Attempt at giving back: 6L6GC SE amplifier

[AmadeusMozart]

I have learned heaps while being part of this forum and this is more or less my goodbye giving back present: this weekend I'll be moving into our new house but unfortunately the subdivision has not yet wired broadband so I'll be going back to dial up (yuk!) (With the hills around mobile boradband is not an option either)

It may be a year to 18 months before I get hooked up so I'll say thanks to all who have helped me move forwards during the time I've been part of the forum.

During the past week I've been learning LtSpice and modelling the different options for my 6L6GC SE amplifier. The power supply is not yet on paper but I'll be using a CCS feeding a OA2 and OB2 gas voltage regulator (more stable and less noise than zeners and the heat is above chassis) and use that voltage reference through a 10 turn put to "fine tune" the screen voltage. (The pot will be feeding a high voltage mosfet which will be acting for the current supplier to the screen)

I've mentioned elsewhere the existence of a self adjusting voltage regulator (with a mosfet too) so I am not tied to using a fixed B+ voltage. (Basically it becomes an electronic choke)

I did search long and hard for some decent 6l6GC SE amps but they are thin on the ground. In the end I found Alex Kitic's RH807 but I did not like the screen voltage dropping resistor which goes against recommendations made by KenRad for longevity of the tube. Some others remarked that the 12AT7 was not the best choice so in the end I wanted to find out what was true and what was old wifes tales.

I am attaching two sets of schematics - I am imposing a 30V pp limit on the 12AX7 output voltage swing since above that level distortion increases a lot (just my personal view on the matter, others are welcome to their views.)

The 12AX7 turned out to have a lower noisefloor (depending on design between 5 and 10 dB) than the 12AT7 plus the THD for the 12AX7 was about half to two thirds of the 12AT7. Confirmed that the 12AX7 is the lower distortion tube. There is also some interaction between the feedback resistor value and optimum performance. Came up with two OPT configurations: one for 2500 Ohm and the other for 5000 Ohm. The latter is the most flexible and can be adapted to different voltages with decent performance yet the 2500 did not exhibit that same flexibility. I ran out of time trying to find a 2500 solution with other voltages. It yielded the lowest distortion though.....

All transformers are James that I obtained directly from Taiwan.

I ran out of time to model properly an OPT so I made a "simulated" fix by using a choke and a resistor. Since it is not a part of the feedback loop I can see no issue with this. (having seen it elsewhere)

Enjoy and goodbye for now
AM

PS design is for personal use only and not for commercial use

[SY]

AM, have you investigated satellite? Here in the US, it's a godsend to those in rural areas.

[Shoog]

The main issue with Kitics designs is the use of a triode as the driver. This cannot deliver the best that this circuit can offer. It is so so simple to substitute a 6AU6 or similar for the triode that you really should consider it. A very simple transitor regulator for the screen is all that is needed. This allows you to pass at least 5mA of driver current and makes the driver relatively immune from the cripplingly low input impedance of the output stage (think about 1K load for your poor triode).
This feedback mechanism was never intended to work with triode drivers. Anything you do with the output stage screens is a waste of time if you don't address the driver problem. I know this from personal experience having built various implementation of screen circuits for the RH807 and found almost no sonic difference - so its not old wives tails

Shoog

[danzup]

Originally Posted by Shoog

The main issue with Kitics designs is the use of a triode as the driver. This cannot deliver the best that this circuit can offer. It is so so simple to substitute a 6AU6 or similar for the triode that you really should consider it. A very simple transitor regulator for the screen is all that is needed. This allows you to pass at least 5mA of driver current and makes the driver relatively immune from the cripplingly low input impedance of the output stage (think about 1K load for your poor triode).
This feedback mechanism was never intended to work with triode drivers. Anything you do with the output stage screens is a waste of time if you don't address the driver problem. I know this from personal experience having built various implementation of screen circuits for the RH807 and found almost no sonic difference - so its not old wives tails

Shoog

I am totally agree with this !
But so few understand the problems this days ... as it was not in the past .

[leadbelly]

Originally Posted by Shoog

so its not old wives tails

Watching MILF porn, are we?

BTW I also agree with your post.

[AmadeusMozart]

Originally Posted by Shoog

The Anything you do with the output stage screens is a waste of time if you don't address the driver problem.

.... found almost no sonic difference


Shoog

Politely disagree - it has a major impact on the longevity of the output tube.

Pentode or triode - that is a personal choice but don't forget that the not bypassed cathode resistor ensures that the triode takes on a pentode like behaviour. Happen to have a stash of 12AX7 and am not going to cut more holes in the chassis, the layout will becoming awkward.

PS Be aware that not all 6L6GC are the same, some that are marketed as a 6L6GC (e.g. Ei) have no higher ratings than the 6L6 / 6L6G /6L6GB and have only a 19 Watt plate rating as opposed to 30 Watts for the real 6L6GC(-STR).

AM

[Shoog]

It may have an effect on the life of the tube, but it will have very little if any on the sonics.

It is your choice what you do about the driver, but using a triode is an abuse of the Schade principle and you will not get the best out of your design. I have built about 10 schade amps so far (RH807's version been about three), and can tell you that a pentode is a better option. Still the amp will sound fine with a triode - just not as good as it could be.

Shoog

[rdf]

Is that solely a function of Rp?

[AmadeusMozart]

Originally Posted by Shoog

.....a pentode is a better option......
Shoog

I happen to like the triode sound and the lower background noise level of a triode. THD figures are good enough for me.

However the sound was not the the only objective - it is all a balancing act between different priorities.

My primary objective was to build an amp that would still be going long after I've gone and hence I like tubes that are in the top 10 of most sold tubes. The 12AX7 is #1 and IIRC the 6L6GC is # 7. So if any they are having the best chance of surviving for quite some time.

Peace and I am signing off now, have to pack stuff up before the move.

All the best
AM

[aardvarkash10]

good luck with the shift AM - I hope you aren't heading to the cold end of Stokes Valley!