ST 70 Triode mode question

I understand the mod to convert the ST to triode operation producing much less output power but if I understand correctly, the amp still operates as push-pull, not class A, is that correct?  If that is true, is there any mod that would place the amp in pure class A at some further reduced output power?

I am interested in fooling around a bit with tube class A but would rather not purchase another amplifier if there is a way to mod the ST 70 to do it for "messing around" purposes?

MexicoMike wrote:I understand the mod to convert the ST to triode operation producing much less output power but if I understand correctly, the amp still operates as push-pull, not class A, is that correct?  If that is true, is there any mod that would place the amp in pure class A at some further reduced output power?

I am interested in fooling around a bit with tube class A but would rather not purchase another amplifier if there is a way to mod the ST 70 to do it for "messing around" purposes?

It's easy enough, don't turn the volume up!

At some point one of the tubes will cut part of the cycle. An oscilloscope on one of the
anodes should show when. Until then you are running in class A

I agree with peterh. As Peter said, you operate most of the time in class A in the normal class A/B amp.
If you want a 100% push/pull pure class A a lot of things have to change. First making the tubes run in class A is easy you just change the bios so that idle current equals maximum current, however the power supply from transformer on must be beefed up to handle the massive draw of power and you will be dissipating a lot of heat from the tubes because any power that is not used to produce music must be burned off by the tubes. Specifically the tube plates. The net outcome is even less maximum power, short tube life and a very high electric bill. But in the winter you can heat your house. My guess is the power transformer would need to be about 300% to 400% larger.

Thanks, I was hoping there was just some simple minor wiring - like the triode mod - that could be easily reversed and that would allow me to experience triode + pure class A operation.

Mike, You can listen to class A just keep your volume down and you are in class A.
Let me give you an example. If you looked at just a load line for a push/pull amp. and it was class A and you think of it as a ruler 8 inches long the bios point or continues idle point would need to be at the 4 inch spot so the voltage and amperage swings a maximum of 4 inches each way. When one tube is swinging up [lower voltage, higher amperage] the other tube is swinging down [higher voltage lower amperage]. Now if you changed it into a class AB amp. you would bios it at 2 inches so that each tube would swing together but opposite of each other from 0 to 4. This is the class A section of a class AB amp. When one tube reaches 4 the second tube reaches Zero [which by the way is zero amperage and highest voltage] and that tube is in cut off or shut down. The other tube however continues on and drives the full load by itself past 8 inches to maybe 10 or 12 inches. Why is this possible? Because when pushed hard one tube is shut down and the other is pulling the full load. When it swings the other way that hard working tube gets to shut down and cool off. IN class A it never shuts down or gets a chance to cool off. So if you have a 17 watt triode amp and you keep it in the 4 watts and under you are in the class A range of the amp. If you measure actual output with a volt meter you will find that most people listen to their stereos in the one watt and under area or Class A. Power hungry Magnepans would be an exception.

Interesting - so If I had some 96+db efficiency speakers (I don't - 87s are my most 'efficient') the amp would probably be in class A at even fairly loud levels, right?

Hmmm, I've been interested in some DIY plans I've seen for such speakers.

Mike that's louder than you think. You have to exceed about 91db with your speakers to be out of the class A mode. If you put a volt meter on one of your amps output and measure the voltage. 4 Ohms speaker 4 volts output is 4 watts. 8 ohms is about 5.7 volts.
I bet if you measure it most of the time you will be around 1 volt. Or well in the Class A range.

Sailor...thanks for the info
But seting your Amps output tubes to run in triode well sound diff for sure.....even if you push the amps hard out off A1 into A/B... most like the sound of triode over UL......good luck have fun with tubes




Thanks for the site Bob....................

Hi ZX, my calculations of about 4 watts is based on the amp being set on triode. I didn't mention it because that was what we were discussing and I assumed it was understood. In UL the class A would be more in the 7 watt range.

A simple circuit could be made that detects when an anode goes below , say 50V ( one has to
see the tube-data or use a scope) . Then one could "blink" a led to indicate that the amp
leaves CLASS A and starts using CLASS AB.

As an alternative one could measure across the cathode resistor and detect when
less then some small voltage is across. Normally 0.4V, a reasonable guess could be
anything less then 0.1V would indicate class AB.

Did some checking...

My McIntosh 7270s (270WPC) running a pair of AR LSTs (86dB efficiency) typically indicate around 3W output on the power meters at typical listening levels in my room. So even with those LSTs, it would seem that the ST70 in triode mode would be in Class A for normal listening of pop/rock/jazz.

MexicoMike
I am interested in fooling around a bit with tube class A but would rather not purchase another amplifier if there is a way to mod the ST 70 to do it for "messing around" purposes?

Go over to the ARC site sroll down ck out the ST70-C3 Schematic....
See how Thay setup there Cathiods....you can do this for better sound some say..i have done this mod on my MK3s..there more info on the site.....have fun with tube sound....



http://www.arcdb.ws/ST70C3/ST70C3.html#



Thanks for the site Bob.......

First Mike that sounds about right. If you have a Diana Krall album lying around it is the perfect piece of music to play and hear class A. If you go into any high end stereo shop and listen to a single ended amp. [All class A] your chances of hearing one of her albums is about 50%. It shows off the best while hiding the weaknesses.
Second, My brain works in mysterious ways that sometimes scare even me. It just hit me that if a smart amplifier builder wanted to make his amp sound special he could drop the class A/B and call his amp. a all class A amp. that switches from push-pull in the quiet passages to switching single ended class A in the more demanding passages.
OK, I know Class A means operating at max 100% of the time but that is why I add switching in front of single ended.
Oh and if you think such [stuff] is above a reputable company guess again. Quad speaker in the 70's claimed it made an amplifier that was 2 amps in one. A pure class A for low level listing but a back class B amp when you needed the extra punch. Really??? Isn't that the definition of a Class A/B amp.

Mike, I am running Audio Nirvana 12" ceramic speakers in large DIY cabinets. I'm pretty sure I stay in Class A most all the time as they are 98-99DB efficient. The Audio Nirvanas are great speakers for a DIY project.

OK...another question: The "tipping point" for an AB amp, like an ST 70, to go from class A to B (AB) is based on the bias current, right? SO, in theory, the bias could be increased to ensure the amp stays in class A longer..or never leaves it? I'm not suggesting this is a good idea because I assume that doing so to any noticeable degree would raise the current to greater than what the transformer can provide as well as reduce tube life. But I'm interested in understanding the concept. Could you, IN THEORY, increase the bias to a level where the amp would never go out of class A?

Now you've got it. Pure Class A is about bios point which must be exactly half way between cut off and max amperage to be pure class A. And the power supply to sustain it. If you don't have a large enough power transformer a class A amp. will clip at idle. It uses just as much power at idle as it does at full power. As we call it, increasing the bios point increases the amount of class A from any grid bios amp. but more heat, amperage, shorter tube life and the possibility of frying the power transformer may happen.
But let's clear one thing up that many have a miss conception about. We all refer to increasing the bios as increasing the amperage or operating point. The bios is a negative voltage when applied to the grid holds the plate to cathode amperage at idle at a set level. This idle point sets the operating point and load line for that tube. As this neg. bios voltage goes more negative the amperage decreases. So saying increasing the bios is really a misnomer. Look at this EL34 tube grafts below, notice how as the grid is bios more neg. the amperage goes down.
At idle the amperage is held steady by the grid bios current you set, but as the signal comes in from the driver circuit the voltage starts to swing at the grid which in turn cause the amperage from plate to cathode to swing. However that bios voltage is always present and is added to the incoming voltage. This addition of this neg.voltage is how the bios keeps the tube on the set load line and operating point.
One last thing, most small tubes and some power tubes are what is called self biosing or cathode biosed. In this case the cathode resistor is sized to cause a set neg. voltage at the grid in which case the bios at the grid is fixed unless the cathode resistor value is changed. Don't want to get into this here as how it does this is a story in itself.
Hope that made sense, if not ask any questions and I will try to answer them.
http://www.drtube.com/datasheets/el34-sed2002.pdf

MexicoMike wrote:OK...another question:  The "tipping point" for an AB amp, like an ST 70, to go from class A to B (AB) is based on the bias current, right?  SO, in theory, the bias could be increased to ensure the amp stays in class A longer..or never leaves it?  I'm not suggesting this is a good idea because I assume that doing so to any noticeable degree would raise the current to greater than what the transformer can provide as well as reduce tube life.  But I'm interested in understanding the concept.  Could you, IN THEORY, increase the bias to a level where the amp would never go out of class A?

Yes, but then you have to reduce B+ to keep within the tubes maximum wattage. This lowers
the available power output. This would make the existing power supply unsuitable.

peterh wrote:A simple circuit could be made that detects when an anode goes below , say 50V ( one has to see the tube-data or use a scope) . Then one could "blink" a led to indicate that the amp leaves CLASS A and starts using CLASS AB.

Hmmmm ... got me thinking there. Be similar to what I see out of my McIntosh equipment. When I push the amps, I get the powerguard lamps flickering. Same thing, only different, as in this case the lights would just indicate heavy draw that shows the amp going out of pure A mode. More or less. Wouldn't be surprised if McIntosh doesn't use a similar circuit, albeit with the protection circuitry triggered as well.

~ ~ ~

Maybe worth noting. Technically, triode mode has nothing to do with how the power side of the amp operates. What you're doing is basically rerouting the signal through the grids on the driver tubes. This is what adds a unique sonic signature to the driver output. And ya, a side benefit (?) of this is reduced input to the power section, resulting in reduced power output from the amp, resulting in a higher volume setting before triggering A/B operation ... point being, you can get the same effect by turning down the volume on a pure pentode amp, but you don't get the darker, sweeter sound of triode that's popular for jazz and orchestral. Then again, you can get real close to that by rolling the drivers. I can go triode, or get a very similar sound running my ST120 with 5963's, yet I still get the punch and power of pentode. Whatever blows your hair back, eh.

Anyway - Lots of options. I'll take em all. He who dies with the most options wins!  

PS - typically, a tube likes to operate and sound best around 60% ... true or false? I do know the KT120's prefer around 60mV bias - they sound (to me) a bit pinched at the usual 55mV ...

Example: 300B
http://www.emissionlabs.com/datasheets/EML300B.htm
It shows B+ from 300 to 450 of course the biosing point has to be picked where the maximum watts dissipation is not exceeded. Mike, if you look at the chart that says maximum conditions, notice it is followed by not possible, this is because 450 volts times 100ma. is 450 times .1 amp. equals 45 watts which exceeds the maximum dissipation of this tube which has a max of 40. Tubes should be run conservative for the longevity of the tube. There Single ended class A example use 300 volts and 56 ma. which get a plate dissipation of 17 watts [well under the 40watt limit]. A bios of -58 volts was used to get this operating point. If you look on the chart you will see where 300 volts and 56ma. cross at -58 volt point. The load line is then drawn which is not pictured.
Below that is a big pile of other operating points volts and amps always cross at the bios setting.