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Old 10th March 2005, 01:43 AM   #1
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Default dissimilar triode for White cathode follower?

I've been trying to come up with a simple, cheap OTL amp, for headphone use but don't want to use any already published designs. I'd like to try my own hand at designing... I've been looking to build a tube amp, but I don't want to sink much money into a project I am not sure will work how I want it to. So, I am going cheap and figured OTL would be easiest if I went for headphones only. I've got a pair of Sennheiser HD580's, which are 300-ohms, so the amp will get good use.

I haven't drawn up a circuit yet, but I'd like to bounce this idea off some people here to get some opinions on whether it would be worthwhile or not. I am wanting to use a White Cathode Follower for the output and was really hoping I could get an output impedence lower than 32-ohms while still maintaining use of the "optimized" version (optimal value for Ra). I've found only a few tubes that would work for me thus far, but they are very hard to find and expensive.

So, I thought maybe I'd just not meet my goal and use some very cheap $3 per tube 6FD7 dissimilar triodes. The idea would use two of the tubes per channel, for four total for a stereo amp. I'd use the two low power sections for an SRPP input stage and the two power halves for the output stage. Does that sound as though it would work well? I am not sure if mixing and matching SRPP-type stages is a good idea, so I am asking here. Would I need to match the tubes?

Thanks for any pointers you may have.
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Old 10th March 2005, 03:10 AM   #2
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It should be okay to use them in an SRPP stage. Close matching really only becomes critical in differential pairs and the like.

Using dissimilar dual triodes with a WCF output seems like an excellent idea for a headphone amp. You might find Gregg's headphone amp of some interest.
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Old 10th March 2005, 03:45 AM   #3
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Needtubes, I've noticed you have made quite a few posts here enquiring about amps. Just out of interest, how many of them actually made it to the construction phase?
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Old 10th March 2005, 04:41 AM   #4
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Ahem... not many of them.

I've done some work with the 6GM8 and have built a couple of amps with it. I've always been wanting to try out a higher voltage, better quality one, but have always been too afraid to take the plunge and order parts... With this ultra cheap approach, though, I think I'll be much more likely. I have most parts in mind... just need to order a few. I have a fear of losing money and failure...

I was afraid that using unmatched tubes in push-pull configurations would yield not so good results (most I see always use dual triodes where both halves are the same and use them in push-pull). If it is not so critical, I'll attempt this. It is definitely cheap enough the way I have it invisioned... And hopefully if it works out well, I'll finally get enough courage to attempt something better.

Thanks for the reply. I am now reading the link you posted...
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Old 10th March 2005, 05:03 PM   #5
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I read some articles in TubeCad last night about SRPP and the WCF...

Now I wonder... If I can locate several cheap dual triodes, could I employ a parallel WCF of sorts? Run two tubes on the output in parallel, both as WCF, tie their plates together with one resistor to B+ and their cathodes together with one resistor and a cap, if necessary, to ground. It seems as though it should work... but often times there are reaons it won't... If anyone foresees disaster, let me know. Many thanks!
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Old 10th March 2005, 05:56 PM   #6
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The Optimized Morgan-Jones on the Headwize.com site will drive 300 ohm phones with authority, using just three low-price TV-Tuner tubes for a stereo amp.

If you are not building much because you are not throwing money around, build this one. It is well-proven.

As one of the TubeCAD essays explains and implies: SRPP is not a lowest-cost design for driving a cathode follower. The load is so light that a single triode volt-amp is plenty, uses less plate and heater power, and has enough gain for headphone use.

Mis-match WCF will work but loses much of the distortion cancellation of a proper WCF. Considering the vast range of twin-triodes, why try?

Single-triode volt-amp and twin-triode WCF gives two channels in three bottles and good performance. Stop over-thinking. Warm up your credit card and your soldering iron.
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Old 11th March 2005, 01:30 AM   #7
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I'd use the two low power sections for an SRPP input stage and the two power halves for the output stage. Does that sound as though it would work well?
Provided your tubes are reasonably matched the idea would certainly work fine.

If I may I'd suggest an Aikido style amp for the application, it has the advantage of upping the PSRR of the amp considerably and with the same stroke would do exactly what you need plus relaxing the PS requirements to boot:

A single SRPP stage providing a gain of mu/2 (most of these dissimilar triodes contain a voltage amp with a mu around the 50-70 mark), followed by a WCF stack that would both deliver ample power and a Zout to brag about...........

Note that for the Aikido to work it's magic it is imperative that Ri of both sections of the SRPP are as closely matched to each other in value as is practically possible, for the WCF this is less critical here.

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