Accidentally been running 56k (not 5.6k) grid stopper resistor in 1987 amp...

[whalecalls]

Hello all...
     I've been heavily playing and touring with a plexi-ish 1987 50 watt MV style Marshall I built about 4 years ago. All in all its been very reliable, but I had to change my power transformer about ~8 months back as I was losing B+ voltage  and the PT seemed to be getting hotter and hotter as time went on. It seemed as if a small internal short or something of the sort had been developing. This amp has seen some abuse with flying and has been knocked off a full stack etc.....
      I was inside my amp yesterday just to take B+ readings of my newish PT to make sure B+ is stable (which it is) and to figure out a developing buzz in in V1a... However, I just noticed that I've had 56k grid stoppers resistors installed on pins 5 instead of the "standard" 5.6k this entire time. (insert Homer Simpson Doh! sound)....  I have 1k installed on pin 4.
      Im still all in all a newbie with tube theory, but would this 56k be at all problematic in terms of the PT health? Please note that I play LOUD. Like MV dimed and pre volume around 50-60%. Also, I've always felt my amp was a bit "dull" in some ways especially considering I'm playing a somewhat bright telecaster  through it as well and often crank the treble control.... Also, would the 56k have any affect the volume/headroom? Any insight appreciated. I have the parts on order, but simply curious. Thanks to you all who provide so much valuable information and support. I've learned a ton from this site. Cheers.

[sluckey]

It's all good. There are much worse places to mess up a resistor value.   

But while it's on your mind go ahead and replace them. You'll sleep better.   

[whalecalls]

Lol. Thanks sluckey!

[SILVERGUN]

Also, I've always felt my amp was a bit "dull" in some ways especially considering I'm playing a somewhat bright telecaster  through it as well and often crank the treble control...

Maybe there is something else wrong 'cause I thought the only way you could get a Tele to be considered dull through a Plexi is if the speakers were in the swimming pool. 

[pdf64]

With EL34, as long as the total control grid circuit resistance (ie to the cathode) is less than 500k, then it should be fine.
I seem to remember that early Marshall DSLs were built with 470k grid stoppers, which didn’t, per se, cause a problem. Whereas their conductive circuit boards on the other hand, often did / do 

[whalecalls]

Silvergun,
     I don't disagree. I currently have a persistent buzz that I've isolated to V1 that is controlled by the preamp vol control. I've changed tubes, checked wiring, everything tests fine on the meter, voltage is proper, grounds seem fined, reflowed joints etc... I even changed the (de)coupling cap coming out of the gain stage as I think maybe it was leaking a smidge of DC, but its still there. This amp does have a relay in it. The output of the pre-vol controls goes into the relay and can be switch/cascaded into another gain stage that I seldom use. I now suspect that the relay is somehow the culprit of the buzz and maybe there some weird grounding issue there. Please note that this buzz is a newish phenomena after being knocked off a full stack by someone who drinks too much in my rehearsal room....

[SILVERGUN]

Silvergun,
     I don't disagree. I currently have a persistent buzz that I've isolated to V1 that is controlled by the preamp vol control. I've changed tubes, checked wiring, everything tests fine on the meter, voltage is proper, grounds seem fined, reflowed joints etc... I even changed the (de)coupling cap coming out of the gain stage as I think maybe it was leaking a smidge of DC, but its still there. This amp does have a relay in it. The output of the pre-vol controls goes into the relay and can be switch/cascaded into another gain stage that I seldom use. I now suspect that the relay is somehow the culprit of the buzz and maybe there some weird grounding issue there. Please note that this buzz is a newish phenomena after being knocked off a full stack by someone who drinks too much in my rehearsal room....

The relay itself is simple enough to test by just removing the coil control voltage. Noise goes away, there's your answer. Noise stays it's on to something else.Is the relay wired so that the coil is dis-engaged during normal operation and only engaged when your extra stage is kicked in?

[whalecalls]

Silvergun,
     I have to check, but Im 90% sure the coil is only engaged when using the extra gain stage. I built the amp a while back and ran out of time during my last time inside the amp and when I started suspecting the relay.  I will remove the voltage and see what happens. Thanks for the response.

[tubeswell]

Hello all...
     I've been heavily playing and touring with a plexi-ish 1987 50 watt MV style Marshall I built about 4 years ago. All in all its been very reliable, but I had to change my power transformer about ~8 months back as I was losing B+ voltage  and the PT seemed to be getting hotter and hotter as time went on. It seemed as if a small internal short or something of the sort had been developing.

What sort of voltages are you running with the new PT? Vintage Plexis typically operate at punishing voltages (500V B+), and EL34s are finicky tubes which are prone to running hot because the grid is wound in fairly close proximity to the cathode (in order together higher gain and transconductance). Most modern production EL34s are not up to much punishment (with Svetlana Winged Cs no longer in production, about the only robust modern production ones I have found are ST EL34B)


So check that you have good negative voltage on your EL34 grid pins and that tube current isn't creeping up when you turn the amp on. It may be that you need to adjust the bias supply -ve VDC source, or check for a leaky coupling cap, or a bad EL34.

[Backwoods Joe]

Please let us know if there is a tone or "feel" difference with the proper grid stoppers....

[whalecalls]

Tubeswell,
      I don't have the amp/notes in front of me atm, but I now have around 430vdc on the plates of the El34 which is I believe a safe operating range. With the old PT and the same tubes the voltages in the pre, heaters, and power amp were just slowly dropping as more time went on and the amp started losing headroom and was sounding off. The PT would be HOT after an hour or so of play time. Everything is now peachy with the new PT and sounding better and bias is stable. I was really hesitant to accept the fact that I had a problem with the PT and literally went through everything else possible, but alas, it seemed to somehow be the PT.
      Now I have this hiss/buzz in the preamp after my amp was knocked over. Hopefully I can get down to the practice room today and see if I have some grounding or other issues in the relay. I thought there was something going on with the (de)coupling cap coming out of V1a to the pre vol, but the buzz/hiss remains after changing the cap....

[pdf64]

...With the old PT and the same tubes the voltages in the pre, heaters, and power amp were just slowly dropping as more time went on and the amp started losing headroom and was sounding off. The PT would be HOT after an hour or so of play time...

What was anode or cathode current at idle while the above was happening?
As those symptoms align with a gradually falling bias voltage.
I can’t think of a PT failure mode that could cause it.
Hence the eventual failure of the PT would seem to be a symptom, not a faulty PT being the root cause.