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In Reply to: MingDa MC805-AA Experience posted by Mpoes on January 26, 2007 at 07:41:52:
do you think it is a good idea to use power tubes (like the 6L6) for drive? For SET design, using power tube as drive always hum I thought.
Follow Ups:
I have heard SET amps that use power tubes like 10, 801, 845, 45 and 2A3 for drivers. These all work quite well.I have also heard quite a few that use twin triodes like 6SN7, but never pentodes to drive triodes. I don't think that is especially impossible to do, just may not yield the best possible sound....particularly in staging and scale/immediacy of presentation.
Later
D
"....do you think it is a good idea to use power tubes (like the 6L6) for drive? For SET design, using power tube as drive always hum I thought..."If designed properly, this would not be an issue with the larger triodes such as 211, 805, 845 tubes. I have worked on at least a dozen of the Ming Da 805 based amps as well as the 300B driven unit.
I also have experience with other similar designs such as the Jolida Envoy and JD3000 mono blocs which use EL34's to drive each of the two 211 output tubes. These are also dead quiet.
One the biggest causes in the past for noise on the Ming-Da 805 amps has been the overbias of the 805 tube often causing an orange glow of the plates. They do not provide enough adjustemnt to compensate for this and the choice of driver tubes greatly effects the bias.(the bias is actually adjusted at the driver tube). I would recommend these amps only to well experienced DIY'er as I believe some circuit changes are needed in the bias stages.
These amps have the potential to sound fantastic but tube selection is critical as even the small input signal tubes can be a source of noise. Maybe a DC filiment voltage would be a better choice for input tubes of these amps.
They do sound great but can be problematic and one should proceed with caution when making a purchasing decision. You must also keep in mind that you are working on an amp with approximately 1000 volts at the plate of the output tube and you have to go inside to play with the bias pot or any other tweaks. One false move and you will feel it in your toes.
I have decided not to get involved with these amps anymore as you simply never know how any particualr unit is going to work out.
I've heard this Amp, and greatly admire itChinese amps are designed around the tubes these are supplied with; and replacing with ( often expensive ) NOS doesn't always give the desired results
I 100% agree with the "biasing" comment: biasing any tube Amp with potentially deadly voltages isn't for amateurs; 805's and 845's fall into this category by default
"......Chinese amps are designed around the tubes these are supplied with; and replacing with ( often expensive ) NOS doesn't always give the desired results....."So true. I don't know why they cannot build a better biasing circuit as everyone likes to tube roll not to mention the fact that not all tubes survive shipping.
In the case of the 805 Ming-Da, I know quite a few who have gained great enhancements with NOS tubes, especially the RCA 805's but have to also agree it is often hit and miss if they tubes work properly in the circuit.
As has been my experience with NOS tubes. I currently have 4 different types of 12ax7 NOS tubes, 6 different types of modern Russian or Chinese tubes, and 4 different types of NOS 12au7's. The Chinese 12ax7's and 12au7's were the nosiest here, and the NOS Sylvania 12au7's were next. However I did think they sounded a little sweeter. The Sovtek 12ax7LPS was the best new tube I tried in it, but the Tungsol was also really nice. However, the RFT 12ax7 is definatly the quietest tube I have used in it for that position. I'm using a Telefunken 12au7/ECC82 and RFT 12ax7/ECC83, with the russian 6l6, and Chinese 805's.As I believe I mentioned earlier, this setup sounds great, and noise is now unbelievably low. I have very very little hum, a slight bit of Hiss, and the noise floor of my modified phonostage has now become apparent. None of these noises are even remotely intrusive on the music, only noticable if you get right up and listen for them next to the speaker, which really doesn't matter.
As for the 805's, I'm trying to get a set of NOS 805's, the chinese ones are just junk. I couldn't get any of the new style ones in the US, so I ordered them direct from china, we shall see how they last in shipment. The old ones overheat the top caps, and the poor quality epoxy used to secure the cap dries out and falls apart. I believe the top caps are repairable, though nobody has been willing to tell me this. I fixed mine by using a mixture of conductive epoxy, and a 200 watt soldering iron. I secured them with the epoxy, and then soldered the the cap so that it was actually secured to the pin. The cap is made of Tin, so it actually will melt just like the solder. This is a careful process, but works quite well. Hopefully the higher quality epoxy I used will last longer.
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