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PostPosted: 01 May 2018, 17:58 
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Joined: 01 Mar 2017, 16:49
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Location: Pacific Northwest
Hello all! I have read many many threads on here, but have never posted...

I am very inspired by the many projects and designs for what appear to be some great DIY tube amplifier builds. The post about Matt's Marblewood Amp really got me thinking. Is it possible to build a push pull amplifier as versitile as the Marblewood amp?

After reading through over half (lots of good information, but still exhausting...) of the 175 pages on the Oddwatt Push Pull Tube Amplifier Projects thread, it looks like the Oddwatts can accept a variety of tubes. I also like the 4S preamplifier, as it can be used with a variety of 12A?7 tubes. Is it possible to combine these two into a single integrated amplifier? I also saw that the Poddwatt runs with a 5751, and has a volume potentiometer on the input, so maybe the 4S is not needed...

I have two output transformers from an older Fisher KX-200. They are not UL. The have a primary impedance of 6.4K and secondaries of 4, 8, and 16 ohms. I would like to use these cause I have them...

I also have a power transformer with 600 vct (300-0-300), as well as 6.3v and 5v windings. The power transformer I believe came out of an older Conn organ. I don't plan on using a tube rectifier, but i am open to it. If more voltage is needed, I have other power transformers to choose from.

I would like to use these transformers cause they were free, and I have them ready to go. Could you please help me determine the values I will need, so I can draw up a schematic and build an amplifer?

Thanks in advance! The knowledge base here is amazing, and I'm very excited to build a great amplifier.
-Jesse


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PostPosted: 02 May 2018, 16:12 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Lots of questions.... You can use the output transformers in pentode mode and make something close to an Oddwatt. Output power might be a tad greater and unfortunately so will distortion. Not bad just a bit more. (2% vs 1% at 1 watt) The power transformer probably can't be used to power the bigger tubes as the design requires a lot of current as well as higher voltages. A 300-0-300 transformer with solid state rectification would deliver about 420 vdc. Too much for an EL84 Poddwatt, A stereo KT88 version would need over 400ma of dc current at that voltage. An EL34 about 300. The transformer most likely can't deliver that either.

Some thoughts...If you used tube rectification and a choke input B+ filter it would probably get the B+ voltage down low enough for the EL84 version. Certainly worth a try. Up to about 300 vdc can work. 250 is a better value in these amps though. A minor adjustment in the idle current for the tubes might be needed, but that is easy to do. You need to be sensitive to the maximum dissipation rating of the EL84s as they are running in class A mode and the dissipation is maximum when the signal is minimum (think idle). 6.6K loading is a tad low for them in this circuit, but not so much as to cause a problem. I hope this helps.

Good listening
Bruce

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PostPosted: 03 May 2018, 16:17 
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Joined: 01 Mar 2017, 16:49
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Location: Pacific Northwest
Bruce, thanks for getting back to me. There are lots of questions, so maybe I need to focus on smaller chunks... :-) Focus on the output tubes, phase inverter, and power transformer to start with.

Good point on the power transformer. It is rated at 250 ma, so I might have to start looking for a different one. On the other hand, what if I used tubes that have a lower plate current, like a 6V6 or 6L6? If I understand correctly, when using the SIPP output, the tubes are not run at 100%, so might I be able to get away with those?

Thanks
-Jesse


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PostPosted: 03 May 2018, 17:30 
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Hi, If the transformer is rated at 600CT and 250ma then there are lots of choices. If it is only 250ma after rectification of the 300 then the choices are more limited. With 250 ma you could run two pair of 6L6s, KT77s, EL34s in class AB with no problems. Essentially you can ignore the current demands of the rest of the circuit, they will be minimal. That level of current would run the same tubes in an Oddwatt if you set the idle current to about 50ma per tube. The reduction in output power would probably not be noticeable as loudness is a logarithmic function of power. The 6L6 would be a good choice in a marginal situation as the heater current is lower than the others. Less stress on the power trannie. I would not drop down to a 6V6 as they would put out significantly less power and IMO you would notice that.

In my designs of SIPP I run the tubes typically at 85% dissipation. This allows long life and good power output. For (each tube in a pair) EL34, KT77, KT88, 6550,KT120 I typically use a B+ voltage of between 425 and 475. The 300-0-300 transformer under load would likely deliver about 400 volts DC. EL34s run at 60-62 ma (cathode currents in all cases) as do KT77s. KT88s at 90-92ma, KT120s at 125-135ma (some don't meet full spec), 6L6GCs at 400v and about 50-55 ma. By comparison EL84s I run at 250 volts and 40-45 ma. Some 6V6s will run there, but to be safe I would probably drop it to 35ma.

In class AB operation the idle currents would be much less...figure about 1/2 but the peak currents could well be more.

In any case more info is needed on the power transformer.

Good listening
Bruce

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PostPosted: 05 May 2018, 11:10 
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Joined: 01 Mar 2017, 16:49
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Location: Pacific Northwest
Okay, thanks to Google I was able to uncover a bit more information on the power transformer. It is stamped with the code C300071G1 which turns out to be from a Magnavox 142AA. The tube line up on that chassis was 2 5y3, 4 6v6, and a 12ax7... The transformer is 600vct (300-0-300) at 250ma, 6.3 at 6 amps, and 5 at 4 amps. Seems to be a pretty stout piece of iron.

Does that help?

Thanks
Jesse


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