Ampex 351 Preamp Modification Info!

[greg]

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.

[wow&butter]

thanks

[danmaksym]

Thanks Greg, you're the best!

[aurelialuz]

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

alex

[danmaksym]

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

[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?

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.

[danmaksym]

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

[greg]

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.

[danmaksym]

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

[greg]

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.

[Miko]

I've been away from the forum for a while, as I was building a new internet machine. But now that I have come back to this delightful thread I would like to say.

THREE CHEERS FOR GREG!

And thanks, too.

[danmaksym]

Sigh...More Modification Questions...


What type of bridge rectifier should we buy?

What is the miniature switch next to the green lighted one? Is the green light only an indicator? I found one from Newark that is a switch also...


Thanks.

Dan

[danmaksym]

Oh, and so that I'm not always a "taker," here's a tip:

When removing the old components from the boards, a Dremel moto-tool with a disc cutting attachment makes quick work of all of them. It should save you some major time, since you'll only have to melt the solder on the bottom of the boards and pull out the metal pins from the components (which are now straight because you cut them). Trying to melt, suck, and unbend metal socket pins is a major pain in the ass.

Hope this helps a little...

Dan

[greg]

I just noticed a typo-

The attenuator is not a Davin but a Daven. Sorry for all you guys trying search engines.

[wow&butter]

here is some more restoration info....if anyone is interested

http://ampex.hughescatalog.com/

jrod

[greg]

Sorry Dan. I just noticed this.

What type of bridge rectifier should we buy?

None. Keep the 6x4 tube where it is, and if you want to replace that selenium rectifier (radiator looking thing behind the output tranny) just use two 1N4001 diodes.

What is the miniature switch next to the green lighted one? Is the green light only an indicator? I found one from Newark that is a switch also...

This is the 600 Ohm output termination switch (see schematic). I usually leave it on all the time, but you can switch it in and out to see how it effects the sound.
The green switch does carry audio, and serves as the output attenuator engage switch.

Don't let me get away with saying the word "tranny" again.

[greg]

I tested a new logarithmic pot for replacing the gain pot. The pot sucked and the gain/rotation relationship was irregular. I'm leaning towards the switched gain option.

Do any of you have an opinion? Pot or Switches?
Keep in mind the output attenuator being like a fader.
I'm thinking for the switch having 12 to 16 positions. The first being "Off" then having gain stages of 5dB.

[danmaksym]

Okay Greg,

Having just asked a bunch of questions about the MS Matrix project I thought I should ask a few clarifying questions about this one.

First, the output attenuator switch schematic is confusing me. You say that you used a 6P2T switch. Do they even make those? Is that contained within the green lighted switch? And, above the "600 Ohm Term" switch symbol there seems to be something written next to the resister. I can't make it out. What does it say?

Finally, what kind of switch did you use for the -18dB Pad switch. I assume it's a rotary one. Which one was it? How many poles and stops?

Thanks Greg. Hopefully I'll find some way to repay you for all of your help someday...

[kevinink]

Hello,
I have been using and repairing 351/350's since the 80's.
A common problem on 351 circuit boards is moisture trapped in the boards under the traces, it causes sputtering similar to a leaky coupling cap. To remedy you will need to heat the traces with a hot iron and drag it along the complete length of the trace, do this to all of the traces on all of the boards.
If your 351 has lost all low end and sounds tinny, more than likely the output tranny is gone. Don't despair I (and many others) have replacements.
Refrain from turning the "rec cal" pot all the way up, it's just too much gain and noise.
I leave my units stock with the exception of changing the resistor values for the line input to mic pad values.
The original units make for the best tape echo bar none, a properly maintained 351/350 recorder is a excellent machine and I hate to see them permanently altered.
Kevin Ink

[greg]

Okay Greg,
...
First, the output attenuator switch schematic is confusing me. You say that you used a 6P2T switch. Do they even make those?
Is that contained within the green lighted switch? And, above the "600 Ohm Term" switch symbol there seems to be something written next to the resister. I can't make it out. What does it say?

Finally, what kind of switch did you use for the -18dB Pad switch. I assume it's a rotary one. Which one was it? How many poles and stops?

...

They do sell that switch. I believe it was bought from Newark Electronics.
The # next to the resistor is 604 meaning 604 Ohms.
The switch we use for the pad is a Grayhill rotary switch P/N 71BD30-02-2-AJN configured for DPDT operation. You can determine the numbers of stops.
Sorry it took so long to answer this.

Thanks for the info Kevin. I didn't know that about the traces. We haven't had a bass response issue yet. Maybe we inadvertently did our own heat treatment when we cleaned the boards. I don't like the idea of permanently altering the units either, but these particular 351s were on their way to the landfill when we picked them up. Steve caught a bunch of hell from people on the Ampex list when he talked about it.