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Dynaco ST70 KIT - your opinion please

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9th January 2013, 08:37 PM

#1

zimmer64

diyAudio Member

Join Date: Sep 2012

Location: Switzerland

Dynaco ST70 KIT - your opinion please

Hi,

I am new to the hobby and in the
process of trying out and learning about various amps. So far I have tried
a few kits (gainclone and class D). Now it is tube time :-)

By
coincidence I came across this Dynaco ST 70 kit:
Triode
USA Dynaco ST-70 Tube DIY Amp Kit

I want to try it with various
speakers, DIY Fostex FX120, DIY Jordan JX92s, DIY Wild Burro Betsy,
commercial Vroemen Diva superiore. Frontend will be Bottlehead Quickie, 47
Treasure CD and vinyl.

What are your thoughts and recommendations
about this? Any input is highly
appreciated.

Thanks

Michael

___________________
Michael's Project
Page

9th January 2013, 10:08 PM

#2

indianajo

diyAudio Member

Join Date: Jan 2010

Location: Jeffersonville, Indiana USA

I've got an original dynaco ST70 with the
7199 driver tubes. 1961 build with the fabric transformer insulation. It
is a rugged beast with tens of thousands of hours on it, and decent
sounding for 1961. I recently replaced the electrolytic caps (again, 3rd
time) and the output tubes (third time) and it is back up to 35 W/ch and
sounding original on my Peavey SP2-XT speakers. I've recently beaten the
quallity of sound of the ST70 with 7199 drivers with a transistor amp
modified for better cold bias current control, as does my road weary
CS800s, so the 1% distortion Dynaco claimed for their circuit is about
right. The triodeelectronics kit uses EF86 copy driver tubes, which might
sound better than the unavailable 7199. The original design transformer
had extra filament capacity to run two outboard two tube preamps, so the
design has plenty of capability to run 4 driver tubes instead of two. The
design has lots of aftermarket support, for example kevinkr has designed a
4 tube triode driver board sold by classicvalve.ca that he claims has a
lot less distortion. My ST70 has been modified to derive the grid bias
voltage from silicon rectifiers, not the original selenium one. I find the
aftermarket high capacity cap boards a bit silly, as on classical music in
my living room I'm running about 1.5 Vpp output max, so the semi-original
power supply with 39 22 22 22 filter caps produces B+ just fine with no
sags. This stays within the parameters of the 5AR4 rectifier tube, which I
am still using due to the rumors about "cathode stripping" with silicon
instant on B+ voltage. I've used the ST70 as a PA amp in a church
occasionally, but didn't try to derive the high power out of it that you
would want out of a bass guitar amp, for example. 35 W/ch works out to 17
Vac on 8 ohm, so the ST70 has plenty of power with fresh tubes and B+ cap
for my 101 db@1W speakers. It was fine with my previous speakers, Peavey
T-300 HF projectors, and KLH23 two ways. If you try to use acoustic
suspension speakers like the old AR1's, the power out might be a little
light for drum or cannon hits. I found over the years that if the ST70
sounded polite instead of real, the B+ voltage was sagging due to tired
main caps, or the rectifier or output tubes were tired. I got about 8
years at 4 hr/night on the 5AR4, about 10 years on sets of 6CA7's, about
10 years per B+ cap.
One huge advantage of the tube design, mine has
been struck by a lightning induced surge and only shorted across the power
switch, and burned up the .01 uf turn off pop eliminator cap installed by
me. No damage to the transformers. Try that with transistors without MOS
supressors on the input. With the weather changing to be more extreme, you
may soon have significant lightning storms in Europe.
I've bought
parts form triodeelectronics.com, and find them helpful and nice to deal
with. When they shipped me a mono volume pot instead of the stereo I
ordered, they shipped the replacement to me at no charge.
So anyway,
have fun.

__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2
ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's,
Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 9th January 2013 at 10:26
PM.

9th January 2013, 10:46 PM

#3

cotdt

diyAudio Member

Join Date: Oct 2005

Do you have any goals in mind as to what
sound you're after? You can't go wrong with a Dynaco ST70 or its variants
as they are good amps. Personally, I prefer the EL84-based PP amps, and
single-ended triode amps based on 45, 2A3, or 300B. They are very
transparent and musically involving. Just giving you some more ideas.

9th January 2013, 10:55 PM

#4

bear

expert in tautology

diyAudio Member

Join Date: Apr 2002

Location: New York State USA

Strap the outputs for triode.
Add more
capacitance to the second main B+ filter, like 4x or more.
Reduce the
cathode resistors by a factor of 10x. (you need a DVM to set the bias, it
will be 10x lower than the stock value, put a sticker on the chassis to
show that. This will improve the bass.

You might consider buying
the parts and getting an "aftermarket" driver board that uses something
OTHER than the 7199 or the Russian equivalent for the input stage. That is
to say the kit without the driver board...

These are the basic
things that can be done to improve the basic amp.

I'd go for the
Teflon wire option, IF you can figure out how to strip it without nicking
the wire and/or cutting it accidentally (unless they are precut and
stripped, doubt that).

__________________
_-_-bear
http://www.bearlabs.com/
-- Btw, I don't actually know anything, FYI -- [...2SJ74
Toshiba bogus asian parts - beware!
]

Last edited by bear; 9th January 2013 at 11:12
PM.

9th January 2013, 11:42 PM

#5

Bob Richards

diyAudio Member

Join Date: Feb 2011

Location: Portland Oregon, USA

This kit sounds like fun for more experienced
people who like to try many variations. A good learning tool for a newbie.
I use an EF86 in a guitar amp I built, and the measured gain is about 240,
which is way more than I needed. In this amp you would need to either
attenuate it's output or use rather large amounts of negative feedback.
More feedback isn't necessarily better sounding in tube amps.

I
researched, designed and built a similar amp using 6SN7's for the front
end and phase splitter, (EL34 outputs) and use only about 12dB of negative
feedback, and it sounds great. The hard core tube guys I know all seem to
think low or no feedback, and triode mode output stage is the best
sounding. I tend to agree. Triode mode output means lower source impedance
driving the speaker, so tighter control of the speaker, and less
un-calibration of a passive crossover, when feedback in minimal. More
feedback makes the issue of phase margin harder to predict (tendency to
oscillate or ring). Some people say high feedback makes an amp sound
"constipated"... Check out my amp (the "musicbox") if interested at
Bob's Website

http://www.spiritone.com/~rob_369
My front end tube
is the earlier equivalent of the 6SN7 (I forget the number). I thought the
older tube shape with the grid cap was cool looking, so used it instead.
Although not shown on my website, I since replaced that front end tube
with a regular 6SN7 to reduce hum, and replaced the plate resostor with a
constant current source for better power supply hum rejection, better
linearity, and more gain. Don't bother with silver wire or Teflon
insulation wire. Speaker wire that costs more than $50 is a joke too. Use
14 or 16 AWG wire for the speakers. It's the connectors that could be a
weak link. I use gold alloy plated banana connectors (gold doesn't oxidize
over time), and I solder the wires to these connectors rather than using
the set screw that comes loose over time.

10th January 2013, 12:07 AM

#6

bear

expert in tautology

diyAudio Member

Join Date: Apr 2002

Location: New York State USA

May I disagree?

Teflon insulated wire
is a good idea for the inside wiring of the amp, IF you can properly strip
it. There are a few methods that work.

For one thing, the simplest
point, it doesn't melt and stink when you solder it.

_-_-

__________________
_-_-bear
http://www.bearlabs.com/
-- Btw, I don't actually know anything, FYI -- [...2SJ74
Toshiba bogus asian parts - beware!
]

10th January 2013, 01:15 AM

#7

indianajo

diyAudio Member

Join Date: Jan 2010

Location: Jeffersonville, Indiana USA

Quote:

Originally Posted by bear

You might consider buying the parts
and getting an "aftermarket" driver board that uses something OTHER
than the 7199 or the Russian equivalent for the input stage. That is
to say the kit without the driver
board...

Triode doesn't sell a 7199
driver board, because nobody makes them anymore. They sell a pentode board
with the EF86 layout. Others sell straight triode triode driver boards. I
bought an "EF86" from triode for my organ, it was actually a Russian 6 pi
something tube.
Teflon insulated wire is great for newbies, keeps you
from burning the insulation off when you move your eyes to the print or
something and wave the soldering iron around. I'm not that new, but used a
some 600V electrical tape in my H100 organ recap to repair damage to PVC
insulation. I'm buying bulk teflon insulated wire from a suplus house,
much cheaper than new silver plated stuff. Not legal in Euorpe,
though.
I find the bass of my stock unit entirely adequate, actually.
There are a million variations, the one I like the most adds two pots and
splits the bias adjustment for the push-pull tubes and allows you to set
them balanced again after they wear some, unevenly. Another thing I have
done is run the B+ to a blank pin of the preamp power plug so I can
monitor B+ while it is playing without taking it apart and tipping it on
its back. That is now I know I don't have a B+ problem now at the
listening levels I am using with 39-22-22-22 caps. Even when the cannon
goes off in 1812 overture.

__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2
ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's,
Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 10th January 2013 at 01:22
AM.

10th January 2013, 01:58 AM

#8

bear

expert in tautology

diyAudio Member

Join Date: Apr 2002

Location: New York State USA

ok, a different driver circut, non
pentode.

afaik
almost all teflon insulated wire is silver plated copper.
there is
some botique wire made now that is copper with teflon
insulation.

_-_-

__________________
_-_-bear
http://www.bearlabs.com/
-- Btw, I don't actually know anything, FYI -- [...2SJ74
Toshiba bogus asian parts - beware!
]

10th January 2013, 02:19 AM

#9

Eli Duttman

diyAudio Member

Join Date: Apr 2004

Location: Monroe Township, NJ

Some comments, If I may. The Triode
Electronics driver board gets populated with 2X EF86s and a single twin
triode. That twin triode may be an ECC99, 12BH7, or 12AU7. I advise
installing an ECC99, for its high gm. High gm in designs employing
GNFB, like the ST70, provides resistance against slew limiting associated
with the HF error correction signal.

As the ST70 uses "fixed" bias
and an instant on, SS rectified, C- supply, even the slightest delay in B+
rise is sufficient to protect the O/P tubes against cathode stripping. A
NTC inrush current limiting thermistor provides the short delay needed, if
either SS diodes or a directly heated vacuum rectifier are/is employed in
the B+ circuitry.

IMO, it's perfectly acceptable to SS rectify ST70
B+. You turn the excessively "tall" B+ rail from a liability into an
asset, by employing combination bias. "Stand" the cathodes of each O/P
tube pair on a 100 Ω/470 μF. network. A very convenient "idle" current
test point is available and the original single bias adjustment for each
channel is quite satisfactory. Matching requirements are reasonably
relaxed. As is always the case, closeness in gm is a must
for each pair. However, small differences in cathode current are
acceptable.

An additional benefit of SS rectifying the B+ is the
cooler running power trafo obtained by holding the value of the 1st filter
cap. down to say 10 μF. Replace the overworked C354 choke with a Hammond
159T.

__________________
Eli D.

10th January 2013, 02:36 AM

#10

rongon

diyAudio Member

Join Date: Jul 2009

Location: Across the river from Rip's big old tree...

Hi Eli -- I'm interested in adding those
thermistors to my mutant amp in an ST70 chassis. I'm using SS diode
rectification, getting about +460V. What specs to look for in the NTC
thermistors? What resistance spec? Small in value, like 100R or less? Or
more like 1k? Thanks for the idea.

Oh, and where do they go in the
circuit for slow ramp up of B+? Perhaps in the center tap of the power
transformer?

Last edited by rongon; 10th January 2013 at 02:41
AM.

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