In studying the specs for JTM50, JTM45 and JMP power transformers I see the HV current ratings listed as 150mA by more than one manufacturer. There is a ton of conflicting info/opinion out there with people saying everything from this being fine up all the way to saying you need almost twice that amount to safely build one of these amps. Yet, I'm sure Hammond and Mag Components would have changed their spec if their PTs were melting in people's builds, and there would be a lot more info regarding their failure.
I own a JTM45 clone with Marstran transformers and while I don't know the specs of the PT I would wager a bet the HV secondary is like the others and rated at 150mA. From what I've been reading the size of the PT itself plays a significant role in the transformers ability to 'sag' under demand but not melt down, and this PT sag is a part of the sound of these amps.
Long story short I obtained a Stancor PT which, on paper, has excellent specs to sub-in for a build of this type. I want a low-ish B+ and it looked like this would get me there.
117 VAC primary
325-0-325 secondary @ 150mA
6.3V @ 5A
5V @ 3A
Looks good, right? So I thought. But when the transformer arrived it's size left a lot to be desired in comparison to the Marstran PT (which is large and in charge). It's almost comparable in size to the Marstran OT albeit with a thicker core. I'm extremely hesitant to build with this PT, and my gut tells me it would be better suited for something like a Deluxe Reverb perhaps.
Am I just being paranoid?
ps - I'm selling the JTM45 clone soon. It's extremely well built, has the top notch Marstran iron, Mullard mustard caps, etc. Im trying to build myself one 'on the cheap' to take it's place. New baby in the house means I need $ (and is also why I'm typing this at 3:51am!)
JTM50 esque PT question (Stancor strikes again)
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Stevem
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Re: JTM50 esque PT question (Stancor strikes again)
When you go and look at a EL34 tube spec chart you will clearly see that just the two output tubes need 140ma to make some 37 watts of clean power!
If you want a 45 clean watt amp with good peak power dynamic range, then yes you need at least 200ma once again just for powering the two 34s!!!
140 ma would get you a very compressed sounding 32 watt amp!
If you want a 45 clean watt amp with good peak power dynamic range, then yes you need at least 200ma once again just for powering the two 34s!!!
140 ma would get you a very compressed sounding 32 watt amp!
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: JTM50 esque PT question (Stancor strikes again)
So in a JTM50 or Marshall JMP is this why the breakup comes on quickly because the PT can barely meet the demands of the '34s combined w/the preamp tubes?Stevem wrote:When you go and look at a EL34 tube spec chart you will clearly see that just the two output tubes need 140ma to make some 37 watts of clean power!
Stevem wrote:If you want a 45 clean watt amp with good peak power dynamic range, then yes you need at least 200ma once again just for powering the two 34s!!!
You can see here that the Classic Tone PT for a 50W plexi is rated for 150mA. This must be a deliberate under rating to match vintage specs?
http://www.classictone.net/40-18089.html
(click 'Specifications')
In light of this I might opt for a PT which was intended for a Fender Super Reverb or Twin. Built with 'Marshall' values I'd imagine it might allow me good cleans but still have nice bite when cranked? I know that the JTM45 is just a Fender Bassman made with parts available in Britian at the time
I'm also not dead set on it being 100% Marshall. I like some of the simpler Marshall influenced amps being made today by Dr. Z and Satellite.But would it be in danger of melting/catching fire?Stevem wrote:140 ma would get you a very compressed sounding 32 watt amp!
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Re: JTM50 esque PT question (Stancor strikes again)
part of the plexi magic lies in having a PT that delivers relatively high voltages at idle that sags significantly when the amp works hard
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Re: JTM50 esque PT question (Stancor strikes again)
Sag in the power supply may act towards protecting the power tubes from overdissipation at high demand, especially overdriven.
If all sag was eliminated from the supplies to the plate / screen grids, the amp may put out twice the power overdriven as at max clean.
Whereas amps favoured for their 'touch sensitivity' may have a smaller ratio between the clean and overdriven power capabilities.
If a part is advertised as being suitable for a particular application then we may have a reasonable expection that it won't be damaged when used as such.
The classictone PT may give us a VB+ of 440V at its rated 150mA, but the bottom of p2 http://www.mif.pg.gda.pl/homepages/fran ... e/EL34.pdf shows that we only need 375V to get 48W from a pair of EL34 with a 3k5 p-p load.
The B+ demand will be over 260mA!
OK, the conditions aren't exactly the same as a Marshall (the Marshall plate voltage will likely be higher, and screen grid voltage lower, at max clean output) but they give a view of how it might work.
The high current demand will result in the PT B+ winding voltage sagging rather below the 625V spec, but we can see that there's plenty of scope for some sag whilst still getting full power.
The sag will result in the PT getting hot, but as the PT is advertised for this application by a well respected manufacturer, we can assume that its insulation class will accommodate the max foreseeable temperature without damage.
It may be that the 'real' manufacturing spec for the B+ winding is a lower voltage at a higher current with poor regulation, eg the unloaded voltage may be >10% higher than full load, whereas a well regulated / low resistance winding will be more like 5%.
The advertised spec perhaps being mainly to assure potential buyers that it's performance will be equivilant to the PT used in the original Marshall.
If all sag was eliminated from the supplies to the plate / screen grids, the amp may put out twice the power overdriven as at max clean.
Whereas amps favoured for their 'touch sensitivity' may have a smaller ratio between the clean and overdriven power capabilities.
If a part is advertised as being suitable for a particular application then we may have a reasonable expection that it won't be damaged when used as such.
The classictone PT may give us a VB+ of 440V at its rated 150mA, but the bottom of p2 http://www.mif.pg.gda.pl/homepages/fran ... e/EL34.pdf shows that we only need 375V to get 48W from a pair of EL34 with a 3k5 p-p load.
The B+ demand will be over 260mA!
OK, the conditions aren't exactly the same as a Marshall (the Marshall plate voltage will likely be higher, and screen grid voltage lower, at max clean output) but they give a view of how it might work.
The high current demand will result in the PT B+ winding voltage sagging rather below the 625V spec, but we can see that there's plenty of scope for some sag whilst still getting full power.
The sag will result in the PT getting hot, but as the PT is advertised for this application by a well respected manufacturer, we can assume that its insulation class will accommodate the max foreseeable temperature without damage.
It may be that the 'real' manufacturing spec for the B+ winding is a lower voltage at a higher current with poor regulation, eg the unloaded voltage may be >10% higher than full load, whereas a well regulated / low resistance winding will be more like 5%.
The advertised spec perhaps being mainly to assure potential buyers that it's performance will be equivilant to the PT used in the original Marshall.
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Re: JTM50 esque PT question (Stancor strikes again)
The rating 150mA is correct, I'm guessing Marshall just copied what they saw being used in the 5F6 Bassman, in which that Triad PT beleve it is not slightly less than 150mA!WRC34 wrote:In studying the specs for JTM50, JTM45 and JMP power transformers I see the HV current ratings listed as 150mA by more than one manufacturer. There is a ton of conflicting info/opinion out there with people saying everything from this being fine up all the way to saying you need almost twice that amount to safely build one of these amps. Yet, I'm sure Hammond and Mag Components would have changed their spec if their PTs were melting in people's builds, and there would be a lot more info regarding their failure.
I own a JTM45 clone with Marstran transformers and while I don't know the specs of the PT I would wager a bet the HV secondary is like the others and rated at 150mA.
As an FYI, that 325 @ 117V Stancor with modern voltages will be around 336 @121V and around 347 with 125V mains.
TM
Re: JTM50 esque PT question (Stancor strikes again)
I have the Mag Components JTM50 PT in my gigging amp (1987 clone with 6L6GC and extra preamp tube). It gets warm but has been fine going on 4 years now. Unless you're running a constant sine wave through it, I think you'd be fine with the appropriately spec'd Stancor.
Re: JTM50 esque PT question (Stancor strikes again)
Hmm...ok guys, thanks for the insights here. I still feel a little queasy about using this PT in a JTM50 style amp.
I'm thinking maybe going the lower wattage route and building with fixed bias 6V6 power tubes instead. I know that on paper 6V6 are rated for a much lower max plate voltage than the 450 I'm likely to get with this PT using a 5AR4, but I've heard that it's really about plate dissipation more than just the plate voltage as a stand alone value.
I just sold an old Bogen CHA-20 that ran a pair of 6V6s around 460V on the plates and was cathode biased to boot, the horror! I bought that amp years ago and used it quite often with an old pair of 6V6G ("ST" or Coke bottle shaped) which are still in relatively good health and being used in a more reasonable amp now.
I'm thinking maybe going the lower wattage route and building with fixed bias 6V6 power tubes instead. I know that on paper 6V6 are rated for a much lower max plate voltage than the 450 I'm likely to get with this PT using a 5AR4, but I've heard that it's really about plate dissipation more than just the plate voltage as a stand alone value.
I just sold an old Bogen CHA-20 that ran a pair of 6V6s around 460V on the plates and was cathode biased to boot, the horror! I bought that amp years ago and used it quite often with an old pair of 6V6G ("ST" or Coke bottle shaped) which are still in relatively good health and being used in a more reasonable amp now.