Custom console rebuild

[Dmichel123]

We recently purchased a late '50s/early '60s custom-built tube console from eBay. It has 5 Hi-Z mic inputs with a single 5879 per channel. The "program" line amp consists of a shared plate 12AU7  as a mixer (one half for mic mix bus, other half for line/TT/tape/radio bus), and another 12AU7 as a gain stage and cathode follower for the output.

I have started work on customizing the console for our needs here in the studio. I've rounded up some input transformers for the mic pres and likely have a suitable transformer for the output (600:600?) around here somewhere. I have removed the second row of knobs (TT/tape/radio/etc. inputs) and will make these aux sends for the mic pres. Each aux send pot will get it's signal straight from each channel volume pot, into aux bus, straight to aux send jack. The aux return will come in at the 12AU7 mixer. 

I am working on drawing a schematic for the full console. For now, I have drawn up the preamp circuit with as-built values, plus aux send pot and input transformer. Originally, the pres fed into the volume pot at the wiper. I've moved that to the top and added a 250k resistor after the wipers into the bus feeding the 12AU7 mixer. I will add 100k Rs before the AUX bus.

Any tips or corrections are greatly appreciated! If you see any changes that could improve the pre circuit, let me know!!

Thanks
Danny

 

[Dmichel123]

Forgot to add, R6/R7 and R8/R9 are pots.

 

[ruffrecords]

Is there no decoupling across R5?

Some pics would be nice too.

Cheers

Ian

 

[Dmichel123]

Is there no decoupling across R5?

Some pics would be nice too.

Cheers

Ian

No screen bypass caps as originally built. I can upload some photos we took before modding started, it's in pieces now. We had the faceplate sandblasted and powdercoated the other day.

 

[Dmichel123]

Here's a photo we took before disassembly.

 

[Dmichel123]

Here is a quick schematic. R13/R14 is a pot. I think I can delete R21 in the circuit.

 

[Dmichel123]

I meant to add in the original post that this console will be a constant work in progress. I plan on eventually adding another tube and OT to every mic pre and doing the mix bus 600ohm and balanced, with inserts on each channel that can be used as direct outs if needed. 

We record and mix live, mostly. We typically run our tube consoles/mixers into our 2-track Ampex 350, with the band on one tape track and the vocals on the other, and the band is mixed live. Vocal/Band mix is done digitally, as well as any overdubs. Occasionally, we will record and mix everything live to a single track.

 

[bluebird]

I would watch out for those high value pots after the pre amp stage and after the mixer. You may have some top end being rolled off with a 1M pot there.  Might be fine but something to be aware of.
I would also consider transformer summing,  you could get away with lower value summing resistors. Dont know how much of a load the pentode can work into without a lot of distortion, but it seems like theres going to be a lot of noise and cross talk in the circuit as it stands.

 

[Dmichel123]

I would watch out for those high value pots after the pre amp stage and after the mixer. You may have some top end being rolled off with a 1M pot there.  Might be fine but something to be aware of.
I would also consider transformer summing,  you could get away with lower value summing resistors. Dont know how much of a load the pentode can work into without a lot of distortion, but it seems like theres going to be a lot of noise and cross talk in the circuit as it stands.

Thanks for the tips. I plan on adding at least one triode and an OT to each channel, eventually.  For now, these pots are there and I think will suffice until we upgrade the console.

 

[Dmichel123]

I just noticed that I messed up the filter arrangement in the PSU. Here's the corrected version.

 

[ruffrecords]

Since the mic pre stage is a pentode, its output impedance is essentially the plate resistor or 100K in your case. This is loaded by a 1Meg pot which in turn is worst case loaded by 250K so it could look like a 200K load. Also you have a 500K aux send pot which in turn is loaded worst case by 100K so it could look like  an 83K load. With both pots turned up full the worst case total load could therefore be as low as 200K in parallel with 83K which is about 59K. This is just over half the pentode output impedance whereas in the original design without the aux pot is would have been 200K worst case which is twice the pentode output impedance. This is likely to be detrimental to the performance of the pentode stage.

Pentode stages without decoupled screen voltages are rare. It does reduce the gain, and maybe that was the intention, but undecoupled stages also inject any HT noise straight into the tube. Talking of gain, you have 20dB from the input transformer and perhaps as much as 40dB from the pentode stage. You lose 14dB in the passive mix but gain about 20dB in the output stage giving an overall gain of around 66dB. Nearly all this gain is in the mic pre which means it will most certainly have headroom problems, especially driving a 59K worst case load.

The output impedance of the AUX send bus will at best be 20K (100K divided by 5). This means whatever you feed it to must have an input impedance several times higher than this. The AUX send pot is post fader - is this what you intended? The 0,022uF cap feeding the AUX send pot has a worst case -3dB point of 87Hz. Is this what you intended?

The HT supply provides about 57dB of ripple attenuation for each of the three separate HT feeds. For the pentode stage this is almost certainly not enough. You could usefully increase this by 20dB for that stage  by increasing C9 to 220uF

Cheers

Ian

 

[Dmichel123]

Ian, thanks for your help.

The schematic actually isn't EXACTLY as originally built. The mix bus WAS wired directly to the top of all the 1 Meg pots(no 250k), with the coupling cap coming in at the wiper! I knew enough from working on guitar amps to rewire the volume pots and that I needed the resistors after. 

Would you suggest lowering Ra or raising the 250ks/100ks on buses to fix load impedance? Both? What size bypass caps on screens? .1uf big enough?

Any suggestions for getting the best performance in this configuration? I have lots of upgrades planned for the future, adding OTs for Insert/direct out capabilities, and more...

I have deleted the .022uf caps at the aux sends, I was indeed trying to cut bass there. I figured I can do it with one cap after the bus instead of one at each pre. The echo send will be used, for now, into a small tube amp to drive a plate reverb.

I will look into better filtering if I notice hum. It seemed very quiet when we tested it.

 

[PRR]

> The mix bus WAS wired directly to the top of all the 1 Meg pots(no 250k), with the coupling cap coming in at the wiper!

That's perfectly valid. See Fender DeLuxe. Also some TEACs. Works better with pentodes.

 

[Dmichel123]

> The mix bus WAS wired directly to the top of all the 1 Meg pots(no 250k), with the coupling cap coming in at the wiper!

That's perfectly valid. See Fender DeLuxe. Also some TEACs. Works better with pentodes.

You're right. But with the new arrangement, I can crank up the reverb with the dry signal down. I'm thinking I wouldn't have been able to do that with the stock mix circuit. 

PRR, do you have a favorite circuit for a single pentode mic pre? Any suggestions on tweaks to these pres?

 

[ruffrecords]

Would you suggest lowering Ra or raising the 250ks/100ks on buses to fix load impedance? Both? What size bypass caps on screens? .1uf big enough?

Any suggestions for getting the best performance in this configuration? I have lots of upgrades planned for the future, adding OTs for Insert/direct out capabilities, and more...

Depends how far you are prepared to go. If you keep the pentode configuration your options are limited. For instance  this stage has already had its load doubled and adding a direct out transformer will only load it more. Basically all the problems stem from the first stage being a pentode so the first option to consider is rewiring it as a triode stage. This will drop the gain to 20 something dB which with the 20dB in the input transformer gives you a more reasonable 40dB or so first stage gain. The other benefit is the output impedance drops to less than 20K so driving the pots you have in there is no longer a problem. However, driving an output transformer is still not really an option. For that you would need even lower output impedance and a much higher current drive capability.

Cheers

Ian

 

[Dmichel123]

Thanks Ian.

I have considered triode strapping the 5879s. Nearly every old preamp or console we've had with pentodes (Gates, Raytheon, RCA) had them wired as triodes. This is good for noise, right? When I mentioned adding OTs, that mod will include adding a triode (maybe 6C5/6J5). I should mention that we use ribbon and dynamic mics almost exclusively(we use an Altec M11 for drum overhead).

 

[ruffrecords]

Thanks Ian.

I have considered triode strapping the 5879s. Nearly every old preamp or console we've had with pentodes (Gates, Raytheon, RCA) had them wired as triodes. This is good for noise, right? When I mentioned adding OTs, that mod will include adding a triode (maybe 6C5/6J5). I should mention that we use ribbon and dynamic mics almost exclusively(we use an Altec M11 for drum overhead).

A triode strapped pentode is almost always quieter than the pentode version because the pentode suffers from partition noise which the triode does not. There are lots of cources of noise in a mic pre and they all need to be managed if you want to minimise noise.

Ribbons and dynamic generally have a low output compared to condensers like the M11 but how much gain you need depends on how loud the source is and how close you mic it.

Cheers

Ian

 

[Dmichel123]

Ian,

We record pretty quiet here and the closest we get a mic to anything is 4"-6" on upright bass. Guitar/harmonica amps are mic'd at 12"-18" normally, drums are almost always overhead only, and very rarely we will mic kick or snare with an EV 666R . No hard rock bands, and we make bands play almost uncomfortably quiet. They always "get it" when they hear the monstrous sound at playback.

I'm going to go ahead and triode strap the 5879s and finish up wiring the FX pots bus and get back with my impressions tomorrow after we test it out.

Thanks a bunch!
Danny

 

[ruffrecords]

Ian,
Thanks a bunch!
Danny

No problem. What sort of music do you record and where are you located?

Cheers

Ian

 

[Dmichel123]

No problem. What sort of music do you record and where are you located?

Cheers

Ian

Mostly Blues, pretty much all early roots stuff. We've had a couple cumbia bands in the studio and we do some jazz and R&B. We're in Hayward, CA. Bigtone Records. We recorded a couple tracks on Kim Wilson's (of The Fabulous Thunderbirds) newest release, "Blues and Boogie".