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[ dmfraser ]

VTA
upgrade board

knowledge base for the classic Dynaco
ST70

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VTA
upgrade board

by
Brinkman
» Tue Dec 04, 2007 5:55 pm

Hey everyone.

I'm perfoming a complicated (for me)
upgrade of the PCB for my ST70. I'm using the VTA board and trying to follow the
directions, but I'm getting confused with them:
1) The instructions that
came with the VTA board say to connect the 2nd & 3rd section of the quad cap
with a piece of wire and to replace the 22k resistor between the 1st & 2nd
sections with a 2.2K resistor...
BUT:
2) the directions at the VTA installation page say:

"the 6.8K and 22K resistor on the quad cap can be removed and thrown away.
The 2nd, 3rd, and 4th section of the quad cap can be connected together with a
piece of wire. The choke will remain connected to the 1st and 2nd section of the
quad cap." No mention of the 2.2K resistor.
HOWEVER:
3) the accompanying
photos at the same website show a
connection not between the 2nd & 3rd section of the quad cap, or even the
2nd, 3rd & 4th sections, but the 1st, 4th
& 3rd sections. No mention of the 2.2K resistor here, either.

Here's a helpful pictoral
from the same VTA site of the last quad cap wiring scheme I've mentioned, this
time including a 2.2K resistor between the 3rd & 4th sections.

Can
someone who's installed the VTA driver board help me out here?

Best,

Benjamin

Brinkman

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by
TomMcNally
» Tue Dec 04, 2007 8:20 pm

Hi Benjamin -

I don't have any personal experience
with the boards,
but having looked it all over, I'd go with the photographs

and ignore the text and pictorial.

You're basically just paralleling
the capacitor sections
for a higher total value.

... tom

TomMcNally

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by
Brinkman
» Tue Dec 04, 2007 9:10 pm

TomMcNally wrote:Hi Benjamin -

I don't have any
personal experience with the boards,
but having looked it all over, I'd go
with the photographs
and ignore the text and pictorial.

You're
basically just paralleling the capacitor sections
for a higher total
value.

... tom

I have no idea how old the
photos are and if my board is a revision of it. Basically, what I'm trying to do
is find out how the hell the VTA board wants the quad cap to be reconfigured so
I can make sure the wiring of my SDS cap board mirrors it.
I already
modified the ST70 with the SDS cap upgrade, so I am no longer using the quad
cap, but to install the new VTA driver board, I'm going to need my can cap
replacement to be wired in a similar fashion.
I'm probably over-thinking
this.

Best,
Benjamin

Brinkman

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by
mesherm
» Tue Dec 04, 2007 10:32 pm

Remove both resistors and replace them with pieces of solid
wire or just solder a wire across each resistor. This connects the 2nd 3rd and
4th sections together so you have essentially two filter caps. The VA and PI
have their own filter networks on the VTA board so you don't need the 3rd and
4th filter section. By connecting them to the 2nd section you increase the
capacitance at the B+ feed to the power tubes where it does the most good. You
could just leave the 3rd and 4th unconnected but why waste good usefull
capacitance. The wires from the 3rd and 4th sections of the SDS that went to the
original driver board can be removed. You will also need to unsolder the bias
tap wire from the SDS board and reconnect it on the VTA board. You can remove
all the bias wires from the two bias pots as they are not used. The 4 bias
trimpots on the VTA board will be used instead. You will need to add an extra
two bias check tap points also and add a 10 ohm cathode resistor at each EL34
socket for a total of 4.
I have used the VTA driver in 4 amps so far and
have been quite pleased.
If you have any other wiring questions just email
me directly.

Mike's N-1 Rule: When looking for N number
of components to finish a job, you have a 95% chance of only finding N-1 of
them.

mesherm

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by
Brinkman
» Wed Dec 05, 2007 9:53 am

Excellent! Thanks for clearing up the quad cap issue. I am
also pleased my own conclusions regarding the disconnection of the bias portion
of the SDS board were echoed in your post. Am feeling confident.

If all
goes well today, my updated Forte crossovers will arrive, my new driver board
will be installed, and I'll have a backlog of tubes to roll for the driver board
(NOS trios of 12bh7s, 12ax7s, & 12au7s & a trio of JJ ECC82s) and the
output tubes, some Winged =C= EL34s.

Will email you with a negative
feedback question.

Best,
Benjamin

Brinkman

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Re:
Correct VTA quad cap hookup

by
Bob01605
» Thu Dec 06, 2007 2:32 pm

mesherm wrote:Remove both resistors and replace them with
pieces of solid wire or just solder a wire across each resistor. This connects
the 2nd 3rd and 4th sections together so you have essentially two filter caps.
The VA and PI have their own filter networks on the VTA board so you don't
need the 3rd and 4th filter section. By connecting them to the 2nd section you
increase the capacitance at the B+ feed to the power tubes where it does the
most good. You could just leave the 3rd and 4th unconnected but why waste good
usefull capacitance. The wires from the 3rd and 4th sections of the SDS that
went to the original driver board can be removed. You will also need to
unsolder the bias tap wire from the SDS board and reconnect it on the VTA
board. You can remove all the bias wires from the two bias pots as they are
not used. The 4 bias trimpots on the VTA board will be used instead. You will
need to add an extra two bias check tap points also and add a 10 ohm cathode
resistor at each EL34 socket for a total of 4.
I have used the VTA driver
in 4 amps so far and have been quite pleased.
If you have any other wiring
questions just email me directly.

Although what you
say above will work > this is not exactly what Roy Mottram had in mind for
his board regardless of what his instructions say. I have Emailed him directly
and below is what he recommends.

Section 1 of the quad cap - TWO
connections
1. A wire to pin 8 of the rectifier tube
2. One of the choke
leads

Section 2 of the quad cap (that faces the back of the amp) - FOUR
connections
1. The second choke lead
2. Red wire from LEFT output
transformer
3. Red wire from RIGHT output transformer
4. One end of a
jumper

Section 3 of the quad cap - TWO connections
1. The OTHER end
of the jumper wire
2. One end of the 2.2 K ohm 1 watt resistor

Section 4 of the quad cap - TWO connections
1. The OTHER end of the
2.2 K ohm 1 watt resistor
2. One end of a wire that brings the B+ to the
driver board

The reason for the jumper is to tie together sections 2 and
3 and add the capacitance of both sections together. On a STOCK quad cap 20 + 20
= 40 Mfd. The reason for the 2.2K resistor is to drop the B+ by 20 volts to
about 420-440 volts which is the voltage that Roy Mottram has specified as the
correct operating B+ for his board. Check link to photo below ..

http://i9.photobucket.com/albums/a78/Bob01605/VTAQuadcapsetup.jpg

Bob Latino

Bob Latino Custom Dynaco amplifiers

http://www.tubes4hifi.com/bob.htm

Bob01605

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by
mesherm
» Thu Dec 06, 2007 5:03 pm

The reason for the 2.2K resistor is to drop the B+ by 20 volts to about
420-440 volts which is the voltage that Roy Mottram has specified as the
correct operating B+ for his board.

The voltage after
the choke on a stock ST70 with all four EL34s running at 40-50 ma each should
not be over 440 volts unless the PT has been replaced or the rectifier tube
replaced with diodes. Solder a 2.4k 1w resistor and a bare wire in parallel with
the 22k and check the voltage at normal operation. If it is too high then just
snip the bare wire out and the 22k+2.4k combo wll give a nominal 2.2k.

Mike's N-1 Rule: When looking for N number
of components to finish a job, you have a 95% chance of only finding N-1 of
them.

mesherm

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by
Brinkman
» Thu Dec 06, 2007 6:43 pm

With all due respect, Roy Mottram's instructions are
incorrect; insofar as he has mislabelled the quad cap sections.

I have
found a high-resolution scan of the ST-70
wiring pictorial. This image is courtesy of Bob Latino from an Audio Asylum
thread.
This is the exact same pictorial that I used to construct my recent-production
ST-70 kit (albeit with color & a selenium rectifier), and I assume it is
consistent with the original-production ST-70 kit wiring pictorial.

Look
at the quad cap. Clockwise, 6 o'clock, 9 o'clock, 12 o'clock & 3 o'clock are
labelled Sections 1,2,3 & 4 respectively. I was able to install my SDS cap
board with this as a reference (without screwing up), so I assume this labelling
is historically correct.

Perhaps there's other ST-70s with a wiring
pictorial that is different. If this is the case, than I imagine that one of
these may have been Roy Mottram's reference and thus the source of the
discrepancy between the VTA driver board instructions and my point of reference,
which is the above linked wiring pictorial.

I will email Kevin at
Dynakit (whom I purchased my kit from) to see if he can verify the authenticity
of the pictorial he is supplying.

Perhaps the section designation of
the quad cap is somewhat arbitrary and can be wired several ways. I wouldn't
know. What I do know, however, is that the integrity of the ST-70 as a teaching
tool is compromised when basic labelling of components are not consistent.
Accuracy is everything, especially with potentially lethal voltages present.

FWIW, with respect to the wiring pictorial I am familiar with, the
corresponding cap sections on the SDS cap board are, from left to right, 3, 4, 1
& 2.

Hopefully this can explain why I was so confused at the start
of this thread.

Everyone's input has been integral to sorting out this
mess, please add more!

Best,
Benjamin

Brinkman

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Stock
Dynaco and VTA quad cap connections

by
Bob01605
» Sat Dec 08, 2007 5:19 am

Much of the confusion about the quad cap connections are
because Dynaco (stupidly) marked them in a way that IMHO leads to confusion.
Dynaco marked them starting with "1" in the six o'clock position and then went
around clockwise - BUT - the ACTUAL FIRST section of the quad cap is the section
that receives current from the rectifier tube and that is section "2" on the
Dynaco pictorial. Dynaco went around clockwise with their numbering but the
current flow is in the opposite direction - COUNTERCLOCKWISE

The "FIRST"
section is the one at 9 o'clock that receives power from pin 8 of the rectifier
tube

The "SECOND" section is at 6 o'clock

The "THIRD" section is
at 3 o'clock

The "FOURTH" section is at 12 o'clock

Check the
link below which shows a comparison between the two numbering systems

http://i9.photobucket.com/albums/a78/Bob01605/Quadcapdifferences.jpg

Bob Latino

Bob Latino Custom Dynaco amplifiers

http://www.tubes4hifi.com/bob.htm

Bob01605

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quad
caps and VTA mod

by
tubes4hifi
» Mon Dec 31, 2007 10:24 pm

yes, Bob is correct on this
also, note the problem
originates with the fact that the ORIGINAL quad cap and the new replacement
UPGRADED quad cap are oriented differently.
I'm always amazed that when
customers have a question they ask everyone else first instead of asking the
source.
Also I must apologize that some of the photos and instructions on my
website are 3-4 years old, I've been selling this mod for 20 years now, and
there have been a few variations!
Roy www.tubes4hifi.com/st70.htm

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by
skidave
» Thu Jan 03, 2008 11:28 am

What are the circuit differences between the VTA board and
Shannon’s DIY Tube board?

skidave

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Re:
quad caps and VTA mod

by
Brinkman
» Thu Jan 03, 2008 6:24 pm

tubes4hifi wrote:I'm always amazed that when customers have
a question they ask everyone else first instead of asking the
source.

It's not that amazing,
really. I just didn't expect you to be familiar with the SDS cap board which I
was already using in lieu of the can capacitor your installation instructions
assume is in place.

I'm sure you'd agree message boards such as these,
which draw from the collective experience of a multitude of folks, are a lot
more useful a place to troubleshoot problems such as mine, as opposed to a
private back-and-forth on my behalf between both you and Triode Electronics.

I'll post a photo soon, detailing how to install your VTA board with the
SDS board in place as well. Both modifications are highly recommended, and I
look forward to trying out a DIYtube driver board in the near future. Perhaps on
a pair of Mk. IIIs...

Brinkman

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by
mesherm
» Fri Jan 04, 2008 2:29 pm

What are the circuit differences between the VTA board and Shannon’s DIY
Tube board?

Shannon's is made to pretty much drop into
an ST70 power supply wise. His PI stage uses a long tail pair with a constant
current source, LM234, as the phase inverter tail. He uses the stock ST70 bias
pots and a trim pot on board for each pair of output tubes to balance the idle
currents of the output tubes. Shannon's uses a 12AU7 type for VA and a 12BH7
type for PI duty

Roy's VTA has its own on board VA and PI power supply
network and only needs voltage from the output tube supply. His PI is a Schimdt
type long tail pair and has a trim pot per side to precisely match the phase
inverters output. There is a trimpot onboard for each output tube so only raw AC
bias voltage is needed. Roy's uses three 12AT7 types for both VA and PI duty.

If I were to upgrade an ST70 I would probably use (and have used)
Shannon's but if rolling my own ST70 clone I would probably use (and have used)
Roy's board.

Either one is an improvement over the stock circuit.

Mike's N-1 Rule: When looking for N number
of components to finish a job, you have a 95% chance of only finding N-1 of
them.

mesherm

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