Low Watt amp Marshall

[mresistor]

I've been eyeballing and considering building a Marshall JTM-1. It's got three tubes, 2 12AX7's and a 12AU7 it uses for the power amp. Problem I see is finding suitable iron for a build. The schematic shows that the PT is 180V and puts out 180VDC through the solid state full wave recitifer. There is another winding on the PT for 12VAC filaments. Sort of a hard PT spec to find. Short of asking Edcor to make one for me (spensive) I can't find anything like it.  For an OT there are a lot of inexpensive choices.  Only thing close that I can find is a Hammond 269AX. Guess I could use a tube rectifier to bring the HV down some.

[mresistor]

Well I should have looked at Edcor prior to posting. They have both a PT and and OT suitable for this build. 


https://www.edcorusa.com/xpwr058


https://www.edcorusa.com/xpp10-8-10k




schematic is here  http://s26.postimg.org/yxk7lyxhl/Jtm1_01_07_2015.png

[Willabe]

Look at these Hammond's, they are simi torid, I've used them and like them.

229B230 for B+, 229A24 for 12.6 heaters.

PT's;
http://www.hammondmfg.com/229.htm

OT's;
http://www.hammondmfg.com/125a.htm

[Willabe]

 Those look like they will work but both are way over kill.

PT; 180V at 200mA and 12.6V (6.3-0-6.3) at 4A center tapped, OT 10w.

That amp will only draw like ~10 to ~15mA for B+ and only 450mA (.450A) for the heaters since they are wired for 12.6vac.

Probable only 1w, maybe 2w at the most, full tilt.


 

[mresistor]

Thanks Willabe - that Edcor PT is exactly the right voltage. Wonder how much the Hammonds cost?


Studying the schematic I am wondering what/why the SG-1, SG-3 grounding circuits off the mains on the right. I see they connect to some of the HV capacitors, but why these extra circuits?  And Where is SG-2?

[sluckey]

Hammond 269EX (190-0-190) looks very good. Or 269 (150-0-150) for a little lower B+. Use a SS conventional FW rectifier for either. Or use EZ80 for that tube magic.

[Willabe]

that Edcor PT is exactly the right voltage.

Yes but it has ~20x the B+ current available and almost 10x the heater current you need. Because of this both secondaries voltages will run high. And that PT is much larger size and weight wise. So is the OT.

Sluckey's suggestion is very good.

[mresistor]

Sluckey  -  on the 269EX  one would just use one leg and the center tap? 


Willabe   there is an Edcor that is even cheaper. That 269EX is $50.  Look at this  http://www.edcorusa.com/xpwr085
Granted, I'd like to find a smaller lighter PT but cost is also a concern. Overspec'd isn't really a problem is it?

[Willabe]

Granted, I'd like to find a smaller lighter PT but cost is also a concern. Overspec'd isn't really a problem is it?

Yes it can be. There's over spec'd and then there's OVER SPEC'D.   

PT's are designed to supply a voltage on there secondaries at a certain current/load.  If you under load a secondary it will run higher that what the spec sheet says.

The more you under load it, the higher it can run, to a point. This may be good for B+, sometimes, but heaters (for 12A_7's) are only supposed to be run at a max of +10%.

So at ~20x the B+ and almost ~10x the 12.6acv, they will run high. Almost as if they don't have any load on them.

I think the 12.6 secondary will run higher than the 10% max. You can run a pair of low value R's in series (1 in each leg) with the heater secondary leads, but it's easier if you can avoid it.

Larger PT's and OT's also cost more.   

[Willabe]

Willabe   there is an Edcor that is even cheaper. That 269EX is $50.  Look at this  https://www.edcorusa.com/xpwr085.

Link doesn't work.

[mresistor]

Link works now. The edcor is a 170v PT with 6.3 vac filaments  for only $39. To me that is a steal.  If it is going to be lightly loaded then at 170v it should work
well. But it is 4.5 pounds. That is on the heavy side.


So  anyone have a chance to look at that schematic concerning the grounding? And I am wondering if connector 104 mates to connector 2 and connector 101 mates with connector 4?

[Willabe]

The edcor is a 170v PT with 6.3 vac filaments  for only $39. To me that is a steal.  If it is going to be lightly loaded then at 170v it should work well. But it is 4.5 pounds. That is on the heavy side.

That's better, but still over kill. Yeah 4.5 lb's is heavy for a 1w amp.  Keep looking.   

Try Antique Electronic Supply and Mouser for the PT's Sluckey suggested. And mouser has the PT's I suggested.

   

[Willabe]

Studying the schematic I am wondering what/why the SG-1, SG-3 grounding circuits off the mains on the right. I see they connect to some of the HV capacitors, but why these extra circuits?  And Where is SG-2?

Look again for SG2, it's at the top in the middle.

There not extra circuits, their telling you what they want grounded to what.

Slow down and spend some time just studying the power supply B+ rail, it's R's, filter caps, it's B+ nodes, what they hook up to and where/how they are grounded.   

And I am wondering if connector 104 mates to connector 2 and connector 101 mates with connector 4?

Same with this, slow down and think about it. Looks to me that they go together where they are, top to bottom.

It looks like this amp is made with a PC board and their also using plug in wire connectors instead of being hard wired.

[mresistor]

I was talking about the gnd circuitry on the RIGHT side on the mains.   I know what the filter caps are.   Talking about where those filter cap grounds "connect to".




Matec    ---  well that explains it. this is a Chinese amp, made in China. Guess there are improvements to be made. I'm going to go study some JTM preamp
                    schematics.

[Willabe]

I was talking about the gnd circuitry on the RIGHT side on the mains.   I know what the filter caps are.   Talking about where those filter cap grounds "connect to"

This what you said;

Studying the schematic I am wondering what/why the SG-1, SG-3 grounding circuits off the mains on the right. I see they connect to some of the HV capacitors, but why these extra circuits?  And Where is SG-2?

You asked about 'grounding circuits' and where is SG-2' (SG= star ground). This was my answer;

Look again for SG2, it's at the top in the middle.

There not extra circuits, their telling you what they want grounded to what.

I know you know what filter caps are, but your asking about their ground connections and you can't find SG-2.

All I'm saying is I think your not taking your time to see the whole picture. If you were you would see GS-2 and see the 'wired' (could be/probably a PC board trace connection) ground connections. At this point you know enough about tube amps and reading schemos to be able to find SG-2 and see the ground connections for SG-1 and SG-3.

The only extra circuit I see is they have 2 diodes in anti parallel across a filter cap. I'm not sure why? Might be a safety of some kind? Maybe for a short. But you did not ask about those.

[SILVERGUN]

I remembered having an older copy of this schematic...here it is...says Rev 1.3 and is a little blurry.
 
Check out these cheap toroids. I used a couple and they're still going strong after a year.
http://www.antekinc.com/as-05t200-50va-200v-transformer/
 
I like the size for the application of fitting them in a tight spot.

[mresistor]

Willabe - I was asking   where is the corresponding  SG-2 on the right side off the mains, SG-2 is plainly visible on the bottom of the HV filter cap.


Silvergun   thanks a bunch for that older schematic.  Those torroids look like the ticket.

[sluckey]

Sluckey  -  on the 269EX  one would just use one leg and the center tap?

No. Wire a conventional full wave two diode rectifier, not a bridge, just like this...

http://home.comcast.net/~seluckey/amps/warbler/warbler_rev2.pdf

[mresistor]

So what is the resultant rectified voltage? using a 290-0-290 PT.

[Willabe]

Willabe - I was asking   where is the corresponding  SG-2 on the right side off the mains, SG-2 is plainly visible on the bottom of the HV filter cap.

I'm very sorry, now I get you.

I don't see a corresponding  SG-2 note. They also haven't noted where the tube circuits grounds go, ie, to which GS.

Now I see the extra circuitry 

Red = some type of circuit ground lift.

I think this is how the other grounds would go together, colors group the GS's. 
 

[mresistor]

Willabe - yes   that's what I'm talking about, wonder where they hid SG-2.   But Silvergun's schematic makes it a non issue I believe.   


Also  those torroid transformers look nice and light and small.  Also on SG's schemo  the connectors make sense. So I guess that schemo I had
is kind of weird.   

[sluckey]

So what is the resultant rectified voltage? using a 290-0-290 PT.

Unloaded B+ would be 290 x 1.414 = 410V

I had meant to reference two Hammond PT in my earlier post but the second one only showed up as 269. Here they are again...

269BX, 150-0-150, unloaded B+ is 212V
269EX, 190-0-190, unloaded B+ is 269V

Both of these PTs have a 115V or 125V primary so the actual B+ can be raised/lowered a bit, depending on which tap you use.

[mresistor]

Silvergun - have you built one of these?

[SILVERGUN]

No, I was using that xfmr in another project.
 
I think I remember another thread where someone was discussing this and that's where I got that schematic.
 
Here's one of the threads I remember:
http://el34world.com/Forum/index.php?topic=16536.0

[mresistor]

a pair of anti-parallel diodes will drop voltage .7 volts    and I see this in the first schematic.  wonder why there were using so much filtering an lifting ground?

[vibrolax]

One more option for a power transformer is the following Antek toroid:
http://www.antekinc.com/as-05t200-50va-200v-transformer/

$28+shipping

It has 180 VAC and 200 VAC HV taps. 2 x 6.3 VAC @ 2A taps.

In my experience, the regulation with the Antek filament windings is good, i.e. the voltage won't be much higher than 6.3 even when it's lightly loaded.

I wish to heck Antek would make a 35 VA toroid that would fit cleanly into a 1U chassis.
The 50VA's supply more HV current than necessary (130 mA in this case).

I like Hammond PT's and OT's ok, but they always seem to be one of the more expensive options.

[DummyLoad]

21 bux.

https://www.edcorusa.com/xpwr024


--pete

[SILVERGUN]

http://www.marshallforum.com/workbench/69572-jtm1h-jtm145h-mod.html


http://www.handmades.com.br/forum/index.php?topic=5841.0

[mresistor]

what it sounds like   
https://soundcloud.com/matec-br/sets

[mresistor]

layout    not proofed     thanks DL for that PT find


http://s26.postimg.org/fhwquobrd/HTM1_3_TURRET_LAYOUT_01.png

[EL34]

Consider the plexi 6V6 also

http://el34world.com/schematics.htm#Hoffman_Plexi_6V6_