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Old 28th February 2008, 12:42 PM   #1
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Default Upgrading my Audio Research VT 130 SE Tube amp

Hi Everyone,
I was hoping to get some advice on upgrading my VT 130SE. It has 4x 6550 , 1x 12BH7 driver
tube and 2x 6922 input tubes per channel. It also has 4x large mallory 800 Uf 450V Capacitors
screw type. I was told by Audio Research to upgrade my MIT Multicap coupler caps it has 2 per channel, to the infinicap signature ($30.00 each .68 Uf) or to the Infinicap M2A ( $50.00 each .68 Uf), WOW these are more expensive than the Mundorf Silver Gold and I am not so sure they are better than the Mondorf ?? it also has a fixed power cord in which is the original.
Any advice is appreciated.

Thanks

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Old 18th March 2008, 05:16 AM   #2
prakit is offline prakit  Thailand
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Hi,
I also have this amp. I've upgraded those 4 coupling caps to InfiniCap. Not sure if they are the signature serie since I did this 3-4 yrs ago. I could discern some improvement on clarity and extension but it also sounds less fluid in the mid. I'd try film and foil type if I were you. They sound much smoother. I have tried the older Infini cap in other applications and had the same conclusion on it's character.

I have done quite a few modifications to the amp. I suggest you replace the film cap at the regulator output with some film/foil cap. I used Hovland. The improvement was much greater than replacing the coupling caps for me. The amp sounded fuller and more dynamic.

The biggest improvement I achieved was from implementing constant current sources to those input and driver stages. The dynamic and bass slam and extension is among the best I've heard from a tube amp. You can check out the schematic of the VT100 on this.
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Old 18th March 2008, 10:59 AM   #3
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Hi Prakit,
Thanks for the information. You know, I was debating about the the film and foil. Solen makes
some real nice "Film and Foil" caps. I used them in a crossover for a pair of Linn Keilidh , in the
tweeter section and they were beautiful, and I tried many, these were crystal clear at a fraction of the price , I have the main distributor near buy. Also there has been a lot of talk about using
The Mundorf "Silver in Oil" What are your thoughts?

Prakit,
I would like to replace the film cap at the regulator output but I am not sure which one it is ?
If you could point it out that would be great, I will definitly try it. if you could explain any other
mods in detail it would be a ton of help to me especially the section on adding
"constant current sources to those input and driver stages"

Thanks
Slate
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Old 18th March 2008, 12:03 PM   #4
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Quote:
Originally posted by Serratoga
I would like to replace the film cap at the regulator output but I am not sure which one it is ?

It seems to be C38 on my circuit. ARC don't seem to be too happy about loading this regulator with capacitance that's why they use a snubber. A different type of capacitor might need the snubber readjusted. Careful as it may start oscillating.

I'd also improve upon C8, C9, the caps filtering the fixed bias.

The VT100 circuitr seems altogether more attractive but obviously means a major surgery.
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Old 18th March 2008, 12:56 PM   #5
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Hi analog,

"It seems to be C38 on my circuit. ARC don't seem to be too happy about loading this regulator with capacitance that's why they use a snubber. A different type of capacitor might need the snubber readjusted. Careful as it may start oscillating."

I checked the schematics on my ARC VT 130 SE and the c38 is a (Rel Cap PPMF 2.0 Uf @450v)
Analog are saying replacing this cap with a (Solen Film & Foil 2.0 Uf @ 400 v) may cause it to
oscillate ??

Thanks
Slate
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Old 18th March 2008, 01:27 PM   #6
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Highly unlikely. I only noticed that ARC are cautious with the capacitive loading, so a very different cap (ceramic?) may indeed lead to instability.

Otoh, why would a Solen "film & foil" be better than a Relcap PPMF? What film? And what foil?
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Old 18th March 2008, 01:34 PM   #7
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Anolog,

I thought that Film & Foil was superior to Metallized Polypropylene???

Thanks
Slate
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Old 18th March 2008, 04:04 PM   #8
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If it is teflon film and tin foil then it is very likely better

Unless you discuss specific series it is difficult to make a general statement.

I would simply go for Multicap RTX which will largely preserve the sound character and bring in some improvement but then again i never go crazy with caps.

Not to mention that opamp (and associated capacitors) responsible for dc balance is probably a bigger liability then some coupling caps.
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Old 18th March 2008, 09:18 PM   #9
prakit is offline prakit  Thailand
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Hi Slate,

I replaced C38 with a 0.47uF Hovland and later I added a 2.0uF Hovland across C38 and R71. The latter was soldered underneath the PCB. The improvement from doing this was very similar to when I added a choke to the raw supply of my diy preamp. The sound became fuller and palpable. Hard to explain.

About adding the constant current sources(CCS), besides the input stage it's quite easy to do. If you want to try this, I suggest you do it with the second stage and the driver stage. The R13A+B are two 5.11K resistors connected in series. One end of these are connected to cathode of V2, the other to ground. There's about 70V drop across these two resistors. I replaced the one next to the cathode with a CCS configured for about 7mA of current sinking for each triode.

For the driver stage, R23 and R24 were replaced with ccs configured for about 5.5mA of current sinking. I added a 15K in series with this ccs so that the voltage drop across the ccs inself is about 20V and no heatsink is needed.

I used LM334(low voltage current source) for the second stage but used IXCP 10M45S(rated 400V) for the driver stage. The improvement was shocking.

I didn't have experience with the oil imprognated caps but have a plan to try Mundorf Supreme on my newly acquired M100s.



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Old 19th March 2008, 07:19 PM   #10
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[QUOTE]Originally posted by prakit
[B]Hi Slate,

I replaced C38 with a 0.47uF Hovland and later I added a 2.0uF Hovland across C38 and R71. The latter was soldered underneath the PCB. The improvement from doing this was very similar to when I added a choke to the raw supply of my diy preamp. The sound became fuller and palpable. Hard to explain.

Prakit,
when you added the .47 Uf hovland on top and the 2.0 Uf on the under side was because of space constriction? Is it OK to increase the values from 2.0 Uf to 2.47 Uf ???
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