EE question - tube preamp
js
eBay for 6 bucks. Just as I hoped, it has an awesome tube pre built into
it - 2 12AX7s, a 6X4 and a 6AQ5
I yanked it out of the chassis and began modding it to use as a stand alone
preamp. It sounds great, except that when I plug a guitar or bass in, there
is a very small window of input gain before the thing goes into total
distortion. (I'm tapping of the record head, BTW). Overall, never gets very
loud, as the tubes co into total saturation/compression about 1/2 way up.
I want to attenuate the input a bit to give me some more play. The
distortion is nice and creamy, but I would like a good bit more clean gain
before distortion. I know passive guitars are already low level devices, but
I guess I need to go even lower.
If anyone knows an easy way to accomplish this, or you have a better idea,
I'd appreciate it. TIA. I don't have any specs on the current impedances or
anything, so take your best guess.
PS one other question: If I keep a lot of ventilation around the tubes in
their new enclosure, do I still need a fan? If so how much will it interfere
with the output signal?
Nigel Goodwin
>I recently picked up an old Silvertone "project" reel to reel recorder off
>eBay for 6 bucks. Just as I hoped, it has an awesome tube pre built into
>it - 2 12AX7s, a 6X4 and a 6AQ5
>
>I yanked it out of the chassis and began modding it to use as a stand alone
>preamp. It sounds great, except that when I plug a guitar or bass in, there
>is a very small window of input gain before the thing goes into total
>distortion. (I'm tapping of the record head, BTW). Overall, never gets very
>loud, as the tubes co into total saturation/compression about 1/2 way up.
>
>I want to attenuate the input a bit to give me some more play. The
>distortion is nice and creamy, but I would like a good bit more clean gain
>before distortion. I know passive guitars are already low level devices, but
>I guess I need to go even lower.
Passive guitars are quite high level devices, probably 1000 times or so
higher than a tape head.
A tape preamplifier is completely useless for you, not only does it have
far too much gain, it's also no where near flat, as it has to compensate
for the characteristics of the head and the tape.
If you really want to build a valve preamp, then strip parts out of it
and build it from scratch.
Personally I've never seen the attraction?, high distortion, poor
frequency response, high noise levels - why bother?.
--
Nigel Goodwin
C.Farmer Ltd.
Matlock
Pt
Try using a 5751 or 12AT7 in V1 (first preamp tube).
That will cut down the gain.
If you want to cut even more try a 12AY7.
I picked up a small all plastic fan for my amp that doesn't cause any
hum.
Walgreens for about $20.
Some people use computer fans.
Pt
js
does that already. I want it to be dirty and quirky and all that. It's gonna
be a special effect, that's for sure - possibly a "stack in a box".
That first part was good info though. So is there some sort of attenuator
circuit I can buy/build to make them match better?
"Nigel Goodwin" <Ni...@cfarmerltd.co.uk> wrote in message
news:HZx90mDA...@cfarmerltd.co.uk...
js
"Pt" <pea...@yahoo.com> wrote in message
news:844ce818-109c-4362...@b1g2000hsg.googlegroups.com...
Todd H.
> I don't want it to be flat; I use a ton of very high end tube/SS stuff that
> does that already. I want it to be dirty and quirky and all that. It's gonna
> be a special effect, that's for sure - possibly a "stack in a box".
>
>
> That first part was good info though. So is there some sort of attenuator
> circuit I can buy/build to make them match better?
A simple L pad will do--2 resistors. No different than turning down
the bass or guitar going into it, however.
http://www.uneeda-audio.com/pads/
--
/"\ ASCII Ribbon Campaign | Todd H
\ / | http://www.toddh.net/
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/ \ http://www.toddh.net/netiquette/ | http://myspace.com/bmiawmb
pTooner
"js" <not...@nothing.com> wrote in message news:g8ce8k$5r5$1...@aioe.org...
>I don't want it to be flat; I use a ton of very high end tube/SS stuff that
> does that already. I want it to be dirty and quirky and all that. It's
> gonna
> be a special effect, that's for sure - possibly a "stack in a box".
>
>
> That first part was good info though. So is there some sort of attenuator
> circuit I can buy/build to make them match better?
>
You could buy one of these: http://www.coutant.org/matching/5.html
Or you could just stop by the shack and investigate a 1000 to 1
transformer. OTOH, most of these old recorders had a line in or at least a
microphone in (usually high impedance) so I don't know why you can't use
that. If you post the model number information I'll try to find a
schematic.
Gerry
Brian Running
> preamp. It sounds great, except that when I plug a guitar or bass in, there
> is a very small window of input gain before the thing goes into total
> distortion. (I'm tapping of the record head, BTW). Overall, never gets very
> loud, as the tubes co into total saturation/compression about 1/2 way up.
Will you allow some non-EE intervention? By "tapping off the record
head," do you mean that's where you are taking the signal to the power
amp? If so, I'd think that's going to be a problem, since the signal
there is meant to go to tape, not to a speaker. As Nigel mentioned,
tape recorders apply absolutely radical EQ, the EQ curve is like a
straight 45-degree line. Not only that, I've got to believe some
serious compression was applied, to avoid tape oversaturation, which you
would not want in an instrument pre-amp. What kind of inputs and
outputs did the original recorder have? I used to use an old TEAC
reel-to-reel as a bass pre-amp, and using the tape monitor outputs
produced a really sweet sound -- at that point, the signal is meant to
be listened to, there's no EQ and no compression. Aren't there tape
monitor outs, or is it strictly built-in speakers? If it's only
built-in speakers, even then, you can take the signal from the speaker
wires and attenuate it to input into a power amp.
js
It's the record/playback head. I'm using it as the "input". The built in
speaker leads are now the "output jack"
There's a slider switch - a piece of perfboard with a bunch of metal
contacts on it, with solder pads/clips on either side that don't move. As
you slide the switch, the contacts hit different sets of clips, changing the
wiring of the recorder from Record to Erase to Play. the schematic is very
vague as to what pad goes where in the different modes. Through
experimentation, Record modes seems to give me the best sound/noise level
though.
There are no tape monitor outs or anything like that. I don't see any
compression circuit or anything either, outside of the tubes. This is
strictly old school. I don't have a date on it, but I have to say late 50's
/early'60's
I was hoping to use the mic input, but it actually sounds kind of wimpy. I
don't think it's running through exactly the same signal path as the record
head, at least according to my amateur schematic reading skills.
Again, I think the problem is on the input as opposed to the output. When I
plug a guitar in and BARELY turn the knob, I get a nice tube-y tone.
anything beyond that is in full distortion.
I like Todd's "L"pad idea. I might wire one of those up at like 40db with a
push button switch. Pt's tube switching idea is cool as well, but building a
perfboard pad is a lot cheaper than going on an NOS tube search, at least
for now.
If you've got any other ideas, I'm all ears.
"Brian Running" <brun...@XXameritechXX.net> wrote in message
news:p8mqk.19620$89.1...@nlpi069.nbdc.sbc.com...
Pt
Pt's tube switching idea is cool as well, but building a
> perfboard pad is a lot cheaper than going on an NOS tube search, at least
> for now.
You don't need NOS tubes.
New tubes will work fine and usually cost between $10-$15.
Even some NOS preamp tubes on ebay are cheap.
NOS Mullard's are a different story.
Pt
js
couple resistors for $1.49. I think I'll wire up the pad first, record some
stuff, and if it's worth pursuing, I'll swap out the tube.
BTW, here's a newbie question - the 12AX7's have a metal cover around tem
that's riveted to the chassis. I've seen these before, but they usually just
slide off. What's the best way to get it off without damaging the amp?
"Pt" <pea...@yahoo.com> wrote in message
news:59425439-481f-4453...@34g2000hsh.googlegroups.com...
Nigel Goodwin
>Yeah, I know. But I can go down the street to the Nerd Shack and buy a
>couple resistors for $1.49. I think I'll wire up the pad first, record some
>stuff, and if it's worth pursuing, I'll swap out the tube.
Like I said before, you need to remove the filtering if you want to use
it as a bass preamp, it's no where near flat for a tape preamp.
>BTW, here's a newbie question - the 12AX7's have a metal cover around tem
>that's riveted to the chassis. I've seen these before, but they usually just
>slide off. What's the best way to get it off without damaging the amp?
They usually push, twist, and slide off - I doubt the screening can is
riveted, just the base it fastens to.