ST70 power supply questions

ureche wrote:Hello everybody,

I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

Thank you.

peterh wrote:
The rectifier ( if you use tube ) is the limiting component in the st-70 power supply

ureche wrote:Hello everybody,

I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

Thank you.

deepee99 wrote:

ureche wrote:Hello everybody,

I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

Thank you.

Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

Bob Latino wrote:
If you reduce the bias to 40 milliamps per each output tube then this is not true and is one of the most often repeated falsehood about the original Dynaco ST-70. What is true is that Dynaco (for whatever reason) chose to operate the amp right at the limit of a 5AR4's ability to supply DC current. A 5AR4/GZ34 can supply steady state about 225 milliamps of DC current. Dynaco operated the 4 EL34 output tubes at 50 milliamps each which is 200 milliamps. I am guessing each of the two 7199 driver tubes used maybe 8 to 10 milliamps each SO > we are up to near 220 milliamps. Yes - this is at the limit of a 5AR4's ability to supply DC current. To get around this all you have to do is reduce the bias level on the original Dynaco ST-70 to 40 milliamps per each output tube. To this, set the bias level at about 1.25 VDC for each channel instead of 1.56 VDC per channel.
[/size]

Bob Latino wrote:
On the VTA ST-70 we use an 80, 40, 30, 20 uF quad cap and a driver board which has its own power supply. The driver board on the VTA ST-70 does not have to draw DC from the quad cap. This leaves the quad cap free to power both output transformers or about 85 uF per each of the two output transformers which is about 3+ times the 25 uF per each output transformer on the original Dynaco ST-70. Set up this way, the VTA ST-70 will make its power level of 35 WPC from 20 Hz to 20 KHz. The VTA ST-70 can put out about 45 WPC @ 1000 Hz in pentode ultralinear mode before clipping. See test link below.
[/size]

ureche wrote:

I saw in a post somewhere, the recommended bias for KT88 in ST70 would be 50ma. So i was wondering if setting the bias to 40ma would have an impact on the sonic output?

ureche wrote:

deepee99 wrote:

ureche wrote:Hello everybody,

I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

Thank you.

Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

I would have bought the kit in a heartbeat, but as i am located in Europe, the shipping cost and customs tax are setting me back a lot.

MarcVBelgium wrote:

ureche wrote:

deepee99 wrote:

ureche wrote:Hello everybody,

I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

Thank you.

Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

I would have bought the kit in a heartbeat, but as i am located in Europe, the shipping cost and customs tax are setting me back a lot.

Hey Ureche,
I live in Europe (Belgium) too. I have the ST-120 from Bob.....With shipment, import duties and VAT it comes to approx. 1.750 €
with tubes (matched quad of KT-120)
All I can say is that it is a magnificent amplifier and.... doing things right from the start, usually saves money !
There is no tube amplifier that I know of that is this good at this price level.

I can recommend the Bob Latino kit.....

All the best, MarcVBelgium

Tube Nube wrote:If you watched that mk III restoration video on you tube that was posted on this forum 2 weeks ago, you can steal the only good idea in the video, which was the way the fellow used a spreadsheet to detail a few different cost projections for  his rebuild. Like him, you could incorporate links to all the suppliers.

Its a fun way to use spreadsheets, if that kind of thing turns your crank. You'll have all your research organized and stored in one place, so you can fret and deliberate, and come back to it later without having to re-assemble your understanding of where you were when you left it.

peterh wrote:I'll aggree with both Marc and depee, by the kit and you will have every little component.
Tubes can be bought in europe ( they are made there :-)

Save your build desires for you next amp !

deepee99 wrote:
I would second MarcVBelgium's comments. Consider the time you're going to spend finding parts, chassis, and transformers alone and bill yourself an hourly rate for all that effort. It will easily better the VAT and customs charges even if you're paying yourself minimum wage. I built a pair of Bob's M-125s and they are easy to assemble. Yes, I got stumped a couple of times but Bob is an email away and never seems to sleep, so you get an answer pretty quickly and a wonderful-sounding amp at the end with bullet-proof chassis. Plus you get Roy's boards which are top-drawer. I would think in Europe you could find tubes for them very easily, locally. There's a lot of good former military tubes out there. Bob's chassis and transformers, and Roy's boards, are hard to duplicate.

Bob Latino wrote:

ureche wrote:

I saw in a post somewhere, the recommended bias for KT88 in ST70 would be 50ma. So i was wondering if setting the bias to 40ma would have an impact on the sonic output?

You can use KT88's in the VTA ST-70. Understand though that if you do use a KT88 in the VTA ST-70, the KT88 is not running in a range that is fully optimal for this tube. A KT88 does work better in the VTA ST-120 with its higher B+ voltages ..

Bob

ureche wrote:

As i didn't order the PT and i can order one according to my specs. Theoretically speaking, what if i order one with 400V-0-400V main voltage? This will raise the B+.

Are there any other aspects that i have to take into consideration ?

ureche wrote:Hi, i was wondering, if there is any sonic difference in using a solid state vs tube rectifiers?

I was thinking of going with ss rectifiers, this will also have an impact on PT cost, as i get rid of the 5v heater.

So, if i go solid state, i will add a switch for B+, or a 60 seconds delay circuit.

Should i increase the capacitance ? What would be the recommended values for 80/40/20/30 ?

Thanks,

ureche wrote:I read somewhere, that ss rectifier on cold tubes could lead to something like cathode stripping. Well i didn't find a post, yet, where this has happened, but i thought i should do the delay to be safe.