50w Plexi bass (now JTM50)

stickyfinger

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Been following and cool build! Nice to have a great chassis available to the public now.

Can you tell me who makes the green screen resistors and what wattage they are? I need green ones for the B+ in my Trainwreck build but cant find them!
 

AndyD

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Been following and cool build! Nice to have a great chassis available to the public now.

Can you tell me who makes the green screen resistors and what wattage they are? I need green ones for the B+ in my Trainwreck build but cant find them!
Thank you! The screen resistors are 5w. I'm sorry, I can't remember where I got them from!
 

neikeel

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I tend to use the 3w green enamel Welwyn 1k. They are the ones Marshall used in the early 70s so look right.
 

neikeel

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https://www.ebay.co.uk/itm/313636584630
s-l1600.jpg

They’re on my” to find” list!
 

coolidge56

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I tend to use the 3w green enamel Welwyn 1k. They are the ones Marshall used in the early 70s so look right.

What's the voltage rating? From Randel Aiken's site...

Q:What about screen grid resistors?

A: I recommend 5W 1% 750V metal oxide flameproof resistors for this application. The flameproof "cement box" resistors are also good, but are usually only available in 5% and 10%, and must be rated for at least 7W or 10W in order to get an appropriate voltage rating, because the 5W and lower units are usually only rated for 350V. In normal operation, the screen grid resistor only has a few volts dropped across it, so a 350V resistor is overkill. However, in the event of an internal tube short, the voltage drop can be much higher, up to the level of the supply, and the current through the screen resistor will increase dramatically, increasing the voltage drop across it. Normally, the fuse will blow fairly quickly, but sometimes not before the screen resistor is toasted. There are two possible failure modes, over-voltage and over-dissipation due to the increased current from the short. If the voltage rating is too low, the resistor element will flash over and either burn up or carbon track and change value, leading to eventual failure down the road. If the voltage rating is made higher than the supply voltage, there is only one possible failure mode of the resistor, over-dissipation. The metal oxide flameproofs or OX/OY ceramics can usually take a higher-than-rated surge for a short time before they burn up. If the fuse is appropriately rated, it will blow before this over-dissipation can harm the resistor. A quick fuse and tube swap and the amp is back in business, instead of having to replace the screen resistors.


There's one 5W wire wound rated for 500v out there, the last of its breed. I'm probably going to make the switch to metal oxide soon.
 

AndyD

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What's the voltage rating? From Randel Aiken's site...

Q:What about screen grid resistors?

A: I recommend 5W 1% 750V metal oxide flameproof resistors for this application. The flameproof "cement box" resistors are also good, but are usually only available in 5% and 10%, and must be rated for at least 7W or 10W in order to get an appropriate voltage rating, because the 5W and lower units are usually only rated for 350V. In normal operation, the screen grid resistor only has a few volts dropped across it, so a 350V resistor is overkill. However, in the event of an internal tube short, the voltage drop can be much higher, up to the level of the supply, and the current through the screen resistor will increase dramatically, increasing the voltage drop across it. Normally, the fuse will blow fairly quickly, but sometimes not before the screen resistor is toasted. There are two possible failure modes, over-voltage and over-dissipation due to the increased current from the short. If the voltage rating is too low, the resistor element will flash over and either burn up or carbon track and change value, leading to eventual failure down the road. If the voltage rating is made higher than the supply voltage, there is only one possible failure mode of the resistor, over-dissipation. The metal oxide flameproofs or OX/OY ceramics can usually take a higher-than-rated surge for a short time before they burn up. If the fuse is appropriately rated, it will blow before this over-dissipation can harm the resistor. A quick fuse and tube swap and the amp is back in business, instead of having to replace the screen resistors.


There's one 5W wire wound rated for 500v out there, the last of its breed. I'm probably going to make the switch to metal oxide soon.
The voltage rating is only 100v. However they are milspec.
 

AndyD

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This build has got me thinking and I have decided to deviate onto a JTM50 spec build! A bass spec amp with valve rectification. I’ve got a few minor adjustments to make, so photos to follow soon………
 

stickyfinger

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The Black Flag I built a few years ago for a friend was one of the best sounding amp I've ever played. Never played a real one though.
 

AndyD

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Help please! Could @neikeel just check over this scheme, please. I have tried to amend it for valve rectification. Couldn't find a complete schematic anywhere.
9NFcEDG.jpg

"B" is the single reservoir can (prefer it after rectifier and before the standby ), 500v. "C1/C2" is a dual 32uf can inside the chassis. "A" is a dual 32uf cap on the board. Any input much appreciated.
 

neikeel

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Looks about right.
The mains can on top should be 32-50uF (absolutely no higher than 64uF or your GZ34 will strop). I personally like to use a 16/32 can up top in parallel and same inside chassis 16 for screens and 32 for pi but 32 across the amp will work fine.
I have an old hand drawn layout of an original JTM50 somewhere. I’ll see if I can find a web version (I’m on call at work right now).
 

AndyD

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Looks about right.
The mains can on top should be 32-50uF (absolutely no higher than 64uF or your GZ34 will strop). I personally like to use a 16/32 can up top in parallel and same inside chassis 16 for screens and 32 for pi but 32 across the amp will work fine.
I have an old hand drawn layout of an original JTM50 somewhere. I’ll see if I can find a web version (I’m on call at work right now).
Thats fantastic. Thank you for the input . I will be interested to see your layout drawing.
All the best.
 

AndyD

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jqWKFtA.jpg

Thank you again to @neikeel for sharing the layout drawing. Here is an amended schematic which is referenced from the layout. I was curious to see that the 500 ma HT fuse was attached to the centre tap instead of in line with the rectified HT which is what I am used to seeing. Is one better than the other????? Please shout out if I have missed something significant.
 

neikeel

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Tend to use the fused centre tap with valve rectifiers.
The ss models have fusing after diodes and first cap with choke and CT wires soldered to body of HT fuse holder.
I think the centre tap ie ac fuse is marginally better.
 
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AndyD

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Tend to use the fused centre tap with valve rectifiers.
The ss models have switching after diodes and first cap with choke and CT wires soldered to body of HT fuse holder.
I think the centre tap ie ac fuse is marginally better.
Thats interesting to know. I will go with the fused centre tap. Cheers.
 

Pete Farrington

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Here is an amended schematic which is referenced from the layout. I was curious to see that the 500 ma HT fuse was attached to the centre tap instead of in line with the rectified HT which is what I am used to seeing. Is one better than the other????? Please shout out if I have missed something significant
The 1k screen grid resistors are missing?
A CT type HT fuse seems to be better, as it additionally protects against an 'anode to cathode' short on the GZ34.
Even better would be the CT fuse in conjunction with 1k5V's worth of silicon diodes in series with each GZ34 anode, as that will also protect against 'anode to anode' shorts on the the GZ34. And as a free bonus, in either case of GZ34 short, it keeps the amp operational!
If the hot switching standby arrangement is avoided, an F500mA (quick blow) HT fuse can be used. That will respond much faster if an EL34 happens to short, thereby taking stress off the PT. Just move the standby to after the reservoir cap, as it was on post #31.
 


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