OT: speaker impedance and old fenders

[aron]

I personally like to see 6L6's and 5881's at around 35-36...

That's where I like them too.

When I have time to look up TUT, I will post the findings but for myself I also agree with Ammscray in that I run my Bassman at 8 ohms instead of 4.

I remember asking if this was ok, and later confirming it by reading TUT and other books.

I did measure current draw as well many times and it was fine.

Finally, yes, the tone is better with the mismatch (to me).

[Peter Snowberg]

I also like the high 30s for bias.... I tend to tune by ear and the lack of glowing plates.

I  just listed those numbers as the maximum that a 6L6 would take before meltdown since that was the question at hand and my first answer was more conservative.

I also listed 4mA as a max on the screens, but I don't think you would ever want to run like that. Svetlana 6L6s (who brands that tube now?) could handle 10mA (5W) on the screens and/or 30W on the plates before meltdown. 2.5mA on the plates is more like what you should see in a typical amp.

As long as everything is balanced (esp. thermal issues), the numbers don't mean too much by themselves.

http://www.svetlana.com/graphics/products/pdf/6L6GC.pdf  

Take care,
-Peter

[csj]

Cajununicorn,
As you've probably seen before this topic is open to alot of debate. Here we have pretty much the same four responses.

1.) Don't do it at all...always match as closely as possible. Better safe than sorry.


2.) Mismatch up but maybe not down. (The Stromberg Carlson example - assuming that the original OT primary was built to match the Raa load specs of 2 6L6's...which I did assume at the time).
This might increase the flyback voltage spikes the plates suffer with the higher than expected impedance. If so, run a string of 1N4007s from the plate to ground (anode side to ground) to absorb the excess voltage. As far as I'm concerned this should be SOP anyway.

3.) Mismatch down but maybe not up. (The Matchless example). "May cause tube stress".  This I wonder about. As far as I know the 6L6 won't really care...it'll keep putting out it's set amount of plate current into the lower load. More current isn't "drawn" out of it somehow. Of course, current can be increased by rebiasing the tube or simply just turning up your amp. This increase in current isn't related to the transformer though. The OT is only like a tennis player smacking back and forth whatever comes it's way. (I hope this metaphor doesn't get me in trouble too...wait, I think that was more like a simile...phew...)

4.) Mismatch and move on. (Myself, Ammscray and Aron). The heck with it...what's it sound like?


So what's the answer still?
I don't really know.  I respect the other opinions here and I've seen where even the expert speaker builders, tranformer winders, power amp designers - though generally in favor of matching loads - waffle a little when it comes to saying exactly what the critical limits are. The debate gets heated, especially with audiophile types, when it comes to operating tube amps into what they call "grossly" improper impedance matches. Is 8 into 16 (or any of the other several guitar amp possible mismatches) grossly improper? I honestly really don't know.

As Peter points out, there are so many variables. If you've got a truly valuable vintage something or other amp and you want to keep it pristine...then, sure, I'd say "play it safe".

But, like Ammscray, I'd wonder just exactly how "matched" it is or even was in the first place. And sometimes I just can't help but wondering... "what would it  sound like if I plugged, hmmm... THIS into THERE!"  :wink:

This got way too long... sorry for the rehash of stuff I'm sure most of you already know.

[cajununicorn]

csj,  i appreciate all the info...very interesting which twist and turns the topic took. it's a vintage amp, but i've long since decided to use it as my main road/club amp. it handles the stress very well and gets used a lot. i'm pretty utilitarian when it comes to gear.  i wasn't even looking for a better tone due to impedence matching, missmatching, ect... i just wanted something that worked and sounded right. i love using celestions in fender amps. BTW, my hiwatt sounds awesome with the greenbacks. thanks for all the input from everyone.   jon

[jplaudio]

I personally like to see 6L6's and 5881's at around 35-36...

That's where I like them too.

When I have time to look up TUT, I will post the findings but for myself I also agree with Ammscray in that I run my Bassman at 8 ohms instead of 4.

I remember asking if this was ok, and later confirming it by reading TUT and other books.

I did measure current draw as well many times and it was fine.

Finally, yes, the tone is better with the mismatch (to me).

Impedance matching in tube amp circuits is probably one of the most mis-understood subjects. This question frequently comes up along with "can I remove 2 power tubes from a 4 tube amp and how does it affect impedance?"
Kevin O'Connor has posted the some answers on his FAQ page for those interested. http://www.londonpower.com/faq.htm#JAN04
From a personal stand point I have operated all of my Fender amps with mismatches, both up and down, for over twenty years with no blown transformers or melted tubes. Some of these amps are operated 8 hours a day with no problems at all. While mis matched loads do change the tonal character, and you will lose some power, it is just another choice available to the musician. For years Fender produced amps with extension speaker outputs with no load requirement. Other manufacturers like MESA have included "half power" switching on thier amplifiers with no requirement to "match" the speaker load or adjust the bias. Fender actually suggests setting the impedance selector on the ToneMaster to whichever setting you prefer. I do not know of any guitarist that has damaged an amp due to mismatched loads. I have actually seen a fender twin,with a shorted voice coil in one speaker, provide enough power to heat the voice coil red hot and ingnite the speaker cone and grill cloth. After replacing the speaker the amp was fine  Mismatch and move on