Using a Pre-amp in a Guitar Tube amp as a Microphone Pre-amp

[Mo Facta]

rkaczano




rkaczano,
I'm not sure if you were planning on unplugging your guitar combos internal speakers during this experiment or not, but if so: Don't because you can damage tube poweramp sections when they're operated without a speaker or resistive load attached

Yup, I was just going to say that. If you run a tube amp without a speaker (load) attached, you will blow your output transformer and maybe more.

Believe me, I speak from experience.

Cheers

 

[ecc83]

Yes, give it a do. But if you use that 560Ohm output you will I am sure need a 50W min load to replace the speaker. Then one of these...
Microphone Transformer XLR to 6.35mm Mono Plug : Mic Transformers & Mixers : Maplin Electronics

Will be a great help. First it will unbalance the mic "properly". Then it will give up to 20dB of noise free gain. As has been said, a guitar amp might have enough gain if well cranked but it is likely to be very noisy and probably hummy. The traff would give you nearly a x10 start on that. Lastly the transformer should kill any RFI which a wide open gitamp is prone to.

There are in fact considerable differences between a rough ass'ed guitar input and a properly designed valve mic pre.
The input traff is of course the first difference. The one shown in the link is pretty cheap and cheerful but a decent pre would have something a lot better with a wide frequency response and low distortion. The input VALVE is also likely to be different. Not an ECC83 but an ECC82 or better an E88CC . Both these last valves have a high gm not a high mu and this is what is needed for a low noise amplifier.

Then of course a pukkah mic pre is designed for a wide, flat response (probably will also have at least a HPF) gitamp stages are usually very limited in their LF response but worse than that they are usually "voiced", for guitar tho' not vocals!

But don't let me put you off!

Dave.

 

[Lt. Bob]

Neither of those outputs will hurt your interface, which is why I say just try it. ?

if he runs that Rivera without a speaker he might learn about tube amps blowing up if they're run without a load.

 

[Ainaudio]

ECC83 comment is right, you will win a lot of gain and reduce preamp noise.
Use mic Lo Z to instrument Hi Z transformer/"de-symmetrizator" if mic signal is to weak.
However - some condenser mic signals are strong enough without transformer (but you need P+48V DC Phantom power adapter before amp for them).
This case you can short XLR pin 3 and pin 1 together, and use pin 2 as hot (+).
* some condenser mics can not work this way, because their impedance convertor/preamp can have pseudo-symmetric output.
Transformer is solving all questions principle, just find right ratio transformer for concrete mic, application and guitar preamp.
** Any transformer is more or less, but little bit degrading frequency response, phasing signal, and adding distortion, but if you are looking for warmer signal it can be even better !

 

[Russ56]

One thing I am going to try for a vocal effect is to plug a good microphone into my Behringer micro mixer, then take the output to line in on my Boss Digital Guitar Pedal, then the output will go from the pedal to the Vox valve amp as per normal. I will then mic the amp and record. Note I am only doing this for thickening effects, not the primary sound, unless it is surprisingly usable and suits a particular song. Experiment.