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.