JCM900 4500 EL34 Red plating/run away bias current

[cozmacozmy]

The cap and tubes just came in the mail today. USPS shipping has been way unreliable since this pandemic hit... anyways, I'll start working on replacing that cap here in a little bit and see how it works now.

XTRXTR, that is a bit above my level of working on these amps. Once I get this repair working I do plan on replacing all the caps for maintenance as they are 30 some years old now. But I wanted to make sure this amp even works first before I head down that path. Thank you for the tip though.

 

[mickeydg5]

I don't think the 250v cap is that problematic my 6100's use the same and have going strong for ever. Prolly they just ran out of 250's and all they could get was 630's or it's shut down the line, so thet used them. That said a 630v isn't going to hurt anything.

But but but, you have a full wave in action with both sides working together.

The 4500 has a half wave taking the full brunt every time the cycle hits.

I have stated before I would not use less than 630V but would just as well use 1000V.

 

[mickeydg5]

That's what I've seen many say. But that this is a 630v cap in later models and that Marshall made a mistake putting is such a low voltage rating safety cap.

Is that right?

The 630V rating is best but 1000V would have been better.

Capacitors can fail and open, even an X type.

If your bias supply voltage goes away after the Class X then that is the culprit.

Honestly, there is not reason to use a Class X there.

 

[mickeydg5]

I don't see a bleed resistor on the reservoir cap on the schematic. You may have one not shown on the schematic. You should add one about 100 to 220k 2w. This bias circuit should have one according to Merlin. Since there is no CT on the HT a path needs to be provided to discharge the filter caps or you could experience a problem if the power blinks out or an "On Off On" situation happens - the negative bias voltage would not be present when the amp is back on but the HT would still be present. This will cause an over current condition on the anodes of the power tubes. If your in the middle of some power chords on the guitar in this condition that could be enough to blow the tubes.

The cap and tubes just came in the mail today. USPS shipping has been way unreliable since this pandemic hit... anyways, I'll start working on replacing that cap here in a little bit and see how it works now.

XTRXTR, that is a bit above my level of working on these amps. Once I get this repair working I do plan on replacing all the caps for maintenance as they are 30 some years old now. But I wanted to make sure this amp even works first before I head down that path. Thank you for the tip though.

@XTRXTR

Tell Merlin not true.

The reservoir/filter capacitor will provide path as it charges as well.
It may be beneficial to have bleeder resistor but not necessary.

 

[cozmacozmy]

Thanks Mickeydg5, I just finished putting in a Xicon .047uf and I got the negative bias voltage back to -39v now. While I was removing the safety cap on the board, as I put pressure on it to remove it, it just fell out. It looked like it smoked itself internally and just fell apart. The new one went in with no problems and I was able to set the bias at a 37ma on both of the new Mullard El34's I ordered with the new cap. The bias pot is maxed out though... I have a plate voltage of 425v so I should get me a 70* bias adjustment @ 41ma??? So it looks like I am running about a 60% dissipation.

I have talked to another 4500 model owner just before I found the bad safety cap and he had a similar issue where the original 56k bias resistor wouldn't let the bias adjust enough and to swap in a 47k in its place. Now that I have this up and running and she sounds awesome to by the way! I'll get the can capacitors to replace for maintenance and I'll replace that bias supply resistor to the 47k when I have it open for that.

Thank you all for your help in this. I appreciate it very much!

 

[cozmacozmy]

Here are some pics I took of the cap and its replacement along with setting the bias.

 

[66 Kicks]

I have talked to another 4500 model owner just before I found the bad safety cap and he had a similar issue where the original 56k bias resistor wouldn't let the bias adjust enough and to swap in a 47k in its place. Now that I have this up and running and she sounds awesome to by the way! I'll get the can capacitors to replace for maintenance and I'll replace that bias supply resistor to the 47k when I have it open for that.

You are talking about R30, right? R7 is also 56K, but reducing it to 47K will have very little effect on the bias voltage.

 

[cozmacozmy]

Yes R30 that controls the bias pot resistor. I am not sure what changing R7 would do to this. R30 comes right off the bias pot.

I was going off of information supplied by a member named milfordcubicle from this post linked here. I had talked to him just days before I made my post in this thread as I noticed I had the same 2.2k resistor on pin 5 as he did and was wondering if he found the reason why. During that conversation he mentioned that the 56k may not work on getting the bias set just right and to swap to a 47k to get a closer adjustment to get that 70% dissipation. It is so close now and it may even drift as these are new tubes. I'll recheck it when I get the cap kit in to replace in a few weeks when I can get some cash again. I have a few other bills that need attention first before I can get those F&T can caps. When I go the new tubes from TubeDepot I paid a little extra to get a matched pair but didn't pay the extra to have them burn these tubes in. I figure I can do that myself.

Which reminds me I forgot to post in his thread about the problem I had right after talking to him. I had just opened up my amp and was looking around doing a visual inspection and saw I had that same resistor on pin 5. So I replied to him before finding out I had no negative bias voltage.

https://www.marshallforum.com/threa...ered-to-pin-5-of-v5-el34.119613/#post-2171982

 

[cozmacozmy]

I have been following this forums for years and just reading what others post for the most part absorbing what you all say. Always wondering if I would ever have a reason to add a post a repair issue to give back any kind of help someone else might need in the future.

 

[Spanngitter]

Not the first time i see one of these white (or grey) block caps just falling off the PCB on a Marshall, seen that on late JMPs, 800s,900s and also on TSL/DSL.
I consider them not being consistent in quality at all and usually replace them with green MKS1822 (as long as my supply lasts) or WIMAs...
Just did a complete JVM with MKT1822 on customer request (did this on a few others also before) and for me (and my(!) ears) this made him sound even better then before...