Ampex 600/601 Tube mic & Line amp

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moamps

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6.9V is on high limit, you can insert a resistor in series to drop it, or two anti-parallel diodes.
Using AC for tube heating in a mic preamp is sub-optimal IMO,  you can get better results using a good filtered DC voltage.
Have you installed hum-balancing resistors or pot in heaters AC line?
 

extempo

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Thanks!

I thought about doing DC for the heaters, but wanted to try AC first and see how bad it actually was.

I have a 100 ohm pot for hum balance of the AC heaters, just like in the stock design (schematic attached).
 

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extempo

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I found and fixed one issue, which was that I forgot to ground the wiper of the  hum balance pot. That explains why the pot wasn't doing anything!

I also used the anti-parallel diode trick for the AC heater and it brought the  voltage down to just under 6.3 VAC. Thanks for the tip moamps.

I still have the issue with loud hum at the output unless the 250k pot feeding the 12au7 is set just so. This is actually 120Hz, not 60 Hz.

I'll upload some high res pics ASAP.
 

moamps

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extempo said:
I still have the issue with loud hum at the output unless the 250k pot feeding the 12au7 is set just so. This is actually 120Hz, not 60 Hz.
This hum comes most probably from not so good filtered power supply high voltage B+, and maybe wrong grounding scheme. You can check the ripple on B+ with DMM set to AC or scope. The minimum of hum you get at one position of the pot is because of summing hum from first stage with hum in second. They are flipped in polarity so this summing became subtraction.
 

extempo

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OK! Fixed the issue. Had a funky solder joint on the ground lug for the multicap can housing the B+ filter caps. All good now. So often it's the simple stuff. Many thanks  :)
 

thomasdf

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Thanks for joining my thread :)
My experiment ended up in the pre sounding great and then thin, and sitting on a shelf ever since.
Can you post pictures of your build ? I'd like to achieve something like you did.
Do you have enough gain for old ribbons?
 

thomasdf

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Hi everyone

Just a quick update and a question :

I finished rebuilding the preamp from the ground up. I went with the FixThatMix schematics, but used the PSU we designed here, as well as a 5879 preamp tube from the Ampex 600 design. The input and output transformers are Altec 15095a, wired 150 > 15K for the input and 15K > 600 for the output. I don't have a 12AY7 in my stash so I replaced with a 12AU7 for now.
I have a huge amount of gain, but microphonic tubes and I think I miss some low end extension going straight into my AD converters.
I tried with a 15K : 15K Altec output transformer, it's better, yet it's not perfect. I could be tempted to blame it on the input transformer, but I have the same sensation when using the DI input with my P bass.
Where should I look ? I don't think the 12AU7 instead of 12AY7 could be guilty...

I am considering finding a better input transformer than the Altec 15095a (by better, I mean a dedicated mic input transformer with maybe a 250 primary to match my RCA Varacoustic)

Thanks for reading, and thanks all for your help
 

thomasdf

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I'll have to do some measurements then, but maybe this design is not as thick sounding as the 351. I was expecting more beef. But it sounds super pleasing anyway. I'll put it to use out there in the recording studios, and see if I can report back with measurements and/or audio.
 

mjrippe

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I'll have to do some measurements then, but maybe this design is not as thick sounding as the 351. I was expecting more beef. But it sounds super pleasing anyway. I'll put it to use out there in the recording studios, and see if I can report back with measurements and/or audio.
I was never a fan of the 351, but a properly restored 350 will make an SM58 sound huge. When I describe the 600 series to people I tell them it sounds like what they think a tube preamp would sound like. Transformer in and out, distortion approaching 0.5% at reasonable settings, all the warm fuzzy feelings that you expect when looking at glowing tubes and a big VU meter. Certainly not lacking in low end, but not massive sounding.
 

thomasdf

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I am not certain what the differences are between 350 / 351 preamp wise (external PSU?) but they sounds absolutely massive even on the average dynamic mic even with poor mic placement... agreed. Maybe I was expecting a bit of that, haha, but it's true that the designs are quite different.
I'll see how it behaves in actual use.

I have a old dual 60 -2 suitcase that this thread makes me want to go through and get working.
Go for it !
 

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