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PostPosted: 18 Jun 2008, 23:27 
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Hi Everyone, There seems to be a bit of interest in the various versions of the Oddwatt amp EL84 project I posted a few months ago. So I thought this forum would be a good place to talk about other versions completed or in the works. First I don't claim to have originated the basic circuit of the output stage. I can track its origins back about 55 years and then it seems to be lost. The idea to use a CCS in the cathodes was posted in a project on the diyparidise site a while ago. SRPP stages have also been around a long time as well. I suppose if I can take credit for anything it is the combination of the two circuits. It would appear that neither by itself is all that wonderful. The pair though work very well together, far better than I would expect. Both parts have things in their favor. The SRPP is highly immune to power supply noise and can drive difficult loads easily. The output stage is simple in the extreme and also is rather immune to power supply hum and noise. Together they make a quiet amp. It can only operate in class A mode, but is very adaptable to other tubes, and modes (triode, UL, pentode). So much for the background and disclaimers.

The amp I use most now is the KT77 (OD225) version. It has a robust and refined sound. I won't go into all sorts of other terms as we probably couldn't agree on what they mean anyhow. I personally think it sounds better than anything else I own (3 vintage SS amps by Marantz, a Kenwood, Pioneer, Super Ts, a LM3875 gainclone, a modified K-12 tube amplifier kit, and a number of lessor ones). The OD225 works well with 6L6GCs too. It has an even more robust sound with a little more distortion (about 0.3%) and a little less refined sound. Great for rock music.

My present project is a variation that uses KT88s. It is about 50% completed. I'll see if I can post images of the progress.

For those of you who may find tube amp building a parts collection nightmare, myself and another diyer (Hi Rodney) are working on putting together parts kits complete with PCBs for the KT77 and KT88 versions. Since only quality parts are being included it won't be cheap, but then no reasonable tube amp in this size range is. Stay tuned on the kit issue, we are still a few months away from that.

Good listening
Bruce

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PostPosted: 19 Jun 2008, 10:34 
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Greetings All, This is a follow on to the earlier post. First, I want to mention that I am glad to work with anyone on a diy project. You can email direct to gofar99_AT_hotmail_DOT_com if you wish. I check the email often, but not everyday. I consider the present KT88 project as an evolution of the earlier ones and differing only in size and detail from them. So comments I make on the one will generally apply to the others.

First and most important, tube amps like this contain high voltages and can cause serious harm and potentially be fatal. Be careful at all times.

The present amp is being built in three pieces. A common power supply and two amps. This was partly for looks and partly because a single chassis would be very heavy and awkward to handle. It would weigh around 60 pounds. I have attached two photos. The one is off a prototype of the wood base (since replaced with one with clean tube holes) and the other of the bottom before most parts are in place. Note that I used turret boards with built in gold tube sockets. This makes the build much easier.
Attachment:
KT88_Bottom.jpg
Attachment:
KT88_side.jpg

Good listening
Bruce


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PostPosted: 22 Jun 2008, 09:37 
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Hello. I was looking at the Ultra Compact Amp and stumbled upon this thread.

I liked the shadow box idea. I thought of using one on several occasions. My preference at present is silverware chests. Just knock out the bottom and glue the lid to the (former) bottom's sides. They are generally strong enough to withstand quite a bit of weight without deflection.

The reason I was checking out the compact was that I scored a batch of SV83's (actually, 6P15P-EV) and am looking for something to do with them. Only problem is that according to some folks, they can only take 200V on the screen. I think that the data sheet says otherwise (http://tubedata.itchurch.org/sheets/113/6/6P15P.pdf). This would mean running in pentode or triode strapped with a zener string if using the 300V B+ in the circuit at DIY Paradise. I see you have the plates at 240V and think that this would probably be OK.

Has anyone had any experience with these tubes?


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PostPosted: 23 Jun 2008, 14:48 
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Hi Tad, There are several similarities to the ultra compact amp and others before and after it. Their aim was simplicity with good sound and they deliver. I saw some opportunities for even better performance, so the circuit is not as simple. I am not familiar with the 6P15P-EV, but from the spec sheet it shows up to 330 volts on the plate and screen. If you wanted to use them in an "oddwatt" style output I would try 250 for B+ and limit the current at idle to stay within the ratings for disipation of the tubes. A plate to plate load of between 8 and 10K would be about right. All that would be required is to adjust the resistor in the CCS for the right current. I see no particular reason they wouldn't work. I would expect output in the same order of magnitude as a pair of 6AQ5s or perhaps a little more. Say about 5 watts in class A per channel. The feed back circuit would probably have to be tweaked for optimun response, but should be a good place to start. Observing the output on a scope would be best. Considering that the EL84 version doesn't need feedback at all, you could see how it sounds without any.

If you have lots of these tubes, you could try push-pull parallel for greater output. Let us know how it works out. Feel free to email me for more info.

Good listening
Bruce

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PostPosted: 24 Jun 2008, 16:42 
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Hi Everyone, Progress is slow but tonight I expect to fire up the first of the KT88 amps. I did a bunch of what ifs with tube cad and decided to try 12AX7s for drivers at 300 volts B+ in place of the 12AT7s at 220. A small amount of rewiring of the PS was needed to accomplish this. The 12AX7s (at least in cad) were more linear and had more drive capability. We shall see. I'll post some new photos if everything doesn't blow up. Might post them even if it does.
Good listening
Bruce

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PostPosted: 24 Jun 2008, 22:29 
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Hi Everyone, No smoke, no fire, no blown fuses. Great. All measurements are within expected ranges. First scope checks show flat response from below 20 to past 20K. The KT88s were running 150 and later 195ma per pair without stress (at 350 B+). With all the wires hanging out, I didn't get to listen to it. That is for tomorrow night.

I attached a photos of the amp as it is now.

Good listening
Bruce
Attachment:
OD88 side 3-4 with tubes.jpg


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PostPosted: 24 Jun 2008, 22:31 
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Second photo


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PostPosted: 24 Jun 2008, 22:55 
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Bruce, bang up job. What are the 4 red knobs on the top view?

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PostPosted: 25 Jun 2008, 14:16 
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Hi Gio, Everyone, The 4 red "knobs" are actually pin jacks so I can set the tube balance externally. The number of millivolts measured in each pair equates to the number of milliamperes of curent in the tube cathodes. The control between the KT88s is the adjuster. It gives me a range of about 10 milliamperes swing either way. It gets around the problems of needing matched tubes and tube aging. The circuit is self biasing so actual tube bias controls that are common in many designs are not needed. The CCS determines the current flow and thus automatically "sets" the cathode bias.

As part of the initial run in on the amp I determined that the CCS needs to be above the chassis. The wood is too much of an insulator and traps the heat. I plan on mounting it behind the output transformer tonight.

Good listening
Bruce

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PostPosted: 25 Jun 2008, 16:12 
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Very cool work. I'll be watching for the finished project.

Gary

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