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Vintage threads from the first ten years

Re: DR update3 - nearly there...

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5/25/2005 12:26 PM

Re: DR update3 - nearly there...

Yes, that's the basic idea. It is supposed to be easy to try even after an amp has been wired conventionally.

You may actually need the hum balance control to get the most out of it, and it really only helps if the hum is coming from the first 12AX7 having a heater-related problem.

When a temporary application fixes the hum, then for a permanent version I prefer a single terminal strip to be added to the chassis within distance of the original heater leads supplying the first 12AX7. Then put the cap, diode, and resistor on the terminal strip and run a new pair of leads to carry the DC to the tube.

Since the DC part of the heater circuit is floating, you depend on the ground reference from *somewhere else* upstream. You never ground the DC lines in most cases but if you did you would need to remove the previous heater system ground point, for instance in my drawing the hum balance pot is the single place where the heater system is grounded.
Once you have no ground for the heater system at all, then you could try grounding the DC subcircuit to see if it helps better. In that case when the whole heater system is floating you could actually choose either the positive or the negative DC terminal to connect to your common or chassis ground, as long as there is not some other ground point where the heater current, whether AC or DC can find its way past the filaments and short out.

Mike

5/25/2005 1:47 PM
Ian Anderson	Thanks to everyone who's helped (again), I might try out the DC heater thing on my next 'brew. Those -3dB points are interesting - being able to get the same tone by changing one of three components relative to eachother, I need to try that out. ... Ian

6/9/2005 1:03 PM

final update...

ok, after alot of listening and jamming I changed the circuit over to a SLO100, it's ALOT nicer imo - smoother and more versatile range of gain.

I've upped the mid pot by putting a fixed 47k resistor between the 22k mid pot and ground, it really helps the amp cut through with the gain cranked. For more sensible amounts of gain, I can back the mid pot down to zero and find a nice retro vibe with my P90-loaded SG jr.

I tend to run the gain on around 8, just enough to bring the tone into focus and dial-out the 'slight' muddyness with the gain on 10 (I think this is just the bright cap coming into play).

This does seem a fairly dark sounding amp, I have to run the treble up full to get enough cut - some of the coupling caps are definately coming down in value.

Despite all that, it does sound killer - I'm talking 99% 'there', the final 1% arsing around is the hardest bit.

... Ian

6/9/2005 4:13 PM

quote:
"I changed the circuit over to a SLO100, it's ALOT nicer imo"

Does this mean, that you've finally ended up again with the 39K cathode resistor on the 3-th gain stage?

Larry

6/10/2005 12:58 AM
Ian Anderson	Larry, I just left the 33k resistor on the 3rd gain stage - 'slight' compromise in value (heh heh). ... Ian

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