JCM900 4500 Low output, grid voltage issue

[Mr_bibbles]

Hello,

I have a JCM900 4500 50w Dual reverb (JMP50B PCB like the one in this photo: Photobucket )

The issue is low output, preamp out sounds fine, power amp in produces almost nothing.

With light bulb limiter, grid voltage is -10vdc, and plates are around 330. So, I checked and replaced 10uf Bias capacitors, checked resistors in that section, still low output, grid voltages remain too high (we are shooting for -45 yes?).

Other stuff:
-Signal sounds fine out of PT, signal is present on pin 5 of output tubes. ( I don't have a scope but I injecting signal to another amp with capacitor / ground clip.)
-Checked IC voltages, + and - are all present.
-ran caps in parallel to large Filter caps one at a time, no change.
-Tested bridge rectifier, all diodes measure fine but I have .012 AC at the BR's exit point.
-OT measures well through simple checks.



SO here are my questions:

1. Can I assume that the issue is caused by the grid voltage not being negative enough? Any idea what could bring this about?

2. The bias resistor / bias trimpot / bias diode / associated resistors are all measuring well. Is it possible there is a short from the PT secondaries, supplying incorrect voltage to the bias circuit itself?

3. Should I be worried about the small AC present after the BR?

Thanks as always for your expert wisdom.

Thank you for your help.

SCHEMATIC: (not exact, but closest I could find to the 4500 version I have: http://www.thetubestore.com/lib/thet...-Schematic.pdf)

[g1]

The proper voltages will only occur if you have 120V coming out of your light bulb limiter.
Was it blowing fuses or any reason you are not plugging it straight in to the wall?

[Chuck H]

+1 to what g1 is asking about. You seem to think output at the PI is correct? Yes or no? So what about the power tubes? Bias should be measured WITHOUT the limiter or power tubes in the circuit. Then, unless a fuse is blowing, without current limiter and WITH the power tubes in place. The low output needs to be isolated in the circuit. Any blowing fuse should be corrected first, THEN the circuit can be traced to isolate the problem. If no fuse is blowing and the power tubes don't seem to be melting down, ditch the current limiter for now.

[Mick Bailey]

I would remove the output tubes and check the voltages at the sockets (without the limiter). Also check the cathode fuse is OK on the output tubes - if this fails you have virtually no output - just some tiny signal due to leakage through the LED circuit. Check you have no resistance from pin 8 to ground with the amp powered off.

Insufficient bias voltage will cause excessive current draw and red plating in an otherwise working amp. You say you checked and replaced the bias caps - so were these faulty? Did you double-check the polarity when you replaced them? Positive goes to ground.

The residual AC on the rectifier output is just PSU ripple and wouldn't concern me.

[Mr_bibbles]

Thanks for your replies-

g1- I'm using a limiter as the JCM is red plating the tubes with the Grid Voltage being this low. The current draw is not EXCESSIVE but the lightbulb is slowly getting brighter when standby is off. Without the limiter, the grid voltage is -13vdc, still crazy low. 120v is coming in at socket.

Chuck-
What should I expect from the output measured across the speaker outs? I'm getting .004 AC from tip to sleeve, which I'm guessing is crazy low?
I'm familiar with biasing procedures and plate voltage measurements, but my guess is the high Grid Voltage is the culprit- I just don't know where else to look other than the bias circuit. The Tubes are red plating, (my 'test' el34's, though I've tested it with multiple tubes with no change in the grid voltage.)

Plate voltage (NO LIMITER)
With tubes: 350vdc
Without tubes- 450vdc
On standby- 35vdc????? Shouldn't this be 0?


Mick:
See measurements above for plate voltages. As for Cathode Fuse, I'm guessing you mean the 100k resistor next to the fuse / LED indicator? It measures 99k
I have less than 1 ohm resistance to chassis from pin 8 with amp off. I did replace the bias caps, one was ker-ploded, I replaced the other just to be safe. The polarities are correct, the only to note is I replaced one 10uf cap with two 22uf's in series. I also replaced the c15 film cap next to the electrolytics.

Any ideas of where to start looking?

[Chuck H]

I'm suspecting that something is loading your power supply even without tubes installed and drawing current. 450V B+ would be low for a 900 series amp IMHE. Also, your bias voltage is low. That may be a bias supply issue, but it may also be indicative of some fault loading the power supply.

Did you replace electrolytics as a matter of maintenance or as an attempt to fix a problem? If the work was just maintenance then you'll want to recheck all wiring, because, if the amp was "working", but really was due for new caps, and now it doesn't work, then something that happened during the course of the work is the most likely cause of the new problem.

[g1]

You can continue without the limiter with power tubes removed until you can get the bias right.
What is the AC voltage at the side of the standby switch that connects to C15 when in standby mode?
DC voltage at D1 anode?
And you were sure to put the positive end of the bias filter caps to ground? (C13 & C14)

[alexradium]

You have a bias problem, in standby it's got to be in the 80v range, then it goes down to whatever is right for your tubes, 40/50 i guess.
Leave out power tubes and give full voltage, check out the voltage after the c15 cap and 56k resistance.

[minim]

This may be useless info however I was mildly curious so here are the readings from my 900, Model 4502:

Mains is 127v.

With tubes, in standby:

Plate voltage 130.
Bias @ pin 5: -87.

With tubes, in play mode:

Plate 474
Bias @ pin 5: -53

Without tubes, standby:

Plate v 126
Bias @ pin 5: -87

without tubes, play mode:

Plate 507
Bias @ pin 5: -51

[alexradium]

exactly

[Chuck H]

Plate voltage without tubes and without limiter is reported at 450. Bias voltage under the same conditions is -13.

DID THE AMP EXHIBIT THESE PROBLEMS BEFORE YOU CHANGED THE BIAS CAPS???

and/or: Under what circumstances did the symptoms manifest?

The amp isn't providing correct voltages without tubes installed. This needs to be addressed before any measurements with tubes installed can be relevant.

Something is drawing current such that the B+ is down over 50V and the bias voltage is down to almost nothing. Since the bias supply was worked on, I'll repeat... If these symptoms manifested AFTER the work on the bias supply you should recheck that work for accuracy.

And please provide the readings requested by g1 as well.

[Mr_bibbles]

I'm suspecting that something is loading your power supply even without tubes installed and drawing current. 450V B+ would be low for a 900 series amp IMHE. Also, your bias voltage is low. That may be a bias supply issue, but it may also be indicative of some fault loading the power supply.

Did you replace electrolytics as a matter of maintenance or as an attempt to fix a problem? If the work was just maintenance then you'll want to recheck all wiring, because, if the amp was "working", but really was due for new caps, and now it doesn't work, then something that happened during the course of the work is the most likely cause of the new problem.

Thanks Chuck- When I got the amp, the symptom was extremely low volume, which I thought would be caused by this low grid voltage. I then assumed the grid issue was from a faulty exploded cap in the supply, so I replaced that. The same low volume / red plating is still plaguing the amp.

You can continue without the limiter with power tubes removed until you can get the bias right.
What is the AC voltage at the side of the standby switch that connects to C15 when in standby mode?
DC voltage at D1 anode?
And you were sure to put the positive end of the bias filter caps to ground? (C13 & C14)

Thanks, I will remove the tubes and forget the limiter for now.

VAC at c15- 350
VDC at D1 Anode:
Standby -18vdc
Play mode -10vdc

Yes I am sure the caps were installed correctly, the symptoms remained the same before and after the caps were replaced.

Plate voltage without tubes and without limiter is reported at 450. Bias voltage under the same conditions is -13.

DID THE AMP EXHIBIT THESE PROBLEMS BEFORE YOU CHANGED THE BIAS CAPS???

and/or: Under what circumstances did the symptoms manifest?

The amp isn't providing correct voltages without tubes installed. This needs to be addressed before any measurements with tubes installed can be relevant.

Something is drawing current such that the B+ is down over 50V and the bias voltage is down to almost nothing. Since the bias supply was worked on, I'll repeat... If these symptoms manifested AFTER the work on the bias supply you should recheck that work for accuracy.

And please provide the readings requested by g1 as well.

Hi chuck- to clarify, those measurements were another poster for reference . As written above, the symptoms remain unchanged after work done, so at this point can we assume there is a load on the power supply somewhere? Preamp Out sounds fine....

[g1]

Which side of C15 has the 350VAC? How about the other side?
You're sure D1 is good?
I'm inclined to agree with Chuck that the 450V with tubes removed is low, but they are only using 500V caps there.

[Mr_bibbles]

Which side of C15 has the 350VAC? How about the other side?
You're sure D1 is good?
I'm inclined to agree with Chuck that the 450V with tubes removed is low, but they are only using 500V caps there.

C15 at the point connected to the standby switch climbs slowly to 350VAC when the power switch is flipped. On the other side of the cap it climbs to 50VAC. This is the film cap I replaced, and D1 is also a brand new in4007.

Measured the plate again at 460VDC with no tubes. It does seem low... is there any other measurements I can provide, or is there any likely culprits on your list?

[Chuck H]

I wrote:

Plate voltage without tubes and without limiter is reported at 450. Bias voltage under the same conditions is -13.

You responded:

to clarify, those measurements were another poster for reference

When actually:

Without the limiter, the grid voltage is -13vdc,..


Plate voltage (NO LIMITER)
With tubes: 350vdc
Without tubes- 450vdc