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denny9167

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denny9167

Posts: 130
Join date: 2011-05-09
Age: 47
Location: Texas

Post n°1

PCB
Question

by denny9167
on Tue Dec 06, 2011 5:33 pm

I am considering a rebuild of the 120W amplifier found in one of the many
catalogs Dynaco put out years ago,and I am wanting to put everything on one
board,and was wanting to know if there are any drawbacks,in terms of stress,or
other considerations ex: heater traces,etc. would love the input,I have a pic of
the board below.

[img][/img]

Bob
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Post n°2

Re:
PCB Question

by Bob Latino on Tue Dec
06, 2011 6:46 pm

Can you post a
schematic of the circuit for this board ?

Bob

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Post n°3

Re:
PCB Question

by denny9167
on Wed Dec 07, 2011 6:40 am

Bob Latino wrote:Can
you post a schematic of the circuit for this board
?

Bob

Hi Bob,

Here is the
schematic for the board,let me know of any changes,your input would be greatly
appreciated.Thanks

Denny

[img][/img]

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Post n°4

Re:
PCB Question

by Bob Latino on Wed Dec
07, 2011 7:31 am

The driver
portion of that circuit is almost the same as an original Dynaco Mark III
circuit. Not just the use of a 6AN8 driver tube, but the resistor and capacitor
values are almost the same at various points in the circuit. A 6AN8 tube is a
pentode/triode design and (IMHO) there are better driver circuits out there. The
use of a pentode/triode driver front end driver circuit allows the use of just
ONE driver tube. If you use an all triode design front end you need a tube and a
half or three triode sections to do the job. Whoever designed this circuit is
basically using a Mark III front end mated to an output stage of their own
design.

Also, the power supply caps C7 - C12 according to the circuit are
all 47 uF @ 400 volt. Since C7 and C8 are in series and C9 and C10 are also in
series then this part of the circuit is good since you have an 800 volt rated
power supply up to this point. BUT - C11 and C12 are only 400 volt rated. The
problem is that running the Hammond secondaries of 400-0-400 you are going to
get a B+ in the range of 480 to 500 VDC which is well beyond the range of these
two (C11 and C12) 400 volt rated caps. This to me is the worst part of this
circuit and IMHO those 400 volt rated caps won't last long looking at 480 - 500
VDC. Also with a 23.5, 23.5, 47, 47 uF you have just 141 uF of DC power storage.
This is not enough IMHO to run a 120 watt mono amp and trying to feed 4 X 6550
tubes. IMHO before the amp ever reaches 120 watts it will run out of DC storage.
In comparison, the VTA M-125 monoblock, which is a similar amp running a single
channel parallel push- pull circuit, has 502 uF of DC power storage (with 550
volt or higher rated caps) which is more than 3 1/2 times what this circuit
provides.

Another thing is that the designer attempts to bias all four
output tubes with just ONE bias control. If you do this, all you need is just
one bad or mismatched output tube to throw the circuit out of
balance.

The one good thing I see is the use of the Weber WZ68 solid
state rectifier. This WZ68 can flow 450 milliamps which is really more than this
amp needs.

Below is the VTA M-125 amp circuit which I think is a better
120/125 watt parallel push-pull circuit
...

Bob

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Post n°5

Re:
PCB Question

by tubes4hifi
on Thu Dec 08, 2011 10:55 am

gasp . . . !!! you want to run 4 6550 tubes with an 6AN8? I'll 2nd and
3rd everything Bob mentioned,
but on the other hand, if you want to do that
from scratch, go for it, that's what DIY is all about!

denny9167

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Post n°6

Re:
PCB Question

by denny9167
on Thu Dec 08, 2011 4:30 pm

I really appreciate the input,I was out of electronics for several years
only to get interested again just within the last few,so I'm still on a learning
curve here! If I'm not mistaken doesn't Audio Research use a pair of 6H30's in
the driver stages of some of their amps? I know it is a very hardy tube from
what I understand.

I will take these suggestions and re-work the curcuit.
Thanks again!!

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Post n°7

Re:
PCB Question

by tubes4hifi
on Thu Dec 08, 2011 4:42 pm

just as a reminder, if you want to save a ton of time and work and KNOW
that you're gonna wind up with a really great amp,
consider buying one or two
of our M125 (or M70) monoblock kits.

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Post n°8

Re:
PCB Question

by denny9167
on Sat Dec 10, 2011 2:33 pm

Could this be used as a good driver stage for the quad's?[img][/img]

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