diytube.com

Does your power supply board have increased capacitance? I will probably receive some flac over this but ....in my opinion the standard driver scheme just doesn't cut it and neither does the power supply. The Dynaco sound totally bad in stock form.

My Dynaco's with the exception of one have all received power supply modification. All have received new driver boards. I always laughed at the dynaco stereo 70's in the past because they sounded pretty bad. With enough modifications they hit hard and with authority and have a pleasing accurate midrange and high frequency response. I have put mine up against some high dollar tube gear and have come out ahead. I would suggest a new driver board scheme before trying to re-invent the output section. As mentioned class A would require a hefty transformer and a brute power supply. The class A operation is fine in solid state amplification where you can turn up the bias make a few circuit modifications and come out ahead and as an added feature you can help heat your room. I would suggest that if a more pleasing and accurate result is your goal that the first thing you try is to change the output stage to triode followed by the driver board change.

Either way, a driver board upgrade to Shannon's board or Roy Mottram's VTA70 driver would be a great step up from the original 7199 (or 6AN8) driver arrangement

Dynaco designed the ST70 (and the ST35 & SCA35) to be just "good enough" and at a reasonable cost, though the output iron is excellent, the driver circuitry is minimal and uses hard to get (in 2010) tubes. The original PSU designs were also minimal, although the PT is plenty strong enough, the caps are just barely "enough" to ge tthe job done.

/ed B

The PSU board from Joe Curcio has increased capacitance and voltage capacitor voltage ratings. As has been pointed out, the dynaco PT will survive with the amp running in Class A mode. So I have no fears of killing it. I also have a spare. :D

Part of this hobby is about experimenting. That is what I am doing at the moment. A different driver board will come about. later on.

Can you post a lik to there webb page .

http://www.tnt-audio.com/ampli/dynaco2e.html

The above article says something about the Dynaco running class A up to about 15 watts. I cannot say that I could agree with that after looking at the output on a scope. It looks like it falls short. In another response it is stated the Dynaco's power supply is good for 200mA. In my estimation it doesn't have the necessary force available to put out 70-85mA that I believe necessary to put it into class A operation. Secondly as you increase the current to the tube you may find it necessary to decrease its operating voltage in order to stay in line with the capabilities of the tube. I fully believe that if your looking for a sense of sonic purity that you find lacking then the best way to go about this is a driver board swap. Frankly there is a night and day difference between the junk driver board in the stock Dynaco and about anything else out there. As I stated in an earlier post I have put several of my Dynaco's up against what should have been much better amplifiers. The modified Dynaco outshined several 3ooB's a 45, and a 2A3.

As stated by Ty_Bower
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.

Slartibartfast I don't see where you are finding that the transformer is capable of handling it.

Here is the link:

http://www.kandkaudio.com/poweramplifier.html


The only parts of a donor ST-70 that are used are assorted hardware, the PT and OPTs. The rectifier tube is also used, but, only for a soft start. It does not provide rectification.

So clearly the PT is up to the task.

From reading Radiotron 3rd edition, "for biasing a PP pair for Class A, the grid bias may be anywhere between the value for single tube operation and equal to 1/2 grid bias voltage required to produce plate current cutoff at a plate voltage of 1.4 Eb." Page 288.

My point is the plate dissipation is only 25 watts. If you crank up the bias your going to have to lower the plate voltage. Example 400 volts on the plate and the tube biased at 70mA is going to run the tube at 28watts dissipation and that is over the 25watt rating.

Looking at the lititure , it is Push pull class A1 and not class A .

A1 bias for push pull is where there is an over lap at the cross over point where both tubes conduct and are linear .AB1 & AB2 there is less or no over lap .

"Class A" is shorthand for Class A1. Class A2 is when grid current flows. Class AB introduces crossover distortion, how much and is it noticeable, is up for debate. This is due to the tubes going into cutoff. The overlap prevents that.


Here is a good read on the different classes WRT SET and PP.

Went through my RCA RC-28 and The Audio Cyclopeadia Both showed push pull class A1 amplifier for 6L6 . The RCA manual shows ( page 388 )
B+ of 270 volts and a bias of -17.5 for A1 and B+ of 450 volts and a bias of -37 for AB1.

I think your reading too much ! into the advertising .

battradio wrote:The RCA manual shows ( page 388 ) B+ of 270 volts and a bias of -17.5 for A1...

I could believe that. Too bad the primary impedance of the A470 output transformer is most likely not ideal for operation at that voltage.

battradio wrote:
I think your reading too much ! into the advertising .

How so? It is either Class A or A1 if you prefer, or it isn't.

Ty_Bower wrote:

battradio wrote:The RCA manual shows ( page 388 ) B+ of 270 volts and a bias of -17.5 for A1...

I could believe that. Too bad the primary impedance of the A470 output transformer is most likely not ideal for operation at that voltage.

It is close. The primary impedance is 4300 ohms. There is a short thread on diyaudio, talking about using 6L6s and the A470.

Slartibartfast wrote:How so? It is either Class A or A1 if you prefer, or it isn't.

Class A1 and Class A are two different things, the K&K mod
clearly states: "As a result of using a true differential output stage,
the output stage operates in Class A1."

Calling it Class A at the beginning of the thread sent everyone
on a wild goose chase.