How do you balance your bias setting on your slightly out of balance Power Tubes

[Ed_Chambley]

Sorry Platefire, I missed the point too.  I was wondering since you aren't a rookie, why you were concerend.  Now I understand.  Truthfully, on a guitar amp I am more likely to adjust signal level from the phase inverter to achieve the tube tone I am looking for.  We cannot know it that LTPI with that "balanced triode" will do it.  Also, transformer winding not balanced generally.


I have left the balance pot on a silver fender and simply bias with a resistor.  Completely fixed.  I did copy a Hifi Macintosh bias balance circit, but there again, I have not use it on a Guitar amp.  Hifi generally doesn't run tubes as hard. 


I just received a 4 point auto bias board same as used in some newer Hifi gear.  It works well, but before I installed I checked signal level and current from the PI, 6sn7 and it had drifted, so I had to reduce resistance on one side a smideg. 


I did use a Dual Bias pot on a build and I am sort of split in this.  At IDLE you can match well for as long as the tubes will hold.  The main problem is we generally do not play at idle.  I like analogue needle Simpson 260 meters, because they really are cool, really!  Anyway, everytime I have dummyloaded and increased the signal gain I can measure i get imbalance, sometimes one tube will pass more current at half signal and the other will at full signal.  Swap places.


The best new production tubes I have bought are from Upscale Audio.  The guy that owns the place is Kevin Deal and he is so straightforward you will think he is not very nice, but he just wants to make sure you do not have any problems.  So I guess what I am saying is I wouldn't be concerned with balanced tubes unless I want the cleanest output power and even then I need a balanced signal.


I have played guitar many times using a Hifi amp, but usually I am playing something hollow or semi hollow. 


I hope this reply is more in-line with your topic.

[Platefire]

Hay Sluckey and Ed
No problem. At no time was I mad or offended even though maybe my text language might of led you to think that. Yeah! besides slightly off balance OT's, PI's and supposedly matched tubes that are not, I've come to realize there is a lot off-sets in Push/Pull to try to pull together by hook or crook. No I'm not a Rookie but for sure I'm technically inept compared to you guys. I've never even thought about balancing a PI or have I tried the dual bias pot thing. My taste in amp tone keeps changing and seems like I'm leaning more and more to cleaner amp tone. I just let my pedals do the dirt and want my amps clean. So like you say, better balancing I reckon is the path to more pure clean tone. So who knows if I keep pursuing the path of purer clean I may find myself trying to pull the imbalance closer together.

I spent the first 15 years of my tube amp experience just building new tube amps and converting/re-purposing other old tube amps. Seems now I reached the end of my building, re purposing and now I'm just playing them and finally getting around to fine tuning them. Evey time I pick up a guitar I find myself fine tuning the setup and same thing with my amp builds going back over everything. So every time I rotate an amp out 
you might see a thread pop up on it:>) Thanks, Platefire

[jjasilli]

Required reading:  http://www.aikenamps.com/the-last-word-on-biasing

This covers the various points mentioned in this thread. 

I submit that it is ultimately impossible to know if your power tubes are in balance to a high level of precision.   

*  If you measure ea side for cross-over distortion, the measurements are inherently imprecise.

*  Real tubes are not bogeys.  Actual plate impedance, etc., will vary from tube to tube.  If you balance bias by using the cathode current method, the operating points of each tube may differ, yielding different gain & harmonic characteristics.

*  If you equalize output signal voltage of ea side, then, do you have equal signal voltage into ea tube, or will you adjust that per the KOC method?  To get equal signal voltage out of ea power tube will require that they be biased differently. 

*  Will you leave your amp on for 20 min's or more before you play it?

There is no demonstrable reason to believe that highly precise balancing, even if possible, improves clean tone. 

[Ed_Chambley]

There is no demonstrable reason to believe that highly precise balancing, even if possible, improves clean tone. 

Obivously I misunderstood something somewhere.  I can certainly demonstrate causing distortion by imbalance of a cathodyne very easily.  I can reduce it again as well.


I do not disagree with any of your comments and I am quite sure tubes are all somewhat different.  It must be in your definition of what highly balanced means, but I agree it seems impossible to perfectly balance. 


I was not speaking to a guitar amp when writing and should not have mentioned the balance.  The reason I did is I have a Stereo KT88 and have improved the clean output wattage and balanced so preceived volume is the same with both pots positioned the same.


Now, I have no way of know if you could even hear the actual distortion or the decrease, but my pots are "balanced."  This is a different type of balance, I get that.  It is basically a dual monoblock.  That is why I shouldn't have mentioned it.  The topic is Bias.

[ginger]

Sorry Platefire, I missed the point too.  I was wondering since you aren't a rookie, why you were concerend.  Now I understand.  Truthfully, on a guitar amp I am more likely to adjust signal level from the phase inverter to achieve the tube tone I am looking for.  We cannot know it that LTPI with that "balanced triode" will do it.  Also, transformer winding not balanced generally.


I have left the balance pot on a silver fender and simply bias with a resistor.  Completely fixed.  I did copy a Hifi Macintosh bias balance circit, but there again, I have not use it on a Guitar amp.  Hifi generally doesn't run tubes as hard. 


I just received a 4 point auto bias board same as used in some newer Hifi gear.  It works well, but before I installed I checked signal level and current from the PI, 6sn7 and it had drifted, so I had to reduce resistance on one side a smideg. 


I did use a Dual Bias pot on a build and I am sort of split in this.  At IDLE you can match well for as long as the tubes will hold.  The main problem is we generally do not play at idle.  I like analogue needle Simpson 260 meters, because they really are cool, really!  Anyway, everytime I have dummyloaded and increased the signal gain I can measure i get imbalance, sometimes one tube will pass more current at half signal and the other will at full signal.  Swap places.


The best new production tubes I have bought are from Upscale Audio.  The guy that owns the place is Kevin Deal and he is so straightforward you will think he is not very nice, but he just wants to make sure you do not have any problems.  So I guess what I am saying is I wouldn't be concerned with balanced tubes unless I want the cleanest output power and even then I need a balanced signal.


I have played guitar many times using a Hifi amp, but usually I am playing something hollow or semi hollow. 


I hope this reply is more in-line with your topic.

Not " completely fixed "  you should have put a Hoffman 50k bias pot on the bias board... for the next set of output tubes installed... what you did , is what we all hate.. no adjustable bias .

[Platefire]

Tube amp Fixed Bias, the most highly charged and controversial subject on Tube amp Forums.
sluckey I'm appointing you to be the Chief bias referee and Arbitrator!

[sluckey]

enjoy! 

[Platefire]

 

[Ed_Chambley]

 Not " completely fixed "  you should have put a Hoffman 50k bias pot on the bias board... for the next set of output tubes installed... what you did , is what we all hate.. no adjustable bias .

It warms my heart knowing you are bothered by this.

[shooter]

Tube amp Fixed Bias

that's why I became good at xSE and using trannies for PI in PP, simple, clean, well received, life is good 

[Platefire]

Shooter
I recall being really surprised first time seeing an old 40's amp schematic with a transformer for a PI. So I learned that to be pretty common back in the day. So are you saying your doing builds doing that again??? And what is xSE? Platefire

[davidwpack]

Yeah, I'm interested in knowing more about using transformers as a PI. Only stuff I've found so far pertains to radios. Haven't found much yet on guitar amps.

[jjasilli]

Checkout: Gibson GA15RVT

In the '40's preamp tubes were low mu.  Output tubes were triodes with low mu, and needed huge input signal voltage, like around 115 VAC.  A transformer could boost signal voltage better than lo mu tubes to drive a power triode.  Also, signal bandwidth of OT's was then limited to about 100 Hz - 8000 Hz. So interstage tranny's didn't need to do better. 

With the advent of tetrodes, then pentodes, power tubes became less signal hungry.  And hi mu preamp tubes arose.  A preamp tube is cheaper and smaller than a hunk of iron.  By the early '60's OT's could pass the full audio bandwidth of human hearing: 20 - 20,000 Hz.  You need to wind a high grade, expensive interstage tranny to match that -- which a cheap tube can do.  Interstage tranny's all but disappeared.

Lately, hi quality interstage tranny's are making comeback in a small niche of tube hi-fi amps, for purists who want to avoid caps in the signal path.

[davidwpack]

Thanks for the info! I'll check that out.

[sluckey]

Yeah, the Gibson GA15RVT and Gibson GA20RVT from the '60s both use interstage phase inverter transformers. Those are the latest amps that I know of that used them. Some of the much older Gibson BR series used them. I'm sure there are others too, but I can't recall any.

You can find all the Gibson models mentioned in Hoffman's schematic library.

[davidwpack]

Cool, thanks! I found an old post on a forum that one of the members mentioned some Fender 400W amp that used them also. Edit: Fender400 PS?

[shooter]

 

your doing builds doing that again

yup, jj got close, the idea came from one of the 23 variations of the GA-5_
I built a fenderish pre, USA-1 tranny pi and 6V6s, never bonded
went to the plexi-ish and a KT88 SE (no PI  )
was liked, but just didn't get quite there
married the plexi-ish and the tranny PI with a matchless clubman PA, it became a hit, sold with in 2 days of demo day

what I found poking around, the iron adds a unique distortion, on a scope you get a leading edge spike, ("overshoot" in my old life).  It's slower than a "digital" transient.  I think this smacks the crap outta the PA tubes, that are already biased at MaxPdiss thenthe main signal gets there and the tubes are "pre-bent"   

from the guitar folk; "You nailed it"  The owner toured and opened for  Foghat, even filled in when someone from foghat got sick.  so i'll take his word it works, for everyone, no.

xSE is single-ended, no pi needed  x is "how many tubes"
I soulda said self-biased instead of SE.
 I like simple, but good, I've found you can get great sound from simple amps, will they trem-a-verb at 100W, no, that's why PA's and Pedals were designed 

[jjasilli]

Not hijacked -- an interstage tranny PI might in theory be better balanced than a tube PI, and won't drift.  But (I think) except for special toroidal windings, there will be some difference in the 2 windings, because one is wound over the other.

[Platefire]

Good discussion. Not concerned about hijacking. I did see Foghat many
times in the 70's. All lead playing was slide on an SG through a Marshal.
I just wonder if this is the same guy using Shooters amp? Thanks
for sharing the schematic. Amazing how old technology sometimes
comes back around. Platefire

[shooter]

is the same guy using Shooters amp

He was in the opening band, forgot name, but in the 2nd go round from the '80's

might in theory be better

I like it because it's simple, low part count, hard to screw up, cost's less than tube PI, if you're into marketing, could be a selling point.
the downside, it smokes easy, 50mA is about max, hence the cap coupled, which "adds" its own tonal kinks

can't find the link, but there is some well written smart guys that explain the deep weeds "logic" for why it's still a viable thing

[Platefire]

Well I ordered 5 of the 1 meg, 1%, 3 watts, 750 Volts working Resisters that 92Volts referenced on post# 35. I will install the Plate Voltage test point in one of my amps to test the work ability of it. If it worksas good as hoped, will install it on my other amps that already has external bias test points also. Platefire