Hi From A New Member and Long Time ARC User

[mep]

ARC is all over the place with their biasing schemes for their amps. Some are easy to bias, some are dangerous to bias, while others are just plain off the charts wacky to bias. The D-79 is one example of the super-easy, the D-70 is a good example of the dangerous, and the VT-100 is a great wacky example. ARC should be ashamed of themselves for some of their biasing schemes and there is no valid reason why biasing can't be simple and safe.

 

[Steve Williams]

I would prefer auto biasing as in the VTL

 

[MylesBAstor]

I would prefer auto biasing as in the VTL

cj's biasing scheme is really idiot proof too Just turn a pot until the LED lights, then back it off. Problem is that don't know which tube has failed when the fuse blows in the back.

That's where the beauty of the VTL biasing comes in. My friend has the 450 mk 2s and the light goes off for the tube that is failing!

 

[Steve Williams]

Luke makes some good stuff

 

[microstrip]

ARC is all over the place with their biasing schemes for their amps. Some are easy to bias, some are dangerous to bias, while others are just plain off Mthe charts wacky to bias. The D-79 is one example of the super-easy, the D-70 is a good example of the dangerous, and the VT-100 is a great wacky example. ARC should be ashamed of themselves for some of their biasing schemes and there is no valid reason why biasing can't be simple and safe.

Although not advertised, the D-70 is very easy to bias - you just ground the negative tip of your voltmeter (4 ohm tap or ground) and adjust the bias measuring the DC voltage at the 0 and 16 ohm taps.

 

[MylesBAstor]

Luke makes some good stuff

Yes, I was impressed with the 450s and know Albert swears by his big puppies Real classic tube sound without sounding tubey.

 

[DaveyF]

Although not advertised, the D-70 is very easy to bias - you just ground the negative tip of your voltmeter (4 ohm tap or ground) and adjust the bias measuring the DC voltage at the 0 and 16 ohm taps.

Micro, I asked my tech about your suggestion....he thinks that this is NOT the way to bias the amp and that you are most likely getting a false reading doing it that way Tech says that the DC balance is not accurate with your method and therefore the bias readings at the driver tubes are also in-correct!
Also, the AC balance is needing to be corrected if tubes are replaced and your method is again, according to my tech, not appropriate

So, back to the drawing board

 

[mep]

Micro, I asked my tech about your suggestion....he thinks that this is NOT the way to bias the amp and that you are most likely getting a false reading doing it that way Tech says that the DC balance is not accurate with your method and therefore the bias readings at the driver tubes are also in-correct!
Also, the AC balance is needing to be corrected if tubes are replaced and your method is again, according to my tech, not appropriate

So, back to the drawing board

Back to the life threatening method called out for by ARC.

 

[RogerD]

Welcome Sparky and good to have someone from AudioKarma. You will like it here,as everybody is very passionate and knowledgeable about their systems and audio.

 

[karma]

Micro, I asked my tech about your suggestion....he thinks that this is NOT the way to bias the amp and that you are most likely getting a false reading doing it that way Tech says that the DC balance is not accurate with your method and therefore the bias readings at the driver tubes are also in-correct!
Also, the AC balance is needing to be corrected if tubes are replaced and your method is again, according to my tech, not appropriate

So, back to the drawing board

HI Davey,
Yes, I agree with your technician. Whoever came up with that scheme just does not understand basic electronics. It can't work. Mate, check mate. You have to do it following the instructions in the manual.

Since the "off grid" method would depend on the output transformer passing DC in direct proportion to the DC bias current passing through the tube it is impossible. Transformers are not DC devices but the bias current is a DC signal.

Jeeze, no wonder some techs have bad reputations. It makes me cringe.

I do agree that many of the ARC amps requires more technical ability to adjust the bias than many people have or want to learn. In fact, if anyone even hesitates over their their ability to do the bias adjustment, they probably need help and I would suggest it. It's very simple to a tech, an adjustment they make several times every day, but could be overwhelming to someone with no technical background and does not even know how to operate a multimeter. Not to mention understanding what the bias current actually does though this knowledge is not really necessary to make the adjustment.

Over the years ARC has tried to correct this situation without changing the basic circuit design. It's not enough that an amp has "Auto Bias". Such a feature is a basic circuit change and is one of the reasons that different amps have different "house sounds". I'll go with ARC's sound every time in spite of biasing difficulties.

I think ARC has an attitude that says "If you can afford to buy these expensive amps in the first place, you can afford to have a tech do an occasional house call to adjust the bias or whatever". Some might call this an attitude of exclusivity. But really folks, you are buying a Ferrari. How many Ferrari owners do their own automotive repair? Not many I'd wager. ARC is probably right.

I had well heeled customers with just that kind of an attitude and resources. They did the listening and I did the adjustments and repairs; money be damned. It was good for them and good for me.

Sparky

 

[jadis]

Hi Sparky,

Nice reading your posts. We have a bit of gears in common too. Though my handle is Jadis, I have a ton of experience with ARC electronics too. D-115MkII, SP-10MKII, D-125, SP-14, VS110, PH3, and my present amp VS-115 which I got last April. I used to have a Jadis JA200 but I sold it due to reliability problems but I still retain the Jadis JPL line stage. I also have, like you, an ET2 tonearm, I never got to upgrade it (I'm not a mechanical tweaking guy though almost my whole career I was working in a mechanical factory). I had been using it since 1987 on my VPI HW19MKII turntable. My cartridge is a Koetsu Rosewood Signature. The ARCs do relay an 'objective' balance to the sound, and it's a good mix and match with my more 'romantic' Jadis and Koetsu. I've heard a few Oracle/ET2 combinations in the old days too and it sounds very good as well, though I did hear a problem would be a certain tilting of the suspension. Welcome to the forum too.

 

[jadis]

cj's biasing scheme is really idiot proof too Just turn a pot until the LED lights, then back it off. Problem is that don't know which tube has failed when the fuse blows in the back.

That's where the beauty of the VTL biasing comes in. My friend has the 450 mk 2s and the light goes off for the tube that is failing!

The Jadis JA200, and the JA500, have individual fuses for each tube and red lights for each tube as well. They are auto biasing amps. Very user friendly. But the problem we encounter here are not blown tubes but blown capacitors and some small parts in the power supply. Other than this, they're a thing of beauty.

 

[mep]

Sparky-IMO, ARC made deliberate choices in their bias set-ups to force most owners of the wacky bias schemes to take their amps in to their dealers for service. ARC has proven they know how to make it simple and they have proven they know how to make it ridiculous. I have already mentioned examples of easy, dangerous, and wacky. Here is another example, the D-76. The D-76 was relatively easy to bias. You just had to stick your fingers between blistering hot 6550s and hope your skin didn’t touch the tubes. Next comes the D-76A. Why oh why did ARC flip the ¼” bias jacks upside down so now you have to remove the bottom cover to jack in your meter? Oh, and the bottom cover has standoffs that fall off and make reassembly a real joy.

Owning a tube shouldn’t be harder that it already is. Having to pay a tech to come to your house to set your bias is ridiculous. Look at the Quicksilver V4 amps. I owned those and I could set the bias on both amps in less than 2 minutes. Marantz had it right the first time out of the chute. Build a meter in to the amp and make the bias pots easily accessible. If I ever buy another tube amp, part of the purchasing decision will be predicated on ease of setting bias. I just sold my Jadis Defy 7 MKII and it was a pain in the ass to set the bias. The amp weighs 100 lbs and you have to flip it on its side, remove the bottom cover, and then you can access the 4 bias pots. You only have one bias pot for each bank of 3 tubes so you end up setting an average bias value for the bank of 3 tubes instead of being able to dial each tube in exactly. It’s always a compromise with the Jadis because *matched* tubes are never perfectly matched so each tube will always measure slightly different than its neighbors.

Mark

 

[microstrip]

Micro, I asked my tech about your suggestion....he thinks that this is NOT the way to bias the amp and that you are most likely getting a false reading doing it that way Tech says that the DC balance is not accurate with your method and therefore the bias readings at the driver tubes are also in-correct!
Also, the AC balance is needing to be corrected if tubes are replaced and your method is again, according to my tech, not appropriate

So, back to the drawing board

DaveyF,

Sorry, the method is 100% accurate if you know how to do it properly. Ohm's law is independent of the technician. The output transformer is symmetrically connected to the tube cathodes, and has its middle point grounded. It is very easy to do - inject a 65mA (if I am remembering well the value) through tap 0 and tap 16 and measure the DC values for reference. Surely if you do not know how to calibrate the readings it does not work - you have to correct for the small resistive imbalance of the transformer . BTW, AC balance of the drivers should be checked looking for the lower value of the second harmonic . I have done it for tens of years in my ARC equipment, including the D70. Static adjustment is only a compromise.

 

[microstrip]

HI Davey,
Yes, I agree with your technician. Whoever came up with that scheme just does not understand basic electronics. It can't work. Mate, check mate. You have to do it following the instructions in the manual.

Since the "off grid" method would depend on the output transformer passing DC in direct proportion to the DC bias current passing through the tube it is impossible. Transformers are not DC devices but the bias current is a DC signal.

(...)

Sparky

Sparky,
Can I suggest that you study the D70 diagram before going on? BTW, although I am not involved in electronics anymore I did nuclear electronics development for some years. Perhaps it is not exactly basic electronics ...

 

[tdh888]

HI Davy,
While one amp hardly constitutes a statistical truth, I would have to disagree with the validity of your reliability statement concerning the D250. With the exception of a few early blown Chinese 6550's, my amp has been totally reliable over a period of over 20 years. Impressively so, in fact. If the reputation you speak of is true, (not sure it is) I belive it is entirely due to unreliable 6550's.

You see, this amp uses 20 6550's. Around the early to mid 1990's, when the amp was still a current product, just getting 6550's was a challenge. Almost all tube manufacturing had gone out of business. Even NOS 6550's were unobtainium. I was fearful that I might have to replace my beloved D250 because of a lack of tubes. I never bought from ARC which probably would have solved the problem but I think they too were suffering from the lack of good tubes. About the only source for 6550's were the Chinese. These tubes were very unpredictable. Of the 40 Chinese 6550's I bought from third parties, about a third of them failed early in their life. Once I managed to get set of reliable tubes, life was good and I got good life from them.

I do understand that from the point of view of the average end user, a failed amp is a failed amp, no matter the cause. They will blame ARC and that's fair. But it may not be accurate. Consider that all the D series (and the M100) amps used the same audio signal circuit design. The main differences can be found in the number of 6550's that were used and the complexity of the power supplies. They all used the same bias current value and each tube produced the same amount of power. So, more tubes, more power. With the 6550's being so problematic, the more 6550's used, the more problems that could be statistically expected. And the D250 had by far the most of any of the D series. So, it's entirely possible the D250 experienced more failures than the other D series amps. But, again, this would be due the number of 6550's used in the design. Once the 6550 problems were fixed, the D250 has proven itself to be reliable.

Now that the Russian's are producing good 6550's, I have had zero problems.

I consider the D250 to be a VERY reliable amplifier which I find somewhat amazing given they have a total of 32 tubes inside.

As for bias setting, the D250 is actually rather easy. True, one does have to remove the cover and make a measurement with a voltmeter but it is easy and the test points are readily accessable. Of course, there are 16 values to adjust, one for each output tube, but that's the price for such a great amp. And the bias values are very stable so it does not have to be done often. The D250 version I have with the Servo feature actually compensates bias according to remaining tube life.

Now, if a person (you?) has absolutely no electronics experience or confidence, the matter is more complicated. In this case the only thing that makes sense is have a tech make a house call since lifting this brute of an amplifier is a multiperson job. One really would not want to take it to a shop.

Bias setting is really not a big deal considering the stability of the bias values.

Thus speaketh one who knows!

Sparky

Hi Sparky,

Welcome to this forum. Met you at the asylum forum ,Im Mondial. I also have a D250 and I totally agree w/ Sparky that the D250 is a reliable amp when using reliable Russian 6550c tubes .In my 6 years of using the D250 I only had one episode of break down when a single 6550 tube died and took w/ it the 100 ohms bias resistor. The resistor and tube were replaced and everything is back to normal. I still have 4 other ARC amp in my stable and I can say that my D250 is the most reliable .

To Davey F I think you are referring to the D250MK1 w/c was really unreliable and was notorious for breaking down. Sparky and I are using the D250Mk 2 Servo . The reliability of the D250 greatly improved when they came out w/ the D250MK2. Maybe Sparky can give us a rundown regarding the difference between these 3 models. All I know is the difference of the Mark 2 from the Servo model was the presence of an auto adjusting DC balance for the input tube of the Servo model.


tdh888

 

[karma]

Sparky,
Can I suggest that you study the D70 diagram before going on? BTW, although I am not involved in electronics anymore I did nuclear electronics development for some years. Perhaps it is not exactly basic electronics ...

HI microstrip,
I printed out the D70 Schematic from the ARCDB. UNfortunately, the D250 data base entry does not include a schematic. My own schematics are buried in a stack of stuff in the garage and are not accessable. So , I will have to go with memory, basically unreliable.

The audio signal path seems to be identical to the D250 including the input phase inverter and the cross coupled output stages. Of course there is only one pair per channel of 6550's where as the the D250 has four pair.

The D70 power supply is so complex that I don't want to guess. It's basic outlines appear to be very similar to the D250's But it may vary in detail. The main difference is the number of screen voltage regulator 6550's. The D70 has one where the the D250 has four. The biasing seems to be identical. My D250 Servo does have the servo circuits but they are not involved with amplification.

In general, the main differences seem to be with scale. There is just much more D250 in a D250.

I'm not sure what more you want me to do.

I will say this. I had a brand new D115 Mk II, which is very similar to the D70 except for power, for five years. I loved it. However, the D250 Mk II Servo is a much better amplifier. Both amps were driving the same speakers and had the same preamp. And it's not just power differences. The D250 has a level of refinement and subtlety that far surpasses the D115. There really is no comparison. The D115 was good. The D250 is great.

Sparky

 

[DaveyF]

Sparky. I think micro is talking about the ability to bias the D70. I haven't confirmed what he suggests in his last post with my tech, however, when I did suggest the 'microstrip scheme' to him, he was pretty amused and as stated didn't believe that this was an appropriate way to bias the amp. Instead, he stated as you did, that the manual is the correct way. I have heard all of the D series era amps with the exception of the M300's. IMO, the D70 and the M100's are the sweet spot. The D250 has more power and is better in the bottom end retrieval, but looses out to the D70's and the M100's in the critical midrange. The D115 is less capable in the mids than the D70 or the M100's and the D115 has more of the ARC 'whitish grain' than any of the other D series amps, again IMHO. It is true that my friend's D250 that had all of the problems was a first itineration, so if the later versions were improved in this regard, that was a very good thing.
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.

 

[karma]

...................I have heard all of the D series era amps with the exception of the M300's. IMO, the D70 and the M100's are the sweet spot. The D250 has more power and is better in the bottom end retrieval, but looses out to the D70's and the M100's in the critical midrange. The D115 is less capable in the mids than the D70 or the M100's and the D115 has more of the ARC 'whitish grain' than any of the other D series amps, again IMHO.

HI Davey,
Yep, I've heard them all too except the M300's. I'm here writing at a disadvantage. I don't know what the term "whitish grain" means. Oh, the continuing problems with audiophile terminology! So, with that ignorance in mind, I'll try to continue.

The M100's were impressive. I came very close to buying a pair. All comparisons are difficult when the system context is left out. All I can say for sure is I have lived with the D115 Mk 2 and the D250 Mk 2 Servo both used in the same system context and in the same room. That's very important to note.

My speakers were (are) the infamously difficult to drive Martin Logan CLS IIA's and the preamp was (is) an ARC SP-11 Mk 2. Cables were the same. In this context, my D250 was (is) simply a window into reality. It seems to have no signature and nothing you could call grain. And, may I add, neither does the SP-11. Can we actually evaluate either the amp or the preamp without considering the other? I don't think so. They must be considered as a symbiotic pair. After all, they never work without the other. So, my judgements consider both, always. My comments will always use a system context of preamp, power amp, speakers, and room.

The speakers need a little explanation. The CLS's have limitations. They have no deep bass and when attempting to use them full range, they are limited in dynamic range due to the low frequency equalization that is part of the design. Consequently, I use biamped dual subwooers, the Kinergetics SW800's, crossed over at 67 Hz (a very important frequency). Not only does this solve the problems with the CLS's, it changes the work that the D250 has to do. It basically does no work below the crossover frequency. I have, as an experiment, used the CLS's full range with the D250 and I love the quality of the bass but there is just not enough of it. This is a panel issue. And dynamic range becomes a real problem. There is no doubt that biamping with good subs is the way to go. With the subs, I have excellent dynamic range and bass that goes to sub 20 Hz with power and low distortion. The integration between the subs and the CLS's is, well, perfect. It really is and I worked hard to get it that way.

I should note, for your inderstanding, that I have modified my D250. All coupling capacitors have been replaced with modern equivalents made by MIT. There's a story here but I won't elaborate. Suffice to say that these caps were custom made for me. Expensive may I say. They did not change the basic character of the D250. But they did allow the amp to do better what it does best; namely resolution, low level detail, transparency, and transient response. The new caps made it a better amplifier.

With this context and comments in mind, the SP-11 and the D250 as as close to perfect as I have heard. Are there better out there? Probably. It is a question I often ask myself. There are certainly newer and more expensive stuff. But Better? A good question for which I have no answer.

Sparky

 

[MylesBAstor]

Sparky. I think micro is talking about the ability to bias the D70. I haven't confirmed what he suggests in his last post with my tech, however, when I did suggest the 'microstrip scheme' to him, he was pretty amused and as stated didn't believe that this was an appropriate way to bias the amp. Instead, he stated as you did, that the manual is the correct way. I have heard all of the D series era amps with the exception of the M300's. IMO, the D70 and the M100's are the sweet spot. The D250 has more power and is better in the bottom end retrieval, but looses out to the D70's and the M100's in the critical midrange. The D115 is less capable in the mids than the D70 or the M100's and the D115 has more of the ARC 'whitish grain' than any of the other D series amps, again IMHO. It is true that my friend's D250 that had all of the problems was a first itineration, so if the later versions were improved in this regard, that was a very good thing.
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.

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