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Ampex 601 preamp hiss/buzz very faint sound (bad tube?)
Ampex 601 preamp hiss/buzz very faint sound (bad tube?)
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Old 3rd January 2018, 06:47 PM   #1
jomaco is offline jomaco
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Default Ampex 601 preamp hiss/buzz very faint sound (bad tube?)

I have an Ampex 601 that has been converted to a preamp (reel to reel part removed).

It has been working fine for some time (for me) and was used in a studio before I got it.

The sound produced by preamp has suddenly become extremely faint, the mic level has to be turned up high just to hear it, and much louder than that sound is a very loud hiss/buzz.

I would be grateful for any ideas of what to try first. I am hoping it's just a bad tube, I haven't opened it up to take a look yet, but can I just start pulling tubes to find the culprit?

I have tried basic things e.g. tried different cables, mics and audio interfaces.

Last edited by jomaco; 3rd January 2018 at 07:03 PM.
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Old 3rd January 2018, 07:24 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by jomaco View Post
can I just start pulling tubes to find the culprit?
Label each tube for its position, and check to see if they are all identical types.
Do they all light up? If so, start swapping if they are the same type to see if one is bad.
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Old 3rd January 2018, 08:42 PM   #3
jomaco is offline jomaco
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Thanks, after I check to see if they are lit and what type they are, do you mean get one new tube of that type and replace each one in turn?
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Old 3rd January 2018, 09:04 PM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by jomaco View Post
Thanks, after I check to see if they are lit and what type they are, do you mean
get one new tube of that type and replace each one in turn?
Schematic: https://web.archive.org/web/20060929...n/601schem.gif

The 601 uses several different tube types, so don't install the wrong tube anywhere.
It's unlikely a small tube's filament is bad, but check anyway to see if light is visible
somewhere inside the tube, usually near the top or bottom. Sometimes you can't
see the filament light in a good tube, though.

The problem could be a number of things, bad connection, bad coupling capacitor,
bad power supply, etc. Remember this was modified, so the schematic isn't right.

Last edited by rayma; 3rd January 2018 at 09:07 PM.
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Old 3rd January 2018, 09:08 PM   #5
jomaco is offline jomaco
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Ok, I'll take a look in more detail in the next couple of days.

The only basic thing I haven't checked outside the unit itself is the step down transformer that I am using - is that also potential thing that could cause noise? - although it's more than just background noise really as the sound of the mic is very quiet indeed.
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Old 3rd January 2018, 09:11 PM   #6
rayma is online now rayma  United States
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Quote:
Originally Posted by jomaco View Post
is that also potential thing that could cause noise? - although it's more
than just background noise really as the sound of the mic is very quiet indeed.
If the 230/115 transformer is working normally, it should not cause noise.
Don't worry about the noise yet, find the gain problem first.
The noise may fix itself when the gain is restored.
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Old 4th January 2018, 10:21 PM   #7
jomaco is offline jomaco
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I removed and replaced the EF86 and also swapped two of the 12AU7's but that didn't seem to bring about any change. The writing is rubbed off a bit on two of the tubes, but it looked like there was either a 12AX7 or 12AY7 in one of the 12AU7 sockets (it could be a 12AU7, but it does look like an X), the other unknown one I am guessing is a 12AU7.

I put the 12AX7(?) in the 12AY7 position, and moved the 12AY7 to the 12AU7 position (with the second unknown (12AU7?) tube in it). I put the 12AU7(?) in the 12AX7 position - a lot moving around but I kept it like this as in this combination the hiss disappeared, and gain was restored.

However, the problem returned (low gain, hiss much louder than mic volume) after a ~5-10mins - perhaps after the tubes had warmed up?). I removed the 3 above tubes again, and blew on the sockets and replaced them and the issue went away again, so it seems that the problem is either caused by or bad tube, or is otherwise intermittent.

I also tried the second tube with writing rubbed off in the 12AY7 position, and the mic level had be to turned very far up to hear anything, so I am guessing that one is indeed a 12AU7 (which I gather has a lower gain factor) - also there was still very loud hiss/buzz in this configuration.

I have a hunch the problem tube might be the 12AY7. I have some 12AY7's, so I'll try putting a new one in once I'm back home. If that doesn't solve it, I guess I need to get some 12AU7's and an EF86.

I guess my question at this stage is whether finding that moving the tubes fixed the issue temporarily tells us anything? I suppose it makes it more likely that it is a tube even though it came back.

Last edited by jomaco; 4th January 2018 at 10:23 PM.
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Old 4th January 2018, 10:26 PM   #8
rayma is online now rayma  United States
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Quote:
Originally Posted by jomaco View Post
I guess my question at this stage is whether finding that moving the tubes fixed the
issue temporarily tells us anything? I suppose it makes it more likely that it is a tube
even though it came back.
More likely a bad/dirty tube socket or other connection.
Small tubes are seldom intermittent themselves.
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Old 6th January 2018, 05:02 PM   #9
jomaco is offline jomaco
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I visually inspected all the tube sockets and internals, but it looks fairly clean. I then cleaned all the tube pins and sockets, but the hiss and low gain remained.

However, I swapped out the 12ay7 and it seems to have solved it - going fine for about an hour now.

Can't hurt to have cleaned the sockets/pins anyway. All of the valves look pretty old apart from the Electro Harmonix EF86.

Hopefully job done. Cheers.

Last edited by jomaco; 6th January 2018 at 05:05 PM.
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