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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Mu stage question posted by Spantou on January 27, 2019 at 19:46:15:
Here are the cct and details of the heaters.
The mains transformer is 280 - 0 - 280, heavily over spec i.e. rated at something like 150 mA or so if I remember. I use a pair of 6CJ3 damper diodes which are very good, these need 6.3V 3.6A total AC and this is provided from the same transformer. Also 6.3V 1A from this transformer for the E83F heater.
Of course you could use a GZ34 or even schottkys if you like.
I use a 6V 50VA transformer for the 27 heaters to keep the spikes from rectification of this high current away from the cct.
The volume control is now different from that shown. By the way, I have two of these preamps, virtually identical, so that I can try out changes in a controlled way. This change of volume control worked well.
I need a fine balance function or I'd use a stepped attenuator. I have heard how a conventional balance pot affects the sound (ruins it), so I use two mono volume controls.
I still use a series resistor and pot as in the cct diagram, but the series resistor is now 33kohm with a 10k Cermet pot. I find the cermet is highly transparent (seems about on a par with an attenuator), and I would not say it is hard or cold at all; recommended!
Take care over the pentode pinouts as they are different to most and I have not marked them on the diagram.
The gain of the 27 is about 10 or so; the volume control in effect reduces this to about 4 which I find excellent in my system. It can be adjusted by choice of resistor and pot resistances, or use a simple pot cct for about 10 gain.
I used Mundorf SGO for couplers and outputs - but in light of a good suggestion in this thread I will try a lower value coupler and try polystyrene or Teflon. The high power resistors (27K, 39K) are Mills.
The anode load resistor is a decent value 27K; having this fairly large means the cct is highly sensitive to tiny changes in anode current which is exactly what is wanted to make the cct work well; I believe it also reduces Zout but I have to say I don't understand why. :-)
Another comment. I have found that mu stages seem to take ages to run in, and change significantly for the better eventually. Generally I guess at about 100 to 200 hours 'hard' running in, 20 hours a day, CD player source into a dummy load; followed by 3 months of normal use. I'm guessing this may be due to the tiny currents in the coupling cap or something to do with the G2 cap.
So if anyone builds this and is disappointed, don't change anything! Just run it in properly.
Anyone is welcome to build this. One thing I do is this; the cct is very compact so I put it close to the input and output RCSa and use extension rods on the selector and volume controls; recommended.
Fire away with any questions.
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Follow Ups
- My 27 / E83F Preamp - Andromeda - Mike C 01/30/1903:09:50 01/30/19 (0)
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