Reflected Impedance

[mwelch55]

I built an amp a while back with an output transformer primary impedance of 6.6K.   This is similar to the Trainwreck where you can use EL34, 6L6, 5881, or 6V6 without any bias adjustments.  The bias is within range regardless of which tube I use.

My favorite tube to use in it is the 5881. It has a beautiful breakup when overdriven.  When I install the 6V6s, the bias is still in range, but the breakup is not as nice as the Tung Sol 5881.

I was thinking that the 6.6K primary impedance is slightly above the ideal load for the 5881, but slightly below the ideal load for the 6V6.

I decided to see what would happen if I plugged my 16-ohm cabinet into the 8 ohm jack of my amp with 6V6s installed.  Now the 6V6s breakup nicely.

The way I see it, plugging !6-ohm speaker into 8-ohm output jack makes the reflected impedance 13.2K, which is slightly above the ideal load for a 6V6.  That is the same degree of mismatch as when I use a 5881 and plug 16-ohms into 16-ohms.
It sounds great.  I built this amp 2 years ago and I am just now discovering this.

Will this hurt my amp?  Is my thinking correct?

Mike

[Williamblake]

Lower power output tubes need higher load to work on. You can probably take this much further but never get beyond "sounds nice". Guessing you didn't draw a schematic. 

[mwelch55]

Here is the schematic with voltages.

[mwelch55]

Tung Sol 5881s need around 4K primary impedance and the output transformer is 6.6K.  I think this is what gives it the nice breakup when overdriven.

6V6s need around 8K primary impedance. I thought that a reflected impedance of 13.2K would be about the same amount above the  ideal 6V6 load as using 6.6K with a 5881.

Mike

[brewdude]

“Needs” seems a bit drastic to me.  The “ideal” will also vary with plate and screen voltage.

[Williamblake]

I see you like distortion with low cut. How about a sound clip?

[66Strat]

Here are Ken Rad load lines for the 6L6 at different operating points to demonstrate Brewdude's point.

[mwelch55]

I don't have any sound clips.  It sounds like the Ceriatone Chupacabra, but only about 20-25 watts. The harmonic content seems to be a little more than if I had matched the output impedance exactly.  I can use EL34, 6L6, or 5881 and they all sound good, but the Tung Sol 5881 seems to bring out the best tones from the preamp.  It was only recently that I discovered that using 6V6 with the 16-ohm speaker plugged into the 8-ohm output gave me similar results.

Can someone tell me if plugging a 16-ohm into an 8-ohm output will double the reflected impedance?

Please forgive that my language such as "needs" is a bit drastic and "sounds nice" is a bit vague and not very descriptive.  I don't post very often, so please be patient.

[mwelch55]

I understand brewdudes point and I agree.

[66Strat]

A 16 ohm speaker plugged into the 8 ohm jack will provide a reflected impedance or 13K2 ohms.

[mwelch55]

Thank you.  That's what I thought, but wanted to confirm.

I wanted to make sure before I went much further with this.  I know that if the mismatch is too far in one direction, I am in danger of damaging the OT.  Since the reflected impedance is 13.2K and "typical load" for a 6V6 is 8K, that is not much of a mismatch.  I think I'm safe.

[66Strat]

You may want to consider bumping up the screen resistors to 2K2 ohms or so. The screens will draw more current at the higher load. The added resistance will help to protect the screens when the amp is overdriven.

[mwelch55]

Thanks JT.  Nice tip.  I'll do that.

[PRR]

The tube does not have a "preference" but it has limits and a bias point.

360V on shared 250Ω cathode resistor sure looks like it puts 6V6 over its thermal limit and may demand more peak current than a 6V6 can give.

Double impedance reduces the peak current problem but are you sure it is not over-dissipating 6V6es at idle?

[mwelch55]

I haven't checked in a while, but if I remember correctly, the 5881 was about 85% and the 6V6 was about 100-105%.  You are right, I should check it again.

[Williamblake]

So are you happy? There is no such thing as bad sound clips. You can dial in lots of distortion and sustain with your circuit, i am guessing. Why not put the cathode bypass cap at the dc follower on a switch, too? Maybe you wont like it but i think it is worth a try.