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Ampex 351 Preamp Modification Info!

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Postby greg on Thu Jan 19, 2006 6:43 pm

Idoia wrote:Me again,
I am sorry but I cannot find ANY Rel-Cap polypropylene 0.15uF 450V and 0.02uF or 0.022uF 600V. I had checked the links you gave me already Greg and they don' t do them. Can I replace them with Mallory, AudiCap or Multicap with the same values ?


Yes. You can also try the single wind polystyrene and foil caps they sell at those values. It is just one of those audio-file choices that people geek out on, but in all likelihood, wont be pivotal.

Also the electrolytic ones, I can find some Sprague-Atom or Vishay-Sprague with the needed values but none operating above 85 degrees. Will that be okay if I use them or should I priviledge 105 degree caps to them regardless of the brand ? I have found some Nichicon Snap-In Aluminum Electrolytic Capacitors, will they do ?
Alex


You should try to get 105 degree caps if you can. The 85 degree ones will work, but not as long.
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Postby Idoia on Fri Jan 20, 2006 5:24 am

Hey Greg,
I found some panasonic EB and TS-ED 105degree for those value and I tried to edit my post but you replied at the same time :).

The last one, I promise: You said that we should round up to the next standard value, I need a 4000uF 20V cap and there is 4700uF 20V but there is also 3900uF, should I go for the 4700uF anyway ?

Thanks
Alex
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Postby greg on Fri Jan 20, 2006 10:56 am

Idoia wrote:Hey Greg,
The last one, I promise: You said that we should round up to the next standard value, I need a 4000uF 20V cap and there is 4700uF 20V but there is also 3900uF, should I go for the 4700uF anyway ?


Yes.
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Postby Idoia on Sun Jan 29, 2006 5:29 am

Hi Greg,
I am doing your modif for a preamp that I will mainly use in Europe. I have to change the input voltage of the device.
I have two solutions:
Either I change the transformer and I found one that can output the voltages needed inside the preamp (which I am not sure about).
Or I can insert a step down transformer that goes from 220V to 110V just before the main one. That seems to be the easiest.
What I need to know is how many Watt should the step down transformer provide. I have tried to make it work with a travel adapter that ouput only 45W but it does not seem to be enough.
Thanks
Alex
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ampex 351 mod

Postby jholston on Thu Jun 08, 2006 3:23 pm

I just got 4 ampex 351 electronics and I want to mod them for mic pre's. I tried to d-load the schematic but apparantly it's not there anymore. Anyone got a copy I can snag? Thanks in advance.
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Postby greg on Thu Jun 08, 2006 6:44 pm

I updated the schematic link. Note the changes on the first post.
Good luck!
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Postby greg on Thu Jun 08, 2006 7:03 pm

Idoia wrote:Hi Greg,
I am doing your modif for a preamp that I will mainly use in Europe. I have to change the input voltage of the device.
I have two solutions:
Either I change the transformer and I found one that can output the voltages needed inside the preamp (which I am not sure about).
Or I can insert a step down transformer that goes from 220V to 110V just before the main one. That seems to be the easiest.
What I need to know is how many Watt should the step down transformer provide. I have tried to make it work with a travel adapter that ouput only 45W but it does not seem to be enough.
Thanks
Alex

Don't change the transformer out if you are not sure of what you are doing! Try to get someone to help you with that.
I am not certain on the exact current consumption of the 351s. A 45W commercial stepdown transformer should be big enough though. See if it gets warm to the touch.
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Postby Idoia on Sun Jun 11, 2006 3:21 am

Hi greg,
I have put another transformer before the main one so that the current is transform into 110 before. The only thing is that the transformer should at least provide 100W unless the ampex won't power up.
I have done two of this units and have for both of them the same problem:
Looking at your schematics, on the amplification board, the middle one, I got all voltages right.
On the record one however, I get 13.5V between R25 and R28 and 15.4V between 4 and 5 of V1.
I triple checked all resistors and cap values and got them matching your schematics. Is there a slightly chance there are still an error on the schematcis ? For ex on the photos you gave us, I can see that you kept the original R60 which is on the original schematics 3ohms. On the modification schematics you put 1.5 ohms. I might be wrong but at this level this makes some difference, no ?
Has anyone here tried to do this modification appart from myself ?
I'd really like some help here.
thanks.
Alex
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Postby Idoia on Sun Jun 11, 2006 3:35 am

Maybe one question that could really help me, on your schematics, both 9 pin of V5 and V4 are connected together with the GN/Y of the main transformer. Am I reading well ?

Least but not last, on the same part of the board, pin 5 of V5 is connected to pin 4 of V4 but on the original board, both pin 5 of V5 and V4 are connected to eachother, which one is it ?

I know you have done this long time ago but unless you match this against one of your device I have little chance to find the correct way to connect those tubes.

Thanks a lot Greg.
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Postby greg on Fri Aug 11, 2006 1:12 pm

rambi10 wrote:Hi Greg

I got twelve AMPEX 351 preamp and i want to transform it into a mic preamp.
I saw on the electrical audio forum that you already did this kind of thing.
I would like to know if you have a complete wiring scheme because it's too difficult to wire only from the forum's pictures, I can't get all the wiring's details.
I already cleaned everything and took out the tube sockets and caps like
C1,C2 and C10 on the first PCB(layout).
I will change all the components you suggested on the forum like the resistors, caps and tube sockets.
In fact, the hardest for me is the wiring between the transformers,the switches, the pots and the IN and OUT XLR's.
Do you have any tips to help me out ?

Thanks in advance

Aurelien RAMBACH
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49 boulevard du lycée
PARIS
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If you want, call me at 0155959820 or m

I moved you to the right topic.
I don't have a wiring diagram other than the schematic, which you can see the updates on this thread. There are some drawings that the last tech made for wiring in some of the peripheral components, but they are irrelevant and would be confusing to you now.
What is troubling you about wiring the in/out and the transformers?
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Postby Idoia on Fri Aug 11, 2006 1:25 pm

Hi Greg,
Did you have a chance to look at my post right above rambi10's one.
I still have not touched them since I posted this thread and would like to know if my questions make sense.
Thanks
Alex
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Postby greg on Sun Aug 13, 2006 12:08 am

Idoia wrote:Hi Greg,
Did you have a chance to look at my post right above rambi10's one.
I still have not touched them since I posted this thread and would like to know if my questions make sense.
Thanks
Alex

Sorry Alex. I have been in the recording part of the studio all summer (that is why I haven't posted much on the forum). Your questions are very specific and I would need a preamp, the schematic and a bunch of time to answer some of them. I'll try when I can to get some info off to ya, but I don't know when that will be.
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Postby greg on Sun Aug 13, 2006 12:22 am

Idoia wrote:Maybe one question that could really help me, on your schematics, both 9 pin of V5 and V4 are connected together with the GN/Y of the main transformer. Am I reading well ?

Least but not last, on the same part of the board, pin 5 of V5 is connected to pin 4 of V4 but on the original board, both pin 5 of V5 and V4 are connected to eachother, which one is it ?
Alex

The schematic shows pin 9 connected to gn/y. This does not need to happen. I should draw a new schematic but probably wont.
The pin 5 of V5 to pin 4 of V4 .... is from the old Ampex schematic. I don't know why they wrote it that way but it doesn't matter. As long as there is roughly 12 volts AC between pins 4 and 5 on both tubes, they will work.
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Postby greg on Sun Aug 13, 2006 12:32 am

Idoia wrote:Hi greg,
I have put another transformer before the main one so that the current is transform into 110 before. The only thing is that the transformer should at least provide 100W unless the ampex won't power up.
I have done two of this units and have for both of them the same problem:
Looking at your schematics, on the amplification board, the middle one, I got all voltages right.
On the record one however, I get 13.5V between R25 and R28 and 15.4V between 4 and 5 of V1.

I don't understand, R 25 and 28 are on the repro/middle board. 13.5 V is only a little high. Was that with tubes in and being powered? With the tubes in, the voltage will drop a bit.
The voltage across V1 will also drop in value if a tube is installed.
I triple checked all resistors and cap values and got them matching your schematics. Is there a slightly chance there are still an error on the schematcis ? For ex on the photos you gave us, I can see that you kept the original R60 which is on the original schematics 3ohms. On the modification schematics you put 1.5 ohms. I might be wrong but at this level this makes some difference, no ?
thanks.
Alex

I don't know of any more faults in the schematic. R60 should be 1.5 ohms, 1W though.
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Postby rambi10 on Thu Sep 14, 2006 7:10 am

Hi greg

I have got few questions about the wiring of the output scheme.
In fact, what is the output attenuator with the three pots just before the XLR output?
I don't understand the principal of this attenuator pot, I just think that it's for the VU METER position and to adjust the output level on the VU meter. Is it possible to wire without this pot? that's to say yellow and green wires directly on the lugs near output transformer and after on XLR output, VU meter and headphones jack?
If I do that, can I always adjust the VU meter?

After, who did you find the DAVEN output attenuator GP2T because on the NEWARK site, there isn't this kind of pots.

To finish, there is two GN wires for V4 and V5 filaments, I must wire on GY and V on the repro board?
But GN/Y where is it wired?
R 25 and R28 are wire on GY and V tracks on the repro board?

Thanks for all greg, there is lots of questions but I don't want to do a big error and your tips are very welcome for us.

THANKS

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Postby danmaksym on Thu Sep 14, 2006 8:45 am

Hi

The output attenuator does just that- lowers the level of the entire output. This is not simply for the VU meter (though the readings on the VU meter will be affected by the output attenuator since the VU meter monitors the output of the 351).

The Daven attenuator is no longer available. You'll have to look in vintage parts stores or eBay to find one. It'll be expensive also- probably $70 or more. A company called STATE ELECTRONICS will make reproductions of the Davens, but only in very large quantities at a very high price ($100 or more).

You can build the 351 without the output attenuator, but then you won't have the option of overdriving it- unless you have some sort of pad on the device following it.

As far as your other questions go: Since I don't have the schematic near me at this moment, I would say just look at the schematic and compare it to the wiring and traces on the 351 circuit boards. This is a complicated project (as far as wiring goes) and it takes time to logically figure out where everything goes. Just go slow and you'll discover the solution eventually.

Good luck.
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Postby greg on Thu Sep 14, 2006 6:29 pm

rambi10 wrote:Hi greg

Hi
I have got few questions about the wiring of the output scheme.
In fact, what is the output attenuator with the three pots just before the XLR output?
I don't understand the principal of this attenuator pot, I just think that it's for the VU METER position and to adjust the output level on the VU meter. Is it possible to wire without this pot? that's to say yellow and green wires directly on the lugs near output transformer and after on XLR output, VU meter and headphones jack?
If I do that, can I always adjust the VU meter?

You can simply connect the o/p trans to the XLR, and you will have a normal preamp. The output attenuator is just there to trim down the output when you are driving the tubes harder with the i/p gain.

After, who did you find the DAVEN output attenuator GP2T because on the NEWARK site, there isn't this kind of pots.


Some hifi resale place. They were bought before I worked here.
To finish, there is two GN wires for V4 and V5 filaments, I must wire on GY and V on the repro board?
But GN/Y where is it wired?
R 25 and R28 are wire on GY and V tracks on the repro board?


Sorry for the confusion. The 12VAC is only used for the meter lights (one G + GN/Y = 6 VAC for lamps). V5, V4, and R25, 28 are in parallel with V1 (12VDC supply). However you do that. The color coding gets confused when you start rewiring so our 'green' may be something else for you. The color code of the transformers are true. Try to keep the color to board connection true if you can, otherwise make note of your changes on the boards. We would install jumpers in some places to keep it consistent.
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Postby punk on Sun Sep 17, 2006 9:53 am

hey greg,

I have seen these kind of stepped attenuators on ebay and am wondering if they would they work as output attenuators for the 351?

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... 0032487174
Bah.
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Postby danmaksym on Mon Sep 18, 2006 12:05 am

punk wrote:
I have seen these kind of stepped attenuators on ebay and am wondering if they would they work as output attenuators for the 351?

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... 0032487174


No. Those won't work. Those are meant to replace normal potentiometers. What you need is a balanced "T" pad attenuator- not a "ladder" type.
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ampex 351 mod power supply

Postby bigfatchampion on Sat Feb 03, 2007 7:47 pm

hey greg,

i wanted to know if you have any schematics for the power supplies you built for EA 351's? i'm just getting started on the mod and i'm trying to gather as much info before i start.

thanks
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