Latest Design - Tube channel strip

[BluegrassDan]

Howdy Gents,

Here is a draft of my next design front panel.

The EQ is sort of based on Ian's Helios 69, but with a fixed 60Hz low boost and 75Hz cut. Mid frequencies are Neve 1073. High is 10kHz. All boosts and cuts are stepped in 2db increments. Makeup gain will be the same schematic as the preamp section, except with a 12AU7 instead of 6072 in V1. V2 is 12AU7 cathode follower.

Compressor is LA2A with all of the radio-era stuff removed (stereo link, preemphasis, etc.). Will be 12.6 vdc, so using a 12AQ5 in place of 6AQ5. Also, gonna do the CJ Mod with 12AY7.

There are still a few things to add...

1. EQ and Compressor "in/bypass" switches.
2. Clearer labels for the EQ and Compressor sections.
3. A +4/GR switch (but, c'mon, does anyone really use anything other than GR?)

Questions:

1. Does it need a "EQ before/after Comp" switch? Am thinking of just having it wired up to be EQ after Compressor, but I'll consider it if everyone thinks it's a deal breaker.

2. Who likes the LA2A "limiter" switch?

3. Any other suggestions? Design-wise, or graphics-wise?

 

[ruffrecords]

Do you need to switch the VU between GR and output?

I tend to avoid using the 12AU7 because of its relatively high distortion. The only thing it has going for it (for me) is its 12V heater option.

Cheers

Ian

 

[merlin]

I tend to avoid using the 12AU7 because of its relatively high distortion.

12AU7 isn't too bad, really. Typically ~1% THD at 10Vrms with a resistive load, or ~0.25% with CCS load.

 

[5v333]

i really like the tri-color design!

i also thought about having a switch for GR/IN/OUT or something.

 

[BluegrassDan]

Ian, is there another 12v, 9-pin, med-low gain tube that you like instead of 12au7?

 

[ruffrecords]

12AU7 isn't too bad, really. Typically ~1% THD at 10Vrms with a resistive load, or ~0.25% with CCS load.

Well, I did say relatively. Almost any other double triode will be better than that. It is a pity really because other than its  poor intrinsic distortion it is a handy tube for output stages.

Cheers

Ian

 

[ruffrecords]

Ian, is there another 12v, 9-pin, med-low gain tube that you like instead of 12au7?

Yes and no. My favourite output tube is the ECC88/6DJ8 and its industrial grade friends but they all have a single 6.3V heater because pin 9 is used as an inter-triode screen. There is a 12V heater version which many years ago were available quite cheaply but someone realised how good they were and bough them all. Now they are very rare and expensive. The tube number is 8416.

http://www.r-type.org/exhib/aag0108.htm

Cheers

Ian

 

[G-Sun]

Looking sexy

 

[rock soderstrom]

If there is enough space, you could use an octal 12SN7, the 12V version of the 6SN7.

 

[BluegrassDan]

How do you guys find the 12au7 works for output as a cathode follower? I have been quite pleased for my purposes, but always eager to hear.

Otherwise, what considerations are there for replacing V1 (6072) with 12AU7? Swapping some resistors?

 

[BluegrassDan]

12SN7 could certainly be doable. Thoughts on distortion, noise, and microphonics versus 12au7?

 

[rock soderstrom]

I had no problems with distortion, noise, and microphonics. I guess 6sn7/12sn7 are among the best medium mu audio tubes you can get. Drawbacks are price and availability (12sn7 is afaik out of production).

 

[BluegrassDan]

Are they that much better, sonically, than 12au7?

 

[CurtZHP]

Very nice!  I've kicked around doing a channel strip as a project, but found the idea of scratch building an EQ and compressor daunting.  (Just getting the hang of preamp stages!)

Not seeing the schematic, does the compressor tube stage also act as a second gain stage for the preamp?

How about an effects loop, in case someone wanted to patch in a reverb or such?

The "EQ before/after Comp" switch is a cool idea, but sounds like a beast to wire.

 

[rock soderstrom]

Are they that much better, sonically, than 12au7?

I don´t know. I had never an opportunity to compare them directly because of their different sockets.

 

[mjrippe]

How about 12BH7, like an LA-2A output stage (White Cathode Follower)?

 

[ruffrecords]

How do you guys find the 12au7 works for output as a cathode follower? I have been quite pleased for my purposes, but always eager to hear.

Otherwise, what considerations are there for replacing V1 (6072) with 12AU7? Swapping some resistors?

12AU7 has mu of only 17 compared to 44 for a 6072. That's the main problem.

edit:  You could always consider a 12DW7 - this has one half of a 12AX7 and the other half is a 12AU7. Use them together with some NFB to combat the 12AU7 distortion and you could have quite a nice gain stage. Apparently they are back in production now.

Cheers

Ian

Cheers

Ian

 

[ruffrecords]

12SN7 could certainly be doable. Thoughts on distortion, noise, and microphonics versus 12au7?

6SN7 and friends beats the pant off the 12AU7 for distortion.

Cheers

Ian

 

[ruffrecords]

Are they that much better, sonically, than 12au7?

Much better. Anything from 10 to 20dB less distortion.

Cheers

Ian

 

[ruffrecords]

How about 12BH7, like an LA-2A output stage (White Cathode Follower)?

That is not a bad contender. Definitely an improvement on the 12AU7 but its is a proper big power tube. Much taller than the AU7 and takes twice the heater current. I used it once in a headphone amp.

Cheers

Ian