DIY Audio Projects Forum

Oddwatt considerations
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Author:  Soundbrigade [ 03 Mar 2009, 11:32 ]
Post subject:  Oddwatt considerations

I am rather thrilled by the Oddwatt design as it is VERY simple and though I loose a lot of power compared to running the tubes in AB, some 17 - 25W is a lot more that the 3W I will get from my 2A3s.
However, as you will notice, I am a person who is very ambivalent and cannot decide that easy, besides I have a tendency of starting projects without finishing them ....

However I have (as I have written in a sort of presentation) two sets of OP trannies; a pair of Edcore 25W and another pair of 50W and have some ideas ... The smaller ones could come in hande in a future project involving some hard to find and bl**dy expensive power triodes, the 50CA10, whereas the bigger ones could be used together with some 6550 in an SIPP configuration. However, I still have a pair of EL156 and could get two more. The EL156 have a Pa of 50W whereas the 6550 stops at 35W. This could mean that an SIPP EL156 would give me 30-35W - PURE CLASS A, IF they will work. :confused:

Another idea is concerning the inpit stage, the SRPP. Will anything work? I mean, I have a bunch of Russian tetrode, 6EE5P, that have a fair gain (very high) and can make almost any tube sing. I've seen them in applications with GM-70, 6S33S, 6P45S, GU-50 etc ... Or what is required of the driver?

Author:  CrazzyAbtTubes [ 03 Mar 2009, 20:05 ]
Post subject:  Re: Oddwatt considerations

There are many amplifier designs to choose from in the range of 35-40 watts. The design I am using now is based on the W5M Heathkit high power Williamson, which can provide up to 35 watts using KT66's if I recall correctly.

My amplifier is using the OddWatt amplifier CCS circuit, but instead of the LM317, I've decided I would try the TL783 voltage regulator which is good up to 125V or so at 700mA. In class "A" mode It has been predicted that I would get around 25 watts out of my design, hopefully.

You are lucky to have multiple pairs of Edcor tranny's on you hands as you can decide on a larger beefier amplifier or a nice compact EL84/6V6 amplifier.

Good luck!

Author:  Soundbrigade [ 04 Mar 2009, 07:04 ]
Post subject:  Re: Oddwatt considerations

If you have a look at my introduction and presentation (my Curriluscum Whiskey, I know I know!!!), you notice that apart from my Past Audio monoblocks (60W each), most amps I have or am building has a power of 10W or less. That is no problem as I at the moment have a pair of horns based on speakers from Hungarian Sonido ( and even makes some big noise even at low input levels. However I am about to finish a pair of 2-way ported speakers that aren't that sensistive and I felt it would be nice to have a 30 or 40W amp, just in case. Therefore my first idea was to use 6550 in the Oddwatt design, but I could consider a more powerful tube like the EL156. I also have a heap of Russian pentodes GU-50 that are rated 50W. They shouldn't be that kinky. The look nice too. 8-)

I noticed that someone recommended the TL783, apart from you and when I checked my local resistor dealer, I saw they were in stock.

Author:  gofar99 [ 04 Mar 2009, 13:15 ]
Post subject:  Re: Oddwatt considerations

Hi, I'm glad you find the Oddwatt design interesting. Here are some thoughts that might be of use to you. The Edcor 25 transformers are easily able to do 35 watts. I use them with KT88s without problems. I have talked to Edcor on them and their ratings are very conservative. A pair of KT88s at 475 B+ and 180 ma will deliver approximatley 30 watts. Actually a bit more with increasing levels of distortion. For EL156s I would definately go for the bigger trannies. I have a set of them as well and they behave well in the circuit. The CCS using a LM317HV for the bigger tubes is a rather cost effective way to do the job. The voltage across it is essentially what is required by the tubes at idle. It would seem that because of signal fluxuations it would increase and result in failure of the LM317. This has never to my knowledge occurred, even when using the standard LM317s on 6650s. I recommend the HV version mostly as a precaution.

Other tubes can be used for the SRPP. I have found that some work better than others driving the output section. The 6SL7/12SL7 are my favorites but a close second is the 5751. I have used ECC802S as well with good results. Others that would seem obvious choices 12AT7, 12AX7 etc, just don't seem to sound quite as good even when the circuit values are optimized for them. I have not bothered to find out why. I would be interested to hear about others expeiences with different tubes. There is no particular reason that the SRPP need be only triodes. I have seen designs that use a triode for the bottom section and a pentode for the "load" section. IMHO there may be advantages, but as well as a dual triode works there doesn't seem to be enough benefit to ofset the possible problems. The basic requirements of the SRPP are determined by the output stage. You need enough gain to drive the outputs fully with about 1 volt input signal. Second it needs to be able to develop enough voltage swing to actually drive the outputs. For many tubes this is in the +/- 25 volt range. With the SRPP the gain is usually fairly close to the rating or the tube. So without NFB a 12AU7 can be used with KT77s (a little over a volt is needed here). For the KT88s you need around +/- 35-40 votls of drive. The other recommended tubes will do this.

One thing that makes the circuit perform well is simplicity, so I certainly encourage experimentation, but caution on making it more complex than necessary.

I use a KT77 version to drive some modified Altec A-7s (95 db/w) and have used it on Dayton MTMs (87db/w) In my listening room the one set can not be used at full power and the other is quite acceptable. The KT88s are overkill with either set.

Good listening

Author:  Soundbrigade [ 04 Mar 2009, 17:28 ]
Post subject:  Re: Oddwatt considerations

Weill ... I slightly unexpected recieved a package today that solves parts of my SIPP-problem. And I am also very happy to hear from you Bruce, that Edcor's transformers are underrated. I know that Hammond's (BIG) trannies are tested with 2kV and also they are underrated.

So what am I talking about? Well I recieved a special wound transformer from Germany that is almost normal but has a 50V tap for heaters :o . When I order it some three or four months ago it was intended for a Luxman clone. And I will re-continue that dreamproject of min, WITH the Edcor transformers. However I will definitely go on with the bigger transformers, they may derive from a Swedish tubeamp manufacturer, cannot remember the name right now. :? Well I will try to take a tuesday, which is my day off, and go and visit a tube-friend and grab a pair of 6550.for a future Project.

I love simple stuff. The Oddwatt is simple, amps using 6BM8 or 6T9 also tend to be simple. My 2A3 and Darling are simple. My 6AQ5-PP is simple using a 6AX8 as input/phase splitter. However the 50Ca10 is not that simple, but not that complicated either. An 6267 as input device and an 6DT8 as phasesplitter/driver, but I've seen other configerations.

Author:  CrazzyAbtTubes [ 04 Mar 2009, 18:59 ]
Post subject:  Re: Oddwatt considerations

I am not sure what is a better bargan for me. I decided on the Williamson amplifier because I am very much familiar with it and it is most commonly used in allot of PP type amplifiers. I also liked the sound of it too.

It was not easy to decide between that or the Odd Watt design. When I thing back I would have saved allot more money going for an Odd Watt amplifier, but I have decided that it would be fun to use the salvaged parts from my Harman Kardan amplifiers to build a 6V6 version later on. ;)

I personally find that 25 watts of pure tube power would be more than enough for me, especially since my amp will be stereo, and the speakers I use have been said to be very inefficient, like 80 or 85db per watt. My 10 watt Harman Kardan amps some how drove these with ease as though I had a 30 watt solid state amp driving them.

Hey, Bruce if you don't mind, I would like to know what improvements(if any) there would be by using the TL783, just so I know that is what I want to use, I'll bet it would need a pretty big heat sink for a 35-50 watt amplifier.

Author:  Soundbrigade [ 05 Mar 2009, 10:16 ]
Post subject:  Re: Oddwatt considerations

I build my (bigger) amps in a rather traditional (and typical) way, se pic below. Will the chassis top be enough to cool two regulators or do I need extra heat sink? As there's only one driver tube per channel, it's easy to make some arrangement for a heat fin on top of the chassis.


(I will make a new chassis where the tubes are more spaced).

Author:  CrazzyAbtTubes [ 05 Mar 2009, 17:19 ]
Post subject:  Re: Oddwatt considerations

Holy crap!! that picture looks like it was taken of some audio magazine! :o

Back to answering your question, if that is a solid piece of copper there should be sufficient heat dissipation, but I am not one to judge, as I do not have any experience with the OddWatt design. In my case I shall ask Bruce the same question and post a pic of the heat sinks I can use rather than using the top plate itself...
Amplifier CCS heatsinks(small).jpg

Nice and compact compared to my amp, if it were me I would keep it that way. 8-)

Author:  Gio [ 05 Mar 2009, 22:33 ]
Post subject:  Re: Oddwatt considerations

Hi Magnus, that looks great. What is the tube compliment?

Author:  Soundbrigade [ 06 Mar 2009, 09:49 ]
Post subject:  Re: Oddwatt considerations

I have to make a new top plate for the amp, guess I have no copper left so I am left with heavy aluminium ( :x ).
The tubes are triodes 50CA10. IF I can find more of them they are like £100 each, so I wonder why I didn't go for a 300B-PP .... :censored: .
I will use an EF86 (6267) trioded for IP-device and a 6DT8/ECC85 as driver, or maybe a 12BH7.

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