[R201, R202 Values Confirmed] Tube-tech MEC 1A R201 & R202 ohms? (and other questions)

[babisbabis]

Hi all!
I am Babis from Greece.
A little of history first, I'll try to be short:
I have a friend's Tube-tech MEC1A (Lydcraft) for repair. In fact he gave it to me for adjustment.
But I realized that something is wrong with it, when I noticed (adjusting it of course) that the "gain trimmer" was at the "minimum gain".
Anyway, it seems to me that the final stage with tube ECC82 doesn't work as it should. The output isn't stable. For example, if you input a sinewave signal of -20dbu (like the adjustment procedure recommends) and you set the output gain potentiometer for an output of +4dbu (0vu on most vu-meters), after a while it varies between +/-2db...
I've measured the psu voltages of course. The 12VDC Test Point for 12VDC (heaters) gives me 11.2VDC (?). +/-150VDC is ok (adjustable), 240VDC is ok (adjustable), and 270VDC behaves a little strange... I remember my first measurement was about 256VDC. I noticed that it's not stabilized. Yesterday I measured it 270VDC. The tube (ECC82) was in its base. I saw from the PCB that these 270VDC, are for the ECC82's anode(s) only (via the output transformer).
Well, the 2 resistors that I mention in the title, R201 & R202 are on this rail of the 270VDC. Someone has replaced them and I guess he/she has replaced them with other values than the original ones. I suppose so, because he/she has written their values on the PCB. R201 is 1.5k 5W (or 3W), and R202 is 470R 5W (or 3W).
Anyone knows what is the right value for these resistors?
I wish I had the schematic, but I guess this is from difficult to impossible...
I've found here the schematic of LCA-2B in which the PSU section for the 270VDC is very similar to the MEC1A's. I've included an image with this part of the schematic with the few differences.
*Without the ECC82 on it's base, the "270VDC" measures about 320VDC (i.e. without load) if I remember well. Ok, this is unstabilized voltage, rectified and filtered with two 100uF capacitors (through 2 resistors). But isn't it a high voltage for this tube? Just a question...
*The ECC82 turns really hot. The other 6 tubes have considerably lower temperature. Maybe this is normal, I dont know...
*I am afraid that maybe the output transformer could be suspect too...
*And of course the question of the title: What is the real value of R201 & R202? If someone knows, I'de be glad.

I tried to be short, but I think I didn't manage to...
Sorry for the big post, any help welcome!

 

[scott2000]

The output isn't stable

Guessing you let it warm up for a while before trying calibration?

I remember my first measurement was about 256VDC. I noticed that it's not stabilized. Yesterday I measured it 270VDC

Is that supposed to be regulated?

Someone has replaced them and I guess he/she has replaced them with other values than the original ones. I suppose so, because he/she has written their values on the PCB. R201 is 1.5k 5W (or 3W), and R202 is 470R 5W (or 3W).

Can you be certain they didn't write the correct values ?

But isn't it a high voltage for this tube?

Seems close to the typical operating conditions?

Have you tried new tubes?

 

[babisbabis]

Hi and thanks for replying.

Guessing you let it warm up for a while before trying calibration?

Yes of course I did.
Also, same issue after 2-3 working hours and even more.

Is that supposed to be regulated?

No. I just mentioned that, after checking the PCB and the components (reverse engineering). For example +/-150VDC are regulated.

Can you be certain they didn't write the correct values ?

I am not certain of course. I don't have the schematic. But if they were the correct values, why would they write them on the PCB? Would you? But I wish they are the correct ones...

Seems close to the typical operating conditions?

Have you tried new tubes?

The first one is a question to me? Well, it could be close to the typical operating conditions but I'm confused... I saw in a ECC82 datasheet specifications for 200VDC, 250, 300, 350 and even 400VDC...
Also I found some schematics with this tube, but the anode voltage was 175 - 250VDC (depends on the schematic). I really don't know...


As for if I have tried new tube...
No, I have not tried a new tube. But I found an old one (philips ECC82), in an old amplifier. I feeled very lucky for this. I put it on and when I switched the MEC1A on, the tube became "red" for a while (like 0.5 sec) and then nothing. I dont know what this was, but it was not good for sure... But I let it switched on, and then I measured the output again. Same results... It was working like the previous tube, with the variances in the output...
Was that lighting the disaster/damage to the tube that causes these alternations to the output? I don't know..

 

[scott2000]

Hard to say really. Guess trying to draw out the circuit would be helpful.
Leaky caps is always a favorite thing to check? Maybe a leaking coupling cap messing with the bias? IDK...

 

[babisbabis]

@scott2000 you're right. Drawing the circuit is very helpful. I've done this for the "amplifier" section by hand. I mean the driver tube (ECC83) and the following final tube (ECC82) with the output transformer.
But it's quick-made by hand and I don't dare to upload it here, it's really awful
I'll try to "edit" the existing schematic of LCA-2B (amp section) so MEC1A's one doesn't seem so ugly...

P.S. The wiper of the "Output gain" potentiometer goes (via a 470nF) to the driver ECC83 (pin2, grid) and the AC mV on the wiper is stable. That means to me that the issue is in the amplifier section (including of course ECC82, output transformer AND ECC83 with all the passive components around them).

By the way, here are the "typical characteristics" from Philips' ECC82 datasheet



Below the "typical characteristics", there are "operating characteristics" of the tube as "af amplifier" with power supply voltages 100 - 400VDC. But all projects of course, have a resistance in series between power supply and the anode (47k - 220k in the datasheet). That's normal.
I attach the Philips ECC82 datasheet.

 

[scott2000]

The wiper of the "Output gain" potentiometer goes (via a 470nF) to the driver ECC83 (pin2, grid) and the AC mV on the wiper is stable. That means to me that the issue is in the amplifier section (including of course ECC82, output transformer AND ECC83 with all the passive components around them).

Maybe checking some dc voltages around the output stage while it's goofing up would be useful for others to chime in.
Yeah I can't draw so know what you mean. But other's here can get a good idea regardless because that's the language they speak best.
You'll get it figured out...

 

[babisbabis]

Maybe checking some dc voltages around the output stage while it's goofing up would be useful for others to chime in.

Yes, I've already done it but I didn't write the results down. I guess I'll do it again.

Yeah I can't draw so know what you mean. But other's here can get a good idea regardless because that's the language they speak best.
You'll get it figured out...

I'll post the "amp" schematic when it's "more presentable"..
Thanks for your help Scott. I appreciate it

 

[babisbabis]

I've almost done the schematic of the "final stage" of the MEC1A. It's not well drawn, it's made in "windows paint" () but at least it's a schematic...
I'll post it soon.
Also I've sent an email to Lydkraft & to their service in Germany for help about the output issue, but still no response (I don't expect something to be honest...). But this issue is something that I should open another thread for, I think.
Meanwhile, anyone knows the original values of R201 & R202 as the title says? Or anyone just to confirm that the ones that are in place are the correct ones?

 

[scott2000]

and 270VDC behaves a little strange... I remember my first measurement was about 256VDC. I noticed that it's not stabilized

Looking at the manual , I can't see a mention of 270vdc?
What stage of the calibration are you working on?
Looks like member/guru Matt Syson has worked on a lot of Tube- Tech stuff. Maybe he'll have some direction for you.
You'll get it figured out.

 

[babisbabis]

Looking at the manual , I can't see a mention of 270vdc?
What stage of the calibration are you working on?

270vdc isn't mentioned in the manual, you're right. Also 270v are not included in the calibration process, they are not adjustable. But there is a test point on the PCB for it (TP201, +270VDC).

Looks like member/guru Matt Syson has worked on a lot of Tube- Tech stuff. Maybe he'll have some direction for you.

I wish he will, this would be nice!

After some (in fact a lot of) reading here in the forum about Tube-tech devices, I will replace all the electrolytics, all the rectifiers and (maybe) the Evox MMK caps. The tube bases are already replaced with porcelain ones but without gold round female "receptables" for the tubes. Also I'll try (meaning I'll ask my friend) to replace all the 7 tubes, too.
I think there is nothing I should replace on the front panel PCBs.
The Tube-tech devices suffer from high temperature, as I read here, so I'll try the above and take measures about it. Then re-adjustment of course, and check if the issue still exists.
But I still don't know the values of R201 & R202. Maybe they are the right ones.

Thank you very much once again for the response Scott!

 

[gyraf]

..you can run the output tube a bit gentler by lowering the output-stage voltage - by upping the drop resistor(s) to the "270V". This is probably what was done to some degree already, and is what you see in there. Yes, you'll only loose a wee bit of output drive headroom - but this unit dosen't sound good when driven to such levels anyway, so it's a small or no loss for gaining long-term stability of output tube. I'd have no fear of going even as low as 240V DC at the transformer

Best way forward is probably to replace output tube with known-good ecc82/12AU7 or 5963 (my current favorite variety) and see what happens. These wear systematically when subjected to such load, so it's a good investment for the owner even if this dosent fix it.

For gain variations after fully heating up, make sure it isn't 1) dirty switches and pots (massage throughly) 2) the output tube socket connection (bend into shape AFTER discharging the HT PSU)

Attaching the adjustment procedure, just in case you don't have it

/Jakob E.

 

[babisbabis]

Hi Jakob! Thanks for your reply
I have absolutely no problem of going down to 240vdc to the transformer, and in fact my next step was to add another resistor in series, just before the transformer. And after taking my measurements, I will replace the components I said in my previous reply. But for sure, I wanted to know the original values of these two resistors... (R201, R202).
I leave the device enough time to get fully heated, each time I want to measure/check something. Sometimes I forget it over an hour switched on, because I am busy with other things
As for the issue with gain variations, the audio signal (sinewave input in xlr) before the driver ecc83 is very stable and it seems ok, so the various switches, potentiometers etc. are out of question for me, as they are all before the output gain potentiometer. Also I've switched them to be "off", something like "bypass".
Post #5:"The wiper of the "Output gain" potentiometer goes (via a 470nF) to the driver ECC83 (pin2, grid) and the AC mV on the wiper is stable. That means to me that the issue is in the amplifier section (including of course ECC82, output transformer AND ECC83 with all the passive components around them)."
I have the adjustment procedure, else I could not do it. But thank you very much.
And of course, thanks again for your reply!

 

[babisbabis]

Also I've sent an email to Lydkraft & to their service in Germany for help about the output issue, but still no response (I don't expect something to be honest...). But this issue is something that I should open another thread for, I think.
Meanwhile, anyone knows the original values of R201 & R202 as the title says? Or anyone just to confirm that the ones that are in place are the correct ones?

Update: The values of R201 & R202 are the right ones.
Lydcraft replied to my email confirming it.
So, R201 is 1.5k 3W and R202 is 470R 3W.
As for the issue, he could not help of course just reading the symptom...
I've ordered new caps, rectifiers and we'll see...