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.
Between bands, and computer/scanner problems (which Russ cleared up for me) I have been putting this off. Sorry guys.

This is the letter I would fax to people when they wanted an overview of what we did. It includes a new hand drawn schematic that you can refer to.

http://greg.electrical.com/351.pdf

It is Acrobat, so enlarge and print as you wish. I recommend saving it to you computer for speed.
Here is a list of corrections to the schematic-
R55- is 6.8k instead of 1.5k
R58- is a 20k instead of 15k
R49- does exist and it should be replaced with a 39k resistor (this resistor is in between pin 3 of V4 and Red/Yel connection on the middle board.
No tubes use the 12.6V AC for their filaments. They only use the 12.6 V DC.
The headphone jack is isolated from ground/chassis. Tip and sleeve each have a .001 uF cap to ground
The attenuator is not a Davin but a Daven.

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. 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 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. Otherwise just get a nice Clarostat pot for the main gain.
I have been replacing the selenium rectifier (for the 12 DC power supply) with a bridge rectifier, because they have caused problems in the past.
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 of the board when stripping them. Removing the Tube sockets will be the worst part. 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 the plugs reversed. I recomend getting a solder sucker ($10 at radio shack), solder braid, and screwdriver shaped tip for your iron. I like to have the heat high (if you have a soldering iron with that kind of control) and desolder fast. Practice on a part of the board that isn't going to be reused.
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

thanks

Thanks Greg, you're the best!

thanks a lot greg, really cool of you to do.

alex

Greg,

Any suggestions on how to implement the 48v phantom power? Also, why did you construct a new backplate? Isn't the original one sufficient?

Thanks

Dan

Any suggestions on how to implement the 48v phantom power? Also, why did you construct a new backplate? Isn't the original one sufficient?

We made a remote phantom power psu that powers several 351s at once. There might be a way for you to derive 48v from the given voltages. Since your not using as many tubes, the B+ voltage might have enough current left to build a 48v regulator. I haven't tried that. You need 48-52 volts with 10-20 mA of current available.
We built a pannel to keep it tidy and have the correct placement of connectors (IEC power conn. and fuse neer power supply, and audio near the input circuit). It was also less work than retrofitting new components on the old plate.

Greg,

In the process of planning, I thought of a few questions:

1. Where did you find a piece of metal to fabricate the backplate out of? Is it an item someone might find in one of the parts catalogs?

2. How do you feel about Solen "Fast Caps" in positions requiring less capacitance? Any experience with them? If not, what do you recommend?

3. Do you happen to have any photos of the inside of your completed 351's?

4. Did you keep the original tubes that came with the 351 or did you replace them with something else?

5. What is the layout of the XLRs on the back panel? Maybe I'm interpreting the drawing wrong but are there 3 jacks?

6. As far as metal-film resisters go, what was your brand preference?

Thanks Greg.

Dan

1. Where did you find a piece of metal to fabricate the backplate out of? Is it an item someone might find in one of the parts catalogs?

We had a machinist friend make them out of aluminum sheet metal. I think it is 1/8" thick.

2. How do you feel about Solen "Fast Caps" in positions requiring less capacitance? Any experience with them? If not, what do you recommend?

I haven't used them. Russ has, says they are fine. I don't have strong feelings about boutique components. If they are reliable, that's all that matters.

3. Andrew took one the other day. I'll post it soon.

4. We kept the same type of tube but used new ones.

5. What is the layout of the XLRs on the back panel? Maybe I'm interpreting the drawing wrong but are there 3 jacks?

You interpreted right- 1 jack for female 3 pin XLR (input), 1 jack is a 3 pin male XLR (output), and 1 jack a 4 pin male XLR (phantom power feed)

6. As far as metal-film resisters go, what was your brand preference?

We used nothing fancy. Out of the Newark catalog, Multicomp. Higher operation temps better. We bought a set of Dale resistors that didn't have the right ratings before I started. I don't know what was wrong with them. They were precision ones that may not have been rated for the high volts.

Thanks Greg.

I should've remembered what you told me when I was there: "I just use whatever has the lowest noise." Makes sense...

As far as the back panel goes, I totally forgot about that phantom power input. Sorry.

And, when you say you used new tubes, do you mean you found NOS G.E. (the original brand in mine) tubes or used something like Sovteks? I didn't mean to ask if you used new tubes in the sense of whether or not you stuck with 12AX7s or something- just wanted to know if you trusted the original tubes or felt better using new components.

Thanks alot Greg... How's your studio coming along? Web site up yet?

Dan

danmaksym wrote:And, when you say you used new tubes, do you mean you found NOS G.E. (the original brand in mine) tubes or used something like Sovteks? I didn't mean to ask if you used new tubes in the sense of whether or not you stuck with 12AX7s or something- just wanted to know if you trusted the original tubes or felt better using new components.

Thanks alot Greg... How's your studio coming along? Web site up yet?

Dan

We use new and old, mostly new, whatever doesn't make noise and conforms to the original schematic. I think the 6189Ws are from a batch of NOS Philips tube we bought a while ago.

My studio is going great. I just got to pick a few lights to put in but that's about it. No website. I don't know if there is going to be one.