Hi From A New Member and Long Time ARC User

[tdh888]

IMHO, the two nicest ARC vintage amps were the M100s and the Classic 150

Both these amps are also easy to bias. The M100 on the front panel. On the Classic 150 bias is done on the main circuit board of the rear of the amp. The cage has to be remove in order to perform the adjustments.

tdh888

 

[jadis]

the D115 has more of the ARC 'whitish grain' than any of the other D series amps, again IMHO.

BTW, I had a chance to listen to a Ref 150 a couple of days ago, this amp seems to be the way ARC is going these days; more refined and perhaps accurate than the D series, BUT in some ways less musical than the D series. Although I was fairly impressed with the Ref 150 and would say that it is definitely a better amp than the REf 110 that it replaces.

Hi Davey,

I agree with your assessment that the D115 has a more whitish grain than the other D series, specifically the D70 which can sound a bit sweet by comparison. The D115 can sound 'cold' by comparison although I remember it to have one of the tightest bass sound specially in kick drums and can go down deep in organ music. Having heard a Ref110 in a friend's system, I also can sense that their direction is towards the less musical but accurate camp though the overall sound is quite refined. I have now a VS115 which ARC says is based on some of the Ref series' designs and I feel in fact they are a bit lean on the vocals, though on instruments they sound really clear, crisp, and fast. Bass reproduction is by far the best improvement it has over my old VS110 amp. I really am curious what the sound of the present day VS115 KT120 amp is. Mine still has the regular 6550s and I loved the KT88 sound when it was driving my old VS110. I am tempted to re-tube to KT88s in my present VS115 in the near future.

 

[DaveyF]

Hi Davey,

I really am curious what the sound of the present day VS115 KT120 amp is. Mine still has the regular 6550s and I loved the KT88 sound when it was driving my old VS110. I am tempted to re-tube to KT88s in my present VS115 in the near future.

Jadis, at the Newport show, I had an interesting talk with the ARC rep. Specifically, I had asked him about replacing my 6550's in my D70Mk2 with KT 120's. He believed that not only would they work in the amp, But would be a probable improvement over my winged C 6550's. So, I am now considering the replacement.....problem is the 6550's are essentially new
Anyhow, why would you want to replace your 6550's with KT88's and not go straight to the KT120's?

 

[Jay_S]

Davey, please keep us posted on your KT120 upgrade. I have a D70 II and am very interested in hearing how the KT120s work (and fit).

 

[jadis]

Jadis, at the Newport show, I had an interesting talk with the ARC rep. Specifically, I had asked him about replacing my 6550's in my D70Mk2 with KT 120's. He believed that not only would they work in the amp, But would be a probable improvement over my winged C 6550's. So, I am now considering the replacement.....problem is the 6550's are essentially new
Anyhow, why would you want to replace your 6550's with KT88's and not go straight to the KT120's?

Good to know that the KT120s would work well in your D70MKII. Like yours, my 6550s are brand new as well so the tendency is to use it more till the next tube change.

I had used KT88s before with my VS110 prior to trading it in for my VS115. I like the KT88s sound character very much. Now, I have not heard the KT120s in action so I'm not sure if the sound quality is closer to the 6550s (punchy bass, more clarity and attack) or the KT88s (lusher mids, more refined highs). IF the KT120s have the mids of the KT88s and the bass punch of the 6550s, then I'm definitely going in that directions.

 

[tdh888]

Please do inform us if you replace your tubes from 6550c to KT120 and how it turns out. Im hesitant to do this in my amps (D250 mk2 servo, Classic 150, D79B) because when I talked to their customer sevice representative late last year ,he told me the company has not tried the KT 120 's with the old /vintage ARC amps. So he cant say nor recommend the use of the KT120 in our old amps.

 

[jadis]

From Tubedepot's site:

Tung-Sol KT120
"Tung-Sol KT120 This is the most powerful audio power tube in production today. With a plate dissipation rating of 60W (compare that with the 35W of the 6550, the 42W of the KT88, and the 50W of the KT90), a pair of these KT120 tubes in push-pull configuration can deliver 150 watts or more of audio power. And when used in a vintage circuit designed for 6550 or KT88, the KT120 will deliver unbelievable amounts of clean headroom, low end rumble and dynamic punch all without overloading. The glass of this tube is a full 1/2" taller with the internal plate structure 3/8" longer than its nearest KT88 cousin. The internal control grid cooling fins are twice as large as the KT88 adding to the amazing performance of this tube. This particular tube draws between 100mA (.1A) and 300mA (.3A) more filament current than a standard KT88, so check with your amp manufacturer to insure you have the additional filament headroom to power these tubes. This new KT120 is the steamroller of audio tubes, prepare to be flattened."

 

[jadis]

Just a caveat re changing the power tubes in ARC amps.

When I changed the stock 6550s of my VS110 into SED KT88s, after a few months, I noticed one KT88 tube's bias would always run to 70ma after I set it to 65ma. It reoccurs after further readjustment and finally, the biasing resistor gave in and I had it serviced and the tube was replaced as well. I would have thought that my case was isolated until several years after, a friend of mine who retubed his ARC 610 reported the same thing happened to him. In his case, he even ordered from ARC an original set of 6550 tubes. Mine was from tubedepot.com. Now, I'm a bit weary of changing the stock tubes of my new VS115 though the temptation really lingers.

 

[karma]

HI All,
I hope you all realize that the only way you will get more power from the KT120 is if you increase your power supply voltage. Ohm's law rules. Increasing the power supply voltage is nearly an impossible job without major changes which probably can't be done. For example, replacing the power transformer.

Shortly after the KT120's became available, I called ARC to discuss their placement in vintage amps like my D250. At the time, ARC was not suggesting the replacement becuse they had not tried it. That's a good, safe stance.

We also discussed the wild rumors dancing around the internet about increased power. They agreed that without an increase in power supply voltage, such power increases were not possible.

What is possible is increased tube life because in vintage amps, the KT120's are stressed less than the 6550's. This may make them ecconomically competitive with the 6550's in spite of their higher price. This is where my interest lies. I don't need more power but I do need more hours.

Also, don't forget that ARC is not using the KT120 as power supply regulators. They continue to use 6550's. I don't know why.

Sparky

 

[DaveyF]

Hi Sparky, That's very interesting. When I spoke to the ARC rep at the Newport show, he told me that the KT120's should work well with the D70Mk2. He wasn't certain if the cage would still fit, BUT who really cares about that. He also mentioned that I would most likely hear more bottom end and a greater ease overall. Additionally, he felt that the amp would be able to portray dynamic contrasts better.... along the lines of what Jadis said. Jadis and Jay_S, I too am probably not going to try this mod until my 6550's are more worn.
However, to re-iterate, the ARC rep was pretty enthusiastic about the KT120's and actually seemed to think that the upgrade to the D70Mk2 would be very well worthwhile.

OTOH, I cannot also help but wander why ARC doesn't use the KT120's in the power supply regulation.....perhaps it's the additional cost of the KT120 vs. a 6550 ( But we are NOT talking a lot of money here)

 

[jadis]

OTOH, I cannot also help but wander why ARC doesn't use the KT120's in the power supply regulation.....perhaps it's the additional cost of the KT120 vs. a 6550 ( But we are NOT talking a lot of money here)

I now recall something. A few years ago, a friend of mine changed the power tubes of his D79 from 6550s to KT88. I distinctly recall the technician saying that the lone 6550 which acts as the power regulator cannot be changed into the KT88 without fiddling or revising something in the circuitry. The owner did not heed the advise and went on to use KT88 for the power regulator, and after a few weeks, he had trouble with his amp. It had to be serviced.

 

[jadis]

My VS115, btw, if bought past November 2010 , would have been fitted with KT120s rather than 6550s for prior units. As such, I understand the tubes to be interchangeable, as with the KT88s or KT90s, they are direct replacements, I was told. But the price of the November 2010 VS115, with the KT120, have increased by $500 on the SRP, and I imagine the price increase is just for the tube change and not with the circuitry or other parts.

 

[tdh888]

I now recall something. A few years ago, a friend of mine changed the power tubes of his D79 from 6550s to KT88. I distinctly recall the technician saying that the lone 6550 which acts as the power regulator cannot be changed into the KT88 without fiddling or revising something in the circuitry. The owner did not heed the advise and went on to use KT88 for the power regulator, and after a few weeks, he had trouble with his amp. It had to be serviced.

When I retubed one of my D79B I installed KT88's but did not replace the 6550 voltage regulator. The EH Kt 88's ( it took more than 3 months to break in) had a different flavor when installed in the D79B it sounded more powerful compared to my other D79B w/ 6550's, has a romantic midrange ,highs that are less glassy but bass that is a little bit plump. My other D79B had Sovtek 6550's the bass is more robust a more leaner or is it neutral mids, w/ glassy high's . I really wanted to try the KT120's in one of my D79B because the design of the P.S. is overkill (550 joules for a 75 w/ch amp ).The power supply energy storage is as large as the D250. If you check out the power transformer of the D79B its as big as the D250. So I was thinking if I shift to KT120's there will be an increase in power.

tdh888

 

[karma]

When I retubed one of my D79B I installed KT88's but did not replace the 6550 voltage regulator. The EH Kt 88's ( it took more than 3 months to break in) had a different flavor when installed in the D79B it sounded more powerful compared to my other D79B w/ 6550's, has a romantic midrange ,highs that are less glassy but bass that is a little bit plump. My other D79B had Sovtek 6550's the bass is more robust a more leaner or is it neutral mids, w/ glassy high's . I really wanted to try the KT120's in one of my D79B because the design of the P.S. is overkill (550 joules for a 75 w/ch amp ).The power supply energy storage is as large as the D250. If you check out the power transformer of the D79B its as big as the D250. So I was thinking if I shift to KT120's there will be an increase in power.

tdh888

HI tdh,
Size, per se, is not the issue and neither is energy storage. Size and energy storage are related to current capacity. It's transformer output voltage which translates to the B+ voltage that has an effect on output tube power. There is only one way the KT120 can give more power without a change of the B+ and that's if they have lower internal resistence. Nothing I have read, so far, (I do have the KT120 data sheet) indicates the resistance is lower than the 6550's. Thus, for the same B+, the KT120 will give the same power output. True, if we were able to jack up the B+ voltage, then the KT120 could give more power. But we can't do this with an existing amplifier.

I fear that many are misinterpreting the KT120's potential for power output verses the actual power in an already existing design. That's not to say that the KT120 does not not have other qualities that makes the change make sense. Just not power. This is analogous to the KT88 vs. the 6550 wars. The KT88 was never touted as giving more power, just a different sound.

Sparky

 

[jadis]

I fear that many are misinterpreting the KT120's potential for power output verses the actual power in an already existing design. That's not to say that the KT120 does not not have other qualities that makes the change make sense. Just not power. This is analogous to the KT88 vs. the 6550 wars. The KT88 was never touted as giving more power, just a different sound.

Sparky

Sparky

That is my view too. My goal in changing to KT88 or KT120 is not to get more power. I would get a Ref 150 if I wanted more power. My goal is to possess a combination of sound qualities that I seek that is not inherent in the 6550. But in so doing, I realize there will be pros and cons in the tube change, given the so many parameters and qualities we seek from our system.

 

[tdh888]

Thank you Sparky for the technical advise .Thats the sort of response I wanted to read in this forum. But until ARC customer services gives its go signal regarding the use of KT 120's in our vintage amps I wont install them in any of my amps. Have you tried using KT88's in your D250?

 

[karma]

HI tdh,
You are welcome.

No, I've never used KT88's. I've been very happy with the 6550's. I think you are wise to wait. Honestly, I don't think the KT120's will cause any harm. Some have mentioned the added heater current that may cause problems. I doubt that. The difference is not great and I'm fairly certain there is enough head room built into the heater transformer windings that no harm will come.

Still, I'm going to wait for a definitive statement from ARC concerning my D250. And even then, I may not switch especially with a new set of 6550's in my amp. Much depends on ARC's research into the life cycle of the KT120's.

Sparky

 

[tdh888]

Maybe the reason for ARC to shift to KT120's on their new equipment is because of the shortage of svetlana wing C w/c are no longer made because I think it stopped production because of breakdown of the machine that makes the tubes. I read it in a magazine or the net months ago. I think the company that manufactures the Tungsol KT 120 is the same company that makes the Electro Harmonix ,Sovtek, and the other Svetlana non winged C.

 

[tdh888]

Maybe the reason for ARC to shift to KT120's on their new equipment is because of the shortage of svetlana wing C w/c are no longer made because I think it stopped production because of breakdown of the machine that makes the tubes. I read it in a magazine or the net months ago. I think the company that manufactures the Tungsol KT 120 is the same company that makes the Electro Harmonix ,Sovtek, and the other Svetlana non winged C.

 

[microstrip]

HI microstrip,
I printed out the D70 Schematic from the ARCDB. (...)

Sparky

Sparky,
Sorry for the delay, I was away for a few days.
I attach a basic explanation of why checking the DC at the output of the transformer can be used to set the bias of a D70II - so any one knowing Ohm and Kirchhoff laws can understand it (I hope that even those not knowing it will also understand it) .
In the D70 the cathode resistors used to check the bias current are mounted in series with half the secondary winding of the output transformer that has very low DC resistance. As the current in the R10 6k2 feedback resistors is very low compared to bias it can be neglected and we can consider that all the bias current flowing through the bias cathode resistors goes through the secondary where it develops a DC voltage proportional to the current. Measuring this DC you get the value of the current - as easy as that.
This method can be used with many amplifiers having output cathode coupled transformers - and they were used in several ARC models.