Tube amp question

I am a novice tube amp owner. I have had my Black Ice Audio Fusion F22 for not quite a year. I want to understand plate voltage and how it impacts tubes or vice versa. I want to roll the tubes to the KT150's which Black Ice has said can be used without issue. The plate voltage of the amp is 500v. In other posts, someone said that KT150's require a minimum of 600v at the plate. Since Black Ice says their amp can use them, does this rating depend on the circuit? I have some basic understanding of electronic circuits and I googled plate voltage but I'm still not clear on what it is exactly. Can someone knowledgeable with tube amps help me understand this? Thanks!


I'm not expert but I think if you run the KT150s at the 6550 milliamp setting you may not be getting everything out of that tube, power wise. It will still work and last a long time but you perhaps will not get the benefit of higher power. Somebody smart will come on line and post the accurate details....

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KT150's require 600 volts at the plate. If you are not able to supply that voltage, i cannot believe you would be getting the best SQ from this tube. Bigger question is whether the transformers on your amp will suffer?

KT150's require 600 volts at the plate.

@daveyf @falconquest This statement is false. If you look at the tube's specs

you'll see that for class A operation (single tube) 400V is recommended. This would be the most current you would want to run on the tube for nominal lifespan. When running 500V you would run less current and even less if running at 600V.

IOW the class of operation is playing a role. If in class A when the tube is always conducting you don't want to run as much plate voltage if you want the tube to be reliable.

Contact Black Ice for the recommended bias point when installing this tube in their amp.

Thanks Atmasphere! this is from an email directly from Jerred at Black Ice...

Here’s a list of the tubes you can use with F22.
EL34, 6CA7, KT77 (Bias is 40mA)
6550, KT88, KT90, KT100, KT120, KT150, KT170 (Bias is 50mA)
So this tells me that the amp transformers should be compatible with this tube. With a 500v plate voltage will I be realizing the full potential of this tube? (it appears yes) It is a fully balanced differential push/pull style, class A/B amp. Can someone also explain "plate voltage"? Thanks for all the responses!


@atmasphere If you place a KT150 on a tube tester, it will not get to spec unless 600 volts is available. As a result, there are few tube testers that can actually test this tube.

@falconquest Plate voltage is the voltage on the tube from cathode (usually at or near ground) to the plate of the tube. The plate is exactly what it sounds like; it receives electrons that flow from the cathode. The audio signal is imposed by the grid (another element of the tube) over the much larger electron flow from cathode to plate; in this way it can amplify.

@daveyf If it were me I'd be testing a KT150 as a KT88 since our tester is an older Hickok. I would also not be surprised to find that a KT150 might not test properly under those circumstances.

That does not mean that the tube does not work at 400V. It simply points at a limitation in the tester.

Thanks @atmasphere that clears it up. I have done enough research to understand the parts of the tube so what you say makes sense to me. I appreciate your thoughts. BTW, I have to say I am intrigued by the new class D unit. Thanks again!

@atmasphere While I agree with you that a tester that cannot properly test a KT150- shows the limits of the tester, ( BTW, yours- nor any Hickok- I am aware of can test these tubes--and your KT88 spec is no good for them) it also points to the fact that they need more voltage than 400 volts to come to spec, IMHO.

So @daveyf are you saying the 500v plate voltage is not enough to realize the full potential of the tube? If that's the case then why does Black Ice indicate they will work? I'm not taking sides with anyone, just trying to learn. May the debate continue!

If that's the case then why does Black Ice indicate they will work?

Because they will "work". You just might not get all the power that tube can produce.


If you use the KT150, I would bias it to 80ma. Based on the amp power specs for the EL34 and 6550, the bias settings of the tubes and 500V at the plate, the amp operates in Class A/B and the output transformer is about 6500 ohms in Ultralinear. The manual also shows the ammeter's upper limit as 80ma.

The EL34 operates at about 5 watts Class A, and A/B to its rated power. The 6550 runs about 12 watts Class A. 

The KT150 at 80 ma with a 6500 ohm xfmr at 500V will operate in Class A for 20 watts and max out in Class A/B at about 75 watts. The bias point is a little less than 60% of the tube dissipation which is on the cool side but within the sweet spot envelope for Class A/B. The tube should work fine. But... save your money. The KT120 operates identically biased at 80ma and runs 70% dissipation, which is the upper limit for Class A/B operation.


If I may, three different scenarios to share. The first two are friend’s amps I helped them with when they were ready to make tube changes. The last mono block amp(s) mentioned are mine.

1. The Jolida amp my friend Alan had KT150s in it, worked fine, sounded neutral, lacked the velvety rich sound. Swapped to PSVANE KT88s. More engaging now. My friend loves it, everything he was looking for from the sound in this circuit.

2. The Black Ice F22 my friend Richard has came new with lower grade EL34s. He swapped in new production Genelex Gold Lyon KT77s. He truly enjoys the change and sound after there was 50-70hrs on the 77s. He listens to it daily now. I don’t think he’ll ever try KT150s after hearing this setup.

3. My own tube mono block amps have massive transformers, came stock with KT150s, sound very linear, super clear, neutral, amazing sound stage and detail. The designer recommends running them as low as 40ma and high as 60ma. I run them at 50ma per tube. Sometimes I’ve felt it was lacking velvet tube sound, kinda missed KT88s. Can’t run 88s in my amps, too high of plate voltage. Split the difference with KT120s, a more lush tone. After the KT120s had 100hrs on them, I have no desire to put the KT150s back in immediately. KT120s were as close as I could get back to 88 sound. Been enjoying them past three months. 

Your F22:

fwiw, I’ve heard your F22 amp and it can sound really nice moving from EL34s to KT77 or KT88s. Hey, KT150s are expensive. FYI, my four KT150s are sitting nicely tucked away in their boxes. Bigger is not always better, IMO. You can also try extra nice KT77s or KT88s s in the F22 if you want to stay closer to a rich full sound without being overly neutral. Trying to save you from skipping a good in-between option to consider. Some of my other friends rotate from 150s back to 88s, 77s - often.

I have the Black Ice F35 and I tried running it with a matched quad of new KT120 tubes and at around 60-65 watts the amp shut off and went into protection mode.

I talked to a Mr. Michael Allen, who told me it was OK to run the KT120 tubes and to bias them the same as the 6550 tubes, but when I talked to him again to tell him about the amp shutting down with the KT120 tubes he informed me that though the amp will run with KT120 and even KT150 tubes that the best performance and sound quality will be had with the EL34 or 6550 tubes, that no extra wattage will be achieved since the transformers are the limiting factor.

He also stated that future Black Ice amps might possibly be designed from the get go to run the KT120 and KT150 tubes to achieve more power.

So I just removed the KT120 tubes and reinstalled the 6550 tubes and that is what I run currently, but I did buy another matched quad of KT120 tubes and run all 8 in my Primaluna Dialogue Premium HP integrated amp and they sound great and really add more detail to the top and bottom end.

Maybe your results will vary with the Black Ice F22, see for yourself and PM me with your results.

OP, I would ask Mr. Allen to confirm that there will be no damage to his transformers by running the KT150’s...and maybe have him put that in writing. While the amp may supply ’enough’ voltage for the KT150’s to work, as russ69 pointed out above, you will not get all the power that the tube can produce.

Personally, I would be more concerned about using a transformer that was not originally designed for this tube, but if the manufacturer is ok with be it.

Just a little more background here. The amp came with the EH EL34's. Having read a lot about the Genelex Gold Lions, I decided to give them a try (same EL34's). The sound quality was improved and I really liked them but they seemed to lack a bit of dynamics that I'm looking for. I am driving a pair of Legacy Audio Signature III's which are 4 ohm and 93db sensitivity. I'm looking for a bit more kick from them that I know they can provide.I purchased a quad set of KT120's and after allowing them to warm up, I biased them at 50mv per the manufacturer spec. I slowly raised the volume and after about 10 seconds the amp shutdown in protection mode. I emailed the factory and they believed that I must have a bad tube. This happened just after the Russian invasion of Ukraine and the run on tubes. I could not locate another set of KT120's anywhere. As I researched more I stumbled across a guy who had four remaining KT150's that are actually ARC matched. I am waiting for those to arrive. It will be interesting to see if the amp reacts the same or how it will perform. This damned Russia thing happened right in the middle of my experiment. But I don't at all want to devolve the conversation into politics. I appreciate everyone's insight

When I spoke with Jarred about this while helping a friend on his, they were open to the idea of running other tubes in the F22 yet there was little feedback and what the amp was really optimized for other than EL34s, originally.  For the little extra "kick" you are looking for, that's why I was recommending a slight jump up to nice KT88s [for this particular amp]. Who doesn't have 3-4 different sets anyhow, lol. :)   

So now we hear that the OP and one other member has had the same problem when running KT120's. The amp went into protection mode. I would think if it does that with KT120's...nothing good can happen when you roll in KT150's.

What concerns me is that some of you are telling this person to bias to a certain level on his amp. It should be noted that bias levels are not universal from amp to amp. No Hickok ever made will properly test most power tubes which would include 6BQ5s. Now this doesnt mean that a tube wont continue to be linear as the plate voltage is increased from tester to tester, but most suitcase testers will not give you an accurate reading because they dont test at real world operating levels. 


For the record I do not like amplifiers that can accommodate a wide range of power tubes. Some have pointed to the possible problems and shortcomings of such designs. However, when in doubt it is a good idea to listen to what Ralph at Atma-sphere has to say on the subject.

best performance and sound quality will be had with the EL34 or 6550 tubes, that no extra wattage will be achieved since the transformers are the limiting factor.

As I suspected. 

I biased them at 50mv per the manufacturer spec. I slowly raised the volume and after about 10 seconds the amp shutdown in protection mode.

Even biased at 50mv, it sounds like they are drawing to much current for the transformers to supply. Stay with EL-34s, 6550s or KT88s. 

FWIW I rolled from stock EL34s to KT150s on my PrimaLuna Dialogue Premium HP integrated and it revolutionized the SQ from that integrated. Very happy with the decision to roll to the KT150s. 

Hello Falconquest. Tubes work by making electrons flow from the cathode to the "Plate" (or anode). "Plate voltage" refers to the amount of attraction (positive voltage) we apply to the plate to get the electrons to flow. The higher the voltage, the more eager the electrons are to flow. We control the amount of "normal" flow by establishing the "bias" (the no Music) flow by appropriate circuitry. By applying a signal (the music) to a fine screen (like the one that keeps the flies from coming in your window - the grid)) in between the cathode (a cylinder of metal coated with a substance the produces a cloud of electrons when heated) and the plate. Physical construction of the tube's innards vary, but you get the idea. The normal current flow is changed by the signal on the grid. It's that change that interests us. That changing electron flow goes through a transformer. The transformer has two sets of wires wrapped around it's metallic core. The Primary Winding carries the current going through the tube on its way to the source of positive voltage. The current varies with the music and produces a voltage in the Seconday Winding of the transformer. The transformer's secondary is connected to your speakers and produces the sound in your room. There are ways of eliminating the output transformer, but it's quite a trick. The easiest (and cheapest) way to get more power out of the amp is to raise the Plate Voltage. But that shortens the life of the tube. So, since it's the change in the current flow through the tube that produces the musical output, if we can get the same amount of current change at a lower plate voltage, we get less wasted heat, lower power consumption, less expensive output transformers, and longer tube life. Bob Carver produces certain models of amps with tubes that run so cool that you can put your hand on the tubes while the music plays and not burn your hand. Those tubes last for decades, not years. Happy listening.

While I agree with you that a tester that cannot properly test a KT150- shows the limits of the tester, ( BTW, yours- nor any Hickok- I am aware of can test these tubes--and your KT88 spec is no good for them) it also points to the fact that they need more voltage than 400 volts to come to spec, IMHO.

I don't think the Hickok subjects tubes to full operating voltages. There is no B+ voltage adjustment for that purpose. It seems that you have to explain why 400V is specified as an operating point for that tube when in class A mode.




I just wanted to close the loop here. I received the KT150’s yesterday and carefully installed them, I turned the amp on and allowed them to warm up and adjusted the bias (which took minimal adjustment) then let it settle in for several minutes, started some music and s-l-o-w-l-y raised the volume. They worked just fine. Not only did they work fine but they sounded great right out of the box. I could tell the music had more authority over the entire audible spectrum. I was very taken by how clean they sounded and the level of detail.

I had some work to finish up so I sat typing on the computer listening to Tubular Bells that randomly came up. Near the end of the piece Oldfield introduces the different instruments. The first time he spoke I nearly jumped out of my skin because his voice sounded so real. There was someone in the room with me! I only have about an hour or so on the tubes but already they are sounding fantastic! I don’t know if I am indeed getting everything out of them, I’ll let that debate rage but I sure do enjoy what I’m hearing!

Plate voltages some mfg companies run nearer to max on the plate for they may sound a bit better ,but at the expense of tube longevity.

i have owned black ice f35 and has very good size transformers ,

ican see the F35 have big enough power supplies  but the F22  is pushing it 

for sure it will tax the power supplies much more  kt88, or maybe even  KT 120..

Did you speak with tech support at Black ice about using KT 150 s

Jarred too should know  thsts a Bigstep up from stock EL34 s


I absolutely asked Jerred. That was the response I posted earlier in the thread.