10k Pr Z too high for 2 x 6V6 in PP?

[tubeswell]

Plate voltage 370, screens 368, bias around 20mA-26mA. fixed bias

Any opinions on the load resistance?

[stokes]

I think Fender ran a pair of 6V6's at around 6k or so,so I dont think 10k would be terrible.If you alter the secondary load it will affect the reflected primary impedance.Give it a try and see how it sounds.If you dont like it try a different speaker load.If your 10k primary is with an 8 ohm load,using a 4 ohm load will give you a reflected 5k primary.

[tubeswell]

Thanks Stokes. (I knew that ). I am running a 5k reflected load at present in my 5G9 thingy, but the stock 5G9 had an 8k load AFAICT (which is Irish for 'a fact'). My one is a wee bit on the quiet side for gigging, altho' it does sound uncommonly good and is okay when mic'd.

But I figured if I put a 270R 5W resistor across the secondary in parallel with the 8R speaker (for protection), that would effectively drop the speaker to 7.7R, which is a reflected load of 4k8 (625:1), which if I ran the 8R speaker in the 4R socket would be 9k7, which is a bit closer to 8k than 5k is. I would expect it to be a bit louder and the tone maybe to change slightly. I guess it'll be alright, so I'm just going to have to give it a go and see... unless anyone can think of a screeching reason not to.

[Wilder Amplification]

According to my calcs I get that the optimum Zp-p load for that plate voltage and 6V6s is 5.7K.

Keep in mind that increasing plate load will increase screen current.

[stokes]

According to my calcs I get that the optimum Zp-p load for that plate voltage and 6V6s is 5.7K.

Keep in mind that increasing plate load will increase screen current.

Sometimes calcs and optimum,at least as far as guitar amps dont mean squat.Fender typically ran a pair of 6V6's at 6.6k.

[Wilder Amplification]

Sometimes calcs and optimum,at least as far as guitar amps dont mean squat.Fender typically ran a pair of 6V6's at 6.6k.

Depends on whether you're speaking of "tonally optimum" or "electrically optimum".

[stokes]

If you look at how various amp designers vary the primary impedances with the same tubes,there is a wide range as to what constitutes "optimal" in this situation,makes it totally subjective.If we were talking a hi-fi amp the "textbook" definition of optimal would apply,but guitar amps always seem to sound better when things are a bit off center.Sometimes close enough is good enough.

[Chuck H]

If we were talking a hi-fi amp the "textbook" definition of optimal would apply,but guitar amps always seem to sound better when things are a bit off center.Sometimes close enough is good enough.

+1 on that. At least as my observations go There are many great sounding designs that use an off ideal priZ.

But as my personal building experience goes I have to back Jon. I seem to get my best tones using a primary impedance at or close to the optimal loads. Hmmmm. I've always wondered why the big amp firms used other than optimal priZ, with the exception that I know you can eek a couple more watts out of a pair of big bottles (at the expense of THD) if you run them into a low priZ. Other than than that I'd guess the OT's were chosen because they were what was around and cheap. Even when OT were ordered custom they were probably electical copies of whatever off the shelf or other amp model OT was around when the new amp was designed. They just weren't that concearned about it. And though I don't have as much experience as some in this area I would speculate that any tonal advantage to running a priZ off ideal was a happy accident, not intentional.

Chuck

[stokes]

Happy accident or trial and error?I think the "optimal" things work for hi-fi because you want sound reproduction,no colorization at all.Not at all what you want with guitar amps.I dont think there was any "accident" involved when Leo chose whatever priZ he chose,it was intentional after trial ane error.

[Wilder Amplification]

Personally, I've had bad experiences with catastrophic failures when running above/below optimum load. Now I'm sure that you can go slightly one way or the other. But lots seem to favor running their amps with either double or 1/2 the primary Z via mismatching their cabs and that is where I personally have had the bad experiences.

Now up until recently I didn't know the engineering stuff I know now, such as the plate load lines, setting the load/screen voltage/screen resistor so that the load line crosses the Vg1=0 line (or the screen voltage is pulled down via the screen resistor induced sag to shift the curves to that point), etc etc. Interesting shit to know and it makes it so much easier when designing an output section and selecting an output transformer, plus it really helps you understand exactly what's REALLY happening in a tube output section. But the drawback is that knowing this stuff always seems to make you more "electrically anal" about shit.

Although I am a big proponent of a quote on another message board from Randall Aiken -

Do you want your amps to be "period correct" or do you want your tubes to last a long time? You know...guitar players are a funny sort. They'd probably all say "Damn the dissipation...full steam ahead!"

Now if you have a power supply that sags enough at full load you can get away with running a lower than optimal load without over dissipating. However, you can never really "plan" how much a particular power tranny will sag without taking a known PT and placing an equivalent load on it that it will see at full output and taking measurements. Of course, then again there's the "trial & error, reverse engineer, and under engineer it until it blows up, then go back a step" method.

[stokes]

Mismatching cabs,although it really is the same in the end,is a different approach.Some OT's can handle mismatches better and depending which way you go can actually make the tubes run a bit cooler.Fender's OT's can generally handle a 100% mismatch with no issues,except to the tubes if you use,say, a 4 ohm load with an 8 ohm out,the tubes will run a bit hotter and theoretically have a shorter lifespan,but I've never seen old stock tubes suffer catastrophically.I have never seen/heard of a Fender fail from such a mismatch,Mesa as well.I have seen older Marshal OT's fail miserably in this regard,they dont like a mismatch at all.

[Wilder Amplification]

Mismatching cabs,although it really is the same in the end,is a different approach.Some OT's can handle mismatches better and depending which way you go can actually make the tubes run a bit cooler.Fender's OT's can generally handle a 100% mismatch with no issues,except to the tubes if you use,say, a 4 ohm load with an 8 ohm out,the tubes will run a bit hotter and theoretically have a shorter lifespan,but I've never seen old stock tubes suffer catastrophically.I have never seen/heard of a Fender fail from such a mismatch,Mesa as well.I have seen older Marshal OT's fail miserably in this regard,they dont like a mismatch at all.

Yeah it's mainly on Marshalls that I've had the issue.

Back when I played through a Pro Reverb I ran an 8 ohm cab on it, and the OT was set up for a 4 ohm load. Had no problems.

I'm thinking that it has something to do with the fact that Marshalls use too low of a load to begin with for the plate voltage they run, on top of the fact that most Marshalls are typically biased closer to their 70% mark whereas you don't have to bias 6L6s all that hot relative to their 70% mark to make 'em sound good. This is just a theory of mine though...no actual test/measurement data to support it as of yet.

In regards to the Marshalls, it could also have something to do with the fact that EL34s are screen current hogs compared to 6L6s, along with the fact that they use a bigger screen resistor than 6L6s as well, which would cause screen current to increase at a lower plate drop than 6L6s.

[stokes]

It was discussed here a long time ago,the Marshal thing,that is.I always took it as "common knowledge" that the Marshals didnt stand up to a load mismatch well.There were a lot of opinions and very little fact tossed about in that thread,but I always just assumed the Marshal OT's use a lighter gauge wire or inferior insulation on the wire.Have to open one up one day.Used to see it a lot more back in the '70's,I think players have gotten more savvy about cab mismatching nowadays and dont tend to just plug any cab in thier heads and hope for the best.