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Mixing Tubes and Semiconductors
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Author:  billp [ 12 Aug 2008, 23:16 ]
Post subject:  Mixing Tubes and Semiconductors

Bruce, I have thoroughly enjoyed reading your tube amplifier projects. Do you find you are getting better performance using a switching power supply and voltage regulators? I'm sure a lot of builders would be bothered by the idea of mixing tubes with semiconductors.

Author:  gofar99 [ 15 Aug 2008, 11:17 ]
Post subject:  Re: FE206 BL Horn recommended tube amplifier

Hi Billp, A great question and certain to cause contraversy. I guess that there are a couple parts to the answer. Two things I consider key are that I like well reproduced music, second as far as I am concerned there is no absolute way to do that. So I will use whatever seems to do the job be it SS or tube. All that said, I personally prefer the sound of tubes. I have done a number of projects (not all make it to the web) using all tubes (no SS parts) and have not really be able to notice any difference between them and ones with selected SS parts. SS is clearly more efficient in doing some things. I use SS rectifiers all the time now and voltage regulators as needed. With proper filtering I personally hear no difference in the sound. The key is proper filtering. Either SS or tube power supplies, that are poorly implemented will cause sonic problems. Now to SMPS. I only have one non heater usage of such at present (Beauty preamp). That being a commercial design that I modified a bit, is very quiet and well behaved (others BTW have had problems with that SMPS though). The place I do use SMPS is dc for heaters. This application has several plusses and a few negatives. Regulation is of course excellent. Things to watch for are switching noise in the output (and back into the line), isolation of line and output (I usually have the heaters well above ground or floating), start up fault indications (not all SMPS like to start with a heavy resistive load). I always add filtering at the SMPS outputs for RF and spike reduction. You really need to use an O'scope to verify the existance of these though. Personally, I find the advantages outweigh the problems. The one amp on the web (OD225) uses a convential supply and a pair of mono-blocks I am presently building are quite similar which will have SMPS for heaters. I'll post any conclusions. The EL84 amp (Oddwatt) works very well with the SMPS. A question you didn't ask that ought to cause more discussion is the use of the LM317 as a CCS in the cathodes of the output tubes. All I can say is that it works. The OD225 has the best sound of anything I own (I have several amps both SS and tube). The amp that measures the best (a gainclone) is not the best sounding. For a tube purist mixing a 317 in a tube stage is a real sin. For me, it seems natural. I am glad to go into more details if you want. Just send a PM my way. This is not a easy or hard and fast type of question to answer. Feelings and perceptions get in the way.

I will now don my body armor,

Good listening

Author:  RiGuy [ 15 Aug 2008, 11:31 ]
Post subject:  Re: FE206 BL Horn recommended tube amplifier

My God man!! How could you?!?!

Author:  billp [ 16 Aug 2008, 11:07 ]
Post subject:  Re: Mixing Tubes and Semiconductors

I'm sure the semiconductors in your tube circuits measure better. Now you make a interesting comment. Your gainclone is your best measuring amp, yet it does not sound best. Call me old fashioned, but I like the sound of a beefy tube rectified power supply and a choke or two. On paper, the CCS seems like a great idea. It is simple enough to implement that I will try it.

Author:  alexg [ 21 Aug 2008, 10:41 ]
Post subject:  Re: Mixing Tubes and Semiconductors

I had great result from using LM317 as cathode current sink in my 832A PP amp. I also used LM317 on my 7193/6j5 preamp.

I have used transistor CCS on my 12au7 preamp, and it resulted to better dynamics, clarity and bass.

I have used LEDs on the 6n1p driver of my SE KT88, and it sounded great.

Author:  gofar99 [ 22 Aug 2008, 17:18 ]
Post subject:  Re: Mixing Tubes and Semiconductors

Hi Everyone. A tip that I probably should have mentioned before. I suggest you use the LM317HV instead of the standard one. The HV can go to 57 volts and the normal one 40. The extra margin is significant if you use KT88 tubes. The CCS will in any case require a heat sink. Small ones will do for the EL84 pairs at 80 ma per pair, but as you go up in power (to as much as 200 ma for the KT88s) you need to increase the size of the heatsink. If the CCS overheats, it will shut down on you. No damage, just no music either.

I got my replacement KT88 in yesterday and will get back to testing mods this weekend.

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