Arc ref 75 w/ kt150s

[CORALX8]

I have just acquired an arc ref 75 amp. It is to my understanding that the previous kt120s that came with the amp was running low on life so the previous owner swapped them out for tung sol kt150s and did the proper biasing (he kept the bias a bit lower than normal just incase the tube draws too much). I did a bit of research online and it seems that it is best to run the 150s with the SE ver. only? Should I swap back to KT120s or should it be fine running it as is?

 

[techguy0192]

I would not use KT150 tubes in that amp. The SE is more than just a badge; changes were made to the amp to make using KT150 tubes possible.

ARC no longer offers the upgrade to the SE version. If I were you, I'd switch back to KT120 tubes and enjoy the amp. The Ref75 is a stellar amp; I've always wanted one.

Was there more info from the previous owner than....."running low on life"?

 

[CORALX8]

I would not use KT150 tubes in that amp. The SE is more than just a badge; changes were made to the amp to make using KT150 tubes possible.

ARC no longer offers the upgrade to the SE version. If I were you, I'd switch back to KT120 tubes and enjoy the amp. The Ref75 is a stellar amp; I've always wanted one.

Was there more info from the previous owner than....."running low on life"?

He just said when he purchased it around 2 years ago the tubes seemed to have some issues and he determined they reached the end of their life so he swapped them with tungsols

What kt120s would you recommend?

 

[CORALX8]

I have just acquired an arc ref 75 amp. It is to my understanding that the previous kt120s that came with the amp was running low on life so the previous owner swapped them out for tung sol kt150s and did the proper biasing (he kept the bias a bit lower than normal just incase the tube draws too much). I did a bit of research online and it seems that it is best to run the 150s with the SE ver. only? Should I swap back to KT120s or should it be fine running it as is?

P.S. I Just noticed it is running absurdly hot (Ive owned plenty of tube amps and ive never felt any run that hot on standby) and all the tubes are glowing extremely red right now, even the 6h30. I will assume that is not a great sign

attached pics of the bias adj (The quick bias checking system doesnt exactly seem super reliable, i think the knobs are like a bit old or something because when i twist them a bit more in or out the meter flicks wildly, esp when turned to the left. right side is a bit more stable.)

 

[techguy0192]

Tung-Sol is the only option for KT120 tubes.

The SE and non-SE version have the same 65 mV bias point but that's not the real issue here. The KT150 has a much higher in rush current at power on and the filament current is higher. What that means, is more stress on the transformers.

 

[gadget73]

Easy choice when there is only one

 

[spantou01]

and all the tubes are glowing extremely red right now, even the 6h30. I will assume that is not a great sign

I’m not sure what you mean by this. Are the plate glowing red? I.e. red-plating? Of all the tubes: power and small signal tubes? Are you familiar with how hot tubes typically get? My guess is that the glow you observe is from normal heater operation.

Red-plating would be a big concern. I would not run the amp before getting a handle on the cause of the red-plating. If you are able to trouble-shoot do so, else it will be worthwhile taking it to a tech that knows this amp.

I did a bit of research online and it seems that it is best to run the 150s with the SE ver. only?

Did you find explanation for the taboo to run KT150s in the ARC ref 75 (without SE). What makes the SE different?

The KT150 has a much higher in rush current at power on and the filament current is higher. What that means, is more stress on the transformers

I am no expert on ARC ref75 amp, but I think this incorrect in three aspects. 1. Inrush current is determined by the charging power supply caps and management thereof, not by power tubes if properly biased. 2. I checked the tube data sheets and the heater requirements for KT120 is listed as 1.70-1.95 amp @6.3 V, while KT150 is listed as 1.7-2.0 amp. So, if the ARC ref 75 can light up four KT120s, it can do the KT150s. 3. No, KT150s will not stress the transformers if the bias/cathode currents are set properly.

Unless @techguy0192 has information to corroborate his advice, I would suggest OP @CORALX8 sets his bias correctly and enjoys the KT150 tubes.

 

[techguy0192]

Don't go sticking KT150s into a standard, non-SE Audio Research Reference 75. It’s a bad idea, and the official "SE" factory upgrade path exists for a reason; it's fundamentally required to make KT150 tubes work safely.

The Ref 75 designed around the KT120's electrical parameters, heater current demands, and plate dissipation characteristics. Shoving higher-draw KT150s into a stock circuit pushes the power supply and operating points past where they were ever meant to go. We're talking about Audio Research here, a company that listens to the difference between different types of solder and when/where to place damping material on capacitors. When component design reaches that extreme level of detail, you can probably be sure that care was taken in the electrical and transformer design, not off the shelf stuff from PartsExpress.

If anyone tries to wave away the risks by pointing at overlapping minimum and maximum bounds on a generic datasheet, they're ignoring the reality of worst-case manufacturing tolerances, cumulative headroom, and the actual transformer specs ARC built into the unit.

Consider the heater current alone. A quad of power tubes runs on a shared heater winding, which means small per-tube variances add up fast. If a quad of KT120s leans toward the lower bound of its tolerance while a quad of KT150s sits at the absolute maximum, your total heater current draw jumps by over 1.2 amperes across that 6.3V winding.

Anyone arguing that KT150s won't stress a non-SE Ref 75 is focusing too much on quiescent bias current as if it's the only metric that matters, completely glossing over dynamic plate dissipation, output transformer impedance, and core saturation limits. The 65 mV we're talking about is not a constant value. Connect a DMM and it's easy to see the voltage swings while music is playing.

Again, don't drop KT150s into a Ref 75 that hasn't been updated to SE status. Don't take my word for it, call ARC directly and try to order a set of KT150s for a stock Ref 75 without the SE update. They won't sell them to you.

 

[spantou01]

I am eager to hear what changes the ARC SE version actually made over the “standard Ref. 75, to accommodate the KT150 tubes. I bet that does not include a power transformer or output transformer swap. Not needed, because surely the ARC designers would allow for the worst case scenario of 4 KT120 tubes that all draw 1.95 amps each. The worst case with Kt150 draws only 0.2 amp (2.75%) more than KT120s. How do you get 1.2 amps?

It is possible that ARC’s SE modification made some improvements to the circuit, and sounds better, but how does that affect the suitability of KT150 in a Ref 75 without these “upgrades”. Power transformer and output transformers stay the same, so the power tube operating conditions stay the same. From what I can tell, the performance curves and parameters are basically the same for KT120 and KT150. So, I still see no actual reason why OP could not continue to enjoy his Ref. 75 with the set of KT150s, that he already paid for. I would like to hear the facts in engineering terms of why @techguy0192 advise against it.

Comparison KT150 versus KT120

Caused by some agressive advertisement around the KT150 ("up to 300W out of a pair in PP") I have made a quick evaluation of that tube and compared it with the KT120. I have used may PP powerstage bench - picture and schematic below - able to drive AB1 and AB2. The OT (250W, 20Hz-20KHz , - 3db...

www.diyaudio.com

 

[gadget73]

no schematic and no upgrade instructions posted, so its hard to say what the difference really is. Looks like newer ARC doesn't include schematics and parts lists in their owners manuals like they once did.

 

[techguy0192]

I am eager to hear what changes the ARC SE version actually made over the “standard Ref. 75, to accommodate the KT150 tubes. I bet that does not include a power transformer or output transformer swap. Not needed, because surely the ARC designers would allow for the worst case scenario of 4 KT120 tubes that all draw 1.95 amps each. The worst case with Kt150 draws only 0.2 amp (2.75%) more than KT120s. How do you get 1.2 amps?

It is possible that ARC’s SE modification made some improvements to the circuit, and sounds better, but how does that affect the suitability of KT150 in a Ref 75 without these “upgrades”. Power transformer and output transformers stay the same, so the power tube operating conditions stay the same. From what I can tell, the performance curves and parameters are basically the same for KT120 and KT150. So, I still see no actual reason why OP could not continue to enjoy his Ref. 75 with the set of KT150s, that he already paid for. I would like to hear the facts in engineering term of why @techguy0192 advise against it.

Comparison KT150 versus KT120

Caused by some agressive advertisement around the KT150 ("up to 300W out of a pair in PP") I have made a quick evaluation of that tube and compared it with the KT120. I have used may PP powerstage bench - picture and schematic below - able to drive AB1 and AB2. The OT (250W, 20Hz-20KHz , - 3db...

www.diyaudio.com

Your post is making a fairly definitive engineering claim, but the information you've presented doesn't establish that conclusion.

The fact that the published KT120 and KT150 heater-current specifications are relatively close doesn't, by itself, demonstrate that a standard Ref 75 was designed, tested, and qualified by Audio Research for KT150 operation. Likewise, setting the bias correctly doesn't establish that every other operating parameter and component margin is appropriate for the KT150.

I'm confident enough in my recommendation that I'd suggest the OP do the simplest thing: call Audio Research on Monday and ask them directly whether they consider KT150 operation acceptable in a standard, non-SE Ref 75 and what the SE modification changed to accommodate the KT150. They can explain the engineering rationale far better than either of us can infer from a tube datasheet.

Until then, I don't think "the heater current is close and the bias can be adjusted" is enough to conclude that KT150s are safe for a non-SE Ref 75. Given that the OP has already reported the amplifier running extremely hot, I'd err very strongly on the side of following ARC's specification rather than experimenting with it.

 

[spantou01]

Fair enough! @CORALX8 please do so and let us know what the gurus say, and why.

 

[gadget73]

its also not impossible that the running hot has little to do with the KT150, and there is some underlying issue. Or the heat is just normal for this amp, but greater than OP expects.

 

[Blue Shadow]

How do you get 1.2 amps?

Minimum KT120 heater A is 1.7 x 4 tubes = 6.80A
Maximum KT150 heater A is 2.0 x 4 tubes = 8.00A

1.2A difference based on what 0192 said could happen wrt tube heater current for the two tube types.

 

[CORALX8]

Fair enough! @CORALX8 please do so and let us know what the gurus say, and why.

m not sure what you mean by this. Are the plate glowing red? I.e. red-plating? Of all the tubes: power and small signal tubes? Are you familiar with how hot tubes typically get? My guess is that the glow you observe is from normal heater operation.

By all the tubes I literally mean all the tubes, both the kt150 and the 6h30s are glowing pretty red (especially the 6h30). Im not familiar with tube amps and more familiar with preamps and ive never seen signal tubes run that red ever. I dont know if i should power it on again and take pics just to show because im worried itll straight up damage the amp

Did you find explanation for the taboo to run KT150s in the ARC ref 75 (without SE). What makes the SE different?

I think the SE had larger spec caps to handle the kt150s and they changed some other irrelevant stuff (wires etc), not sure about other stuff

 

[CORALX8]

its also not impossible that the running hot has little to do with the KT150, and there is some underlying issue. Or the heat is just normal for this amp, but greater than OP expects.

As far as sound goes the amp sounds fine, I do see 4 small burnt marks at the top where the kt150s are (In fairness, i dont know if theyre left by the 150s or 120s) and im not sure if thats normal or not

 

[CORALX8]

The amp now has a slight hum but only when the tubes ar eheated up (e.g. when I first turn on no hum, after tubes begin to heat up the hum starts)

I think theres a biasing problem?

 

[Blue Shadow]

You can look for images of tubes red plating, the metal in the tube glowing red, not just the heaters which are usually inside the inner works and you only see part of the heater glowing. It should glow, most of the rest of the tube guts should not. You can also see why it is called red plating, it is the plate glowing red.

 

[whoaru99]

By all the tubes I literally mean all the tubes, both the kt150 and the 6h30s are glowing pretty red (especially the 6h30).

A picture is worth a thousand words. Right now it's a bit speculative what that really means.

Red, as in like the tube just right of center in this pic?

 

[techguy0192]

The amp now has a slight hum but only when the tubes ar eheated up (e.g. when I first turn on no hum, after tubes begin to heat up the hum starts)

I think theres a biasing problem?

Is the hum coming from the transformer or are you hearing a hum at the speakers?

Do you have an infrared thermometer? actual temperature readings would be helpful.

Lastly...

"I think theres a biasing problem?" ----- I think you should contact the ARC service department and consider using an different amplifier until everything has been sorted out.