Fine Tuning My Tube Mic Pre - Transformers Rewired Update II

[NewYorkDave]

This is becoming hard to follow--actually, it's been kinda hard to follow from the beginning. Didya ever make sure the secondaries of your transformers were terminated properly? Unterminated transformers often yield a rise in high frequency response and a rolloff of low frequencies.

If you post your schematic and data on your transformers, that would make it much easier to advise--as opposed to trying to extract or extrapolate the info from your postings. Please take the time to create a complete schematic rather than making us hunt around and put together fragments.

 

[analag]

[quote author="NewYorkDave"] Please take the time to create a complete schematic rather than making us hunt around and put together fragments.[/quote]

Actually you could just post the part of the schemo you need help with and leave the rest out if you don't care to share. Fair enough?

analag

 

[beatpoet]

OK... Thanks for volunteering to help sofar. It always lifts a ton of frustration off of me when someone volunteers new things to try or think about. I'm still relatively new at this, so here goes:

At first I tried 150R:15kR strapped as specified by the Altec 15095A can. This resulted in both bass and treble loss pushing that 150k load. -3dB @55Hz - 8.3kHz

Then I strapped 3 & 4 to try for a 600R input. Same thing - losses galore. My output transformer (BPI - University Sound 500/500:15k) also experienced -3dB @65Hz strapped like this, in reverse of course. This is after pulling the old 3:1 transformer for -3dB@70Hz

My latest try evened out the frequency response - but I can see that it's loading the preamp oddly - The low end is tubby and distorted, and I can see obvious distortion in my waveforms. My output is wired this way also, using only a single winding.

This is a pretty simple cascade mic pre - similar to NYDAVE's 2-bottle and a few old Altec Schemos I've looked at sans the global NFB - - No K bypass either.

[quote author="NewYorkDave"] Didya ever make sure the secondaries of your transformers were terminated properly? Unterminated transformers often yield a rise in high frequency response and a rolloff of low frequencies.
[/quote]

My grounding seems OK - even redundant in places, but time to double & triple check. I hope this clarifies things a little!

 

[jaygunn]

When you said you were using a 1K grid stopper resistor, you ACTUALLY meant 1MegOhm, right? If not you're way off of NYD's one and two bottle designs I think...

John

 

[skipwave]

John - Forgive me if you know the following already, but it seems there is some confusion that I might be able to clear up. The grid stopper resistor and the grid resistor serve two distinct functions. The grid stopper is in series with grid pin, whereas the grid resistor is hanging off the grid to ground. Take a look at NYD's MILA schematic, he uses both in that design.

Grid stoppers are common in tube guitar designs were gain is very high, and oscillation more likely. The grid stopper helps to quell any oscillation, and the maximum recommended value is 68k. You will never see a 1M grid stopper, but 1M is a typical value for grid resistor for instrument inputs (DI). The reason is that the grid resistor defines the impedance seen by instrument connected to this input.