• 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.

Affordable KT88

Hello again.

I'm considering building Mr. Mikael Abdellah's KT88 amp.
As always €€ are the problem.
Are there any cheaper equivalents for the KT88 tube (like a 6B4G to a 2A3, PCC88 to a ECC88...you get the picture) or a more affordable manufacturer?
I've seen some chinese KT88 for around €35 each, but from what i've searched here, they are not an option.

Mr Abdellah, if you happen to read this post, could you please share the PS schematic?

Thank you very much.
 
Tuga, podia-me informar dos preços dum par (não é necessário serem matched, como já deve ter notado:D )

Sherman, thank you for the information about the tubes(what's your PS), as well as to Marcello1 for the link... nice prices!

I'm considering a national manufacturer of output trannies, they seem to be of very high quality, and at around 90 euro each with covers etc, etc. I will give them a try, as they were so helpfull determining the necessary tranny for this circuit and offered help on the PS. It will hurt on the wallet, but i'm not going down on this.
 
Simpleton said:
Tuga, podia-me informar dos preços dum par (não é necessário serem matched, como já deve ter notado:D )

Sherman, thank you for the information about the tubes(what's your PS), ...


Here is my PS schematic. Really dead simple. The caps are ASC motor run (not start ) caps. Cheap and very effective. My PTX is a Hammond 273BX (remember I made monoblocks so each PTX only has to power one channel). The choke is a Hammond 8 Henry.

My mains voltage is a little high so my B+ is closer to 425 than 400 but still well within 10%.
 

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TugaTweaker said:
Simpleton . Have a look . Make no mistake ,it is a Jean Hiraga design. I built this one six years ago, never ceased to improve it with good components until today .

http://www.bonavolta.ch/hobby/en/audio/kt88_1.htm

Regards

TT

Hi TT,

That schematic isn't really the same as Mikael's. Mikael's schematic is much simpler and uses only a single triode (1/2 a 6N1P) in the input stage rather than two triodes (or both halves of a dual triode). Also this schematic has a feedback loop, Mikael's does not. All in all a quite different amplifier (but it does look interesting!). I do like the decoupling of the PS, something I ended up doing with my monoblocks.

Also there is a problem with the PS schem in that design. The 5U4-G has only 4 pins, not 5. Also the maximum value for the 5U4G is 40uF for the first cap in a cap input PS.
 
Also there is a problem with the PS schem in that design. The 5U4-G has only 4 pins, not 5. Also the maximum value for the 5U4G is 40uF for the first cap in a cap input PS.

One , two or none . The schema shows four pins for the 5u4G .
The other problem : No music lover " audiophile " or not will deny the superior musicality of the new SiC Schottkys diodes as rectifiers against any rectifier valve , so the cap can be 100uf and
its even cheaper , inexpensive as you like .

Finnaly Jean Hiraga deservs some respect .


TT
 
TugaTweaker said:


One , two or none . The schema shows four pins for the 5u4G .
The other problem : No music lover " audiophile " or not will deny the superior musicality of the new SiC Schottkys diodes as rectifiers against any rectifier valve , so the cap can be 100uf and
its even cheaper , inexpensive as you like .

Finnaly Jean Hiraga deservs some respect .


TT


You are right about the number of pins! I don't know what I was looking at as a second quick look confirmed it for me. However the schematic does still show a 100uF cap following the 5U4G.

As for SS vs. valve rectification I won't even comment there. :D

Finally I didn't do anything to disrespect Jean Hiraga. Just pointed out that his schematic and Mikael's are very different amplifiers.
 
Also there is a problem with the PS schem in that design. The 5U4-G has only 4 pins, not 5. Also the maximum value for the 5U4G is 40uF for the first cap in a cap input PS.

I have a 5U4GB here, and it does, indeed, have five pins. This one is branded: "Realistic Lifetime", and says in finer print: "E. T. A." within a small ellipse, and "Japan F4". Two pins connect to the cathode, the other two to each diode plate, and that odd pin has a small metal tab spot welded to it on the underside of the lower mica spacer. The rest protrudes about 1/4" above the spacer, and is bent some 30 deg towards the right-hand diode plate. Looking at it through a magnifying glass, it's obvious that it was simply snipped off with a pair of dykes.

I don't know what that's all about, but it looks like connecting that thing to anything is just asking for a flash-over. :bigeyes: You'd think that if they needed an extra pin for stability in the socket that there'd be no need for an extraneous metal/glass seal for an apparantly "useless" connection.

I agree: 110uF in the P.S. is way too much. A 5U4GB may be one of the beefier vacuum power diodes, but its surge rating is still just 1.0A. I doubt that a pair of them with plates in parallel could handle 110uF. :whazzat: That supply will go through 5U4GB's pretty quickly.