Subject: Add a 9V guitar tuner to at tube amp .
From: "fffcj"
[email protected]
Date: 10/22/2004 8:51 PM Central Daylight Time
Message-id: <
[email protected]>
I have a nice tube amp .
I'm modifying the case to lighten it etc ...
While I'm at it , I'd like to incorporate my guitar tuner into the amp .
Regarding the audio circuit , I'll probably just place the tuner into the
FX loop .
What I'm unclear on is how to find some 9V dc within the tube amp to
power the guitar tuner .
I guess I'll need maybe some more components to rectify / smooth the power
from the amps own power transformer .
I've not tested any ac voltages yet .
What circuit should I be using ?
Thanks
Chris
Hi, Chris. If you don't need a lot of current, find a couple of small 6.3 VAC
secondary transformers and do something like this (view in fixed font or M$
Notepad):
.------.
120VAC 120VAC 6.3VAC | | +
o---. ,-o-----------o-. ,------o--o~ +o-----o-----o
6.3VAC )|( )|( | | |
)|( )|( | | +|
o---' '-o-----------o-' '------o--o~ -o-. --- 8VDC
| | | ---
'------' | |
| | -
'---o-----o
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Pick off your 6.3VAC to the tube heaters (which will almost always be grounded
-- you can't use that) and use it to backfeed the first transformer, and then
use that output to feed the second transformer. You can take the output of the
second and rectify/filter that to give you a little over 8VDC. Use schottky
diodes instead of a standard bridge rectifier for a slightly higher voltage.
First, you have to oversize the first transformer if you're backfeeding -- It
should be rated at least 4X the VA you are using at the DC side. So, if you've
got two 3VA transformers (
[email protected]), you shouldn't expect to get more than
80mA or so out of your DC without getting the first transformer really hot.
And you do have to be careful about the heater power -- don't take too much, or
you'll smoke the main xfmr. I like the concept of borrowing a couple of VA --
that's about 1 extra tube's worth of current -- it's usually safe.
Second, you'll have to use a brute force cap to reduce ripple on the output.
The 120 Hz ripple might not be too good for your application, so you might want
to use a 12VAC secondary transformer for the second one, and then feed that to
a standard rectifier/filter/linear regulator like the LM317.
.------. ____
120VAC 12 VAC | | | |
--o-. ,------o--o~ +o-----o---| 317|--o----o
)|( | | | |____| | |
)|( | | +| | .-. |
--o-' '------o--o~ -o-. --- |240 | | |
| | | --- | | | |
2nd Transformer '------' | | | '-' +|
| | | | ---
| | o-----' ---
| | | |
| | .-. |
| | | |1800 |
| | | | |
| | '-' |
| | | |
'---o-----o ---------o
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
Be sure to have a second transformer of less than or equal VA rating to the
first.
Also, be extremely careful here to avoid a situation where you connect a higher
potential in to this circuit. It'll kill the LM317, and potentially cause a
lot of other damage inside the amp.
Good luck
Chris