VTA ST-120 power cord question

ARC looks good on paper .......but sounds ...well to me lost in space...an a old rusted out 50 year old Dynaco, eico, Harman Kardon, McIntosh.......can sound much better...all Stock even!
gofig...........justsaying


Thanks for the site Bob.........

I know Roger builds his amps a bit differently than everyone else.  Here's a quote from the original RM-200 review:

"In describing his circuit to me, Modjeski advocated the use of low screen voltages and higher-than-usual plate voltages in the output stage. The RM-200 runs the screens and plates of its carefully matched set of four output tubes at 350V and 750V, respectively."

He is able to achieve somewhere around 10,000 hours of life from his tubes due to running them at half of max dissipation.

He gets similar results from his RM-10 which gets 35 watts from a pair of EL84s.  

I swear I'm not trying to sound like an ad for his gear but I've done a lot of research on his amps and his tubes and I'm convinced the guy knows his stuff.  

Just my opinion but 750 VDC on the plates of KT88's is way too much DC voltage on the plates. Today's Russian, Chinese and Slovak Republic made KT88's won't stand up to that much voltage on the plates for very long. As a comparison, the VTA ST-120 and the VTA M-125 monoblocks run about 490 to 515 VDC on the plates of the KT88's or KT120's. Doing it this way gives not only good output tube life but also good life on the resistors and capacitors in the amp. I am not surprised that poster zx had 4 different Modjeski designed amps flame out in his home ...

Bob

I wonder about zx's experience as it differs from the majority of positive reviews of Roger's amps.  His RM-9 and RM-200 were both rated Class A from Stereophile.  Roger has been in the business for a long time and is well respected for his products and his tubes.

As for the higher voltage on the plates here's what Roger has to say on the subject:

"High plates are permissable when the screens are low. The reliability is excellent and tubes are very happy under these conditions. Because the plate voltage is high, the cathode current is low and the saturation is very low, thus the good efficiency."
http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=61139

"...It's not the plates that mind the HV, it's the screens."
http://www.audioasylum.com/cgi/t.mpl?f=tubes&m=61248

"I was testing a RM-200 MK II the other day and took some measurements for you. You can get 14 amps peak from the 1 ohm tap. That corresponds to 196 watts peak or 98 watts on an RMS basis. You can connect a one ohm load to the 2 ohm tap and get 17 amps which is 289 watts peak or 144 watts on an RMS basis.

These are quite high figures if one notes that the 8 ohm tap on a typical 100 watt amp delivers 5 amps peak and the 4 ohm tap delivers 7 amps. An important aspect of the RM-200 is its ability to drive difficult loads which often drop to 1/2 or 1/3 of the nominal value."
http://www.audiocircle.com/index.php?topic=48373.msg435819#msg435819

"Over the last 10 years I have written several new applications to get higher power out of some common tubes that I thought could do more. If I were employed by Sylvania in the 1950's-70's I would have done the same thing. There are an infinite number of applications that can be written for any given tube. They involve variations in B+ voltage, current, grid drive, screen voltage and load impedance. The data books list only a few to give amp designers some ideas. Sadly, that's as far as many designers go. David Manley was a stickler for applications believing that the only ones allowed were the ones in the data books and they must be adhered to the letter. Sadly, he took the ambitious 560 Volt Ultra-linear application from MOV as reliable  and made some amplifiers that damaged a lot of tubes. Not even MOV could make tubes that consistantly held up in that application. To this day there are no tubes that hold up well at those voltages on the screen.

I research my applications very carefully with dissipation studies on all elements. And, although I have a replacement tube business, I do not want to waste valuable tubes (or your money) on poor applications.

One application that caused a lot of comment in the Asylum was my getting 35 watts out of a pair of 6BQ5/EL84's. They said it couldn't be done, yet it is done and has proved reliable over the past 10 years in the field."
http://www.audiocircle.com/index.php?topic=23867.msg211356#msg211356

(Regarding his RM-10 amp...)
"This is my favorite amplifier and there is nothing else in the industry that equals it in power output for a pair of EL-84/6BQ5. It consumes less electricity (60 watts at idle and 65 at normal levels) than any other 70 watt amp. It costs less to re-tube than any other 70 watt amp. Being handwired it sounds better, is easier to substitute components (the latest super cap or resistor), gain and damping can be easily altered by a changing a few resistors, almost anyone can fix one and the failure rate is very, very low. The output tubes are fused giving them multiple chances to fart (ionization, spark, or blast a lint short). Amps without output fuses usually destroy one or both output tubes if there is an intermittent overload caused by a tube fart or user error. Bias is very easy to set and can go years without need for adjustment. I just check mine twice a year after the tubes are burned in.

The amp has been on the market for 10 years with excellent reviews and user satisfaction. I hear universally "Gee I didn't think that little amp could drive my speakers so well both in volume and woofer control".  This is because the power is quite real and the damping is higher than most small amplifiers. This is not your typical EL-84 amplifier. The RM-10 has twice the RMS power, more than twice the peak power due to the higher voltages employed, runs cooler and the tubes last 3-5 times longer.  This was achieved with a new application of the tube that was tested for 5 years in my system driving electrostats and dynamic speakers.

The driver tubes are good for more than 10,000 hours, about 10 years. Outputs last 5,000 hours at normal playing into matched loads below clipping, the way we should play our music. The amp also has an unique mono switching system that does not have the typical drawbacks of conventional mono configurations that involve adding resistors to the output terminals. I find it odd that Dynaco would put a mono switch on the ST-70 (the same could be accomplished with a Y connector) but provide no such switch for the output terminals. You have to get some power resistors and Y together the L & R outputs. Makes no sense at all."
http://www.audiocircle.com/index.php?topic=66336.msg610218#msg610218


As far as zx's experience I can't say for certain but perhaps he used poor quality tubes in his amps or he has them confused with someone else's amps.

The 4 Counterpoint Amp i have owned...... most from the 80s-90.......
SA80,SA200,Both amps started blowing fuses.....never new why...the SA200 tuck out one of my B&W801 tweeter..........
Next was the SA220 this was to replace the SA200.....worket fine
Next the SA-4s....At the time my dream amps on paper ......Driveing a pr of Acoustat 2+2 ESL one amp Smoket , only one of the OTL amps flamed out......I did not mean to say all 4 amps smoket!
RM-200.......an i stan by what i have posted about the sound......an how it could not drive my Apogee Stages.......

Corndog71


That's an RM-200 Mk II by Music Reference. I WISH this was my amp!

As for what others say.....Just look up ARC amps an pramps you find raveing about the great sound!
But till you put these amps in your home.... you well never know if there for you.....

If you dont go you wont NO.......All this is just one mans O-pine

Thanks for the site Bob..............

zx wrote:The 4 Counterpoint Amp i have owned...... most from the 80s-90.......
SA80,SA200,Both amps started blowing fuses.....never new why...the SA200 tuck out one of my B&W801 tweeter..........
Next was the SA220 this was to replace the SA200.....worket fine
Next the SA-4s....At the time my dream amps on paper ......Driveing a pr of Acoustat 2+2 ESL one amp Smoket , only one of the OTL amps flamed out......I did not mean to say all 4 amps smoket!
RM-200.......an i stan by what i have posted about the sound......an how it could not drive my Apogee Stages.......

Corndog71


That's an RM-200 Mk II by Music Reference. I WISH this was my amp!

As for what others say.....Just look up ARC amps an pramps you find raveing about the great sound!
But till you put these amps in your home.... you well never know if there for you.....

If you dont go you wont NO.......All this is just one mans O-pine

Thanks for the site Bob..............

It seems Roger licensed the circuit for the SA-4 OTL amp for Counterpoint.  But the rest of the amp was designed by Michael Elliott.  I found a link to the most common issues for the SA-4.  http://www.altavistaaudio.com/SA4_Known_Issues.htm

The other Counterpoint amps were designed by others.  Even Roger conceded the company made a lot of bad equipment.

As for your experience with the RM200 all I can say is maybe it was a bad match.  On paper it should've driven your speakers.  But I've experienced mismatches before and they can be disappointing.  Were you using RAM tubes in it?

Yes.....  Look new the guy i got the RM-200 from was driving Klipsch K-horns
An the amp only had maybe 100 hr....... thats why i got the amp....

Look you need to get your self one...see for your self.... you may like the amp.....What other say is just a small pic.....

The only way i know this amp was not for me was to put it in my home....Same as with the ARC,an the Jadis Defy 7 an minny others Amps i have owned....One thing is for sure the Old Dynaco MK3s kik ass, there keepers.....
I have no regrets....it just $$ an a lot fun
I Was in the high end Audio Biz in the 80s-90s...............Life is short....have fun with tubes Big bang for the Money.....if you like the sound!
I say we need to move on from this....this is not what this post is about....good luck

Thanks for the site Bob.....

tubes4hifi wrote:I've never heard an ARC that I like, they use so much SS in the power supplies that it kills all the tubes in the main signal path!
That's why I won't build any tube preamp without a tube rectifier.    Now I do use an SS regulator in there, but it acts as a shunt on the voltage,
and it drops the noise level 60db, and sounds better than a huge bank of capacitors, and doesn't seem to change any sonic flavors like some other designs.

Just an esoteric question for tubes4hifi, if it's been answered elsewhere delete this post and steer me to a link, please.
We do quite a bit of yakking about rectifiers on the power amps but I've seen little discussion of rectifiers for the SP/PH preamps. Is there an argument in favour of the NOS RCAs (or sumpin else), or will any flavour rectifier tube suffice in that hole?

Deepee99 --

If you are talking 12X4 type tube rectifiers, as used in the PAS and Roy's SP series preamps and if they color the sound in any way.....  

I have found that NOS Sylvania, TungSol, GE, RCA, Raytheon are all fairly consistent assuming that the test Good.  When I looked thru my old PAS 2 and 3's I found that I used the Sylvania 12X4 rectifier.   I don't think they colored the sound in any way.

If I remember correctly, the two prebuilt SP-type preamps that I bought from Roy both used the Raytheon 12X4.  

Hope that helps,
Stew

stewdan wrote:Deepee99 --

If you are talking 12X4 type tube rectifiers, as used in the PAS and Roy's SP series preamps and if they color the sound in any way.....  

I have found that NOS Sylvania, TungSol, GE, RCA, Raytheon are all fairly consistent assuming that the test Good.  When I looked thru my old PAS 2 and 3's I found that I used the Sylvania 12X4 rectifier.   I don't think they colored the sound in any way.

If I remember correctly, the two prebuilt SP-type preamps that I bought from Roy both used the Raytheon 12X4.  

Hope that helps,
Stew

Stew, it does, and thanks.