Ideas for a mod for more gain on this tube pre?

[jdmcgee]

According to the couple of replies when I first posted about any info on this pre, there's not many of these out there but here goes...

I've got a 1991 Groove Tubes MP-1 Mic Pre. I don't want to change the tone of it, I like that juuuuust fine but I would like to get a little more clean gain out of it if possible.... Tried tube rolling & that didn't get me enough of a boost. Thought I'd at least get opinions. I'm attaching the schematic for review.

Thanks In Advance,

jd

[Jim Williams]

You can replace the input transformer with a Jensen JT-115KE 1/10 ratio. The 10 k resistor from the cathode to ground can be bypassed with a 47 uf cap to lift gain. You may want to insert a resistor in series with it to set it where you want.

[jdmcgee]

You can replace the input transformer with a Jensen JT-115KE 1/10 ratio. The 10 k resistor from the cathode to ground can be bypassed with a 47 uf cap to lift gain. You may want to insert a resistor in series with it to set it where you want.

Thanks for the reply Jim,

What voltage for the 47 uf cap? 250V?

Thanks Again,

jd

[lassoharp]

If you go 1:10 on the input tx you will reach overload on the 1st stage a little sooner.

You'd mentioned wanting to have more control on how clean the amp stays as well as gain. These amps were somewhat designed to be paired with the original series GT mics - one of which- the Model MD1A has a whopping amount of gain to begin with. Probably explains the low ratio input transformer and lower overall gain.

As for their tone/color - I'd say they're just like the MD1A mic - designed to sound that way. This may mean operating the tubes on a more non-linear section of the curve or just lower supply voltages - maybe both. I have a MD1A mic and I'll take a wild guess and say your pre amp has a kind of soft mellow (dare I say mushy) mid voiced sound. It may be a case of just having to enjoy the amp for what it is.

My best advice would be to post this over at Prodigy in the Lab. PRR can give you a bottom line breakdown on cathode followers.

[Jim Williams]

You can replace the input transformer with a Jensen JT-115KE 1/10 ratio. The 10 k resistor from the cathode to ground can be bypassed with a 47 uf cap to lift gain. You may want to insert a resistor in series with it to set it where you want.

Thanks for the reply Jim,

What voltage for the 47 uf cap? 250V?

Thanks Again,

jd

No, not that high. It's probably about 2~3 volts at that point. Use a 25 volt cap, it's safe. I would use a Panasonic 47 uf 25 v FM.

[emrr]

Is there a quoted gain figure for these?

Interesting it has a treble boost at every stage; is it really necessary?

As Jim said, larger cathode bypass caps on the first two stages should buy you about 6 dB per stage, but at the cost of that small treble boost they're doing with the 10K/330pfd combos.

The 47K in parallel with 100 pfd at the grid of the cathode follower stage is also a tiny treble boost. Look at it the other way, it's a tiny pad on everything other than top end. You can strap some test leads across that and see if you like it bypassed. Probably only on the order of a dB or so.

I agree that transformer ratio change won't get you much here; and any gain is a trade-off against max input (headroom).