
Alright,
For those of you who have been waiting a few months, you past the first test. Patience. You're going to need it to complete this mod.
Updated Electrical Audio Mod Schematic for August, 2009
Schematic Updates:
*R60 (series resistor after filament supply rectifier), can be increased in value to compensate for the lower voltage loss resulting from replacing the old selenium rectifier with 1N400X diodes. I've had to increase it to 3Ω on several to keep the filament voltage at or below the 12.5V.
It's good to have the polarity reverse switch located on the output (between the attenuator and XLR connector) for low noise switching. I like to use a sealed relay controlled by the toggle switch.
These are interior photos of one from our studio 351s
I should answer-
I know you're really busy, but could you also include a sheet listing the components needed for the modification? It would be a great help to all of us who have a 350 and want to gather the parts... Thanks
I don't have such a list, but it is simple. Copy down all the component values (on the schematic) and order new parts. For all the electrolytic caps use 105 degree rated caps for replacement (Sprague, Panasonic TS series are good). Don't change the uF value, round higher on the voltage rating. For the resistors, get 1% 1/2 watt metal film ones (with appropriate peak voltage specs). We used Rel-Cap polypropylene film caps for the interstage DC blocking caps (C1,18,21,19 etc.) C20 is a polystyrene film cap, the .47uF cap bypassing with the 10uF cap on the B+ is polypropylene, and C4 is an orange drop cap.
The Daven o/p attenuator is not an off-the-shelf item anymore. What you need to get is a o/p attenuator that will have an input and output impedance of 600 Ohms through its rotation. You might be able to find this at a hi fi audio builder site.
We are in the process of replacing the gain pot with a 12 or 16 position switch for better tracking of gain per degrees of turn.
I have been replacing the selenium rectifier (for the 12 DC power supply) with a bridge rectifier or 1N4004 diodes.
We added jumpers when needed to bring signals and voltage to specific spots on the boards. If you get a cap that needs a bigger pad/hole to attach to don't be afraid of hacking a bit. If you are committing to this, be only concerned with making it a preamp. It most likely will not be converted back to being a part of a tape machine.
Take care and not burn the traces off the board when cleaning them. Removing the Tube sockets will be the worst part (not totally necessary if clean and socket pins have good tension). Keep good track of the wires, where they plug in to the board, and what direction the plugs are facing. I shocked Soren (old tech) once when I had some of the plugs reversed. I recomend getting a good de-soldering gun.
You're going to constantly have to retrace you're steps making sure things are where they should be. It is a simple cicuit to follow, but a mess in the box.
Some places for parts include:
Digikey (caps, pots, switches, resitors)
Newark (for the crazy green lit o/p attenuator switch)
Allied Electronics (has the same as everyone else but cheaper)
Mouser (same)
New Sensor (tube related goodies)
Angela Instruments
Velleman (hi fi kit place)
Antique Audio (hi fi)
CE Distribution (tube amp catolog)
I'll add to this post as I get more info.
ta ta
2008 and beyond:
Picture of the simple phantom power board (includes the full-wave rectifier for 12vdc filament supply, replacing the selenium). The phantom power circuit is a voltage quadrupler tapped off the 12vac winding (Green) clamped down with a 47 volt zener diode.

State Electronics L24338 is a suitable replacement for the output attenuator.