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Ampex 351 Preamp Modification Info!

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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Wed Jul 20, 2016 3:54 pm

Hi Greg. Thank you SO much for creating this forum and posting the schematic and pics! I'm an amateur when it comes to anything more elaborate than DIY mic pre kits, etc., so please excuse my lack of knowledge.

The record and repro board mods are super straight forward, but I'm getting hung up on what VU1, VU2 and the 3 "H" (maybe heater?) components that are on the power supply section of the schematic. I know VU normally stands for Volume Unit, but in this case I'm a little baffled...

Also, there have been so many posts about the output attenuator. Is it safe to assume that the original attenuator is not suited for this mod (not constant 600ohm)?

Thanks so much for your time!!

Paul
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Mon Jul 25, 2016 3:16 pm

First of all, Greg, thank you so much for doing all of this work! I'm learning way more doing this mod than I have with any DIY 500 series kit I've built!

So, I'm a little stuck drawing out the traces for the separate diode based rectifier board. :shock: Sorry I have to ask this, but what are the VU1 and VU2 components? I can't imagine them being meters...

Thanks! :mrgreen:
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Mon Jul 25, 2016 3:26 pm

First of all, Greg, thank you so much for doing all of this work! I'm learning way more doing this mod than I have with any DIY 500 series kit I've built!

So, I'm a little stuck drawing out the traces for the separate diode based rectifier board. :shock: Sorry I have to ask this, but what are the VU1 and VU2 components? I can't imagine them being meters...

Thanks! :mrgreen:
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Mon Aug 01, 2016 5:08 pm

Hey,

Sorry for the delayed account approval.

VU1 and VU2 on the schematic are lamps for the VU meter. They run off of 12 volts AC.
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Fri Aug 05, 2016 2:09 pm

greg wrote:Hey,

Sorry for the delayed account approval.

VU1 and VU2 on the schematic are lamps for the VU meter. They run off of 12 volts AC.


No worries at all, Greg. Thanks for approving me! :D While I was waiting, I started gutting one of my 351's and figured out all sorts of questions I had, iiiiincluding the 2 power wires to the lamps... Perhaps I should have started the gutting process before asking questions... whoops. :oops:

With that said, I have a question about C20. Apparently, there's a bit of debate about polystyrene caps. Some are big fans of their sound. Others say they're extremely heat sensitive, once heat changes the farad value they stay changed, etc. How is the C20 capacitor being used in the circuit and can I use a ceramic disc or mica cap instead? (I'm showing my inexperience with these questions) Due to it's microphonic and antenna-like nature, would a ceramic disc cap inject too much noise into the circuit?

Also, I know from your schematic that C301-305 are all 63v. What are all the rest of the caps' voltage ratings, 500vdc or higher? Just want to make sure I order the right values.

On a side note, I'll be using the Gas 351 replacement circuit boards. I borrowed from George's BOM spreadsheet layout (from the Gas 351 circuit board manual) and have been putting together a new BOM list specifically for this mod. Once I purchase/assemble everything and get both units fully functioning, I'll be more than happy to share the BOM list with everyone.

Thanks so much! :D
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Fri Aug 05, 2016 3:45 pm

Interesting point about C20. When everything's done there's more space in the box, less tubes and other radiating parts. It actually never gets that hot. I could see heat being an issue if the original parts were left in there. The polystyrene caps have worked for us.
As far as voltage ratings go, 450v and above for everything other than the filament and phantom supply, if I remember correctly. You can refer to the original Ampex 351 schematic for specific cap ratings.
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Wed Aug 10, 2016 3:22 pm

The State Electronics L24338 potentiometer is $65.10 per unit and they bang you with a $50 fee when ordering less than 25! That's $180.20 for 2 pots!!!! Thats not too far off from what the rest of the build costs!! F@#k!!! :?

Does anyone have stock of these and are willing to sell two to me or has anyone found another pot that works?
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Mon Sep 26, 2016 3:43 pm

Hey, Greg. Quick question: What's the voltage rating you guys used for C306 (on the phantom power board). I built a board with C306 being 450v (based on the majority of caps in the box), but now I'm realizing that doesn't make sense with the circuit it's in... not to mention it looks ridiculously HUGE! Haha!

I'm guessing C306 is 50v or 63v, yes? Getting SO close to finishing these beauts. SUPER excited!! :D

Thanks so much!

Paul
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Mon Sep 26, 2016 4:53 pm

I used a 63v cap.
That'll work, but as you guessed it's overkill.
Good luck!
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Thu Oct 20, 2016 1:48 pm

Okay all! My two units are up and running and they sound pretty darn good! My only concern is how ridiculously HOT the power transformers are running. :? With an infrared thermometer one tranny is registering at 275 Fahrenheit and the other tranny is at 282! It does take a good hour to get up to this temp, but 30+ minutes after power up the wax on the wire jackets coming out of the trannies starts liquifying!! Is this normal?

I've looked over the schematic numerous times and all my wiring checks out. All grounds that the schematic calls for are in place. I upgraded to 14 gauge ground wire and all of the power related grounds are grounded to the same spot near the IEC plug/power tranny (both XLRs are grounded to the other side of the box). Am I grounding out something I shouldn't that might be putting too much of a load on the tranny? Any ideas? Any input would be greatly appreciated.

MANY thanks for everything, Greg! :D
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Fri Oct 21, 2016 5:39 pm

That's too hot. Did you lift the green/yellow winding (secondary on the power transformer) from the ground post? It should be floating.
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Tue Oct 25, 2016 8:48 am

Son of a B!+ch! That was it!! Thank you, Greg!!! I was using the 3 point solder strip/tabs that the 2 green and 1 green/yellow tranny leads were already attached to and my tunnel vision didn't notice that the center tab was grounded. DOH!! Now the trannies are running at a more respectable 140 fahrenheit. Thanks so much!!

Man, I'm glad I didn't fry the transformers! I can only imagine how hard it is to find replacement trannies for these units... :?
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Re: Ampex 351 Preamp Modification Info!

Postby ericrasmon on Sun Oct 30, 2016 11:21 am

Hi Guys,

I'm in the process of rebuilding a 351. Recapped it, at least all the ones in the mod schemo. But, I haven't removed all the parts like the mod seems to have. I'm having a bit of a hum issue. The hum is there when the record control is all the way off.

Greg, you have said "to not have a tubes pin 9 at ground" V3 on mine has pin 9 at ground, or at least very close 1-2 ohms. If I remove the heaters or the tube, I get a HUGE amount of noise and hum.

Is part of the mod to remove the DC filiments from all the tubes? The output transformer seems very close to the power transformer. How much hum does that cause?

With the record control all the way off, I'm getting hum at about -65db on my PT rig.

**Additional info**

While waiting for this post to be approved I found the 12AX7 (v4) was defective in someway causing a lot of the hum. Replace that dropped it about 20db. Is there a grounded set of instructions some place?
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Thu Nov 03, 2016 10:10 am

ericrasmon wrote:I'm having a bit of a hum issue. The hum is there when the record control is all the way off.
The output transformer seems very close to the power transformer. How much hum does that cause?


I was having the same issue. When the input pot and the output attenuator were turned all the way down, I was still getting a 60 cycle hum. In my case, it turned out that the output tranny hated being near the 120 volt power wires that go to the on/off switch. Even tiny movements of the power wires helped remedy the issue. If you're using those solder strip tabs/points between the OTran and the faceplate, the same applies as well. I did detach the OTran from the wall momentarily to see if it's proximity to the power tranny made any difference. It didn't seem to differ at all. I hope this helps.
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Thu Nov 10, 2016 1:12 pm

I might be putting together an order for more output attenuator, L-pad pots (L24338) used in this project.

Let me know if you are interested in getting one (email please, my first name@this website). The price would be $55 each + shipping to you. The minimum is 10 and I need 4.

Note that these are useful for lots of DIY projects, especially if you're cloning Urei gear.
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Re: Ampex 351 Preamp Modification Info!

Postby Paul Hariu-Powell on Wed Apr 19, 2017 2:18 pm

Ok, gang. I'm a little reluctant to post this, due to my lack of experience, but I said I would, so here it is! The BOM spreadsheet (with product links) for the EA351 (no tape related guts) that I made while working on this project. It's broken down based on each circuit board. Although it's based off of the GAS 351 circuit boards, there should be no reason why you couldn't use this on the original boards:
https://docs.google.com/spreadsheets/d/ ... sp=sharing

Here are links to my saved Mouser "projects" related to this build. Just like the spreadsheet, each "project" is based on a specific circuit board. Once you open the link you can add each project to your cart. The majority of components were purchased through Mouser. For the components purchased outside of Mouser, use the links in the spreadsheet (highlighted in yellow).

Gas 351 Record Board EA Mod 10-28-16:
http://www.mouser.com/ProjectManager/Pr ... 87D8C3411B

Gas 351 Repro Board EA Mod 10-28-16:
http://www.mouser.com/ProjectManager/Pr ... a7b7bdc4a2

Gas 351 Power Board EA Mod 10-28-16:
http://www.mouser.com/ProjectManager/Pr ... 4b820dfa98

Ampex 351 Phantom, -20dB pad and Power boards Mod 10-28-16:
http://www.mouser.com/ProjectManager/Pr ... 8ea344bb67

(Side note: If you use the 2" x 2" PCB breadboards I have in the last project for the added phantom power circuit, please be aware that there is a ground layer on the board, which is connected to 3 solder points in each corner. Be very mindful when planning the orientation of the build. Also, If you scratch the board you could potentially expose the ground layer. With this said, you might want to consider using a different board)

Please let me know if you find any issues with any of the links, if you have any questions or if I need to correct anything.

MANY thanks to Greg for all his help and time making this mod possible! And thanks to George at recordingwithgas.com for designing new, high quality 351 boards and for inspiring me to make a BOM specific to the Electrical Audio 351 Mod!!

Disclaimer: Use these links, lists and parts at your own risk. Again, I'm not a professional and take no responsibility for malfunction or specing incorrect components. (with that said, I've been running my two 351's for months now with no issues and they sound great, so I'm assuming I did it right. :lol: )

Cheers,

Paul
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Re: Ampex 351 Preamp Modification Info!

Postby greg on Fri Apr 21, 2017 10:47 am

Thanks for sharing the info. We bought a couple sets of those boards for our 351s. Still have a couple carcasses to fix up.
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Re: Ampex 351 Preamp Modification Info!

Postby fazeka on Wed Oct 11, 2017 3:48 pm

Hi,

Any update on this? I have a similar issue with heavy distortion with one of my 351s.

Cheers,
Chris

outoftune wrote:I'm troubleshooting one of these units and wondering if anyone has seen the same issue. This is a stock 351.

It's passing signal and functioning but heavily distorted. I've traced the issue to V4 (injecting a signal directly into V5 passes cleanly to the XLR).

With no tube in V4 I can measure 175 VDC at the junction between R40 and R42. When I insert a tube into V4 the B+ goes away. I've tried multiple tubes with the same results.

I'm thinking to replace all the resistors in the circuitry surrounding V4. Any other suggestions of what to look for?
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