• 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.

Mikael's KT-88 question

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My first tube amp experience. After a HUGE learning curve, I have gotten close to finishing my KT-88 after Mikael's design. I solved most of my issues, but my greatest problem is lack of power.

If I boost my pre-amp signal, I get distortion before any appreciable gain. (I am basing this on output from a small 8 ohm test speaker). As it is, volume is fairly low.

So checking my B+ I find 380v. Can I safely assume that I lack enough B+ voltage to get a reasonable output? I know some builders on this forum have used significantly larger voltages for greater output.

Values of components are identical to the schematic posted here many times. I followed Sherman's power supply fairly close. I have a 360-0-360, 5U4G, 40uf, 10H, 80uf power supply circuit.

Any helpful advice would be greatly appreciated. I should state that I am extremely grateful for all the advice posted on these forums and the education it offers us newbies. Now I am going to hope I don't get asked too many questions that will expose my inexperience. Be gentle.:)
 
Hi Cryo-
I've built a stereo version of this amp. My schematic with voltage readings is located here: Glenn's KT88 stereo build

I get B+ voltages anywhere from 392-418 volts. What is your line voltage?
I think you may be getting low volume due to the "test speaker" you are using. How efficient is it? It also won't sound good without an enclosure. I had noticed this when testing my build with just an open frame speaker on the bench. I would suggest you try a complete speaker enclosure.
Also, are you using the correct output transformer, and do you have the speaker on the correct output tap? A little more information on the OT would help.
I'm sure you'll be happy with this amp, I sure am :)
Glenn
 
380V B+ should be fine. The first clue to the problem is that you are using a preamp. You shouldn't need a preamp with this build (unless it is for phono use). That means you probably eliminated the 100K volume pot. If so what values did you use for the resistors between the input and the 6N1P? You might be killing your input signal there.

If you used the volume pot perhaps it is wired wrong?
 
Porkchop 61-

Thank you very much for the schematic and voltage readings. I will compare readings and report back. It may take me a day to find the time to get out to the shop.

My line voltage is a steady 120 (I have it on a voltage regulator). I have a James 6115HS output transformer.

I have the output on the 8 ohm tap, but now that problems have arisen I am going to go through meticulously and re-check everything. I did try it on a bookshelf speaker with the same low output result. I am encouraged that the 380 volts should be adequate, now I just need to find the error of my ways.

Tubewade-

I would be very grateful if someone could inform me how to measure the output power, and what kind of values I should be getting.

Sherman-

I only hooked up the pre-amp to try boosting the signal, which did me no good. I have connected direct since.

I do have a 100K Alps pot. I also will double check the wiring on it, thanks for the advice.
 
And the winning guess is.......

Pilot error by yours truly. When I transferred everything from the breadboard to the final build, I made an error. I creatively swapped a 470 ohm resistor for the 470k resistor on the KT-88 grid. (I think I said that correctly).

Amazing how much difference that makes when your signal isn't going to ground!

Thanks to porkchop61 for the voltages. When I checked around, I did not have any voltage after the coupling capacitor. That's when I checked the resistor, and did a Homer Simpson "duh".

I am EXTREMELY please with the sound at this point. I will finish things up and get a picture of it posted ASAP. Thanks to all of your replies, it meant a lot to me.

Now if I can only disappear into the shop for another day without my wife noticing.......
 
cryo said:
And the winning guess is.......

Pilot error by yours truly. When I transferred everything from the breadboard to the final build, I made an error. I creatively swapped a 470 ohm resistor for the 470k resistor on the KT-88 grid. (I think I said that correctly).

...

Now if I can only disappear into the shop for another day without my wife noticing.......


You are on the slippery slope now my friend! :D Soon your house will be filled with glowing bottles, projects in the planning stage, projects half built and maybe even a few completed!

Have fun and enjoy the amp!
 
Thanks Sherman, I really appreciated following some of your posts regarding the power supply for this amp. It helped a lot.

Now, I am looking at Glenn's schematic posted on this thread. I am thinking of incorporating a three-way switch so I can play around with the different modes.

What I do not know is if I should get a make-before-break or break-before-make? And I assume this should only be switched with the power off? Opinions out there?

And how did you know that I have a few unfinished projects on the shelf?
 
Thanks! I'm no expert in either amp design or PS design but I've managed to build quite a few hum-free amps. So I feel that if I've built it and it works I can say something that might be constructive.

As far as mode switching goes I've installed switches on a few amps to be able to run them amp in triode, ul and/or pentode mode. In a couple of cases I've used simple DPDT toggle switches (only two modes can be switched with those of course). I only switch modes with the power off. In another case I used a rotary switch which gave all three choices. Again I only switched it with the power off.

If switching with power off it doesn't make any difference if the switch is shorting or non-shorting (if you wait for the caps to discharge!). I don't have any recommendation for switches to be used with the power on. Maybe others have more experience with that.
 
Cryo-
I used a rotary switch from Radio shack for both my mode selection, and output speaker impedance selection.
As Sherman said, you should only switch the mode switch with the power off.
The Radio Shack p/n is 275-1386 http://www.radioshack.com/family/in...Type/Rotary+Switches&fbn=Type/Rotary+Switches
It's a six position switch, but you only need three (UL, Triode, Pentode).
Glad you found your problem and got your amp working.
Good luck.
Glenn
 
A couple of months have passed, and I have been fairly slow at finishing this amp. (of course I started a couple more which tends to slow down the finishing process).

But I wanted to show the almost finished state of affairs. The only remaining task will be to finish the bubinga base with a varnish, and enjoy the music.

Thanks again to all who helped, many of whom are unaware.

Cheers!:)
 
Sorry, I will try the picture again......
 

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I'd be interested to see some photos of your implementation of this now famous KT88 amp. I'm about to start work on a stereo amp with 5K 10W Transcendar outputs, with the KT88's triode wired, fixed-bias at 400V 70mA. Power output at 5% simulates to a bit over 9W in SE Amp CAD...

edit: Saw the photo, looks quite impressive! I like the motor runs. My layout is going to be a bit more minimal and small. What kind of speakers are you using?

also, do you have a shot of the wiring? :smash:
 
Cryo,

Your amp looks awesome! I wish I can do metalwork and joinery as good as you do.

I built this amp, just like you, as a first tube audio project, and Sherman is right, this will be the start of your tube audio diy projects. After I finished this wonderful circuit, there was no turning back. I already built three of these amps for friends.

My latest project is a PP version of this amp. I used 6n1p driver, KT88 output tubes, autoformer (center tapped choke) as phase inverter and, inspired by Shoog, a Talema mains toroidal transformer for OPT. It sounds great.
 
Thanks guys, you swell my head. I'm sure I'll discharge a large cap on myself tomorrow.

Sorenj07-I will try and get out to the shop and take a pic of the underside tomorrow, if not this week. Hopefully I won't show off too many erroneous wiring techniques. (remember-maiden voyage).

I am initially using some Tyler Taylo's, a very good 2-way. It sounds great with them.

Alexq-If I got paid by the hour this would be worth a fortune. I am the tortise when it comes to building. But thanks. Your PP version sounds very intriguing, do you have a link of the schematic anywhere?
 
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