Homemade 5F2A Tweed Princeton amp, first build

ivan H said:
For pots, I use "Audio taper" for the volume & bass controls & "Linear" for the treble, middle & presence controls. Some do like to use an Audio taper pot for the treble control too, though the schematic usually calls for a Linear.
I would recommend metering the pots to make sure none are below the stated value.
Maybe try the "stock" values for the slope resistor (56k for a JTM45) & the PI tail resistor first. You can easily change them out later if you want to. Cheers
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I’ll add that info to my notes. Thanks.
 
It is another rabbit hole. I've been sourcing parts for a 1987 JMP 50 build now that my 800 is done. I used Classic Tone transformers in that, but now you guys have me thinking....
 
Cadorman said:
It is another rabbit hole. I've been sourcing parts for a 1987 JMP 50 build now that my 800 is done. I used Classic Tone transformers in that, but now you guys have me thinking....
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Classic Tone is a very good product. Unless you're ready to cough-up some extra bucks for premium quality and maybe a bit more authenticity.
 
syscokid said:
Classic Tone is a very good product. Unless you're ready to cough-up some extra bucks for premium quality and maybe a bit more authenticity.
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Well, I'm not really going for dead on accuracy. Got a bunch of Dale metal foil mil spec resistors on the way and I am looking at using Solen Fast Caps for coupling and bypass capacitors. Circuit wise it will copy as close as possible a 1968 to 1970ish 1987. I don't know if tranny choice makes much difference in this scenario?
 
John I did not intend to cast confusion on the values on the Bassman video it's more like that's where the JTM45
was born more of a history lesson. What made that tweed bassman sound so great 1, Traid transformers 2, Astron capacitors 3, Mallory electrolytic
capacitors and the tubes were top notch back then. My 1959 Fender Concert Amp is a Marshall killer it has the Traid transformers.
I have used Classic tone transformers in the past for repairs they run hot and the last three were trash shorted out from the factory
are they using China copper wire now I don't know.
 
67plexi said:
I have used Classic tone transformers in the past for repairs they run hot and the last three were trash shorted out from the factory
are they using China copper wire now I don't know.
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I have used one Classitone made Marshall PT in the past & it supplied a higher than spec B+ & ran hot. I had PM'd John earlier to tell him of this, as I didn't want to rubbish the product here, but, well, the cats outta the bag now, isn't it.
Cadorman, Heyboer or Marstran (wound by Heyboer) iron will serve you well. You want a 1202-118 PT, C1999 filter choke & 784-139 OT.
Cheers
 
67plexi said:
I have used Classic tone transformers in the past for repairs they run hot and the last three were trash shorted out from the factory
are they using China copper wire now I don't know.
Click to expand...
ivan H said:
I have used one Classitone made Marshall PT in the past & it supplied a higher than spec B+ & ran hot. I had PM'd John earlier to tell him of this, as I didn't want to rubbish the product here, but, well, the cats outta the bag now, isn't it.
Click to expand...
These comments and experiences are very interesting. I have two 50 watt JCM800-style amps. The amp with EL34s has a Marstran PT (JMP164 PT) and a Marstran OT (784-128), and a Mercury Magnetics choke (Toneclone MC10H). Trannies get warm, but not hot. B+ is 365vdc to 370vdc. Amp sounds excellent. The amp with 6550s has a Classic Tone PT (40-18023), a Mercury Magnetics OT (O50JM), and a Classic Tone choke (40-18032). The trannies get warm, but not hot... including the Classic Tone. But the B+ is 465vdc to 470vdc. This amp sounds excellent, too. Just sharing my personal observations. about the trannies.

I will say this though in regards to specs and B+ of the two different brands of PTs: When I loosely calculated the Marstrans B+ potential by looking at the its specs, the real world results were just about dead-on. Calculating from the specs given for the Classic Tone PT, the real world results were about 20vdc higher than expected.

I've never understood what values of "henries" means in a choke's performance. I always wondered why a 10 henries (250mA) choke was installed in place of the original Drake 3 henries choke in the EL34 amp I described above????
 
syscokid said:
These comments and experiences are very interesting. I have two 50 watt JCM800-style amps. The amp with EL34s has a Marstran PT (JMP164 PT) and a Marstran OT (784-128), and a Mercury Magnetics choke (Toneclone MC10H). Trannies get warm, but not hot. B+ is 365vdc to 370vdc. Amp sounds excellent. The amp with 6550s has a Classic Tone PT (40-18023), a Mercury Magnetics OT (O50JM), and a Classic Tone choke (40-18032). The trannies get warm, but not hot... including the Classic Tone. But the B+ is 465vdc to 470vdc. This amp sounds excellent, too. Just sharing my personal observations. about the trannies.

I will say this though in regards to specs and B+ of the two different brands of PTs: When I loosely calculated the Marstrans B+ potential by looking at the its specs, the real world results were just about dead-on. Calculating from the specs given for the Classic Tone PT, the real world results were about 20vdc higher than expected.

I've never understood what values of "henries" means in a choke's performance. I always wondered why a 10 henries (250mA) choke was installed in place of the original Drake 3 henries choke in the EL34 amp I described above????
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A "Henry" is a measure of inductance (a choke is an inductor). The mA rating is exactly that, the amount of current the inductor can safely continously flow. A choke impedes AC current while the low resistance of the winding offers very little resistance to DC.
The Marshall type Classictone PT I used runs noticeably hotter than a similar model Marstran item I have, but hasn't failed yet (& it's seen quite a lot of use). The Classictone power transformer in my 5F1 Champ build doesn't seem to run hot, but the supplied By+ is a little high, probably on the borderline of the -/+ 10% tolerance. Cheers
 
67plexi said:
John I did not intend to cast confusion on the values on the Bassman video it's more like that's where the JTM45
was born more of a history lesson. What made that tweed bassman sound so great 1, Traid transformers 2, Astron capacitors 3, Mallory electrolytic
capacitors and the tubes were top notch back then. My 1959 Fender Concert Amp is a Marshall killer it has the Traid transformers.
I have used Classic tone transformers in the past for repairs they run hot and the last three were trash shorted out from the factory
are they using China copper wire now I don't know.
Click to expand...


Thanks Steve, my questions weren't out of confusion but more out of trying to understand everything I'm throwing at myself. Since the Bassman and JTM45 are similar circuits, except for obvious stuff like where the components were sourced, and what they ended up being; i.e. Radio Spares versus Triad, etc. That's also why I asked about slope resistors and tail resistors affecting the phase inverter. I remember that the voltage between the A and B side of V3 is very critical of a good sounding Marshall. Another reason why I'm doing these builds is to also better understand electronics and reading schematics to help me with my job (even though everything in modern electronics seems to be replace the entire board, drive, amp, whatever).

I definitely want to build an early JTM with KT66s, and I want to do it right the first time. That's why I had the questions regarding chokes. I read on the Amp Garage that people were having problems with the later style JTM/JMP style chokes running too hot, when used with KT66s. The problems were not brand specific, nor as bad with EL34 or 6550 power tubes. From what I have been reading, it seems the 3Hy, 250mA, 110 Ohm choke is the way to go with KT66s. A search for an RS 10Hy, 150mA choke that you used in your '63 JTM has come up empty so far.

When Merren gets back to me on transformers, I'll ask about chokes. Since I'm not making road money any more, extra cash isn't there anymore. I can spend the extra up front to get the "right" stuff for this build, but not to buy stuff twice. Ivan's PM's have been helpful, as well everyone else's replies, and reading through past posts on different threads. I have been taking notes, and hope to build a sonic monster of a JTM. Fortunately, I'll also have the M.I.T. guys watching over my shoulder.
 
Didn't like the way the tone knob was working. Come to find out, I did the microfarad to picofarad conversion incorrectly, and had a 500 pF capacitor on the tone pot. Put the correct .0047 uF cap on it today, and it made a world of difference. It has a much nicer sound to it, and you can actually use the tone pot now.
 
SG John said:
Didn't like the way the tone knob was working. Come to find out, I did the microfarad to picofarad conversion incorrectly, and had a 500 pF capacitor on the tone pot. Put the correct .0047 uF cap on it today, and it made a world of difference. It has a much nicer sound to it, and you can actually use the tone pot now.
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An easy mistake to make. One zero can make a big difference. A "uf/nf/pf" convertion chart printout is a handy thing to have, as well as a resistor color code chart. Glad you got it sorted. Cheers
 
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there is a WHOLE LOT of stuff in this thread I dont understand..................and soldering .....

I call DIBS on the amp-----
 
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