Transformerless Tube Pre - looking for advice

[leigh]

Hi y'all,

I've got a Harmon Kardon ME 40 (mini tube pa) that I'm trying to clean up. There's no documentation, either in my hands nor on the web (afaik). It's a simple scheme, 2 mic ins, 2 aux ins, mix to a single channel for the power stage and output.

Last night I traced one of the mic input circuits. The path does not have an input transformer, which of course is pretty unusual these days for a tube pre. But it was most likely designed for high impedance mics (the mic connectors are center-pin screw-on Amphenols), which wouldn't mind so much going direct to a tube (right?).

The thing is really noisy, and recapping the power supply is definitely in the plans.

But I'm wondering what I might do to make the inputs more friendly to modern mics, besides stick a transformer on there. The input path goes:

in -> 0.1 uF cap -> pin 2 of a 12AX7, with a 3M3 resistor to ground between the cap and the tube

I know this is a very open ended question, but I couldn't find any practical info about using a tube input stage without a transformer.

Cheers and welcome to the (new) jungle,
Leigh

 

[gyraf]

Hi Leigh,

I would start by simply mounting an input transformer directly before the input as it is now. Something like 1:5 to 1:10.

Depending on the transformer you may or may not want to terminate the secondary with a bit lower impedance than the 3M3. A good transformer - like the LL1530 1:7 - would behave nicely as is.

Jakob E.

 

[PRR]

> does not have an input transformer, which of course is pretty unusual these days for a tube pre.

It was dead-standard in the old days.

> But it was most likely designed for high impedance mics

We did a lot of small PA work with crystal mikes, specifically because they avoided the cost of a transformer.

But even before that, it was very common to use dynamic mikes with internal transformers to step them up to nominal 50K. That way you could aford to buy a "big" 4-input PA system, but not pay for all 4 transformers up front. You might do your first gigs with just 1 or 2 mikes, and earn the money to expand to use all 4 inputs.

This business of running 150 ohms everywhere was only for the rich boys who ran mike lines hundreds of feet. You can run "50K" (really closer to 10K) lines 40 or 50 feet without treble loss. (Well, not more treble loss than the low-cost mikes and speakers of the time.)

With modern mikes and close-mike techniques, you can probably just wire a XLR jack without a transformer. Impedance does not have to match. Close-mikes often give signals as hot as the older hi-Z mikes at traditional distances. It will be unbalanced but I've done a lot of work that way.

XLR-Amphenol transformers used to be a standard part. I have a few.

Last I saw, Radio Shack still stocked an XLR-1/4" 1:7 transformer. Cheaper than buying an XLR jack and a "good" transformer. Response is decent, and overload level is well above 12AX7 grid level. Peel the label. Open it up. Disconnect the leads to the 1/4" plug. Now use your big wire cutters (saws are for wimps) to cut-through the 1/4" plug, in the insulator between tip and sleeve. All the guts inside the sleeve fall out. Find a piece of coax that barely fits inside the sleeve. Stuff it in with a little silicone caulk or shoe-goo. Connect the inside end to the transformer and close the body. Connect the other end to an Amphenol cable jack or just butt-solder to Amphenol jack on the chassis or to tube grid and ground. Use conduit clamps to mount the XLR bodies to the chassis. Or drill holes in a block of oak, nail it to the chassis, wedge the XLR trannies in the holes.

Over-build the power supply. Most low-price tube PA systems had very marginal supplies. The transformer was usually excellent, but the caps were small so it would sag under overload and fry slower. I've had happy results upgrading from 30uFd to 470uFd.

That's for PA use. Ya know, those direct-grid inputs are perfect for Fender guitar (guitar amps are just old PA amps with quick-connect phone plugs instead of screw-in Amphenols). As guitar, you probably do not want to super-size the power caps.

 

[Mark Burnley]

Yes,

My all time fave guitar amp is an old Fantavox 35W PA head. A quartet of EL84's provide the output, and it doesn't half "sag" when you hit it with a big chord :grin:

The input is straight to a grid, and I just tweaked the grid resistor from 47k to 1M. A certain Mr Cobain was a big fan of using old PA amps for guitar.

Right, I'm gonna have to go and play guitar now...all this talk of sagging HT lines with big power chords is getting me going...



Mark

 

[clintrubber]

from Mark:

Fantavox 35W PA head. A quartet of EL34's

4 EL34s only giving you 35W ? Or EL84's ?

Whatever, happy powerchording.

And funny to see that name Fantavox again - only ever saw it on my first Japanese mono cassette-recorder (solid state).

FWIW...

 

[Mark Burnley]

Ha!

Keen eyes- I just spotted my mis-type! Corrected now.

Yes, I couldn't find a schem for it so I drew it out. It sounds amazing cranked up. I got it in a junk shop for £15, and replaced a few caps and raised the value of the input resistor.

I've only ever seen this amp and a re-badged RIAA pre amp with the "Fantavox" name.

Yep- off to play guitar now!

Mark

 

[mcs]

[quote author="PRR"]I've had happy results upgrading from 30uFd to 470uFd.[/quote]
But don't make the first cap after the tube rectifier too large - 33-47µF should be OK with most rectifier tubes.

Best regards,

Mikkel C. Simonsen

 

[SonsOfThunder]

Well lets see if we can start a fight:

[quote author="PRR"](saws are for wimps)[/quote]
:shock: You gonna just take that CJ?


:green:
Charlie

 

[CJ]

Saws are kind of wimpy, thats why I switched over to he Bridgeport!
:grin: