Which Tube Mic Preamps??

general questions, comments and ideas about recording, audio, music, etc.
frank
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Re: Which Tube Mic Preamps??

Post by frank » Sat Nov 29, 2003 1:06 pm

hey audiopirate, the lulusonic 77-f looks pretty cool, let us know how it turns out...but I don't think it's a mic tube pre(no xlr jacks on the list), right? I guess you could you use it as a tube DI...kinda...either way let us know. I'm printing out the parts list/schematics as we speak.

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Derrick
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Re: Which Tube Mic Preamps??

Post by Derrick » Tue Dec 02, 2003 8:08 am

audiopirate wrote:Derrick,

I happen to think building this stuff is fun, so check this out too.

http://home.earthlink.net/~altaflux12ax ... reamp.html

I am working on this right now. I am a bit curious how its going to sound without audio transformers, but it's cheap enough to experiment, right?
Anyways, the aplification is actually done with valve and not an IC which i expect will have pleasant distortion qualities.

only one way to find out!

-Allen
Allen, Have you built this circuit yet? I just built it myself over the weekend with surplus parts lying around the house. It is a distortion monster! At least mine is. I'm emailing the author to get his take on it and suggestions. I may be sending too much B+ into the circuit as I'm using an older transformer that was diesigned back when AC wall voltage was still around 115v. Anyhow, I'd really like to compair notes with you when you have your preamp built. I'm doing alot of research and finding a few things here and there that I could share. I have a hunch that for this circuit to work well, it will need an output adjustment added and a resistor or two changed to bias the bubes for a bit less gain. If there ends up being no hope for this circuit, I will get some audio transformers and continue with slightly more complex circuits. Anybody else have any experiance with building a tube preamp?
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Re: Which Tube Mic Preamps??

Post by mjau » Tue Dec 02, 2003 10:57 am

For my two cents (literally), I found the HHb/TL Audio Radius 50 tube preamp to be very respectable. It's single channel with a very simple, subtle compressor on it, and has done a fine job with my Beta 57 on a guitar amp, my Oktava 319 on vocals, my c1000 on acoustic guitar, and with my p-bass direct. Somewhat colored, round, and warm, and for $400 new, it wasn't a bad deal.

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Re: Which Tube Mic Preamps??

Post by drunton » Tue Dec 02, 2003 11:09 am

Derrick,
I've played around with some tube stuff before (but I am not an expert). What tubes are you using? If you have AX7's, then maybe try 12AU7 or 12AT7 instead. They have less gain.

Regarding the plate voltage, you are better off having higher voltages. The gain is adjustable by playing with the resistor values, but I have never been successful getting anywhere near balanced (upper and lower) distortion unless you have 200+V rails.

Don't forget to check out www.hamptone.com or his DIY Tube article from about a year ago in TapeOp to get some ideas. or go to jensentransformers.com, they have some good info there too.

Also, you won't get much drive from just the 2 common cathode stages. You really need a voltage follower to get any decent drive. Or, run it into an opamp as a buffer. Use an o-scope to look at an unloaded output waveform, that might tell you where the distortion is coming from.

Lots of fun!
Dave
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Derrick
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Re: Which Tube Mic Preamps??

Post by Derrick » Tue Dec 02, 2003 1:12 pm

Dave, I tried the 12AX7 first, then the 12AT7 with similar results. Are you saying that it will be eazyer for me to adjust gain with the bias resistors if my B+ (high voltage rail) voltage is greater then 200 volts? The Hamptone tube kit seems pretty expensive and isn't for sale yet so I wanted to try this one first. The Lulusonic F-77 was my first try at a tube mic Preamp and a place to learn a little. At first build, it's pretty unimpressive, but I havn't tweeked it at all. I would like to see if there is a way to make the harsh clipping (almost transistor guitar amp like) a bit smother. To use ajictives, I want this to be able to go from a clean sound to a saturated sound with warmth and fulness, not fuzz like a pedal which it does now.
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Re: Which Tube Mic Preamps??

Post by alba359 » Tue Dec 02, 2003 2:18 pm

Anyone have experiance with the Peavey TMP-1 tube mic preamp. I understand this is a properly designed tube micpre. I wouldn't expect much from it since phantom power is only 24v and list price is preaty low. I don't know anyone who has used the TMP-1.

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Al
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Re: Which Tube Mic Preamps??

Post by Al » Tue Dec 02, 2003 2:30 pm

I dont know about the Peavey one, but the HDB audio one is a REAL hand built tube pre, and not overly expensive either!

Al

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magritte
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Re: Which Tube Mic Preamps??

Post by magritte » Tue Dec 02, 2003 2:35 pm

how much of a difference is there between a boards preamps (tascam m-2516) and a dedicated pre amp? is it worth an investment?

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Re: Which Tube Mic Preamps??

Post by drunton » Tue Dec 02, 2003 10:13 pm

Derrick, do some poking around with a voltmeter and see what the B+ is. If I read the schematic right you should be at sqrt(2)*115V=165V or so. Having more voltage on this rail allows you more voltage swing before clipping.

You really need an o-scope to check where the signal is clipping. Look at the output of tube 1 and then output of tube 2 without it connected to anything - remember no current drive from these 2 stages. If it looks clean without the load, then you just need to put a buffer op-amp in place. It doesn't have any gain, and if you get a good one, will be dominated by the tube sound.

Try checking out the hamptone page again. On the picture links on the left, choose the one up from the bottom - pictures of knobs. This is the support page for the original TapeOp article. He has the article, pictures, and schematics. Figure 1 is the preamp, the others are the power supply. BTW: he is running B+ at 225V.

Do a search on google for 12AT7, you can find the original tube datasheets online. They will help you with biasing. There are also a lot of tube preamp schematics out there.

Want to build a really good one from a kit, get the Hamptone. Want to buy one premade - Peavey VMP-1. $600 used - but very nice.
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Re: Which Tube Mic Preamps??

Post by Derrick » Thu Dec 04, 2003 8:14 am

Thanks, Drunton! I looked at the Hamptone page again. Yes, I do get around 165v for my B+. Also I do have an o-scope and a signal gen. Can you tell me:

* What is "sqrt(2)*115=165v"? Either my math needs a refreasher, or I'm not seeing somethig.

* I do have an o-scope. When you say "look at the output of tube 2", I assume you are reffering to the second half of my 12A*7 (cause there is only one tube)?

*You really need an o-scope to check where the signal is clipping. Look at the output of tube 1 and then output of tube 2 without it connected to anything - remember no current drive from these 2 stages. If it looks clean without the load, then you just need to put a buffer op-amp in place. It doesn't have any gain, and if you get a good one, will be dominated by the tube sound.

1- Do you mean inject a signal, check 1st half and then 2nd half of tube for clipping with out the output connected to a load (ie: without a mixer, tape deck,etc. connected)?

2- What do you mean by no curent drive by these two stages?

3- If it is clean without the load, I need a buffer (ie: an op amp) in place at the output?

Thanks so much for your time and heplp!
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