Poor Man's Tube Mixer

[ruffrecords]

I just saw the PNG image of your layout ,
You have the gain blocks down to a very tidy footprint , that should pay dividends

The idea about the heaters also makes good sense , it allows the heaters to be run in the traditional manner ,twisted pair ,looped up and away from sensitive circuitry and on to the next valve . It also means less chance of any leakages or coupling between heaters and other electrodes .

One thing in the PNG above  is some of the electrolytics are very close to valves , of course on one hand physically adjacent to the components or electrodes your bypassing is good news , life expectancy of electrolytics in very close proximity to valves might be somewhat limited though . One possible way around this is to mount the electrolytics on the backside of the board ,there no further away than right way up but more or less completely shielded from the direct heat of the valve , that could increase the lifespan by years.

I take your point about electrolytic capacitors. There are two of them quite close to V1. However V1 is a 12AX7 running at about 0.5mA plate current  and 90V plate voltage so its dissipation is quite small. The only real heat is from the heaters which is only 3.6W. All the other tubes are 6922s running at about 6mA and 125V plate and I keep electrolytics well away from them

Cheers

Ian

 

[ruffrecords]

I just finished the first version of the main PCB and placed an order for five boards. I have moved the electrolytics away from the 12AX7 just to be safe. I have also added some more mounting holes, finished adding all the component values plus a bunch of other labelling to make assembly easier (I hope). Should have the boards back in about a week.

Cheers

ian

 

[Tubetec]

I was going to mention extra mounting holes seeing as the board is bearing the weight of lots of transformers , but you thought of it already . Would the mounting holes need to be seperate and isolated from the ground plane on the board  to avoid loops in the case of metal stand off's on a metal chassis ?

 

[ruffrecords]

I was going to mention extra mounting holes seeing as the board is bearing the weight of lots of transformers , but you thought of it already . Would the mounting holes need to be seperate and isolated from the ground plane on the board  to avoid loops in the case of metal stand off's on a metal chassis ?

If you look at the PCB pic I posted you can see the top and bottom rows of mounting holes are outside the ground plane (the red line) so there is no problem with them. The ones across the middle do go through the ground plane so they should be fitted with nylon stand offs and screws.

Cheers

Ian

 

[Tubetec]

Brass at the corner of the board , nylon in the middle ,that would build in a lttle 'curtesy' on behalf of transformer  mass when your bouncing down the high roads and byroads to earn  your daily bread and strain the solderjoints way less too ,

 

[ruffrecords]

Prototypemain PCB arrived this morning:

I started populating it this evening and found a couple of errors. I forgot the 100K bleed resistor on one of the output caps (easy to fix). I also made a mistake on the OEP A187A15C mic input transformer footprint. Fortunately I just got the can pin at the wring end so I can drill a hole for this and hand wire it to analogue 0V.

I thought I would splash out a bit so I went for blue solder resist and opted for gold flash on all the pads.

Cheers

Ian

 

[tk@halmi]

Looks delicious! RS America just raised the price on the A187A15C with 20% when I was about to buy a couple.

 

[ruffrecords]

Looks delicious! RS America just raised the price on the A187A15C with 20% when I was about to buy a couple.

They were £40 plus tax when I bought them the other day which at current exchange rates is a little over $51.

Cheers

Ian

 

[ruffrecords]

Just finished soldering in all the passives and most of the transformers:

I made a bit of a dog's breakfast of the footprint for the OEP mic transformer. The pins are all in the right relative positions and the two rows are the correct space apart. Just the spacing of the pins within each row is wrong. I managed to get one to more or less fit by bending its pins but after soldering it in the primary and secondary inductance values are not what they should be, Don't know if it is a PCB fault, damage caused by bending/soldering pins or just a duff transformer (I bought 4 and the other 3 test OK). I am not going to risk bending another so I will need to find an alternative mounting/wiring method for this prototype and fix the footprint in the next rev.

Next step wire up the heaters and get some tubes glowing.

Cheers

Ian

 

[ruffrecords]

I just completed wiring up the heater and HT supplies. Tested first with just the heater supply connected. Tubes all light up as expected. Then added the HT supply. HT drops from no load value of 355V to about 285V when the tubes are all heated up which is about right. No smoke so far.

I still cannot remove the OEP mic transformer that I forced to fit the erroneous footprint. I cannot even get a pair of snips under there to cut its legs off. Time to rethink that.

Cheers

Ian

 

[scott2000]

I still cannot remove the OEP mic transformer that I forced to fit the erroneous footprint. I cannot even get a pair of snips under there to cut its legs off.

Maybe some solder holding it in somewhere? Have you tried some wick??

 

[Winston OBoogie]

...  all in the right relative positions and the two rows are the correct space apart. Just the spacing of the pins within each row is wrong.

As Mr Preview and a Wise (pun) man were once told:

"I'm playing all the right notes, but not necessarily in the right order."

(I suppose you have to be at least 50 and from the UK to understand this, sorry!)

 

[ruffrecords]

Maybe some solder holding it in somewhere? Have you tried some wick??

I have tried the solder sucker but not wick. I will give it a go.

Cheers

Ian

 

[ruffrecords]

As Mr Preview and a Wise (pun) man were once told:

"I'm playing all the right notes, but not necessarily in the right order."

(I suppose you have to be at least 50 and from the UK to understand this, sorry!)

Ah, Eric and Ernie. Sadly missed but still regularly on TV (thank god).

Do you remember Morny Stannit?

and of course the stripper.

Cheers

Ian

 

[Winston OBoogie]

Ah, Eric and Ernie. Sadly missed but still regularly on TV (thank god).

Do you remember Morny Stannit?

and of course the stripper?

Hahaha, yes of course.  That stripper music routine while making breakfast was pure Gold. 

Eric Morcombe could make me laugh without even saying a word,. Maybe a certain look, even just pushing his glasses up his nose... 
Sadly missed indeed

 

[ruffrecords]

Hahaha, yes of course.  That stripper music routine while making breakfast was pure Gold. 

Eric Morcombe could make me laugh without even saying a word,. Maybe a certain look, even just pushing his glasses up his nose... 
Sadly missed indeed

"What do you think of it so far? Ruggish.

Cheers

Ian

 

[Tubetec]

Just wanted to add in , Im not bothered in the least by the few teething issues you had with the first run of the board .
I have other input transformers that arent pcb  mounted at all and I think I might end up using them if I can .
The space issue with the output transformers is also not a problem for me  ,but I would like to try the new dual bobbin design at some point if you have an extra pair to hand .

Would postal order in £ STG be an acceptable way to pay you ,
I may also need some of the small parts like the pins for the tube mountings , possibly some of the header plugs also . 

Flooding all the pins on one edge of the transformer with solder ,and using a big iron might get you unstuck , sometimes stuff like that needs to be 'walked out' heating one edge and lift then the other side and lift until its comes away from  the board.

 

[ruffrecords]

PCB debugging update. I I managed to remove the mic transformer after a little persuasion.

I have now fed signal through all 6 pre-amps and they all work. I have tried all four output transformers and they all work except for T5 which for some reason does not. I have checked continuity from the transformer to its two connectors and that is OK. Possibly a duff transformer,.

Current caveats with the initial layout.

1. Mic transformer footprint is wrong. (fix in PCB revision but find alternative fixing means for prototype).
2.Output caps are too close to their bleed resistors which means that big fat 10uF film caps will not sit flat on PCB. They fit at an angle so OK for prototype. (fix in PCB revision)
3.Output cap bleed resistor missing from OUT2 . Hard wire under PCB for prototype. (fis in PCB revision)
4. Legend on G1 and G2 swapped (G1 and G2 are the connectors to gain setting resistors on channels 1 and 2). (fix in PCB revision)
5. T5 not working.

So with those caveats I am happy to release the remaining 4 prototype boards to those who asked for one. I will be contacting each of you individually to discuss shipping address and payment. The basic cost of the board is £22 plus shipping and Paypal fees.

Cheers

Ian

 

[ruffrecords]

Just wanted to add in , Im not bothered in the least by the few teething issues you had with the first run of the board .
I have other input transformers that arent pcb  mounted at all and I think I might end up using them if I can .
The space issue with the output transformers is also not a problem for me  ,but I would like to try the new dual bobbin design at some point if you have an extra pair to hand .

Would postal order in £ STG be an acceptable way to pay you ,
I may also need some of the small parts like the pins for the tube mountings , possibly some of the header plugs also . 

Flooding all the pins on one edge of the transformer with solder ,and using a big iron might get you unstuck , sometimes stuff like that needs to be 'walked out' heating one edge and lift then the other side and lift until its comes away from  the board.

I will pm you shortly about this. The gold pins are available from Farnell:

https://uk.farnell.com/harwin/h3161-05/socket-pcb-gold-knurled-1mm/dp/149319

And the two way 0.1 inch pitch 2 way and 3 way Molex connectors are available form lots of places.

Cheers

Ian

 

[ruffrecords]

Quick update. I found a satisfactory method of mounting the input transformers on their backs so they could still be used on the prototype despite the  cock up on the PCB footprint. Tested one and it works OK. Next job is to fit the other three. Also discovered I managed to wire in one of the output transformers in backwards. Should be fun getting that out.

Cheers

Ian