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Old 17th October 2004, 11:38 AM   #1
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Location: The Netherlands
Question Audio Research CA-50 tube rolling

Hi There,

After reading the tube rolling experience in Joe's Tube Lore at audioreview . com in FAQ-sections I bought 4 nos-siemens
E88CC's.
When removing te sidepanels of my AR CA-50 integrated i dicovered 2 adjustment poinst (low/high voltage).
My question is: can i just plug in the new Nos tubes instead of
the Sovtek 6922's or do i have to adjust the voltages to the
Nos Siemens pre- and driver tubes?

The nos-tubes work fine but i want to make shure
that the NOS-tubes will have a normal tubelife instead of wearing
them out in short time due to faulty adjustments. The sovteks i'm using are five years of age now, same as the 6550 output tubes, but they are still good condition!

Does anybody have a schematic of the CA-50 available?
I did find a schematic of the VT100 which design is supposed to
be of the same design principle of the CA-50...

Anybody else having a CA-50 with some experience in
tube rolling with this amp?

greetings,
Sander
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Old 17th October 2004, 12:17 PM   #2
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Assuming that your circuit is indeed similar to VT100 there should be no need for adjustment. The input, driver and feedback stages are CC sourced and tube variations have no effect. The cathode follower (also CC sourced) requires -60v at the grids but tube variations will also have no effect.
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Old 17th October 2004, 12:38 PM   #3
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Hi Frank,

Thanks for your reply! All stages in the CA-50 have Constant Current sources indeed you are right...what i did not know is
that this degins principle would make tube rolling easier this
way.
Anyway, after 5 years of use i think it would be a good idea to check if these voltages are still correct: a schematic of this amp
would make this easier so i'm still looking for this..Anybody?

It would also be nice to hear some experience from
other CA-50 users doing some tube rolling!

Greetings,
Sander
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Old 17th October 2004, 02:33 PM   #4
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Default Ca 50

Hello,

from my experience it is not easy to change the 6922, because they must be highly matched. The first two diffamps are DC-coupled and only little variation leads to malfunction.

As I know, at ARC they use a CA 50 to test the matched tubes.
The CCSīs are stable and donīt need trimming.

Reinhard
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Old 17th October 2004, 08:35 PM   #5
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Default Re: Ca 50

Hi Reinhard,

The 4 Siemens E88CC I bought some time ago (at www.machmat.com) are matched pairs, is the matching that much critical or does Audio Research do matching another kind
of way?
Could you please explain your post in more detail, what
do you mean with "the first two diffamps" for example
and what malfunction are we talking about, the tube itself or
the rest of the amp?

Quote:
Originally posted by reinhard
Hello,

from my experience it is not easy to change the 6922, because they must be highly matched. The first two diffamps are DC-coupled and only little variation leads to malfunction.

As I know, at ARC they use a CA 50 to test the matched tubes.
The CCSīs are stable and donīt need trimming.

Reinhard
Thanks,
Sander
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Old 20th October 2004, 06:03 AM   #6
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Default Ca 50

Hello,

only little variations in plate voltages of the first diff amp brings the amp out of symmetry. Power is reduced and the amp is sounding very non dynamic.

Have a look at the plates of the second diff amp. When plate voltages are mor than say 30 Volts from each other, this effect occurs.

Hope you understand what I mean.

Reinhard From Germany
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Old 20th October 2004, 12:06 PM   #7
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Hi Reinhard,

Thanks for your info...
At the moment the amp is doing fine... with its original ARC
tubes (5 years old) and still sounding very well, so i think the
original tubes are still ok.
Recently i measured the bias voltages of the 6550's and after
five years the are still very well adjusted (left 64mV right 66mV)
Now, after five years of use i trimmed them at 65mV for each channel. This was rather simple to do for a non-technical guy like me.
Measuring the plate voltages of the first en second differtial amp
is another thing...I do have some knowledge about lay-out en how tube amps work and build myself a kit (Vanderveen Williamson kind of tubeamp) before but i'm not an expert so this is over my league...the CA-50 has a hybrid pramp/driver and that don't make things easier!

Although the NOS Siemens E88CC's are matched, i don't think they will be matched in the critital way ARC does, so i think i won't use them for the CA-50...too risky, don't want any troubles
with it...
I tried them by just pluging them in but the soundstage became more lifeless althoug somewhat more smoother as with the original sovteks, the difference wasn't that much. (Maybe ARC's claim that the amp is designed around the sovtek 6922 is true after all?)
I think this is what you mean with your reply saying that different plate voltages will make the sound very non-dynamic... In the kit amp i made they were playing fine by the way and each channel had one E88CC as an upgrade for the standard 6n1p for phase inversion and driving the output stage. I think i will use these Siemens for a headphone amp design i'm planning to make in the near future for my DT880 headset....

Greetings from The Netherlands,
Sander
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