Help understanding output transformer primary impedance

[Gainzilla]

Hi all,

This is an area I know very little about but would like to gain better understanding of. I *think* you select an OT based on the transconductance of the desired output tubes. If true, this would explain the difference in say a bassman and a 50 watt plexi.

Another point is why is the impedance for a 100 watt OT lower than in a 50watt? Is it because the transconductance is similar with multiple tubes like running resistors in parallel?

How does the difference affect tone? Would I choose in a safe range for a desired sound similar to biasing hotter or colder?

Thanks in advance for any insight!

- cheers

[Alan0354]

You choose the primary impedance according to the load line of the tube.

1) Make a copy of the plate curve of the power tube you use.
2) choose the idle operating point base on your idle current and the +B available from you power supply.
3) Draw a straight line from the operating point from 2) to the knee of the 0 grid plate curve.
4) From the straight line, you can find the change of voltage to the change of plate current. Your primary impedance should be R=V/I of the straight load line.


the idea is to have the primary impedance to give you the max voltage swing corresponding the the max current swing.

[R.G.]

That's close, but there is a bit of finesse to be applied.

First, remember that we've been standing on the shoulders of giants for so long we almost forget there were giants.

As Alan notes, you pick a load line that will use as much of the possible voltage and current swing on the plate characteristic of the tubes. Yes, putting more tubes in parallel doubles the transconductance, but more importantly it doubles the available plate current sum for the same voltage swing. At a crude estimate, you cut the plate to plate impedance by the number of paralleled tubes.

However, the giants used to build and tinker these things in laboratories. It turns out that for pentode and beam power tube amps (today: EL34 and 6L6) there is a maximum power load line, and a maximum power with minimum distortion load line, and these are different. Guitar amps always went for most power, hifi always went for minimum distortion, as the different customer sets paid more for their speciality.

On the other hand, the tubes themselves are not terribly fussy about the plate to plate load, and differences of 2:1 still work, albeit with different frequency responses into particular speakers, different distortion curves, and mildly different power outs - generally not enough difference in power out to make a huge difference to apparent volume.

[Alan0354]

I just talk about the basics, the finesse part has to be done by experience and with time spend on it. We all know by now electronic theory get you into the ball park, it's important to know. But once you are in the ball park, that's where mojo and finesse take over. No body can tell OR are willing to tell their experience that they spent so much time to acquire.

This is just the ABC guide to load line.

[J M Fahey]

No body can tell OR are willing to tell their experience that they spent so much time to acquire.

Why do you say so?

You picked right here the advice you just wrote , and you got it from generous Forum Members, who had no problem or asked nothing in return for answering your endless questions.

Doubt many have milked this Forum as much as you , what any old threads search can show.

FWIW you had no clue about calculating impedance or load lines just a couple Months ago.

[Gainzilla]

Thanks for the info guys! I've calculated load lines in the past for preamp stages so the concept makes sense to me. I get that choosing the voltage is a by-product of your given design, but how do I go about finding #2 from Alan's post? I'd like to give it a try with something off the beaten path (like a pair of kt88s, for example), just for the sake of practice.

Cheers!

[R.G.]

As a practical matter, that's not all that easy to do. The plate to plate impedance is actually the slope of the I-V load line. In general, changing the bias current doesn't change the slope of the desired load line, it just overlaps the two tube conduction characteristics. The chosen max-power, or min-distortion impedance is still just about right, but you get less peak to peak voltage swing out of the two combined tubes before you hit maximum current for the active tube. I'm sure there are special cases, but I think this is true for most practical Class AB tube amps.

And this is good, because in a real tube amp, you're almost never going to get tubes that match that printed plate characteristic curve. They'll all have slightly different transconductance and emission characteristics, and they'll all need a bit different bias to get them balanced and operating at an acceptably low distortion point.

Think of the composite plate characteristic as taking the plate characteristic for one tube, then copying it and flipping it up/down and right-left and overlying THAT onto the first one. Slide the flipped one left-right/up-down until the intended load line on it is co-linear with the load line on the first. Sliding the two so the load lines are always overlaid is a visual representation of what happens as you change bias. For instance, completely overlapping the two so that neither tube ever goes to 0 current is Class A. Sliding them so that they have places where one tube turns off is Class AB. Load line slope doesn't change, bias does.

[jazbo8]

Thanks for the info guys! I've calculated load lines in the past for preamp stages so the concept makes sense to me. I get that choosing the voltage is a by-product of your given design, but how do I go about finding #2 from Alan's post? I'd like to give it a try with something off the beaten path (like a pair of kt88s, for example), just for the sake of practice.

Cheers!

Read this for starter. Bear in mind that typical push-pull guitar amps operate in Class AB, not Class A (which the preamps always use). So the load lines is a composite of Class A (1/2 Ra-a) and Class B (1/4 Ra-a), as you will see from Merlin's page.

[Alan0354]

Why do you say so?

You picked right here the advice you just wrote , and you got it from generous Forum Members, who had no problem or asked nothing in return for answering your endless questions.

Doubt many have milked this Forum as much as you , what any old threads search can show.

FWIW you had no clue about calculating impedance or load lines just a couple Months ago.

Because what you said about load line is standard electronic theory. What I reply to RG is the small little variations from the standard optimizing that people really dig into deeply. Read RG's post. I am sure you have little tricks up your sleeves that you never willing to share.

Yes, I thank you for telling me about load line. But that is the standard method, RG is talking about something more. That's the mojo part that people are not willing to share and I DON'T BLAME THEM EITHER.

Yes, I learn a lot from this forum, I specially thank Chuck H that gave me the guidance at the beginning about cutting the lows at the first two stages and raise the voltage of the preamp to improve definitions.

Remember you did said you don't want to give out how you make the unbalanced PT output to balanced as your quota of freebies has reached?

[Gainzilla]

Thanks again for thoughtful replies (I especially enjoyed the RG's standing on the shoulders of giants analogy). However, I feel like I may have stepped into the deep end of the pool. I'll read the Merlin info and see if that helps, but honestly all I'm after is a way to get the right OT for whatever tube selection... For things like 6v6, 6l6, el84, el34, etc, I can just look at existing designs. If I wanted yo look at kt88, would I be safe using marshall values for either 50 or 100watt?

Cheers!

[Alan0354]

There is not too many OT primary impedance to choose and it's not particular important to be exact. Standard common ones are about 2K, 4K, 8K, 3.4K and I think 1.7K. For common +B of 400 to 450V, 2 6V6, use 8K. For 2 6L6, use 4K. 4 6L6, use 2K. Then for 2 EL34 use 3.4K like in Marshall Plexi.

We talk way too much here already, in truth, unless you go search all over the place, play tricks or wind your own transformer, that's the most common choices from after market vendors like Classic Tone, Weber etc.

[eschertron]

@Gainzilla

Alan0354 is absolutely correct that there are a limited number of "standard" options to be had from the major manufacturers. I go to the hammond website Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Output Transformers and look over the replacement models to see what the typical primary impedances are for given tube models.

If you're a visual learner (I sure am!), you might be interested in Interactive Valve Data Sheets, a resource that I go back to again and again. Pick a tube, output configuration (push-pull transformer), and set the plate and screen voltages; you can see the load line for any given OT primary impedance. It'll show you just how the load line matches up to the max dissipation curve, among other things.

[Alan0354]

On top of everything, are we over thinking about the impedance matching. I use a Classic Tone Vibrolux OT with 4K for 6L6. But I use 6V6, so I play a trick by connecting the 8 ohm speaker to the 4 ohm tap of the OT to make the primary impedance to 8K. It works very well. But on my second amp, I miss read the color of the wire, I connect the speaker to the 8 ohm tap by mistake. The primary is then 4K. It sounds just as good. I had been using it like that for a week or so. I did not discover until I measure the output power and it was only about 15W. I even post a thread and people scratching their head here too. It was like a day after before I read the schematic of the OT and discovered my mistake. I switched the wires and I got back my 22W. IT DID NOT MAKE A DENT OF A DIFFERENCE IN THE SOUND due to the mismatch, just lower power. In all do respect, I doubted people can hear the difference in power of the amp. I would have never discover if it was not because I measured the power. Using a speaker with slightly less efficiency can drop the sound level more than this mismatch.

[Gingertube]

Too much attention is often paid to this in these DIY Git Amp Forums.

A typical guitar speaker which is listed as an 8 Ohm speaker will have an impedance vs frequency profile which is something like this:

It will be about 6 Ohms at very low frequncies, say below 10Hz - this not of particular interest since we don't use this frequency range.
It will rise rapidly with frequency to a peak of greater than say 30 Ohms at the speakers resonant frequency, around 100Hz
It then rapidly drops back to the nominal 8 Ohms by say 200 Hz.
It then rises gradually with frequency until it hits say 30 Ohms again at around 10kHz - above this it continues to rise but again this is a freqencies above those we use.

So your 8 Ohm speaker is best described as 6 to 30 Ohm speaker.

If you are using say a Raa = 8K to 8 Ohm output tranny (typical for a pair 6V6 or EL84) then reflected impedance will be from 6K to 30K.

So in general, whether you use a 5K, 6K, 8K or 10K Raa tranny will make a rodents hind quarters difference.

I generally use the RECOMMENDED value from the tube data sheets. Keep in mind that recommended operating voltages etc. on those data sheets are generally lower than what is used in guitar amps and so err on the high side. I routinely use 10K Raa for 6V6 and EL84 Guitar Amps (20% higher than 8K datasheet recommendation). This results in lovely unstressed sound and long tube life. - Oh and a couple of watts less output but the watts I do get are unstressed/relaxed and you DO notice that in the sound.

AS with all guitar amps stuff always keep in mind a significant percentage of your final sound will depend upon your speaker and cabinet choices regardless of what you do in the amp.

If you really want to persue this topic down its Rabbit Hole then there is plenty of stuff around on the web,
Patrick Turner has written some good stuff on it here:
loadmatch4-pp-beamtetrodes

Cheers,
Ian

[Gainzilla]

Thanks for all the info guys! The valve wizard reading was extremely valuable, as was the interactive data sheets (I'm definitely a visual learner too! Bookmarked!), not to mention the Raa real-world approach (thanks Ian!), and matrix of impedances (Alan0354).

After reading through this thread again I keep coming back to the pearl of wisdom dropped by RB. We've all been standing on the sholders of giants for so long we forgot they even existed. What I get from that is that we have a selection of available options worked out long ago by the "scientists tinkering in laboratories." We can use those options as best practices, but as Ian mentioned, push it slightly for our evil guitar purposes.

I'm trying to learn as much as I can about building amps. I always feel like for every 1 thing I learn, I discover 2 new things I didn't even know about. I felt like understanding how to pick a transformer set based on my design goals was a necessity if I ever want to be more than a paint by numbers guy. Which I don't. I want to make creative choices from a place of knowledge and understanding. It's actually a little refreshing to learn that I don't actually have to learn a pile of ancient formulas, or use the ABC step-by-step ballpark Alan suggested at the top.

So, to test myself what would I use for a pair of KT88s in push-pull? If I look at the interactive data sheets, just plugging in a KT88 in P/P without changing any voltages, would I be shooting for an OT with a 5.6k primary? If so, what if I wanted a quartet of those (yikes... what is that? like 180 watts?)

Thanks again for all your help!