Page 1 of 5 12345 LastLast
Results 1 to 35 of 149

Thread: Push/Pull Pair Removal Theory & Questions

  1. #1
    Member
    Join Date
    Mar 2017
    Posts
    24
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0

    Push/Pull Pair Removal Theory & Questions

    So I found this majorly-conflicting-with-everything-i-know post

    Click image for larger version. 

Name:	Discrep.png 
Views:	115 
Size:	572.7 KB 
ID:	47107

    Can someone give me a basic run down of the "what happens" when you pull 2 tubes and half the impedance switch on a typical push/pull w/ 4 power tubes? Like.. the theory of what happens?

    Voltage and current draw on the 2 tube plates left running go up, the overall impedance doubles, the overall resistance goes down (I think?). Things like that. My understanding is fuzzy but would love a quick professional and fast to-the-point of what exactly happens here and what if anything additional needs to be done. (Parts that get more stress/less stress?)

    I was under the impression of puling two, half the impedance, and you're good. You might have to adjust bias but as bias is mostly a preference within acceptable range thing you can get away without having to adjust after pulling two given that there is no red-plating happening. Is this inaccurate? Not should you still adjust the bias (we know we "should") but do you have to? Does anything negative happen to the amp with the change, specifically, will it "actually" cause any problems by consistently running 100 watt heads w/ only 1 pair?

    I was told at an expo event that this will always damage an amp because of the change in current and stress on some parts. I have also been told the opposite, that it's fine, you don't need to adjust bias (but you should) and that it will not fail because of running at half power. Then, I see a post as above, which also conflicts.

    I'm hoping to get a solid understanding of what happens when you pull a pair in a push/pull and what changes if any are necessary after the fact (and to avoid misinformation).

    Thanks guys!

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,829
    Thumbs Up/Down
    Received: 72/0
    Given: 184/1
    After a brief glimpse of the rant you linked - where the author claims 'that's bogus', '[they're] wrong', 'dead wrong', etc. - I'd say that you're better off taking the argument with a grain of salt.

    There are circuit-dependent factors that can affect the usefulness of any thumbrule. Any determination aught to be done with a specific amp as an example, only then can any general case be extrapolated. Some amps can operate in the envelope with two tubes pulled, others may be over-dissipating. A cathode biased amp will change the bias operating point (not sure if there are any examples of a cathode-biased 4-tube amp?) and need compensation.

    As a general rule (after advising you to be suspicious of general rules) pulling two tubes and moving the load to the next lowest impedance (thereby doubling what the reflected primary Z sees) should keep the amp inside the designer's operating envelope.

    If your objective is to optimize the amp's behavior with 2 tubes versus 4, then all the technical suggestions come into play. Bias, check plate and screen dissipation, check output signal, etc. But you'd likely take some of those steps with any simple tube change.. if it's the tone you're chasing.

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  3. #3
    Senior Member
    Join Date
    Aug 2017
    Location
    Portland, OR
    Posts
    351
    Thumbs Up/Down
    Received: 37/1
    Given: 36/1
    So you are asking strangers on the internet to comment on a comment by a stranger on the internet in response to another stranger on the internet?

    I actually think it would be great to get some data on these questions, but any experiment you set up would just cover that one circuit design with that transformer, those tubes, etc. Whenever you use something differently than it was designed there is a chance that things break faster than they would normally. But there seems to be enough overdesign usually to let you get away with such things.

    0 Not allowed! Not allowed!

  4. #4
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,593
    Thumbs Up/Down
    Received: 136/1
    Given: 273/6
    Quote Originally Posted by eschertron View Post
    A cathode biased amp will change the bias operating point (not sure if there are any examples of a cathode-biased 4-tube amp?) and need compensation.
    Anyone care to offer up their AC30 on the altar of amp science?

    Justin

    0 Not allowed! Not allowed!
    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  5. #5
    Senior Member
    Join Date
    Aug 2017
    Location
    Portland, OR
    Posts
    351
    Thumbs Up/Down
    Received: 37/1
    Given: 36/1
    Quote Originally Posted by Justin Thomas View Post
    Anyone care to offer up their AC30 on the altar of amp science?

    Justin
    Crate VC30 would be less of a sacrilege, but I like them anyway and wouldn't do it.

    0 Not allowed! Not allowed!

  6. #6
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,829
    Thumbs Up/Down
    Received: 72/0
    Given: 184/1
    A cathode biased amp will change the bias operating point (not sure if there are any examples of a cathode-biased 4-tube amp?) and need compensation.
    Quote Originally Posted by Justin Thomas View Post
    Anyone care to offer up their AC30 on the altar of amp science?
    Quote Originally Posted by glebert View Post
    Crate VC30 would be less of a sacrilege, but I like them anyway and wouldn't do it.
    So let the OP beware that I clearly don't know all the amps

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  7. #7
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,593
    Thumbs Up/Down
    Received: 136/1
    Given: 273/6
    Sorry, no offense was intended on my part.

    Justin

    0 Not allowed! Not allowed!
    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  8. #8
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,321
    Thumbs Up/Down
    Received: 265/0
    Given: 234/0
    The language in that .png rant in the first post looks vaguely familiar. Just enough knowledge to appear to know what he's talking about mixed with an "I know everything" attitude. Hmmmmm, there used to be a guy around here that sounded like that. Can you tell us the source of that screen shot?

    0 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  9. #9
    Senior Member
    Join Date
    Aug 2017
    Location
    Portland, OR
    Posts
    351
    Thumbs Up/Down
    Received: 37/1
    Given: 36/1
    Quote Originally Posted by The Dude View Post
    The language in that .png rant in the first post looks vaguely familiar. Just enough knowledge to appear to know what he's talking about mixed with an "I know everything" attitude. Hmmmmm, there used to be a guy around here that sounded like that. Can you tell us the source of that screen shot?
    Did a google search for the text.
    https://www.seymourduncan.com/forum/...age&styleid=28

    0 Not allowed! Not allowed!

  10. #10
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,593
    Thumbs Up/Down
    Received: 136/1
    Given: 273/6
    But I think That Guy used the same name everywhere... And there's no mention of hating Crate-Peg or having invented sliver mica caps or using an ice axe to bias your Mesa...

    Justin

    0 Not allowed! Not allowed!
    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  11. #11
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,200
    Thumbs Up/Down
    Received: 343/1
    Given: 672/0
    Trip, your understanding is mostly correct as far as I can tell.
    The 'responder' makes an argument based on an error, that "the impedance of the primary winding does not change".
    The impedance of the primary actually depends on the load on the secondary, as the ratio stays the same.
    The power delivered is dependent on the primary impedance being matched to the power tube impedance.

    0 Not allowed! Not allowed!
    The voices in my head are idiots!

  12. #12
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,829
    Thumbs Up/Down
    Received: 72/0
    Given: 184/1
    Quote Originally Posted by Justin Thomas View Post
    Sorry, no offense was intended on my part.

    Justin
    Oh! None taken!

    I could claim that I was thinking about the AC30, but didn't know for sure...

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  13. #13
    Member
    Join Date
    Mar 2017
    Posts
    24
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Quote Originally Posted by g1 View Post
    Trip, your understanding is mostly correct as far as I can tell.
    The 'responder' makes an argument based on an error, that "the impedance of the primary winding does not change".
    The impedance of the primary actually depends on the load on the secondary, as the ratio stays the same.
    The power delivered is dependent on the primary impedance being matched to the power tube impedance.
    Thanks! That was my understanding of what I remember, that's it's based on a ratio so removing two tubes would alter that ratio, so his response made me question everything I've been told haha I thought the info didn't seem correct, but I just wanted to lose any misinformation and gain a better understanding.



    Appreciate the responses from every one guys, I know there is no one-answer-fits-all but just a general rule of thumb as to what happens internally when you pull the two.

    & Yes, I guess I am asking strangers on the internet to comment on a comment by a stranger on the internet in response to another stranger on the internet lol

    To be a little clearer, thinking along the lines of fixed bias 100 watt. Most of what I own are Peavey's, but I own way too many tube heads, most are the typical push pull 4 power tube variety. (My VK doesn't fall in line w/ the power tubes in series) If it costs me half as much to tube everything than it's worth always running half power. I'm not too concerned on tone as I don't hear too much of a difference in most (though the JSX sounds more saturated to my ears), it's mainly a money / wanting fresh tubes without spending 10x more than what I spent on the amps in the first place lol

    For reference, this is more or less what i'm working with (as applicable) when I have these thoughts in mind. Ideally I kinda would want to run everything at half power except maybe the 5150 or 6505 (and BV, just because it makes me stuck to 16 ohm cab):

    B52
    AT-100 - Likely a bias issue happening but running 2 currently
    AT-100 - Modded for el 34 - running all 4

    Bugera
    333XL Infinium - Running at 2

    Crate
    BV120 - Running all 4 - Set the bias but it still feels like this guy runs too hot, as the other one runs with barely any heat
    BV120 - Running at 2, kinda a b*** because I can only run it into 16 ohms at half power, my good cabs are 8, never risked a real mismatch
    Palamino V32 Custom - 4 el84s, never attempted to pull or even looked at the circuit but of course thinking about it

    Line 6
    Spider Valve HD100 Mki - Worked great w/ 4, lost a little punchy-ness going to 2 but have been running at 2 for most of its life, concerned as lots of forums talk about how crazy in spec SVs have to be or else they fry something

    Peavey
    Triumph 120 - All 4, would attempt two, love the tone and saturation on this amp, but with the layout of the tubes it's questionable which to pull (I don't remember the schem)
    JSX - 2, loving how the tonal color changes, fave amp, would love to always run at 2 tubes, the idiot before sent it to me red-plating, this is the only amp I haven't ever actually measured and set bias for, I just set it arbitrarily to not be red-plating like when I bought it and have left it as is (for $250 I wasn't THAT concerned).
    3120 - All 4, 6l6
    5150 Signature - Something in the bias chain is failing but when I fix that I'd like to switch to 2, have always ran 4
    6505 Import - Have to run this guy through a 110 to 220 converter, all 4 tubes, not sure of any potential issue with it being imported spec
    Valveking VK100 (Can't, Tubes in Series, would love to, see lots of mods for a half power switch similiar to mesa, kinda wanting to implement that because I really don't have a need to run 100 watts)

    & that's my cluster of stuff. I care about all of these and want to keep them operating forever long as I own them (and not blowing up components) so understanding the why and logic behind what happens would be pretty helpful. And honestly I'd prefer to run everything at half if I can so I can always have fresh tubes and not spend 2x more on retubing everything I own. (And also before I find the issue with the b52 and 5150 bias I would rather not fry 2 tubes in troubleshooting than not fry 4 lol)

    0 Not allowed! Not allowed!

  14. #14
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,908
    Thumbs Up/Down
    Received: 54/0
    Given: 0/0
    I thought the guys name should have been "Brown Spot" instead of "Brown Note." But I'll try not go go down the path of bullying somebody who isn't even here to defend himself. To that end I'm not going to make snarky comments from the sidelines without at least making a useful contribution to the thread:

    Getting back to the OP's question:

    Things are always more complicated than we like to admit, and people like to work with rules of thumb because that's easier than deriving the correct answer to these types of questions. That's not all bad, because the rules of thumb can be very useful. As useful as those guidelines may be, it's still worth understanding the potential pitfalls of following those rules so that you won't get into trouble.

    Can someone give me a basic run down of the "what happens" when you pull 2 tubes and half the impedance switch on a typical push/pull w/ 4 power tubes? Like.. the theory of what happens?

    Voltage and current draw on the 2 tube plates left running go up, the overall impedance doubles, the overall resistance goes down (I think?).
    Suppose that a pair of 6L6 are going to be run in AB1 at an anode voltage of 450 VDC. The GE data sheet suggests Raa of 5k6 and a bias voltage of -37, and a peak load of 232 mA for the pair.

    Going to a quartet at the same B+, the recommended Raa will be halved to about 2800 (5k6 in parallel with 5k6) and the peak load will increase to 2 x 232 mA = 464 mA.

    Now suppose that you had an quartet type amp, like a Twin Reverb. It might have an OT with a turns ratio of about 700:1, so that when you place a 4R load on the amp (two 8R speakers in parallel), it reflects a primary Z of 2800 to the quartet of 6L6: 4R x 700 = 2800R. That puts us right in the area of the design center values for running the quartet.

    If you pull two tubes out of the amp, that doubles the target Raa for the tubes design center values, taking you back to an optimum value of 5k6 for a pair. A problem pops up though, that you've still got a 700:1 turns ratio in your OT. Your optimal load would be 5600R/700 = 8R; in order to reflect a "proper" load impedance through the transformer to the pair of 6L6, you'll need to connect an 8R load. If you have two 8R speakers wired in parallel, disconnecting one of them would solve that problem.

    As a matter of practice, most people don't bother to do this, and instead they just rely upon the fact that the amp's engineers designed the amp to match design center values, which provide a fair amount of leeway for mismatch errors. In that case you've got some safety margin to work with, so most people won't worry about changing the speaker load, even though doing so would be optimal.

    Unfortunately, there are a lot of people who develop a false sense of security about the safety of proceeding to pull tubes without worrying about Z-matching, because they've gotten away with sloppy practices for a long time. Because amps like Fenders were designed around optimal design center values, and are built with robust components, many people have developed a cavalier attitude about mismatching. They haven't been burned (yet) so they're inclined to recommend what I would not consider good engineering practices.

    Those rules of thumb work great for amps that were designed around centered values and were built with robust components. They don't work so well though, if you've got an amp that was deliberately designed away from the tubes' design center recommendations. An example of this could be a boutique amp, which could be deliberately designed to alter the amp's distortion characteristics by deliberately moving it to an off-center load resistance. In cases like that the mismatch could push things a little too far, and being cavalier about mismatching could result in getting burned. In that scenario Z-matching when pulling tubes would be a good idea.

    That covers the Z-matching question. The voltage/current/bias problem still needs to be addressed. More coming...

    0 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  15. #15
    Senior Member
    Join Date
    Aug 2017
    Location
    Portland, OR
    Posts
    351
    Thumbs Up/Down
    Received: 37/1
    Given: 36/1
    Quote Originally Posted by trip View Post
    Palamino V32 Custom - 4 el84s, never attempted to pull or even looked at the circuit but of course thinking about it
    FYI, this is Class A.

    0 Not allowed! Not allowed!

  16. #16
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,908
    Thumbs Up/Down
    Received: 54/0
    Given: 0/0
    the forum seems to be having a latency problem with the page loads. it took a long time to commit my last post.

    Quote Originally Posted by trip View Post
    I was under the impression of puling two, half the impedance, and you're good. You might have to adjust bias but as bias is mostly a preference within acceptable range thing you can get away without having to adjust after pulling two given that there is no red-plating happening. Is this inaccurate? Not should you still adjust the bias (we know we "should") but do you have to? Does anything negative happen to the amp with the change, specifically, will it "actually" cause any problems by consistently running 100 watt heads w/ only 1 pair?

    I was told at an expo event that this will always damage an amp because of the change in current and stress on some parts. I have also been told the opposite, that it's fine, you don't need to adjust bias (but you should) and that it will not fail because of running at half power. Then, I see a post as above, which also conflicts.
    You should always check bias on the amp when you're servicing it. Always. It's just good practice and it's easy to do.

    There are a few things that can cause problems with pair-pulling that most people don't think about. Although most people are familiar with the Z-matching problem, most people aren't aware of the voltage problems that can ensue with tube pulling.

    Your PT is designed to provide a specific secondary voltage, and that secondary voltage will vary somewhat depending on the load being placed on the transformer. An amp's transformer is designed to provide relatively constant voltage supply with a design range of varying loads. The extent to which that voltage does not vary much within the range of specified loads is determined by the "regulation" specification of the transformer.

    Going back to our 6L6 data sheet, the idle current for a pair of 6L6 at 450 VDC is about 122mA, and as I mentioned in the previous post the peak current is about 232mA. For a normal pair of 6L6 you might expect to see load vary by about 110mA between the idle and peak operating points.

    Those numbers would be doubled for a quartet of tubes: about 2 x 122 = 244mA at idle and about 2 x 232 = 464 mA at peak. That's a range of 464 - 244 = 220mA.

    From a practical standpoint the 2x6L6 PT should be designed to maintain a reasonably constant voltage when loaded with about 110 to 232mA of current, and a 4x6L6 PT should be optimized to deliver twice that. When you pull two tubes, you're asking a PT that's designed to be fairly constant in the range of about 244 - 464mA to work in the range of about 122-232mA. Chances are that your transformers regulation won't be as good at half of it's intended current, so voltages may rise. How much? Hard to say with hand-waving, but it could be significant.

    That increased voltage could cause problems. Anything that ends up operating outside of it's proper operating voltage range could experience a shorter lifespan. If you go pulling tubes, you should expect that the B+ voltage would rise, and the bias voltage should change along with it. IMO re-biasing is important if you pull a pair of tubes.

    Other things to look at would be the heater voltages. If your heater voltages were to rise by say 10%, that could put them outside of their normal tolerance range and shorten the lifespan on all of your tubes.

    There are a lot of things to consider, but most people turn a blind eye to them. If you really wanted to be optimal about pulling a pair of tubes, you might want to consider replacing them with a dummy load so that your PT remains functioning in it's intended load range. I'm probably the only guy who bothers to do this.

    0 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  17. #17
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,432
    Thumbs Up/Down
    Received: 418/0
    Given: 455/0
    I don't think that post reads so much like the Crate-peg guy as the "I invented led cathodes" guy (and it could be he). At any rate, it's not so much wrong information as terribly incomplete with all it's caveats to the normal mo of matching the impedance to the tubes. So at face value it should be taken to the top of a freeway overpass and thrown at a passing car. Keep balancing your push/pull sides and keep matching the impedance when pulling tubes.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  18. #18
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,029
    Thumbs Up/Down
    Received: 18/0
    Given: 0/0
    This person does not understand that if you add tubes in parallel, you have to change the turns ratio of the transformer in order to take advantage of the additional available power. No need read or comment further.*

    *That is, the person who wrote the response in the attachment to the first post of this discussion.

    0 Not allowed! Not allowed!
    Last edited by Mike Sulzer; 02-15-2018 at 07:22 PM.

  19. #19
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,200
    Thumbs Up/Down
    Received: 343/1
    Given: 672/0
    Quote Originally Posted by Mike Sulzer View Post
    This person does not understand that if you add tubes in parallel, you have to change the turns ratio of the transformer in order to take advantage of the additional available power. No need read or comment further.*
    Agreed Mike, if you want to stay with the original load impedance. Alternately, you can maintain the original turns ratio and change the load impedance. This is the most common, simple solution when the amp is already built.

    0 Not allowed! Not allowed!
    The voices in my head are idiots!

  20. #20
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,432
    Thumbs Up/Down
    Received: 418/0
    Given: 455/0
    What the guy in the OP's attachment DIDN'T say is that you can't run tubes out of spec like that at any sort of stressful levels and also can't expect the same level of performance when doing it. The average guitar monkey armed with only that post for advice is going to blow up parts in their amp. (<that's a period)

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  21. #21
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,029
    Thumbs Up/Down
    Received: 18/0
    Given: 0/0
    Quote Originally Posted by g1 View Post
    Agreed Mike, if you want to stay with the original load impedance. Alternately, you can maintain the original turns ratio and change the load impedance. This is the most common, simple solution when the amp is already built.
    But that solutions side effects:

    1. The percentage losses in the transformer approximately double. That is, winding resistance is bigger than it should be because there are too many turns and the wire is too small.
    2. The leakage flux becomes more important. This can cause a loss of high frequencies.
    3. The magnetizing inductance is larger than it needs to be, and so the small signal bass response has gotten better, but the power handling capability in the bass has not improved.

    So if you really want to double your power, you need a bigger transformer wound correctly to specs.

    0 Not allowed! Not allowed!

  22. #22
    Senior Member
    Join Date
    Nov 2013
    Posts
    145
    Thumbs Up/Down
    Received: 21/1
    Given: 2/0
    he said that primary impedance doesn't change,i'd say there is no absolute primary impedance value,only a primary/secondary relationship that can also change depending on the load/ no load condition.
    That guy is a total BS and should present himself with his real name to prevent people from disaster.

    0 Not allowed! Not allowed!

  23. #23
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,432
    Thumbs Up/Down
    Received: 418/0
    Given: 455/0
    Quote Originally Posted by alexradium View Post
    he said that primary impedance doesn't change,i'd say there is no absolute primary impedance value,only a primary/secondary relationship that can also change depending on the load/ no load condition.
    That guy is a total BS and should present himself with his real name to prevent people from disaster.
    Yes, in that statement he was dead wrong. He probably meant, thinks or believes that because the transformer turns ratio is constant. But a transformer is an coupling device. The primary to secondary are not mutually exclusive. Altering the circumstances on the secondary absolutely does have an analog to the primary and if you don't correct for it then you get inefficiency and losses. And, as already pointed out, you get losses even when you DO correct for impedance, but not usually a big deal for guitar frequencies and not such that you shift into dangerous operating conditions or noteworthy power losses.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  24. #24
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,647
    Thumbs Up/Down
    Received: 577/1
    Given: 0/0
    Transformers may be designed for certain impedance ratios, but the transformer itself has no impedance, it only has a ratio. If someone says they have a 6k primary OT, it is not something you can measure with a meter. It ONLY presents 6k ohms impedance to the tubes IF the specified load is connected.

    So the primary impedance changes with the load impedance. That is why they wind OTs with multiple output windings or taps.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  25. #25
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,908
    Thumbs Up/Down
    Received: 54/0
    Given: 0/0
    Quote Originally Posted by Mike Sulzer View Post
    This person does not understand that if you add tubes in parallel, you have to change the turns ratio of the transformer in order to take advantage of the additional available power. No need read or comment further.*

    Quote Originally Posted by Mike Sulzer View Post
    ...So if you really want to double your power, you need a bigger transformer wound correctly to specs.
    All valid points, but wasn't the OP's question directed at pulling a pair of tubes from a quartet type amp, in order to halve the power, not adding a pair to double power?

    0 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  26. #26
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,432
    Thumbs Up/Down
    Received: 418/0
    Given: 455/0
    It's just discussion of peripheral aspects about the subject at hand Bob. g1 and Mike drifted into some general considerations. It's all food for the forum.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

  27. #27
    Better Tone thru Mathematics bob p's Avatar
    Join Date
    May 2006
    Location
    Chicago, USA
    Posts
    4,908
    Thumbs Up/Down
    Received: 54/0
    Given: 0/0
    Oh, I understand the food for the forum thing. But the OP's question was about tube-pulling, not modding a PP pair amp to a PP quartet amp. From a practical standpoint, there are other things in an PP pair amp that will preclude the idea of adding 2 tubes to increase power, like lack of space, PT limitations, OT limitations, etc. I understand the value of theory, but I guess I'm too grounded in practical application to understand the deviation toward the physically impractical/impossible considerations. I hope that hasn't confused the OP.

    0 Not allowed! Not allowed!
    "Stand back, I'm holding a calculator." - chinrest

    "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

  28. #28
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,029
    Thumbs Up/Down
    Received: 18/0
    Given: 0/0
    Quote Originally Posted by bob p View Post
    Oh, I understand the food for the forum thing. But the OP's question was about tube-pulling, not modding a PP pair amp to a PP quartet amp. From a practical standpoint, there are other things in an PP pair amp that will preclude the idea of adding 2 tubes to increase power, like lack of space, PT limitations, OT limitations, etc. I understand the value of theory, but I guess I'm too grounded in practical application to understand the deviation toward the physically impractical/impossible considerations. I hope that hasn't confused the OP.
    Sorry to be confusing. So let's pull out one tube (of two) on each side. We continue to drive the same transformer. Entirely independent of how the taps to the load are adjusted, or the load changed, there is a potential problem. We have only one tube per side providing the magnetizing current to the primary inductance. Since this current increases with decreasing frequency, it becomes a problem at twice as high a frequency, and we lose bass.

    If you include the low frequency specification, transformers are designed for a specific impedance.

    0 Not allowed! Not allowed!

  29. #29
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,200
    Thumbs Up/Down
    Received: 343/1
    Given: 672/0
    Agree with the theoretical concerns, differ on the practical ramifications.
    I'd think this would be more of an issue with hifi than guitar amps, but point taken. It's still real and I imagine the tx designers have a specific primary z to build around.

    0 Not allowed! Not allowed!
    The voices in my head are idiots!

  30. #30
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,029
    Thumbs Up/Down
    Received: 18/0
    Given: 0/0
    Quote Originally Posted by g1 View Post
    Agree with the theoretical concerns, differ on the practical ramifications.
    I'd think this would be more of an issue with hifi than guitar amps, but point taken. It's still real and I imagine the tx designers have a specific primary z to build around.

    I agree that it could be of no practical concern in a guitar amp. Or maybe it could sometimes. Output transformers used in guitar amps can be smaller than those used for equivalent power levels in HiFi, and so the safety margin for 80 Hz might not be as big as it might seem.

    0 Not allowed! Not allowed!

  31. #31
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,829
    Thumbs Up/Down
    Received: 72/0
    Given: 184/1
    Let me offer this completely unproven assertion:
    When trying to get less 'volume' out of a 100W amp by pulling two tubes and moving the impedance tap, the resulting loss of LF capacity in the OT can actually help the perception of a quieter amp.

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  32. #32
    Supporting Member
    Join Date
    Sep 2007
    Posts
    3,029
    Thumbs Up/Down
    Received: 18/0
    Given: 0/0
    Quote Originally Posted by g1 View Post
    Agree with the theoretical concerns, differ on the practical ramifications.
    I'd think this would be more of an issue with hifi than guitar amps, but point taken. It's still real and I imagine the tx designers have a specific primary z to build around.

    Consider The Magnetics Components Classic Tone 40-18025, a fifty watt transformer. The impedance across the full primary is 3.6K (http://www.classictone.net/40-18025.pdf). I measure the primary inductance at 6.56H. The frequency f at which the inductive reactance equals the primary impedance is f = 3600./(2.*pi*6.56) = 87.34Hz. If you use this where you should be using a 7.2K primary, you have about 175Hz. This might matter; it depends on what you want.

    0 Not allowed! Not allowed!

  33. #33
    Supporting Member
    Join Date
    May 2006
    Location
    Staffordshire UK
    Posts
    3,426
    Thumbs Up/Down
    Received: 86/0
    Given: 83/0
    6.56H seems low?
    The Hammond equivalent is spec'ed at >119H (tested at 240V 50Hz) http://www.hammondmfg.com/pdf/EDB1750N.pdf

    0 Not allowed! Not allowed!

  34. #34
    Supporting Member
    Join Date
    Mar 2017
    Location
    Massachusetts, USA
    Posts
    1,108
    Thumbs Up/Down
    Received: 49/0
    Given: 923/0
    What a great discussion. So, the answer is . . .
    Pop 2 tubes out (assuming you're popping the correct set out),
    then pop in a different OT designed for the task.

    0 Not allowed! Not allowed!

  35. #35
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    13,432
    Thumbs Up/Down
    Received: 418/0
    Given: 455/0
    Quote Originally Posted by mikepukmel View Post
    What a great discussion. So, the answer is . . .
    Pop 2 tubes out (assuming you're popping the correct set out),
    then pop in a different OT designed for the task.
    Or just pop out the tubes (in a four tube amp) and either double the actual speaker load impedance or half the impedance switch into the existing speaker load. Consequences not withstanding. Most OT's aren't so cheesy that you'll suffer noticeably for it at guitar and guitar speaker operating frequencies.

    0 Not allowed! Not allowed!
    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "A shot gun delivers a force that exceeds the operational range of most systems, such as pumpkins." Antigua

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. GU-50 Push-Pull AB OT Impedance/Load Line Questions
    By defaced in forum Theory & Design
    Replies: 35
    Last Post: 06-04-2015, 06:21 PM
  2. Battery powered Push pull amp
    By iccaros in forum Build Your Amp
    Replies: 12
    Last Post: 12-24-2011, 03:33 PM
  3. Designing push pull stages - many questions!
    By exclamationmark in forum Theory & Design
    Replies: 10
    Last Post: 10-12-2011, 03:08 PM
  4. Will a pair of 6L6's take 3200 ohm load push pull?
    By priesty in forum Theory & Design
    Replies: 6
    Last Post: 08-03-2010, 03:14 PM
  5. Lopsided Push-Pull 7591S Pair, 6U8A Splitter
    By ktritty in forum Debugging Your Build
    Replies: 13
    Last Post: 06-25-2009, 07:09 AM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •