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Old 1st February 2012, 06:14 PM   #1
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Default triode and pentode 6v6 in parallel

I have 2 6v6's in parallel. one triode connected. one pentode connected. the output voltages are similar to a gibson ga-5. 345v plate approx 290v screen approx on pentode.

they both are connected to one output transformer.

weird thing is that the triode can output more power than the pentode... each output tube has seperate vol control on grids.

470 ohm cathode resistors on both.

using hammond 125FSE output transformer.

The pentode will only swing like 2v neg into a 10ohm load. Connected with the green and black wires on OPT.

Triode will swing WAAAYYY more.

sooo... what gives?

This is for a GUITAR amp.. but i assumed pentode would give me way more power than triode... but its backwards.

HELP!

Thanks. let me know if you need more info.
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Old 1st February 2012, 07:05 PM   #2
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if i pull the triode tube out then amp pentode works fine. if i pull pentode tube then triode works fine.

would it be the total current through opt causing saturation? or interaction between the 2 output tubes?
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Old 1st February 2012, 07:55 PM   #3
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Hi,
I strongly suggest reading some literature.
A Triode has a far lower plate resistance than a Pentode. If you put them in parallel operation, the triode will smoothly govern the pentode without a problem. I hope you understand the situation here. The resistance when you look into the anode of the pentode down to ground is extremely high compared to that of the triode. Because of its inherent feedback, the triode will work against any changes the pentode does. But when the pentode is working, it can't do anything against the low rp of the triode.

Furthermore: Cathode resistor is a bit high. Datasheet says something between 270R and 380R for that anode voltage.

Next: -Triode ~ 10 and -Pentode ~ gm*R-Load = 0.0041*5000 ~ 20
Your Pentode has twice the amplification of the triode.

Don't get mad at me, but your concept is crap ;-)

You could try tons of negativ feedback to get the rp down. Why not try something different for a guitar amp ? Unsymmetric push pull or something. One 6V6GT and one EL84 or something.
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Old 1st February 2012, 08:19 PM   #4
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i changed the cathode resistor allready. maybe i just do a triode pentode switch.
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Old 1st February 2012, 08:30 PM   #5
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soo... yeah.

probably do this instead:

parallel output stage pentode with triode switch and switchable cathode bypass resistor.

maybe on input do 6SJ7 and a 6SL7 and have a mixer on input.
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Old 1st February 2012, 08:45 PM   #6
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Quote:
I have 2 6v6's in parallel. one triode connected. one pentode connected
I never tried that one myself.....never thought about it.

Quote:
A Triode has a far lower plate resistance than a Pentode.
Yep, the triode is going to win that fight.

Want to make some cool sounds in a guitar amp....Plug in two different tubes, both wired as pentodes, then play with the bias on each tube until you find the spot.

I used a 6L6GC and an EL34, or a 6V6GT and an EL34. The trick is to find a bias point where both tubes are working and making a clean sound, but just a little more volume throws the whole thing into distortion. A skillful player can dance on the edge getting clean notes and distorted notes just by picking a bit harder. My first experiment along these lines was done in a Fender Bandmaster. Later I was building my own amps using a Marshall circuit. An undersized OPT helps too.
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Old 1st February 2012, 09:33 PM   #7
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ok i switched it to parallel pentode 6v6's each with its own vol control. maybe i throw in a pentode / triode switch too.

so question is: should i wire the OPT as if 1 single tube or 2 tubes in parallel?

hmmm.. maybe i went overboard on OPT. I used the 90mA DC version. 2 pentode 6v6s are about 90mA total.
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Last edited by AudioGeek; 1st February 2012 at 09:35 PM.
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Old 1st February 2012, 11:11 PM   #8
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You can simulate the range between triode and pentode by varying what goes to the screen grid. But that is mostly like varying the feedback. To really change the character of the distortion, it might help to have adjustable bias. You'll get more even order distortion with the grid more negative. When an amp distorts just after being way overdriven, it is called blocking distortion. The bias gets forced negative by the charging of the coupling capacitor that occurs when the peak audio goes postive and grid current flows. Since the transconductance of tubes normally rises with current, if you bias way negative you'll not only get lopsided compression (the negative peak heavily reduced or clipped), but you'll also get a sort of expansion on the positive side as it will increase by MORE than the amount that the drive increases when you push the audio harder. As for triode and pentode behavior at once, the range of variation would be greater with using push pull, perhaps just changing what's on the screen of one side. Perhaps a high value pot in the middle of a resistor divider across the plates, the the slider, through either a cathode follower or emitter follower with no resistor, but the cathode or emitter driving your screen. (careful, very dangerous voltages contact only plastic or grounded shaft, also parts can go up in flames if values are too low) Put Zeners in series if the screen voltage is too high. (consider duplicatating for both tubes) Again effects will be more dramatic (and tube life better) if the grid bias is more negative than normal. Otherwise the overlap tends makes it take more signal to break out of class A. I'd also be tempted to try a direct coupled class-B capable driver stage. With that, high plate voltage and lower screen voltage you can also get very high output power without frying the grids. With a screen pot swung towards the other plate a bit (reversed screen phase compared to both ultra-linear and triode mode), you get some positive feedback, and the screen voltage rising while the plate falls allows the plate to pull more on negative peaks boosting power (keep an eye on how hot the grids get). If you go too far it will oscillate. It should take on some strange resonances and more distortion just on the threshold of oscillation. If there's no negative feedback elsewhere in the circuit you'll barely be able to go in the positive feedback direction before it goes nuts. If that all sounds crazy, don't mind me. I once ran an 807 at 1800 Volts. It was great for many months until bias was lost... then the glass formed an inward bubble as it melted!
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Old 2nd February 2012, 02:56 AM   #9
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I've seen an article somewhere where they used a triode and a pentode in parallel on each side of a P-P OT. The triode was supposed to dominate at low power and the pentode dominate at high power. (apparently the pentode was biased off a bit more than the triode at idle.) The pentode eventually dominates because it has more effective transconductance than the triode due to the triode's internal neg. fdbk. from it's plate/screen combo cancelling some of the g1 gm.

In SE mode here, it looks like your problem is the DC in the OT. Try biasing the tubes down enough to use the existing OT, or use another OT with twice the DC capability.
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Old 2nd February 2012, 03:09 PM   #10
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well i made them both pentode and now have no problem.

so i can put in a 6v6 with a 5881 or a el34 etc... but get this... as soon as i put in a 6550 i get weird feedback / hum noise.

oscilation?

should i have shared cathode resistor or 2 seperate cathode resistor for parallel operation?
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