[NJARC] Help with AMP

[Philip Harris]

My name is Phil.  I'm having some trouble finding information on the 6V4
tube.  The RCA tube manuals I have for 1966 and for 1959 do not list it.  I
doubt this tube came out after 1966.  I don't know who has info on this
type.  In later tube amps than yours, Bogen chose to go with solid state
rectifiers, I believe.  This would be a good choice for you, permanently or
until you can find the correct tube if you want it authentic.  1N4007
rectifier diodes would be fine in this application and cost just pennies
each..  (1000 PIV @ 1 Amp)

You will need two and they can simply be solder tacked in place on the
bottom of the 6V4 socket.  The anode of each diode goes to the plates of 6V4
and the cathodes would be connected together and soldered to the tube
cathode if it's a indirectly heated type of rectifier.  (You may want to
disconnect the filament power to the 6V4 socket if it is a directly heated
type of rectifier tube.)  This is all assuming it is a full wave rectifier
circuit.

If you just replaced the electrolytics  it is a good idea to put in a
current limiting resistor to match the characteristics of the voltage drop
in the 6V4 tube.  Not knowing what this is, I would start with 150 ohms 5
watt wirewound.  Measure the high B+ voltage (B+ feed to output transformer)
after the tubes fully warm up and see what it is.  Compare this with what
you think it should be (from the schematic).  Adjust the resistor
accordingly.  The voltage drop of silicon diodes is very low compared with
most vacuum tube rectifiers.  This will protect your new electrolytics from
high surge current when turning on amp also.  If this is a real concern, a
thermistor can be applied to the AC input to the transformer.  This is real
good at protecting the filament surge current for the whole amp, as well as
the B+ circuit.

A 47 lamp is fine placed across the filament string of the amp.  If you use
diodes in the power supply you will be loading the transformer filament
winding less anyway.  A 47 lamp draws very little current, I believe about
50 MA or so.

Microphone audio levels are down in the -60 to -70 dB voltage level (in the
microvolt range).  You will not be able to get away with any unshielded
wiring of any kind at these low levels.  The gain of the first pre-amp
stages are just too high.  A 1/4 inch phone jack is fine for use with a
guitar cord and can be paralleled with the microphone input.  (Don't try
using a microphone at the same time as a guitar).  A grounding jack is a
good idea if you can find one, to eliminate hum when nothing is plugged in.

You may find the equalization of the microphone input will not sound correct
with a guitar.  Thus, the tone controls will not be able to give you enough
range to work with to get the desired sound.  This is a short coming of the
microphone pre-amp circuit which is looking for a certain load impedance
(probably a dynamic microphone).  If the Bogen was designed to work with a
crystal microphone, you will be very unhappy with the sound.  Crystal
microphones had a very high impedance output with higher voltage.  You will
notice a distinct lack of gain in the amplifier if this is the case.  You
will need a separate pre-amp for your guitar pick-up.

Good luck on your project.


----- Original Message -----
From: <RADIOTUBE@aol.com>
To: <njarc@mailman.qth.net>
Cc: <RADIOTUBE@aol.com>
Sent: Sunday, June 16, 2002 9:16 PM
Subject: [NJARC] Help with AMP


> I have a Bogen Challenger Pa amp, I want to add a 1/4 inch MONO Phone Jack
to
> the front panel allowing me to connect a guitar to it. Should I use an
open
> or closed circuit connector? can I bridge the wires from the phone jack to
> the terminal strip connecting the MIC input, or is it better to disconnect
> the MIC wires? Should I use shielded wire connecting the Phone jack, or is
20
> gage stranded wire from Radio Shack?  Is it OK to replace the 6V4 full
> rectifier tube with 6CA4 full wave rectifier? The substitute book shows
that
> you can replace it but the filaments must be parallel. The way the tubes
are
> wired are 6V4 pin 5 "H" goes to pin 5 "H" of the 7868 pin 5 "H" of the
7868
> goes to pin 5 "H" of the 12AX7. Pin 4 "H" of the 6V4 goes to pin 4 "H"of
the
> 7868 pin 4 "H" of the 7868 goes to pin 9 "Hc" of the 12AX7. "H" being the
> heater pin. Is this a series circuit?  can I add a 47 6.3 bulb for a power
on
> panel indicator by connecting it to pin 4 and 5 of the 6V4 to tube or will
> this cause a problem and just connect a NE-51 glow lamp to the A/C switch.
> I picked the bogen up for a $1.00 at the Kutztown radio meet auction with
no
> tubes, and replaced the electrolytics and any out of tolerance resistors,
> cleaned it up and seems to work nice. By the way anyone who did not attend
> the Kutztowmn meet missed a real good event. Good thing I brought the
station
> wagon!
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