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Ampex 351 Preamp Modification Info!

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Postby danmaksym on Fri Apr 09, 2004 12:25 am

Thanks for the info Greg. That's a big help.

On a side note, since you'd mentioned Newark, I have to voice my strong approval of that company. They have nearly everything you could ever need, their web site and catalog are great, and they have your stuff to you in 2 days. I really couldn't imagine a much better component supplier. I definitely recommend them to my fellow DIYers.

My experience with Digi-Key has been less satisfactory. They took a relatively long time to deliver my stuff and, when it did arrive, the packaging wasn't nearly as nice as Newark's. For instance, Digi-Key simply throws the components you ordered into clear plastic bags with few, if any, identifying labels. On the other hand, Newark lovingly places each different type of component in sturdy little boxes which are explicitly labeled. That goes for even the smallest number of components. Say you were to order one resister: You would get one individual box for it. Now that's what I call attention to detail, concern for product safety, and caring for the customer.

So, my recommendation is to buy all of your components from Newark and only use another company if Newark doesn't carry a certain item.

Good luck everybody...
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Postby greg on Fri Apr 09, 2004 1:29 pm

I agree they are definitely pro with their business. They are a little more expensive than say Allied or Mouser but I think that care is what you are paying for. When they had a will call in the ghetto we would order from there more often. It was convenient to have a same day, in town supplier.
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Postby greg on Tue Apr 20, 2004 6:00 pm

HEY!
There are corrections I need to post for the schematic. Eventually I'll draw a new schematic but these are the ones I've found.
R55- is 6.8k instead of 1.5k
R58- is a 20k instead of 15k
No tubes use the 12.6V AC for their filaments. They only use the 12.6 V DC.

Sorry for the problems this may have caused. I'll try to keep updating as I learn more.
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Postby greg on Mon Apr 26, 2004 3:49 pm

Here are some photos of the inside of our modified Ampex 351s
http://www.electricalaudio.com/~greg/351-photos/
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Postby tarandfeathers on Mon Apr 26, 2004 5:51 pm

greg wrote: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.


I built some preamps recently with 12 way switches in. I like them a lot. I'd recommend it, definitely.

Just how 'coloured' are these pre's? The ones I built myself are super clean, so my next order of business is probably something with a bit more character.
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Postby danmaksym on Mon Jun 21, 2004 10:22 pm

Hey Everyone,

I've finished the 351 mod and am trying to get it to work. The problem is, the rectifier tube is glowing much brighter than it should and I'm getting highly elevated B+ voltage readings. For example, I'm getting about 370V going into R6. Does anybody know what could be wrong? I've checked the values of the resisters and they're all correct. Capacitors seem fine as well.

Thanks.

Dan
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Re: Ampex 351 Preamp Modification Info!

Postby Chris G on Mon Aug 23, 2004 2:24 pm

greg wrote: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 exhist 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 attenuator is not a Davin but a Daven.



Also: The "gentile rise" in frequency response mentioned in the penultimate paragraph of Greg's introductory note should probably be a "gentle rise."

Unless he's making some sort of joke about Christian erections.
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Postby lachiehawk on Mon Sep 06, 2004 11:32 pm

Hey there,
How easily would it be to convert these to Australian 240v power?
Im considering modding 2 of these...
thanks
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Postby Rspaceman on Wed Sep 08, 2004 1:10 pm

Hi Greg,
I have looked over your plans for mod-ing Ampex Tape machine pre's to use as mic pres, I would actually like to use a couple without modification if possible and I have a couple of questions.

What cable is it that gives the top preamp power since the power cable only hits the bottom preamp?

Will I be able to use the mic/line knob to tell the preamp to expect a mic instead of a mixer signal or do I need to make a modification there?

I work at a UC here in California and we just found a great looking Ampex 351 and they are letting me use the Pre's. The shop tech said he'd help me to slowly bring it up to power and test all the tubes for me. I'm hoping to rack them and use them in my project studio.

Thanks for your help if you have a chance.
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Difference between 350 and 351

Postby Idoia on Thu Jan 06, 2005 6:59 am

Sorry if this has been asked already but I am a bit confused.
In the list of equipment you said to have a bunch of Ampex 351 but when reading the description you said that you bought a bunch of 350.
What are they 350 or 351 ?
Thanks
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Postby greg on Sat Jan 15, 2005 11:01 pm

sorry,
they are all 351s (with the black dot)
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Postby Idoia on Mon Jan 24, 2005 2:32 pm

Greg,

Thanks for sharing this on the forum.
I have another general question, what did push you to choose the 351 for modifications ? why haven't you chosen the 350 ? Many people devalue the 351 in favor of the 350 for some reasons. In theory what you have done on the 351 could be done on the 350, could not it ? what is your views on that, would you approve the decision on starting the same kind of work on a 350 ?
Thanks a lot.

Alex
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Postby Idoia on Sun Mar 13, 2005 7:13 pm

Hey greg,

Me again with another question, I am in the work of modifying on 350 with your schematics (thanks for this):

On the schematics, you kept the record calibrator, may I ask what is this for ? (it might be a stupid question but...). And since the pot must be kept, how do you have it set ?

Okay thanks for this.
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Postby danmaksym on Sun Mar 13, 2005 9:05 pm

There are three reasons that might've prompted Greg to stick with the 351's. First, the 350's require a dedicated power supply. The 351's have one built-in. The second reason might be that Greg found it easier to do a "mass modification" with preamps that employed circuit boards rather than the 350's point-to-point wiring. Third, it might be simply that the studio came across a large number of 351's at some point available from the same seller at a decent price.

The record calibration pot is there to provide an internal set gain function. I believe Greg has his set to provide up to 65db of overall gain (see his Tech Journal for more info).

Hope this helps a little...
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Postby greg on Wed Mar 30, 2005 8:01 pm

I don't have any experience modifying the 350s. I would assume you could do it, but I don't know how different the job would be. I wouldn't use my schematics alone. Try to get the original 350, and 351 schematics so you can compare and contrast.
http://www.recordist.com
There are no stupid questions, just stupid people.
Pretty much what Dan said is true. That record cal is there to control the full gain of the preamp (which we set to 65 dB). I suppose I could get rid of it now, but we used it to tinker with in the past. You can cause early clipping if that is turned up.
I wasn't part of the decision process that led to the 351. That was done before I arrived. I think Steve and Bob were using an unmodded 351 as a preamp and liked it. We then bought a batch of them for really cheap.

What do people say is worse about the 351?
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Postby Idoia on Wed Jan 18, 2006 7:56 am

Hi,

I have difficulties to find the RELCAP 0.02uF/600V polypropylene (C 18 19 & 21).
Actually I can't find any. I found in TEFLON but it is very expansive.
Can you sugest me a dealer or a replacement ?

Also is C1 450V or 600V ?

Thanks a lot.
Alex
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Postby greg on Wed Jan 18, 2006 11:48 am

Idoia wrote:Hi,

I have difficulties to find the RELCAP 0.02uF/600V polypropylene (C 18 19 & 21).
Actually I can't find any. I found in TEFLON but it is very expansive.
Can you sugest me a dealer or a replacement ?

Also is C1 450V or 600V ?

Thanks a lot.
Alex


They may not be available anymore but you can get .022uF as well. Let google be your friend. Quick finds-
http://www.hndme.com/storeindex.html

https://secure.tubesandmore.com/

C1 is 400/450V
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Postby Idoia on Wed Jan 18, 2006 4:33 pm

Thanks for the fast reply.
While I am at it,
- right above the meter the resistor is 3K9, is it a typo error and you meant 39K ?
- on the right of this there is the headphone Jack which has a capacitor in between its connectors, below is written 0.01 assuming this is uF what voltage is that cap and is it a polystyrene ?

Thanks for your time Greg..
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Postby danmaksym on Wed Jan 18, 2006 8:10 pm

The meter resistor is 3.9K

The headphone cap can be anything you want. 50V should be fine. Polypropylene is probably the best choice as it will be cheaper than a polystyrene.

Good luck with it.

For those who are interested in having an Electrical-style 351 without the hard work, my company is about 2 weeks away from releasing a modern copy of it with all the bells and whistles. Check out http://www.AnthropicAudio.com in a few days for info.
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Postby Idoia on Thu Jan 19, 2006 4:31 pm

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 ?

Also, I cannot find the electrolytic 4000uF 20V. You said that we should round up to the next standard value, so actually there is 4700uF but there is also 3900uF, should I go for the 4700uF anyway ?

Thanks again for your input, sorry to be such a pain.
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