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Understanding circuit in my Dynaco MKIII's

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#16

06-20-2012, 05:18 PM

dcgillespie

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Posts: 5,532

Simply installing EL34s should have
done no harm to your amplifier. The predecessor of the MK III (the MK II)
in fact used EL34s. It would likely have in fact sounded thin, as the
tubes would have been well over biased, producing too little quiescent
current flow. But no damage should have been done.

Now if one of
the tubes was bad and damaged some components, that is a different matter.
Check that the bias is correct with the original KT88s reinstalled, and go
from there. If there were no output tube arcs associated with trying the
EL34s (pops in the speakers, or flashes in the tubes), then it's doubtful
anything serious has happened, so just be systematic when you go through
it until the problem shows itself.

Dave

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#17

06-20-2012, 05:33 PM

Kegger

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Location: Michigan,warren

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To add more to Dave's assessment to
your circuit and questions.

Input sensitivity and output power are
NOT interrelated.

You can have 2 amps that both produce 40wpc but
one say .5v sens the other 1.5v.

What that means is 1 amp only
needs .5v for full power any more input the amp won't
get any
louder/make any more power it will just clip and distort.

The other
amp needs 1.5v input for it's full power, and any more input that amp
won't
get any louder/make any more power it will just clip and
distort.

What you will see with these 2 amps, 1 gets louder with
less turning up of the volume
from the preamp, so one may be loud say 2
on the volume knob, while the other would
get loud at say 9 on the
volume knob, but both would produce that same output power
just at
different levels on the volume knob from the preamp.

One is just
more sensitive to the input voltage being sent to it.

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#18

06-20-2012, 05:39 PM

Audioexcels

AK Member

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Posts: 100

Quote:

Originally Posted by dcgillespie

Simply installing EL34s should have
done no harm to your amplifier. The predecessor of the MK III (the
MK II) in fact used EL34s. It would likely have in fact sounded
thin, as the tubes would have been well over biased, producing too
little quiescent current flow. But no damage should have been
done.

Now if one of the tubes was bad and damaged some
components, that is a different matter. Check that the bias is
correct with the original KT88s reinstalled, and go from there. If
there were no output tube arcs associated with trying the EL34s
(pops in the speakers, or flashes in the tubes), then it's doubtful
anything serious has happened, so just be systematic when you go
through it until the problem shows itself.

Dave

One of the tubes was bad is my only guess, though it's hard to say. This is the one
that burned a little through the top, but it never lit up bright red and
no smoke or any other stuff came of this...it did get excessively hot to
the point that it literally cooked the decal/logo! No Arcing or abnormal
behavior besides what I just described. These are brand new tubes, too
(blame it on China!...only a joke of course though that's what I tried in
there). The other tube didn't do anything abnormal at all! Volume is
relatively decent, but the bass output is very weak and the sound coming
through really does sound out of phase due to it being thin/not full and
robust as it should be (like the other amp).

One of my KT88's, in
that same socket that hit the EL34, has its black plastic bass off (thing
just came off of the tube maybe from me pulling it out being too hot???).
Output tubes are getting extremely hot, even just turned on for a very
short bit.

Bias is at .500V's=50ma's.

Last edited by Audioexcels; 06-20-2012 at 05:50
PM.

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#19

06-20-2012, 05:52 PM

Kegger

Anything can be S "MODed"

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Location: Michigan,warren

Posts: 11,333

Your other question, how can the VTA
board supply so much more gain
an drive then your 2 triode driver when
the VTA only adds 1 more there.

Your circuit has 1 Gain stage, the
first triode all of the gain is done there
the second triode has no
gain, actually slightly less than unity, but pretty
much whatever the
gain is in the first tube comes out the second "minus"
just a hair, so
most call it unity on that second stage/concertina splitters.

The
VTA board uses 1 triode for gain just like yours, then it uses a
splitter
type that uses 2 triodes and gets roughly half that gain of
the tubes used
and in that configuration whatever you send to the
second stage/triodes is
then multiplying it to get even more
gain.

So for a quick an dirty breakdown, say you get 50 of your
triode gain stage
and say the VTA does as well, yours is done, but say
you have the 12AT7's
in the second stage of the VTA and you get half
that 50 so 25, for every 1
volt you get from the first stage on the VTA
board it adds 25 to it, so take
whatever you get out of the first stage
an multiply it by 25, that's a LOT of
gain "possible" out the VTA
board, this is a gross oversimplification of how it
works and it will
take a lot of power supply voltage for the VTA to do any of
it but it
gets the basic circuit points across.

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#20

06-20-2012, 06:00 PM

Audioexcels

AK Member

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Posts: 100

Quote:

Originally Posted by Kegger

So for a quick an dirty
breakdown, say you get 50 of your triode gain stage
and say the
VTA does as well, yours is done, but say you have the 12AT7's
in
the second stage of the VTA and you get half that 50 so 25, for
every 1
volt you get from the first stage on the VTA board it
adds 25 to it, so take
whatever you get out of the first stage an
multiply it by 25, that's a LOT of
gain "possible" out the VTA
board, this is a gross oversimplification of how it
works and it
will take a lot of power supply voltage for the VTA to do any of

it but it gets the basic circuit points
across.

Thanks a ton. I really
appreciate Dave and your response. Would using a second tube be possible
in my design and would it affect the sound/circuit?

See above about
my amp issue. Not only is one of my amps not working correctly, but I
just pulled a KT88 out of the socket, and the very bottom of the tube
(black plastic) literally broke off in the tube socket. I can glue it
back on, but why would this or any tube break the plastic off (is it due
to pulling the tube while it's still excessively hot?).

Thanks!

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#21

06-20-2012, 06:08 PM

Kegger

Anything can be S "MODed"

Join Date: Apr 2005

Location: Michigan,warren

Posts: 11,333

I saw your post about your amp, yah
that sucks.
All I can say is check everything out, I have no
suggestions at this point.
(kinda hard for me to diagnose without being
there and checking it out)

As to adding to yours, yes it could be
done, but not enough room on your
board for all the extra circuitry, it
would turn yours basically into williamson
circuit and you'd need 2
more triodes so it would look something like this.

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#22

06-20-2012, 06:19 PM

Audioexcels

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Join Date: Jun 2011

Posts: 100

Looks like a great input stage,
that's for sure. But it would completely take away any point in having
the input stage that I have, well, I would no longer really have that
input stage and rather have this one.

I'm no Tube amp builder so
now I'm without sound except for my one amp. I will have to figure out
what to do. I may just dump these and all tube gear off the local
cliffs...it's such a headache with some odd-ball thing going wrong that
totally fails the entire device. I did this to my pre-dac
recently.

Time to invest in a digital multi-meter and suck it
up...and glue the bass of the KT88 tube so it's hopefully not completely
worthless shall I ever sell the amps.

Kegger, do you think with my
existing circuit that I should just do an ST70 w/EL34's and the power
output should be the same so as to not really hear any difference/point in
having the MK3's unless I like the KT88 sound over the EL34 sound, etc.?

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#23

06-20-2012, 06:26 PM

Kegger

Anything can be S "MODed"

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Location: Michigan,warren

Posts: 11,333

Quote:

Originally Posted by Audioexcels

Here are the words the
designer used regarding this circuit and the input part of
it:

"Without feedback, the amp preserves all the gain of the
input stage. A few triodes gave sufficient gain and maintained
linearity over 40V P-P signal, which both of the input stages
(voltage gain and a unity amp/splitter) have to maintain for
sufficient drive to the output stage to develop its rated power. The
selection of a suitable triode is further reduced by the limitation
of a single 9-pin socket. Many dual triodes would bias wrong for
this application. Ideally one would find a dissimilar 9-pin dual
triode. Converting to octal socket would have opened up a whole new
world, but that's more involved. Once that decision was made, I
chose the 12AT7 based on the bias point and gain, with the
additional requirement of a relatively low output impedance. I
checked a few tubes and only a slim fraction of them worked at the
40V P-P output level without severely clipping in this circuit with
250V supply for the first stage. You can adjust component values and
the gain will drop or you will have to bypass the cathode of the
first stage for a fraction extra gain. Bottom line, for a simple
circuit this is "on the edge".

I
basically agree with everything he said there,
(but there realy is
more options in 9pin tubes than octal though)

And his last line basically sums it up, (for a simple
circuit this is "on the edge").
(I agree, I've used/built the same
circuit with the 12at7, it works pretty good)

The 12at7 is the most
suited for this of any of the TWIN triodes, what this type
circuit
needs is GAIN in the first stage and DRIVE in the second stage, the 12at7

is decent at both but not the best at either, there are tubes with
more GAIN and
tubes with better DRIVE as well.

Gain is pretty
easy to see/determine, tubes with mu/gain of 20 like a 12au7 are an
low
gain tube, tubes with a gain of 100 like a 12ax7 are a high gain tube, an
neither
by it'self would be good at doing Both Gain and Drive in this
circuit, the 12ax7 would
be great for the gain portion but crap at the
drive/phase splitter portion with a 12au7
being great at the
driver/phase splitter portions but crap at the gain portions in
there.

A good tube here would be 12ax7 gain an 12au7
splitter/driver, well you know what, it
is possible get such a tube,
the 12dw7 is a 12ax7/12au7 in the same bottle there.

For a beefier version than a 12dw7 I've built this same
basic circuit using a 6JT8 which
is triode with gain of the 12ax7 but
half the plate resistance I use for the gain stage an
the second half
of the tube is a high transconductance(killer drive) pentode that what
I
do is strap it as an triode and use low loads to DRIVE the larger
outputs pretty darn well.
(probably about the best your going to get
for this type of circuit setup using 2 "triodes")

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#24

06-20-2012, 06:37 PM

Audioexcels

AK Member

Join Date: Jun 2011

Posts: 100

Quote:

Originally Posted by Kegger

A good tube here would be 12ax7
gain an 12au7 splitter/driver, well you know what, it
is possible
get such a tube, the 12dw7 is a 12ax7/12au7 in the same bottle
there.

For a beefier version than a 12dw7 I've built
this same basic circuit using a 6JT8 which
is triode with gain of
the 12ax7 but half the plate resistance I use for the gain stage
an
the second half of the tube is a high transconductance(killer
drive) pentode that what I
do is strap it as an triode and use
low loads to DRIVE the larger outputs pretty darn well.
(probably
about the best your going to get for this type of circuit setup
using 2 "triodes")

This sounds like
it can retain the fundamental circuit while adding the gain needed to
truly power the output tubes and open up the amp more. Am I correct in
saying this tube would not "compromise" the circuit in any way, or is it
going to be compromised somehow?

I wish I had the enthusiasm I
usually have, but after two recent tube related issues (my tubed pre-dac
is out of service and now my one amp is not working correctly), I wish I
had some cheap secondary amp to put in because now I do not have music to
listen to...sigh...

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#25

06-20-2012, 06:44 PM

Kegger

Anything can be S "MODed"

Join Date: Apr 2005

Location: Michigan,warren

Posts: 11,333

Quote:

Originally Posted by Audioexcels

Kegger, do you think with my
existing circuit that I should just do an ST70 w/EL34's and the
power output should be the same so as to not really hear any
difference/point in having the MK3's unless I like the KT88 sound
over the EL34 sound, etc.?

I would
just stick to the MK3's.. A little less than full power but there
you
get mono blocks and 2 beefier power supplies, I would look at the

things to do to lower voltage an with that driver stage then run
EL34's.
(in other-words set the amps up to best run EL34's)

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#26

06-20-2012, 06:47 PM

Kegger

Anything can be S "MODed"

Join Date: Apr 2005

Location: Michigan,warren

Posts: 11,333

Quote:

Originally Posted by Audioexcels

I wish I had the enthusiasm I
usually have, but after two recent tube related issues (my tubed
pre-dac is out of service and now my one amp is not working
correctly), I wish I had some cheap secondary amp to put in because
now I do not have music to listen
to...sigh...

Well that comes with
owning/running tubes quite often, and more so when you tinker with
them.
(which to me makes it more imperative to understand how they
work)

Sorry I can't be any more help then that, but without being
there and checking it out I have no
clue what may be wrong with your
particular amp. (Dave may have some ideas I can't think of)

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#27

06-20-2012, 06:56 PM

Kegger

Anything can be S "MODed"

Join Date: Apr 2005

Location: Michigan,warren

Posts: 11,333

Quote:

Originally Posted by Audioexcels

This sounds like it can retain the
fundamental circuit while adding the gain needed to truly power the
output tubes and open up the amp more. Am I correct in saying this
tube would not "compromise" the circuit in any way, or is it going
to be compromised somehow?

No it
would not compromise the circuit, it would let the circuit run basically
as best it could
while retaining the original circuit topology but
tweaked to run it better, it may sound better
to you as well in the
frequency response and lower distortion, but it is possible you could be

one who prefers the sound of the previous circuit though if it sounded
the way you liked it to.
(upto about half power or so that 12AT7 runs
good driving KT88's, should sound nice doing it)

Me I'm one who can
never leave "good enough" alone, an I wouldn't run the 12at7 here in
the
circuit it's in to drive these output tubes, I would be compelled
to make it run as best it could
and if keeping this type driver setup
then it would mean changing out the 12at7.. (That's me)

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#28

06-20-2012, 07:06 PM

Audioexcels

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Quote:

Originally Posted by Kegger

No it would not compromise the
circuit, it would let the circuit run basically as best it could

while retaining the original circuit topology but tweaked to run it
better, it may sound better
to you as well in the frequency
response and lower distortion, but it is possible you could be

one who prefers the sound of the previous circuit though if it
sounded the way you liked it to.
(upto about half power or so
that 12AT7 runs good driving KT88's, should sound nice doing
it)

Got any time on your hands and
want to have some fun playing around with these amps to see what you can
do with this circuit to keep the fundamental sound, but give it the
boost/kick to fully drive the KT88's?)

What's the best thing to deal with the voltage in
order to run the EL34's in this particular circuit/MK3 design or would it
be better to leave things alone and not bother running the EL34?

The ST70 I have is also a kit, all beefed up trannies, PS, etc. etc...so I
have that option if the power output using the existing circuit in the
MK3's does not yield any further volume output to be had with EL34's in
the modern kit based ST70.

Thanks a ton for your replies. Makes me
feel better though I guess I need to find/buy some cheap $20 garage sale
receiver or whatever to get me by whenever things like this come up.

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#29

06-20-2012, 07:16 PM

Kegger

Anything can be S "MODed"

Join Date: Apr 2005

Location: Michigan,warren

Posts: 11,333

I'll think about ways I would go
about lowering voltage on the MK3.
(as for now I'm backed up with work
to take any more on right now)

For ST70's I prefer the Low Gain VTA
based front end as a really good
setup for them and would be my
suggestion for the way to go on them.

-------
Not scolding
but just a headsup for you or anyone else along the lines.

In the
future it's best to send PM's to someone on work related or any
money
changes hands type scenarios to keep within the AK rules there.

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#30

06-20-2012, 07:47 PM

Audioexcels

AK Member

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Quote:

Originally Posted by Kegger

I'll think about ways I would go
about lowering voltage on the MK3.
(as for now I'm backed up with
work to take any more on right now)

For ST70's I prefer the
Low Gain VTA based front end as a really good
setup for them and
would be my suggestion for the way to go on
them.

VTA is out of the question.
It is far from acceptable in terms of what I am after for sound quality.
I guess it would be a fine second amp, but I'd rather use a $20-$100 used
one if that was the purpose. I wish I never put those EL34's into the
MK3's. From what I am hearing, unless the tubes are being driven insanely
off the charts and I will not be getting the same output if I had them in
the ST70, is they are just fine volume wise. My question is this...would
I have this same level of volume with them in the ST70 kit w/newer
iron+larger capacitance, etc. OR is there some reason the volume would be
louder still in the MKIII's?

One thing for sure, the KT88 is
getting a bit more "full/clean/better sounding" volume than the EL34, but
the MK3 amp is also biased/set for the KT88 tube and not the EL34.

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