New JTM45 build blowing fuses.......

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stelligan
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New JTM45 build blowing fuses.......

Post by stelligan »

Finally turning to y'all for some ideas. Bought a pre-populated JTM45 board from a forum member(thanks, solderstain) a couple years ago and am finally getting at it.

Tight squeeze in an aluminum Bassman chassis(thanks, stretch 2011) and a low power Tweed Twin cab from eBay(thanks for the heads up, Milkman sound). First ever tolex job went well(thanks, amplifiednation). One G12H 55Hz and one G12H 75Hz(thanks heisthl).

Got it all together and working first on bulb limiter. Sounding good with KT66 but, would blow fuses occasionally when coming off standby. Checking that power section all was good. Then, I thought was related to tight proximity of input jacks to board because it happened when switching jumpers a couple times while on. That area checked out fine.

I swapped all tubes Power tubes several times: rectifiers, changed to 5881 then JJ 6V6, shotgunned preamp tubes.

Initially thought was power inrush on switching standby because it always powers up. Did not do the diode business I have seen on some tube rectifiers. Did not put a 100k resistor at standby switch. Put a switch at the 16ohm tap to defeat negative feedback(presence).

Right now I have stopped using the standby, have JJ 6V6 biased at 22mv each running at 455 volts. More stable but, now every once in a while blows HT fuse?! Was previously blowing both.

Has always sounded very good when running whatever tubes were in. Will post pics of ugly guts if you want 'em. Housed within is a heretical Ceriatone main board.

Got the feeling there is an intermittent connection problem. I have been being bold and just playing the hell out of the thing and swapping fuses when they go......

Help me if you can I'm feelin' brown........
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M Fowler
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Re: New JTM45 build blowing fuses.......

Post by M Fowler »

I had to change from JJ GZ34 recto to Sovtek 5AR4 tube to stop a Dr Z amp from blowing the fuse. Advise came from Dr Z tech they claim JJ recto tube quality went down hill.

Problem solved and the customer has been gigging weekly since without problems. I did add the diodes as safety measure because he is on the road weekly with no backup amp.

But most likely the inrush is exceeding blowing the fuse or need to rewire the standby switch. I have also switched from 2A to 3A fuse.

Mark
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stelligan
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Re: New JTM45 build blowing fuses.......

Post by stelligan »

Mark, Thanks for the help!

Funny thing is I started with a Ruby 5AR4/GZ34(from parts drawer) then bought a new JJ. Guess I need to move from Chinese to Russian.... Are the Sovteks the tube of choice for this amp? I'll order one today.

I have a 3 amp Slo Blo in there and a 500 mA at the HT.

I've triple checked the other wiring in the amp and am thinking it has to be a tube...

Thanks again
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M Fowler
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Re: New JTM45 build blowing fuses.......

Post by M Fowler »

I changed over to Sovtek recto tubes and no problems since.

Either poor recto tube or power tubes. Not sure if you have changed out the power tubes.

Mark
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stelligan
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Re: New JTM45 build blowing fuses.......

Post by stelligan »

M Fowler wrote:I changed over to Sovtek recto tubes and no problems since.

Either poor recto tube or power tubes. Not sure if you have changed out the power tubes.

Mark
Tried many power tubes. Have a Sovtek & Tung-Sol coming in the mail. Thanks again
pdf64
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Re: New JTM45 build blowing fuses.......

Post by pdf64 »

Standby is just plain bizarre / perverse, there's is no technically valid reason for it.
I suggest that if an amp has it then don't use it.
If building an amp don't fit it.
To me, if defies rationality why someone would choose to repeatedly invoke plate damaging surge current that is caused by these early standby arrangements (ie hot switched reservoir cap on to a tube rectifier), rather than the slow smooth HT ramp up that a GZ34 provides.
Last edited by pdf64 on Sat Nov 19, 2016 5:33 pm, edited 1 time in total.
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M Fowler
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Re: New JTM45 build blowing fuses.......

Post by M Fowler »

pdf64
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Re: New JTM45 build blowing fuses.......

Post by pdf64 »

That blog piece seems to contradict itself, eg if hifi tube amps are fine without standby, then why advise
Always use a standby switch when dealing with a solid state rectified amp. The immediate surge of high voltage on a tube isn’t the best for its long-term life
And how can the HT of a silicon rectified supply be said to 'surge'?
It just rises to its normal 'no load' level in short order.
To 'surge' implies a significant overshoot?

I suspect that DrZ would find that JJ / any rectifiers would have a rather better operational life if their unfortunate 'hot switching' standby arrangement was eliminated.
stelligan wrote:...Right now I have stopped using the standby, have JJ 6V6 biased at 22mv each running at 455 volts. More stable but, now every once in a while blows HT fuse?! Was previously blowing both.

Has always sounded very good when running whatever tubes were in. Will post pics of ugly guts if you want 'em...
Yes, some gut shots may help.
Does the HT fuse tend to blow in any particular conditions, or does it just seem random?
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stelligan
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Re: New JTM45 build blowing fuses.......

Post by stelligan »

New Tung-Sol rectifier tube is no longer blowing fuses.

I still rarely use a standby on my amps. More concerned that something wasn't right with the build. All good now - no blowing fuses when using amp with standby use or not......

FWIW Henry @ RedPlate shares pdf64's disdain for the standby switch......
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JazzGuitarGimp
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Re: New JTM45 build blowing fuses.......

Post by JazzGuitarGimp »

I didn't realize it isn't safe to transport your amp if the filter caps are charged! :-)
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frankdrebin
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Re: New JTM45 build blowing fuses.......

Post by frankdrebin »

with gz34 you don't need standby anyway,so why bother...
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stelligan
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This damn thing again..

Post by stelligan »

OK. So this problem child has been acting up again. I have never used the standby since finding a decent rectifier tube in winter 2016. Over the last year or so it would sometimes power up but be silent. With ever so little power amp noise when listening to the speaker. I tried shot-gunning preamp tubes and that was never the issue. It would just work some days - and sometimes not. Now will never pass audio. The intermittent problem is now continuous. Opened it up finally to get to the bottom of it and getting nowhere. Swapped some known good rectifier tubes and solid state plug tube with no luck. Heater voltage is OK, Bias supply and pot work with expected negative voltage. ***Voltage at power tube plates is 70v(pin 3)*****. KT66 tubes are good. Swapped in 5881s - same. 690 volt power transformer reads 726 volts across red leads. Suspect the power section of this amp has always had something wrong and can't pinpoint it. Is the output transformer bad? And how would I test that? Rectifier socket?

Thanks in advance.
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pdf64
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Re: New JTM45 build blowing fuses.......

Post by pdf64 »

70VDC on the power valve anodes? How about at the OT CT / reservoir cap?
Needs a full voltage survey really, eg as per https://el34world.com/charts/Schematics ... ad_45w.pdf
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stelligan
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Re: New JTM45 build blowing fuses.......

Post by stelligan »

pdf64 wrote: Mon Feb 08, 2021 9:50 pm 70VDC on the power valve anodes? How about at the OT CT / reservoir cap?
Needs a full voltage survey really, eg as per https://el34world.com/charts/Schematics ... ad_45w.pdf
Thanks for looking!
Here is a quick chart of power section with new rectifier tube. Yes, that low voltage reads at OT reservoir cap as well..
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pdf64
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Re: New JTM45 build blowing fuses.......

Post by pdf64 »

What’s the VDC at rectifier pin8?
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