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Ampex 351 Preamp Modification Info!

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Re: Ampex 351 Preamp Modification Info!

Postby greg on Thu Dec 18, 2008 1:26 am

recordinggeek wrote:I also did some noise measurements. With a 150 ohm terminator at the mic input, at full gain the output noise is about -37 dBu, which gives an equivalent "referred to input" noise level of just below -126 dBu... pretty good actually, considering the crazy high gain here. Switching in the input attenuator actually RAISED the noise just a hair, and cranking the record level to minimum (24 dB attenuation) gave a reading of about -41. I scope and listen to the output as well as metering it, and it sounds like the hiss actually drops more than that. RFI may be a factor here, as I live only a couple of miles from an AM transmitter site (which I worked at some years ago). When I try to look at really low level stuff, I can often see modulated RF on the scope.

One other thing I notice is a tendency toward microphonics. Tapping on any tube or either amp card "rings" a bit. A rap on the front panel will do this also (but not as much). Again, I am not certain whether I am seeing a real problem, or if it is just a case of "well yeah, crank up 90 dB of gain and you WILL get stuff like that".

I would still like to know whether my results are in line with what should be expected when you feed the second stage directly to the input of the line amp.

We haven't had any problems with RF, or microphonic tubes (other than the occasional microphonic tube).
I've never bypassed the "cal" pot. I can't tell if what you're seeing is normal. It is possible that the C-R filter that it creates somehow stabilizes the signal at the input of the inverter.
So far, I am considering 3 alternatives to drop the maximum gain to reasonable levels:
1) Bypass the second gain stage, feeding the output of the record gain pot directly to the line amp.
2) Change the plate load and cathode bias resistors of the first two stages to reduce the gain of each stage.
3) Use a lower gain tube (with changes to resistor values if needed) to reduce gain of the first two stages.

Any one of these options, though, might materially affect the sound of the preamp, which might reduce its value (I will eventually sell this thing, as I have a hard time with the idea of using up 4U of rack space for a single channel mic preamp, no matter how "cool" it is).


See if you get good results with the unit calibrated to 65 dB max gain (Record level all the way up, loading the output with 600 ohms). It has worked well for us for 12 years now. Maybe that extra stage is the magic sound.
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Re: Ampex 351 Preamp Modification Info!

Postby recordinggeek on Thu Dec 18, 2008 7:44 pm

greg wrote:I don't think you're missing anything. After playing with more gain, we settled on 65. It is rare that we ever need more than that from any preamp. We kept that second stage because it is part of the circuit (I'm guessing. That decision was made when I was a lowly intern here). I suppose if we had problems with noise, we would have looked at it closer.
You should try bypassing that second stage and see what you get.
I disconnected the output of the second stage and jumpered across from the record level wiper to the line amp input (2nd stage grid still connected). Hooked up that way, I measure a maximum gain of 67 dB. Kicking in the input pad drops it to 42.5 dB, and cranking the record gain to minimum drops the total gain to 19 dB.

With the pad in, the input stage clips at just above 1.5 volts input. With the record gain cranked all the way down, the input stage clips before the line amp does. I can crank the output pad down by about 12 dB, which puts the line amp clipping level very close to the 0dBfs point of many A/D converters.

All in all, leaving out the second stage gets me a pretty reasonable gain range with a nicely balanced gain structure. As for noise measurements, the meter readings are now swamped by RFI from the local AM radio station (50 gallon rig about 2 miles away).

The downside here is that my perverse sense of economy is offended by leaving half of a tube unused. After having gone to the trouble of building a FET DI input buffer, I am now seriously tempted to find a way to re-task the "spare" triode stage as a boot-strapped cathode follower (instead of the FET buffer). Now I'm off to do some more research... :smt002
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Re: Ampex 351 Preamp Modification Info!

Postby Dan Zellman on Fri Jan 16, 2009 8:19 pm

Hi Greg, Nice job. The Daven line was taken over by Shallco switch. All of Daven's products are available. http://www.shallco.com/attenuators.html
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Mon Jan 19, 2009 3:23 am

Dan Zellman wrote:Hi Greg, Nice job. The Daven line was taken over by Shallco switch. All of Daven's products are available. http://www.shallco.com/attenuators.html

Hey Dan,
Our attenuators were causing some problems, so I opted for a custom replacement. That is great to know. Are you still working in that awesome building in Queens?
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Wed Aug 12, 2009 1:03 pm

Again, with the Updated Schematic
One day I will get things right on the first try. One day...
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Re: Ampex 351 Preamp Modification Info!

Postby lookn4tone on Sat Nov 14, 2009 11:41 pm

Hi Greg

Can you confirm that the bypass cap for C2A should be .047 uf (as indicated in the new schematic). On your original, hand drawn schematic, it was listed as 0.47 uf (which would seem to be appropriate, being roughly 1/100 the value of the 33 uf lytic cap value).

Also, any thoughts about using the AudioCap polystyrenes (RT series) for the DC blocking caps? I have all the values on hand, and was wondering if I should just use them or is there a compelling reason to use polypropylene.

One last thing: You mentioned above that you replaced your attenuators. Can you explain what made you do so. I have a couple Mallory's (same type as the Davens that you used) that I have on hand, which I would like to use instead of ordering new ones.

Thanks for taking the time to make this project available, and for drawing the schematics. I have two 351's on the bench right now undergoing your mods. Can't wait to get them buttoned up and in service.

Best regards,

Steve
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Wed Nov 18, 2009 11:11 pm

.047uF is correct.
I would go ahead and try whatever caps you like. I have used Multicap, Auricap, and Solen before with good results. The ones in Electrical's 351s are from Reliable Capacitors.

Our attenuators were becoming noisy, and intermittent. We replaced them with custom pots from State Electronics, part # L24338. It is a popular item, used in the urei 1176, and many others.
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Re: Ampex 351 Preamp Modification Info!

Postby axelh4110 on Mon Sep 20, 2010 4:49 am

Hello every body,

I'm working on a PCB design for a double unit of the AMPEX 351....

But when i compare the original schematic and your's i find some different value of component

R11 3K3 => 4K7
R42,43 100K => 220K
R38,41 2,2K => 1K5

Do you change this value for having a global gain around 60/70 dB

and R75 is not on the original schematic .... can you tell how you have calculate it. (Is it the equivalent part of the rest of the circuit of the original schematic). I'm very intereted to understand why you use a 680K

This original schema is here at the end of the doc : http://recordist.com/ampex/schematics/351man/351man.pdf
but my PCB design is based on the schematic of electrical audio site

Sorry for my bad english

Thanks
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Wed Sep 22, 2010 4:59 pm

The gain is a little different than stock. I set ours for 65dB max. There are some new values since the B+ is higher (without all the loading of the other components). I need to look at things a bit further to see what was the idea behind R75. It might be an upgrade Ampex had, I can't remember.
I think the gain balancing on that first 12AX7 is a little weird for this purpose, but it works, and sounds good, so why mess with it.

You would be the second to manufacture a new unit based off this. The other dude used to post here runs a company called Anthropic Audio. I don't know if he still does it...
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Re: Ampex 351 Preamp Modification Info!

Postby henryb12 on Sat Jan 29, 2011 11:22 pm

Is R201 = 3k ohms or 3.3k ohms?
<br>
thanks Greg!
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Sun Jan 30, 2011 1:57 am

henryb12 wrote:Is R201 = 3k ohms or 3.3k ohms?
<br>
thanks Greg!

3.3kΩ
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Re: Ampex 351 Preamp Modification Info!

Postby schmidlin on Wed Feb 09, 2011 5:55 pm

Hi Greg, nice work!

I followed your schematic, but in the gutting, forgot to note where the grounds ideally go. I am assuming the PS board ground goes to the original post by the PT. Anything special going on at the front end? What about pin 1? I have the front end (input board)grounded at the gain pot.

I'm getting some 60 and 120 cycle hum, and guessing at grounding being the issue.

Thanks In Advance.
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Wed Feb 09, 2011 7:40 pm

Pin 1 of the xlrs should go directly to your star ground, which should be adjacent to the safety ground pin on the AC power inlet. You can use that post near the power transformer if it still has good contact with the chassis. The power supply ground should be tied to that star ground as well (short black wire). There should be jumpers (original) that carry ground from the power supply board to the record and repro boards. Make sure the contacts are clean and there is sufficient friction to the slide connectors.
The casing of the input pot isn't a good ground reference for the record (input section) board. Grounding through the chassis isn't ideal.
If you still have hum issues, try to make sure everything that needs to see ground sees it securely, with a short path. Replace the selinium rectifier as well (with some 1n4004 diodes or diode bridge).

Good luck and be careful.
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Re: Ampex 351 Preamp Modification Info!

Postby schmidlin on Thu Feb 10, 2011 2:46 pm

Thanks Greg!

Now that you mention the selenium rectifier, I have what seems too be a ghost in the machine. I wired up the heater circuit and left it floating. Why? As is, the ground side measures -13.0 volts DC and the pos side measures -1.17 DC. Here's the kicker: when I ground the ground side I get no voltage: dead short. I can successfully ground the pos side, (~-13VDC) but does nothing to alleviate the hum.

IIRC, I swapped out the diode with a couple new ones and no love, but I could be mistaken. Swapped AC leads: nothing. I tested for internal shorting, but tested as isolated. I'm very tempted to snip the center tap and go bridge. Very frustrating, hope you can help.
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Fri Feb 11, 2011 8:55 pm

schmidlin wrote:Thanks Greg!

Now that you mention the selenium rectifier, I have what seems too be a ghost in the machine. I wired up the heater circuit and left it floating. Why? As is, the ground side measures -13.0 volts DC and the pos side measures -1.17 DC. Here's the kicker: when I ground the ground side I get no voltage: dead short. I can successfully ground the pos side, (~-13VDC) but does nothing to alleviate the hum.


Hmm. You're talking about the Blackk-Black/Yellow-Black winding right? Is it wired like the schematic shows? The Black/Yellow winding should be grounded, and the two Black windings go straight to the diodes.

Also, check that there are no connections to ground via pin 9 of any tube sockets.
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Re: Ampex 351 Preamp Modification Info!

Postby danmaksym on Sat Feb 12, 2011 6:12 pm

You would be the second to manufacture a new unit based off this. The other dude used to post here runs a company called Anthropic Audio. I don't know if he still does it...


Thanks for the mention Greg. I'm on a bit of a hiatus from building them. I think I made about 15 or so in total. When you build them like I was building mine (using only the best parts available) they're so costly to build that a huge cash outlay is necessary to even build a few. I suppose you could skimp on a lot of things and get the price down, but that's not my style.

Anyway, good luck to all who are attempting this project. It's fun, but can get hairy.
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Re: Ampex 351 Preamp Modification Info!

Postby schmidlin on Tue Feb 15, 2011 9:07 pm

I got an update for you, Greg (I hate when folks fix an issue and slink off not publishing the results):

It's a bit embarassing, but when I installed the IEC power jack, the lower (hidden) bolt snagged a purple wire under it and it's lock-washer grounded it to the chassis. That wire happened to be the lead from the heater diodes on thier way to the PS board. I'm amazed I had heater voltage at all or didn't fry something. Doh!

So thanks for all your help and excellent work!
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Tue Feb 15, 2011 10:44 pm

schmidlin wrote:I got an update for you, Greg (I hate when folks fix an issue and slink off not publishing the results):

It's a bit embarassing, but when I installed the IEC power jack, the lower (hidden) bolt snagged a purple wire under it and it's lock-washer grounded it to the chassis. That wire happened to be the lead from the heater diodes on thier way to the PS board. I'm amazed I had heater voltage at all or didn't fry something. Doh!

So thanks for all your help and excellent work!


Awesome, and congrats.
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Dead output transformer and T-pad questions

Postby cherry on Sat Feb 23, 2013 8:11 am

this message got duped..
See the one below.
Last edited by cherry on Thu Mar 21, 2013 4:17 am, edited 2 times in total.
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Re: Ampex 351 Preamp Modification Info!

Postby cherry on Mon Feb 25, 2013 4:13 am

Hi Greg,

Received for free an Ampex 351 model 30750 1.
Glad for getting such a beauty I started reading about it, and reached your thread.
It's highly appreciated sharing your important experience with this unit utilizing it for a microphone preamp.
This is exactly what i want to try.

Line output transformer
Checking the 'heart' of this unit, the transformers I noticed the measurements of the 30764-01-B line output transformer show it's probably dead:
Primary:
brown-red-blue show the following readings:
blue-red: 538 ohms
brown-red: no resistance, dead.
Secondaries:
yellow-black:13.9 ohm
black-green: 16 ohm
red/yellow-green/yellow:182 ohm

So it's dead, isn't it?
Could the replacement only be a costly Sowter 9084?

T-pad
And now for a T-pad question:
Is it possible to use a Bourns 600 ohm T-Pad Attenuator, instead of the rare hard to find Daven?
http://classicapi.com/catalog/product_i ... cts_id=256

Thanks,
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