Vortexion 4/15/m

[cpaxton]

Hi There!
This is my first post to the forum...unfortunately it is with cap in hand looking for help!
I Have recently got hold of an old vortexion 4/15/m 4-1 mic mixer. It uses ef 86 valves (1 for each channel and a final output one.) All channels are working, but there is a hum  centred around 100 Hz on the output. The hum is present without anything plugged into the input and also present with something plugged into the input.
I have read on other forums that the unit itself should be pretty quiet if in good condition. The capacitors that are used  are the brand Hunts. I bought replacement capacitors and have replaced some (starting at the output.
I was wondering do these units produce this hum always? it is not unusable at the moment, I was just wondering if I should fire ahead and replace all capacitors in the circuit..or might the problem lie else where?
I have checked the valves to see if they are microphonic and they seem to be in good condition.
I can provide any pictures should anyone need them.
Any help would be greatly appreciated.

Cheers
Conor

 

[JohnRoberts]

I am not a tube guy, but first thing to check for a common problem is probably power supply.

JR

 

[cpaxton]

Hi JR,

Thanks for taking the time to reply. I will have a look. The power transformer seems to be In a screened box so I was hesitant to open it but, I know now I should.

Cheers,

Conor

 

[JohnRoberts]

be careful messing around mains voltage, while tube PS rail voltage can be dangerous too.

The first step might be to look at the rail voltage for low ripple?  But I repeat I am not a tube guy.

JR

 

[PRR]

Welcome.

> I have read on other forums that the unit itself should be pretty quiet if in good condition.
> I was wondering do these units produce this hum always?

You already read that it shouldn't buzz. You want a second opinion? The internet is full of all opinions.

Your wall-power is 50Hz AC. The rectifier makes DC with a 100Hz ripple. The *main filter caps* take off this ripple.

So it is not the power transformer, or the individual stage (except slightly). Find the rectifier and follow it toward the tubes. There will be some large roughly 33uFd 400V caps (possibly several in one can). Replace those.

 

[cpaxton]

Thanks for information PRR, I really appreciate it and will get straight on that.
cheers,
Conor

 

[Rob Flinn]

Hunts are the original caps in the Vortexion, & will be very old now.  I remember one old boy technician saying to me that they spelt the name wrong, implying that he had replaced a lot of them & they don't age well .........  Replacing them should sort it.   

 

[Tubetec]

Some of the Vortexion units go back as far as the late 40 .
I have a couple  of the poweramp/mixer units with the transformer inputs and EF86 input stage , they really added a lot of smoothing/decoupling in these maybe 7 large can hunts caps. Be carefull if you remove the mains transformer ,mark is so when you replace it the orientation remains as it was or otherwise you might just end up with even more hum .Check the rubber seals on the end of the power supply filter caps ,I'll bet these caps have vented and dried out any sign of leakage from the cap will be a telltale, in any case like the advice from the other guys replace all the electrolytics at very least in the power supply ,any others that look suspect ,replace them too . Bear in mind its most likely also that the input transformers are 30/50 ohms ,which is a bit useless for matching anything modern in mic terms ,many old dynamic and ribbon mics were in the 30/50 ohm range ,lustraphone, reslo,tannoy and grampian are some of the better ones to keep an eye out for cheap ,so really to get the best use from your unit you either need to change input transformer/circuitry to a more suitable impedence/gain or find some vintage mics to use with it , as far as replacing the transformer you'll need to find something with a highish secondary impedence for it work right ,NFB is also applied from the ef86  stage output back to input via the lower end of the input transformer secondary to grid. I should have the schem for the Ef86 input stage around here somewhere ,I might have posted it here before .

Probably some simple mods would need to be done to the input stage to make it more suitable for modern useage,in any case it will be a good exercise to bring it 'back to factory' specwise. The warlike casework of the Vortexions is something to behold ,your house could litterally fall down on one of these things and after a quick dust down ,and a slap of a hammer to get the dents out ,you'd be back up and running ,the vortexion marque saw all kinds of industrial uses ,I believe the BBC used one of the power amps fed by a signal generator to allow pitch control of large synchronous mains turntables and maybe even taperecorders,the 100volt line tapping on the output transformer being used to feed the motor with 50/60hz etc.Post a few pictures of the unit if you can .

 

[Rob Flinn]

Attached is a crap quality scan of the circuit diagram (sorry all I had on my pute) , but you can get the gist of how it works.

 

[cpaxton]

Thanks to everybody so far for all the information. There is some solid gold here. I am currently on the 'Hunt' (pun intended  ) for some 33uF, (four in total) and 8uf(single) mains filtering capacitors.  not looking for any NOS stuff or anything...would like to get ones with the same diameter, but its not a deal breaker. the main thing is, i want it working sans hum!
That is really interesting re: the input impedance of each preamp. Would that mean that the microphones intended to work with this box have even lower output impedance? i kinda figured the input impedance should at least be an order of magnitude greater than the output impedance of the microphone? (but i openly admit i don't know so much about these things), so it doesn't have to drive a hard load.
I am definitely going to lookout for the brands of mics mentioned. I have no real want to modify or replay the input traffos. I am quite find of some of Joe meek's recordings, in particular, 'The Cryin Shames: Please Stay', so i can pretend to myself that i will achieve such things with this preamp, hahaha!

 

[cpaxton]

@ Rob Flinn, wow, thanks for the schematic! really appreciate it

 

[Rob Flinn]

Use 10u for the 8u & yu can probably use 47u for the 33u.  Personally I wouldn't touch nos electrolytics with a bargepole.  I have the 3 channel version that has a PPM meter in place of the knob for channel 4.

 

[PRR]

> currently on the 'Hunt' ...for some 33uF,.. and 8uf.. mains filtering capacitors.

Old-time caps started at 4uFd and went 8u 16u 32u

At a later time the odd-numbers 32 64 etc were "rationalized" to 10u 20u 40u etc.

At a still later time they were "rationalized" again to support closer values, more like what was already common in 10% resistors, so 10u 16u 22u 33u 47u....

Remember an "8uFd" cap was rated +100%/-50%.... it might really be 4u or 16u. Modern caps fall closer to their marked values.

10u and 33u should be very common sizes.

The original design was compromised by the high cost (then) of e-caps, and the fact that the mixer would be obsolete and set aside in 5 or 10 years; why spend a lot? Make it just-good-enough. Now that you are finding life in the old box 50 years later, and e-cap prices are low (until tariffs catch-up; even then), you may just up-size. 22u and 47u. Watch voltages, round-up on these also.

> would like to get ones with the same diameter

Today's caps run very much smaller than the 1950s.

If you are into "looks", it is a thing to hollow-out old caps and hide new (smaller diameter) ones inside.