5E3 OT Load Resistance

[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]

"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]

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]

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]

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]

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]

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]

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.