OK,
First off, there is way too much gain for most practical applications. You need to decide what type of mic is going to be used with this? If it's a condenser, then 40dB will do, or 50-60dB for a dynamic. Only a low output ribbon at a distance from the source would ever need 75dB.
To start with, I would change the EF86 grid resistor for something 10x lower, like 51k or even 22k. The noise of the tube is equivalent to about 25k, so the 500k resistor is contributing 20 times more noise than the EF86!
You don't need to ground the CT's of the OPT. If you ground the CT of the primary you screw the ratio and make the load too heavy. The CT of the secondary should not be grounded and be left at 600 ohms.
You really don't need the gain stage of the ECC81 either, it's great as a cathode follower, but non linear as a gain stage.
Lets look at the gain situation in more detail:
Your IPT is giving you about 23dB, your OPT is losing 12dB so your net gain is 11dB plus the gain of the amp. The EF86 will give you around 45dB with the values chosen, so that makes 56dB as is. Perfect!
You should not follow a pentode with a triode normally, as the Miller capacitance of the triode will lose your top end. A bootstrapped CF has a very high input impedance so the coupling cap only needs to be around 22nF, not 100nF.
So re-wire the ECC81 doubled up as a cathode follower (CF) or use one half for each channel if stereo.
Ditch the gain control as it will still contribute some noise and use the bootstrap CF as described here:-
http://www.valvewizard.co.uk/accf.htmlRead everything this guy has to say to fill in the gaps in your knowledge.
The gain of the CF will be around 0.9 something so it will leave you with a fine 55dB mic pre.
I have found from experience that the input tube will give less hum if you provide a DC heater supply.
I hope there is enough advice there to sort out your problems.
DaveP
July 23, 2004, 07:48:09 PM