Re: Valve preamp


"Dave.H" <the1930s@xxxxxxxxxxxxxx> wrote in message
news:1b4426d2-160f-4fca-a3f1-293899c63bae@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jun 20, 12:42 am, "ian field" <dai....@xxxxxxxxxxxx> wrote:
"Dave.H" <the19...@xxxxxxxxxxxxxx> wrote in message

news:404015e4-00e7-40f0-badc-1a5beb73ba39@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



On Jun 19, 1:46 am, "ian field" <dai....@xxxxxxxxxxxx> wrote:
"Dave.H" <the19...@xxxxxxxxxxxxxx> wrote in message

news:5a943c3d-af76-42d7-8745-2dfc65a2bc3c@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On Jun 19, 12:23 am, "Dave.H" <the19...@xxxxxxxxxxxxxx> wrote:
I have found a valve preamp kit in an online store, to go between
an
amplifier based on an LM380 chip, and a crystal or regen radio, but
it
uses a microphone where I want an audio input. If anyone can tell
me
how to connect an audio in one this or give me a schematic for a
low
voltage preamp using a single 12AU7 tube, I'd be grateful

Thanks
Dave

Oops, forgot the link to the preamp kit,
http://secure.oatleyelectronics.com//product_info.php?products_id=722

They do have (had) another similar preamp with an audio input, but
none are in stock now (discontinued)

Can you upload the regen schematic to a picture host and post the
link?

http://www.oldradios.co.nz/hikers/, it's the 1951 Improved Hiker's one
valve

Replace the headphones with a logarithmic pot - the kit instructions
should
indicate the resistance value to use, you need to make sure the pre-amp's
input capacitor's working voltage is rated to withstand the full HT on
the
regen, or you could add a coupling capacitor to the pot's wiper.

I just built a two transistor (BC549,BC558) preamp kit that works
well, will use that. I have a question regarding another regen I
have. The 45 volt HT is placed across the headphone terminals, how
would I connect an IC based amp to the radio, without frying the chip?

Put a resistor of the same resistance as specified for the phones in place
of the phones, then couple the plate/anode to the top of a volume pot via a
non-polar capacitor (0.1 - 0.47uF) rated to withstand the HT and take the
O/P to the IC from the wiper - the bottom of the pot is obviously taken to
ground.

You may find the IC input impedance is a little on the low side (often less
than 47k) resulting in not much volume, an easy quick fix is to add a simple
JFET source follower between the wiper and the IC amp chip - take the Vdd
from the IC supply rail.


.

Relevant Pages

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    ... amplifier based on an LM380 chip, and a crystal or regen radio, but it ... uses a microphone where I want an audio input. ...
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  • Valve preamp
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    (sci.electronics.basics)
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