Calculating Output Transformer Primary Impedance

[nashvillebill]

For common power tubes (6V6, 6L6, EL84, etc) I select an OT by finding an amp that uses a similar configuration and then going to Classictone to see what they sell for that amp. A pair of 6V6's with cathode bias? That's like a Fender 5E3, there we are. A quad of 6L6's, that's like a Fender Twin so there we go. (If we don't want to use the Classictone OT's but maybe a Hammond, use the impedance values from Classictone as a working point.)

The times I could use a Selecting an OT for Dummies book: I've got a gazillion less-common tubes, along with scavenged PT's and OT's and conversion projects. So I wind up thinking: gee, what would be a good OT for a 6SN7 running push-pull? (I've actually got a Webcor tape recorder conversion that does just that) Or whatever--say a 6AK6 in PP. The books don't give design values, or handy little power and distortion curves for that configuration (such as the chart guitician shows for the 6L6). Although the tube charts list Class A SE (but not PP) design value load resistance for my 6AK6 example, I'm lucky to even find that for some of the other tubes. I don't need calculations to 3 significant figures, just a general idea of "something around 10K" give or take a few K.

[jazbo8]

As Merlin pointed out, there are just too many parameters and tube types for anyone to compile and come up a comprehensive table. But it should be possible to go through each tube's datasheets and get the "ideal" load using multiple charts and tube constant conversion factors. To get the distortion estimate, the process is even more involved, but it can be done as well, you can get the detail steps for the above, from RDH4.

Going through the above steps is a real chore, which is why the whole process can be simplified by using a computer program like Excel or SPICE. For example, I could draw a plate charactersitic chart with load lines for the 6AK6 (or any other tube), estimate the output power and distortion with just a few clicks on the keyboard and mouse - much faster than I can if I had to go through the whole datasheet and doing everthing manually.

OTOH, just because it is easier to use such program to get the optimal load impedance, it does not mean it is the best way to design the output stage. In fact, I think it is better if we stick with standard off-the-shelf OPTs, and simply adjust the output stage and the speaker load to get the desired results (within limits of course).

[Chuck H]

For common power tubes (6V6, 6L6, EL84, etc) I select an OT by finding an amp that uses a similar configuration and then going to Classictone to see what they sell for that amp. A pair of 6V6's with cathode bias? That's like a Fender 5E3, there we are. A quad of 6L6's, that's like a Fender Twin so there we go. (If we don't want to use the Classictone OT's but maybe a Hammond, use the impedance values from Classictone as a working point.)

I think the selection process needs to be taken one step further. Running a pair of 6V6's in cathode bias isn't necessarily like a 5e3, unless it also has no NFB. 5e3 primary impedance 8k. The Deluxe Reverb uses NFB and a lower primary of 6.6k. NFB significantly reduces the optimum primary impedance. Add all the different NFB options as well as "presence", variable and "resonance" controls, as well as the likelihood of power tube clipping and the whole shebang becomes incalculable. I dunno... I figure if your going to clip the power tubes often then just choose the primary from the chart since NFB is sort of obliterated when the power tubes are clipping. If the amp will be run clean, or at least with unclipped power tubes and NFB maybe it's best to go with the 4k per pair for big bottles. Just guessing though.

[Gingertube]

If you look at the speaker impedance vs frequency plot and check what range of impedances that reflects to the primary side you will quickly see that impedance ratio is not that critical.

In general - a higher Raa will give less power and less distortion, that is will be more suitable for HiFi.
conversely
- a lower Raa will give more power and more distortion.

The output tube datasheets will generally give you a recommended Raa value for a recommended B+ voltage.
- For a guitar amp we will usually use a B+ higher than the datasheet recommendation which would lead me to suggest that you use the recommended Raa or a little higher.

I see 6V6 examples quoted above. I have built 2 off 6V6 Amps and am extremely happy with both.
1) A single pair of 6V6 into a 10K Raa (that is a bit higher than the data sheet recommended 8K but then I'm running B+ at 340V which is a bit higher than the data sheet 285V). The Output Tranny is a Chineese HIFI Output Tranny intended for 2 x EL84 in Ultralinear and rated at 10W down to 30Hz. Fine for Pentode Mode 6V6 with 14 or 15 Watts since I'm not trying to deliver power down to 30Hz. I would have happily used a Hammond 1608 (8K Raa, 10W HiFi tranny) but had the Chineese Trannies "in stock".
2) A quad of 6V6 into 3K4 (that is a bit lower than the 8K divide by 2, once again I'm running B+ 340 to 350V). Output Tranny was a spare JTM45 I had on the shelf.
So this tells me that +/- 50% of datasheet recommended Raa make a "rodents hind quarters" difference.

My favourite EL34 amp runs cathode bias, B+ of 380V and Raa of 3K4 which is ALL straight from the datasheet.

The datasheet aplication data will usually be a little conservative (lower B= and Higher Raa than often seen in Guitar Amps) but I am convinced that you get superior sound by using those conservative opearting conditions. Many of teh "typical" Guitar Amp operating conditions have resulted from what Chuck calls the "Power War" Era. My 60W Amp must be better than yor 50W amp... right? Well in actual fact I would be willing to risk a small wager that the opposite is true.


And always keep in mind - this is an amp design forum and we tend to concentrate on that, but the choice of speaker(s) and cabinet will be at least as important in forming your final sound as all the hours you spend designing and building the amp itself.

Given a choice of tube and required output power etc. then Kevin O'Connor gives design procedure to determine required Raa.
Given a particular Output Tranny he also gives a procedure to arrive at required tubes and power supply.
This is covered in "Principals of Power" (POP) and from memory in TUT2.

Various guys webpages deal with load matching, example: Patrick Turner's webpages, see loadmatch4-pp-beamtetrodes

If in doubt use Power Supply B+ and Output Transformer Raa which are close to the application data on the tube datasheet.
Duncan Amps, tube data site gives typical operating conditions as well as teh datsheet links.

For example the 6L6GC
TDSL Tube data [6L6-GC]
If I were designing a quad of 6L6GC from scratch (in ignorance of all the Fender, Mesa, etc. designs) I would look at that second entry for Class AB1 Push Pull which shows 55W into an Raa of 5K6 for a pair of 6L6 and then go looking for a 100W rated tranny with an Raa of 5K6/2 = 2K8. Then going to the various transformer manufacturers and seeing that they are all offering Raa of 2K I would just say "close enough" and use that.

For a quad of 6L6GC most replacement output trannies offer an Raa of 2 KOhms. The "ToneMaster" replacement offers Raa of 2K35 and might be worth a look
Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Output Transformers

Tube Amps were traditionally built with +/-20% components and in fact teh tubes themselves are a +/- 20% component. Don't obsess trying to get an exact value for any component and output tranny Raa probably least of all. "Ballpark" values will invariably be good enough.

Cheers,
Ian

[Chuck H]

Bitchin' seriously. And nice links. But...

What about NFB??? My JL Hood book indicates that the use of NFB reduces ideal Raa. And you mention in the macro sense that a lower Raa generally gives more watts and distortion while higher gives less watts and less distortion. But the posted chart is obviously contrary to that? Why? And what about the affect of NFB vs. no NFB on Raa???

Not putting anyone on the spot. Quite the contrary. Inquiring "mind" wants to know.

[eschertron]

Bitchin' seriously. And nice links. But...

What about NFB??? My JL Hood book indicates that the use of NFB reduces ideal Raa. And you mention in the macro sense that a lower Raa generally gives more watts and distortion while higher gives less watts and less distortion. But the posted chart is obviously contrary to that? Why? And what about the affect of NFB vs. no NFB on Raa???

Not putting anyone on the spot. Quite the contrary. Inquiring "mind" wants to know.

So what do you mean by the "book indicates that the use of NFB reduces ideal Raa"? Do you mean that you can get by with a lower PriZ if NFB is used? If that's the case, I'd suggest that NFB increases the 'apparent' Raa and results in the text book value seen by the tubes even with a lower actual OT impedance. Does that make sense?

[Chuck H]

It might, yes. If the need for damping is reduced. And this could well be the case with the application of NFB where the output tubes are part of the loop. I'm not tech enough to prove or disprove anything though. So I'm asking.

[nashvillebill]

I tried to follow the chapter on NFB in RDH4 and got a headache pretty quickly. But then again, I'm just a mechanical engineer, not a EE

[Gingertube]

NFB has the effect of lowering the internal (rp) of the output tubes which means that you CAN get a better power match to an output transformer with a lower Raa.

To talk about NFB I beg your indulgence as I go off on a seeming tangent and talk about amplifier output impedance.

The Amplifier Output Impedance will be determined by the output tubes internal impdeance rp reflected from the primary to the secondary of the output transformer.

HiFi guys talk about amplifier Damping Factor, this is the ratio of speaker impedance divided by amplifier output impedance. It determines the ELECTRICAL "damping" of the speaker provided by the amplifier.

HiFI gurus brag about their damping factor = 50 amps and say that any amp with a damping factor of less than 10 is No Bloody Good (NBG).

My experience with HiFi Tube Amp design and build tells me that a Damping Factor of 3 is fine and with speakers which have a good MECHANICAL damping characteristic then a damping factor of 2 is fine.

Guitar Speakers need to be able to "flap in the breeze" more than HiFi Speakers, you don't want to overdamp their response. That means that an amp with lower damping factor agiain (say 0.3 to 0.5) is just fine.

Back in the 70's there were a pair of Ozzie Boffins called Thiele and Small. They worked aout all the maths for handling speaker damping.

They talked about 3 types of Q (Q is a "gain" parameter in a resonant system - so it is the inverse of damping, OR 1/the damping amount)
Qts - Q of the total speaker system (although the ts probably stands for Thiele Small not Total Speaker)
Qms - Q of the speakers mechanics (stiffness of spider, mass of the cone and all that stuff)
Qes - Q of the speakers electricals (voice coil resistance etc.etc)

Now the HiFi guys tend to overlook that the output impedance of the amplifier works with the Qes parameter to provide electrical damping of teh speaker but does absolutely nothing about the Qms, so their dampin factor discussions are only addressing part of the story.

Speaker Total Q or Qts = RMS of Qms and Qts (= square root([ Qms squared + Qes squared]/2))

Speakers have these parameters and importantly speakers mounted in thier working cabinets (that is the speaker "system") have parameters of their own only partly determined by the speaker driver parameters.

For Closed Boxes there will be significant mechanical damping (low Qms value), that means we can get away with less electrical damping and we don't need a low output impedance from the amp which in turn means we don't need NFB or at least we need very little of it.

For Open Backed Boxes (or Combo's) there will be little mechanical damping which means we need more electrical damping, that means a lower amp output impedance and therefore needing more NFB.

"Loose Tuned" boxes are in the middle.

This is all complcated again because we can choose the actual speaker drivers to have more or less Qms vs Qes etc.. The adverts will say suit COMBO or OPEN BACK Box or suit Closed Box if they have the decency to say anything about this at all.

My experience has been than with loose tuned boxes and using real intended for guitar speakers then a damping factor of 0.3 to 0.5 is fine (for a 4 Ohm Speaker that infers an amp output impedance of 8 to 12 Ohms. You can achieve that with no negative feedback but will probably need a little.

So I would suggest that you avoid NFB if you can and use no more than you have to. You will need more in a combo or when using open backed cabinet.
The advantages of having some NFB is the ability to add PRESCENCE and RESONANCE controls on the power amp section.

Cheers,
Ian

PS the "moral of the story" is that NFB affects amplifier output impedance which affects only one aspect of speaker damping characteristics. You can choose speakers for more or less Qms vs Qes depending upon whether you want to put them in a combo or a closed box or whether you want to damp them using a lower output impedance (more NFB) amplifier.
Like many things its all "swings and roundabouts" but if you have a general idea of whats going on then you have a general idea of how to fix things which are not quite right.

Example of what I'm raving about:
On the Junkbox Trainwreck I did for the local Guitar God I ended up nailing a board across the back of his speaker box to convert it from half open back to 1/3 open back. That is, rather than up the NFB in the amp to get better electrical damping of the speaker (which was sounding a little loose) I part closed the back of speaker box to get better mechaniocal damping and left the amp alone. That was because he occasionaly uses the amp with a closed quad box and I did'nt want to make the amp any tighter.

Appologies for the OF-Topic'ish Rave.

[Chuck H]

No apologies necessary! Very relevant, real world experience on the matter

[g1]

Earlier in the thread, Chuck H mentioned "choosing high" when confronted with a choice of higher or lower than ideal impedance primaries.
From strictly a safety and reliability perspective, that is the better choice, in order to stay within safe operating parameters for the tube. Also, when having to choose between a load that is too high or too low for the tap, the higher impedance load is preferred as it will reflect a higher impedance back to the power tubes.
It is explained pretty well by Aiken here: The Last Word On Biasing
"In some cases, if the voltage is high enough, there is no bias setting that will result in safe operation without exceeding the maximum plate dissipation of the tube. For example, in a 100W EL34 amp with 480V on the plates and a 1.7K primay impedance, there is no bias setting that will keep the output tubes from redplating at some point in the power curve, even if you bias the amp at 0mA! If you double the primary impedance to 3.4K, however, the amplifier will operate fine at all power levels, and you can bias it all the way up to 52mA without ever exceeding the plate dissipation at any point in the operating curves. This may sound odd, because the bias point is right at 100%, but it works because of the very high plate impedance load, which never allows the tube dissipation to increase above the idle level. It also may not sound as good, because the screen voltage should ideally be decreased so the loadline intersects the "knee" of the curves if not, the nonlinearity increases drastically. The bottom line is that you have to take into account not only the plate voltage and plate current, but also the primary impedance to find out the safe bias area."

Please note the remark that "it may not sound as good". If tone is your prime directive, and tube life is of no consequence, you may want to "choose low" .

[Bob M.]

A Great Output Transfx thread...

Hi all,

What an informative thread this is!

I was gonna start a new thread about output transformers but this is so on-point, I thought I would just add on here.

I have an original design that I built and it sounds great.

It is: 2x 6L6GC (NOS GE med/small bottles), UL output transformer (Hammond 1620), free running - no neg feedback, open back cab, similar to a Fender type amp cab, 440 plate voltage, into 8 ohms (my favorite 12" Weber spkr). I happened to have this Hammond 1620 in stock, gathering dust so it got pressed into service. It fit my design parameters pretty well: 6,600 primary impedance, 158 mA and was recommended by Hammond for 2x 6L6 usage. It's not too large or heavy and this is a small chassis and amp, suitable for a small or medium club with about the power one would expect from that, appropriate for the rooms mentioned. In my experience, it 'feels' like about a 35 to 40 watt amp. And, yes, I am employing the ultra-linear taps.
,
The Hammond website tell me that the primary impedance is 6,600 and, of course, I've heard that 4K number in relation to a pair of 6L6GC tubes so I though I'd hookup an output switch so I could experiment between the 8 ohm taps or the 16 ohm taps. This Hammond 1620 has 4, 8, and 16 ohm hookup possibilities. I was assuming that running the amp on the 16 ohm tap would cut the primary impredence to 3,300. So a switch was wired and I've used this amp for about a year now, switching this impedance switch every few months to see if I could detect a performance difference of any sort from the available two taps I chose. (I'm a busy player and play often so this amp has had frequent workouts). Well, I cannot hear any difference in power, feel, distortion, clean sound or anything at all. The switch was mounted temporarily and experimentally so I decided to choose and hardwire a permanent tap, which turned out to be 8 ohms. The Hammond 1620 has a goofy wiring scheme as there are two discreet windings on the secondary. By choosing 8 ohms, I tried to pick the ohmage that used as much of the transformers windings as possible. More recently, Hammond now makes another version of the 1620 (1620A) with a more traditional hookup of the secondary winding but my transformer is the older type.

Anyway, I was wondering if any of the posters had any comments at all about my little output transformer journey. I think I was in search of some of the same answers to question that have been posed in this thread.

Thanks and I hope this adds to the discussion,

Bob M.

[jazbo8]

UL connection is much more tolerant to changing loads than the pentode connection, so may be that's reason you did not detect much of a difference between the various settings. BTW, what was the B+ voltage?

[Bob M.]

UL connection is much more tolerant to changing loads than the pentode connection, so may be that's reason you did not detect much of a difference between the various settings. BTW, what was the B+ voltage?

B+ = 442 Vdc
Plate = 440 Vdc
Screen = 440 Vdc

Bob M.

[jazbo8]

Sorry should have read your eariler post more carefully, you already showed the B+ voltage, what are the 6L6s biased at? And are you asking what is the optimal Zp-p or why there wasn't more difference between the various settings?