# 6S19 Circlotron (OTL)

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#### Fuling

Hi,

Irrational as it might be, I´ve become interested in Circlotron amps lately. I have a big stash (40+) of 6S19 tubes, 20 noval sockets and some interesting power transformers (two 2x6V 50VA toroids for the heaters and four 68V 100VA EI transformers for the floating output stage rails).

Some questions:
Can an OTL circlotron output stage operate in class AB?

Would 95V rails be waaaaayyy to low (using 8 x 6S19 per channel*)?

Is a tube or a solder joint goes south, would it mean the end for my speakers?

* With 4x6S19/rail, ie 8/channel I should get a peak current capacity of about 1,2A which should give almost 6Wrms @ 8R.

#### flg

Fuling said:
Irrational as it might be, ...
I happened to have worked up a Circlotron design but it's mosfet I have a sim file so if you feel comfortable with answers from it, , The topology would not be exactly the same but I'll bet you'll get most of your answers...

Can an OTL circlotron output stage operate in class AB?
In the definition of AB, do we have some small Iq current and with signal one side goes to 0 current and the other 100% of the current? Then, I think yes, is the answer. For instance, my basic output circuit wil drive 8 ohm just fine up to about 22Vp(44p-p). It uses 2.5A Iq in each side. You can see the non linearities of each side in the sim display of current. At 22V out one side drops at the peak point from 2.5A (Iq) to about 1.25A while the other side increases to about 4A. Not very linear The resultant is 2.75A through the speaker (22V/8R=2.75A)... Well, if I were to use a 2 ohm speaker that would be 22V/2=11A But, lets see what the Sim display of current looks like under these conditions...
As one side of Circlotron lowers it's current it hits rock bottom. Not 0 (400uA) but in this post Zero. Current in the other half turns on to essentially the 11A the speaker wants. This is all subject to no power supply current limiting being reached. That sounds like class AB dosent it?

Would 95V rails be waaaaayyy to low (using 8 x 6S19 per channel*)?
Dunno

Is a tube or a solder joint goes south, would it mean the end for my speakers?
Well, I simulated with a 1M resistor in the source of one side and the output will swing to full voltage of only one polarity. 22V or so in my case. Dose not sound dangerous so far... So then I shorted drain to source I don't think any of my supplies could actually do it but, my simulator supply is peaking at 800+Amps Not the answer you wanted to here. Me either, believe me, 90%+ of the time a mosfet fails shorted Do tubes fail shorted often?
Well, theres my 2C

#### Miles Prower

Fuling said:
Irrational as it might be, I´ve become interested in Circlotron amps lately.

Yeah, me too.

I have a big stash (40+) of 6S19 tubes, 20 noval sockets and some interesting power transformers (two 2x6V 50VA toroids for the heaters and four 68V 100VA EI transformers for the floating output stage rails).

No idea what a 6S19 is, I did a preliminary design with 6AS7s. Dunnow if I'll actually do that, or use lateral MOSFETs instead. Many VTs per phase means mucho heater power.

Some questions:
Can an OTL circlotron output stage operate in class AB?

Absitively!

Is a tube or a solder joint goes south, would it mean the end for my speakers?

You should add a DC offset detection circuit regardless of whether you implement this with SS or HS. Without an OPT, speaker poofage is always a possibility.

#### HollowState

Any of you guys want the Atma-Sphere MA-1 diagram? (6AS7s) Email me.

Victor

#### CarlyBoy

Circlotrons are also interesting me lately, OTL or not. I'm sure it's common knowledge to most members here, but it was only last week that I became aware that Wiggins = Electro-Voice and that they marketed transformer-coupled circlotron amplifiers from 15-100W. Given that the output impedance is 1/4 of that used in push-pull, these should be awesome sounding amps. (Anyone have/heard any of these?)

I recently exchanged email with Robert Turnbull, operator of this site:

www.audiocraftworks.gr/gr/pages/arthra/internet/circlohist.htm

in an attempt to verify the OPT impedance for the A30 (uses 6BG6G or 1614) as I was considering constructing from the design - well either that or Al Stewart's circlotron circuit (AudioXpress, May 2002).

Sooner or later I've gotta put together a transformer-coupled circlotron and an OTL. (So much time, so little money..)

Roky, I think the tube you're referring to might be more commonly known as a 6C19. From what I've read it's like half a 6AS7 (ie single section) but I don't know how the ratings compare.

Victor, I have the MA-1 circuit as well. Do you know how to go about adjusting the bias and balance pots?

#### Tubelab_com

No idea what a 6S19 is

It is the Russian equivalent of a 7233 (not pin compatible) which is roughly half of a 6AS7. I bought 20 of these from an Ebay seller in the Ukraine. I have been experimenting with them for use in cathode followers at low plate voltages. For \$2.50 USD each, I can experiment on these, 7233's are \$10 EACH and much more fragile.

Would 95V rails be waaaaayyy to low

I don't think so. I did some experimenting to see how much I could crank through this little tube. The maximum steady state plate current spec is 140 mA. I can reach this with 50 volts on the plate and -1.6 volts on the grid. At a plate voltage of 75 volts (my chosen voltage) 285 mA of plate current can be reached at a grid voltage of -1 volts. The plate shows a very dim red color in a totally dark room at this 21 watt dissipation. At 95 volts on the plate, -1 volts on the grid gives 440 mA of plate current and a gright red glow! You should be able to run these guys in AB1 with a reasonable idle current and still hit 400 mA per tube on peaks without drawing grid current at 95 volts.

As a side note I saw my first tube instantly and violently shatter last night. It was one of the 6S19P's. I must admit that is was a totally dumb blonde mistake on my part that caused it though. I had been testing some Bendix rectifier tubes so I had the filament wires soldered to pins 3 and 6 of the 9 pin socket. All other connections were made with clip leads, which I removed. Without thinking I plugged a 6S19P into the socket and switched on the power supply. The tube instantly popped sending glass fragments all over the place. After using a lot of bad language, I figured it out. Pins 3 and 6 are both plate connections on the 6S19P. My old Fluke power supply probably cranked 10 to 20 amps into that dead short. The weakest link was the wire that goes through the glass from the pin. As the wire vaporized it must have built up some serious pressure inside the glass, causing violent destruction.

#### HollowState

CarlyBoy said:
(Anyone have/heard any of these?)

Victor, I have the MA-1 circuit as well. Do you know how to go about adjusting the bias and balance pots?

Recently I got to hear the Joule Electra Grand Marque 160 watt OTL that uses 6C33Cs at the home of a high end dealer who's a member of the NJ Audio Society I belong to. Man, are they nice...and expensive!
http://www.joule-electra.com/index2.htm

CarlyBoy,
I've never owned that MA-1 but would think that the DC balance is set to provide the least DC offset (differential) between the upper and lower sections as measured at the output. The bias adjustment sets the operating level of the output tubes since the driver is DC coupled to them. This level is 750 mA per side, or 125mA per tube.

#### woody

I have been thinking about using a few of those 6c19pi tubes
in a circlotron to. As you see I already have a few tubes mounted.
I already have the 4 transformers for the 4 floating power suplies.
I bought some 110-230v autoformers that i was able to convert to
110v to 130v transformers. One of these is in the picture.

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#### Milen

6S19 Datasheet

max. anode current is about 140mA, written in another datasheet...

#### Fuling

Thanks guys for helping me out with some answers!!

(Just for clarification: I´m not Roky Erickson, this is Roky:
http://en.wikipedia.org/wiki/Roky_Erickson)

440mA at 95V on the plate is better than what I estimated by looking at the datasheet.
According to this Pout might end up much higher than expected.

#### poynton

Re: Re: 6S19 Circlotron (OTL)

Fuling said:

Is a tube or a solder joint goes south, would it mean the end for my speakers?

Miles Prower said:

You should add a DC offset detection circuit regardless of whether you implement this with SS or HS. Without an OPT, speaker poofage is always a possibility.

Funny, this question is asked whenever OTL is mentioned but rarely with SS.

What is the commonest failure mode for tubes , open or short.?
How often does it happen?

Andy

#### 316a

How about a push pull cathode follower OTL ? You could get away with using the 115V primaries of a mains transformer as your load and hang the speaker directly off the cathodes , alternatively use a lower ratio toroid and wire as an autoformer . Circlotrons are on my 'to do' list , I need to get my head round the 'floating' topology first I've got as far as a choke loaded SE 6C33 OTL which is currently breadboarded , I'll be building 72 ohm line arrays for this at the end of the week

316a

#### woody

Tubelovers Anonymous did a good study of the different otl output
arangements. Acording to info there the output impedience of a
push pull cathode folower is high 2 Rp/1+u and both sides must
be operated in class A. Back in 1951 one was built with 8 6as7g
tubes per chanel this gave a whopping 6.32 W into 16 ohms!

The Tubelovers Anonymous web page is now gone. A google
brought up a cached version of the page though.

#### Fuling

The reason I bought all those 6C19s and the sockets was to build a big choke loaded SE OTL with 8-10 tubes in // per channel.
I already have one with 4xPL504 per channel that sounds great, but lacks power.

The idea of a circlotron just popped up when I found those four power transformers. I thought they would be suitable for a circlotron as they have dual chamber bobbins and thus low capacitances between the windings.

I like the symmetry of a circlotron output stage but designing a good driver stage which allows symmetrical feedback doesn´t seem easy.
Building an inverted Futterman using the same tubes and power supplies would be easier, but I don´t like the idea of having to compensate for the output stage asymmetry.

#### Zero Cool

I have TWO of Ralph Karsten's (Atma-Sphere) early prototype Circlotron amps!!! Both use 8 6AS7G tubes per channel.

These amps are inherently stable. But, if you should lose one supply rail, you suddenly have 65V at the speaker terminals. If you build one of these. i would highly suggest building a relay system for the HV rails that will disconnect BOTH rails should either the + or the - rail fail.

Ralph has stated that in 30 years of building these amps. he has only had that happen once or twice. and the first prototype of Ralph's i have is a true prototype and ugly as sin. at first look you say "im not putting power to THAT!" but it has been running for nearly 30 years non stop! so these are pretty good amps.

Zc

#### icarex69

circlotron OTL

I have been working on one of these too. I am planning to use 4 6336As. If anyone has any original schematics I would be interested in seeing them. I still haven't decided on my driver tubes...maybe parralled 12BH7s or 6CG7s. JC

#### 316a

Fuling said:

The reason I bought all those 6C19s and the sockets was to build a big choke loaded SE OTL with 8-10 tubes in // per channel.
I already have one with 4xPL504 per channel that sounds great, but lacks power.

Low power into a low load is the reason why I'm building a line array with 12 x 6ohm drivers in series per side

316a

#### ulibub

Hi all,

To avoid data redundancy ;-) I just would like to remind you of an earlier thread covering questions about 6C33C circuit designs, where I also presented my circlotron amp - also with some practical experience during construction and operation, and - with a circuit diagram. In the meantime, this amp has run now for more than 3 years almost every day without any problems.

The amp has one pair of the big 6C33CB-tubes per channel, I think these could be replaced easily by several (matched) 6S19 tubes in parallel. It is -not- a true OTL design, I use a toroidal output transformer with a 4:1 winding ratio.

It sounds wonderful, I've never heard such a powerful an well-defined bass with an Tube amplifier. Not an easy project, but worth the effort!

Uli

#### nafanja

Hello people - if You like 6S19 You can find Russian 6S41S which can change 3 *6S19P.

6S19is not have analog.
6N15 like straight analog 6AS7 is note can to change 6s19.

6S19 have a good linearity and can give good Alfa( it's Rout/Ra).
Three 6S19 can give 13 Alfa in 230V in Anode with Io=40ma*3=120ma in SE sweetching and 6W that is enough for work with 94db acoustic amps where 1 300B with 13 Alfa can give just 1.5W
If SRPP it's 5 W with 200V in anode

But I don't know what os transformer less with transformer...
It's joke?

#### nafanja

Hello

Schemes is have somes variants
1) SRPP
2)SEPP

You see in a picture- it's SRPP.
SEPP it's the same but higher tubes is regulated from defferencial (cascade)

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