Looking for Mikael's KT88 Schematic

[alexg]

Re: Re: Noob questions

Jeff,

Earl posted my PS so you have that. It is for a monoblock, one 6N1P and one KT88. If you are building in a stereo chassis you only need 3 tubes- one 6N1P and two KT88. The 6N1P is a dual triode (two triodes in one envelope).

Calculating the current needed from your PS is not hard. First if you are doing tube rectification you need to supply sufficient current to the rectifier heater. The 5U4-G heater takes 3A at 5V.

Next you need to supply the heaters for your other tubes. The
KT88 takes 1.6A at 6.3V and the 6N1P is probably about 1A (I forget the exact value).

Last you need to know the current draw of your tubes for the B+. The KT88 is biased at 75mA in Mikael's circuit, I don't remember what the 6N1P draws, I think about 15mA? Add those up and give yourself some headroom.

If you are doing a stereo chassis then you'll have to supply heater current to two KT88's so you'll need 3.2A for those and still need the 1A or whatever for the 6N1P.

For the B+ you'll need to double everything from the monoblock. 150mA for two KT88s and 30mA for both triodes in the 6N1P.

Pick a PTX with enough guts.

As for power output- at full-tilt a single ended KT88 might make 8-10 watts (I think the sheet value is 11 watts). In this circuit, keeping distortion reasonable and running ultralinear rather than pentode mode a good guess would be between 4 and 7 watts. I haven't put the scope on mine yet to really measure it. I think Mikael was estimating 4-5 watts on his amp.

For the 8H choke, what is the DCR or is this insignificant?

Thanks.

 

[Tubelab_com]

Alex, typically 8-10W. But I know George likes to run 'em hard How much did you squeeze from them George?

I typically run a higher than normal plate load, which gives me slightly lower power and lower distortion. I also tend to run more voltage than others do, so that helps get some of the power back. On most amps I use 5K transformers. I get about 7 watts in triode and 12 to 14 in UL with a 5K transformer and 465 volts of B+ (about 400 across the tube). The distortion, DF and sound quality seems to get better with higher current, so I am currently running 6550EH tubes at 90 to 100 mA. The EH tubes handle this without complaint. I built the amp in the picture with 3K transformers, but I have decided that I prefer the sound with my 8 ohm speakers on the 4 ohm taps, affording a 6K load. With 96 db speakers it is plenty loud, but I haven't measured the power yet. Higher tube current doesn't change the power output much though with a cathode biased amp. Cranking up the current reduces the tube voltage, so the power stays relatively constant. Measurements on earlier amps gave 5 to 9 watts in triode mode depending on B+ and load impedance.

Poor CD player Why do they always get such treatment?

That "poor" CD player has lived a sheltered life on its own private shelf in my "tower of power" for at least 5 years. I dragged it out to the living room for some "testing" when my wife wasn't home.

 

[alexg]

Alex, typically 8-10W. But I know George likes to run 'em hard How much did you squeeze from them George?

ps.

Poor CD player Why do they always get such treatment?

Thanks Arnold.

I have a bunch of 6550 and KT88 tubes, I might go for this design. I can source 125 series Hammonds in Makati (phasetron) so I might wire for triode operation and upgrade later to UL with a better OPT.

 

[arnoldc]

alex, when you prepare your chassis, consider the upgrade path. You can see how I did it with my 2A3 amp. You can check DIYParadise...

 

[alexg]

alex, when you prepare your chassis, consider the upgrade path. You can see how I did it with my 2A3 amp. You can check DIYParadise...

Arnold, I will keep that in mind. Thanks.

 

[alexg]

Can I make this circuit work with a 70ma, 5K OPT by lowering B+?

Thanks.

 

[Tubelab_com]

I run my amps at 70 ma when using NOS 6550's or other tubes that can only dissipate 30 watts. Leave the B+ where it is, and raise the value of the cathode resistor. I also use 5K transformers because to me they sound better than 3K transformers. The 5K load works best with a lot of voltage swing, which you get from running the B+ as high as possible.

Beware of the very early Shuguang KT88's (10 to 15 years old) they work great at voltages below 400 volts, but turn into fireworks displays if fed 500 volts!

 

[jayme]

I found a similar schematic on the Lundahl site, which (of course) uses a lot more iron in the design (grid chokes and a plate choke).

Simple SE Design

I think I might build Mikael's design as-spec'd, but I'd like to use it as a starting point....upgrading over time.

I assume the plate choke on the driver would make the most noticeable difference....

 

[arnoldc]

I assume the plate choke on the driver would make the most noticeable difference....

In my case it did. I started with resistor load on both my 45 and 50 amps. Then I experimented with choke and CCS loads. I ended up with a choke load on the driver of my 45, and diyAudio CCS load on the driver of my 50 amp.

Both driver tubes are 5842/417A, and oh, both amps use grid chokes.

 

[alexg]

I run my amps at 70 ma when using NOS 6550's or other tubes that can only dissipate 30 watts. Leave the B+ where it is, and raise the value of the cathode resistor. I also use 5K transformers because to me they sound better than 3K transformers. The 5K load works best with a lot of voltage swing, which you get from running the B+ as high as possible.

Beware of the very early Shuguang KT88's (10 to 15 years old) they work great at voltages below 400 volts, but turn into fireworks displays if fed 500 volts!

Thanks, how much higher should I make it? (this will be my first tube amp build).

BTW, how many ma of current does the original design put out? (I will try to compute for the added resistance).

 

[Sherman]

...BTW, how many ma of current does the original design put out? (I will try to compute for the added resistance).

Mikael's orginal schematic called for 75mA at each output tube IIRC. My implementation follows his schematic (except for being monoblocks). My mains voltage is a little high 124VAC or so and as a result my B+ is about 425 V, using the original value of cathode resistor on the KT88s I get 82mA. The amps work great at that voltage and current.

 

[Tubelab_com]

Thanks, how much higher should I make it? (this will be my first tube amp build).

Look to see what you can get for a power transformer. I use the Allied 6K7VG because it is cheap. It puts out 750 volts center tapped at 150 mA (50Hz) and 175 mA at 60Hz. That gives me about 440 volts of B+ with a tube (5AR4) rectifier and 465 volts with a solid state rectifier. Choosing a B+ this high requires 500 volt capacitors. Capacitors for more than 500 volts are hard to find and expensive, so this places an upper limit of about 475 volts for your B+. I hooked my amp up to a variable power supply (with no caps on the PC board) and cranked it up. The EH tubes were still behaving happily at the limit of my power supply (550 volts).

700 or 720 volt center tapped transformers are more common yielding just over 400 volts of B+ and possibly allowing the use of 450 volt capacitors if your line voltage isn't too high.

You can build a good amplifier with any of these choices, see what is available to you. Look for a transformer rated for at least 200 mA. Your amp will draw somewhere between 150 and 180 mA depending on tube and bias choices. My 175 mA transformer is marginal, and gets quite warm, but I have building amps with these for at least 10 years with zero failures so far.

If you used a 3K output transformer you would want a lower B+ (about 360 volts) and higher tube current to give a better match.

 

[alexg]

Mikael's orginal schematic called for 75mA at each output tube IIRC. My implementation follows his schematic (except for being monoblocks). My mains voltage is a little high 124VAC or so and as a result my B+ is about 425 V, using the original value of cathode resistor on the KT88s I get 82mA. The amps work great at that voltage and current.

Could I use a Hammond 125ESE for testing, the KT88 wired for triode since the 125ESE does not have ultralinear tap, and is rated at 80ma, and should I bump the cathode resistor from 470 to 500ohms which should reduce the current to about 70ma.

Thanks.

 

[Sherman]

Could I use a Hammond 125ESE for testing, the KT88 wired for triode since the 125ESE does not have ultralinear tap, and is rated at 80ma, and should I bump the cathode resistor from 470 to 500ohms which should reduce the current to about 70ma.

Thanks.

That would work. The 125ESE is a pretty decent OPT for the money, I've got a pair on a SE EL84 amp and they sound very good.

I haven't heard them myself but the Edcor SE OPTs have also gotten good reviews here on the forum. They have UL taps and cost less than the Hammond 125ESE. If the Edcors have a drawback it is that they have only one primary Z where the Hammond can be configured with one of several making them a bit more versatile for testing either triode or pentode designs.

 

[alexg]

Am looking into the Hammond 125ESE since the 125 series are readily available on one of the electronic stores in Manila. They also carry 125FSE and 125GSE.

Since I plan to upgrade to a James later, I can use the 125ESE for prototyping other tube projects.

Anyway, I already have a custom power transformer being wound and got a used Hammond 193J choke from a fellow DIYer in Manila (a more advanced DIYer, actually he redesigned my PSU and did a minor revision onf Mikael's circuit). I will post it soon.

Thanks!

 

[Tubelab_com]

I have tried the 125CSE and the Edcor XSE15-8-5K in my amps. Both of these are smaller than the 125ESE that you are asking about. Both of them sounded far better than I expected. I prefer the Edcors for 3 reasons, they sound better, they have UL taps, and they cost less. I don't know if you can get them locally, or if shipping from the US would be too expensive.

There was a long thread about these transformers here:

http://www.diyaudio.com/forums/show...72654&perpage=10&highlight=edcor&pagenumber=6

 

[navin]

Originally posted by mikey115 ... im currently using spendor s3/5's and my ella kit which is a el34 pp powers it up quite nicely.
so the question is, what speakers are you folks using to hook up the amp to? im leaning in the direction of an open baffle design - only because its easy [/B]

I thought you were using KT88s.

for a speaker you should try this...http://audiotropic.netfirms.com/Projects/speakers.html

the wood work will keep you busy for a while

 

[alexg]

I started building this amp, all iron was custom wound by a local company here in the Philippines (those with red endbells), except for the hammond 193J choke, here is a picture of the amp with the topside populated, but I have not done any connection yet:

 

[mus]

Rw: http://audiotropic.netfirms.com/Projects/speakers.html

Is there anyone had experience about the sound of the speaker mentioned above?

 

[showdown]

Not to thread-jack, but this is related to Mikael's amp and I need some brain assistance -

I've built it and everything works OK with the KT88s... but I had a couple of EL34s around so I popped them in without changing anything. It sounds great, but it had me thinking about the max. values for the EL34. I'm just calculating out loud here, and please correct me if I'm wrong:

Right now I'm running in UL mode with a 470R screen resistor, at idle the voltage across the resistor is 3.4V so this means there's (3.4/470) 7.23mA screen current. And with a voltage of 425, give or take a few volts, there's about... 3.1W screen dissipation? Should be within the limits for an EL34, no?