• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

KT-88 amplifier

I'm in the process of building my first tube power amplifier. I've built several solid state amplifiers but never a tube one. I always considered tubes to be inferior to transistors but nevertheless I really liked reading and learning how tubes work (Morgan Jones's book and several websites). All this reading made me wanna build one really bad :). I stumbled upon Bob Cordell's KT88 amplifier presented in Linear Audio. I really like this design and I decided to give it a go. He uses the kt88s in pentode mode and I will use them both in pentode and ul mode. I already have the tubes but now I'm looking at buying the transformers and I'm in a bit o a pickle. I want to use an output transformer with a primary inductance of 6600 OHMs due to the higher turn ratio that yelds lower output impedance of the transformer. I know that output power will not be very high with 6600 ohms but I'm ok with this. I do not know what type of transformer to buy. Toroidal or classic EI ones. I can buy good toroidal transformers from Toroidy (used them in the past for solid state amplifier power supplies) or classic ones from Hammond. Are there any disadvantages of using toroidal transformers for output? I know that toroidal transformers will saturate a lot quicker due to DC but I'm not sure if this is a reao problem or not. Price wise, both solutions are at the same level. Any advice is highly appreciated ( regarding the transformers and the project itself).

Thanks.
 

wg_ski

Member
2007-10-10 5:21 pm
If it is your FIRST tube amplifier, I’d probably use EI core OPTs. The stock answer is because they tolerate DC imbalance better. This doesn’t matter much on a well executed properly functioning amplifier. But slugs of DC can happen unintentionally too, especially during development or when making newbie mistakes. The EI is far less likely to get permanently magnetized (and is more reversible when you do magnetize it) if something goes horribly wrong. If it is out of balance or misadjusted, it will still work with an EI while you’re tracking down the problem - the toroid will be less forgiving.
 

Francois G

Member
Paid Member
2004-06-16 8:36 pm
Illinois
The cautions mentions by wg_ski and aesthetic issues by OldHector deserve your consideration. It sounds like you are quite adept at amplifier building, and with some prudence I don’t expect you will have any issues dealing with a toroidal output transformer. However, the Toroidy TTG-EL34 with 6600 Ohm primary is rated at 50VA - I don’t know if that is conservative.
 

horias2000

Member
2010-10-07 3:46 pm
Thanks all for you input. Highly appreciated! Regarding the aesthetics of the amplifier, I don't really care too much how the amp looks. I like the looks of toroids as much as the EI core ones. It's also a matter of availability. The Hammond is only available in 21 working days (from don-audio) and the toroidy is available sooner. Indeed the toroidy is rated for 50W of output power but the amp I'm looking to design/build will have a rated power output of about 2x35W. So I think these 50W transformers should be enough.
 

Francois G

Member
Paid Member
2004-06-16 8:36 pm
Illinois
Thanks all for you input. Highly appreciated! Regarding the aesthetics of the amplifier, I don't really care too much how the amp looks. I like the looks of toroids as much as the EI core ones. It's also a matter of availability. The Hammond is only available in 21 working days (from don-audio) and the toroidy is available sooner. Indeed the toroidy is rated for 50W of output power but the amp I'm looking to design/build will have a rated power output of about 2x35W. So I think these 50W transformers should be enough.
Then you might be quite happy with the Toroidy TTG-EL34, especially if you prefer low distortion to “tubeyness”. Member Bfpca built a Baby Huey KT88 using this transformer. IIRC he actually measured the bandwidth in his amp.

He wrote:
”I used the Toroidy 6.6k in my EL34 BH. Bandwidth is impressive at greater than 200khz on the top end. Bass performance is also much better than the typical EI transformer with lower distortion levels and flat response to below 20hz.
I don’t think the Toroidy has a typical tube amp sound signature, being lower distortion in the bass. The DC resistance values are lower as well, meaning you get a bit more power, less heat and slightly higher damping factor than an EI transformer.”

See post #950: https://www.diyaudio.com/community/threads/el34-baby-huey-amplifier.326920/page-48#post-6458352
 
Then you might be quite happy with the Toroidy TTG-EL34, especially if you prefer low distortion to “tubeyness”. Member Bfpca built a Baby Huey KT88 using this transformer. IIRC he actually measured the bandwidth in his amp.

He wrote:
”I used the Toroidy 6.6k in my EL34 BH. Bandwidth is impressive at greater than 200khz on the top end. Bass performance is also much better than the typical EI transformer with lower distortion levels and flat response to below 20hz.
I don’t think the Toroidy has a typical tube amp sound signature, being lower distortion in the bass. The DC resistance values are lower as well, meaning you get a bit more power, less heat and slightly higher damping factor than an EI transformer.”

See post #950: https://www.diyaudio.com/community/threads/el34-baby-huey-amplifier.326920/page-48#post-6458352
OH wow, that is really incredible bandwidth. I knew toroidy were good butI was not expecting them to be so good :). I will read that thread, thanks for sharing.
 
I do not know what the circuit is like of Bob Cordell's KT88 amplifier presented in Linear Audio.

In any / every case, I recommend very well matched KT88 tubes. That can help to keep the DC balance of the push and pull tubes to be very good.
Even with well matched KT88 tubes, then . . .

Either use Adjustable Fixed Bias for each tube (and individual 10 Ohm cathode resistors to ground, so you can measure the voltage, and calculate the current V/10 Ohms = current).

Or, use Individual Self Bias Resistors with Individual Bypass Caps.

I purchase my JJ KT88 tubes from Eurotubes.com.
They are extensively re-tested by Eurotubes, after they arrive from JJ in Slovakia.
Eurotubes is in the USA, but ships to many countries around the world.

Happy Building!
Happy Listening!
 
The only difficulty with using toroidal xfmrs for OPT positions is that a toroid can tolerate very little unbalanced DC current, which quickly magnetizes the core to saturation. Because of this problem, they are not recommended for SE amplifiers, and with push pull designs, the two currents must be very carefully balanced to cancel out the DC-caused magnetism in the core.
 
I do not know what the circuit is like of Bob Cordell's KT88 amplifier presented in Linear Audio.

In any / every case, I recommend very well matched KT88 tubes. That can help to keep the DC balance of the push and pull tubes to be very good.
Even with well matched KT88 tubes, then . . .

Either use Adjustable Fixed Bias for each tube (and individual 10 Ohm cathode resistors to ground, so you can measure the voltage, and calculate the current V/10 Ohms = current).

Or, use Individual Self Bias Resistors with Individual Bypass Caps.

I purchase my JJ KT88 tubes from Eurotubes.com.
They are extensively re-tested by Eurotubes, after they arrive from JJ in Slovakia.
Eurotubes is in the USA, but ships to many countries around the world.

Happy Building!
Happy Listening!
The project is available online for free. just google search "Bob Cordell KT88" and you will get the linearaudio.net PDF with the article. I attached two images with the schematic for simplicity. The output tubes do have 3.16OHM (I know, weird value) resistors for measuring the current through each tube. I also purchased JJ tubes directly from the JJ website and the KT88s are "matched quad". So I'm expecting that all four tubes will behave similarly.
 

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For KT88 PP a 5k transformer would be more suitable, even 4k is more typical than 6k6.
Toroidy have 5k PP transformer. I have used it with good results.
Yes, I've seen that most design use 4.3k or 5k transformers. As far as I can tell, this is so that you can obtain as much output power as possible. Based on my calculations, with a 6.6k transformer, I will get somewhere around 35W per channel. This is more than enough for my needs. Are there any other implications of using lower impedance transformers?
 
I remember Bfpca saying that he used 6k6 to keep the damping higher, even if that would give the same output power with E.G. EL34 of KT77.
I also remember that he preferred his EL84 version over the KT88.
Yes, I've seen the thread where Bfpca used the Toroidy 6.6k transformer. The conclusion was that it is a very good performance transformer and that it had very good bandwidth and low distortion. So much so that it didn't sound "tube like" anymore :). This is quite ok for me as I do not intend to build a distortion generator :). I want to have as much fidelity as possible. Even B. Cordell says in the LinearAudio article that if the user wants, the amplifier can be adjusted so that it generated high amounts of 2nd order distortion so that is sounds "tube like" :)
 
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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
I'm building an MA-1 (My modular amp design) with KT88 today. I'm using 4k3 Hammond 1650N OPTs and a Hammond 278CX PT. It'll be UL connected, and cathode biased for ease of use/service/reliability for whoever buys it - I just want to get rid of the parts anyway.

One of my first amps used 4 6P3S per channel instead of 2 KT88. There's a thread about it on here somewhere but that amp has been scrapped and rebuilt 3 times so far - today will be it's 4th iteration.