Latest Design - Tube channel strip

[ruffrecords]

No reason why not as long as you are careful with the layout.

Cheers

Ian

 

[BluegrassDan]

I feel smarter now than I did a while ago!

 

[alexc]

I did my tube opto based  channel strip as :

- AY7 in front, one half as a mic/line/DI high gained stage (120K plate load) , other half as a regular gained (67K plate load) eq makeup amp.

-AU7 interstage, one half regular gained (47K plate load)  driving a hiZ line output traffo (20K:600) as well as the other half, setup as a cathode follower (no transformer)  ...  driving the limiter section.

- standard LA2A limiter

All works out pretty well - noise can be very low if done well; I used the drip version 1 la2a pcb - very low noise in a compact area.
(his later boards are twice as big!)

The psu traffo is hanging out the back - small Hammond unit good for la2a current needs plus an extra 20mA of HV and 600mA of heater ...  for the front end tubes.

The small 'outriding' psu traffo is good with respect radiated hum into the traffos and hiZ circuit portions .....

...  but to *really*  get the most sonics out the thing, I would redo with an external PSU with multi-pin umbilical

I do the external psu thing with all my many-transformer'd boxes nowadays ... the extra space in a psu module also allows the use of a psu choke/rectifier tube  and/or    voltage regulator(s) as optional extra. 8)

...  it's that extra bunch of dB hum relief that really makes complicated tube builds sing *most* nicely!

As well as giving a lot more space to use in the main signal unit - which all translates to lower noise and improved usability.

ps - I prefer a 'hard wired' preamp+eq+limiter  signal flow -  I did the 'flip' eq/limit switch and found (in my case) the extra wiring and switch degraded performance somewhat, so it was scratched.

pps - if you have a 'passive' vu meter hanging on the outputs, make sure it is 'switch defeatable' - it adds quite a bit of mid-band distortion, and not the nice kind! Or you can use a buffer module, like the JLM unit (which has a single supply rail but  fully balanced inputs)

....

My 'Vintage Channel #1'  has been most most used diy box of the last 10 years  8).  It records exceptionally well with some very nice 'crunch' available  in addition to 'clean', using  the 'Gain' and 'Volume' pots of the preamp as well as the 'Level' pot of the la2a section.

good luck with it

 

[BluegrassDan]

That sounds like a really cool channel strip. My basic channel is as follows (at least, as of tonight).

Preamp is both sections of 6072 followed by double cathode follower 12AU7 (pretty close to the Hamptone schematic with some changes).

After the coupling CAP, have an "EQ/Compressor bypass" switch.

Next, the DCF goes straight into the LA2A circuit (minus the input transformer) which uses one half of a 6072. The LA2A's 12BH7 cathode follower goes into the EQ section. The EQ makeup gain is the other half of the 6072 followed by a DCF 12AU7.

Essentially, it's like having another section of the mic preamp to serve as makeup gain for the compressor and EQ. Except each half of the 6072 serves either the EQ or compressor, and each one has its own 12AU7 double cathode follower. (Although, maybe I'll halve-up the 12AU7 and do it that way. We'll see.)

 

[merlin]

You must have tested some really bad 12AX7s. The ones I tested were a lot better than that.

IIRC you were using a CCS load(?) whereas into a typical 100k-200k resistive load you'll get around 0.4 to 0.5% @10VRMS, assuming a decent HT voltage and a decent tube. If you're measuring much better than that, suspect your instruments.

 

[ruffrecords]

IIRC you were using a CCS load(?) whereas into a typical 100k-200k resistive load you'll get around 0.4 to 0.5% @10VRMS, assuming a decent HT voltage and a decent tube. If you're measuring much better than that, suspect your instruments.

I see we are measuring different things/topologies. All my measurements were of mu followers in order to get closer to the intrinsic distortion of the tube. So when I say the distortion of the 12AU7 is a lot worse than the 12AX7 I mean the intrinsic distortion. Of course the topology you use can make a lot of difference to the distortion in an application but the intrinsic distortion at least tells you how low you might be able to go. As you know, triode distortion is inversely proportional to anode load and with a 12AX7 with an rp of 80K or more it is hard to achieve a plate load much greater than two or three times rp with a simple resistive plate load. With tubes like the 12AU7 with a much lower rp it is much easier to build a circuit with a much greater anode load to rp ratio. This tends to reduce the 12AU7 distortion whilst the same topology tends to increase the 12AX7 distortion.  However, if you compare intrinsic distortion the 12AX7 is much better. I seem to recall measuring figures of around 0.1% for the 12AX7 in a mu follower at 10V rms output. I was surprised because it was much lower than I had expected but I tried several tubes and they were all much the same. I will see if I can find my original results.

Cheers

ian

 

[alexc]

Sounds like you have the biz 

If you do with care, a tubey  channel strip built around an la2a is about as good as it gets. Period.

I do also like discrete front ends for sure ...  I've been getting some great results with dfe modules driving tube modules. 
No shortage of excellent doa modules out there!

It's all good - I'd like a  redo on an la2a with  ay7 in place of each of the ax7s for sure .. with adjustment of the gain staging to accommodate.  The first ax7 stage is where the replace would do the most.  Even more 'cj' cool, would be a build with octals

 

[5v333]

Ian, i found your text on intrinsic distortion in different 6sn7. cool to see that EH tubes holds up so good.

are there more experiments like this in your archive?

would be interesting to see how good other new brands  holds up to spec´s

 

[ruffrecords]

Ian, i found your text on intrinsic distortion in different 6sn7. cool to see that EH tubes holds up so good.

are there more experiments like this in your archive?

would be interesting to see how good other new brands  holds up to spec´s

Yes, I tested a whole range of different types and makes of tube in the same basic way. It is a simplified version of the method used in Morgan Jones' book. I recorded them in a cheap accounting book because it already had nicely laid out columns. I think I posted it already here but heaven knows where.  I will have a look for them.

Cheers

ian

 

[5v333]

you can read alot on forums about what people think about different tubes they have bought for their hifi and studio stuff.

everything is a bit chaotic i think because
alot of people talk about sound. this is highly subjective and can be missguiding.
there is the issue with new tubes that either fail very early in life or dont live up to specs.

i guess studdys like yours are great in knowing what/who to trust when buying tubes.

my local shop started selling tubes quite recently. he is not experienced with tubes but recently, customers have reported bad stuff about the tubes he sell. so his planns are to use a tube tester of some sort, buy quantitys directly from factory, test tubes and send the poor ones back. this is great i think.

i wonder if this is something missing from the old days. testing of tubes before they go to the shop. perhaps also test during 10-100hours to see if they hold up after some burn time. whats your thought about burn time?