the Poor Man 660 support thread

[alexc]

Also, am just going thru my lib of circuits for push-pull traffo coupled stages that have compensating cap/res networks on the traffo primary.

I've seen it plenty of times in different forms. I've also seen it called a 'conjunctive filter' whatever that means!
Generally about correcting some kind of transient/oscillatory behaviour I think.
Worth reviewing - maybe could help understand what might be going here.

Nothing like a riddle in an enigma wrapped in a mystery!

 

[Kingston]

I'm sick and tired of this project and its problems. Time for a branch.

http://www.groupdiy.com/index.php?topic=51478.0

 

[telefunk]

OK, today we checked my PM670 with a scope and it proved it did not have this "moving headroom" or offset DC effect in it. We ran a bunch of sine waves through it with automated mute filters to get huge level changes. But no audible or measureable problems whatsoever. Question is: why does your PM670 have this offset DC which you had to remove with a cap and mine doesn't?

Anyway, great work with your M670! I appreciate it!

 

[[silent:arts]]

I'm sick and tired of this project and its problems

Wow!

Nothing wrong with your opinion or improvement,
but sick and tired?

A little bit rude!

 

[alexc]

Since this is a pretty quiet thread nowadays .... 

and the subject of next gen poorman 660 only occasionally crops up,

here's a pic of the basic setup of mine.

Based on the work of many others - from this thread, as well as the various 'pimp' threads and of course all of the many other vari-mu type threads.

Trying to remain relatively poor, but a marked step closer to the real deal  ;D
Around 1200usd in parts cost with maybe 200 of that in the shipping all the way down here.
About the same ball park as my dual pm670.

You can see where I'm going with it ..  more or less.

I think I'll bump it forward in my queue and do it right now. It's been a couple of years since I did the regular Poorman 660s.

I'll start a thread for it when I have some chassis stuff done and it get's a little more interesting.

 

[shabtek]

alexc, I think I see utc & hammond iron in the signal amp, and beefy edcor for the sidechain out---very nice.

can you put the heatsinks and silicon on the other side of the p.s. card... ???

 

[alexc]

Actually the beefy edcor output traffo is 8K:600 for the signal.

If I had the dough, I'd get one of those big utc LS traffos for this position.
Ideally 10W or so with some happy ability to accept a few mA of imbalance.
And a higher Z.
Like 10K min and maybe even going up to 50K impedance on the primary for the 600 loading.
With a good winding balance
And excellent freq response  too 

UTC for signal in A25 - said it was still a poorman! Better yet the 'other one'.

The little edcor is control in. I'd like to put the A25 here and get a better utc at the input.

Hammond is control out. This is about it for budget stuff.

The tubes stick upwards, the psu pcb(s) are on the underside of lid pointing down.
All the heat is outside - inside is bridge rectifiers, heater dropping resistor(s) and psu pcbs.

Not so much heat really - this is intended for a free standing unit - on top of the wooden rack 

 

[alexc]

The psu works out quite nicely here. Same psu traffo from Edcor as commonly used in PM660, 670

The additional bridge-rectifier+choke+caps are for the sidechain  amp 6V6s B+ and the 12BH7 driver B+.
So it doesn't weigh down the psu pcb at all.

The psu pcb does the regulated hv for the GR tubes and the sidechain pre-driver ax7 tube.

The big caps and bridge rectifier are for unregulated dc used in the sidechain.

If the little red heater regulater pcb can take 2.4A of 6.3V dc of heater for the GR tubes, then all great.
I use a dropping resistor before it to minimize the in-out difference voltage and therefore the power it needs to dissipate.
I think 2.4A at maybe 3V or so required -> 7W. It's a little tight, so maybe not workee. In which case, can get alt.

And the regulated lv +/- rails are for operating point references and general utility supply for relays/lights and vu meter buffer amp.

--

I will try the different methods of  cathode bias - the original Analag  cathode-clamper (Guarranteed to CLAMP you) from the PM660.
And also the original way(s) as researched from other builders descriptions and the schematics - maybe retain a choice.

So in fact it will be a serious but very obvious mkii and retaining some of the building blocks and features as well as 'taking a step further to the right' away from solid state sidechain and/or solid state make up/output amps.

A real upwardly mobile whilst still eyeing 'vintage tradition' kind of a thing.

Will look a lot like an upmarket rack guitar amp mostly. Who knows maybe I'll build a couple

 

[duantro]

I had a little time to work on my pm670 today. It powers up with good voltages. Slow start heaters, Rondo toroid and a Triad 9V 11A toroid for the Heaters. I have Russian 6n5p tubes (unmatched at the moment) also Lolo's Scamp booster. One side passes audio but has a low end cut and its about 10-20 db quieter than in bypass mode, and is fairly grainy. The other channel isn't passing audio. I've been checking the connections and the switches but haven't found anything obvious. Did Kevin/KHstudio ever post all of the voltage comparisons somewhere? I can't find them and this would probably help me trace some errors quicker.
I know it's recommended to build stock first, but after all of the issues and problems, I'd like to minimize tear down and rebuild as much as possible. I'm also curious what Kingston is working on, but in the time being I'd like to get this thing going. Can anyone suggest some areas to search or voltages to check on?
Here are some crappy phone pictures.

 

[alexc]

If one side isn't passing audio then you need to trace where it does pass to.
Thru the T1 traffo, onwards thru the level pot then thru the tubes onto the secondary of T2.

One needs to be well careful not to probe a hv point if you are using a signal-trace tool.

So - trace the audio. Where does it go?

Generally I found the 'rotary switch' assemblies was where I had a lot of issues. And correct wiring of the transformers.

10-20dB lower signal is quite a lot lower, so again you need to trace that signal.

You say your psu is looking OK, so how does your 'operating point' voltages, related to the trimmer 1 and 2 look?
They do quite a bit.

But first check, check and recheck your wiring, psu main voltages and  operating voltages.

If no good, then I would disconnect HV to one channel, leaving the tubes in place and the heaters all going.

Focus on getting the remaining channel working. By tracing signal and with a cro.
It got to be there somewhere!

Once you get one working properly, then bring back the other. Retrace your steps, checking psu voltages all the time until you get somewhere where things are as you expect, then slowly move forward.

Good luck - let know what you see in there.

Cheers

 

[duantro]

Thanks for all of the info Alexc!
I'm using a fluke 179 for voltage testing. Everything at the connectors are pretty much on. The channel that isn't working is only 6.25 at pm board heater connector, with the working one able to adjust to 6.3. All of the others are at 250V, 130V, and -17v. Rv3 and rv6 are -4.5 and -2.5 respectively.
I don't have a tone generator at home, just a small mixer and a mic. Imay have to take it to my studio and run a sine wave thru it to check signal easier. My meters peg to the right regardless of additional resistance added. I'll go over the connections again tomorrow morning.

 

[alexc]

6.25V at one pm pcb compared to 6.3V is not too much variation for an unregulated heater situation at high currents.

For regulated heater not to able to dial up the voltage higher may mean you may have sagged somewhat in the unregulated voltage
ie. the transformer is starting to have trouble supplying the current.

Have you checked the heater pcbs are both still regulating? What's the input voltage to the regulator for those heater voltages you mention.
You said you have a beefy seperate heater transformer, so current supply is not likely to be an issue.
You're not thermally blatting the regulator?

Anyway, if heaters are maintaining voltage and all looks OK and not cutting out and so on, then move on.

Assuming you have good basic voltages at psu and then at the trimmers in idle, then good.
If you are certain you have the transformers properly connected and that the switches are too and not falling apart or anything ..

You should have signal on the switches, on the grids of the gain tubes. Then look at T2 secondary. Is there sound?

If not, and the tubes all look normal  ie. no run away thermal events  you start to look at whether anything is getting out of the gr tubes.
That's hv multimeter on the plates and on the cathodes and check values of 6BC8.  Carefully.

If the cathode volts are too negative, your tubes maybe way shut down. And so now to look at the operating voltages.

But first, discount anything dumb by tracing that audio.

 

[alexc]

Just doing some in-studio quick measurements and observations on my pm670.

So the usual : power up wait a while to warm, set the gr meters for 0dB GR.

I send my test tone direct to the pm670, from my Motu converters.
PM670 is racked, nearby to others in rack powered up and all interconnected.
I receive my pm670 output direct to my Motu converters.

The Motu have a loopback (unity gain) noise floor I measure of -83.5dBu when measured as an SPL on RTA 20Hz to 20KHz, 24bit. That is with the RTA calibrated for absolute voltages at the Motu bal ins and outs. I use an RMS meter, my 2mV/div CRO.

Hardware bypassed PM670 is -80.0dBu noise floor with almost no low freq spikes and a gradually rising HF related to the Motu and the RTA.

So now, I set the GR off with the thresh at minimum, turn off the bypasses and  set the output level switch so that I see a unity gain signal at the RTA. 1KHz, -10dBu sent from the sig gen say.

Now turn off signal and get the no-GR unity gain noise floor, in circuit. I see -79.0dBu.

Now I check the shape of the output waveform on the same RTA display of the PM670 output.
At -10dBu input and no GR : all great, rounded and fine.
Run up to 0dBu, +10dBu etc till the Motu is maxx'd
Checking the signal returned from the PM670 - all still very rounded and starting to gradually triangulate at the highest settings.
But at some reasonable level, say up to +5dBu - all fine.

Now with no GR - repeat  but with increases of the PM670 make up amp as well.

Check the roundness as you go. I saw nothing unexpected and all pretty stable.

Now this is all steady no GR scenario - go back to a unity gain Level setting and this time add GR by turning up the threshhold. Go to something fairly strong - what you would think would be 10dB GR.

And again, check the waveform for any weirdness. Clicking the generator on/off so you get some GR transtions. I think I would see some glitching for an even of >0.5seconds.

For events of < say 0.5s you need to record the resultant waveform and manually inspect cause it would pass your eyes (and you ears?).

Then repeat all that with say 100Hz signal to check for lower freq effects.

Then maybe even some sweeping frequency, pulse and transient type input signals.

Looking for any visually noticeable type of glitches and what not.

-----------

OK - I did a lot of that - did not see anything out of the ordinary.

Driving softer gives less distortion.
Driving harder gives more distortion.
No obvious asymetrical clipping of output wave form I could tell from visual inspections.

Will do the recorded ones next.

------------

OK - and as well, maybe some Rightmark. I'll have a go now.

 

[alexc]

Rightmark reports reasonable noise floors, freq response - nothing great but OK. -95dB noise and dynamic range

Distortion pretty high at 2.3%. Intermodulation distortion + noise 2.4%. IM+noise swept freqs 5.2%
Quite a marked hi-freq droop. Like -2.7dB down at 20KHz - got to check that further. Low freq fine.
--> RTA shows max deviation best to worst is 1.86dB So +/- 0.93dB 20Hz to 20KHz  Not bad.

That was hitting it pretty strong - some GR was occurring even at min Threshhold.
So those test reflect some 3-5dB of GR in play.

Distortion spectrum is mostly 2nd harmonic and then odd harmonics linearly decreasing in amplitude.
2nd is 43dB down from peak odds from 52dB down and decreasing to > 70dB at around 5th harmonic.

You could do a manual weighted sum of those and probably confirm this is a tube amp running moderately strong. ie. greater than 1% probably 2% and driving hard >5% thd+noise.

And that before any dynamic and transient type distortions let alone control punch thru stuf is considered.

Not stellar but typical for a tube unit especially with a real world +4dBu input doing some 5 db GR outputing around +1dBu or so.

Adding more GR surely means more noise due to the makeup amp and more distortion generally due to the imbalances and all that stuff.


---------------

So there it is - noise around what I'd expect. Regular distortion also imho. Not too bad actually.
Could certainly improve, particularly under heavy GR which is where many of the wavemangling happens.

Now this IM and IM swept freqs is not so great  2.5% to 5.2% at fairly modest levels of around +4dBu or so outputs.
Fully pushed (+10dBu or so is where my Motus crap out), it could probably get higher - maybe 7-8% or so!

Could be a big deal. I'm not sure. So far, I am not alarmed. Such is the value of ignorance at times.

So now to look some more with recorded test tracks so I can catch the single cycle and more GR type of stuff.

And then to return to the listening tests again.

CHeeRts

 

[alexc]

Hey Duantro

That's a mighty fine looking build there. And the whole extended version too!

That wiring looks very tidy and providing it's correctly connected, should help stability quite a bit.
I wonder how the noise performance will be - I started with that traffo config and moved to the far side to get the best noise immunity. You may see that yourself when you get that far.

Also, if you don't have a signal tracer, a pair of cheap amplified pc speakers with a homemade probe connected to their inputs makes a handy tester that won't break the bank if you fritz something too hot.

There is probably a circuit where you can add some diode/resistor/cap for protection.

Or not! Mine is just have-at-it.

You also really need to have a sig gen and a cro unless you are very lucky or very good with more basic test tools.
Finally, an RTA on a soundcard is pretty essential to get noise and stuff under control.
Then you could look for really detailed stuff. Or not, if it works to your liking.

Is your GR metering registering any movement or is it just pegging all the time?
My GR meter mod was to add a very low resistance pot in parallel. Like 100R or 250R. Might have been 500R.

Cheers

 

[duantro]

Thanks for the compliments. I plan on tidying up the wiring some, when it is functioning. I'm going to build a signal trace probe now. Thankfully we got 12" of snow yesterday and my studio sessions have rescheduled!
I can use an phone app signal generator to trace. What is a cro?
Gr meters are registering when large signals compression takes it off of being pegged.

 

[alexc]

What is a cro?

Priceless 

 

[alexc]

If the GR meters show some GR then you are certainly getting there.

Interesting scenario - a build which looks pretty good, and the full monty set, mostly working, apparently.

But no CRO and which suggests no detailed checking of everything.

Indeed, no signal tracing either, so this would be a build, obviously not smoking on power up, mostly working 'off the bat' ?

Of course you would know to be pretty careful in there - we all know there are some painful voltages in quite a few unexpected places, such as the meter, centre-taps of some signal transformers and of course, the psu - particularly if poke around adjusting the trimmers in various places. Or with an audio signal tracing probe.

Make sure you know what you are going to poke before you go ahead. And don;t get complacent after not dying for a while.
It can happen any time.


Anyway - intriguing. Look forward to hearing further about getting the PM670 into service.

Cheers

 

[duantro]

I assume you're referring to an oscilloscope, and not this ;D
http://www.trendsofthefuture.com/pandarapper-cro-german-rapper-with-panda-mask-breaks-music-charts/

 

[alexc]

You are correct.

Looking further at your build - you have the 6n5P in there, so thats some 600mAx4 -> 2.4A + 4x5687 so thats 3.6A there,
for a total of 6A total or 3A per slow start heater regulator pcb.

Interesting to know how that little regulator board is coping - thats a pretty little heatsink and not so much resevoir cap.
I can't see the heater supply dropping resistor, but I assume your heat is undercontrol otherwise it probably would suffer thermal cutout on the heater and the the affected channel would stop passing signal until it cools down again.

I would check that area - it may be all good. Best check.