Help with Gates tube mic preamp???

[emrr]

Very cool.  That's officially the first home made SA-70 I've seen.    Play with the #2 feedback stuff too.  Try the C4 low boost first and see if you even notice it.  Sort of like a tape machine head bump on the original.  The C8 boost is a nice bit of air.  I didn't have it in my restorations for awhile, and once I tried it, left it in all channels. 

 

[gary o]

Hehe thats how I likw to see diy I think its the best way to DiY and end up with something that sounds good my SA70 is still naked havent eaten all the biscuits yet...

heres a scary pic the bunch of wire on top of green thing (1176) is the Gates SA71 believe or not it doesnt even hum even like that the wire to its right is the other gates....why do one thing at a time when you can do 25

 

[craptical]

Wow, that beats my "Simple 6SN7 Preamp" by far ;D

 

[emrr]

If you look at the Gates SA-70A pic again, you'll see your simple 6sn7 looks almost like the actual shock mounted frame the circuit is built upon!

 

[gary o]

Ha Craptical I did a double take at yr 6SN7 pre I thought it was one of mine I didnt remember posting the pic ha ..... is that yr own circuit or a Gates too or something else ....does it sound good thats most important

 

[craptical]

Actually it´s NYDave´s circuit (thanks! ), "Simple6SN7preamp", posted a while ago in this thread:

http://www.groupdiy.com/index.php?topic=12637.60

Sounds great, of course, but I haven´t tested it thoroughly. There´s a (very) slight hum, but will be taken care of when built proper! The frame is taken from some old lab gear I tore apart long ago. : So little time, so many pres... I really love the BA2C I built, by the way (thanks for your help, emrr!)!

/Dave

 

[rascalseven]

I'll try those feedback caps, Doug, thanks.  I'll have to get it back from a buddy who just borrowed it for a session tomorrow, though  :

The BA-2 circuit is next....

JC

 

[gary o]

Get soldering JC.... what transformers do have for your BA2 ? Id kiken to hear wahat you think of BA2 sound I like mine I have wrong transformers tho......Im currently fiddling with a Redd47 pre I made a while back sounds good but not best iv heard Iv read lots of people raving about it maybe its because beatle connection.....

 

[emrr]

Note that a BA-2 with this output will be a compromise, since it's not rated for DC current.  Resistive B+ feed will be an entirely different beast.

 

[rascalseven]

Gary, my transformers are UTC HA-100 for input and HA-113 for output.  The 113 can't handle DC on it's primaries, as Doug points out.  So I'll have to resistive couple the B+ and stick a DC blocking cap before the output, so yeah, it'll be somewhat different than the actual BA-2. 

I suspect I'll like it though, and am looking forward to comparing it to the SA-70 proto I just finished.

JC

 

[gary o]

Its great fun trying these circuits I love it I have a modern big Sowter on my BA2 oputput and old RCA for input I like the sound tho I thinl I like the SA71 better as Doug & you say its not gonna be same type a circuit it may nobble it a bit.....your planning some other builds arnt you ....would be interested to hear how they sound....did I read you only want to use octal valves ? if not I have schematics for all I have built about 12 pres most schematics are here somewhere anyway .....good luck let us no how yr doin.

 

[craptical]

I used an old Kenyon rated "6C5 to 500 ohms" ;D as output in my BA2. Next up will be a cleaned up pair with 6C5´s instead of 6(S)J7´s and A25´s on the output (can´t afford LS iron ). Gary, I´d really like to see those schemos if you´d care to share (please!)!

/Dave

 

[gary o]

OK most are already here ifn theres particuler ones you want let me know il see if iv got them id be interested in anything anyones got tube pre wise I think some of mine came from Doug cheers Doug ( I bet his got a load more hehe)

http://www.groupdiy.com/index.php?topic=7292.20 link to layout & schematic in this thread

RCA here
http://www.waltzingbear.com/Schematics/RCA.html

one of Dougs a Clollins in this thread
http://www.groupdiy.com/index.php?topic=3397

rafas MB1
http://www.geocities.com/rafafreddy/DIY/pulteq-mb1/Pulteq-MB1.htm

Mccudy au300 I love this pre lots of gain within this thread is schematic
http://www.groupdiy.com/index.php?topic=5795.0

CJ V72 and more
http://www.groupdiy.com/index.php?topic=17563.0

 

[craptical]

 

[rascalseven]

Yesterday I read some info that stated (and was supported by our own PRR) that the 6C5 may really be a 6J7 internally connected as a triode.  That being the possible case, I swapped the 6J5 (triode) I was using on my SA-70 with a 6J7 (built an octal socket-to-octal plug adapter to do this), and gave it a listen.

Nice, full, round tone -- definitely thicker and with a little more compression it seems.  Not a bad sound at all, though not quite as clear/natural as the 6J7/6J5 combo.  Both would be nice for different things.

My SA-70 doesn't use the caps in the feedback for frequency shaping... just flat.

I haven't built the RCA BA-2 yet, and notice that it too have a filter between stages, so maybe with this bit of HF lift (that's what it is, right?) it might just be a great, colorful, rich, but clear pre.  Still waiting for more sockets to arrive before I can build that one, but just wanted to chime in on my current test.

JC

 

[craptical]

Yesterday I read some info that stated (and was supported by our own PRR) that the 6C5 may really be a 6J7 internally connected as a triode.

Suggested sub for use in the BA2 as well (by emrr), but the other way around:

http://www.groupdiy.com/index.php?topic=26675.0

 

[rascalseven]

So, Craptical,

That pre you built based on the BA-2, you used 6SJ7's?  Are those just a different pinout version of the 6J7's?  Your description of the tone (good midrange detail yet lacking highs) is consistent with my dual 6J7 SA-70.  Did you ever put those 6C5's in?  If so, was there a difference, sonically?

Have you, by any chance, tried 6J5's as well?  If you're running 6C5, I would love for you to try a 6J5 in its place (drop-in substitute) and tell me if you think it clears the sound up a littler, making it more detailed, more natural but still very toneful.

I also added a switch to lift the feedback for 50dB of gain and little more character.  In this feedback-less mode it is quite similar to the BA-2, so I am wondering what your experience is with the different tubes.

When my stuff arrives I'll throw together a BA-2 for some experiments myself (using the same UTC iron I have in my SA-70) and roll the tubes, but just wondering if you have any experience with this to share.

Thanks!

JC

 

[emrr]

I'm not sure that I said it anywhere, but my real SA-70's sound like ass if you use a 6J5 rather than a 6C5.  This surprises me, but it is so.   

I probably also said elsewhere that I've heard at least one knowledgeable collector/historian type swear that 6J7's are garbage compared to 6C5's.  I still have not busted one open to verify.    In theory, the 6J7 has a better shot at low noise and input capacitance with the grid being at the top cap versus close to everything else.  But, the grid wire itself will have capacitance, and if you don't use proper low capacitance wire you will have losses there.  And the grid cap shield could be critical to noise in some environments and builds. 

Your clone SA-70 with UTC's will probably like the high boost part of the feedback circuit.   Mine look totally flat with it in place. 

Are you judging tonal differences based on only 1 or 2 tubes?  If so, you really don't have a real basis of comparison, other than those particular tubes.   You would need to listen to a half dozen of each type to even get started with saying which type is preferred.   And be sure to burn each and every one in for a few days before listening.  And make sure your warm up time for each tube is comparable between comparisons.  Really.   

 

[rascalseven]

Good thoughts, Doug.  I have a few different types and brands of tubes coming for the purpose of more useful testing, so I'm there with you.

I'll also try the filter to see what that does. 

When you say that it sounds like "ass" with a 6J5 I feel I have to take that with a grain of salt.  I may be nitpicking here, but such descriptions, to me, fall into the category of the "plug-ins suck" and "digital is shit" comments made by people on Gearslutz who read too many magazines and have no ears of their own.  You, Doug, clearly have a wealth of knowledge and experience, so an "ass" description from you DOES make me want to investigate the matter more, but is it really, to your ears, that dramatic?  I really enjoy the sound of this preamp with the 6J7/6J5 combo (more so than my G9 and a buddy's VAC RAC preamps).  If this is 'ass', then I want to hear what a good one sounds like!    I have a couple of NOS 6C5's on the way that I'll be trying as well.

What tube compliment do you prefer with the SA-70?  The stock 6J7/6C5?  What about with a BA-2 (I'll wrap up a BA-2 when my sockets arrive this week).

Also, do you have the feedback caps in all of your SA-70's, or just some?  I have yet to get those in, but will shortly.  And what transformers are in yours?  I remember you said UTC's, but which models?

This is all very interesting.  I'm looking forward to rolling some tubes with this and the BA-2 when it's finished.

Thanks, Doug, for you time and for sharing your knowledge.

JC

 

[emrr]

In a direct comparison between 6J5 and 6C5 in my SA-70's, the 6J5's all sounded broken.  Not subtle at all, and it's always been a mystery too me.  It shouldn't be that different, and they list as subs for one another in most sub guides.  I do not at all get tweaky about specific tube types, and run a pretty wild and random mix of stuff.  This specific observation stood out more than any other I've ever experienced with a reasonably similar tube in the same circuit.  My 6J7's are all metal.  They've been less noisy than the glass 6J7GT's I've tried.  I'd use 1620's in every 6J7 position if I had enough, but I save them for higher gain circuits. 

I have all of the UTC feedback options in all of mine.  The larger type seen in the group picture is in a UTC CG type can with a custom part #.  The transformer data sheet reads exactly like an LS-10X, and mentions the LS-10X by name.  The output does not match any other specific stock UTC exactly, as it has an allowable DC spec of 20 mA, but would most closely compare to the LS-27 in all other regards.  Both are definite sonic upgrades over the HA cased Gates iron, which I've had multiple examples of.  Both Gates UTC types easily beat the response charts of the RCA BA-2 iron. 

The SA-70A version with the smaller Triad transformers don't sound as good as the UTC version. I didn't keep any of those, and sold them off as I was able to find more of the UTC variety.