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Looking for Mikael's KT88 Schematic

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#81
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...

I ask arnoldc about using EL34 on Mikael's circuit and he told me it is within specs of the EL34 to be used in the circuit.

I finished my KT88 amp last week and it sounds very, very good. Today, I ordered a pair of EH EL34 to try on my amp.

Showdown, what difference in sound have you noticed when you switched to the EL34?

Mikael, thanks for the great design, my next amp project will be a monoblock version of your circuit.
 
#83
alexg said:

Showdown, what difference in sound have you noticed when you switched to the EL34?
Click to expand...
I don't know, sound is hard to explain... 🙂 But "softer" springs to mind, not as hard and punchy as the KT88s, I think. I haven't A/B tested much, but as soon as I have the time I will test both tube types in both UL and triode connection.

I've also tried an ECC85 in the input which sounded nice, but it was a little "soft". I think I like the 6N1P better.
 
#84
Kofi Annan said:
alexg--

I am working with some oval ASC oil caps for a 300b project, which I believe I see on your chassis. Can you tell me:

1. How did you polish them (or did you)?

2. How did you mount them?

Sorry-- I know this is a hijack with some basic, newbie questions, but I gotta, gotta know.

Kofi
Click to expand...

Kofi,

Polished the oil caps using 400 grit, then 1000 grit sand paper, then I used very fine steel wool.

I cut the holes on the aluminum plate the size of the oil cap, then mount it from underneath going up until the lip catches the underside of the aluminum plate (you will notice that there is some sort of a lip on the end of the cap towards the connectors, see picture below). Then I fashion a pieces of angle bar that pushes the caps upwards and then I bolted it to the wood plinth. (I got the idea from one of the posts here.) I will take picture of how I mounted during the day so I will have enough light and post it here.




showdown said:

I don't know, sound is hard to explain... 🙂 But "softer" springs to mind, not as hard and punchy as the KT88s, I think. I haven't A/B tested much, but as soon as I have the time I will test both tube types in both UL and triode connection.

I've also tried an ECC85 in the input which sounded nice, but it was a little "soft". I think I like the 6N1P better.
Click to expand...

I ordered a pair of EH EL34 today to try on the amp.

I also tried some amperex 6dj8 as substitute of the 6n1p, I like the 6n1p better.

Thanks.
 

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#85
I just got some 6bq7 tubes, checked it out and it is very similar to 6dj8/6n1p, I will try to use it on my KT88 amp.

I noticed that if I use EL34 on Mikael's circuit, am getting quite noticeable hum (with the Electro Harmonix EL34). The other day I have a pair of Mullard EL34 and did not notice any humming.
 
#87
showdown said:

I don't know, sound is hard to explain... 🙂 But "softer" springs to mind, not as hard and punchy as the KT88s, I think. I haven't A/B tested much, but as soon as I have the time I will test both tube types in both UL and triode connection.

I've also tried an ECC85 in the input which sounded nice, but it was a little "soft". I think I like the 6N1P better.
Click to expand...

Hello Showdown, was wondering if you have had a chance to compare the EL34s to the KT88s? Curious to hear what your opinions are on the matter.

Thanks.
 
#88
Tweeker said:
The heater to cathode insulation on the Electro Harmonix EL34s may be suspect.
Click to expand...

Thanks for the information.

BTW, I just noticed today that when I turn on my standby switch (switching the center tap of my B+), am getting quite a big movement of my woofer cone, no sound though.

My main switch turns on the heaters for the 5u4gb, KT88 and 6n1p tubes.

My B+: 375-0-375-->5u4gb-->10uf-->10H hammond-->100uf

Any suggestion how I can remove this, would switching the heater for the 5u4gb together with the B+ help? This way, the rectifier tube is not warm enough to work and will ramp up slowly.

Or should I use a 5ar4 with the slow turn on?
 
#89
alexg said:


Thanks for the information.

BTW, I just noticed today that when I turn on my standby switch (switching the center tap of my B+), am getting quite a big movement of my woofer cone, no sound though.

My main switch turns on the heaters for the 5u4gb, KT88 and 6n1p tubes.

My B+: 375-0-375-->5u4gb-->10uf-->10H hammond-->100uf

Any suggestion how I can remove this, would switching the heater for the 5u4gb together with the B+ help? This way, the rectifier tube is not warm enough to work and will ramp up slowly.

Or should I use a 5ar4 with the slow turn on?
Click to expand...

Just rewired my standby switch to connect the center tap of my HV to ground and turn on the heater for the rectifier. No more turn on thump!
 
#91
tomes said:
Hi Alexq,


I just wiring my set, can i check with ur? can i turn on it together?I mean just one switch, another question is should i terminate the earth to opt speaker binding pt(0ohm) as other i build before?
Click to expand...

What B+ are you using? I have to install a standby switch since some of my capacitors before are only 450VDC and my B+ with load is 400VDC. If the tubes are non-conducting (cold) the B+ will ramp up close to 500VDC and it will kill my 450VDC caps.

So it it is advisable to turn on the heaters of the driver and power tubes for about 30 seconds before turning on the heater of the rectifier and the B+.

HTH.
 
#96
jarthel said:
if someone want to mod the design to have 2 KT88 in PSE config, what change is required?

thank you.
Click to expand...

Or would a PP KT88 be a better design than a Parallel SE KT88?

BTW, I tried 6BQ7 tube for the 6n1p and it sounds good. Also tried, EH EL34 with 6dj8, and the vocals are nice!
 
#98
jarthel said:
if someone want to mod the design to have 2 KT88 in PSE config, what change is required?

thank you.
Click to expand...


As Arnold said your OPT needs to have half the primary impedance of the SE version. In my case I used a 5K OPT so to go with parallel KT88 I would need to use a 2.5K OPT. Next, again per Arnold, you'll have double the current (two output tubes) so your OPT needs to be able to handle that and your power supply has to be able to deliver it.

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).
 
#100
Hey Sherman, I have a question for you:

Not to hijack the thread and to post a question kind of related to the KT88 schematic, 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.
 
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