the Poor Man 660 support thread

[alexc]

OK - so am putting aside the unit I was working on, a dual rca style limiting unit done very similar to the pm660 mkii I showed earlier.
One side big tubes, on side small tubes, and a seperate psu section for each to limit interactions between channels.

Here's a pic just for interest - actually I think it will be more challenging than the single channel one.

As homer said "Bring me two of every animal ! "

 

[duantro]

And don;t get complacent after not dying for a while.

I like this one
So after tracing signal on channel 2, the one that doesn't pass signal, I notice that the signal gets distorted and drops down on the secondary of T1. Actually it is sending signal to ground/ a short somewhere, because this low distorted signal is also showing up on the star ground. I checked channel 1, that is working although poorly (low roll off with 10-20 volume drop and slightly distorted signal), and it is doing the same thing, except tone signal is much louder. So I think I have something wired up wrong or shorted out somewhere. I'll keep looking.
Yes my power supply seems to be holding up so far. I haven't left it on for long periods of time though. I did up the 220uf psu caps to 450V, and changed c6 to a film47uf also.
On the heater boards, I did up the caps to 3300uf 16V here. I seem to remember someone claiming this helping here. I just have the 2x 22r 5W resistors on Moby's boards also. The heat sinks do get warm. I may need to get some bigger ones here.

 

[alexc]

Secondary of T1 means coil or connected across it, the 'Level' rotary switch. Possibly the grid circuit of the GR tubes.

Assuming the primary of T1 is good  , then you have a borked signal before the tubes.
Check that switch and connection to T1. And also T1 connect to the main PM board.

The main thing is if isn't smoking or glowing too red hot and the  psu voltages are reasonably stable and the heaters not shutting down from heat, then You Are Mostly There    There's just not that much in the audio chain after all this stuff!

You should check the voltage into your slow starts and the output. If it's too high for the heatsink, then add a series resistor if you can, tenths of an ohm, probably 10W or so. If neccessary, you can blow off the excess heat there to spare your regulators some.

Anyway - getting there I hope.

 

[duantro]

Yeah voltages seem pretty stable, both of my slow start outputs are at 6.3V now. Not sure if maybe I didn't let it warm up enough before, but appears good now. The input to my slow start heaters is at 9.55V. It does seem like quite a drop. would the 10W, 3r be to large? I also have an adjustable one that is 0-3r.

 

[alexc]

Some other observations for you, Duantro.

Once you button it all up, I would advise you give this plenty of space above, if you rack mount it.

2RU I would say of good ventilating. 1RU absolute min.
And I like to position it in rack where it will have open space below it or above it.

Particularly if you be in a warm climate.
I am not. Generally the heat is not unwelcome.

Also not sure where your meter adjust pot is, but generally you want that to be accessible 
I need to adjust mine each time, but it's pretty good. I now adjust it to be fully on one side with no GR.
So I can see a wider range of metered gr.

 

[alexc]

9.55 to 6.3 is difference of 3.25V @ whatever current the regulator is at plus the similar sort of thing with the transistor.
I need to check the power dissipation some more.    I'm not sure what the spec says for that reg and that transistor. Best check.

338 would be running at < 1A, so say 3W or so there. The transistor current boost would  be around 2A and the voltage differential on it might be 1.5 to 2V or so. I need to check. So 3W maybe on that little onboard heatsink.

Those resistors are also obviously do a fair bit of radiating - they may well have the regulator differential at it's min already.
Same for the other one. So it may only radiate a smaller part thru the transistor heatsink.

That would be around 6W or so on each on the heatsinks, maybe more on the resistors into free space.
But, really I need to test mine properly to know for sure.

Keep it under watch. I think maybe 6W max for each is all I'd feel comfy with.

Radiance had some things to say regarding those afew pages back and the words 'serious heatsinks' were mentioned 

OK - I just saw the heater regs are mounted to the back of the rack?
That would be way better and may well handle the dissipation. Maybe.

I plan to have an offboard heatsink for mine too. I have 4x 16V 4700uF caps on mine. I need it to 2.5A or so.

 

[duantro]

Ok another 10" of snow so I can look into this thing more. My Vu trimmer is temporarily attached to the back of meters. These meters are NOS Lomo Vu meters, that have an adjustment pot on the front. I don't really know anything about them however. I was hoping to get it close on the hidden trimmer and use the front to adjust. May have to drill a hole though. First I need to get the short figured out and then dial the meters so as to not being pegged.

 

[alexc]

Meter front adjust screw is just the basic meter 'pointer at rest' mechanical trim.

Not at all related to the zero gr' adjust you need to make pretty much each time you turn the unit on and it warms up.

I mounted mine 'out back' on my pm670. Subsequent units I make very sure it be front.

Good luck with the snow 

 

[duantro]

Ah, gotcha on meter trim. I'll drill holes above the meters when I get everything working.
So while sorting thru the then 150 pages, to decide how to wire up my attenuators, I realize now that I should have followed Kingston's db attenuator calculations, as opposed to the 15k excel sheet that Silent arts listed. Can't wait to rewire these things!! Ugh. I guess I should at least get signal thru it first. The Switches should have 15k resistance to pin 1 on deck 1, and pin 3 on deck 2, when unconnected to Pm board and T1, correct?
I removed the pm boards and re-soldered all suspect connections and searched for any bridges. Still no luck.

 

[alexc]

I followed some of the instructions eariler in the thread and printed them out and made sure I understood how it was going to go together before doing it.

So - not sure what pinout scheme you need but just be sure you test it and get happy before using.

It was 15K end to end for each deck as I remember.

The pole terminal of each of the 2 decks caries the signal for that phase onwards. So probably check what you have and test it for signal, then each switch position resistance and write it down and repeat for the other. You want the resistances for each position of each deck to match.

It's easier to test than build - a 3 wire device not too complex.

 

[bluesbaz]

Hi all I have the boards for the pm660 rev1 x2 and the psu660 rev02 x1. My schematic is missing a bunch of stuff around the heater can anyone direct me towards a bom of some kind or a complete schematic
-s

 

[shabtek]

see post #2969

 

[bluesbaz]

thats the one i have and its missing the whole heater section. IM mostly looking for the value of r15

 

[duantro]

Bluesbaz,  take a few days and read this whole thread. This is covered in the first ten pages probably. It's value is deducted from trial and error, to achieve the correct heatèr voltages.

 

[alexc]

For my pm670, it wound up at 0.5ohms 25W minimum. 

That's with 240VAC 50Hz supply feeding a 240VAC rated transformer primary.

That is, for 5A of heater current which mine uses, a voltage drop of around 2.5V dc.

The rectified heater voltage was around 11.0V or so unloaded and drops to 8.8V under full load.
(winding was rated for 6A at 9Vac)

So from that 8.8V loaded, I drops 2.5V to give the required 6.3V loaded. Or thereabouts.

It does need to be determined for a particular build, so a 25W or more variable resistor (rheostat) is helpful to adjust to the desired loaded heater voltage. You then read off the rheostat resistance value and try to find a power resistor which is closest.

The common orange aluminium power resistors are fine - if you are lucky you can find one close to the required value.

For me, it was spot on 0.5 ohms.

 

[duantro]

Ok a little update. I've had a tech friend helping me trouble shoot this. We now have both channels working somewhat. I had forgotten a jumper on the scab board, which made channel 2 not work. We are only getting a max of 7db gain out of this. Any ideas what would be causing this? It still gets grainy with a lot of reduction.  I still need a tube matcher and meters are still pegged also. Will do it as soon as I can figure out this gain thing. Do I remember corectly that it should have around 23db?

 

[DerEber]

Check your tubes. I had different values of gain with different brand of tubes. 7db seems low

 

[alexc]

My recollection is the makeup gain is more like 12dB or so. 

I get around 14dB of GR max from mine, so I add a couple of dBs at the desk too.

Meter pegging is quite easy to achieve - have you done the thing of putting a low value pot across the meter?
(take care - the meter has some  high voltage on it)

Low gain could be due to the settings of the two trimmers. One trimmer sets the -ve bias voltage on the bridge, the other sets the voltage of the 'cathode clamp'. The difference between them (less a diode drop) sets the grid-cathode voltage and therefore the gain (or GR) of the tubes.

I think it was -4.5V and -2.4V respectively for the bridge and cathode trimmers.

They can make quite a bit of difference - essentially they set the 'idle' current  (max current , min GR condition).
The higher the idle current,  the more room for the current to go down (min current, max GR condition) without crapping out, thus determining the max GR.

Now the 6BC8 tube can take quite a bit of current - I ended idling a bit higher. My grid-cathode difference voltage was set to around -2V from memory.

Another interesting thing to do is to check the voltage diff between the cathode of any GR tube and the common tie point of the 330R cathoide resistors. That tells you the current  thru the tube. You can use this for a basic level of tube matching, at least weeding out the ones which are way off base.

It's pretty easy to have one or more GR tubes  very different - that can and does contribute to all manner of things bad.

 

[duantro]

Actually, my friend who checked it had said 7db. I have it back now and after checking it, I'm getting 14db of gain. Not sure what he was doing. I am getting 12 db of gain reduction. That sounds close to what you posted Alexc 8).
Yes, I have a 5k trimmer strapped across the + & - terminals of my meters. If I drive the input hard with a drum track I get meter movement away from being pegged. That seems like a good sign! It seems that I need to build a tube tester to eliminate that question mark. It sounds pretty damn good though.

 

[alexc]

5K trimmer is pretty high. I had to use around 200R or so to get adjustment on the GR meter.

I would definately try something smaller - I seem to remember starting at 5K and having to go right down.