abbey road d enfer

Re: Ampex 600/601 Tube mic & Line amp
« Reply #20 on: March 24, 2017, 02:39:37 AM »
Audio Cyclopedia, 1st edition, page 319,
"Unbypassed cathode circuits are more susceptible to hum pickup from AC heater circuits."
I agree with that on a general basis, but what I question is the relevance in mic preamps, where the contribution of heater-to-cathode is about 100 times less than the heater-to-grid. It's a basic subject of node sensitivity to external electrostatic fields; the effect is directly dependant on the node's impedance.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


thomasdf

Re: Ampex 600/601 Tube mic & Line amp
« Reply #21 on: March 24, 2017, 05:54:53 AM »
Wow I didn't know my question would lead to such a deep debate :) Impressed by your knowledge.. I should get back to studying :)

PRR

Re: Ampex 600/601 Tube mic & Line amp
« Reply #22 on: March 24, 2017, 01:17:23 PM »
> external electrostatic fields

The usual superstition is not external field, but "some" heater insulation LEAKS.

It is not theory. Insulation leakage is very erratic. It is a batch of hummy amps which can not be sold until you find a different batch of tubes, or throw a big cap on the cathode. Some designers learn to pre-emptively over-cap all low-level cathodes- the cap cost is less than dealing with production trouble or unhappy customers.

abbey road d enfer

Re: Ampex 600/601 Tube mic & Line amp
« Reply #23 on: March 24, 2017, 02:53:30 PM »
> external electrostatic fields

The usual superstition is not external field, but "some" heater insulation LEAKS.

It is not theory. Insulation leakage is very erratic. It is a batch of hummy amps which can not be sold until you find a different batch of tubes, or throw a big cap on the cathode. Some designers learn to pre-emptively over-cap all low-level cathodes- the cap cost is less than dealing with production trouble or unhappy customers.
Yes, I'm aware of this, but my EF86 datasheet (Mazda Belvu) specifically excludes this; maybe I'm too naive...
And I would think Ampex used only premium tubes; am I twice naive? Anyway, in the 601, the cathodes are decoupled.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Ampex 600/601 Tube mic & Line amp
« Reply #24 on: March 27, 2017, 05:06:00 PM »
How many 12AY7 tubes are there? 
Is there a reason you don't want a stereo mic/line amp?  The thing is almost there. and then you could also have the line in switchable from the pentode to the triode stage for extra options. 

Also, for input trafos, anything 1:8 or above should work fine, like the API inputs, etc. Cinemag, Lundahl, Jensen, etc, whatever you can get that is decent quality. 

My experience is that these things have so much gain, you could get rid of half the 12AY7 (use one valve for both channels, one triode each), add another pentode + input trafo and have stereo mic inputs. 

Re: Ampex 600/601 Tube mic & Line amp
« Reply #25 on: March 27, 2017, 05:41:23 PM »
If you find the mic pre to have too much gain and too much hiss you can wire the 5879 as a triode.  You may get a bit more 2nd harmonic too.

You seldom see pentodes in any position in vintage pro tube mic pres without there being NFB used to lower the hiss.  The 601 was supposed to be 'prosumer' deck relative to their other models.

Re: Ampex 600/601 Tube mic & Line amp
« Reply #26 on: March 27, 2017, 06:09:36 PM »
If you find the mic pre to have too much gain and too much hiss you can wire the 5879 as a triode.  You may get a bit more 2nd harmonic too.
awesome idea.  What about making it switchable Pent/Triod?  Never done it but I've seen it on some pre/power amps.

abbey road d enfer

Re: Ampex 600/601 Tube mic & Line amp
« Reply #27 on: March 27, 2017, 06:21:32 PM »
You seldom see pentodes in any position in vintage pro tube mic pres without there being NFB used to lower the hiss. 
NFB does not lowers the hiss significantly; it just lowers the hiss of the second stage, but not that of the 1st stage, which is, in a well-designed product, dominant.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: Ampex 600/601 Tube mic & Line amp
« Reply #28 on: March 27, 2017, 10:05:08 PM »
NFB does not lowers the hiss significantly; it just lowers the hiss of the second stage, but not that of the 1st stage, which is, in a well-designed product, dominant.


Here's one of the circuits I had in mind.  (It's saying the attachment size is too large)  It may be in our meta but there's plenty of copies out there.  Langevin 116 A

thomasdf

Re: Ampex 600/601 Tube mic & Line amp
« Reply #29 on: March 28, 2017, 05:05:22 AM »
Well, that's a lot of interesting points of view for sure! Thanks a lot!

I have to say, I am still a rookie as I stick to copying schematics with some very slight modification at the moment.
I wanted to have something really faithful to the Ampex 601 in terms of sonic signature.

Regarding the mic pre, I like the idea of having a sh*tload of gain, as I am building it to use with a vintage RCA Ribbon Mic mainly. The other pres I have handy are a bit too weak for that task. I also want to keep the front panel as simple as possible, but I have to admit that a pad and a triode/pentode might be helpful.
If at some point I realize I need a 2nd channel, I'll see how I can add it... but the chassis is already pretty crowded :)

Regarding the stereo line amp, I'd use it mainly on stereo buses or mix bus, so on pretty high line levels.. I am going for a tube / iron flavor, might a bit edgy but certainly not a fuzz box :) Maybe I should use a 12AU7 instead of the 12AY7?
Right now, I plan on using stereo rotary switches for a precise LR matching. One is for the "Line Input" attenuator/pot, the other one is for the "Rec Level".
Do you guys think I should feed only the last 12AU7, have the "Rec Cal" control as a trimpot somewhere, and then having an T Pad output attenuator after the output transformer? I am not sure a single 12AU7 will bring a lot of color / character...


Re: Ampex 600/601 Tube mic & Line amp
« Reply #30 on: March 28, 2017, 07:42:49 AM »
Quote
I am not sure a single 12AU7 will bring a lot of color / character...

A 12AU7 IMO, is quite a colorful 9 pin.  It seems to have a distinct mushy splatter to it when hit hard.  Useful for rock drums (esp snare) but I am replacing the AU7s on my Pultec clones in hopes of  getting rid of some of that type of  'color'.    Also, the Ampex circuit uses Cathode Follower and that is known for a similar type of sound when hit really hard.  And with all that gain I doubt you'll be in shortage of potential overdrive sounds.  An output attenuator is a must for keeping control of it.

Re: Ampex 600/601 Tube mic & Line amp
« Reply #31 on: March 28, 2017, 03:20:25 PM »
I wanted to have something really faithful to the Ampex 601 in terms of sonic signature.
Regarding the mic pre, I like the idea of having a sh*tload of gain, as I am building it to use with a vintage RCA Ribbon Mic mainly.
bypassing the rec. cal stage and associated potentiometer, re-wiring the EF86 as a triode,
retaining the output stage as is:
77 dB of gain with EIN better than -128 dBu (weighted).



thomasdf

Re: Ampex 600/601 Tube mic & Line amp
« Reply #32 on: April 06, 2017, 01:27:07 PM »
Hi everyone

Here is where I am at right now, what do you think? I got rid of everything except the last 12AU7 for the line pre, and kept the rest for the mic pre as I like the idea of having a lot of gain available for old ribbons. I'll use proper pads on the mic input and line outputs.
I also believe I will have to readjust the values of the PSU resistors as there are less tubes than usually planned so the load will be lower, and there may be too much juice?
Edit: I also believe my line won't be level matched that way.. What can I do about this?
Thanks a lot
Thomas

Re: Ampex 600/601 Tube mic & Line amp
« Reply #33 on: April 07, 2017, 01:58:25 PM »
I still think you should do 2x mic pres. 

But keeping with that schematic, unless you already have the Altec output transformers, I would grab a couple of Edcors and call it a day.  The Altecs are overpriced due to all the hype and all. Not saying they're bad, but for the money you could get something better from Cinemag, etc.  Or get Edcors and spend the money on your input transformers instead, where they'll arguably make more of a difference.

I don't get the trafo->pot line input.  Probably better to have two stages with the pot in between.

EDIT: read through this thread for some ideas:
https://groupdiy.com/index.php?topic=41509.0


thomasdf

Re: Ampex 600/601 Tube mic & Line amp
« Reply #34 on: August 18, 2019, 04:34:59 PM »
Hi all !

It's been a while but I finally got back to that Ampex project.
That stereo thingie mic / line amp was a bit over the top, so for the moment I simply rebuilt a single channel of tube preamp and DI, point to point.

I get a very distorted sound, and I realised my main PSU output is WAY above the expected values. Instead of 250 and 275VDC I get around 350 and 370VDC.
Could this be the reason of that distorted sound?
I think this could also be related to the fact that I use less tubes than in the original design without any PSU redesign except for the capacitors values.
How can I lower the voltages? Bigger R148 and R149 ?

PRR

Re: Ampex 600/601 Tube mic & Line amp
« Reply #35 on: August 18, 2019, 10:09:59 PM »
> I realised my main PSU output is WAY above the expected values.... 370VDC.

So? Tubes love voltage. (Until something smokes.) That's not your distortion.

Measure voltages at all points. An unhappy tube usually shows unhappy voltages.

The plan posted earlier has V102B working into a large/infinite voltage divider with the Line Level pot. It may not be able to hump enough level into V105A for good output.

thomasdf

Re: Ampex 600/601 Tube mic & Line amp
« Reply #36 on: August 19, 2019, 04:52:13 AM »
For the moment, there is no line input, no pot. Just the 2 resistors voltage divider. Should I bypass that ?

The other thing I added was DI input : straight into the first tube grid through a 0.1uF 400V cap. 

I must precise : output is very very low, no low end at all and very distorted. Sounds like a filtering thing or impedance mismatch...

EDIT : sound is fine before V105. Wired a croc clip from the voltage divider to the output. So the problem seems to be around V105 because after that tube it sounds crappy. I'll keep on investigating ..
« Last Edit: August 19, 2019, 06:58:09 AM by thomasdf »

PRR

Re: Ampex 600/601 Tube mic & Line amp
« Reply #37 on: August 19, 2019, 09:34:06 PM »
Measure voltages at all points. An unhappy tube usually shows unhappy voltages.

thomasdf

Re: Ampex 600/601 Tube mic & Line amp
« Reply #38 on: August 20, 2019, 11:24:19 AM »
You were right PRR, I realised the heater supply wasn't properly wired on V105.
Now it's fine and the preamp is working. Well its more of a fuzz box than a mic pre (after 2,5 it is in distortion territory). And I have some 50hz hum to get rid of..
Thanks for your help.

extempo

Re: Ampex 600/601 Tube mic & Line amp
« Reply #39 on: July 15, 2020, 02:45:50 PM »
Hi there. Hoping I can piggy back on this thread, as I've just finished building  a 600 into a single channel 601. I totally gutted the chassis and rebuilt with all new parts. Using an RCA MI 12399A for the input transformer and a stock 602 output transformer.

I'm using the fixthatmix.com guy's circuit: https://fixthatmix.com/DIY/main_circuit.html

The preamp works after completing the build, and sounds pretty good! One thing I'm struggling with though is that I get exacerbated 60 Hz hum if I have the 250k pot which feeds the 12AU7 positioned anywhere but just right, close to 50% . With this pot positioned like this, there is very little 60Hz hum. Any ideas on why this occurs? This pot should function more or less as a master volume control as I understand it.

Another questions I have, is how tolerant are the heaters to a bit of overvoltage? I'm using the stock heater supply scheme, straight from the 6.3v windings of the power transformer, and I'm measuring about 6.9 VAC.

I pretty new to tube stuff. Thanks in advance!