50W KT88 PP w/ NFB on Bonavolta - diyAudio
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Old 24th July 2006, 06:54 PM   #1
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Default 50W KT88 PP w/ NFB on Bonavolta

I've been obsessing over this schematic (perhaps unwisely):

http://www.bonavolta.ch/hobby/en/audio/kt88_5.htm

It seems relatively simple to build, uses very common tubes, and has a nice punchy 50W rating. Unfortunately, I've been having problems with finding a suitable transformer. The closest thing I've found for a slightly reasonable price is a 100W Hammond beast.

In any case, does this circuit look any good? I recognize some NFB but I'm not a purist in that sense - I believe that a bit of it can do some good. I have this idea to build two of these into monoblocks. Anyway, I latched onto this schematic because it seemed the best "bang for the buck" - I don't want to build a 10-watt tube amp and never feel like it can get quite loud enough, and have to build another. Also, it was one of the few that actually listed some voltages and currents, so that I can build my own (probably SS-rectified) power supply.
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Old 24th July 2006, 10:19 PM   #2
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Hi Sorenj07

With nowadays sources as CD players you don't need that huge amount of gain, so there are simpler solutions for a PP with KT88. This is a link to a schematic by a - at least in the netherlands - respected designer. http://members.home.nl/triode.dick/p...td/Bill_En.pdf Besides this one he also built a PP with the KT88 using just one ECC99 as phase splitter and driver. Unfortunately no English text to accompany, but a schematic says a lot!
http://www.triodedick.com/monobill/m...versterker.GIF
http://www.triodedick.com/monobill/m...%20voeding.GIF

Hope it helps a bit. I can't say anything about the OPT, though.

Erik
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Old 24th July 2006, 10:36 PM   #3
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It looks pretty sweet, that amp, but it's at least a few classes above the on I am planning to build (in budget also, i'm sure). The schematic also doesn't include any information about the output transformer (not very helpful).. I will definitely lay it out neatly, but that's where the resemblance will stop. For one thing, I seriously doubt that I could ever hear the difference when using silver IC, or the finest film caps... I'm building everything with stock parts (aside from a nice JJ cap to filter B+) and quite frankly I never want to have to worry about power... ever. Besides, should I ever move the amp to a bigger room, I don't want to have it feel inadequate . No 1-watt-of-glory SE amps for me.

Anyway, I'm not sure as to how I'll filter the B+ at least at the outset, because I estimate about 540V immediately after rectification. It'll all depend on actual measurements, then I can get to simulating...

edit: just saw the price tag - 700-800 EUROS - this is $886 to $1012 in real money... about twice my budget.
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Old 24th July 2006, 11:11 PM   #4
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Edcor (www.edcorusa.com) offers "iron" at affordable prices. Their model CXPP60-8-4.2K is suitable for use with PP KT88s and costs $56.

I agree that with CDP drive you don't need as much gain. However, I highly doubt a tube with a mu in the 20s by itself can yield the gain needed to drive KT88s. Replacing the 12AX7 with a 5965 and the 12AU7 with either an ECC99 or a 6n30p seems sensible to me.

BTW, if you would be content with 30 WPC, a nice 2 stage "integrated" amp is possible. A 12AT7 serves as the splitter/driver and PP 7591As are the "finals". Edcor's model CXPP60-8-6.6K, which also costs $56, would be the O/P trafo.
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Old 24th July 2006, 11:16 PM   #5
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Hi SOrenj07

I have shown you this amp mostly for the schematics, which I find way simpler than the schematic at Bonavolta's site. For the stereo Bill there are three 0,33muf caps and a bunch of resistors, while the bonavolta one has 8 caps, some elco's, and two bunches of resistors. So far the differences in schematics...

For trafo's: wheter you build the Bill or the Bonavolta amp, you will need the same basic iron, so no difference in costs here. You are completely free to buy cheaper ones in the US. I don't know all the sources, but you could go with edcor. Here is an example of a trafo, 100W for 75 dollars, each. http://www.edcorusa.com/classx/tube_...pp100-8-5K.htm This is just an example, I am not saying that you have to buy this trafo, or that this one is right for the application!! For the power trafo you can go with a industrial one, as tubelab uses in his projects (said to go cheap on ebay). This is a 120VAC to 480VAC. I am sure that fiddling a bit with PSU designer will give you a PS with a nice 500V for the KT88's.

I couldn't agree more on the use of some standard wire and capacitors (just be sure voltage ratings are adequate, that elco's aren't dried out, etc). When the amp is ready and playing, and you are used to its sound you can always turn it around and change the coupling cap to see if it changes the perceived sound - or not.

So, what do you think?

Erik

I see Eli already jumped in with the EDCOR's. Does anybody know how much it would cost to deliver a pair to the netherlands?
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Old 24th July 2006, 11:53 PM   #6
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Quote:
Originally posted by Eli Duttman
Edcor (www.edcorusa.com) offers "iron" at affordable prices. Their model CXPP60-8-4.2K is suitable for use with PP KT88s and costs $56.
This looks pretty suitable except for the primary impedance - obviously the schematic calls for 5000 ohms, but what effect would using 4200 instead have? From what I've heard, it'll put out a bit more power, but at higher distortion. Likewise with the 6600-ohm one, less power, less distortion? Anyway thanks for introducing me to a transformer maker to compare to Hammonds

Quote:
Originally posted by ErikdeBest
Hi SOrenj07

I have shown you this amp mostly for the schematics, which I find way simpler than the schematic at Bonavolta's site. For the stereo Bill there are three 0,33muf caps and a bunch of resistors, while the bonavolta one has 8 caps, some elco's, and two bunches of resistors. So far the differences in schematics...
The schematic does look quite good. It does indeed look simpler. However the problem of the output transformer remains - what impedance to use? Also, I didn't quite understand how many watts this schematic put out - the writer confused me by talking about different names, mono/stereo output, triode mode, etc.. My main selling point for the Bonavolta schematic, aside from power, was the clear layout of everything, each component explained etc. BUT if I get that same clarity and power from "Bill" amp, in a simpler package, why not?

As for the power transformer, I'm looking at 2 of the following at the moment, they should be good for monoblock operation, and also I just love the look of toroids

http://cgi.ebay.com/720VCT-6-3V-400V...QQcmdZViewItem

Not too expensive and plenty of headroom in the HV to drive the KT88's. If I want to use the Bill, all I have to do is slap a bigger inductor and maybe a power resistor into the power supply and voila, lower voltage.
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Old 25th July 2006, 12:06 AM   #7
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Hi Sorenj07

Eli recommendation for the OPT is also good for the Bill amplifier. Don't take this impedance thing to serious: the give impedance is for a fixed load. You will have this using a resistor while testing the amp, but the loudspeaker isn't anything near a perfect, constant load - it will change it's impedance across the whole audiospectrum.

The version from which I sent you the pdf is rated 50W. The 100W version will be using two pairs of KT88's per channel.

I see you already found some nice iron to power the thing up!

I am going to sleep, two in the morning over here

Erik
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Old 25th July 2006, 12:31 AM   #8
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Ok, thanks for all the info though!

One thing bothers me a bit though - those transistors in what I'm guessing is a constant-current bias setup... I'm not really familiar with them, but I guess I can figure it out. Getting 15V from nowhere might be a challenge though... it seems that he was working off of a very specific transformer. Is there any way to get rid of the transtors here and the need for a separate 15V line?
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Old 25th July 2006, 01:40 AM   #9
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Actually, the more I look at the schematic, the more I object to the tons of random voltages he pulls from the power transformer. Very inconvenient to anyone who doesn't start out with his exact model. Either it means tracking that model down or sticking lots of ugly little transformers to fill the slot - and I do not want that. So it's back to the drawing board, the GEC isn't great, but this new amp schematic is too much of a hassle to deal with.
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Old 25th July 2006, 03:47 AM   #10
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It wouldn't let me edit my previous post... Anyway I'm taking a look at this schematic also, hopefully it isn't a poor design (but since it seems old, it might well be)

http://www.bonavolta.ch/hobby/en/audio/6l6_2.htm

The Hammond 1650G is perfect for this amp, but the missing capacitor voltage values are killing me.

Which brings me to my next point - does ANYONE have a reasonably high-powered, high-quality, easy-to-build schematic for one of the big octal tubes? I'm having real trouble finding one, and it's frustrating
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