Tube amp input for microphone

[Leevi]

How to adapt a "classic" tube amp input/1st amplification stage to microphone?
/Leevi

[sluckey]

Depends on the mic. Some are plug 'n play.

[jjasilli]

Some old, vintage mic's are hi impedance and can be plugged directly into a el guitar tube amp.  Most modern mic's are lo impedance which is a mismatch to your amp.  Some modern mic's like Shure can be switched from lo to hi impedance, sometimes by moving an internal jumper; sometimes by re-wiring its mic connector.  BUT, such mic's perform much better when set for lo impedance operation, which will not match your tube amp input.  If a lo impedance mic is plugged into a hi impedance amp input, it will sound weak and feed back readily. 

The simplest solution is a passive in-line lo-to-hi impedance adapter.  If your mic cable terminates in an xlr plug, then something like this:  http://www.musiciansfriend.com/pro-audio/whirlwind-little-imp-lo-to-hi-impedance-matcher   or  http://www.bhphotovideo.com/bnh/controller/home?O=&sku=438615&Q=&is=REG&A=details. 

More sophisticated solutions are a mic preamp or a mixer. 

[jjasilli]

If you actually want to adapt the amp, one solution is to install a matching transformer.  Vintage one's, available on eBay, can be used with an octal tube socket.  Jensen makes new one's, single or dual, which can be mounted to the chassis.  Instead of an xlr into the amp, a T/R/S (a/k/a stereo) phono plug can be used with a jack to match.  So if the mic cable terminates in an xlr plug, you can re-wire it T/R/S, or get an adapter:  xlr-to-T/R/S. 

[silverfox]

What about a Mic Preamp like the Art.

A single tube- 12ax7 with a drive and master like this: http://www.amazon.com/s?ie=UTF8&page=1&rh=i%3Aaps%2Ck%3Amic%20preamp

Silverfox.

[Leevi]

If you actually want to adapt the amp, one solution is to install a matching transformer.

Yes, I was thinking something like this.
But then what kind of transformer? which depends of course on the mic.

/Leevi

[Willabe]

But then what kind of transformer? which depends of course on the mic.

Your chasing your tail until you decide what mic.


                Brad     

[HotBluePlates]

What about a Mic Preamp like the Art. ...

It's my opinion that small me-too tube preamps like the ART are little more than garbage. Or maybe half-decent opamp preamps. But either way, they do the job with solid-state and have a tube mostly for show.

I have a pair of tube mic preamps based on the UA 610 (but predating UA's current offerings); that's transformer in & out, with a 12AY7 and 12AX7 for a single channel. There are other niceties (some low & high shelving EQ, choice of input impedances, Hi-Z instrument jack, phantom power), but the basic circuit is pretty straighforward. The transformers (2x UTC transformers per channel), packaging and power supply (250vdc, 12.6vdc, external to preamp chassis) are what make it expensive.

Anyway, Willabe's right that you have to know the intended type of mic. And I'd say you have to know the intended performance level. If you actually want quality amplification of a vocalist's mic, you're probably not gonna get that in a guitar amp (unless the guitar is staying clean throughout the amp. If you just need a mic input for a harp player's mic, well that's a different story...

[jjasilli]

If you actually want to adapt the amp, one solution is to install a matching transformer.

Yes, I was thinking something like this.  But then what kind of transformer? which depends of course on the mic.  /Leevi  

Leevi:  I feel your pain having recently gone through this issue myself.   IMHO:  In terms of spec, anything close will be good enough for use of a mic matching trannie into a tube amp.  In the old days Bogen solved this problem using octal plug-in trannies in its tube PA amps, with its own brand of matching trannies.  See e.g. the schematics to the Bogen MX-60A or MX-30.  Any of these Bogen brand plug-in will work:  TM 50 -- 50R in : 50K out; TM 200 -- 200R : 50K;  TM 500 -- 500R : 50K.  Any other brand that matches these specs is OK.  Don't buy if the seller leaves the specs unknown. They are often available on eBay - be prepared to pay about $35 - $100 or more.  So the modern in-line adapter is cheaper & easier.

Or go Modern with a Jensen:  http://www.jensen-transformers.com/datashts/115ke.pdf  tube amp schematic:  http://www.jensen-transformers.com/as/as020.pdf  Other schematics here:  http://www.jensen-transformers.com/apps_sc.html

Bottom line, The Rule of Tens gives you lots of leeway.  Lo imp. mic's typical output may range from say 20R to 200R.  A mic trannie to match can have an input imp. ranging from 20R to 600R and work well enough with any such mic.  Tube amps are happy with the output imp. of a guitar's PU, say 8K.  So, the output impedance of the matching trannie may range from 8K to an extreme of 80K for .  

[Leevi]

Thank you Jjasilli for this very valuable information
/Leevi

[kagliostro]

Ciao Leevi

if you are interested in tube mic preamp

here is a good one by Pete Millet

http://www.pmillett.com/micpre.htm

---

Here in the forum Richard developed a tube mic preamp

http://www.el34world.com/Forum/index.php?topic=10116.0

K

[jjasilli]

BTW:  a quick check shows very reasonably priced Edcor's  @ 600R : 10K, in your ballpark:  http://www.edcorusa.com/p/13/m2m600-15k  about $23.00, or un-housed trannies at the bottom of that page for about $11.00!!!  My personal, purely subjective preference is the old octal plug-in type.  Curmudgeons of the world unite!

[Leevi]

Thanks for your responses!
It seems to be lot of good products available and even for reasonable price.
My interest to this issue is related to the topic I started couple of weeks ago.

http://www.el34world.com/Forum/index.php?topic=16330.0

/Leevi

[jjasilli]

In that case you may want a vintage "bullet" mic which is hi impedance anyway.

[HotBluePlates]

And those are routinely plugged into average guitar/harp amps without any special inputs.

[jjasilli]

Whoops I accidentally posted this on Leevi's bassman thread:

Here's the thread form my bullet mic project:  http://www.el34world.com/Forum/index.php?topic=2474.0  Can't believe it was 6 years ago!

Old fashioned, dynamic, hi impedance mic's don't sound as "good" as modern mic's and have a limited frequency range.  But so do harmonica's.  If the goal is to play Show Tunes on a chromatic mic, then a modern lo imp. mic is the way to go.  Opposite for a traditional blues harp sound.  Leevi did not specify, but if he's plugging into a Bassman, I'm guessing Blues Harp.

[Leevi]

Leevi did not specify, but if he's plugging into a Bassman, I'm guessing Blues Harp.

I'm not playing harp but I was supporting a Blues player who uses different kind of harps and mics
all from 50's/60's. Maybe it's difficult to find one and common solution since there are mics with different impedance.

His harp sound:
What Have I Done Ina Forsman with Helge Tallqvist Band

/Leevi

[jjasilli]

A SW can be used to select either hi imp., or lo imp. wiht the matching tranny in the circuit.  That's what Bogen did.