EZ Tube Mixer

[ruffrecords]

I have been asked what is the tube complement for this design. The answer is:

I have tried to use tubes that are readily available. Each PCB uses a pair of 6922 and a single 12AX7 tubes. The PCB houses two complete amplifier stages that each use a single 6922 and half the 12AX7. Each amplifier can have its gain varied from 6dB to 40dB by altering a single resistor. One stage is tracked to the input transformer and switches (gain,  phantom, pad, phase and mic/line). The other stage is used for gain make up after the external fader and separate passive EQ and has a pre-set pot to set the gain.

The 6922 tubes I have had best results with are JJ E88CC ( a 6922) and Electro Harmonix 6922EH. Both are in current production but the NOS Russian 6N23 also performs just as well. Please note that you cannot use a 6DJ8 (ECC88) instead of a 6922 because the 6DJ8 does not have a high enough plate voltage spec. For the 12AX7 I use a Sovtek 12AX7LPS because it has the lowest distortion of any 12AX7 type I have measured but you can use pretty much use any 12AX7 or ECC83.

I am in the process of drawing schematics and will post them soon. In the meantime here is a pic of what I hope will be the final PCB layout.

Cheers

Ian

 

[rmaier]

Very interested in a couple of these pres, and possibly a few Eq boards as well - just not sure which ones yet. I see you've given a number of options for transformers. Looking forward to hearing your impressions.

Ralph

 

[ruffrecords]

I have just finished the basic documentation for the 3 band Pultec EQ PCB which you can find here:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/3BandPultec/

I shall also be adding this link to my White Market page which gives details of how to buy these PCBs.

Cheers

Ian

 

[ruffrecords]

And likewise here is the basic documentation for my tube version of the Helios Type 69 three  band EQ:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/Helios69EQ/

Cheers

Ian

 

[Kid Squid]

Fantastic stuff Ian !!

I've been away from the forum for far too long, it seems 

Steve

 

[ruffrecords]

Fantastic stuff Ian !!

I've been away from the forum for far too long, it seems 

Steve

Thanks for the kind words Steve - nothing seems to stay still around here for long!!  Have you seen my new White Market page??

Cheers

Ian

 

[letterbeacon]

This is indeed very exciting!  I'm watching from the wings, but I won't be able to buy anything for a little while, not until I a) earn some money (freelance woes) and b) convince my girlfriend we DEFINITELY need a tube mixer in the flat.

 

[ruffrecords]

I just placed the order for the new version of the mic pre PCB that takes the three different alternative mic transformers (Sowter, Cinemag and Jensen). They are already listed in my white market page.

I spent some of today drilling and filing to put together a first prototype of a complete channel with three band Pultec EQ. Here are a couple of pics:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/pultecchannelp2.jpg

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/pultecchannelp1.jpg

Not all the pots are fitted because I discovered two values I had been sent had plain shafts instead of splined.

And below is a screen grab from the final PCB layout of the three transformer PCB.

Cheers

Ian

 

[ruffrecords]

I forgot to mention, here is a pic of my Front Panel Designer layout for the 3 band Pultec EQ version of an all tube channel amplifier.

Cheers

Ian

 

[wave]

Ian,
This is super cool! Can a fader be implemented into each channel? Is there a separate board for a mix bus?
I have been on your White Market page and had a look at your documentation, but you know how long it takes for the penny to drop for me!

Dave

 

[ruffrecords]

Ian,
This is super cool! Can a fader be implemented into each channel? Is there a separate board for a mix bus?
I have been on your White Market page and had a look at your documentation, but you know how long it takes for the penny to drop for me!

Dave

Yes, the channel board designed to be used with an external fader, either rotary or slider. There's basically two amplifiers on the channel board. One is pre-configured as a mic pre with transformer input, gain, phantom, pad, mic/line and so on. Its output can be fed straight to a regular 10K fader and then into the passive EQ (either Pultec, Helios or your own on a separate PCB). The second amplifier is then used as a gain make up amp for the EQ. Its output can go to an external transformer for a balanced out and/or to a mix bus (via routing and pan).

There is no separate board for a mix bus. You just use the channel board but leave out the mic input parts (transformer and switches) and use it for passive mixing and because there are two amps on the board you can do a pair of buses on one PCB. There's provision for fitting pre-set pots for tweaking the gain make up.

That's one reason I call it the EZ Tube Mixer. You only need two types of board, the channel amp and an EQ board.

And by changing a few resistors and a couple of tubes and adding a couple of external output transformers, the channel amp board converts into a nice little all tube stereo headphones amp.

Cheers

Ian

 

[wave]

Ok I think I get it. So for an 8x2 mixer I would need 9 channel boards?
When wiring the channels to the board that is used for a mix bus, are the channels just wired up in parallel to the inputs of the bus channel board?

Dave

 

[ruffrecords]

Ok I think I get it. So for an 8x2 mixer I would need 9 channel boards?
When wiring the channels to the board that is used for a mix bus, are the channels just wired up in parallel to the inputs of the bus channel board?

Dave

Essentially yes. You feed the outputs from your routing (probably a pan pot in an 8x2 mixer) via resistors to the mix bus. The mix bus is just a wire that parallels these all up and then goes into the input of the bus channel board. The mix bus is literally a piece of wire that runs along the connectors that the channel boards plug into.This is regular passive mixing and relies on the bus resistor value being much larger than the impedance driving it.

Cheers

Ian

 

[ruffrecords]

Just updated the documentation for the 3 band Pultec to include a BOM. New pdf is here:

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/3BandPultec/P3bandDoc.pdf

I'll be doing the same for the Helios version over the next few days.

Cheers

Ian

 

[bieckmusic]

Hi Ian!

I have a quick question.  I have boards for the poor man's pultech, with the mid boards.  I could never get the cap calc to work on my computer.  So could I just use the cap values from this new bom, with my poor mans peq1-pme1?  I'm not clear if these cap values would be affected by any changes to your new boards...inductor choice, etc...

thanks!

greg

 

[wave]

Greg,
Do you have a gmail account? The cap calc works perfectly in google docs.

Dave

 

[ruffrecords]

Hi Ian!

I have a quick question.  I have boards for the poor man's pultech, with the mid boards.  I could never get the cap calc to work on my computer.  So could I just use the cap values from this new bom, with my poor mans peq1-pme1?  I'm not clear if these cap values would be affected by any changes to your new boards...inductor choice, etc...

thanks!

greg

Yes can can use the values from this new BOM. All the hi and lo values are the same as the PMEQP1A and the mid ones are chosen to mimic the Pultec MEQ5.

If you have trouble getting the capcalc spreadsheet to work I would recommend downloading the free office suite Libre Office:

http://www.libreoffice.org/download/?type=win-x86

Which will definitely open it successfully.

Cheers

Ian

 

[bieckmusic]

Thank you!!

greg

 

[anjing]

Hi Ian!

  I really interested in this project … Would it be possible to have a partial bom of the most expensive components for the ez mixer eurocard. so i can evaluate an approximate  cost per channel and see were i can get the parts from.

Regards,

Pierre

 

[ruffrecords]

Hi Ian!

  I really interested in this project … Would it be possible to have a partial bom of the most expensive components for the ez mixer eurocard. so i can evaluate an approximate  cost per channel and see were i can get the parts from.

Regards,

Pierre

I'll be drawing up a complete BOM in due course, but in the meantime off the top of my head the most expensive components are:

Mic transformer (Sowter, Cinemag or Jensen)
Grayhill 1P12W 71 series switch
6922 tubes (2 off)
12AX7LPS tube
PCB

which will probably cost around 150 to 200USD in total.

After those, the most expensive component will probably be your front panel.

Cheers

Ian