Need help with my ST70 build.

[JayJH]

I built an ST70 from scratch, point to point wiring for my first project. I used new Dynaco repro transformers from Triode, upgraded cap psb from Triode, JJ Tubes, and the driver board is the DIYTube design.

Here are my problems:
1.) I had to disconnect the 16 ohm leads to the negative feedback because of horrible buzzing. It is now fairly quiet, but I would rather have those leads connected.

2.) It plays music without any buzzing, however there is a quality to the music I guess I would describe as distortion.

3.) I am also a little unclear on where I am supposed to be measuring the 400 then 500 mv when biasing. What terminals exactly should I be putting my multimeter?
Any suggestions on where to look?

thanks.

[TomMcNally]

Hi Jay ...

Take a look at the notes in the beginning of the manual (latest version is here on the forum) and take a look at the part about phase reversal ... the "horrible buzzing" will be cured when you flip the phase of the primaries of the output transformers.

The bias voltage is measured across the 10 ohm cathode resistors from pin 8 to ground on the EL-34s ... there is normally a wire running from pin 5 and 8 on the "preamp socket" so you can measure it from the front panel.

manual link: http://www.diytube.com/st70/diytube_st70_B.pdf

[JayJH]

That solved it! This thing sounds so much better than ss.

Now that I got it working with cheap caps I want to upgrade them. Should I upgrade all 10 caps or will some of them not have much impact at all? Which caps in the circuit will have the most impact?

[TomMcNally]

Glad you got it going Jay !

The only caps that will make much difference as far as I'm concerned are the four coupling capacitors, C-7 8 9 and 10. Some paper in oil type, or boutique type like these would be nice:

http://www.hndme.com/productcart/pc/viewCategories.asp?idCategory=77

[kheper]

That solved it! This thing sounds so much better than ss.

Now that I got it working with cheap caps I want to upgrade them. Should I upgrade all 10 caps or will some of them not have much impact at all? Which caps in the circuit will have the most impact?

The following caps are good candidates for upgrade. Solen fast-caps will be the most economical, especially for C1 and C2. Auricaps will be good for C7, C8, C9 and C10, if you have that kind of disposable cash.

C1, C2 .1 uF

C7, C8, C9, C10 .047 uF

If you want to hook the negative feed-back loop to the 8ohm tap, change C3 and C4 to 330 pf and R9 and R10 to 2 K.


I would put the Solen caps on C1 and C2, alter the values of C3, C4, R9, and R10, hook up the NFB to the 8ohm tap and listen for a while, before going into debt with 'boutique' caps.

Better to invest in some nice NOS 12au7 tubes, than fancy caps.

[JayJH]

If you want to hook the negative feed-back loop to the 8ohm tap, change C3 and C4 to 330 pf and R9 and R10 to 2 K.

What is the advantage of using the 8ohm. BTW, I am using 8 ohm speakers if it makes a difference as to what lead to use for NFB.

[burnedfingers]

As one that has spend $100's of dollars on capacitors looking for that slight edge I would caution you on getting too carried away. I realize also that after saying this I will need to duck and cover.

I spent a bunch of money trying out different coupling caps in a pair of my Quicksilver 8417's that I converted to use KT88's. The truth in the matter is there is very little to be gained in throwing expensive caps into the mix. Most of the difference you hear is going to be a product of your mind telling you that you just spent X number of dollars on that cap so it sounds better.

Having a number of Quicksilver mono block amplifiers it was very easy to conduct some simple tests. In amplifier A I put $4 a piece coupling caps and in amplifier B $100 a piece coupling caps. After conducting a listening test with 4 sober friends the outcome didn't weigh on the side of the high dollar caps. Now imagine having 2) sets of identical amplifiers, speakers, and line stages not to mention source gear and conducting more tests to compare power supply cap mods and other assorted tests.

I conclusion I gained a little in using motor run caps in the power supply and by bypassing those with a film cap. Very slight difference was to be had by using expensive coupling caps that took hundreds of hours to break in so that a slight difference could be heard. I actually heard a greater difference as a result of tube rolling than in tweek capacitors.

Well, to each his own I guess.