Have old ST 70 and fired it up using a variac...I cannot get the 435 volts on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins 3&4 of
the EL34s...I get about 355vdc....Upon closer examination I found multi value
can to be shot...Will changing the can fix problem or is power tranny on it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA

you want to replace ALL electrolytics on this amp (including the big can)
before doing anything else

the trannies on an ST-70 almost never go bad, unless you try to run them
with these old caps

use of a variac is not nearly as important as replacing any and all paper,
wax, and electrolytic caps, which ARE bad after 40 years

PS - one of the world authorities on this amp, Uncle Ned, frequents this
group, and sells everything you need to make an ST-70 like new at his
website he

http://www.triodeelectronics.com


cheers!

cowboy


"Rimfire1022" wrote in message
...
Have old ST 70 and fired it up using a variac...I cannot get the 435 volts
on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins 3&4
of
the EL34s...I get about 355vdc....Upon closer examination I found multi
value
can to be shot...Will changing the can fix problem or is power tranny on
it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA

in article , Rimfire1022 at
wrote on 1/30/05 12:14 PM:

Have old ST 70 and fired it up using a variac...I cannot get the 435 volts on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins 3&4 of
the EL34s...I get about 355vdc....Upon closer examination I found multi value
can to be shot...Will changing the can fix problem or is power tranny on it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA


Sounds like the first section of the filter can is open, which makes this
circuit behave as a choke input filter. Bypass the 1st section with a 20
MFD cap and watch the voltages return to normal, or higher!

If too high, place a 100 ohm 20W resistor between the PS H/V center tap and
ground.

- Jon

The PT is probably as good as it ever was, although it is way too small
for both channels, and also has a primary for more like 110-115 than
today's 120-125 VAC.

Just for fun you might try reforming the cap, but probably even if you
succeed it will die shortly. It should be replaced.

You have to ask yourself if it's worth the effort to rebuild the amp
to stock spec when stock spec was not that great even in its day.

Many ST70 mods exist. Most include a larger PT with more core and a
winding for higher primary voltage.

the original cloth wire transformer Dynaco power transformer is fine, just
use a variac to operate the amp at 115-117 and all will be well once the
caps are all replaced


wrote in message
oups.com...
The PT is probably as good as it ever was, although it is way too small
for both channels, and also has a primary for more like 110-115 than
today's 120-125 VAC.

Just for fun you might try reforming the cap, but probably even if you
succeed it will die shortly. It should be replaced.

You have to ask yourself if it's worth the effort to rebuild the amp
to stock spec when stock spec was not that great even in its day.

Many ST70 mods exist. Most include a larger PT with more core and a
winding for higher primary voltage.

If you use a Variac long term not only will you generate a lot of heat
and waste power, you will kill the Variac. They are not continuous
duty equipment. If you want lower voltage on a continuous basis you
need a transformer or an autoformer with permanent copper connections.

The power transformer in the ST70 is "fine" if you are satisfied with
bad sound and short life. Even at the original input voltage, the
original power transformer runs at the upper limit of design
temperature if the stock bias level is set. If the amplifier is
operated continuously or every day it will fail in a couple of years.
They are as common as they are today because sentimental owners kept
them after they failed or upgraded shortly after buying them, and
because the major agent of disappearance for Mcintosh and Marantz
equipment-Asian buyers-never bothered with Dynacos.

Replacing the tube rectifier with solid state diodes will reduce load
on the power transformer. It will affect power tube life but not
significantly because the stock bias level will kill the emission of
most imported tubes before anything else. Turning down the quiescent
bias current-by turning up the bias voltage-will allow the tubes and
transformer to live longer, but distortion at high signal level will go
up (although with efficient speakers you won't notice typically). The
stock power supply is high impedance, because the power transformer is
operating at high copper losses.

Of course there are a lot of people who are stupid, and will deny
this. They may not be of low IQ but they have 1)no legitimate
electronic background, 2) have gained what little knowledge they have
from sources with an unrealistic emotional viewpoint, or 3) are
interested in selling a hoard of equipment at inflated prices to
gullible hobbyists. The facts, however, are there for those who want
them, in the form of numerous articles in Audio Amateur magazine and in
an article written for Stereophile in 1966 (!) by Ed Dell. And in many
other places.

If you want to operate the stock Stereo 70 circuit as designed, you
will need a bigger power transformer. Several are available to fit the
original chassis holes. If you wish to use the original transformer the
circuit must be altered to let it live.

"Rimfire1022" wrote in message
...
Have old ST 70 and fired it up using a variac...I cannot get the 435 volts
on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins 3&4
of
the EL34s...I get about 355vdc....Upon closer examination I found multi
value
can to be shot...Will changing the can fix problem or is power tranny on
it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA

Listen to Cowboy. ST-70s are very popular amps and run for many years
without problems. It sounds like " is the one with an
emotional agenda to grind out. I would ignore his post and listen to those
who are more objective.
Cordially,
west

"Replacing the tube rectifier with solid state diodes"

Huh?

if you are going to do that, why not just replace those pesky EL34's with
transistors while you are at it?

geez, are you some kind of solid-state sock puppet?

(here is a clue about tube audio and rectifiers, think about the greatest
sounding amplifiers ever made, even guitar amps, they almost ALL used tube
rectifiers)

wake up, son!


wrote in message
ups.com...
If you use a Variac long term not only will you generate a lot of heat
and waste power, you will kill the Variac. They are not continuous
duty equipment. If you want lower voltage on a continuous basis you
need a transformer or an autoformer with permanent copper connections.

The power transformer in the ST70 is "fine" if you are satisfied with
bad sound and short life. Even at the original input voltage, the
original power transformer runs at the upper limit of design
temperature if the stock bias level is set. If the amplifier is
operated continuously or every day it will fail in a couple of years.
They are as common as they are today because sentimental owners kept
them after they failed or upgraded shortly after buying them, and
because the major agent of disappearance for Mcintosh and Marantz
equipment-Asian buyers-never bothered with Dynacos.

Replacing the tube rectifier with solid state diodes will reduce load
on the power transformer. It will affect power tube life but not
significantly because the stock bias level will kill the emission of
most imported tubes before anything else. Turning down the quiescent
bias current-by turning up the bias voltage-will allow the tubes and
transformer to live longer, but distortion at high signal level will go
up (although with efficient speakers you won't notice typically). The
stock power supply is high impedance, because the power transformer is
operating at high copper losses.

Of course there are a lot of people who are stupid, and will deny
this. They may not be of low IQ but they have 1)no legitimate
electronic background, 2) have gained what little knowledge they have
from sources with an unrealistic emotional viewpoint, or 3) are
interested in selling a hoard of equipment at inflated prices to
gullible hobbyists. The facts, however, are there for those who want
them, in the form of numerous articles in Audio Amateur magazine and in
an article written for Stereophile in 1966 (!) by Ed Dell. And in many
other places.

If you want to operate the stock Stereo 70 circuit as designed, you
will need a bigger power transformer. Several are available to fit the
original chassis holes. If you wish to use the original transformer the
circuit must be altered to let it live.

Good evening all
I find the previous posts interesting only because I've been using my
st-70's for the last 5 years with no problems at all. I have
ocassionally run them for 6 to 7 hours straight with very little heat
build up in the power tranny. I guess I'm just lucky.
Regards
Mike M

On Mon, 31 Jan 2005 04:05:10 GMT, mike
wrote:

Good evening all
I find the previous posts interesting only because I've been using my
st-70's for the last 5 years with no problems at all. I have
ocassionally run them for 6 to 7 hours straight with very little heat
build up in the power tranny. I guess I'm just lucky.

IME Dyna Stereo 70 power transformers run hotter than anyone
would want, but rarely fail. One larger bolt-in replacement
is the power transformer from an Eico Stereo 70, strangely.

Uncie Ned could probably fix you up with something modern and
a lot better, though. Life's short.

Chris Hornbeck
"Don't be foolish, like the others." _Lola Montes_, 1955

Yes, I have an emotional issue, I don't like abject stupidity.

The ST70 in stone stock form will run 'satisfactorily' (i.e. as well
as it did when new) for awhile if its components are still in good
order. As the previous poster indicated, the electrolytics are usually
bad at this stage but exact replacements are available. However,
summarily replacing all the other caps is usually not necessary if you
are willing to test them for leakage. Replacing them with stock values
and types probably wouldn't hurt but the "collector value" would
suffer. If you are a collector or want the unit for nostalgia or
occasional "period correct listening" that's fine. However, the stock
ST70 is a poor amplifier for music reproduction purposes as compared to
many other amplifiers, vintage or modern. And it will go through tubes
at a rate I would not accept, and eventually the power transformer will
fail from time and temperature. ("Eventually" all transformers will,
but some will last two years and some a hundred or more...)

As far as tube rectifiers go, there is no doubt a fine amplifier can
be built with tube rectifiers. However the idea that all the good ones
have them is abject horse****. The best sounding McIntosh tube amp is
the MC40 or 240 in the judgment of about everyone, and it has a solid
state rectifier. In the case of the ST70 the goal of conversion is to
unload the overloaded power transformer from the rectifier heater draw.


I'm sure there are people who like the stock ST70 just as is but that
doesn't make it very satisfactory for people who want less alteration
of the signal and more reliability and longevity.

The can cap is no doubt history but an original ST70 needs more than that to
work properly. Do a search for ST-70 on the below link. There should be tons of
info.

http://www.audioasylum.com/forums/dynaco/bbs.html

I would agree with the opinion that many if not most of the universally
agreed upon "great" tube amps do use silicon instead of valve rectification.
After years of really wanting to like tube rectifiers in tube amps, I have
finally started just listening to the difference and I cannot deny the
truth - fast recovery silicon does a better job, is more reliable, cheaper,
and most importantly it sounds better, at least for my tastes. If, on the
other hand, you like mushy bass, less punch and less power, valve
rectification might be the right choice.

There are cases in some lower power (EL84 or similar type amps) where a 5AR4
does a fine job and there is no real benefit to converting to silicon, but
if you want to make the best of higher power tube amps using EL34s, KT88's,
6L6's, etc., then my opinion is that silicon is best.

As far as the ST-70 goes, it's definitely on the low end for audio quality
tube amps, but even stock it's a lot better than most silicon. The iron
is good when not asked to perform at the upper edge of it's capability, as
in the ST-70. I gutted mine and used an all triode design by Kevin Kennedy
and it's now at about 18 wpc with the 5AR4 intact and it is a fine amp, and
very reliable.

If you want much better quality than the ST-70 for less money, the following
are examples I have obtained at a relative low cost on e-bay:

Sherwood S5000-II - An integrated 7868 based amp with SS rectification,
kicks ass!

Heathkit AA-121 and/or AA-50 monoblocks - Power amp with EL34's, SS
rectification and huge iron - similar to the Dyna ST70 with pentode front
end, but far,far better sound and lots more power. My all time favorite
next to the Citation II.

Various Fisher & HH Scott amps using 7591's or 7868's which sound great.
These tubes are now in current production, NOS 7868's are not that hard to
get either.

All of these amps will need new caps and perhaps some resistors to sound up
to par, but since they have virtually no "vintage collectable" value, why
not? Once recapped and tweaked they all compare favorably to current
production "boutique" tube models easily costing ten times more. Enjoy the
music for less $$$ and have fun with the refurb process.

Finally, I think that one example of greatness using SS rectification was
left out of the previous post: The HK Citation II. For my money the best
tube power amp ever made.

Just my opinions.

Kevin Killebrew
Austin, Texas


As bad as the stock ST-70 is, it is still an upgrade over most silicon, and
there are many far better tube amps to be gotten cheap on e-bay or elsewhere
wrote in message
ups.com...
Yes, I have an emotional issue, I don't like abject stupidity.

The ST70 in stone stock form will run 'satisfactorily' (i.e. as well
as it did when new) for awhile if its components are still in good
order. As the previous poster indicated, the electrolytics are usually
bad at this stage but exact replacements are available. However,
summarily replacing all the other caps is usually not necessary if you
are willing to test them for leakage. Replacing them with stock values
and types probably wouldn't hurt but the "collector value" would
suffer. If you are a collector or want the unit for nostalgia or
occasional "period correct listening" that's fine. However, the stock
ST70 is a poor amplifier for music reproduction purposes as compared to
many other amplifiers, vintage or modern. And it will go through tubes
at a rate I would not accept, and eventually the power transformer will
fail from time and temperature. ("Eventually" all transformers will,
but some will last two years and some a hundred or more...)

As far as tube rectifiers go, there is no doubt a fine amplifier can
be built with tube rectifiers. However the idea that all the good ones
have them is abject horse****. The best sounding McIntosh tube amp is
the MC40 or 240 in the judgment of about everyone, and it has a solid
state rectifier. In the case of the ST70 the goal of conversion is to
unload the overloaded power transformer from the rectifier heater draw.


I'm sure there are people who like the stock ST70 just as is but that
doesn't make it very satisfactory for people who want less alteration
of the signal and more reliability and longevity.

The ST70 can be made into a pretty good amplifier if one is so
inclined, and at one time was available at a very right price. Many of
the Fishers, Sherwoods, Scotts, et al, are capable of reasonable
performance if set up properly but they are often a bitch to work on
for the inexperienced, whereas the Dynaco offers some room to work. I
parted out my share and-then-some of them for guitar amp repairs and
upgrades, it doesn't keep me awake at night.

I agree the HK Citation II is a good amplifier by any standard, but
it's not a better amp than several modern high end units. I would put
the Nestorovics, and VTL/Manleys with the later opt ahead of them as
well as some DIY designs with UTC and Acrosound or Peerless outputs. I
would put it as equal with any of the Quicksilvers, c-j's or most any
ARC. I would also say "there's always a better amp possible" if you
have the determination and money. Better OPTs could be built than
anything to date if one had the wherewithal to do so, including the
20-20 Peerlesses...and tube amp ultimateness comes down in very large
part to how big, heavy and elaborate you are willing to make the power
supply and how much time and build cost you are willing to put in
transformers. The tubes, even new ones, are not the limiting factor.

Get a new FP cap ASAP. Low voltages are a surefire indication that
your amplifiers power supply is drawing some current and the
capacitor is about to go.

Once the 'engine room' is in order, then you can tune the rest
of the amplifier

Rimfire1022 wrote in article
...
Have old ST 70 and fired it up using a variac...I cannot get the 435
volts on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins
3&4 of
the EL34s...I get about 355vdc....Upon closer examination I found multi
value
can to be shot...Will changing the can fix problem or is power tranny on
it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA

wrote:
Yes, I have an emotional issue, I don't like abject stupidity.


Well then perhaps you should keep quiet.

wrote:
wrote:
Yes, I have an emotional issue, I don't like abject stupidity.


Well then perhaps you should keep quiet.

A Google search reveals you to be a doofus. You ask dumb questions in
an apparent effort to avoid the simplest research.

wrote:
wrote:
wrote:
Yes, I have an emotional issue, I don't like abject stupidity.


Well then perhaps you should keep quiet.

A Google search reveals you to be a doofus. You ask dumb questions
in
an apparent effort to avoid the simplest research.

For what it's worth, I need to apologize. I was in a foul mood the
other day when I posted that and unfortunately took it out on you. I
can't stand the fact that people take advantage of the anonymity that
internet groups provide to spew all sorts of things that they wouldn't
dare do in person...and now I've gone and done it. Please accept my
sincere apology.

I disagree.

I recently ran into this same problem. Replacing the rectifier tube (which
has presumably gone gassy) with a pair of solid state diodes brought the B+
right back up to where it should be, and it sounds great. And I'm running
original caps. As I understand it, electrolytics like to be charged
frequently; it keeps the dialectric formed.

About the only significant mod I've done on this system was to completely
bypass the tone controls. I found I was loosing highs there.

/kenw


Jon Yaeger wrote:

in article , Rimfire1022 at
wrote on 1/30/05 12:14 PM:

Have old ST 70 and fired it up using a variac...I cannot get the 435 volts on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins 3&4 of
the EL34s...I get about 355vdc....Upon closer examination I found multi value
can to be shot...Will changing the can fix problem or is power tranny on it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA


Sounds like the first section of the filter can is open, which makes this
circuit behave as a choke input filter. Bypass the 1st section with a 20
MFD cap and watch the voltages return to normal, or higher!

If too high, place a 100 ohm 20W resistor between the PS H/V center tap and
ground.

- Jon

Ken Wallewein
K&M Systems Integration
Phone (403)274-7848
Fax (403)275-4535

www.kmsi.net

in article , at
wrote on 3/6/05 8:05 PM:

I disagree.

I recently ran into this same problem. Replacing the rectifier tube (which
has presumably gone gassy) with a pair of solid state diodes brought the B+
right back up to where it should be, and it sounds great. And I'm running
original caps. As I understand it, electrolytics like to be charged
frequently; it keeps the dialectric formed.

About the only significant mod I've done on this system was to completely
bypass the tone controls. I found I was loosing highs there.

/kenw


Jon Yaeger wrote:

in article , Rimfire1022 at
wrote on 1/30/05 12:14 PM:

Have old ST 70 and fired it up using a variac...I cannot get the 435 volts
on
pins 2 or 8 on the GZ34...Also I cannot get the 410 or 415 vdc on pins 3&4
of
the EL34s...I get about 355vdc....Upon closer examination I found multi
value
can to be shot...Will changing the can fix problem or is power tranny on
it's
way out...Transformer looks fine,wires are pliable,nothing burned,no bad
smell,etc...Just want to make sure power tranny is OK...
TIA


Sounds like the first section of the filter can is open, which makes this
circuit behave as a choke input filter. Bypass the 1st section with a 20
MFD cap and watch the voltages return to normal, or higher!

If too high, place a 100 ohm 20W resistor between the PS H/V center tap and
ground.

- Jon

Ken Wallewein
K&M Systems Integration
Phone (403)274-7848
Fax (403)275-4535

www.kmsi.net


Ken,

I believe the OP was referring to a Dynaco ST-70 and it sounds like you're
talking about an Eico ST-70, since the former has no tone controls. So much
for agreeing or disagreeing.

Even though you bought your B+ voltages back up by using SS rectification,
your ST-70 could still have a bad cap. The GZ34 typically drops about 17V
across it; add the fact that line voltages have generally increased since
the Eico was made, means that you may have successfully worked around, but
not solved, the original problem. It's an easy thing to put a 20 MFD across
the 1st section and see if the B+ voltages rise significantly . . .

I've got several mods that I've done the EICO ST-70s to make them sound
better. I'd be happy to share them with you and you can further agree or
disagree as you see fit.

;-)

Jon

The question about solid state rectifiers and tube rectifiers is an
interesting one and I have experimented by running SS diodes in parallel to
the tube rectifier. Voltages rise to a solid state configuration, of course,
but what is interesting is that if I remove the tube rectifier, the unit
definitely does not sound as clear as with the tube in. (Actually, if you do
this, it is fun to watch the expression of a knowledgeable onlookers face
when you grab the rectifier tube and pull it out while the unit is running!)
I most defeinitely prefer the tube rectifier, although when paralleling the
two devices you achieve the best of both technologies.

Stu

in article qMPWd.57330$EL5.12098@trnddc05, audiodir at
wrote on 3/6/05 10:20 PM:

The question about solid state rectifiers and tube rectifiers is an
interesting one and I have experimented by running SS diodes in parallel to
the tube rectifier. Voltages rise to a solid state configuration, of course,
but what is interesting is that if I remove the tube rectifier, the unit
definitely does not sound as clear as with the tube in. (Actually, if you do
this, it is fun to watch the expression of a knowledgeable onlookers face
when you grab the rectifier tube and pull it out while the unit is running!)
I most defeinitely prefer the tube rectifier, although when paralleling the
two devices you achieve the best of both technologies.

Stu


Stu,

Does the combo of SS & tube rectification sound better than tube
rectification by itself, or when you say the best of both technologies do
you mean lower voltage drop?

What would explain the improvement? Have you changed the impedance of the
supply?? Very strange.

Jon

wrote

Please. There's a key, and rather obvious, difference.
Rectifiers are not
on the signal path. I.e., the sound does not go through them.

You mean signal, not sound, of course.

"Signal path" is misleading. A power supply, controlled by a signal
from the previous stage, generates the signal for the next stage.
It's a point often made, I know. More obviously true in the sound of
SS amplifiers I suppose.

IF one accepts that stability of power supply voltage is a good
thing, the
low forward resistance of solid state rectifiers does that
measurably
better.

For class A, "poor" regulation can be a good thing. Measurably
better depends on what you are measuring.

But I do agree that a good, quiet SS supply is more tightly
regulated and more efficient. A good SS supply is not made by simply
swapping out the valve rectifier though.

cheers, Ian

"audiodir" wrote in message
news:qMPWd.57330$EL5.12098@trnddc05...
The question about solid state rectifiers and tube rectifiers is an
interesting one and I have experimented by running SS diodes in parallel
to
the tube rectifier. Voltages rise to a solid state configuration, of
course,
but what is interesting is that if I remove the tube rectifier, the unit
definitely does not sound as clear as with the tube in. (Actually, if you
do
this, it is fun to watch the expression of a knowledgeable onlookers face
when you grab the rectifier tube and pull it out while the unit is
running!)
I most defeinitely prefer the tube rectifier, although when paralleling
the
two devices you achieve the best of both technologies.

Stu

Hi Stu -
Perhaps the difference is due to an *increase* of B+ when you yank out the
rect. tube - after all, it's fils do draw current, and, perhaps, having a
pulsating high voltage from the diodes applied to one end of the fil.
winding does something nasty... I dunno. But.
he toob shouldn't be doing much, when bypassed bi diodes - just for giggles,
why not measure if it pulls any current *ever*, and, if it does, at what
degree(s) of the cycle? could you post what happens on a 'scope? I know
theory & practice could be a bit ... different.
-dim

I can detect no change in the rectified signal when hooked up to my 'scope,
although I do admit it is a smaller and slower Hitachi model capable of only
35 mHz. I do use Hexfreds and other fast recovery ss diodes, BTW. One would
assume that when using the FR diodes or hexfreds that the tube would be
totally out of the loop, so to speak, speed wise. Ripple on the filter caps
seem to be unchanged: B+ voltage is unchanged when the tube is pulled.
Obviously the ss devices are carrying the greater part of the rectified
load.
Change in sound is that when the ss diodes are inserted, bass and dynamics
are improved. With the tube rectifier inserted I get better dimensionality
and fine detail in the mid and upper frequencies. When both are used I get
the characteristics of both devices. Perhaps the tube rectifier acts as a
snubber, but it certainly doesn't sound like any snubber circuit I have
tried.
I would wish the more knowledgeable list members try this and analyze the
results, as this is a bit beyond my understanding.


Stu

audiodir wrote:

The question about solid state rectifiers and tube rectifiers is an
interesting one and I have experimented by running SS diodes in parallel to
the tube rectifier. Voltages rise to a solid state configuration, of course,
but what is interesting is that if I remove the tube rectifier, the unit
definitely does not sound as clear as with the tube in. (Actually, if you do
this, it is fun to watch the expression of a knowledgeable onlookers face
when you grab the rectifier tube and pull it out while the unit is running!)
I most defeinitely prefer the tube rectifier, although when paralleling the
two devices you achieve the best of both technologies.

Stu

If one just connects silicon diodes onto the GZ34 socket,
the B+ will be considerably higher than with the tube only.

One might expect the plate dissipation to rise dangerously,
because for each 20v rise in Ea, Ia will increase about 16 mA
for each tube.

The sound may indeed change for the worse.

However, you are saying that whil you have the amp set up
with Si diodes, and *then* you remove the GZ34, the
sound gets worse.

I cannot see any electronic explanation for that, since the Si diodes
have a small fraction of the 'on' resistance of the Si diodes,
and strapping SS and tube rectifiers together will make very little
difference to where just SS diodes alone are used.

What happens if the value of the C components of the PS are
raised about tenfold?

SS diodes *must* be used then lest we exceed the peak charging
current allowed in a GZ34.

But I expect the SS diodes + seriously good values of C
compared the toy values in the original ST70 will
only improve the music.

Try 50 ohms in series with each SS diode.
What is the sound like then?

Patrick Turner.

audiodir wrote:

I can detect no change in the rectified signal when hooked up to my 'scope,
although I do admit it is a smaller and slower Hitachi model capable of only
35 mHz. I do use Hexfreds and other fast recovery ss diodes, BTW. One would
assume that when using the FR diodes or hexfreds that the tube would be
totally out of the loop, so to speak, speed wise. Ripple on the filter caps
seem to be unchanged: B+ voltage is unchanged when the tube is pulled.
Obviously the ss devices are carrying the greater part of the rectified
load.
Change in sound is that when the ss diodes are inserted, bass and dynamics
are improved. With the tube rectifier inserted I get better dimensionality
and fine detail in the mid and upper frequencies. When both are used I get
the characteristics of both devices. Perhaps the tube rectifier acts as a
snubber, but it certainly doesn't sound like any snubber circuit I have
tried.
I would wish the more knowledgeable list members try this and analyze the
results, as this is a bit beyond my understanding.

Your observations are purely subjective at the end of the day.

I don't ever seek to deny anyone their subjective opinions on sound quality
with tube amps, or variations of circuitry within them, even though there is
little
technical measurement to suppport the sunjective opinions.

I would be one of the technocrats here, and I probably won't
bother to try to analyse your idea any more than you have
because I know I won't find anything that would logically explain what you hear.

I have been using SS diodes in all the amps I sell for years,
but I also use humungous values for filter caps.
I have had no adverse comments from customers so far......

Patrick Turner.



Stu

said:

(here is a clue about tube audio and rectifiers, think about the greatest
sounding amplifiers ever made, even guitar amps, they almost ALL used tube
rectifiers)

Another statement with null value, even if true.


Well, there *is* something to be said for a sagging B+ when plucking
the strings ;-)

--
Sander de Waal
" SOA of a KT88? Sufficient. "

Oh yeah, I forgot to mention that I beef up the PS caps also so that ought
to really favor the ss rectifiers.

Stu

"Sander deWaal" wrote in message
...
said:

(here is a clue about tube audio and rectifiers, think about the
greatest
sounding amplifiers ever made, even guitar amps, they almost ALL used
tube
rectifiers)

Another statement with null value, even if true.


Well, there *is* something to be said for a sagging B+ when plucking
the strings ;-)


Yeah, but then output toobs set up to run on the verge of runaway sound
"just right", & the marshes which happen to hit the plates of el34's with
750V have plenty of character...
--
Sander de Waal
" SOA of a KT88? Sufficient. "

Hi Jon
seems you are the most knowledgeable on this subject.
If I may ask you one?
The 100 ohm 20 watt ressitor. What kind of voltage drop is there from
that.
Both my MK3's are reading 20 to 25% higher on DC voltage. It killed the
quad cap on one yesterday. Will this help to bring voltage back to normal?
Thanks
Mike M

Mike,

I've got my Dyna ST-70 packed up in storage and hermetically sealed, so I
can't easily jump out the resistor and see what the voltage drop is.

Are all of your voltages too high (filament and B+) or just B+?

Basically, I plugged in my amp and experimented until I found the right
value that dropped the B+ to the original specs as shown in the owner's
manual. I put the resistor between ground and the center tap for the B+,
IIRC. (That won't help you if the filament voltages are too high . . . )

If it's just the B+ that is too high, you can try a different tube
rectifier, which won't have the drawback of dissipating heat beneath the
chassis.

The Weber folks have a convenient chart on their site of the various
rectifier voltage drops. See: http://www.webervst.com/ccap.html For
example, the GZ34/5AR4 has a drop of 17V and a max. current of 225 ma. The
5R4 drops 67V and can handle 250ma (also check the 5V filament specs to be
sure that you're not taxing the tranny).

If all of your voltages are too high, you might be able to add an additional
transformer to buck part of the voltage. There are lots of references on
Google that you can dig up on that.

Good luck.

Jon





From: Mike Mueller
Organization: EarthLink Inc. -- http://www.EarthLink.net
Newsgroups: rec.audio.tubes
Date: Wed, 09 Mar 2005 00:57:36 GMT
Subject: ST 70 Question ?

Hi Jon
seems you are the most knowledgeable on this subject.
If I may ask you one?
The 100 ohm 20 watt ressitor. What kind of voltage drop is there from
that.
Both my MK3's are reading 20 to 25% higher on DC voltage. It killed the
quad cap on one yesterday. Will this help to bring voltage back to normal?
Thanks
Mike M