KT120 Amp build

kward

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Thread Index
  1. Post #1: General discussion of design goals, circuit topology, and improvements
  2. Post #238: Build schematic
  3. Post #278: Bill of Materials
  4. Post #343: Chassis layout
  5. Post #351: Construction & Testing
  6. Post #454: Measurements and Tuning
  7. Post #499: Final schematic with 12AX7 input
  8. Post #521: Final schematic with 5751 input

On this thread I intend to document the build for a 60 watt per channel KT120-based amplifier. I want to try the KT120 tube because I've never used it before, and everything I read about it indicates its a really great tube.

This is a continuation of http://www.audiokarma.org/forums/showthread.php?t=492619.

Attached are the schematics. I’d like some feedback on the circuit design. Is there some dumb thing I’ve done that if changed will improve the design? Is there anything missing that would greatly improve the sound or performance? Fire away with your comments.

After I get feedback on the circuit design, I plan to document the build on this same thread with a parts list and pictures as I progress through it. It’ll take a while (2 or 3 months) but I think it will be a good thing to share in the public forum. I hope tubes never die, and I’m doing my part to ensure that they don’t.

I’d love someone else to try this build also. It’s not exactly a beginner build, but you’ll have a great sounding high performance amp when you’re finished.

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Here’s the circuit description and design notes for those who are curious: (warning, the rest of this is kinda long)

One thing you should know is that I’m only showing one channel for the audio frequency amplifier (schematic page 1), so you’ll need to build page 1 of the schematic twice for stereo. The rest of the circuitry (schematic pages 2 and 3) are sized and shown as-is for two channels.

My design principles for this amp were to have no preconceptions of what works or doesn’t work and to make it as absolutely simple as possible while still maintaining very high quality sound reproduction. Input sensitivity is 5V RMS to drive the output stage to full power. (Edit--this has now been addressed. Input sensitivity is now about 1.15V RMS). This amplifier will produce about 60 watts per channel.

Schematic Page 1: (Audio amp)

The first stage is a common cathode voltage amplifier. I’ve tried other configurations for the first stage, such as a mu follower, SRPP, constant current sources, even a pentode, and they don’t make it sound better and just add complexity. I’ve also tried other tubes for this stage such as a 6DJ8/6922, and I don’t like them as much in this position. The good ole’ 12AX7 is a perfect tube for this stage and there are lots of choices for tube rolling, including many good NOS choices. It might be interesting to try a pentode strapped as a triode. If I did that, I’d consider the EF86.

Capacitor C23 is used to reduce HF instability. This is one part of the circuit I haven’t tried before so I’m guessing at the capacitor size. Somewhere between 330 pF and 470 pF seems about right and actually anything in that range will probably work just fine.

Circuit ground (labeled G in the schematics) is expected to be a ground bus. The circuit ground should be connected to the chassis at one point only, near the input tube. When I build it, the ground bus will be a 12 gauge bare copper wire pulled from a length of Romex.

The second stage is a cathode coupled differential pair phase splitter. I’ve tried the cathodyne configuration also, and while it’s a very simple phase splitter, I’d need a lot of headroom to run the output tubes and I didn’t have that kind of voltage available. I’ve also tried other tubes for this stage, including the 6H30p and 12AU7. The 6SN7 is the best tube I know of for this stage. It has inherently low distortion, allows very large signal swings, and has an octal base so it’s easy to work on when wiring it up.

The first stage is direct coupled to the second stage. I’ve tried capacitor (AC) coupling these stages and it doesn’t improve performance and adds complexity when instrumenting the feedback circuit.

The output tubes run at ~485 volts on the plates and are biased at about 60 mA per side. This is a medium idle for these KT120’s (!!) so these tubes should last a good long time. There is a balance adjustment circuit that is intended to be accessible from the top side using a screwdriver and a volt meter to keep each side of the push-pull pair at equal idle current. Since I didn’t make the bias of each output tube separately adjustable (seemed like added complexity that was unnecessary), you’ll need to use a matched quad for best left/right channel balance. You can direct substitute 6550, KT88 or KT90 and the amp should work just as well.

If you don’t change the fixed bias setting, you can even directly substitute a KT66 (but not a 6L6!!) and it will work pretty well, although it will idle the KT66’s at about 60% of capacity and output power will drop to about 45 watts per channel. Maybe try this if you’re really bored and want to tube roll, but ideally it’s not designed directly for a KT66. Just don’t try a 6L6. It’ll likely red plate them or fry the screen grids, or both.

The two output transformers that I know will work well with this amplifier are the Hammond 1650R and the Triode Electronics A431 Dynaco clone. Both should give really good performance, but the Hammond will reduce the output power to about 70 watts per channel since it's primary winding is a little higher impedance.

Schematic page 2 (Power supply)

Here’s where it gets a bit more interesting. The power transformer is a toroid. I’ve never had good luck with stacked EI core power transformers — they are more prone to mechanical buzz and can create hum problems in the audio circuit. Toroids are designed to keep most of the electromagnetic field inside the confines of the core. The toroid I’m using is from toroid.com. I chose it because it’s an “off the shelf” part and is a multi-wind, so I can fit everything on one chassis. R26 is a thermistor that limits current surge so the fuse doesn’t blow when you flip on the power switch.

Capacitors C9 and C10 are special. They are relatively new from ClarityCap, called the TC series. These are film caps and have extremely low ESR so they deliver energy almost instantaneously. Also these caps are rated at 700 volts so there is no need to stack them to get the required voltage tolerance. Downside is they are HUGE! If you don’t want to use the Claritys, you can easily substitute two stacked 220 uF electrolytics rated at 350V in place of each Clarity, and the amp will still sound really good.

I added a high voltage (HV) regulator so that at large signal swings (high output power), the voltage supplied to the input tube and phase splitter tubes will remain more or less constant. This greatly reduces distortion for both the first and second stages. It also improves bass response at high volumes.

Here’s where I may get some questions about why I did it this way. All filaments, including the power tubes, are DC supplied. I did this out of consideration for total heat generation and hum reduction, plus the availability of the windings on the toroid—they were there. I’m running the output tube filaments in series per pair of tubes, so that only half of the current is needed to power all four filaments (about 3.85 amps). A straight up all parallel connection using AC would require the transformer to supply almost 8 amps. This DC configuration will generate significantly less transformer heat and won’t saturate the core as much. The toroid is going to get a little warm, but I intend to mount it top side, so it will dissipate heat easily. The amplifier itself should run fairly cool.

Schematic page 3: (HV regulator and power supply)

The HV regulator is a series-pass design. I’ve tried many tubes for the series-pass element, such as a 6C19p, 6AS7, EL34, and a 6L6. They all work well, but when I tried the 6V6 (a new production Tung-Sol), I about peed my pants! It sounded that good. One thing I would like to try is a cold cathode constant voltage source instead of the 150V Zener. I think that might require a two chassis build though, as it’s probably going to be tight to get everything here on a single chassis. That might be one area I will experiment with in the future. I hear cold cathode voltage sources in voltage regulators can sound really awesome though.

The filament supply for the HV regulator is DC supplied because I needed to keep the voltage between the cathode and filaments within specification, so I’ve raised the filament bias to 266 volts. The 12AX7 error amplifier receives the same raised voltage on its filament.

Well, that’s the design. This should be a fun build and I think it’ll be a cool running gorgeous sounding little amp.

Kevin
 
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I am in! Been thinking about playing with this tube for some time. I am a semi-experienced newbie/ex-ham radio guy.

I'd like to see a design that uses auto-bias and modern semiconductor based PS section, FWIW.

Let's see who else wants to join in!
 
I am in! Been thinking about playing with this tube for some time. I am a semi-experienced newbie/ex-ham radio guy.

I'd like to see a design that uses auto-bias and modern semiconductor based PS section, FWIW.

Let's see who else wants to join in!

Interesting...I cooked up an opamp based self bias circuit that I was originally going to use for this design. I'm envious of other amp builders that have figured this out. I finally decided against it because I'm no good at PCB design and...if for any reason the autobias circuit fails---poof there go your tubes and your output trannies!! But I'd love to see someone tackle that in the public forum also, as I'd love to incorporate it.
 
Hello Kevin. I am in also. I have watched your posts with great interest. Thank you for the schematics. I have Hammond output transformers already as well as two Baldwin power transformers from organ amplifiers. I think that my power supply has to be modified and simplified but this should be easy. So, when do we start?:yes: Robert.
 
HI Kevin,

It appears that you are not afraid of a bit of complexity since you are building tube voltage regulation. I might suggest, based on personal experience, that you use a CCS in the tail of the phase splitter instead of the 20k resistor. This will allow you to use matched anode loads. Is should reduce distortion and provide nearly perfect phase splitting even with unmatched halves in the 6SN7.

Good luck with your project. Should be a good one.
John
 
HI Kevin,

It appears that you are not afraid of a bit of complexity since you are building tube voltage regulation. I might suggest, based on personal experience, that you use a CCS in the tail of the phase splitter instead of the 20k resistor. This will allow you to use matched anode loads. Is should reduce distortion and provide nearly perfect phase splitting even with unmatched halves in the 6SN7.

Good luck with your project. Should be a good one.
John

Yeah, great comment. Tried that!! (forgot to state so in the intro). I tried a pentode, a triode (6CG7), and silicon. (I've been honing this desing for a few years now). Your're right it does work really well. It would be fairly easy to add, but...it didn't sound better to me, so I left it out. (I'm sure the amp measured a bit bitter though).
 
Having a plate dispensation of 60 watts, the KT120 is the most powerful tube of the KT88/6550 types.
one pair of these tubes in push-pull configuration can achieve 150 + Watts. The KT120 tube is one of
the best in newer production vacuum tubes. Its also a superior replacement for KT88/6550 types, if it
will fit in your chassis. This KT120 also draws more filament current (95-280 mA avg.), Make absolutely
certain that your power supply can deal with this increase.

Most fixed bias amps have enough range to dial KT120 tubes in to the recommended current draw for
that amplifier, though some do not. Even if you are unable to bias the tube all the way down to the
recommended current draw you can probably still use KT120 tubes. You'll just have to do a little math
to be sure the tubes are running safely. Simply take the plate voltage and multiply it by the current
measured in mA. For example, if your plate voltage is 500v and you are measuring 70 mA across a
single tube then the math goes like this 500 x 0.070 = 35 Watts. Since this is a 60 watt dissipation tube
and the rule of thumb is to run tubes at 65% of their maximum dissipation (60 x .65 = 39 Watts),
70 mA across a single tube in this case would be just fine even though the manual is likely to
recommend 50 mA for a 6550 tube.

(The above info was mostly gleaned from a TC TUBE text)

"Make sure you know what your doing and have a wonderfully successful build."
 
Hello Kevin. I am in also. I have watched your posts with great interest. Thank you for the schematics. I have Hammond output transformers already as well as two Baldwin power transformers from organ amplifiers. I think that my power supply has to be modified and simplified but this should be easy. So, when do we start?:yes: Robert.

Hey hey Robert! Thought I might see you here. I'm ready to start. I'm working on sketches for the chassis layout and the bill of materials now. Might get them posted tonight before bedtime.

My issue right now is money. I'm going to have to spread the purchase over two paychecks. So that means no drilling the chassis (for me at least) until Feb 15th or so. If I stop buying vinyl for a few months, it will help it move a bit faster :D
 
... the rule of thumb is to run tubes at 65% of their maximum dissipation (60 x .65 = 39 Watts), 70 mA across a single tube in this case would be just fine even though the manual is likely to
recommend 50 mA for a 6550 tube.

"Make sure you know what your doing and have a wonderfully successful build."

Yup, this design will certainly allow you to crank up the juice to run the KT120's hotter. The power trannie and output trannies are sized to handle it. Just adjust R24, to maybe -50v or so. (that's a big FAT GUESS as I haven't built it yet!!).

Oh, and I've had my share of electrical shocks. I usually clip the negative probe of my volt meter to ground, wear shoes and socks, and put my left hand in my pocket when measuring voltages with the amp turned on!!
 
Kevin -- Neat project and nice design! Two comments I would offer:

1. If you're opposed to sand in the tail of the inverter circuit, you might then consider tying the tail to your negative bias supply. That will allow you to maintain the same current draw with a larger tail resistance for improved balance.

2. It appears that you have a DC balance system built into the cathode circuit of the output tubes. At first glance, it would appear that there is not nearly enough range available with the component values shown, and at the very least, varying the resistance in the cathode legs will upset any current readings taken there -- unless you plan to measure current draw directly. It is usually best to accomplish DC balancing at the control grids of fixed bias designs.

So, whadda ya think? have it ready to go by say, next weekend? :)

Dave
 
I'm curious as to this statement as to why:

"The drawback to direct coupling is that you get little or no gain out of the stage."

I would also think that using a high mu 12AX7 you could design this for much more sensitivity than 5 v rms input?

I would triode-strap a pentode here as well but it would be the 6EJ7...
 
I've never tried a tube CCS, just FET's. I now always use cascoded depletion mode FETs.

Nerdorama, My forum etiquette might need some sharpening. What I should have said was that even though I probably won't consider it for the build I'm doing, others might. Would you be willing to post the circuit modifications to the tail of the phase splitter so others could decide if they want to build it that way?

(and sorry about that)
 
I'm curious as to this statement as to why:

"The drawback to direct coupling is that you get little or no gain out of the stage."

I would also think that using a high mu 12AX7 you could design this for much more sensitivity than 5 v rms input?

I would triode-strap a pentode here as well but it would be the 6EJ7...

I've never used the 6EJ7. Why is it your choice?

Concerning the gain or sensitivity of the amp: As the size of the tail resistor (R9 in the schematic) approaches the size of the plate resistors, the closer to unity gain each leg of the differential pair will produce. Using existing resistor values and HV supply voltage shown gives differential gain of 0.98. Thus all the gain needs to come from the first stage tube. The first stage tube is already "maxed" out for gain --I've eeked out all the gain I can, and that's at about 49x right now. I *could have* used a bypass capacitor across the 2K ohm resistor on the cathode of the 12AX7 of the first stage and that would increase gain to about 70x or so, but the stage becomes a little less stable ....and .... you guessed it...it doesn't sound as good. (because now there's a big ole' electrolytic cap in the signal path).

The differential pair tubes needs a voltage difference of -4 volts (in this design) between the grid and the cathode to put it at the operating point I selected. So the design calls for a 20Kohm resistor to bias the cathode at 124 volts which is exactly 4 volts higher than the grid at 120 volts.

If you AC couple the stages, you place a capacitor between the stages and reference the grid of the differential pair to ground. Thus theoretically the tail resistor can become quite small because you just need to raise the cathode to +4V (instead of +124 when direct coupling). That would give you a differential gain of about 5x I expect. BUT then the stage will be *really* unbalanced using such a small tail resistor and it will sound terrible...so the best approach is to use the largest tail resistance you can get. Thus I've just described conflicting requirements...one requirement says make the tail resistor as small as you can, and the other requirement says make the tail resistor as large as you can.

This is why the other poster recommended a constant current sink because it appears as a very very large tail resistance as seen from the cathode but takes only a few volts to drive it. (Dang that silicon is good sometimes).

If you did those things, it would probably increase the sensitivity of the amp to around 1V RMS so you could drive it directly from a CD player for example, or use a passive pre.

That may not have been a very clear explanation. Others may have better answers (feel free to chime in)...but this is my knowledge on the matter.

Since I'm using a preamp anyway for other purposes, it wasn't a concern for me to have a power amp with this lowish sensitivity. Besides, I like direct coupling stages when I can because it removes one capacitor from the signal path (so it sounds better...usually :)
 
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Nerdorama, My forum etiquette might need some sharpening. What I should have said was that even though I probably won't consider it for the build I'm doing, others might. Would you be willing to post the circuit modifications to the tail of the phase splitter so others could decide if they want to build it that way?

(and sorry about that)

No offense taken. Keep it going. This is going to be a neat project.
 
Kevin -- Neat project and nice design! Two comments I would offer:

1. If you're opposed to sand in the tail of the inverter circuit, you might then consider tying the tail to your negative bias supply. That will allow you to maintain the same current draw with a larger tail resistance for improved balance.

2. It appears that you have a DC balance system built into the cathode circuit of the output tubes. At first glance, it would appear that there is not nearly enough range available with the component values shown, and at the very least, varying the resistance in the cathode legs will upset any current readings taken there -- unless you plan to measure current draw directly. It is usually best to accomplish DC balancing at the control grids of fixed bias designs.

So, whadda ya think? have it ready to go by say, next weekend? :)

Dave

Yeah, those are both great suggestions. #1: Done. #2 I gotta think about. It feels better than what I was doing though. :scratch2:
 
Here's the proposed Chassis Layout

Okay attached is the proposed chassis layout.

The chassis I'm going to use is a regular 17" x 14" x 3" Hammond aluminum case. I'm no good with wood and have no wood tools, so a metal case is my only viable option. I do know all the parts will fit in there even though it does look tight from the sketches. If someone out there is great with wood--knock yourself out. Just be sure that the inside dimensions are about 14" x 17" so that all the parts will fit.

You may ask "what's he doing with MDF for bottom rails in a power amp build?" Well, I just need something to add rigidity to the case after it's all done, and to protect the bottom somewhat but still leave room for air circulation.

I don't like metal bottom plates...reacts too much to vibration and you gotta drill the daylights out of them to get enough air circulation. MDF is cheap and easy to cut and if you lay out three strips (each 3 1/2" wide) evenly spaced along the 14" axis, it leaves just about the right amount of open space between the strips for air circulation. Plus I expect it controls vibration pretty well if it's used in speaker cabinets a lot.

But hey, anything would work for the bottom plate.

This is pretty much all I've got time for this week. Next weekend I'll finalize the schematic changes and hopefully get a bill of materials posted with part numbers, recommended vendors, and such. Then it's time to start ordering parts :D. Start sharpening your drill bits.

I'm going to order all my iron tonight though since it takes a while to manufacture and ship it.
 
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Okay attached is the proposed chassis layout.

By the way, Bridge rectifier 1, 2, and 3 (BR1 BR2 and BR3 in the schematics), as well as the 12V regulator are mounted on the side walls of the chassis in their approximate locations shown on the chassis layout.

Something to think about: The ClarityCap MR 110 uF caps are so big that there's not a purchasable capacitor clamp that will fit them. Plus there's no room anyway to put them anywhere else except right beneath the toroid. They've got to be secured some how. I'm thinking wood rails (1/2" thick and the length of the caps) on either side epoxied in place and then mount the caps to the wood rails with long cable ties. I dunno...just thinking out loud.
 
By the way, Bridge rectifier 1, 2, and 3 (BR1 BR2 and BR3 in the schematics), as well as the 12V regulator are mounted on the side walls of the chassis in their approximate locations shown on the chassis layout.

Something to think about: The ClarityCap MR 110 uF caps are so big that there's not a purchasable capacitor clamp that will fit them. Plus there's no room anyway to put them anywhere else except right beneath the toroid. They've got to be secured some how. I'm thinking wood rails (1/2" thick and the length of the caps) on either side epoxied in place and then mount the caps to the wood rails with long cable ties. I dunno...just thinking out loud.

Angela Instruments has a cap clamp (2.0625") that will hold that cap for ya.

Linky: http://angela.com/capacitorclamp20625heavyduty3-screwmount.aspx

I've used them and they hold on TIGHT. Cutting a hole in your case... big enough for a jig saw there.

GLWB

Cheers,
Bob
 
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