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Oddwatt Push Pull Tube Amplifier Projects
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Author:  cannst77 [ 30 Jul 2009, 09:58 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi Robert, in case you your concern about the SMPS came from my earlier comments about mine putting out too low a voltage under load, this turned out to be an error in my multimeter, not the SMPS.

While there is definitely something strange going on with one of my tubes, in my case I don't think this is heater related. The power transformer of the amp with the off-centre balance consistently gets hotter than the other one, and after swapping the tubes over to the other amp, that one now gets hotter. I realise tubes draw more current before they die, so will be monitoring the situation closely. Hopefully it stabilizes, but this is nothing to do with the SMPS. The only real problem I had with mine was finding one that fitted in the case.

Btw, can anyone suggest a minimum value fuse for protecting against tube failure. My mains is 230v, and the fuse is only connected to the HT, and not the SMPS. I can't find an efficiency stated on the edcor site, but I suspect my current 1.6T fuse is much too high.

Author:  robertf [ 30 Jul 2009, 22:22 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Thanks for the responses to my heater question. I am just investigating the power supply options and what transformers I could use. My preference was to build the amplifier on a single chassis, but given the number of transformers required (2 output, 2 power and SMPS) this option is probably not practical. More likely I will go with a separate power supply build as in the KT77 version.

But to get the HT supply with 500mA I will need two Edcor power transformers.

Author:  gofar99 [ 30 Jul 2009, 22:41 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, Yes the B+ requirements for these guys are fairly high. I'll check tomorrow and see if I can locate a single trannie that would work for you. Post back if you don't get an response in a day or so. I have (unfortunately) forgotten some things I said I would do and don't want to slight anyone. If I can find a power trannie you will need to put this one on wheels.

Good listening

Author:  gofar99 [ 31 Jul 2009, 14:18 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, There are two transformers I feel would be OK to use if you built the monos as one unit or used a central SP for two monos. If youdo that make sure you have good interconnects between the PS and the amps. You will also have to put the final two sections of the B+ filters inside the amps. PM me if you need specifics.

The transformers are EDCOR
XPWR023 300-0-300 at 250 ma = about 420 dc at 500 ma and 2 windings of 6.3vac at 4 amps each lists for about $71
XPWR071 350-0 (no CT) at 1000ma = 490 dc at 1000ma and 2 windings of 6.3 vac at 5 amps each. lists for $109
The XPWR071 is the one of choice. It is a 15 pound beast, but I find it better to be a bit larger than on the low side. The smaller one is certainly no light weight at 8.5 pounds. But it is about the minimum you can run a pair of the KT88 amps with and will run rather hot. Your call. On a fairly costly set of amps I prefer to have them work well for a long time and the extra $38 would appear well spent. The big trannie will need a full wave bridge to function and the other only a pair of rectifiers. Everything else is the same and the B+ is in the range where I would not make any other changes to the amps.

Good listening

Author:  Gimp [ 01 Aug 2009, 00:04 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

What is the smallest (power wise) Odd watt any one has done?

I've got 6AK5s, several 115VAC to dual 12.6V@.178A transformers, and an audio out PP transformer that is supposed to have come from a Lafiette amp with 6L6 Drivers. Transformer is a tad big, but it has taps at:





I figure If I put a 8 ohm speaker on the 39:1 tap I'll have a plate load of 12.16K, which is probably a bit low for the 6AK5s but should work.

I can run two power transformers back to back and get 115:12.6/12.6:115 and fwb them to get about 130Vdc at 39ma which should be enough for two 6AL5 tubes at 7.7mA plus a driver at 9ma.

I still need to work out the 6V filaments unless I use a third transformer and run the two outputs in series and use a 12V driver (in parallel with the output filaments).

Next I need to work out the biasing.

Oops, I'll be driving a Klipsch Heresy (circa 1976) with the amp so I don't need a lot of power. I measured it back around 1979 and found I was running 32mW average and 1W peak for normal listening levels.

My guestimation is I should get around 2W out of it with a swing of 7.7mA with a 130V swing (2 X 130X.0077).

Hints, kinks and general suggestions welcome.


Author:  gofar99 [ 01 Aug 2009, 10:56 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi, I have not heard of any real small ones, however I am working on a headphone version using 6J6s (triodes) for a output of about 1/2 watt. BTW there is an Edcor matching transformer for 10K ct to 150 ct that is perfect for this application. Many new phones are in the 150-300 ohm range.

Generally, I see no special reason a small one could not be built. Max input to a pair of 6AK5s would be just about 2.7 watts and allowing it is class A and you would probably use UL mode I would expect the output to be between 25 and 33% of that so figure on just below a watt. It could be OK for your application. I am not familiar with the trannies you have, but the load would most likely be OK. If there are multiple output taps you can use the 4 ohm one on 8 ohm speakers and effectively double the impedance of the primaries. I would certainly try it and see. A good scope and signal generator would tell you which is best. I believe the PS should be about 150 as RCA shows the limits at 180. I would set the CCS for 12-15 ma. I have modified Altec A7-500s and can use amps as little as 5 watts and get sufficient loudness so yours being a bit more efficient could work OK. Other tubes that might work in the circuit for low power are 6CG7s, possibly 12AU7s and 6SN7, 6AQ5, 6AU6, You could make a variation with triode pentode bottles and have the SRPP and outputs using only 2 tubes per channel instead of 3. This would require some research first to make sure the heater cathode voltage is not exceeded.

Good listening, good ideas

Author:  Gio [ 03 Aug 2009, 20:24 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hi Guys, I got my first KT88 OddBlock done. Some photos are below. Sorry, I did not take any construction photos. However, I will take plenty of the next monoblock construction and post them in my Odd Block KT88 Series 1 Tube Amp Kit - Build Log.
The instructions are very detailed and the kit is fairly easy to put together. The amp is dead quiet (no hum) with 96dB/1W speakers (Fostex FE206E). I did listen for a while, but I will wait until I get stereo going before I give my impression of the sound. So far, it seems very positive.

Author:  Dan [ 03 Aug 2009, 21:48 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

great looking amp you have there !!!


Author:  cannst77 [ 12 Aug 2009, 02:53 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

Hey Bruce, I saw your pic of the amps with the blue JJ kt88 tubes.

How did you find the sound? Anything like the kt77's?

As I have the single pair of jj's already I'm planning on getting another pair to see how it goes.


Author:  cannst77 [ 12 Aug 2009, 08:24 ]
Post subject:  Re: Oddwatt Push Pull Tube Amplifier Projects

:mad: Need to share some more problems.... I noticed a slight loudness difference between the two monoblocks, and although it was slight, even when tubes were swapped one amp tended to allways be louder. I cracked it open and measured all the voltages and was alarmed to see the SRPP plate up at 293v (top of 100uFx100uF and 50uFx50uF were 474,458, 349,293). Although not affecting the loudness, swapping tubes DID affect the voltage, and the same amp with the JJs had 466,448,335,272.

The other amp had 475,456,337,267 with its GLs and 464,446,321,250 with the JJs.

Listening to the JJs again, I actually quite like them, so will be interested to hear how they sound in both amps.

I am worried about the heater-cathode voltage though, as I am way over 250v. The amp at 293v had heaters referenced at 108v.

My resistors in the PS are 5% 1w metal oxide (the only ones I could get), except the 100R which is 5% 9w enamel. Any suggestions as to what could be wrong gratefully appreciated, and any quick and easy ways to lower the B+ also welcome.

The discrepency between the two amps is confusing as the components are all the same, includng the PCBs, and there is no obvious sign of component damage....


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