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6550 SE design, please comment.

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L.S.

What do you think?
Haven't done the power supply yet, too lazy now ;)
 

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6550 is 6L6 family, designed with square law (x^2) pentode curve for optimal AB crossing.
EL34 might be a somewhat closer to linear (x^1.5) pentode curve choice for triode strap?

Van Doorne Octode used EL34 for Class A triode strap, and 6L6 for Class AB dumper.
There was a reason he didn't use all EL34 , nor all 6L6. Each behavior having a purpose.
 
Is there higher power version of EL34, more or less similar to 6550?
Perhaps a beam power tube like 6550, but not curved for square law?
What octal pentode of compatible pinout gives the most linear triode?

Is transconductance law so overwhelmed by Mu feedback that the
underlying pentode curve (before triode strap) doesn't really matter?
 
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I have heard that a common failure mode of potentiometers was for the wiper to lift from the track. With a bias setup as drawn, such a failure would leave zero bias on the output tube - very destructive. If you put a resistor from the bottom of the grid leak (220K) to V-. That resistor will normally be in parallel with the pot, but if the wiper lifts the grid will be pulled to the bias supply voltage - much safer...

Check tubelab site for Simple SE for example of switchable cathode feedback with cathode bias.

Cheers,

Chris
 
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I notice that your ECC81 (12AT7) has a rather low value for its plate load. I'd rather see 33k or more. Also, this tube is not all that suitable as a single-ended voltage amp (although it does OK in PP or differential configuration). Maybe a 6DJ8 or 6N1P would be suitable?

If you put a resistor from the bottom of the grid leak (220K) to V-. That resistor will normally be in parallel with the pot, but if the wiper lifts the grid will be pulled to the bias supply voltage - much safer..
.

Yes, indeed. This is nearly always overlooked in commercial circuits.
 
Thanks for the replies everyone!
I chose a ecc81 because I have one here. My intention was to make a prototype this weekend without having to order tubes and wait for them to arrive..
Other tubes I have are ecc83, 5751, 5755, 6072.
I'm still in the dark about what makes a good driver tube (in SE), I stole the ecc81 idea from a PP design.
But I have to buy some small parts, I might as well buy an extra tube, if it's not too exotic for my local shop..
I don't really like 6DJ8, but that's not based on actual experience..

How about an EL84 as a driver? :)
 
The HD spectrum of the 12AT7/ECC81 makes it (IMO) a poor choice for use in a SE amp. The sound will be very "wet". Check the 6GK5 out........I notice that your ECC81 (12AT7) has a rather low value for its plate load. I'd rather see 33k or more. Also, this tube is not all that suitable as a single-ended voltage amp

I have heard the "don't use a 12AT7 in a SE amp" statement repeated here a lot. Because of this I have also received a few emails questioning my use of a 12AT7 in the Simple SE. As with many blanket statements they are not always true in all situations.

The 12AT7 does exhibit some non-linearities that result in a fairly high amount of second harmonic distortion in many circuits. Not all 12AT7's are the same. This is exaggerated by a low valued plate load resistor. The 2H output can be reduced by raising the load resistance, and there is not much 2H if a CCS load is used.

I wouldn't use a 12AT7 as the first tube in a 3 stage SE amplifier (especially with a triode output tube) since the first and third stages are in phase and the "bends" in their transfer functions will be in the same direction such that the 2H will add, resulting in said "wetness". In this case (and in the Simple SE) The 2 stages operate out of phase such that SOME distortion cancellation occurs. Measured data on the Simple SE reveals that the even harmonic distortion of the entire amplifier is less than that of the output stage alone as long as the amplifier is not driven to clipping. Distortion cancellation results when the driver tube and the output tube have similar distortions in their transfer characteristics but are operated out of phase. DC is not perfect and the amount of cancellation varies with signal level and operating point. It is however a measurable phenomenon and has been used succesfully in many designs.

Check tubelab site for Simple SE for example of switchable cathode feedback with cathode bias.

In the design process of the Simple SE, I tried connecting the OPT directly in series with the output tubes cathode as you show. This forces the cathode current of the output tube to flow through the OPT secondary. Depending on the OPT there will be a small DC voltage (about 80 mV on my test amp) developed across the speaker terminals. With most speaker systems this will not be an issue. On some high efficiency speakers (like Lowthers) audible distortion may result.

6550 is 6L6 family, designed with square law.....EL34 might be a somewhat closer to linear

The Simple SE works with 6550, KT88, EL34, and 6L6GC. I use all of these tubes. True the EL34 has a lower inherent distortion and I prefer its sound for some music. It is the closest to a DHT sound and works well for female vocals. It however doesn't "ROCK". If you like to crank up something loud, use the 6550. The 6L6GC falls in between.

Do you really need the feedback with the triode connected 6550?

It depends a lot on the speakers and choice of music. I have a switch on the cathode feedback, and I generally don't use it with a triode connected EL34. I sometimes use it with a trioded 6550, and nearly always use it with a 6550 in UL.
 
6550 SE design

Hi,
The design is nice, but for a low sens tube like 6550, it would better be use cathode driver, use one 12AX7 first part for voltage amp. and the second part for cathode output. this would give sufficient drive voltage to the power tube.

give it a try and see the difference. :eek::eek:


Mitwrong ;););)
 
I have heard the "don't use a 12AT7 in a SE amp" statement repeated here a lot. Because of this I have also received a few emails questioning my use of a 12AT7 in the Simple SE. As with many blanket statements they are not always true in all situations.

George, my remarks were made in the context of the schematic the OP provided, which shows LED bias and, as you indicated, an undervalued plate load resistor. You are spot on regarding CCS loading. Employ CCS loading and an unbypassed cathode bias resistor and the 2nd order HD issue ceases to be problematic.

Further review of the OP's schematic shows an excessively large grid to ground resistance. TungSol's 6550 data sheet states that 50 KOhms is the max. allowable value, when "fixed" bias is employed.
 
Well, I just returned from shopping tubes.
I settled for a 6922 srpp as suggested. (updated schematic coming up)
This is just a prototype, intended for driver experiments. My preamp is quite powerful, at the moment it directly drives my EL84 SE, no driver tubes in the power amp. It's there I got the cathode secondary feedback idea from. In that amp it seriously improved the low end, seemed worth the try, though the bigger power probably will be enough.
 
I copied the bias circuit from a 6550 PP guitar amp I built earlier (not my own design).
This seems quite important, how would you do it then?

TIA!


All of the following applies to the originally provided schematic.

George and I have already provided info. regarding linearizing the voltage amplifier. CCS load (10M45S?) the ECC81 section at 3 mA. and select an unbypassed cathode resistor that yields an anode voltage in the 200 to 220 V. range. Remember: of anode voltage, IB, and grid bias voltage that fixing any 2 of the 3 quantities also fixes the 3rd quantity.

Correcting the 6550 bias circuitry is easy enough. The 220 KOhm resistor becomes 27 KOhms. P1 becomes 25 KOhms/20 turns. The 47 KOhm part is eliminated. To prevent loss of bass info., the 0.22 μF. cap. gets increased to 1 μF.

Unfortunately, correcting the 6550 bias setup causes the load impedance presented to the voltage amplifier to be too low. You resolve the matter by DC coupling a ZVN0545A source follower to the ECC81 section's anode, which isolates the voltage amplifier from the O/P tube circuitry.
 
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<snip>

Further review of the OP's schematic shows an excessively large grid to ground resistance. TungSol's 6550 data sheet states that 50 KOhms is the max. allowable value, when "fixed" bias is employed.

While I completely agree with the above comment I haven't ever lived by it and I have gotten away with 100K resistors with fixed bias for centuries and never had a "good" 6550 runaway in any amp I have designed. I would, however not go any higher than this, and with some tubes even so it may be a problem. 50K ought to be just fine for any 6550 I have encountered and to make the 12AT7 happy load it with a CCS. (Sounds like George's design was pretty spot on..)

I would scale the value of that 47K based on the value of the pot you end up with - its purpose is to prevent the application of insufficient bias voltage to the tube in the event of serious mis-adjustment. For the sake of long term stability I try to use 2W pots minimum and keep the dissipation at less than half this. I also restrict the bias voltage adjustment range to something reasonable for the tubes I am using say +/-25% or so of my target value in order to somewhat reduce the effects of thermal drift and aging in the pot.
 
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