diytube.com

Okay out of curiosity and the fact that I found that the speakers used with my 70 are pretty efficient (I connected my 2A3 SET amp to them, and was surprised), I want to triode strap the outputs and run it in Class A.

Looking at the Tube data, it lists Rk = 220 ohm resistor for each pair of EL34 tubes, for a 130-140mA. What do I do about the bias circuit? Is it still needed?

Thanks,

Robert

Triode strapping the finals is not the same as running pure class A. Triode means you're going to tie the screens of the finals to the plates. Class A means that both halves of the push/pull output stage conduct through 100% of the cycle, with neither tube ever going into cutoff.

The datasheet you are looking at is describing a circuit where the tube is cathode biased. That's an effective technique for biasing a tube, but really has nothing to do with either triode strapping or class A operation. It's also something you probably don't want. The Stereo 70 has a bias supply which provides a fixed negative voltage (fixed bias) to the grid. In this amp, I think you will find the fixed bias topology sounds better than cathode biasing.

I think what you want is to triode strap the output tubes. Simply disconnect the screen taps of the output transformers. They are the ones going to pin 4 of the tube sockets (should be green and green striped). Carefully insulate the bare end of the lead and neatly tape it back. Connect pin 4 of the socket to pin 3 using at least a 2 watt flame-proof (carbon or metal oxide) resistor. The value of the resistor should be between 100 ohm and 1K ohm.

Leave the bias circuit alone, and bias the tubes the same as you normally would. Do not install 220 ohm resistors at the cathodes of the power tubes.

I realize that triode strapping and class A are two different things. The info I am seeking is how to to get the ST-70 into real class A mode. With the bias supply, is it just a matter of increasing the current to the point of bring the amp into class A?

The reasn for this is, I hooked my 2a3 amp to these speakers and was amazed that they were as efficient as they are. The sound coming out of these speakers, with the 2a3 driving them was better than the stock (but updated) pas3 and st70. So... I want to try and determine what the difference is. Is the tube, triode v. UL, probably not class A, as the st70 run in class A for the first 15 watts, or so I have read...

Slartibartfast wrote:I realize that triode strapping and class A are two different things. The info I am seeking is how to to get the ST-70 into real class A mode. With the bias supply, is it just a matter of increasing the current to the point of bring the amp into class A?

...as the st70 run in class A for the first 15 watts, or so I have read.

I read the same thing, probably here: http://www.tnt-audio.com/ampli/dynaco2e.html

In a simple sense, yes, you would have to raise the current until the tubes are forced into pure class A during operation. In reality, you can't just do that. The power transformer can't supply that much current, and the tubes would exceed their dissipation limit. I don't know enough about the way pure class A amps are designed. Maybe you need to lower the B+ so the tubes can handle higher current, but now we are starting to talk about a total redesign of the amp.

Your motivation for this experiment is to determine why your single ended 2A3 amp sounds better than your push/pull Stereo 70? They are different animals, and the sonic characteristics of each is attributed to a multitude of different factors involving design criteria and component quality.

Why would the tube's rating be exceeded? The manual shows triode mode, class A bias, with a current rating between 75 and 140mA.

I did see brief conversation on another forum, talking about using an LM317 to keep outputs always conducting. Of course that thread lacked details, but, I thought it possible, without a total redesign.

The ST70's power supply isn't capable of much beyond 200 mA total, as the 5AR4/GZ34 and power trannie and C354 choke would then be pushed beyond their safe limits. The EL34s could handle somewhat more plate dissipation than the normal ST-70 service, just the PSU can't.



/ed B

Slartibartfast wrote:Why would the tube's rating be exceeded?

Plate voltage in a Stereo 70 is spec'd at 410VDC. Idle current is 50mA per tube. That's 20.5 watts idle dissipation per tube. Pa maximum for the EL34 is 25 watts. You've actually got a little more room than that, since the screen is handling some of the power too and it is rated for 8 watts. Even still, you don't have a lot of room to turn up the idle current before your tubes are going to start to redplate. I doubt you'd be able to get them up to 70mA per tube without having big problems. Even if you could, their life would be short and good tubes aren't free.

Of course, the 5AR4 is already taxed to the limit in this amp. It'll probably give up long before the finals.

I know K&K audio sells a driver/psu board that runs the 70 in class A, triode or UL. This is using the original PT. In fact they use someting similar to the lm317 to keep it in class A. I was just trying to avoid spending $275.

I am not using tube rectification. I upgraded mine with one of Joe curcio's psu boards. So that leaves the bias supply. I do have a spare PT, maybe I could stack them and run one per channel.

If you were to use cathode bias the current could be raised somewhat for more true class A operation, but then there would be some loss of output power, which will also result with triode mode operation.

The cathode biasing will run the cathodes around 35V above zero volts ground, so that would result in a V P-K of around 375VDC (+/-) Keep the current across each EL34 65 mA or less in that mode (60mA would be good) . RK would be around 470- 560 ohms, use 19 watters, just to play it safe. Bypass with 470 uF / 100V caps. This will generate extra under-the chassis heat.

A combination of fixed and cathode bias could also be used, though this won't allow as much current through each tube.

FWIW the Triode PA-060 power trannie has lots of extra current reserve.
You will need a heftier choke, the C354 will cook or burn out with the higher current passing through it, and the larger choke most likely will not fit under the chassis.

(This is about as long as I can go before the poster chokes and pukes)

/ed B

Part II (to accomodate the limited posting size)

The alternate, more complicated approach, though this is getting into some heavy re-work, would be to select one of the 200VA or larger ANTEK toroid power trannies, and aim for a B+ around 378-380V, and run the EL34s cathode biased, and then draw around 70 mA through each.

The simplest "no pain" approach is to try wiring up the EL34s in triode mode as described earlier, and give that a try before trying anything more "radical". You may be pleasantly surprised with the results.

I did similar to this a few years ago with an ST35, in triode mode, and though it's only good for around 5WPC, the sound quality is very sweet.

/ed B

Ed,

At the very least, I will wire it for triode operation, now that I know it will have enough power for those speakers.

Another question, when they say the ST-70 runs in Class A for the first 15 watts, would it not be more correct to say, "the ST-70 runs in Class A for the first x% of the 35 watt total"? Because when I wire this for triode mode and the max output is somewhere around 15 watts, it would not remain in Class A for the full amount, would it?

Are you sure that they are just running the drivers in class A , which would be alot easer to do .

battradio wrote:Are you sure that they are just running the drivers in class A , which would be alot easer to do .

an excerpt from their webiste:

As a result of using a true differential output stage, the output stage operates in Class A1. This results in a very robust sonic presentation that belies the 10 watts per channel (triode connected) test bench power rating. In fact, the kit modified ST-70 sounds far more powerful than a stock unit does. Lest you forget, the "70" in the model number denotes 70 watts output for both channels. Test bench numbers clearly don't tell the whole story! A K&K Audio Kit modified ST-70 provides very dynamic performance in a moderate size room with 89dB-91dB speakers and can credibly drive lower sensitivity speakers, as well.

Slartibartfast

Is your Dynaco stereo 70 in stock form or have you modified it? If you modified it what are your modifications? Power supply? Driver board?
Both?

6bg6ga wrote:Slartibartfast

Is your Dynaco stereo 70 in stock form or have you modified it? If you modified it what are your modifications? Power supply? Driver board?
Both?

I have a new old style driver board, power supply board from dynaco-docator, running SS rectification. new tube sockets and tubes.