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Kegger / Blueglow KT88 ideas for increasing power.

Blueglow KT88.jpg
 

45

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Why do you pick an amplifier that has low power for a KT88 and then modify it to get more? Why don't you pick a design that has already the power you need? I have found it on youtube and honestly, despite what the guy says, he only gets 9.3W (24.4V peak-to-peak) in UL mode before the sine wave gets really chopped. In triode is 7.6W. He gives peak power, not rms, in that video (part 10 or so).....

I honestly don't like that design, in particular that kind of local feedback. In fact, many options are shown for the input stage but only one (the ECC85 run at 226V anode voltage) works well. I suspect that all the others work much worse because of that kind of feedback, not just because of the lower anode voltage. That is already a warining to not go with it if you want to modify it after asking for advice in a forum. I could understand if you wanted to built it like shown and tested.

As guidelines:

If you wanna get more power out of the KT88: run in tetrode mode at 420-450V on the plate, 150-160V on g2 (some types might need 10-30% more) and adjust the bias for 80 mA anode current into 4-5K. Easy way to find the right value is building a regulated g2 supply with adjustable output. That's not difficult out of 450V main supply.

You can get at least 14-15 W rms when the sine wave starts to flatten like in that video. The range on values will also depend on the actual tubes you buy. I could get up 20W with selected JJ KT88s!

Feedback:
1) Could be from KT88's plate to driver's cathode if wanna keep the output transformer out of the loop (no LED bias but just normal RC cathode bias + feedback resistor)
2) or could be just cathode feedback on the K88 if the output transformer has both 8R and 16R (you could use an equivalent Hammond 16XX series). Use 4R or 8R for the speaker and the 16R for feedback.
 
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Thanks 45, appreciate your input. I am not necessarily attached to the idea of this particular design, nor am I a huge fan of the KT88..!! what was attractive was there seems to be a heap of information about it, it’s very accessible and many people are very knowledgeable about this design. I’ve modified plenty of amps in the past but now I’m looking towards a scratch build. If you could point me towards another example or design that is well documented with plenty of pictures or a layout drawing (building from schematic is still a challenge) then I’d be happy to investigate.
 
More power using the same output transformer? Expect saturation at low frequencies.
More power using the same power transformer? Expect a very Hot power transformer.

As others may have advised, start with a completely new set of parts, and a new design.

If you are in-love with Single Ended, it will cost you (high co$t. A much higher dissipation output tube; 845, 211, or other). And, be sure you have Hernia Insurance, expect a trip to the Hospital, especially if you build a stereo amplifier, instead of 2 mono-block amplifiers.

If you are willing, build a push pull amplifier; if it is mono-blocks, Hernia Insurance is not needed.

The old Rule, is either Double the amplifier power, or live with what you have (3dB more power).
Or, get speakers that are at least 3dB more efficient.

Just my experience and my opinions.

Have fun!
 
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Thanks 6A3sUMMER great info. The first two issues you’ve mentioned regarding transformers, I would like to dig into that a little deeper if you could kindly share your knowledge? My questions are based upon the idea of building the amplifier as originally designed and documented;
First question is, I would like to use a OPT that I could deploy elsewhere on future projects, although not yet purchased I would be interested in either Hashimoto or tango, if I was to use a 25 or 30w opt while still keeping the primary at 5k would this have any effect or benefit ?

Second question is, if I match up the PT (brand) to the opt’s the input voltage is 230v, I’m in Australia and we are 240v what would you suggest in regard to further compensating for this to maintain the voltages in the design? Or would you consider this to be within tolerance? I’m not concerned with passive component limits as I always like to increase the voltage/wattage allowance.

Thanks again and look forward to your feedback.
 
You want to double the power ? Then build a pp amp which easily will have a pair of kt88 deliver 50w

Using single end you are stuck below 10w
I have a pair of 5k 40w PP OPT’s I’m not opposed to the idea of PP and would ideally like to use el34’s. Is there any design that’s well documented you are aware of? Sorry I’m not a designer I’m a confident constructor so long as there is a layout drawing to follow.!
 
Whilst the pentode mode may give more power, it won't materially improve the loudness you experience because it's just not enough extra power to make a real difference. And Pentode sounds different, you may not like it, especially when pushed to max output. My 'Yukon Gold' spud amp uses a EL84-type tube that I can switch between triode and pentode and whilst it's louder in pentode I can't listen to it for very long - maybe OK for heavy metal from the 80's !

The suggestion of PP is the best idea, in my less-experienced opinion, by far. And I'd look for a design capable of dipping into Class AB, to give you headroom on peaks. Then you can really get enough power for these lower sensitivity speakers. Somewhere around 40W in Class A would be great.
 
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You might consider Michael Koster's Schadeode for a simple design with some sand. He posted a pc board for this design. Search for his final implementation with a cascoded front end. There is a thread on this site for a PSE version.

posts 62, 75, and 78
https://www.diyaudio.com/community/threads/most-linear-triode-strapped-pentode.165221/page-4

My experiments at post 18. KT88 tested after this post produced 15 watts midband at 3% distortion. Higher at clipping.
https://www.diyaudio.com/community/threads/set-build-recommendation.370350/#post-6599691

Steve
 
Whilst the pentode mode may give more power, it won't materially improve the loudness you experience because it's just not enough extra power to make a real difference. And Pentode sounds different, you may not like it, especially when pushed to max output. My 'Yukon Gold' spud amp uses a EL84-type tube that I can switch between triode and pentode and whilst it's louder in pentode I can't listen to it for very long - maybe OK for heavy metal from the 80's !

The suggestion of PP is the best idea, in my less-experienced opinion, by far. And I'd look for a design capable of dipping into Class AB, to give you headroom on peaks. Then you can really get enough power for these lower sensitivity speakers. Somewhere around 40W in Class A would be great.
I agree, a pentode connection is difficult to listen to (IMO) its just too big!
 
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You are asking for another 50% power from a design that, for a UL KT88 at those voltages, is making pretty decent power. In one video Mark did calculate the power wrong (was calculating peak and not RMS) and later corrects himself saying it makes just under 10W. This amp does have a nice sound signature, which is why some many people have built and enjoy it. Tweaking or redesigning it for more power, isn't likely to sound the same simply because you are using the same output tube.

Kegger, the guy who designed this blueglow one, also posted a KT120 version which might fit what you are looking for if you like the sound of this blueglow amp and just want more power. I'm planning to build a pair of mono-blocks based off this KT120 design. I do know a triode strapped 6EJ7 tube sounds really nice as a driver tube. I'm probably going to tweak a few values, like either raise the value of R15 or the coupling cap associated with it, but otherwise I like the design.

I know there are some folks who really hate that local plate to plate feedback, I personally like the way it sounds and that the resistor value chosen can be used to fine tune the final tone of the amp to taste.

My honest suggestion in the end though is: if you really like the way a single ended amp sounds, buy some speakers that are more efficient. That's a lot easier/cheaper than trying to build something to drive inefficient ones.

kt120_kegger.png
 
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