Making an ST-70 Driver Board

[TerrySmith]

They need to be isolated. I etch the copper off of circuit board blanks, then cut to size and drill holes as needed.

[mesherm]

The PTs in stock ST70s were always marginal and besides the can cap is the second or third thing most people want to replace with a new beefier upgrade. But finances considered, keeping the stock PT in there will not will be any harder on the PT than what its already seen. The stock unit will run hot for sure and you may eventually want to replace it but you wouldn't be taxing it any harder than it was designed for, excluding its age of course.

[Quad]

Kyle,

If they are still available, the scratch-n-dent version of
the KTA from Geek is a tremendous value for a first amp.

The boards have the silk-screen (note the mentioned
R1-R2 error) legend for the components.
Geek seems to have followed the original KTA component
layout.

http://www.classicvalve.ca/surplus.html#KTA

5AR4s may be stressed due to the large capacitors on the cap boards and the tube may arc.

[antiquekid3]

I considered that scratch-and-dent purchase, but I was hoping to really DYI, not buy a premade board. I think I have decided to go down the point-to-point 6SN7 route. I got three fairly well matched GE NIB/NOS tubes today (along with my 5AR4...I got five matched EL34s two weeks ago).

I got most of my components today as well. A few other questions:

With the Blackburn Audio 6SN7 driver board, I found two schematics. One is in the manual, and the other is just an image...both located on the same part of the website. However...there are several components with different values. Both are "Rev. 1" dated 5/7/2008. Weird!! For example, the cathode to ground resistor for the EL34s are 15.6Ω in the manual and 10Ω in the image. There are a few others with different values, too. Quite perplexing...

Also, he has a 10Ω resistor going from the input ground to the circuit ground. What's that for?

With three significant figures, I assume the 15.6Ω resistor needs to be precision. However, can I get away with using a 15.0Ω instead? And what kind of wattage does it need to be?

The feedback capacitor is 390pF. Will 330pF work?

Can you tell I'm very cheap? I'm trying to use local parts that I can get for free or next-to-nothing...! :-) Maybe I'll end up proving (there's a chance!) that it doesn't take a lot of money to make a great sounding amp! Haha!

Kyle

[antiquekid3]

Here's the link in case anyone wanted to check out the differing schematics:
http://www.blackburnaudio.com/dynaco.html

Kyle

[TerrySmith]

If you read the original Dyna ST-70 manual, it explains why the need for a 15.6 ohm resistor. The only thing this resistor is for is to measure bias current using ohm's law.

Now we have precision multimeters and don't need the 15.6 resistor. On EVERYTHING I've built thats fixed bias I use a 10 ohm 1/4 watt resistor on each output tube, this serves several purposes, you can measure each tube individually, 10 ohms makes the math easy- .4v = 40ma, and the 1/4 watt size acts as a fuse when an output tube goes pyro.

I set my bais as followes:

6BQ5 / EL84 types 25ma - .25v with a 10ohm resistor.

EL34 types 40ma - .4v

6550 / KT88 60ma - .6v

[antiquekid3]

I'll go read the manual now. Probably something I should've done already, but I was too busy studying the schematics, to be honest.

So you can use four resistors, one for each EL34, or join the cathodes of two EL34s together and tie them through one common resistor? Electrically speaking, that's definitely different. Does it really make any difference?

Kyle

[Quad]

Like Terry explained in the previous post, having individual
10 ohm resistors for each tube make it easier to calculate/set
the individual tube bias. As they age, even matched
tubes may drift and it is better to
have the ability to bias them individually.
(too bad there's no individual bias control for humans to keep
matched pairs in sync as they age :-) )

[Geek]

If you read the original Dyna ST-70 manual, it explains why the need for a 15.6 ohm resistor. The only thing this resistor is for is to measure bias current using ohm's law.

I think they just got a truckload of 15.6R @ 1% for really, really cheap.... that's why they used them ;-)

Cheers!

[burnedfingers]

As one that has used the Blackburn board I feel that I have the experience to comment. First of all the stock transformer WILL run the Blackburn board without problems. The Dynaco WAS designed to run the Dynaco preamp and there IS enough left in the transformer to run 3 6SN7's. If your not using the Dynaco preamp with the 70 there is no problem. I used mine for hundreds of hours monitored the transformer and laughed at the idiots that said the transformer HAD to be replaced.

Secondly in all honestly I cannot recommend this board. I rolled a hundred tubes at least and never found this board to be any sonic marvel.

If you want a board that actually performs then I suggest that you purchase Geeks board that uses 6SL7 and 6SN7's and has CCS on board.

Another suggestion is you consider the SDS board that uses a 12AU7/12BH7 and 6AU6's. This board is a real sleeper and provide the ability to roll numerous tubes.

Another board that will provide exceptional performance is the Mapletree Driver board. I have made 4 of these boards (point-to point).

Quote:
Can you tell I'm very cheap? I'm trying to use local parts that I can get for free or next-to-nothing...! :-) Maybe I'll end up proving (there's a chance!) that it doesn't take a lot of money to make a great sounding amp! Haha!

Cheap is good within reason.....It makes no sense to be too cheep and then end up re-doing a board and or going to a proven design.

[antiquekid3]

Okay, I read the manual. Now I see...they assumed everyone would have at least one fresh alkaline battery laying around to compare its voltage to the bias voltage!

However, in the manual, it states that two EL34s draw 100mA from cathode to ground. So one would draw 50mA. Where did you get 40mA from? I suppose it's close enough, but I'm just wondering.

What's the smallest wattage pot I can get away with using for the bias? I would like to be able to bias each tube individually, but I don't want to drill any more holes in the chassis. Maybe I'll add some smaller pots in the circuit board area.

I have already decided to try out the Blackburn board. If I don't like, I'll do some more modifications and try another circuit. I'm building this amp with the intent to change it over time to suit my listening pleasures. It'll be an experiment!

I'm glad to hear the stock transformer will be okay. Here's a question: is it safe to parallel (correctly) the 6.3V windings so the load is distributed evenly across the transformer? With an odd number of 6.3V tubes, one will be pulling slightly more. Just wondering if that's an option.

Kyle

[burnedfingers]

antiquekid3

Don't tie the two 6.3 legs together.

Your bias is determined by the voltage across the 15.6 ohm resistor. With a voltage of 1.56 across the resistor 1.56/15.6= 100mA
If you modify the circuit to be able to adjust your bias for each individual tube you will usually use a 10 ohm resistor per output tube. There fore you will usually adjust your bias pots to achieve .500 volts across the 10 ohm resistor or .500/10=50mA. To answer your question to achieve 40mA you will adjust your bias pot for a reading of .400 volt across each bias resistor. .400/10ohm= 40mA

It doesn't take a hellish pot to used as a bias pot

[burnedfingers]

http://www.mouser.com/catalog/catalogUSD/641/712.pdf

Here is a pot suitable for use as a bias pot.

Picture of my Mapletree Dynaco with individual bias pots.

http://s378.photobucket.com/albums/oo228/burnedfingers/

[antiquekid3]

Yeah, I got how to calculate the voltage. But I was wondering why you might want to set the bias for 40mA versus 50mA.

Why's it bad to tie the 6.3V lines together? At least it would distribute the power evenly among all of the tubes.

So four 1/2W potentiometers will work just fine? That's good to hear; now I can fit some small pots (I think I have some 10K 10-turn pots) where the circuit board would go (which will probably end up being a metal plate for me).

Kyle

[burnedfingers]

Quote:
Yeah, I got how to calculate the voltage. But I was wondering why you might want to set the bias for 40mA versus 50mA.

Some don't want to run their tubes that hard. Run them easier and they may last longer.

Quote:
Why's it bad to tie the 6.3V lines together? At least it would distribute the power evenly among all of the tubes.

If that was a prime consideration the engineers of the Dynaco would have either specified a transformer with a large 6.3 volt tap or would have paralled them together like you mentioned. I personally see no need or benefit of paralleling them together. The tubes will get their 6.3 volt fix without a problem and will not care if one side is running an extra 6SN7 tube.

I suggest that you purchase a copy of Morgan Jones second edition of "Valve Amplifiers" and read it.