually provides.

He doesn't understand, but how could he?  If he did he would have done the work himself.  I do not do this for a living, it is a hobby.  I am not doing it for free except for friends.  Word of mouth is the only way anyone knows, however I do have a friend that owns a very nice music store and I have been doing retubing and recapping on used amps for him for a long time.

Come to think of it, I did a cap job and retube on something I have never seen before that came from his store.  It was a head with a brownface that had Super Reverb on it and rough blond tolex.  I have never seen a faceplate like that.  Nothin special, but I would like to know where the faceplate came from.  It had the Fender trademark on it as well.  The owner bought it that way so he did not know.  I am hijacking my own thread, but if someone knows who is making these, I would like to know.

[G._Hoffman]

Are we really that much smarter than the guys that designed, built, and marketed this stuff?   

According to the stats on on IQ tests, we are. 

At least, the average for each generation goes up by a few points.  I'll admit, though, I think that might have more to do with educational priorities than any biological change.

More to the point, though, real musical signals are seldom symmetrical, why would it matter if the tubes aren't?  That's just part of the sound.  And of course, a guitar amp isn't really about making a clean representation of the pickup's output.  That would be awful.


Gabriel

[TIMBO]

Hi ED, I got a mismatch pair of RCA 6L6s one being a black plate. Your topic maybe what I am looking for. I looked at separate adjustable cathode bias but I think the wattage that is needed for the pots I am finding hard to find.

So I am hoping that maybe a dual fixed might do the job. Just as a side note I did put then into a cathode bias amp and I think I had about 40mA difference.

With some of the fixed bias I have done I don't think that I had that much in the adjustment.

Are you able to supply me with a schem of your circuit.Thanks

[sluckey]

Here's a schematic of my only dual bias amp...

http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

[Ed_Chambley]

Timbo,
Sure, here is one.  I am not at my main computer so I found this on an old post.

NOTICE the ERROR.  The dual pot, the lowest one, does not have a line drawn from the lug 3 back to the junction of the cap/resistor.

Sorry, wish I were able to do better for you.  This is how I did it with tremolo.  It is not hard for someone who has tinkered as much as you.  If you think about it a little, you will get it.

I can post other drawings in a few days.  I did successfully create a bias circuit with a balance pot as I have been messing around with pentode octal preamp.

I think Willabe (Brad) may have a schemo of this.  I am sure he will read this soon and will help out as the Dual Tremolo may not be what you need.  Then again, it was the Bias Vary Tremolo curiosity that lead me to understand how a bias circuit works.

[TIMBO]

PERFECT, Thanks guys 

[silverfox]

After building a power amp I wanted to monitor the bias while playing to avoid red plating. It had separate bias sources.

The bias fluctuated while the amp was in operation.

I'm sure both bias sources would have fluctuated differently for reasons such as differences in the components used.

I suppose one way to resolve the question would be to find some amps that sound great and check the bias values of those amps.

A proposed test bed would include a regulated bias supply, (This starts to sound like a Hi Fi circuit), compared with an unregulated supply.

This has already been alluded to and I agree- An amp's sound is a summation of the circuit components and unique to a particular design and period in which it was built as well as the individual components used. Kinda like people and the colour spectrum. Even an improvement in the power supply, from what I've read, can have a negative impact on the sound of the amp.

Just some thoughts.

Silverfox.

[HotBluePlates]

... I looked at separate adjustable cathode bias but I think the wattage that is needed for the pots I am finding hard to find. ...

2w pots, RV4 style (the ones I have gotten are generally PEC RV4 pots). They're kinda expensive, but it's not like you need 5-10w pots, because there are a couple in the circuit and the other resistors also dissipate some of the power.

... you never know if new tubes are matched unless you check ...

RDH4 says power tubes should be matched by testing power output of each tube in a power circuit (similar to the intended application). Show me a tube vendor that does that... Idle current is probably also a useful test, as well as Gm at several plate voltages, etc.

Why should we listen to RDH4? The guy who edited it worked for AWA, which was an Australian tube company who eventually became RCA's manufacturer for that part of the world. Aside from getting double-checking by other folks in the industry, he knew the stuff directly from the tube designers and manufacturers.

... Maybe today we focus too much on idle bias, but at what point does a mismatch cause problems? ...

An interesting section in RDH4 is Chapter 13.5 v, Matching and the effects of mismatching, pp 580-582.

My Hickok tube tester has a Good-Bad "English" scale, where the bottom edge of the Good range is equal to 60% of the Gm for a bogey tube of the type being tested.

Anyway, RDH4 has an example of the effects of mismatching wherein they pair a 2 different output tubes (a 2A3 and a type 45) in a push-pull stage to see what will happen. Amplification factor for these output triodes is 12% apart, but there is a more than 50% difference in Gm and plate resistance. By the standard of the Hickok tester, 50% low Gm would be the same as a "Bad tube".

The end result was 5% 2nd harmonic distortion (2nd normally cancels in matched push-pull stages), and a shift in tube bias due to rectification. All-in-all, you call it "less clean output power".

What other effects might happen in radically mismatched tubes?

Hugely mismatch idle current may cause hum in the output stage (which would normally cancel in the OT), but you'd almost need fault current conditions for it to happen (part of the reason I think you hear hum in the speaker when plate current skyrockets in an amp). Huge imbalance, especially under driven conditions, might cause early saturation of the OT core (which is intentionally small in a push-pull OT) meaning less power at as-low a frequency.

So if you need maximum clean power & maximum bass authority, then well-matched output tubes is a good thing. But for most practical purposes, in a guitar amp (where we intentionally have ENORMOUS distortion by hi-fi standards) it just doesn't seem to matter much.

[Willabe]

Here's a couple I got from Kevin O'Connor's TUT books. I don't see these as secret circuits but he did develop these for a low(er) Z and with safety in mind with the R's across the pots should their wipers fail.

I still think Kevin had and still has a very large impact on tube amp building with both his TUT books and the now defunct    Power Scaling forum. When his TUT (1) came out it was at the very beginning of the 90's custom/self built/modded/hand wired explosion. He helped kick start the whole thing.

FWIW, I also think our host here, Doug did too. He was right there in the beginning of the "I want to know why the old amps sound so good and get your hands wet up to your elbows to find out" in the early 90's and continues on.      

On the dual -bias if the wiper fails then the safety R will still give -bias to the power tube grid. On the -bias/balance if the wiper on either pot fails then it will give full/max -bias to the power tubes grid.    

25K or 22K pots with 25K or 22K R's will work.

These 2 -bias adjust circuits will NOT work with bias very tremolo.

PI coupling caps go to the junction of 220K R, grid leak and the 1K5 R, grid stopper. All pots should be linear.  


          Brad      

[TIMBO]

Hi guys, I must have got caught up in the moment and for got the PPIMV 

[Willabe]

It just goes before the PI coupling caps, same junction feed going to the 220K R and 1K5 R.   


        Brad    

[TIMBO]

Thanks Brad, Something like this.................

[Willabe]

Timbo, I'm sorry but I've never been able to get your files open.

I wish you would post the drawing itself and not just a file link.


            Brad   

[sluckey]

Timbo, I'm sorry but I've never been able to get your files open

Don't you have ExpressSCH? That's what a lot of people here use for schematics.

[Willabe]

Yes I do, but when I try to open Timbos drawings my computer promts me to 1st save it then I can't seem to get it open. Screen says "open with" and then it doesn't give me the option to use sch. Plus then I end up with a file that if I don't want to keep it I have to go back in and clean it out.

I really appreciate and respect Timbo but it would be easer if he would just post the drawing.

It might just be me as I'm not the sharpest knife in the drawer when it comes to navigating on a computer.


           Brad          

[Ed_Chambley]

At what point does a mismatch cause problems? ...[/quote]

The reason I asked this question a while back.  I have a original Marshall 50 watter plexi.  Runs el34's and was made to run them.  Drake OT.  The amp gets a LOT of maintenance attention and I have 3 to 4 additional parts to keep it original.  I have a fair supply of NOS Mullard XF2.

Well the old Marshall began popping the 1/2 amp B+ fuse.  I do not use slo-blo fuses in it. Replaced it and did not thing much about it.  Then the fuse blowing became more often, but still random.  Finally, it immediately blew the fuse when flipping the DC switch.  Gotta pull the chassis now.

Long story even longer.  Power tubes out fuse holds.  Check with Hickok 6000A, decent tester and calibrated well.  One of the tubes has drifted so I got an old =C= that tested match on my little tester.

Well now I have 2 sets of matched tubes.  Back in the amp.  Fuse holds.  Out with those and back with the second pair.  Fuse holds.  Head scratching now.

I did get bias readings and the tube I thought was bad read 22ma paired with the =C= the other tube was still at 35ma.

So my question is what other than a mismatch could cause this?

I put them back as they were and immediately popped a fuse.  Now, I had forgotten all about this till now till the thread reappeared.

[sluckey]

Assuming you have a PC...

Sounds like *.sch files are not associated with the ExpressSCH program. That would keep you from easily opening anyone's .sch files, not just Timbo's.

Try this. Download Timbo's "DUAL FIXED BIAS with PPIMV.sch" file to your desktop. Right click on the file and select 'Open with...'. Check the box that says 'Always use the selected program to open this type of file'. Now click the Browse button and locate the ExpressSCH.exe file. Click OK. Now .sch files are associated with ExpressSCH and should open easily for you.

The sch file on your desktop should now have a black and green icon of an op amp.

[Willabe]

Hehe! It worked!

Thank you!

And yes it was with anyone's sch file post.


          Brad     

[Ed_Chambley]

Yes I do, but when I try to open Timbos drawings my computer promts me to 1st save it then I can't seem to get it open. Screen says "open with" and then it doesn't give me the option to use sch. Plus then I end up with a file that if I don't want to keep it I have to go back in and clean it out.

I really appreciate and respect Timbo but it would be easer if he would just post the drawing.

It might just be me as I'm not the sharpest knife in the drawer when it comes to navigating on a computer.


           Brad          

Brad, just create a new folder on you desktop.  I name mine For Review and right click, save file as, navigate to the folder on the desktop and save it there.  When I am done, I simply trash the folder contents I no longer need.

From here you have 2 options.  You can open PCB EXPRESS and use the open command in the program.  Navigate to the folder you made, open it and open the file.

Total solution is to right click the saved file, "open with" and find Express.  Usually located in program folder.  After you find Express there is a little box you can check that states something like "always do this with this type of file."

I am not on a Windoze machine and I do not know your OS currently or I could give exact instructions.

Just give her a try, I know you will get it.

[Ed_Chambley]

Sorry, Sluckey types too fast