A tube tracking console using ITB’s MkIII concepts

[rjuly]

…I think you may have meant to compare the 9610 with the A187A15C.

Yep, that’s right.

I see you got the lovely big ElectroMag output transformers. The have a lot of windings, Do you have the pin out and how to wire them up info?

Yes, they are indeed handsome.
In fact, I don’t have any of that information - I hadn’t thought past receiving them. I was going to query Electro-Mag, but if you have that information handy, I would take it most gratefully.

I am just now at the point of starting to assemble the chassis frame. I have the extrusions from SRS, and the cheeks, and I am awaiting delivery of metal scribing tool that will allow me to make accurate guide marks to place the holes in the cheeks.

I also have to lay out and drill/punch the holes in the chassis skin sheet, for all the io connectors, and fastening holes.

I’ll post some pics as I go.

- Richard

 

[rjuly]

Managed to cobble together the prototype chassis..







..and with approximate motherboard placement...




I'm using quite heavy cheeks for the prototype phase while I build and test, but will change them and add wood panels for the final buildout.

- Richard

 

[rjuly]

Thinking about the outboard EQ module. I am going with Ian's three band Pultec design with tube gain makeup and stepped cut/boost, for which I will do a two channel layout on a single channel circuit board. Because I will need to go back out balanced, I will need to add an output transformer.



I know from tube guitar amps that having no load connected can cause damage. Will I need to have a dummy load half-normalled to the output jacks of the module?


- Richard

 

[ruffrecords]

I know from tube guitar amps that having no load connected can cause damage. Will I need to have a dummy load half-normalled to the output jacks of the module?


- Richard

You will not need a dummy load. The SRPP output stage is happy driving anything from open circuit down to 600 ohms (via a transformer).

Cheers

Ian

 

[rjuly]

Thinking now about the power supply.

This will be external, and in keeping with the spirit of this project, will use Ian's HT350 and phantom power circuit designs, as well as the Meanwell LRS-100-12 for the heaters, VU lamps and relays. As Ian doesn't currently stock the HT350 pcb, I will build from scratch combining the the HT and 48v circuits onto one board.

I will overspec the transformer for future proofing and use one that delivers 240v at 300ma on the first secondary along with 50v on the second secondary. Not sure how much current for the 50v - I will need to dig around... I believe the Meanwell with its 8.5A will be more than sufficent for 12v puposes.

Does anyone know of an off-the-shelf transformer that will suit, or do I need to get one custom wound?

- Richard

 

[ruffrecords]

There was not much demand for the HT350 so it was replaced by the HT250 which is good for four or so modules but not enough for eight so you are right to resurrect the 350. Adding the phantom to it makes it more versatile.

Deciding phantom supply current capability is less simple. My original phantom supply design, with the specified heatsink, would source 100mA. The big variable is the current drawn by microphones. The IEC standard says it should not be more than 10mA so the design is good for at least 10 microphones. But most modern mics draw 5mA or less but there are exceptions so check the specs of the mics you plan to use.

Cheers

Ian

 

[rjuly]

There was not much demand for the HT350 so it was replaced by the HT250 which is good for four or so modules but not enough for eight so you are right to resurrect the 350. Adding the phantom to it makes it more versatile.

Deciding phantom supply current capability is less simple. My original phantom supply design, with the specified heatsink, would source 100mA. The big variable is the current drawn by microphones. The IEC standard says it should not be more than 10mA so the design is good for at least 10 microphones. But most modern mics draw 5mA or less but there are exceptions so check the specs of the mics you plan to use.

Cheers

Ian

Of the mics I will use, most are about 5ma per output, two are 7.5, so it should be plenty

Many thanks - Richard

 

[rjuly]

@Ian - In looking at your HT350 schematic, I see it has three outputs - 0v, 75v and HT+(~230v). I’m not clear what the 75v is for. I did notice you mention in a couple of places that you have used elevated voltage for your heaters at around one quarter of HT+. Is that what the 75v is for, and if so, do I need that output if I’m using a dedicated power supply for my 12v heaters?
- Richard

 

[ruffrecords]

In the earl days I used an HT voltage of of about 300V. The meant that if the heater 0V was connected to analogue 0V, there was a possibility that the voltage between the cathode of the top triode of the SRPP output stage and its heater could exceed the Vhk rating of the tube. Since this cathode is very close to half the supply voltage, the safest solution was to elevate the heaters to 25% of the HT voltage so neither the top or bottom cathode was more that 25% of the HT voltage above or below its heater voltage. 25% of 300V is 75V and that is exactly what that output was for.

However, I later realised that if I could reduce the HT voltage to no more than 250V then I would not need heater elevation at all. The specified 200V Vhk meant that even the top cathode at 125V could still have 75V of peak signal on top of it and not exceed Vhk.

So my advice to you would be keep the HT to close to 250V and forget heater elevation. An added bonus is you can now use the 12V heater supply for auxiliary functions like LEDs or relays without having to worry if the operator might get 75V on his.her fingers.

Cheers

ian

 

[rjuly]

…so to omit the 75v rail I would do something like this?:

- Richard

 

[ruffrecords]

You should leave a resistor across the supply to ensure the caps discharge. Those 470uF caps can hold charge for ages. If you use something like 300K it will only draw one mA at the most. It will dissipate about 300mW so better make it a 1W rated part.

Also these days I tend to use 5W resistors for the ones between the caps. Inrush current can be quite high and I have smoked the first resistor in the chain before now when its wattage was too low. Use ones like this now:

https://uk.farnell.com/cgs-te-conne...470r-5-4w-radial-wirewound/dp/1173943?st=470r

Cheers

Ian

 

[rjuly]

You should leave a resistor across the supply to ensure the caps discharge. Those 470uF caps can hold charge for ages. If you use something like 300K it will only draw one mA at the most. It will dissipate about 300mW so better make it a 1W rated part.

Many thanks Ian,

to ensure I have it clear, this is what I understand you to have said:

That is: 5W resistors for all resistors, and 300K across the outputs.

Also these days I tend to use 5W resistors for the ones between the caps. Inrush current can be quite high and I have smoked the first resistor in the chain before now when its wattage was too low. Use ones like this now:

https://uk.farnell.com/cgs-te-conne...470r-5-4w-radial-wirewound/dp/1173943?st=470r

These are rated for 4W / 43 volts - is that sufficient, or should I use a higher voltage and wattage?

-Richard

 

[ruffrecords]

Many thanks Ian,

to ensure I have it clear, this is what I understand you to have said:
View attachment 85397
That is: 5W resistors for all resistors, and 300K across the outputs.


These are rated for 4W / 43 volts - is that sufficient, or should I use a higher voltage and wattage?

-Richard

That is the maximum continuous voltage across the resistor calculated a the square root of the power rating times the resistance (see data sheet) which is simply I*R in disguise. For the 470 ohm 4W resistor that page represents, that works out at 43volts. Even if you are dropping 340V down to 250V that is a 90V drop over three resistors which is just 30V each so well within spec.

Cheers

Ian

 

[rjuly]

That is the maximum continuous voltage across the resistor calculated a the square root of the power rating times the resistance (see data sheet) which is simply I*R in disguise. For the 470 ohm 4W resistor that page represents, that works out at 43volts. Even if you are dropping 340V down to 250V that is a 90V drop over three resistors which is just 30V each so well within spec.

Cheers

Ian

Hey Ian - thanks for clarifying that for me.

- Richard

 

[rjuly]

Here is a provisional schematic for the power supply:



..and a rough layout attempt:

View attachment 85793

- Richard

 

[ruffrecords]

I presume there is a TL783 in the 48V supply that for don't have an LTspice symbol for.

Otherwise it looks OK to me.

Cheers

Ian

 

[rjuly]

I presume there is a TL783 in the 48V supply that for don't have an LTspice symbol for.

Otherwise it looks OK to me.

Cheers

Ian

Yes indeed - TL783. Not sure if they have a symbol - I simply forgot to add it... :-0
- Richard

 

[rjuly]

Question about the 120 Ohm resistor on the 48 volt circuit - what wattage rating is it - .25 volt, or does it need to be beefier?

- Richard

 

[ruffrecords]

Question about the 120 Ohm resistor on the 48 volt circuit - what wattage rating is it - .25 volt, or does it need to be beefier?

- Richard

View attachment 85820

The 120 ohm only has about 1.2V across it so it carries about 10mA and hence dissipates only 12mW. A regular 1/8 or 1/4 watt resistor will be fine. The 4K7 also carries this 10mA so it dissipates about 470mW which is why it should be rated at 2W.

Cheers

Ian

 

[rjuly]

The 120 ohm only has about 1.2V across it so it carries about 10mA and hence dissipates only 12mW. A regular 1/8 or 1/4 watt resistor will be fine. The 4K7 also carries this 10mA so it dissipates about 470mW which is why it should be rated at 2W.

Cheers

Ian

Thank you Ian - much obliged