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Old 1st September 2004, 03:01 PM   #1
percy is offline percy  United States
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Default Which ST-70 ?

I think I am inclining towards building a new ST-70. I found a few 'mods' or versions of the driver boards on the web and here are some of them -

Triode Electronics http://store.yahoo.com/triodeel/st70boards.html
SDS Labs http://www.quadesl.com/
Curcio Audio http://www.curcioaudio.com/
Welbourne Labs http://www.welbornelabs.com/st70.htm
(any other ??)

Triode Electronics and SDS Labs versions appear to be very close to the original. They differ from the original in the sense that they use seperate tubes (a triode and a pentode) instead of both in one envelope like the 7199. Looks like the idea is to be able to use better audio tubes. Both TE & SDS use EF86 and 12AU7, whereas SDS has one more version also using a 6AU6, instead of EF86.

Curcio and Welbourne are a bit different. Curcio has two versions both are 'all triode' but one with a CCDA (constant current differential amplifier).
Welbourne Labs also makes a very different type of driver.

Other than this, I dont have much information based on which I can decide what is good, better or best. So here's where I need your help. If anyone can provide some more information or insight on this, it would be very helpful. Especially who have built and used any of these or better yet had a chance to compare!
Looks like SDS would be the most economical way to go.

Secondly, I noticed that all the mods that are out there are related to the driver board only, none for the power stage. Is it because modifying would call for replacing the OPT which is considered to be a very good opt, or there really arn't any great mods that can be applied and its probably best left as is ? I am asking because the option for a different transformer(different from a Dynaco opt clone) is open for me since I would anyway have to buy transformers so if there is a mod for the power stage do let me know.
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Old 1st September 2004, 03:53 PM   #2
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Default if building from new parts...

I'd not replicate an ST-70. Historically, the ST-70 is an ingenious case
study on doing something good on the cheap. It is an artful balance
of tradeoffs to achieve a particular price point, a particular package
footprint and be easy for people to build. It's not worth replicating those
design criteria if you are coming up from new parts because you can
do better for not much more money.

That said, a 6600CT output transformer rated around 50W gives you
alot of flexibility. You can opt in for 6550, KT88, 6L6 as well as EL34s.
There's a myriad of good PP output transformers with these specs.
Next, plan for a stiff 450V power supply. Again, there are plenty of
good new Power Transformers that meet these specification. If you
build one of these - you will essentially have what is out there on the
market (Quicksilver, Rogue, Conrad, et cetera). It will be flexible for
changes and it will last forever so dont flinch at dropping $1000 in
parts to do the project right if you are so inclined. I might add you should
be able to get all the parts you need for less than that.

You can still buy driver boards if you are not comfortable hardwiring
from schematics. Uncle Ned's boards a great. His boards are great
and I've heard a renovated ST70 - with the board equipped with
12AU7s and a 12BH7 which i donated.

-- Jim
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Old 1st September 2004, 06:21 PM   #3
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Off this page @ vacuumstate.com....

Click the image to open in full size.

This is the direction i'm going with mine...

dave
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Old 1st September 2004, 06:22 PM   #4
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This is worth what it's costing you:

I have the Curcio full-blown mod with regulated power supply and CCDA driver stage. WIth "garden variety" type tubes (6922 Sovteks, Siemens EL34 (one pair marked "Made in E Germany", the other "Made in Germany") I was extremely happy on first power-up in 1995, and am still happy (and still using the same tube set after 4 - 5,000 hours).

The image I get out of this amp is un-believable (front to back and side to side). When I first powered it up I was using a Sonny portable CD player (1987 vintage) on the input side, and a "sacrificial lamb" (Jensen Duette's, late '50s) pair of speakers. After about 30 minutes of smelling closely and looking for flames I shut it down and connected ADS L1090's.

I cannot describe how good it sounded. While finishing the amp I had been listening to a Hafler DH-220 (certainly no slouch) and the ST-70 was miles better.

Some of the things you may want to think about are:

1. Curcio is the only one who looks at the whole picture - power supply, driver stage, and output. If you still need a tube rectifier, you are still in trouble. Have you priced real 5AR4/GZ34's lately? Look at a tube manual, add up the current draw for 4 EL34's and compare that with a GZ34 can supply. Then consider that the Fisher EL34 stereo amp had 2 GZ34's. The GZ34 supplying 4 EL34's is right at it's limit.

2. I personally don't care for pentodes in driver stages (you may not care).

3. Support for Curcio is great - he has a forum on AudioAsylum, and he's got a bucketload of troubleshooting documentation on his web-site. Welborne also has a forum on AudioAsylum, but no other documentation that I am aware of.

4. The A470 output transformes are 4300 ohms (not 6600), which is also good for 6550, KT88, 6L6, and I think the Sovtek 6B4G's can also take the 425V B+ of the modified unit. I would think you would have this option with any of the mods (after taking care of bias, etc.).

From the length of this it looks like I got on my soapbox.

Fire away.
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Old 1st September 2004, 06:32 PM   #5
316a is offline 316a  England
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Default PP1C

Quote:
Originally posted by planet10
Off this page @ vacuumstate.com....

Click the image to open in full size.

This is the direction i'm going with mine...

dave
Hello ,
The driver stage is excellent , highly recommended IMO If you only need to swing 30-40V P-P and have B+ >350V this should provide enough gain to only require a volume pot in front of it . Currently working on a more substantial version using 6H30 to drive GM70 , maybe using cathode followers

cheers

316a
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Old 1st September 2004, 06:57 PM   #6
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Default Re: PP1C

Quote:
Originally posted by planet10
Off this page @ vacuumstate.com....

Click the image to open in full size.

This is the direction i'm going with mine...

dave
+1!! especially if 10W or so is enough. I've built a couple of these and they're truly excellent, much better than any standard ST70 or clone.
Quote:
Originally posted by 316a
The driver stage is excellent , highly recommended IMO If you only need to swing 30-40V P-P and have B+ >350V this should provide enough gain to only require a volume pot in front of it . Currently working on a more substantial version using 6H30 to drive GM70 , maybe using cathode followers

cheers

316a
Before you add a CF into such a nice sounding circuit, take a look at Pete Millett's pentode driver designs. Built a few of these too, and they were the best drivers I found for the 813 trioded and the GM70. I don't have my notes in front of me, and the actual amp is in storage 800km away, but iirc, I used 12HG7's in the end.

For both the cascode and the pentode drivers, you want a nice clean PS of course, but I'm sure I'm not telling you anything you didn't already know.
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Old 1st September 2004, 08:36 PM   #7
percy is offline percy  United States
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Having heard good things about it on the forum I did seriously consider the PP-1C. I believe its rated at 4W, and I confess it is the main reason I did not consider this and a few nice SE projects also(I hope I dont start a war here. if you feel I need to be educated on how much power is realistically needed please fire away). But do note that atleast for a start I wont be having any great speakers. Efficiency maybe 87-89db range.

One reason for choosing the st-70 was there is a lot of support available. Many people have built it. This is important as it will be my first time building a tube amp(other than a tube school project few years ago). From whatever I have come across so far, I felt that st-70 is a nice balance between complexity, cost and power. I might be wrong as I have not yet seen enough.

I am open for other designs that are not too complex. Availibity of boards will be an advantage, if otherwise its not a major obstacle.

Thanks!
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Old 1st September 2004, 08:47 PM   #8
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Quote:
Originally posted by percy
[B]Having heard good things about it on the forum I did seriously consider the PP-1C. I believe its rated at 4W
More like 14W
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Old 1st September 2004, 08:48 PM   #9
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If you're looking in the 'get my feet wet' area,
look at http://www.diytube.com.

They have an ST-35 board, and Eico HF-89 board.

Won't sound as good, but, both are well documented, and the ST-35 would be about half the price of a 70.
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Old 1st September 2004, 08:55 PM   #10
Wodgy is offline Wodgy  United States
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The PP-1C looks very interesting. For those who've built it, did you also use the suggested power supply? What mains transformer did you use? I'm having a devil of a time finding one with both 6.3V and 12V taps. Also, did you add any additional filtering (e.g. a choke)? Has anyone tried converting it to use KT88s for more power?
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