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Looking for Mikael's KT88 Schematic

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#101
jduffy said:
Hey Sherman, I have a question for you:

... can you recall what the dimensions were of the the top plate width by length that you used for your KT88?

Putting my pieces together to build the monoblocks. Thanks.
Click to expand...


I used 1/8" aluminum plates that are 12"x12". I bought a pack of about 8 of those plates on eBay for a good price. I used two for the KT88 monoblocks, another for a SE EL84 stereo amp and a fourth is earmarked for a PP EL84 stereo. Guess I need to build at least four more amps of some kind! 😀
 
#103
I have been tube rolling with this amp, and have tried GEC KT88, Mullard EL34, Svet 6550C, Svet 6550B3, Electro Harmonix EL34, Shuguang KT88 and National Electronics 6L6GC on the power tube and Sovtek 6n1p, Amperex 6DJ8, Sylvania 6BQ7A.

Of all the power tubes I tried, the GEC KT88 and the Mullard EL34 are tops, followed by the EH EL34 and the Svet 6550C and then the Svet 6550B3. Forget the Shuguang, the National Electronics 6L6GC is good for rock music.

On the driver tubes, the Amperex 6DJ8 shines, the Sovtek 6n1p has more grit and the 6BQ7A is a good every day use tube.
 
#104
Sherman said:


Next, you need to consider Miller capacitance of two KT88s in parallel. A 6N1P or equivalent might have a hard time driving paralleled KT88 tubes so that would mean adding a driver stage that could handle it. I think (though I haven't tried it) that a simple cathode follower buffer between the 6N1P and the KT88s would do the trick.

In the end you would be adding another tube, possibly another 6N1P or maybe a 6922 to act as the buffer/driver. A single tube would do it since the ones I mentioned are dual triodes. Consider the current draw of your buffer (should be low but there will be current) and the current draw of your buffer's heater when planing the PSU.

I was in the planning stages of a Parallel Single Ended KT88 amp and drew a few schematics that I think would work but in the end decided that if I was going to go through that effort I'd go with a push-pull KT88 setup. Basically the same effort, double the output (in triode, more in UL and even more as a tetrode).
Click to expand...


any chance of a revised schematic with PSE KT88? 🙂

thank you.
 
#105
jarthel said:



any chance of a revised schematic with PSE KT88? 🙂

thank you.
Click to expand...

Here is what I came up with. Keep in mind that I never built this circuit but I believe it should work.
 

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#108
arnoldc said:
The 6N1P provides the gain.

If you have a high gain preamp, you can try driving the KT88 directly from its grid.

Also when you remove the 6N1P, you'll have 20mA off the load, which will bring up your B+

For this design, I would say no. Just keep it and replace the 100K pot with 100K to ground.
Click to expand...

Arnold, thanks.

I really do not know what is the gain of my preamp, so I do not know if it can drive the KT88 directly.

Thanks.
 
#109
arnoldc said:
...For this design, I would say no. Just keep it and replace the 100K pot with 100K to ground.
Click to expand...

Arnold is right on with this advice. I built Mikael's schematic as monoblocks without the volume control. I simply put in a 100K resistor to ground in place of the pot. Works fine.

I've driven with a preamp with too much gain and could only use about 1/5 of the volume control. With a 'passive preamp' it works great.
 
#110
Sherman said:


Arnold is right on with this advice. I built Mikael's schematic as monoblocks without the volume control. I simply put in a 100K resistor to ground in place of the pot. Works fine.

I've driven with a preamp with too much gain and could only use about 1/5 of the volume control. With a 'passive preamp' it works great.
Click to expand...

I built my stereo version the same way (no vol pot with 100K across the input to ground).

I am in the planning stage of doing a monoblock and I was thinking if I can ditch the driver.
 
#113
showdown said:
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?
Click to expand...

Okay, I'm just finishing this amp (finally), and saw this post about using EL34's. This is possible without any rewiring? I'm using the original schematic, UL mode. I assume that g3 on the EL34 is just not used then, correct? I have a pair of EH EL34's that I'd like to try, but I don't want to blow anything up.

Thanks
Glenn
 
#115
Thanks Tubelab, glad I asked!
I thought it was odd that you could use an EL34 in this amp at all, as the KT88 is a beam power tube.

I did ask this question with the understanding that the amp is running in UL mode. Can I run the EL34 in Triode and Pentode mode? I have a rotary switch controlling the connections. Of course I know you can't switch these modes while operating 🙂
 
#116
I have used EL34, KT88 and a good strong 6L6GC in the SimpleSE amplifier which is of a similar design. They can all be used in triode, UL and pentode mode if your B+ voltage is close to the 400 volts called for in the schematic. 450 volts or so (where I run my amp) is pushing it for some 6L6GC's.

Some say that the EH EL34 is not the strongest in the world, but my pair seem to be OK at 450 volts in UL or triode mode. I don't use pentode mode because it sounds terrible on my speakers. I have cranked a pair of JJ EL34's up to 500 volts without issue.
 
#117
tubelab.com said:


No, G3 must be connected to the cathode. Run a wire from pin 1 to pin 8 on the output tube sockets. This will have no effect on KT88 use, but will allow the use of EL34's.
Click to expand...

It has been a while, but I think after I connected pin 1 to 8 on the octal sockets, the JJ EL34 now is very quiet. Shorting pins 1 & 8 will not affect the KT88 because it is already internally shorted.

I already have tried KT88, 6550, EL34 and 6L6 on this amp with great results.

Overall, I prefer the EL34 for vocals and the KT88/6550 for rock and orchestral music.
 
#120
I omitted the input cap since my source would not put out any DC and because I wanted as little components as possible in the signal path. Otherwise I am sure it would be a good idea to put one there. I tried to make the design as basic as possible, and there are many ways to improve on the design or tweak it.
 
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