Marshall JVM205 Cathode Follower Death, Sprial Filaments and Internet Mendacity

[bordonbert]

I can understand your desire to keep to controlled conditions for testing purposes but I'm confused as to how the real life situation within the JVM then relates to that. Surely the gain stage 220k anode resistor has a limiting action on the current available to sink into the grid (<2mA) and the 47k DCCF cathode resistor further limits it. Do they not prevent your most severe overcurrent conditions of >8mA from occurring in the real amp?

[bordonbert]

I'm not sure what has happened here Nick. I did reply to this a fair few days ago now but it seems to have vanished into the aether, (how many of you guys are old enough to remember that one?)

I can certainly accept your findings that cathode/heater overvoltage is not the killer I imagined it would be, neither is grid/cathode arcing a problem within our voltage range. It's good to have actual practical results of sound real world testing to give up on those two issues and sleep at nights again.

I think I can understand that your results also show that it is current which is the deleterious factor with a crucial point being reached around 8mA but I can't see the mechanism by which that current in the grid is actually reached. How is it that the grid current is not limited to around 1.25mA by the preceding anode resistor of 220k and its own cathode resistor of 47k. With the grid/cathode forward biased and acting like a diode surely they limit the current as the only source of grid current is from the HT line through the grid to ground via the two resistors. I thought your results showed that the crucial level for damage was about 8mA or have I missed something there?

[nevetslab]

Nick, you've outdone yourself with the engineering, testing and following the findings to reveal some amazing results. I haven't yet come upon any of the JVM series in our inventory, but do see there are a pair of JVM410's that haven't ever been into the shop. As I'm presently doing preventative maintenance on our rental inventory, all of your work here very much interests me. I'm also curious as to the tube curve tracer you're using. I'm still loosely in the market for one, though being poor at present, I tend to cobble things together that I've obtained over the years to do new tasks. Never did have the funds to invest in proper semiconductor curve tracers, let alone one for tubes.

Excellent thread and contribution from all of ya!!

[bordonbert]

I'm really not sure what is going on here but I have replied to this thread twice in the last 10 days and both posts have vanished into the aether. I get the message that as a newbie my reply must be passed as wholesome first but neither has appeared after 10 days. I can't help thinking they have somehow gone astray internally. I am trying again as there is something I really want sorted out in my own mind in order to fully understand this problem and its solution.

Like Nevetslab I am highly impressed with the info that has been offered here. Of course using a Chinese ECC83 is a working solution and the way to go with a simple follower but for me to just accept and apply that without getting to grips with the entire problem and its causes seems a bit of a fudge to me. It seems the ultimate conclusion is that it is the current which is the killer at levels over 8mA and not the exceeding of any maximum voltage levels. I can't however see how that level of current can occur in the real life JVM circuit. Originally I assumed you meant the grid current with the grid/cathode diode forward biased but then I realised from your description of the cathode involvement you would more likely mean the total cathode current. The DCCF has a cathode resistor of 47k. Surely that alone is enough to limit the total current through the valve to safe levels even under fault conditions at startup?

Is the damaging state under fault conditions such that the entire HT is put across the cathode resistor? This would assume there is no grid current being passed which would drop the grid voltage via the anode resistor either reverse biasing the grid/cathode again or dropping the cathode voltage due to follower action and reducing the anode current contribution. All states I can imagine result in reduced total current through the valve.

What am I missing here?



(EDIT: This time it seems to have shown up. Sorry Nickb, as I said I did reply to you earlier.)

[nickb]

I'm really not sure what is going on here but I have replied to this thread twice in the last 10 days and both posts have vanished into the aether. I get the message that as a newbie my reply must be passed as wholesome first but neither has appeared after 10 days. I can't help thinking they have somehow gone astray internally. I am trying again as there is something I really want sorted out in my own mind in order to fully understand this problem and its solution.

Success

Like Nevetslab I am highly impressed with the info that has been offered here. Of course using a Chinese ECC83 is a working solution and the way to go with a simple follower but for me to just accept and apply that without getting to grips with the entire problem and its causes seems a bit of a fudge to me.

Compare it to using a 20A power transistors instead of a 5A one in an application that needs 4.9A. So fitting an appropriately rated part is a reasonable solution. Also, practically speaking, a solution is needed that can be easily fitted to the PCB.

As far as getting to grips with the entire failure mode, part of the problem with doing that is time. I seldom get enough to indulge in this kind of investigations. Also, to be conclusive, I think I'd need an electron microscope to examine the (suspected damaged) cathode.

It seems the ultimate conclusion is that it is the current which is the killer at levels over 8mA and not the exceeding of any maximum voltage levels. I can't however see how that level of current can occur in the real life JVM circuit. Originally I assumed you meant the grid current with the grid/cathode diode forward biased but then I realised from your description of the cathode involvement you would more likely mean the total cathode current. The DCCF has a cathode resistor of 47k. Surely that alone is enough to limit the total current through the valve to safe levels even under fault conditions at startup?

The figure of 8mA was for a fully heated cathode. For one that is n the process of heating I would expect the figure to be lower and vary from tube to tube.

Is the damaging state under fault conditions such that the entire HT is put across the cathode resistor? This would assume there is no grid current being passed which would drop the grid voltage via the anode resistor either reverse biasing the grid/cathode again or dropping the cathode voltage due to follower action and reducing the anode current contribution. All states I can imagine result in reduced total current through the valve.

What am I missing here?

I don't think you are missing anything. I suspect the sum of grid current with localized current density effects and anode current is important. Experiments need to designed to prove it one way or another and be beyond the scope of a home lab. However I am confident that based on the testing I have done that heater-cathode insulation breakdown is not the cause.

[nickb]

Nick, you've outdone yourself with the engineering, testing and following the findings to reveal some amazing results. I haven't yet come upon any of the JVM series in our inventory, but do see there are a pair of JVM410's that haven't ever been into the shop. As I'm presently doing preventative maintenance on our rental inventory, all of your work here very much interests me. I'm also curious as to the tube curve tracer you're using. I'm still loosely in the market for one, though being poor at present, I tend to cobble things together that I've obtained over the years to do new tasks. Never did have the funds to invest in proper semiconductor curve tracers, let alone one for tubes.

Excellent thread and contribution from all of ya!!

Thanks for the kind words

I used a modified uTracer

I have modified mine for a wider grid voltage range and a 400mA max current. I also wrote a new GUI that was more suited to my needs.

The repeated failure of Sovtek 12AX7 in this function has been a recurrent nightmare for me as that was the only brand I stocked. I now stock Chinese ones for use cathode follower functions. Time will tell but so far so good.

[bordonbert]

The figure of 8mA was for a fully heated cathode. For one that is in the process of heating I would expect the figure to be lower and vary from tube to tube.

AHA! Of course, that just never occurred to me doing all my work in solid state. And I think that point makes sense of the process now for me.

I do appreciate what you say about limited time and resources to throw at the problem and, for my money, you have done some pretty spot on investigatory work Nickb. It should have been enough that you have actually shown the grid/cathode and cathode/heater breakdown voltages are not the bêtes noires they have been thought to be. Aspects like the damage repairing process are also an unexpected bonus for me.

If you ever start passing the hat to fund that electron microscope...

[mikepukmel]

I guess not very many are interested in this. I spoke with Merlin about this. We agree that damage to the cathode during warmup is the probable cause. The cathode can only supply so much current as it warms, demand exceeds supply and the cathode is damaged. It's clear this damage can happen very quickly. Here is a suggestion of a mod to prevent this from happening.

R1 senses the current through the first stage and when high enough turns FET M1 on. C1 prevents it turning on to quickly to avoid a thump and also give a some delay which helps mitigates an early turn on due a FET that happens to have a gate threshold at the the low end of the range.

[ATTACH=CONFIG]47511[/ATTACH]

Very interested, thanks for posting details of your research. A friend from high school, that Ive been able to connect with through Facebook (guitar player, semi retired electronics guy of about 35 years, and Marshall enthusiast) sent a bunch of notes about certain Marshall amp CF problems, and difficult time finding tubes for CF that won't blow. It wasn't a JVM205 though, will find out

Nick, any chance you could mention which brand(s) CHinese tubes you've had success with?
Thanks!
MP

[bordonbert]

Odd again! Just tried to thumbs up your post Mike but it looks to me as though it may have done something with the thumbs down instead. If so it's an accident, my apologies. (EDIT: Nope, AOK, it's just me not understanding the system again.)

I'd be interested in knowing this too. One thing I am aware of is the fact that if you stick a Chinese made model in there it can then place one of its triodes in a gain stage position which many people won't like. I have a Hughes & Kettner GM36 which is in that category. For my money, I've never found much difference if any between makes in guitar amps and I recently found what, to me, is proof online that there is little difference. And it was actually masquerading as proof there is a major difference!

[nickb]

Very interested, thanks for posting details of your research. A friend from high school, that Ive been able to connect with through Facebook (guitar player, semi retired electronics guy of about 35 years, and Marshall enthusiast) sent a bunch of notes about certain Marshall amp CF problems, and difficult time finding tubes for CF that won't blow. It wasn't a JVM205 though, will find out

Nick, any chance you could mention which brand(s) CHinese tubes you've had success with?
Thanks!
MP

Mike, they were Shuguang.

[mikepukmel]

Thanks Nick. This is huge!

[bordonbert]

This is not to try to get Nickb to do any more work on this issue, , it's just to do with its practical effect.

If we have a generic 12AX7 in the DCCF slot and it does succumb to this sort of problem what effect does that have on the sound of the amp? It's only a follower after all, 100% feedback should cope with a lot before it becomes blatantly obvious that something is wrong though the specific DCCF distortion mechanism could no doubt be affected. Does anyone have a first hand knowledge of this problem in a working amp and can suggest what would be heard?

I do have a JVM205H which has a problem of sounding a bit lacking in brightness and it definitely becomes more dull after about half an hour of playing. It has had a complete new set of valves fairly recently, probably about 6 weeks ago, which improved it at the time but I'm sure it is slipping back again into its dullness. I'm wondering if this could be what I'm suffering from. I have a Shuguang to drop in there but it has just arrived and I haven't had a chance to do that yet. I'm trying to get it in place for a gig tonight though I shouldn't really make a change so close to live work.

Any info available?

[nickb]

This is not to try to get Nickb to do any more work on this issue, , it's just to do with its practical effect.

If we have a generic 12AX7 in the DCCF slot and it does succumb to this sort of problem what effect does that have on the sound of the amp? It's only a follower after all, 100% feedback should cope with a lot before it becomes blatantly obvious that something is wrong though the specific DCCF distortion mechanism could no doubt be affected. Does anyone have a first hand knowledge of this problem in a working amp and can suggest what would be heard?

I do have a JVM205H which has a problem of sounding a bit lacking in brightness and it definitely becomes more dull after about half an hour of playing. It has had a complete new set of valves fairly recently, probably about 6 weeks ago, which improved it at the time but I'm sure it is slipping back again into its dullness. I'm wondering if this could be what I'm suffering from. I have a Shuguang to drop in there but it has just arrived and I haven't had a chance to do that yet. I'm trying to get it in place for a gig tonight though I shouldn't really make a change so close to live work.

Any info available?

Those that had failed sounded very quiet and distorted. I can only say that all two types of Sovteks I tired failed and that good RCA, GE and Sino did not. I didn't try anything else.

Wait till your new tube arrives and try it out then we'll know if that is the issue or not. You probably should start a new thread if you want to discuss since it's a different subject.

[bordonbert]

Will do Nick. Thanks for pointing that out.

[R.G.]

As one more extension to the MOSFET Heresies, I could point out that MOSFETs do not have heaters, and they make a dandy-fine DC follower.

See post here and articles on MOSFET Heresies at geofex.com.