the Poor Man 660 support thread

[duantro]

Thanks for the cathode measurement tip, AlexC. I'll do that when I get a free minute! I'll also try a smaller trimmer on my meters.

Edit: Ok I've gone through 10 6n6p's and 8 6cb8's and found some really close matching 6n6p's. On to the meters next free time I find.  8)

 

[creal]

My Pm670 is alive and working well.
I really happy to finished this project after 4 years.
The panel came from Frank Röllen.
I add the SCAMP from loloM, i can't imagine this comp without this mod, thanks lolo
For the time constant, the first five position are the Fairchild, the six other positions of the attack knob are the same used by Kingston and active the release knob.
I also add the Kingston LPF.
thanks to Analag, Volker, LoloM, Kingston and others for this great project and advices.

Cyril

 

[duantro]

Great work Cyril! I like the vintage look. I agree, I can't imagine using this without the scamp board fromo LoloM.  I like the idea of the side chain lpf from Kingston. I will have to look into that. Unfortunately I already have my panel done :-[ I guess it'll have to be a non labeled toggle.
When i get some spare time, I need to find out why my right channel isn't passing signal.

 

[creal]

thanks for the kind words, the kingston's LPF is great feature for the 670, very useful for drums and full mix and it's very easy mods to do.
i can't really help to troubleshooting your right channel, but do you have a working STOCK PM660/670?

 

[duantro]

I had a working non- stock version ( regulated heater boards, seperate 11a heater toroid, scamp boards) and then right channel just stopped passing signal. No worries, i Just need to find some spare time to look at it!

 

[skal1]

Ok lads,

i really need some help with this poorman psu , at the momment i am just power up the amp side of things, ever componet is new acept for the 220uf 450v and the the 1uf film caps.

psu voltages without any load. 250v 133v

simtoms are as follows

as soon as the 6bc8's come on line psu pulls down to a hundred or so volts.

oh could i use a resistor to emulate a load ? and what value of  resistors  should i be using for  this.


Any help would be helpful

Cheers

skal1

 

[alexc]

More specific ?

Are both the +135V and +245V dc rails dropping when the 6bc8 tubes begin to operate after the warm up?
What are the heater rails doing ? Do they remain as expected. And are the bipolar rails OK?

Sounds like you are pulling excessive current from your psu transformer from the 6bc8 tubes on the +135V

They don't usually conduct so much current so I would be thinking the 5683 tubes on the +245V are more likely to load you psu incorrectly.

This can also happen if they other psu windings are overloaded by a fault condition. You can usually hear the psu transformer 'buzzing' when it is passing too much current in any of the windings, such as in a short circtui condition.

What I would do would be to remove the HV rails from one of the main pcbs and examine one main board in operation only.
Then I would pull a 'pair' of 5683 tubes. Then the other.  Check what that does to the HV.

You need to determine whether the signal amp or control amp of which main board could be loading excessively the psu.

Assuming the heater  or bipolar are not shorting or nearly shorting.

Obviously take special care when connecting/disconnecting psu wiring - not only to your well-being but also to the PSU.
Shorts/discharges to ground of the HV WILL destroy components in the HV section.

 

[skal1]

only running the signal amp

More specific ?

Are both the +135V and +245V dc rails dropping when the 6bc8 tubes begin to operate after the warm up?

Yes

More specific ?

What are the heater rails doing ? Do they remain as expected. And are the bipolar rails OK?

steady 6.3v, yes and yes



Ok i have done some testing

signal amp

64 dc @ ct of T2  R6 REMOVED direct connection to tx no meter

The current comsumtion 4.84ma

RV3 set to -2.4v



All psu voltages are correct without the 6bc8 in

skal1

 

[alexc]

If I understand you correctly, you measure the low voltage at CT of T2 when you have the 4x6BC8s installed on both main boards (assuming a 2 channel build) and also a current of 4.84mA thru the CT.

That would indicate that the 6BC8s aren't loading anything at all.

You also say heaters appear good and steady.

So I would say your dropping HV is therefore likely to be in the control amp. If that is drawing excessive current, both HV rails could drop. Pull the 5683s and check with the 6BC8s in.

** correction : it appears that you are already only testing the 'amp side of things' which I take to mean you have not connected the +245V rail already.

If that still indicates HV dropping then it is not current loading based.

My guess would then be faulty PSU or PSU transformer - but I'm a little skeptical.

I would first remove all HV connection off the PSU and slowly re-establish them to each main board in turn to properly rule out mainboard(s) faults.

It's simple to do and would immediately narrow down the problem in the no-signal condtion. If it happens when each of the main boards alone is connected( and with 6BC8s installed), then it's unlikely to be a build error on the mainboards - they are pretty simple. So it would point to bad PSU pcb or traffo.

Is you're build 'stock' or modified?

Good luck.

 

[skal1]

Hi alexc,

My build 'stock'

Just for a laugh i hook up the sidechain amp on its owen without  signal amp and guess what ,HV droops just like the signal amp , so your thinking about the psu as for cause maybe right.

I think i will bread board the q1 ,q2 ,q3 and q4 with all the resistor that bais them and see what happens then.

Thanks for the hints

skal1

 

[alexc]

Sounds like the psu HV is dropping when it delivering any current - that would lead me to think that there is an error in the little transistors of the HV section.

Could be a kind of shorting of traces or more likely incorrect component value. Given that the little transistors' orientation are pretty clearly marked on the silkscreen, I would say it is some resistor values incorrect. Or possibly bad or incorrect zeners.

I've found that when  any of the transistors or mosfets are toast, you won't get anywhere near expected HV, so because you do get there under no-load, I don't think it's them that's bad.

I would remove the PSU, carefully check the solders for shorts, hairline breaks and so on.
Then check again as best you can the values and orientation.

After that, I would start checking the significant PSU HV section voltages - there is a PSU diagram with voltages at various places many pages back from some a generous member

Again - be careful if you poke around - those big heatsinks can easily be at a HV if not fully insulated. And make sure you fully discharge the HV section - they can easily retain a mighty wallop for quite a while when not connected to the mainboards.

Cheers

 

[DerEber]

also consider to implement the psu mods shown somewhere here. It cured all my PSU problems.

 

[skal1]

Ok so i have the regualotor parts are working on a bread board right now every thing is holding up fine at the momment .

i have some  concerns  about the psu, the  245vdc rail is about 253vdc and the 136vdc rail is 130vdc how would i bring this in line with the voltages on the pcb.

i have one more ? why is it when the signal amp is compressing the CT voltage rises.

Thanks again alexc for your help

skal1

 

[alexc]

The voltages achieved by the psu will depend on the loading and the component tolerances, I think.
My PSUs usually give +/- 2V or so unloaded and can go to +/- 5V or so when heavily loaded.

So if your PSU is unloaded and giving 8V and 5V respectively above and below the spec, then I think that might be just beyond what I would expect. Since this is breadboarded, I wouldn't worry too much though. I would switch back the actual PSU pcb and focus attention there. Connect it up again with no load and check the important voltages around each HV rail section.

As far as 'signal amp CT rising when compressing' - I assume you mean T2 primary.
Although I have not noticed the DC voltage shifting by much at all when compressing/non compressing, it is true that the compressing action is the 'least current draw' condition and therefore the loading on the +135Vdc rail is reduced, which I suppose could lead to an increase in voltage fed at CT.

But not very likely unless your 'no compression' condition of 'max current draw' is really excessive.
Which I doubt.

In that case, the sudden shift from excessive current draw in 'no GR' to minimal current draw in 'full GR' might cause a small shift in the regulated voltage.

The 'idle current' of the GR tubes is set by the combined action of the RV6 (bridge negative reference voltage) and RV3 (cathodes clamp reference voltage) - the difference between them in the no-signal condition sets the grid-cathode voltage which in turn sets the current flowing thru the GR tubes.

That current might be some 12mA or so for each stage, each side, so 25mA each phase. Hardly a crazy amount
If you have them red-plating maybe, possibly. But generally, no.

Again, it sounds to me that you have something amiss in the psu or your measurement is not quite right.

 

[skal1]

ok had a look at the RV3 (cathodes clamp reference voltage)  which is -2.4vdc and the  total current draw of signal amp 2x 6bc8 no compression is 10.2ma @ 134vdc at t2 primary , does this sound right alexc

cheers

skal1

 

[alexc]

10.2mA at the primary of T2 in no compression sounds too low to me. I would expect more like 20-25mA.
2x6BC8s is 4 triode stages - at no GR ie. 'idling' should be 3-6mA each I think.

2mA per stage (if all 4 are working) is what I would expect with some reasonable GR occurring. It may be that you have RV6 too high which is putting the 6BC8s a little into GR with no signal.

RV3 of -2.4V (measured at one leg of C2 : one of the big caps) and RV6 at -4.5V (measured at pin 3 of Q1 - careful not to short it!) are a good starting point.

You can also check the voltage of each of the 330R cathode resistors R1-R4 to work out the current thru each 6BC8 stage at idle.

The bottom of all those resistors should be the value of pin 3 of the Q1 transistor cathode clamp, and the top of each is the cathode voltage of the respective 6BC8 triode stage. Current is ohms law.

 

[skal1]

ok done some more testing

first test 2n6107 pin1[ -2.4vdc] pin 2[-17vdc] and pin3[-1.85]  i think this shows that Q1 is doing its job.

second test check the voltage of each of the 330R cathode resistors at junction of cathode and resistor leg.

r1 -0.88vdc
r2 -1.07vdc
r3  0.77vdc
r4 -0.82vdc

so ohms law tell me I=V\R  so

r1 -0.88\330r =0.0026ma
r2 -1.07\330r =0.0032ma
r3 -0.77\330r =0.0023ma
r4 -0.82\330r =0.0024ma

So acorrding to the schem Vb1 and Vb2 are parellel so R1 Current add R3 current = 0.0026ma+0.0023ma=5ma more or less.

so i deduce that i should have aleast 3.4 to 4vdc coming out of q1 into  the bottom of r1,r2,r3 and r4 to get the 5ma per triode section.

correct me if iam wrong please ..

Skal1

 

[alexc]

The Q1 numbers are good. With your cathode resistor numbers, you take the voltage across each.
The -1.85V at pin 3 is the RV3 trimmed voltage less 1 diode drop of 0.6V at the Q1 base-emiiter junction.

eg. r1 -0.88vdc at one end, pin3[-1.85vdc] at the other, so current is 0.97V/330R -> 2.94mA

Similar for the others 2.36mA, 7.94mA, 3.12mA  to give you the total +135V rail current draw.

Your R3 value of +0.77Vdc is very different from the others - I would say check that again.

Assuming that R1, R2, R4 areOK and R3 is the odd man out, that would imply a total current approx 6mA per signal phase phase and 12mA for the signal amp. (not sure if you have 1 signal amp pcb or 2 in this build).

It's a bit lowish but never mind for the moment - you should surely see some output at T2 secondary with 12mA flowing thru it.

The other thing to note is that the GR tubes conduct current according to the voltage difference between the grid and cathode. As mentioned before, in the case of no-signal idle, grid is held at a -ve reference voltage set by RV6 via the CV feed to T1 secondary. That should be around -4.5V as previosuly discussed.

So the bias applied to each of the GR tubes is grid -4.5V minus the cathode voltage -> -3.62V, -3.43V, -5.27V, -3.68V respectively. Cathode 3 is the odd man out (if your measurement is correct).

Looking at the data sheets for 6BC8 with plate of 135V and around -3.5V bias says around 3mA per stage is expected.
So you are pretty much as expected.

Check again that funny R3 number. If it is correct, you may have a dud tube section.

Move it around and see if the funny cathode voltage follows it.
If so, replace it, if not - check the resistor is actually 330R and not some other value and finally check solder for any issues.
They check again.

The reason I said a higher total signal amp current is that I bias mine to idle quite a bit higher - like 6mA (-2.5B bias) .

That's to give more available GR (more current = more gain therefor more  room to compress before crapping out due to low current under GR) , but at the expense of input signal capability at idle (will clip the input at idle at lower inputs - bad) as well as higher idle current dissipation, which can be an issue with heat on tubes, regulators etc.

Not to mention higher noise when not compressing much. AND that the output traffo can handle that amount of dc CT fed thru it

To summarise, that looks more or less OK except for the bad R3 reading. Fix that up them satisfy yourself you get some good signal at xlr output connector then you can start looking at GR issues with the control amp.

Simple

Cheers
Alex

 

[alexc]

Also, from the current numbers for each triode stage, you can get a sense of the 'match' of the tubes at this one current, the 'min GR' current (idle).

You have 2.94mA, 2.36mA, bad mA say 2.94mA again and 3.12mA

So that says stage 2 is a little lowish compared to the others at this ONE current operation.

Now - repeat this for a whole range of applied CV values say from idle -4.5V down to -15V in a few volt steps and you would see them vary differently at each step. The trick is to try to find tubes that give you what you think is the best match at various places along the way.

What I did was do that check and simply weed out any really varying tubes in order to get the total current on each half as close as I could.

If you are super brilliant, as some have described, you can match exactly and all that. EXACTLY.

Personally, I never even came close to having good matches - just as good as I could get with the tubes I had, which was few. Some people have found substituting other tubes like 6N5P to give better match. That's great providing you can handle the different heater and HV requirement. In a stock PM670 build I wouldn't do it. A PM660 is good though.

And there you have it - the quality of the match is in my opinion, one of the main issues in determining the fidelity of the signal amp under GR  ie. distortion. Good performance here is hardest to achieve for lower frequencies.

All in all though, I managed to achieve pretty reasonable performance without superhuman heroics.

Clean and clear when not driven too hard or with time constants too quick with very, very transparent GR.
Crunchier and more noticable GR when driven harder and with faster time constants.

Mastering grade performance expectations is another thing entirely.

That would require much more attention to matching tubes, resistors, Level/Threshold switches and of course higher spec signal transformers.

For that reason, mastering vari-gm commercial units are anything but cheap. And require regular maintenance.

Hence the Poorman 670. It is exactly that. At the time these were offered to the DIY community  there was nothing else in the ballpark. Nowadays, there are more build choices. And everyone has the benefit of perfect hindsight.

 

[skal1]

yes i just check the tube at v2b and i think it is faulty it was giving out  a neg 1.23vdc out of pin 9 so i sub in a different tube and that prob has gone now .

so current at T2  pin 1 to CT is 0.006ma  and pin 4 to CT is 0.007ma  so it is pulling about 12ma as you said .

And yes it yes 1 pm board

thanks for your help again alexc

cheers

skal1