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PostPosted: 01 Oct 2008, 11:51 
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Hi Everyone, A project update. The mono-blocks have been up an running for several days now. Very nice sounding (IMHO). Initial distortion levels at 1 watt / 8 ohms have improved and are in the 0.15% region throughout the entire audio band. Low end response is awesome for a tube amp. It is flat to below my ability to measure (8HZ). I raised the B+ on the outputs to 475 as it was suggested that KT88s didn't run well at lower power levels (was 360). The EH KT88s were no better behaved there than at lower voltages. Still would not stay balanced. The JJ 6L6GCs and JJ KT77s just lock in and stay put. I have Gold Lion KT88s coming (today) and will try them to see if they work properly. The actual article and plans are nearly complete. As soon as I check the Gold Lions I will get the project to the site.

Good listening
Bruce

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PostPosted: 16 Oct 2008, 18:02 
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Hi Everyone, I appologize for not updating sooner, but family issues got in the way. I got the Gold Lion KT88s and put them in the mono-block amps. They balanced immediately and stayed that way. I must conclude that EH KT88s are unsuitable in the circuit. I will not go into finger pointing as why. I am in the process of final testing of a few variations with some of the tubes. The KT77s (JJ) and 6L6GCs (JJ) seem to work quite well at 250 to 350 VDC into either 4 or 8K loads. The Gold Lions seem to work best (testing still going on here) at 475 B+ into 8k. I raised the current for the KT88s to 100ma per tube (the others were at 62 per tube). I plan one more series of tests to try the KT77s at 250 B+ and 100ma each to see how that works. Generally the sound is approximately the same for all combinations. That is - very nice and solid. I'll post more information shortly as well as the long delayed article.

Good listening
Bruce

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PostPosted: 21 Oct 2008, 18:02 
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Hi Everyone, More information. I spent a lot of time comparing tubes, voltages and load impedances over the past week. A full article is ready to post, but here are some highlights. For the bigger Oddwatt style amps (6L6 and up) use a B+ of 450-475 volts. Set the idle current at 125 ma for a pair of the 6L6/KT77/EL34 and similar tubes. Set it for 180 ma a pair for KT88s/6550 and similar. Use an 8K load. Number crunching would indicate a lower value, but actual measurments show more output at 8K. The amps will deliver between 15 and 20 wpc class A UL with the lower setting and 20-25 with the upper. The sound is colored by the tube type and brand. Actual measurements however are quite similar. Do not use EH KT88s. I don't know why, but they don't work well in the circuit. Gold Lions do. My personal favorites are the KT77s by JJ and the KT88s by Gold Lion. If you don't need the extra power, get the KT77s (a lot cheaper). I used Edcor Iron and decent caps and resistors. Look for the posting of the build in a few days.

One thing for sure is that the big KT88s look great even when they are off.

Good listening
Bruce

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PostPosted: 21 Oct 2008, 19:17 
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Thanks for the update. I'll be putting a page together over the next few days and I hope to get it up by the end of this weekend.

Cheers,
Gio

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PostPosted: 22 Oct 2008, 14:53 
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Hi Everyone, Thanks Gio. I ran some tests last night to see what the KT88 version could do. I wanted a severe test so I used square waves. Each channel (amp) was able to deliver 24 watts RMS square waves into 8 ohms from 20 HZ to 10K HZ. They might go higher, but I melted the dummy load case. I now have a couple of power resistors encapsulated in a blue plastic blob. The actual screen shots at the 24 watt level are virtually identical to the ones at one watt. I was rather surprised that a tube amp could deliver that type of performance. When I get a chance I'll run the same tests with the KT77s.

Good listening
Bruce

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PostPosted: 30 Oct 2008, 08:00 
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Bruce,

Very nice, and that is an understatement! The balance of the layout, the copper cage, the clean wiring underneath shows a love and commitment that I really appreciate. I am sure the rest of the group feels the same way.

Please keep us posted on the kits. I would love to add mono blocks to my system.

Regards,

Alan

PS Any chance of getting a photo looking down at the top of the chassis?

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PostPosted: 30 Oct 2008, 15:49 
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Very nice! I really want to hear these sometime.

Please keep us posted on the kit version please.

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PostPosted: 30 Oct 2008, 16:26 
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Hi Everyone, The Odd Blocks project has been posted. I have been using the KT88 version for about 6 weeks now and it has been trouble free and sounds great. IMHO any tube amp that can deliver pretty clean 10KZ square waves at 24 watts rms per channel is OK. Actually they look virtually identical to the ones at 1 watt and 10KHZ. One thing for sure, if you run the KT88s fairly hard it will reduce your heating costs in cold climates. The rest of us (Arizona) will need more cooling. The four KT88s will convert nearly 200 watts of electricity (heaters and B+) to heat. Efficiency as is common with many class A amps, is about 25%.

Good listening
Bruce

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PostPosted: 03 Nov 2008, 13:32 
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gofar99 wrote:
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.

Hi Bruce, I was looking at what I have on hand for parts and it looks like I can put together a 6V6 or 6L6 version on the OddWatt 225. My power transformer is 260VAC @ 220mA and 6.3VAC at 3A and 2A. I will have to confirm the impedance if the output iron I have. I don't have the tubes, so that is open to discussion. I need to keep the total draw (including the driver section) to less than 220mA, but I was thinking in the order of about 200 mA.

Can you suggest some operating voltages and current draws that worked well for you? I was thinking 6SL7 and 6L6GC but 6V6GT is also a possibility.

Cheers

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PostPosted: 03 Nov 2008, 16:23 
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Hi Gio, Everyone. With the transformer you have and a SS bridge rectifier, you would get about 360 volts B+ with a capacitive input filter and about 300 with a choke input. To keep within the current range desired I would use 6L6GCs on the higher voltage with an idle current of 90 ma per pair. With the lower voltage, 6V6s would be suitable at about 80 ma per pair. Some brands would handle a bit more, but I would rather err on the safe side. For an output transformer, one in the range of 8-10k would be my choice. Lower values can be used, but there is a sacrifice in output power. The 6SL7 is a good choice for the SRPP driver. The B+ to it seems to be best in the 210-250 volt range. A little more is OK, but you need to be sensitive to the heater cathode rating of the tubes. With a SS supply it is possible to have the full B+ across the tube before it is warmed up. I like to include a 30 second (more is fine) delay before applying the B+. Additionally it may not be obvious but the resistor values and capacitance values in the last two sections of the power supply filters were chosen so that they charge up just in time for the tube to warm up. The B+ thus does not exceed the H-C rating at any time. The values would need to be adjusted for other tubes and current drains. Once charged up the filters need to remain at the correct B+ when the circuit is working. For the 6SL7 this is in the 1.5-1.7ma range.

Good listening
Bruce

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