Re: EMI RS124
« Reply #380 on: March 13, 2020, 07:03:17 PM »
If I pull the 6AL5 I have the  waveform doing the same thing.     Does this mean that I have little or no compression happening ?  I have tried maybe 3 different 6AL5's.

If you pulled the 6AL5 and it's still the same problem then I would say there's an issue with the audio amp.  I assume you've looked through everything for wiring errors and checked the voltages yes?
Might be best to probe through the audio starting at the input to see exactly where it's getting squirly.   

P.S.  not really the same as having 'hands on' but, you could post a few high res pics to see if another set of eyes sees anything amiss?


Rob Flinn

Re: EMI RS124
« Reply #381 on: March 13, 2020, 07:18:08 PM »
Thanks John.   I will post some pics tomorrow if my time doesn't get utilised by the other half.   

Currently I'm using an Edcor output transformer.  Not sure if it's doing anything funky to the output, but I have pretty much spot on 145v on the anodes of the 6cg7.  Other than that all quiescent  Voltages seem to be good.
« Last Edit: March 13, 2020, 07:25:41 PM by Rob Flinn »
regards Rob

Re: EMI RS124
« Reply #382 on: March 13, 2020, 07:27:45 PM »
OK cool. 
As long as it's not faulty, I don't think your Edcor would cause that issue but I suppose it's possible.  Sounds like both halves of your 6CG7 are biased up correctly anyway.   
As I said before - you'll get there. 

Rob Flinn

Re: EMI RS124
« Reply #383 on: March 14, 2020, 09:51:04 AM »
Seems to have been the output attenuator that I used, which someone posted earlier.      If I just strap a 620 on the output it seems much more comfortable.    I still get the funky waveform at max input, but the control signal will go to -18v.       Maybe the 6cg7 isn't quite ballsy enough for a low load.   

Not quite sure what is a good attenuator for this, there have been a few posted some using precision resistors which I don't really wnat to have to buy.   I may just try a couple of 300R with a 1k log between them, like you suggested to me years ago for a different project.     

Currently I don't have a meter attached, because the only one I have is 500uA F.S.D & that seems to screw with the biasing, somehow.  I have some el cheapo 200uA meters on order, but who knows when they are likely to arrive from the far east at the moment.
regards Rob

Re: EMI RS124
« Reply #384 on: March 14, 2020, 11:11:05 AM »
OK that sounds much better, great stuff.   
I  don't think you can get much more out of a 6BC8 than the 18 odd volts CV you're measuring now.  A 6ES8 will get you to a bit over 20 volts and starts off with a higher standing current (lower anode voltage) so you can squeeze a bit more out of the unit if you want.

The simplest output attenuator would be a 1K audio taper pot across the transformer so you might give that a go.  It's been  years since but I think that's what I did on the Lisson Grove comp that was made for a while.

Edit:  forgot to say that, with  a meter to see what's going on, you'd probably be indicating near enough to -20dB gain reduction with that -18V on the grids.  The 6BC8 just has nowhere left to go as the plates are right up near the rails at that point so that's why your waveform is looking stranger there.   Still, that's a lot of Gain reduction so...   :D

Secondly, I'm not quite sure why your output attenuator was causing the control voltage to only rise to 3.3 volts tbh? 
« Last Edit: March 14, 2020, 11:25:46 AM by Winston O'Boogie »

Rob Flinn

Re: EMI RS124
« Reply #385 on: March 14, 2020, 01:29:39 PM »
Secondly, I'm not quite sure why your output attenuator was causing the control voltage to only rise to 3.3 volts tbh?

Me neither.    Although the transformer is supposed to be 600R out & the attenuator measured 300R in all positions.     I realised I made an error with one of the resistors  but swapping it for the correct value which gave me 600R  still messed it all up.

Anyway thanks for the encouragement.
regards Rob

letterbeacon

Re: EMI RS124
« Reply #386 on: March 24, 2020, 06:38:56 AM »
The simplest output attenuator would be a 1K audio taper pot across the transformer so you might give that a go.  It's been  years since but I think that's what I did on the Lisson Grove comp that was made for a while.

This is a stupid question, but how would you connect a 1k pot across the transformer? Do you mean a stereo pot? I'm using the Sowter transformer designed for the Altec 436.

Rob Flinn

Re: EMI RS124
« Reply #387 on: March 24, 2020, 09:06:11 AM »
This is a stupid question, but how would you connect a 1k pot across the transformer? Do you mean a stereo pot? I'm using the Sowter transformer designed for the Altec 436.

See attached. This is how I've done it in the past.

You take your output across the pot.    You can turn the signal all the way down , but the output transformer always sees at least 600R.  You could probably use just one 620R resistor, but I've always doen it this way.
regards Rob

letterbeacon

Re: EMI RS124
« Reply #388 on: March 24, 2020, 09:11:54 AM »
Oh great, thanks for this - will give it a go.

My RS124 has been sitting unfinished for a while, with similar problems to yours, so hoping to crack on with it now/

Rob Flinn

Re: EMI RS124
« Reply #389 on: March 24, 2020, 09:30:08 AM »
Mine are both now working, but, I only have one output transformer & I've been waiting on the right spec meters from China for sometime now.   I ordered some Sowter outs last week but they had a 4 week lead time when I ordered them which has probably now turned into several months, so I have switched to other unfinished projects while I'm waiting.   Once they are both working I will get some decent panels made for them too.
regards Rob


letterbeacon

Re: EMI RS124
« Reply #390 on: March 24, 2020, 02:38:16 PM »
Just out of interest, did you build the power supply as it is on Winston's schematic? What power transformer did you use?

Rob Flinn

Re: EMI RS124
« Reply #391 on: March 24, 2020, 04:35:15 PM »
A good question.    I have a pair of Permeko vintage mains transformers in there at the moment.  The problem is they are 300v C.T.   I was hoping to build a psu like the original altec one, in the hope that the load is so low the voltage that the 150v would sit up to be correct.  This doesn't work, so I can get it to run with a bridge rectifieracross the 300v a la RS124 but I need a hell of a big dropper resistor.   So, I'm not quite sure what I'm going to do in the long run.   I could either change the transformers or possibly mess around with the 10k & 6k8 resistiors in the psu.   Transformers are so expensive compared to a few years back, which is why I wanted to use these ones.

 Anyway they're on the shelf until the o/p transofmers get delivered.
regards Rob


 

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