JCM800 2204 Phase Inverter issue and other

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This is the strange thing. I've 455Vdc at plate, 452Vdc at SG2. With this values moving the Bias trimmer the values are:
- from 33mA of Bias current with -36Vdc at the node between RG resistors
- to 6mA of Bias current with -44Vdc at the node.
May be something not working well in the Bias circuit?
 
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You should measure the bias voltage for each tube at grid 1. There will be voltage drop across R24/R25 (220k) and grid stopper R31/R32 (5k), depending on internal grid resistance leakage. If the leakage resistance is different, the current will be different even with correct bias voltage supply adjusted. Because it's share bias circuit, you can only get matched tubes to get equal current. First measure the voltage drop and actual bias voltage at grid 1.
 
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You got fairly good match pairs, then you can reduce the bias some more so to increase current to 30mA or to the point where the cross-over distortion become acceptable to you ears. Hope you can get nearly equal current again. For output tube you should also monitor cathode current in each tube when driven beside the DC match.
 
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Thanks Koonw. I increase the current as you suggest and now I've the following situation:
- V4 Bias current 29.5mA, GR2 voltage -35,9Vdc
- V5 Bias current 27,8mA, GR2 voltage -35,8Vdc
A couple of questions (thanks for your patience):
- the GR2 voltage seems quite lower than what I read on many comments and forum (generally they speak about -40Vdc or higher, the Mark Huss schematic report -57Vdc). This can generate issues?
- I still have some red plating when Gain and Master are fully clockwise and I apply a signal (400Hz, sine wave) to the High input (with my Fluke DMM I measured around 240mA of Bias current in this situation). Can this reduce a lot the life of power tubes?

Tomorrow I go ahead with the guitar, to check the sound.
Thanks again
 
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Place the scope across the 1 ohm cathode resistor (noted the ground point) and post the images of waveform for V5 and V6 in dual channel mode if possible. Also HT voltage sage at full output prefer under clipping and heavy clipping and when does the red plate begin? Also the bias voltage when red plate.

The power output is calculated as Output voltage (RMS) sq/load. To convert the peak voltage (on the scope) say 40V peak-peak is 40/2=20V peak, RMS values is 20V*0.7=14, 14*14/8=24.5 rms watts. I strongly advise you install a 150mA fuse on each cathode at least during testing.
 
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In the following the results of the suggested test (in the scope images the trace below is V4 and the above is V5):
1) Pre & Master 12 o'clock
20181015_203557.jpg
very few red plating in the middle, where plate crossing
Grid voltage V4 -37.5Vdc, V5 -36Vdc

2) Pre 12 o'clock - Master fully clockwise
20181015_203604.jpg
more consistent red plating, quite more on V4
Grid voltage V4 -52Vdc, V5 -62Vdc

3) Pre fully clockwise - Master 12 o'clock
20181015_203615.jpg
red plating more than 1) but less than 2), more on V5
Grid voltage V4 -44Vdc, V5 -42Vdc

4) Pre & Master fully clockwise
20181015_203621.jpg
red plating more or less like 2), more on V4
Grid voltage V4 -56Vdc, V5 -61Vdc

What do you mean with HT voltage sage and where to measure it?
Many thanks
 
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What do you mean with HT voltage sage and where to measure it?


That is B+ voltage on CT of OT or before the T3 choke during no signal and when driven and have red plates. So before it's 455V, what is it during red plating? The idea is to establish if bias is affected or not. So if B+ has sagged, so will the bias voltage dropped and hence tube current increased and red plating.

I have yet to study the images, why the bias is being increasingly more negative sometimes? Maybe you need to measure bias voltage before bias resistor between r24 and r25, to avoid picking up the AC signal.


P/S my wrong spelling SAG not sage
 
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That is B+ voltage on CT of OT or before the T3 choke during no signal and when driven and have red plates. So before it's 455V, what is it during red plating? The idea is to establish if bias is affected or not. So if B+ has sage, so will the bias voltage dropped and hence tube current increased and red plating.

I made the test with the same positions of Gain and Master Vol as above.
In idle the B+ at the point you indicated was +459Vdc
In the other condition:
1) Pre & Master 12 o'clock
B+ 414Vdc
very few red plating in the middle, where plate crossing
Grid voltage V4 -37.5Vdc, V5 -36Vdc

2) Pre 12 o'clock - Master fully clockwise
B+ 396Vdc
more consistent red plating, quite more on V4
Grid voltage V4 -52Vdc, V5 -62Vdc

3) Pre fully clockwise - Master 12 o'clock
B+ 403Vdc
red plating more than 1) but less than 2), more on V5
Grid voltage V4 -44Vdc, V5 -42Vdc

4) Pre & Master fully clockwise
B+ 392Vdc
red plating more or less like 2), more on V4
Grid voltage V4 -56Vdc, V5 -61Vdc

I have yet to study the images, why the bias is being increasingly more negative sometimes? Maybe you need to measure bias voltage before bias resistor between r24 and r25, to avoid picking up the AC signal.
During the measurment I heard (I have a small speaker connected to the dummy load) that when I attach the probe to measure the Bias voltage the volume rise slightly up.
 
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I saw other people fixing the red plate by changing out bias capacitors C15 and C20, 10uF each. You can connect another 10uf cap in parallel to see it's doing anything. So a larger cap will reduce bias voltage drop for same B+ sag. From the images I saw one tube clipping earlier than the other, so the B+ sag cause a drop in bias voltage some more to the other, making it even lower and hence more tube current and red plating. The tube that clipped earlier has higher transconductance or Mu than the other, unless the PS output is asymmetry. If you have function generator you can check waveform symmetry with triangular waveform before and after output stage.
 
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i still think there's a problem in the bias supply and leakage in the phase inverter caps ( yes i know they are "new and recently replaced")
the thing i'm finding strange is when your pre is at twelve o'clock and the master is fully clockwise your B+ is at 396 (sagged) but your bias voltage has increased by nearly -20 volts, not sure how that's possible... the output tubes must have supressor grid leakage. in fact can you confirm that the output tube socket wiring is correct that the screen grids are common to cathode and the suppressor grid 1k's are ok and tied to B+? at this point with all the red plating that these tubes have done they may be cooked!
has the B+ line been checked for oscillation? and have the main filters been replaced?
the other thing i would look at is the feedback line off the transformer are you certain it from the proper winding and the right phase to produce negative feedback ( if it' wrong and positive that could be what's behind the red plating with increased gain)
 
I saw other people fixing the red plate by changing out bias capacitors C15 and C20, 10uF each. You can connect another 10uf cap in parallel to see it's doing anything. So a larger cap will reduce bias voltage drop for same B+ sag.
I replace both the Bias capacitors (I suppose you indicate C19 and C20) and also the diode D1 and the 220K resistor R30. No changes.

From the images I saw one tube clipping earlier than the other, so the B+ sag cause a drop in bias voltage some more to the other, making it even lower and hence more tube current and red plating. The tube that clipped earlier has higher transconductance or Mu than the other, unless the PS output is asymmetry. If you have function generator you can check waveform symmetry with triangular waveform before and after output stage.
Below you can see the image with triangular waveform before (below) and after (above) the output stage.
20181016_205130IO.jpg

i still think there's a problem in the bias supply and leakage in the phase inverter caps ( yes i know they are "new and recently replaced")
I haven't new caps at home. Probably tomorrow I find them and try a new replacement

in fact can you confirm that the output tube socket wiring is correct that the screen grids are common to cathode and the suppressor grid 1k's are ok and tied to B+? at this point with all the red plating that these tubes have done they may be cooked!
I triple checked the wirings and I quite sure they are correct. Below a couple of picture of V4 and V5.
20181016_210408V4.jpg
20181016_210516V5.jpg
I suppose the tubes are not cooked because I don't let them the red plate for a lot of time. When red plating arise i reduce immediately the volumes.

has the B+ line been checked for oscillation?
What do you mean for oscillation? there's a little of variation but within the range of +/- 1Vdc when the amp is warm

the other thing i would look at is the feedback line off the transformer are you certain it from the proper winding and the right phase to produce negative feedback ( if it' wrong and positive that could be what's behind the red plating with increased gain)
In the image below you can see the input signal to power tubes (below) and the feedback signal (above). They are opposite so I suppose it's correct.
20181016_205433INF.jpg
 
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Quote:
Originally Posted by turk 182
yes that's oscillation try increasing the horizontal time base.

top trace in the first pic in post 72 shows it
What I've to look for?


The waveform you used is the triangular waveform, is it? You adjust the tone stack to get more accurate waveform.

From the multiple ghost images waveform of first pic, it appears due the high ripple in B+ supply.

From wavy waveform at top (input), it appears there are some low frequencies, 50/60 hum perhaps, causing it unable to display straight horizontally, so you increase the time base you see it better (or without the input signal).

So you hook the probe on CT of OT to see and measure the amount (between low and peak) ripple voltage under load. Use 0.1uf/600v cap in series if your probe can not withstand 400V DC.
 

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The waveform you used is the triangular waveform, is it? You adjust the tone stack to get more accurate waveform.

From the multiple ghost images waveform of first pic, it appears due the high ripple in B+ supply.

From wavy waveform at top (input), it appears there are some low frequencies, 50/60 hum perhaps, causing it unable to display straight horizontally, so you increase the time base you see it better (or without the input signal).

So you hook the probe on CT of OT to see and measure the amount (between low and peak) ripple voltage under load. Use 0.1uf/600v cap in series if your probe can not withstand 400V DC.


Results of measures on CT of OT (between low and peak):
1) With no input signal and gain/master fully counterclockwise (x10 probe, 0,1Vdc/division, 4 division on screen) --> 4Vdc
20181017_201142.jpg
2) with triangular wave input, 400Hz, and gain/master fully clockwise (x10 probe, 0,5Vdc/division, 4 division on screen) --> 20Vdc
20181017_202421.jpg

Frequency (1msec/divisiojn, 10 divisions) --> 100Hz

Thanks again for your help and patience. I've a question for you gurus: the troubleshooting of an amp is always so complicated, or it's due to my inexperience?:confused:
 
i wish i could see the waveform in action. (could be diode breakdown happening here)

just to be certain your saying the ripple voltage is 20 volts?

and yes ,even with experience, troubleshooting some amps can be complicated. that's why it's important to do things carefully, and assemble the clues methodically to get a proper diagnosis of the fault and never jump to conclusions or make suppositions you can't verify.

i mentioned it earlier on a "dim bulb tester" did you ever assemble one? i like supply testing with one powering the device i'm working on as it's a quick visual signal that current levels are changing, i.e. high frequency oscillation (some won't show up until your at the right power level or frequency)
 
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<snip> What do you mean with HT voltage sage and where to measure it?

I'm going back to the question why grid bias got increasingly negative. Look like you're not alone: https://www.thegearpage.net/board/i...m-im-stumped-need-some-expert-advice.1570583/

RussB said: ↑
Yes, do that while the tube goes into runaway
Bias voltage fluctuates on both tubes when playing. It goes to the -50V to -52V range or so (amp on full volume), then stabilizes back to -44V (idle).

#9
The problem is eventually fixed by replacing C19/C20 10uF bias cap, according to above bbs, not sure exactly how. It has to do with clipping, the bias voltage is affected due to a change in grid leakage (resistance) under clipping, and thermal run away set in due to unstable condition herein. Welcome anyone to chime in to brighten this.

I agreed changing the bias cap wouldn't fix the problem but change bias resistor 220k to 100k might fix it.
 
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i wish i could see the waveform in action. (could be diode breakdown happening here)

just to be certain your saying the ripple voltage is 20 volts?
Yes, I confirm 20 Volts of ripple voltage. What do you think about? Is a value so much greater than usual? Or usually there is no ripple at all? Can ripple generate redplating? Do you suggest to replace rectifier diode (all of them?)? Are there different sources for such a behaviour (ripple)?
Thanks
 
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