Ampex 350 mic pre sudden increase in the noise floor

[skaren]

Good evening,

I have this Ampex 350 tape pre that was converted into a mic pre last year.
The original noise floor was about -70db which is fine.
Last the week the noise suddenly increased to about -42db.
At first it sounded like a valve was going but reseating the 6F6G valve sorted the really crazy noise which is what made me believe that was the case.

Unfortunately I was left with this new -42db noise.
https://www.dropbox.com/s/heqyhavxbinhm65/ampex%20noise.wav?dl=0

My question is whether anyone recognize the noise signature.? Could this be due to a faulty valve? Or more likely a ground issue?
The gain knob doesn't affect the noise, it stays the same regardless. Also, the noise is there both when something is plugged to the  the input and when it isn't.

Many thanks

 

[gridcurrent]

I have this Ampex 350 tape pre that was converted into a mic pre last year.

No conversion is necessary.  If someone "converted" it, ask the "converter".

The gain knob doesn't affect the noise, it stays the same regardless. Also, the noise is there both when something is plugged to the  the input and when it isn't.

That narrows down the problem to the line amplifier.

 

[radardoug]

Its mains related, could be a ground loop on the output side. Have you plugged it in to a different socket or changed the output wiring?

 

[skaren]

Thanks for your replies.

I've tried pretty much every socket available to me whilst also unplugging the rest of the studio.

Regards to changing the output wiring would you suggest trying to lift the ground?

I'll have a go at output wiring first. If it's the line driver I'd have to send off the unit to a more qualified technician so will try what I can do myself to begin with.

radardoug and gridcurrent, really appreciate your time!

 

[pucho812]

when did the unit last get new psu filter caps put in?

 

[skaren]

The psu was built last year...

 

[pucho812]

have you tried a different valve/tube?

 

[skaren]

Its mains related, could be a ground loop on the output side. Have you plugged it in to a different socket or changed the output wiring?

Hi again,

I have tried different sockets and different wiring (lifting the ground etc) to no avail.

 

[mjrippe]

But have you tried a different tube/valve?  I had one on my bench the other day and the tube in V401 position had serious hum.  Swapped it and hum is gone.

 

[skaren]

have you tried a different valve/tube?

Hi there,

I haven't tried different valves yet. That would be the next step if there is nothing to be done without some spare parts.

 

[mjrippe]

have you tried a different valve/tube?

Hi there,

I haven't tried different valves yet. That would be the next step if there is nothing to be done without some spare parts.

That should be the first step if you have a unit with all the tubes in place.  Take V405 (which should be a "Select" 12SJ7) and put it in V401 position.

 

[skaren]

There are valves in V-401, V-402, V-406 (all 12SJ7) and V-407 (6F6).

I found some spare 12SJ7 valves and tried to replace them with the valves in V-401, V-402 and V-406 but the noise was still the same.

I just recorded the noise profile from power on if that can shed any new light on the matter. Here is the link
https://goo.gl/hxbIjr

Next would be to source a spare 6F6 valve and see if that is the culprit.

Thanks again for everyone's input

 

[jrowell]

Often people term  these mic pre conversions, and rip half the stuff out and wire around things. The mic pre conversion is stock on the front panel - the selector switch has an input monitor mode - unfortunate to see them with the rest ripped out and messed with. Recap with good parts and film caps too and they sound good, clear, and big with a bit of grit. Just did a pair last year.

 

[skaren]

Hello again,

I ended up sending the unit back to the person who made the conversion and built the power supply after exhausting all more simple potential fixes. (Gridcurrent, your suggestion turned out to be the correct on in the end).

Apparently there was previously only 1 transformer in the power supply for all the various voltages so another 2 were put in (as explained by the tech) and even though that hadn't been a problem previously it resulted in an acceptable noise floor.

Thanks for all suggestions!

 

[mjrippe]

Well, that doesn't make much sense unless the original power transformer had windings that failed.  On the other hand, if it is working then leave it alone!