5E3 OT Load Resistance

[tubeswell]

Well I thought I've have go at one of these

The tube chart for a 6V6 says 8k for push-pull and all the commercial tranny builders seem to do tweed deluxe OTs at 15W with 8k primaries (But what do we care about tube charts? - right?)

I recall reading somewhere that you can go between 6k-7k5 with a push pull cathode-biased 6V6 amp plate voltage of 350-370V idling at 13-14W. I've got a pair of jj6V6s.

Since I live on the dark side of the planet, its easier to get my trannys wound to spec rather than buy ludicrously costly stuff off the shelf, so...

... if I go 6k I'll get more distortion and if I go 8k I'll get more bass. I don't want too much bass and I like a slightly cleaner sound with syrupy distortion. Is 7k5 about right for me? Whaddaya reckon?

Agony Aunties - where are you? 8-)

(I'm only parodying myself, please no-one take this personally)

[tubeswell]

Okay - I'm going to run with the 7k5 primary and she how she goes

[Enzo]

Well, when it comes to bass and distortion, it is not like that OT primary is the whole amp. There is a long slope on those tube curves, and you have to push to the ends of it to get power tube distortion. And just because it is capable of X amount of bass doesn't mean the preamp sent that bass along.

[Chuck H]

Don't be too quick to settle on a value. I probably can't give you the absolute best answer, but someone more experienced will see this and give advice. That is, unless you just want to do costly experiments.

One thing I've noticed about OT primary impedances is that there are a couple of "sweet spots" depending on your goal. Other values can be arbitrary and give the "worst of both worlds" as opposed to the best. The lower primary impedances seen in some guitar amp OTs were typically chosen because they would produce more power and damn the torpedoes on some extra harmonic distortion. But the tonal nuances of different impedances are apparent. You might do better to try a proven value from a known good sounding design, or wait for some advice.

One thing you could do is try a similar amp (one that you know the OT primary impedance of) with different speaker loads. Differing the speaker load will reflect a different primary impedance to the tubes (transformers only know turns ratio, not rules). Then you could hear the difference for yourself before choosing.

I would go with 6600. But thats just me.

Chuck

[tubeswell]

Thanks Chuck H and Enzo. I did understand what you were saying already and I know there are many variables, I guess I should've qualified what I was asking with an "all other things being equal" statement.

I was really after some opinions (dangerous things that they are). I know that we are talking about the turns ratio here, but I'm just being lazy when I talk about the different impedance values (i.e.; assuming that one is going to plug in the 'correct' rated speaker in the appropriate secondary).

I'm curious as to why you would go for the 6k6 Chuck. Does that give you a more 'syrupy-thick' sounding distortion (when you talk about extra harmonics) all other things being equal? I ask this because my understanding so far is (correct me if I'm wrong) that if you increase the plate load (i.e. higher load resistance/OT primary) you get a flatter load line and more equi-distant spacing on the grid voltage curves, but when the grid swing gets higher in that scenario you get more harmonics due to the relative 'narrowing' of the transfer characeristic as the grid voltage hits the negative peak. Whereas if you have a lower resistance, you have more 'compression' because of the bigger variation in grid voltages where they cross the load line.

(Come to think of if I think it might've been Bruce from Mission Amps who said that about the 6k-7k5 load resistance)

[Chuck H]

I read somewhere that 6.6k was used on some of the more coveted Fender amps. I don't remember which ones. Of course, it's worth noting that the best selling 6L6 amps usually use a primary of 4 or 4.2k for a pair. But I tried the 6.6k on a build and it worked out very well for me. The hamonics were well balanced. None of the odd and specific harmonic tendancies I've had using 4k primaries. As a result the overdriven tone of this amp actually sounded like it had MORE hair on it, not less. I had to know if it was the OT making the difference so I swapped in a 7.5k unit I had. That extra little bit made the amp too flat and HiFi sounding. I halved the speaker load to reflect a 3750 impedance and the amp was louder and punchier than with either the 6.6k or 7.5k primary, but it lost the magic harmonic balance and killer overdrive I had with the 6.6k OT. I suppose it's worth noting that I tried this amp with 6L6, 5881 and EL34 tubes. The EL34s seemed to like both the 6.6k and 3750 primary, but did sound different with each. For the 6L6 and 5881 tubes, for me, 6.6k was the sweet spot for this amp. I'm sure that it can also depend on other things like the style of preamp and what properties you want to exploit from it, as well as plate voltage affecting the tubes tonal character. My amp had 460 volts on the plates.

Load lines and all aside, any specific type of compression indicated on paper is just paper compression. The interesting thing about it is that the amp "felt" more lively and compressed with the 6.6k primary due to the balanced harmonic content of the amp speaking the cabinet and guitars language, so to speak. The acid test is always real world playing conditions and your ears. I guess to be fair I never did gig with this amp. So I don't know if I would have missed the extra volume and punch of the lower impedance primary in a band situation.

Thats my humble experience and why I chose 6.6k

Chuck

[tubeswell]

Hmmm... On the strength of that I think I'm going to change my order.

[cminor9]

tubeswell, for what it's worth my 5E3 uses an OT with a 6.6k primary. To my ears, one of may favorite things about that amp is its punchy quality. The other favorite thing is the sound of it getting slammed by a hot neck humbucker. If there's a sweet spot, I'd put my $ on 6.6k. That seems to be pretty standard from what I have read, too, so if you want real 5E3 vibe that seems the way to go.

[tubeswell]

Thanks cminor9 - I'd say from past posts that you like the same sort of sounds as I do - that's conformed it. I phoned the winder and he still hadn't started, so it was no sweat for him to change. 6k6 it is.

[Bruce / Mission Amps]

"That seems to be pretty standard from what I have read, too, so if you want real 5E3 vibe that seems the way to go."

6K6 (with an 8 ohm speaker) is incorrect for a classic, cathode biased 5E3 with tweed amp plate voltages.
Whether or not you like 6k6 over 8k is another matter, but it is not the the standard output primary impedance found in real 5E3 amps.
6K6 is the primary impedance (with an 8 ohm speaker) you'd find in black face Deluxe amps with much higher plate voltages and fixed bias 6V6 power tubes.

8K is the correct Zed for a tweed Deluxe.

[hasserl]

Also, it looks to me like the amps Chuck H is talking about used 6L6 tubes while the 5E3 Tubeswell asked about uses 6V6 tubes.

[tubeswell]

Yep I appreciate that the origianl used an 8k primary winding (all other things being equal) . I am after a compressed, syrupy sort of sound, and from my humble understandings, I think I would get more of this with a lower load resistance (because of the steeper load line), (at least that's what I think I understand).

There was a poster here a while back by the handle of mincku from some eastern european country, and I liked the sound of his 5E3 with a greenback (and I think it used an OT with a 6k6 primary come to think of it). But I can't seem to find his postings when I search. Are old postings removed after a while?

[tubeswell]

But I am in such a whimsical mood that I am going to flip-flop again

(he still hadn't started winding that OT - he was going to start today)

Maybe better to build it with an 8k primary to get a classic sound after all.

[tubeswell]

I should've learned my lesson after the last build - MWJB's advice was "build it stock first". I can always change the OT later (I've changed it 2x already right? )

[tubeswell]

So I'm sitting here thinking (very difficult for me to do) and it occurs to me (all of a sudden - cos I'm slow) that since I am getting an OT wound with varying secondary taps, so that I could plug an 8 ohm speaker into (say) a 9.6R secondary that was on an OT with an 8k primary, and it would give me 6k6 load resistance.

8R/9.6R * 8K = 6.6666666 (say)

And/Or

8R/9.1R 8 8k = 7.0 (say)

So I would get an 8k primary and an 8R sceondary and also a 9.6R tap and a 9.1R tap for 6.6k and 7.0k respectively, if I was wanting to try these different load resistances Does that sound right? (have I got the math correct?)

Because it sure would be alot easier to switch the speaker plug over to another socket than swap out a whole OT just for experiments sake.

[Chuck H]

Since your having it custom wound anyhoo, I think thats a really smart idea (But I didn't check your math).

Also...Hmmm...I misread your original post somehow. I thought you were building with 6L6 tubes. As for 6V6 tubes, well, I've only built one 6V6 amp and I never checked the OT primary as the OT I used was already running a pair. So I'm sorry for any misinformation. Though I did indicate 6L6 and 5881 tubes in my post you WERE talking about a different tube. But hey, if you ever build a 6L6 amp...

Chuck

[cminor9]

Originally Posted by Bruce / Mission Amps

"That seems to be pretty standard from what I have read, too, so if you want real 5E3 vibe that seems the way to go."

6K6 (with an 8 ohm speaker) is incorrect for a classic, cathode biased 5E3 with tweed amp plate voltages.
Whether or not you like 6k6 over 8k is another matter, but it is not the the standard output primary impedance found in real 5E3 amps.
6K6 is the primary impedance (with an 8 ohm speaker) you'd find in black face Deluxe amps with much higher plate voltages and fixed bias 6V6 power tubes.

8K is the correct Zed for a tweed Deluxe.

Interesting, the diagram that came with my OT definitely indicates 6.6k. I could never figure out why my 5E3 doesn't sound like the samples I hear online when it's cranked. I love how mine sounds; it's not all farty when it's dimed. You think the lower primary impedance has anything to do with this?

[Bruce / Mission Amps]

Originally Posted by cminor9

Interesting, the diagram that came with my OT definitely indicates 6.6k. ... I love how mine sounds; it's not all farty when it's dimed.

That's the key right there... what ever sounds good to you, is good no matter what anyone says and you should stick to it.
For that reason alone that should imply there is nothing really "wrong" with a primary zed of 6K6 in a cathode biased pair of 6V6s.
However a "problem" arises when vendors are super lazy and use an inexpensive, reissue Silver Face Deluxe Reverb output tranny (from China or where ever) in their tweed Deluxe kits and tout it as "classic tweed Deluxe tone", etc. ...
All the early fifty's 10 to 25 watt push pull amps (except the tweed Bassman and Twins) used OTs with 6K6 (2x6L6s amps) and 7k5 to 8K5 in the 2x6V6s amps.
When output power and clean head room demands went up, the B+ voltages went up and the biasing changed to fixed bias.
Then the primary zeds of all the push pull amps dropped to 4K (2x6L6s) and 6k6 (2x6V6s).
The only amp that didn't change from 8K5 to 6K6 was the black face Princeton, which in my opinion, is actually the logical evolutionary conclusion of the tweed Deluxe/Vibrolux amps.

[tubeswell]

Originally Posted by Bruce / Mission Amps

The only amp that didn't change from 8K5 to 6K6 was the black face Princeton, which in my opinion, is actually the logical evolutionary conclusion of the tweed Deluxe/Vibrolux amps.

Hmmm.. maybe it's no coincidence that the BF princeton is one on my to-do list. :-)

Bruce, are you able to confirm whether the way I calculated the secondaries would get me an OT that I could plug an 8Ohm speaker into with a choice of 6k6, 7k or 8k load resistances?

[Bruce / Mission Amps]

Originally Posted by tubeswell

Hmmm.. maybe it's no coincidence that the BF princeton is one on my to-do list. :-)

Bruce, are you able to confirm whether the way I calculated the secondaries would get me an OT that I could plug an 8Ohm speaker into with a choice of 6k6, 7k or 8k load resistances?

Impedance ratio with an 8 ohm speaker would be;
6K6 = 825:1
7K = 875:1
8K = 1000:1
Do you want to do it backwards and have a 9.75 ohm tap on the secondary winding but use an 8 ohm speaker to get 6k5 ohms and then also an 8 ohm tap for 8K ohms?
Yes, just tell your winder that and he'll know what to do to get the turns ratios right.

[tubeswell]

Thanks a million Bruce.

[cminor9]

Originally Posted by Bruce / Mission Amps

Impedance ratio with an 8 ohm speaker would be;
6K6 = 825:1
7K = 875:1
8K = 1000:1
Do you want to do it backwards and have a 9.75 ohm tap on the secondary winding but use an 8 ohm speaker to get 6k5 ohms and then also an 8 ohm tap for 8K ohms?
Yes, just tell your winder that and he'll know what to do to get the turns ratios right.

Bruce, thanks as always for the info.

So then if an 8k primary zed will output a load @ 8 ohms, I am actually running my speaker at 6.6 ohms.

I know when I used to hook up car stereos for friends we'd bridge amplifiers to get extra power but also to get a lower output impedance. The effect was that you got the most out of the speaker and thus got the most "bump for the watt", if you will.

I wonder if that same principle applies here?

[tubeswell]

Originally Posted by cminor9

So then if an 8k primary zed will output a load @ 8 ohms, I am actually running my speaker at 6.6 ohms.

I know when I used to hook up car stereos for friends we'd bridge amplifiers to get extra power but also to get a lower output impedance. The effect was that you got the most out of the speaker and thus got the most "bump for the watt", if you will.

I wonder if that same principle applies here?

Hi cminor9

Excuse me if I'm saying what you already know, but I think Bruce was saying was that for an OT with an 8 Ohm secondary speaker, the turns ratio required to get a primary load resistance of 6k6, will be 825:1 (825 turns on the primary for every 1 turn on the secondary). Your 8 Ohm speaker that you plug into the secondary winding still measures 8 Ohms - it just that the winding ratio gives you 6k6 on the primary because the winding ratio is lower than that for an 8k load resistance at the primary. If your winding ratio was 1000:1 and you were getting 6k6 load resistance on the primary, then yes, your speaker would by rights be a 6.6 Ohm speaker.

What I think would happen is that on a 825:1 OT, an 8 Ohm speaker ends up with slightly less current and slightly more voltage than it would do if it was connected to secondary with a turns ratio of 1000:1 to the primary. How much this affects the performance of the speaker I don't know. (I guess it must affect it somewhat/slightly tho'). But I wouldn't think it was the same thing as adding another amplifer into the circuit.

[cminor9]

I'll share my thought process, please critique as needed so I (and maybe other readers) can learn.

Transformers work on ratios. They step down (or up) voltage and step up (or down) current inversely. Bruce was giving ratios of the ohms on the primary to the ohms of the secondary.

Bruce sez: "
Impedance ratio with an 8 ohm speaker would be:
6K6 = 825:1
7K = 875:1
8K = 1000:1
"

What I don't understand is how impedance plays into the equation. I'll crunch some numbers and try to understand it, and let you see my thought process. Since I=V/R, I and V are directly proportional. Since V=I*R, I and R and inversely proportional. What I don't know is if R and V are directly proportional. Well, they are, but how direct? Would that follow the same line as I and V? I guess it'd have to? Let's do a proof.

Let's say we have 400v coming from the power tubes at 50ma (sounds practical to me). If 0.050=400/x, x = 8000. That's going into the primary The OT has an impedance ratio of 1000:1, so the secondary impedance is 8 ohms. Does that mean that the voltage is 0.05 and the current is 400a? That actually sounds right, because that means you get 20W output.

I guess I answered my own question. The "impedance" ratio is the same thing as the voltage and current ratio? Therefore V and R are also directly proportional.

Is that right? Or is there a flaw in my logic? I have a new appreciation of ohm's law. It's like a triangle. If you increase the angle of one angle in a triangle, you have to decrease another. So a transformer ratio applies the same to everything going through that transformer: current, voltage, and impedance.

One correction on what I said in my last post (thanks to doing the math):

If I have a 6k6 OT, I am actually putting out 9.69 ohms and not 6k6 ohms. Got my proportions wrong on my last post. That is interesting, since I usually run two 8 ohm speakers in parallel (my 5E3 is a mini stack with two 1x12 cabs) so I am pushing about 4.8 ohms into each speaker. So my original question still stands of what effect lowering the impedance will have. Wouldn't this effectively increase the wattage of the output by raising the voltage? And why does an impedance mismatch hurt an amp? Because it draws too much current (or resists too much current)?

That's a lot of questions, sorry. Any clarification or information would be appreciated.

This is making my head hurt.

Bruce is right: whatever sounds good, man.

[cminor9]

ooOOOoo, I just found this:

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch

That confirms what I just came up with below.

Beholds, teh p0\/\/urs of teh internets!!1!

[tubeswell]

Originally Posted by tubeswell

Hi cminor9

Excuse me if I'm saying what you already know, but I think Bruce was saying was that for an OT with an 8 Ohm secondary speaker, the turns ratio required to get a primary load resistance of 6k6, will be 825:1 (825 turns on the primary for every 1 turn on the secondary). Your 8 Ohm speaker that you plug into the secondary winding still measures 8 Ohms - it just that the winding ratio gives you 6k6 on the primary because the winding ratio is lower than that for an 8k load resistance at the primary. If your winding ratio was 1000:1 and you were getting 6k6 load resistance on the primary, then yes, your speaker would by rights be a 6.6 Ohm speaker.

What I think would happen is that on a 825:1 OT, an 8 Ohm speaker ends up with slightly less current and slightly more voltage than it would do if it was connected to secondary with a turns ratio of 1000:1 to the primary. How much this affects the performance of the speaker I don't know. (I guess it must affect it somewhat/slightly tho'). But I wouldn't think it was the same thing as adding another amplifer into the circuit.

Sorry cminor9 I worked that out the wrong way (but I still got the same result?),

The impedance ratio is not the same thing as the turns ratio. I forgot to take the square root of the impedance ratio. (That little shock I had a while back must've done a bit more damage than I thought - I gotta slow down)

For an 8k primary load resistance from an 8R speaker with a 1000:1 impedance ratio, the turns ratio is 31.6:1.

So with 400V on the primary, there will be (400/31.6) = 12.7V on the secondary.

With .05A on the primary, there will be (.05 x 31.6) = 1.58A on the secondary.

Those figures look more like what you get on a secondary

12.7V/1.58A = 8R


Translating that to an OT with a 6k6 load resistance having a 825:1 impedance ratio (which has a turns ratio of 28.7:1)

400/28.7 = 13.9V on the secondary

0.05 x 28.7 = 1.44A on the secondary

13.9/1.44 = 9.6R

8-)

(I'm 99% sure that someone will chime in if I still haven't got this quite correct)

[cminor9]

Yeah, you just took the other way around and got the same result. I am glad you got the same result, I don't think both of us would have made the same error. I think we figured it right.

Interesting info on the ratios on the actual turns of wire, too.