Poor Man's Tube Mixer

[ruffrecords]

Hi  Ben,

For the HT supply I use a TRIAD VPT230-110 in the poor man's mixer design. You can get these at Mouser and Farnell. It has a universal primary so you can use in both Europe and the USA. It has two 110V secondaries which you wire in series for 220VAC which should get you comfortably over 300V raw dc for the HT. You can use something like the HT250 PCB design to drop the volts down to 250V and smooth it. Secondary ac current capability is 110mA so this will give you up to  68mA of HT so it will be fine for 30mA - just use larger dropper resistors and get less ripple too.

For heaters I now use a SMPS usually rated for twice the heater current to account for inrush current. For two TLAs I am right now using a Meanwell IRM-20-12 which is available from Mouser and RS. You can probably also run the VU buffers from this supply.

You are correct that tube amps can be dangerous. That is why the TLA boards have an HT discharge resistor built in so that when you unplug them any charge left on the decoupling capacitors drops rapidly to a safe level. The HT250 boar also incorporates an HT discharge circuit for the same reason. Just be very careful when you have power applied.

Cheers

Ian

 

[Tom snich]

Cheers Ian

I have plenty to be getting on with now, with collecting parts, i will put in an order for PCBs to you soon.
I will probably return with more questions as well. 

Cheers
Ben

 

[Tom snich]

Hi Ian

If you got a minute, i found a transformer @Tube Town that i think will work for the two TLAs what you reckon?
I am also struggling to find the DB1 1000v PIV DIL Bridge for the HT250? none seem to fit that i have found.

https://www.tube-town.net/ttstore/product_info.php?info=p4908_toroidal-56va.html&no_boost=1

Pri: 0-115-220-230-240 V (schwarz-grün-gelb-lila-rot / black-green-yellow-purple-red)
Sec1: 300 V @ 0,1 A (rot / red)
Sec2: 6,3 V @ 2 A (grün / green)
Sec3: 12,6 V @ 1 A (blau / blue)
long leads ca 200 mm
Size ca. h=38mm d=90mm
50/60 Hz EN 61558

Found pretty much everything else, making front panels and framework at the mo.

Cheers
Ben 

 

[ruffrecords]

Hi Tom,

The 300V HT secondary is a little on the high side. This will give a peak HT voltage at the HT250 first capacitor of 424VDC. You could make this work but you need electrolytic caps rated at least for 450VDC and the suppressor cap across the bridge needs to be similarly rated. Do you need the 115V primary tap because if you don't there are probably better transformers in their range with 250VAC secondaries for the HT. Also, if you are going to create dc heaters you need the AC winding to be rated about 1.6A to account for the power lost in the rectifiers and regulator. If you plan to use AC heaters then 1A rating is fine.

Someone else emailed me about the DIL bridge rectifiers. There are plenty around. This is the one a normally use:

https://uk.farnell.com/vishay/df10m-e3-45/bridge-rectifier-1kv-1a-dip/dp/3373276?st=dil%20bridge

Cheers

ian

 

[Tom snich]

Thanks for getting back, heads a bit fried at the mo, its dark in Sweden this time of year, should of read your previous posts, i will be more thorough.

Brexit is a real pain now, having to learn german now to find parts, which is fun!

No need for 115v prim.
This toroidal seems right, and the 12.6v tap will power the Vu meter board. I will use the Meanwell IRM-20-12 you recommended for the heaters. 

https://www.tube-town.net/ttstore/toroidal-62va.html

Pri: 0-220-230-240 V (schwarz-gelb-lila-rot / black-yellow-purple-red)

Sec 1: 250 V @ 0,12 A (red / rot)
Sec 2: 6,3 V @ 3 A (green / grün)
Sec 3: 12,6 V @ 1 A (purple / lila)
freie Enden / long leads ca 200 mm
h= ca.52 mm d= ca.102 mm
50/60 Hz EN 61558

I had found that DIL bridge but was just unsure.

Cheers
Ben

 

[ruffrecords]

Thanks for getting back, heads a bit fried at the mo, its dark in Sweden this time of year, should of read your previous posts, i will be more thorough.

I know exactly what it is like in Sweden this time of year. Many years ago (30?) I visited Electrolux HQ in Sweden. We flew to Copenhagen then got the last ice breaking ferry across to Malmo. The next morning the sea had frozen over completely. The Electrolux HQ is shaped much like a huge Viking ship. Inside there was a huge rest room where the walls and ceiling were covered with light boxes to combat Seasonal Affect Disorder (SAD)

Brexit is a real pain now, having to learn german now to find parts, which is fun!

At least most of them have an English language version

No need for 115v prim.
This toroidal seems right, and the 12.6v tap will power the Vu meter board. I will use the Meanwell IRM-20-12 you recommended for the heaters. 

https://www.tube-town.net/ttstore/toroidal-62va.html

Pri: 0-220-230-240 V (schwarz-gelb-lila-rot / black-yellow-purple-red)

Sec 1: 250 V @ 0,12 A (red / rot)
Sec 2: 6,3 V @ 3 A (green / grün)
Sec 3: 12,6 V @ 1 A (purple / lila)
freie Enden / long leads ca 200 mm
h= ca.52 mm d= ca.102 mm
50/60 Hz EN 61558

I had found that DIL bridge but was just unsure.

Cheers
Ben

That transformer should be fine.

Cheers

Ian

 

[Tom snich]

cheers Ian, the sun came out back on track!

 

[Tom snich]

A question about the DC heater supply you recommended a Meanwell IRM -20-12 SMPS with 1.8A, would using a larger one impact/cause problems? this one : IRM-60-12ST - DC Power Supply, 60W, 12V, 5A, MEAN WELL.
I'm thinking about power to the x2 VU meter boards and a little SS headphone amp I'm thinking of incorporating, so there is plenty of ampere for the heater supply. 

I'm also trying to plan for the external PSU (i want to avoid hum) and using some multicore cabling (https://www.thomann.de/se/concab_1100010008.htm) to get the HT+ and OV and the +12VDC and 0V heater supply then use the second set of +12VDC and 0V once inside the mixer to power VU and headphone amp.
Or am I gonna get problems? I want to get the power supply as neat as possible..


Ps. I liked your icebreaking story, the sea froze a bit this year too. They have a SAD waiting room (light box) at the hospital to help people, you go there in morning and get a newspaper and sit in a dressing gown.

 

[ruffrecords]

You need to keep a couple of things in mind when selecting an SMPS for heaters.

Remember the inrush current into cold heaters is usually several times the rated heater current. It drops rapidly as the heaters warm up. Some SMPS have simple foldback limiting and will not power up into a heater load unless the PSU is rated four or five time the nominal heater current. Most Meanwell SMPS have a hiccup mode current limit where they try their hardest to drive the load but give up a short time later if the load does not lighten. They wait a short while and try again. I have found this type of current limit works much better for heaters and as a rule I now rate the SMPS for twice the heater voltage rating if it use hiccup mode current limiting.

At the other end of the spectrum, if you SMPS has many times the required current capability, once the heaters are warmed up they represent a fairly light load for the PSU. In these cases there is a possibility hiccup mode is entered again, not to limit current but to regulate the output voltage. This adds significant low frequency content to the heater voltage which can sometimes escape into the audio path.

Taking the above into account I now choose a hiccup equipped SMPS rated for close to twice the desired heater current. This means it will safely power up cold heaters but also keep out of hiccup mode when the heaters are fully warmed up.

In your case, you are running a pair of TLAs which each consume a nominal 0.45A heater current at 12V so the Meanwell IRM -20-12 SMPS with 1.8A output current is ideal. If you use the IRM-60-12ST rated at 5 amps then the heater current is only 10% of the nominal so there is a definite risk that hiccup mode could be entered.

I understand you want to run other stuff from the same  supply but I think you would be better of with the IRM-30-12.

The cable you have selected for running dc from an external power supply looks fine to me. I use 1mm cross section wire for the Mark 3 mixer and just double up conductors for the heaters. With you modest heater requirements a single pair should be OK.

Cheers

Ian

 

[Tom snich]

Thanks Ian!

Good to know when doing other projects in the future.

Cheers
Ben

 

[matriachamplification]

Today I received the new custom designed metalwork for the poor man's tube mixer. This now has a proper 3U space for modules and backplane plus a sloping front 4U section for faders and misc controls:

I fitted it with a few old EZTubeMixer modules and a fader just to give you an idea of what could be built with it:

Cheers

Ian

That is slick! Well done!

 

[matriachamplification]

Who manufactured your enclosure? SRS?

 

[ruffrecords]

Who manufactured your enclosure? SRS?

Yes, it was SRS.

Cheers

Ian

 

[Tom snich]

In need of some more guidance with my mixer build . Here is my Poor Man’s tube mixer so far. Almost everything is found material, off cuts, old PA amps for vu meters… A poor man’s tube mixer!

 

[Tom snich]

I am starting on the wiring for individual channels now I am using shielded cable throughout, Grey cable is signal, yellow is signal ground also shielded,  is this a bit over the top with shielding?

 

[Tom snich]

The case is clad on the inside with composite sign writer’s aluminium sheet for shielding.
To the right I have built a small box to contain the two TLA’s Fx1-2 and Main outs, I’m thinking it is good to double screen here, or is this a bit over the top with shielding?
I am also going to start on the bus channels and I had planned on using 47k bus resistors with 25k pan pots but changed my mind and went with LCR panning instead, is 47k still good to go with for bus resistors?
The Aux fx pre/post switch is wired as both channels either pre or post with 100k pots, I hope this does not load the TLA’s to much?

 

[Tom snich]

Here is the back, the shelf in the middle I am thinking of using for the input and output transformers.
The bottom left is the house for the TLA’s the right is empty.

 

[Tom snich]

These transformers I pulled from a mixer, I have 24 of them, they were the line in transformers going into NE5532 op amps, I have done a filter sweep 20 hz – 20khz with a DDS tone generator FY6800 it only goes up to 20v with square wave and sine, it holds up well with sine, Square wave rings a fair bit. 1v in gives 1.3v out. I don’t know if they are 10k I did a mock channel build and they dropped the signal by 5db, I am thinking of using these to unbalance signals and see what happens. :/

 

[Tom snich]

This is my external power supply with a multi cable connector 7 pin. Tested and correct so far.
Toroidal Transformer wired for primary 230vac gives secondary 273vac, Meanwells are both 12vdc one for heaters and one for VU and headphone amp. I put a 1.6 amp fuse slow blow, but I think a 2 amp fuse would be better when loaded?

Any help would be much appreciated.

Cheers
Ben

 

[matriachamplification]

In need of some more guidance with my mixer build . Here is my Poor Man’s tube mixer so far. Almost everything is found material, off cuts, old PA amps for vu meters… A poor man’s tube mixer!

That is proper slick! Well done!