• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Looking for Mikael's KT88 Schematic

lets see if this works:
 

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This is a great sounding circuit and is as simple to build as you will find! You'll be happy with it.

If you are looking for OPTs for this amp I can recommend the James 6123HS. Price from Euphonia Audio is about the same as for the Hammond 1628SE and the sound is great. (Plus they look great, finish is definitely top notch!)
 

Toyman_PA

Member
2004-07-23 3:09 am
PA
Thanks Guys.

Sherman, can you recommend some caps and resistors?

I seem to be having trouble finding a 220uF/63V and the 100uF/25V caps as well as the 470Ohm/25W resistor.

Can this amp run without a pre-amp, strictly with a CD source?
What about a SS preamp?

P.S. I'm a noob.

Thanks,
Earl
 
Toyman_PA said:
Thanks Guys.

...

I seem to be having trouble finding a 220uF/63V and the 100uF/25V caps as well as the 470Ohm/25W resistor.

Can this amp run without a pre-amp...

P.S. I'm a noob.

Thanks,
Earl

Earl,

I ended up using 220uF/100V and 100uF/50V caps. Both were non-polar electrolytics. With the caps you can use a higher voltage rating than specified but you should stick with the recommended capacitance.

For the power resistor 20W should be OK.

You can find suitable resistors and capacitors at Mouser.

The amp was designed to run without a pre-amp so any source such as a CD player, tuner or tape player should be fine. For phono you would need a phono pre.
 
Noob questions

Sorry for the extra-basic noob stuff, but I've only built one amp so far, and I gotta learn somehow. :D

1) What's a good way to design a PS for this amp? I can't decide if I think a single supply for a pair of these on one chassis to make a stereo amp would be cooler than a separate PS for each, as in a pair of monoblocks. Regardless, I know I need 400v B+ per the schem, but what about generating enough current to power all 4 tubes? How do I calculate that?

2) For a circuit like this, how does one calculate the output in watts? I've always wondered that...

Thanks for the patience!

-Jeff
 
Re: Noob questions

jgunn5066 said:
...

1) What's a good way to design a PS for this amp? I can't decide if I think a single supply for a pair of these on one chassis to make a stereo amp would be cooler than a separate PS for each, as in a pair of monoblocks. Regardless, I know I need 400v B+ per the schem, but what about generating enough current to power all 4 tubes? How do I calculate that?

2) For a circuit like this, how does one calculate the output in watts? I've always wondered that...

Thanks for the patience!

-Jeff

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.
 
Stereo power transformer for this amp?

Ok-
I just found this thread on this project, and I'm going to build a stereo version of this amp.

My question is, can I use a Hammond 273BX for a stereo version of this amp? It's rated for 175ma not the 180ma as calculated is the above message. Is this close enough?:confused:
Have any others built a stero version of this amp?
Thanks.
Glenn
 
I'm interested in this project as well. Could someone recommend a few different output transformers? I'd thought about using these from Triode Electronics:

http://store.triodestore.com/tfchxfwi48oh.html

However, their bandwidth gives me a little concern. The price is right, and I'd like to give an inexpensive SE amp a try (my only other tube gear is the 11MS8 DIY amp from S5 and a 5687 hand-built preamp that I recently made).

Thanks!
 
motherone said:
...Could someone recommend a few different output transformers?...
Thanks!

As Mikael said those OPTs can't take the current. He biased at 75mA, mine are biased at 82mA.

For my amps I used the James 6123HS which can easily handle the current. I got them from Euphonia Audio and the price was basically the same as the Hammond 1628SE. I don't think you can do better for the price than the James trafos.
 
Re: Stereo power transformer for this amp?

porkchop61 said:
Ok-
I just found this thread on this project, and I'm going to build a stereo version of this amp.

My question is, can I use a Hammond 273BX for a stereo version of this amp? It's rated for 175ma not the 180ma...
Glenn

Glenn,

I put the meter to my amps to double check the actual values I'm running. I have 82mA on each of the output stages and 5mA on the input stages. That gives you 174mA total. If you bias the KT88's at 75mA as Mikael specifies in his schematic you'd be at 160mA.

You should be able to use that PTX as far as the rectified DC current is concerned. However double check the specs for the secondaries. Do the filament windings have enough current capability to run all the heaters? If so you should be good to go.
 
Thanks for checking that Sherman.
The Hammond 270BX has 5.0V/3A, and 6.3V/5.0A CT taps so should be no problem.

The 6N1P takes 600mA (I'm only using one), and 2 KT88's take 3.2A, 5A available so no issues.

I've also heard that Hammond transformers do live up to their claim of "Conservatively rated".

I'd rather just buy one PT if you know what I mean :D

Thanks again!
Glenn
 
porkchop61 said:
...The 6N1P takes 600mA (I'm only using one)...

I'd rather just buy one PT if you know what I mean :D

Thanks again!
Glenn


Glenn,

I had forgotten that since you are building a stereo unit you do only need one 6N1P! It does look like you should be fine with that PTX.

I know what you mean about only buying one PTX. Iron adds up quickly (both in $$$ and in weight)!

Good luck and post your progress!
 

rdf

Member
2004-06-21 8:04 am
big smoke
porkchop61 said:
Oops, I think the choke may be a problem as it's only rated for 150mA (Hammond 193D).
Anyone make a higher current choke in 8 Henries?


The 8H value isn't critical and you could go higher without any problem. Hammond has a 10 Henry 200mA choke (193J) and a 10 Henry 500mA choke (193Q) both of which would work fine and happen to be on sale at Angela (not associated with Angela except as a satisfied customer).

Alternatively you could go with a lower Henry value and add an LC section to the PS.
 
Sherman said:



The 8H value isn't critical and you could go higher without any problem. Hammond has a 10 Henry 200mA choke (193J) and a 10 Henry 500mA choke (193Q) both of which would work fine and happen to be on sale at Angela (not associated with Angela except as a satisfied customer).

Alternatively you could go with a lower Henry value and add an LC section to the PS.

Thanks Sherman.
I think I'll go with the 10 Henry 200mA.
And oh yes, Angela is my favorite vendor for Hammond stuff. I'm also a satisfied customer. :)
Glenn