Hello...and another ST-70 project.

[8vgumby]

I figured I would finally post something, I've been lurking and reading on this website for quite some time now.

I am looking for help and advice where I can find it. I've been playing with audio stuff since I was about 10..11 years old. From that, I've expanded to a full HT arrangement, and have slowly begun to work on a true hi-fi setup.

This has progressively changed from a Carver MXR receiver and Acoustic Research Towers to my current NAD 3155, and Paradigm Monitor 7's.

My knowledge of DIY in this area has been limited to the random PCB projects in highschool/college, as well as general repair work on solid-state stuff (replacing caps, resistors, connections...etc..)

Recently unemployed, and returning to school, I find myself with an absurd amount of time on my hands, and a need to rekindle an old hobby. So I've decided to take my first stab at building a valve amplifier and figured, what better way than the ST-70, which seems to have the most documentation and support of others.

Any help or support you guys can provide is greatly appreciated, and I will certainly keep you posted on my progress.

I was looking around on eBay the other day and stumbled across a Tomiko toroidal PT that claims to be a direct drop-in (minus obvious mounting issues). Has anyone had any experience with this item?

[Eli Duttman]

You should do fine with your speakers connected to the 4 Ω taps.

Have you decided on a driver board? The OEM 7199 is scarce and expensive. FWIW, I like Triode Electronics' board, which retains Dyna's topology, while employing tubes that are in production. AAMOF, TE's board populated with 2X EF86es and an ECC99 will outperform Dyna's original.

"Exact clones" of Dyna's magnetics are available. I know nothing about the toroid you mentioned.

Current production 5AR4 rectifiers are a dicey proposition. If you like, I can provide a method for using SS B+ rectification that does not overly stress anything in the amp.

[boywonder]

If you are set on a ST-70 clone, there are lots of places to pick up parts & iron. Here are a few among many:

There are quite a number of alternative driver PCBs for the ST-70 that use different tubes and rectification, so do a search for that here and on Google......

For ST-70 Dynaclone iron check out Triode Electronics:

Triode Electronics Tube Amp Kits Transformers Tubes Dynaco Upgrades and Parts

The Triode electronics PA-060S power transformer is a great choice, as it is rated for higher current than the stock unit, and it's beefier with more lams. The stock power transformer can be a little marginal, esp. if you want to run KT88's at some point.

And there is an entire forum dedicated to the ST-70 over at DIY tube:

diytube.com :: View Forum - st70

Can you provide a little more info on the Ebay toroidal transformer?

[garage1217]

Also check the stellar stuff located here...
tubes4hifi home page

I am close to picking up a st120 / kt88 kit and DROOLING over the upcoming m125 monoblocks sort of based off the m60's.

[8vgumby]

Probably should of linked that eh? - PT in question: upgrade Dynaco Dynakit ST-70 ST70 power transformer amp - eBay (item 300471198661 end time Nov-20-10 21:10:01 PST)

I actually have sourced a chassis and VTA driver board (tubes4hifi) so far.

[taj]

Originally Posted by Eli Duttman

Current production 5AR4 rectifiers are a dicey proposition. If you like, I can provide a method for using SS B+ rectification that does not overly stress anything in the amp.

Hi Eli,

I'd like to hear about your 5AR4 to SS method that keeps the operating points within reason. I've just seen BIG dropping resistors used with the SS diodes before, and I thought there must be a better way.

I'm going to use Gregg V.'s (Geek's) DynaMutt circuit in my ST-70.

Anyone know where I can buy 716P orange drops for a reasonable price? (Bonus points if it's in Canada).

..Todd

[rmyauck]

I just read there are bad batches of orange drops out there. Don't know which version. Maybe Solen might be better. Russian K-40 are supposed to be a great cap for the buck and Auricaps are well liked for a bit more $.

Randy

[Eli Duttman]

I'd like to hear about your 5AR4 to SS method that keeps the operating points within reason. I've just seen BIG dropping resistors used with the SS diodes before, and I thought there must be a better way.

My method starts with a "stock" ST70 that has 1 bias adjustment per channel. I retain that arrangement, while dealing with minor variations in cathode current in an O/P pair. The fundamental concept is to turn the extra Volts SS rectification yields from a problem into an advantage. Insert a CL150 inrush current limiter between the rectifying diodes and the PSU filter. Select a 1st filter cap. just "tall" enough to keep the rail voltage satisfactorily up. Replace the crappy OEM choke with something actually useful. Pile the energy storage up after the choke.

The "secret" is combination bias. "Stand" each pair of "finals" on a shared bias network made from a 100 Ω resistor and a 470 μF. 'lytic. A resistance of 100 Ω provides a convenient "idle" current test point and "eats" the extra Volts yielded by SS B+ rectification. The shared self bias network compensates for minor tube to tube cathode current variation and allows the single bias trim pot. to do a decent job. Close matching of gm is still necessary in each pair of "finals".

[taj]

Originally Posted by Eli Duttman

Replace the crappy OEM choke with something actually useful.

Can you define "crappy"? My guess is we need a bigger choke, 5+H?

I'll see if I can find out how a combination bias circuit is wired. Any hints?

Does the CL150 slow things down enough to skip using a standby switch at start-up?

..Todd

[Eli Duttman]

Originally Posted by taj

Can you define "crappy"? My guess is we need a bigger choke, 5+H?

I'll see if I can find out how a combination bias circuit is wired. Any hints?

Does the CL150 slow things down enough to skip using a standby switch at start-up?

..Todd

Dyna's choke is not up to snuff, current handling wise. A genuine 250 mA. or greater capable part is in order. An inductance of 2 H. should be enough.

In an amp with "fixed" bias, any delay in B+ rise is enough for an "instant on" bias supply to provide electrostatic protection against cathode stripping. So, a standby switch is unnecessary, when the inrush current limiting thermistor is installed.