tonal effect of output tranny primary impedance ?

[pula58]

Let's say I am building a Fender blackface style amplifier. two 6L6GC's in the output stage.

What is the difference between using an OT with a 4K primary impedance (with 8 ohm load) vs. an OT with a 6.6K primary impedance (with 8 ohm load)?

Is there tonal difference? Volume difference. A difference in "feel" ??

Thanks!

P.

[Jana]

https://tubeamparchive.com/viewtopic.php?t=15517

[David Root]

I tried a 5K6 OT on a pair of 6550s vs 3K4, and didn't like the 5K6 at all, less power and a constipated tone by comparison with 3K4. Don't remember feel, I didn't play it long enough with the 5K6 OT.

490 Va, 360 Vs. Both OTs were custom built toroids. Haven't used toroids for OTs since.

[Andy Le Blanc]

Its demonstrated like this...

This chart isn't valid for anything other than demonstration

[pula58]

Its demonstrated like this...

This chart isn't valid for anything other than demonstration

Is that chart for 6L6's ? Is it accurate? Seems to suggest that a 6K primary Z might be more efficient than 4K.

[VacuumVoodoo]

I tried a 5K6 OT on a pair of 6550s vs 3K4, and didn't like the 5K6 at all, less power and a constipated tone by comparison with 3K4. Don't remember feel, I didn't play it long enough with the 5K6 OT.

490 Va, 360 Vs. Both OTs were custom built toroids. Haven't used toroids for OTs since.

Just like deciding not to taste something again because you got your first tatse from a chef who spoiled it.
5k6 at 490Va/360Vs and 6550 is wrong, 3k4 is better but still not optimal.
When it comes to winding toroid OTs it's a bit tricky, something called micro sectioning is required.
I'm not going back to EI cores, I trained my toroid winder well enough

[Markusv]

FWIW

I just built an 18 watt with a 2Xel34 output
So I guess its more like a big bottle 36 watter
First I had an OT with a primary as 8K andthe secondary as 8 ohm
Then used an OT with the primary as 4K and re-wired my two 10's as 4 ohms

I think I'm way happier with the amp now

I know the load for a pair of el34's is not 8K so the first option was probably not wise

The 2nd option opened the amp up. The amp feels like it is breathing better and it sings frankly. I will upgrade the cheap Chinese OT next and get an Edcor or a Heyboer

Still- side by side to my Liverpool(which I know is a different beast) there is no comparison but that is a different topic. And I think it has to do with the quality of the OT BTW.

Markus

[pula58]

The reason I am asking about this stuff is as follows:

I am considering a 40W build. Two 6L6 output stage.

Typically the Fender amps I have worked-on (Pro reverb, vibrolux reverb, etc) have an OT with a 4K primary Z.

Fishing around for possible OT's I came across this one (Edcor):

http://www.edcorusa.com/products/436-cx ... -6_6k.aspx

And I wondered why the 6.6K impedance? And, how would that makes things different (in amplifier behavior) as compared to the "Fender" 4K value?

[pula58]

See this GE 6L6 GC data sheet.....seems to suggest possibly using a primary Z close to 3.4K to maximize the power and minimize distortion......

[Markusv]

.....................And that underscores (it seems) what I have found
The amp has more headroom and the distortion is more pleasing to my ear at the lower 4K primary

I realize the 6L6's are sometimes used at higher primaries but that was my experience with the 34's

[David Root]

pula 58, only if your plate and screen are at 250V, at 400V that would likely look a lot different.

If you plot 400V plate on the 6L6GC plate curve, (you will have to extrapolate the screen curve up to 400V or use the KT66 curve), you'll see that at the knee of the curve the primary impedance is about 4K. Ideally you might want to be a little to the right of the knee, which I think is why some older Fender OTs are 4K2.

[pula58]

thanks David, right, I forgot to think about the relatively low operating voltage that the plot I posted was derived from.

In any case, why do you suppose that Edcor sells their OT (the one I mentioned a few posts above) with a 6.6K Primary impedance?

[VacuumVoodoo]

Because it's designed for UL operation which usually requires slightly different Raa than straight AB1 P-P operation.