1950s MOODY AMP

[TIMBO]

Hi guys, I picked this one up yesterday and just had to show you.
When I get them I usually get a schem drawn and have a poke around to get an idea of its health.

This one is as stock as the day it was built.

I am always in two minds on how to attack these. Not wanting to defile it with new parts and the usual safety things, I was just going to store it away to preserve it as best as I could.

BUT, WHAT THE HELL.

Checking all the components have them all close to spec, even the Ecaps.

I could not find a reason not to turn it on. 

WELL, to my surprise fired up well.
With nothing plugged in absolutely dead quiet.
With guitar glugged in no hums or buzzes and pots smooth and quiet.

There is a guess with the valves............
Rectifier - I'm thinking 6X4 as the internal structure is similar to photos and they were common for the day.(no marking on valve)
Preamp - These look like 6AU6A again I can only go by they look similar to photos. (no markings on valve)

Its needs a new cloth covered power cord and power switch and I would think a fuse.

 

[TIMBO]

A few more.........

[trobbins]

Tim, if you have an insulation resistance meter, then you can check the coupling caps at say 500VDC.  It would be great if they showed negligible leakage at this age.


IR Testing the PT and OT would also be non-invasive tests to give you some confidence.


Perhaps you could operate it through an external mains fuse.


Is the mains wire insulation pliable?


You could surreptitiously connect tone pot wiper to the end terminal.


The schematic is missing a PT CT.

[TIMBO]

Thanks Tim, I don't have any testing equipment to do a test, just a MM.
The PT and OT look in good order with wire insulation in tact.
The power cord has the rubber type insulation and the Bakelite plug is not good.

The tone does not have much effect as is, will that improve things. Thanks

Schem fixed.

[DummyLoad]

not that that's wrong, but the 1M connected to pin 5 of V2: shouldn't that connect to node "C". circled in red in the attachment.



--pete

[TIMBO]

Thanks Pete, just found it.

When going over the circuit I did find that the 1M resistor was NOT soldered to the tagstrip, when it was put together the 1M and 220k were in the same hole on the tag and when it was soldered the solder did not flow onto the lead of the 1M resistor, but it was just touching enough to make a connection. Just found another mistake, that node is "B"

[PRR]

10K seems odd for a 6AQ5 cathode resistor. 1K?

V2 screen bypass "should" be larger than 0.01u, perhaps 0.1u. However this may be a tweaker's-choice.

[plexi50]

Sweet GEM!

[TIMBO]

Thanks PRR, The problem with cut and paste, Should have a closer look at values, it should be .1u.

The bypass resistor is a bit hard to tell what it is as it is tucked under the cap and has no colour markings but I can just make out some numbers. I'll have to remove the to get a better look. Thanks

[DummyLoad]

Thanks PRR, The problem with cut and paste, Should have a closer look at values, it should be .1u.

The bypass resistor is a bit hard to tell what it is as it is tucked under the cap and has no colour markings but I can just make out some numbers. I'll have to remove the to get a better look. Thanks

measure resistance with your DMM from pin 2 to ground - never mind the cap. your meter will read correctly (unless the cap is shorted). 


--pete

[TIMBO]

OK, There may be some issues

The bypass resistor is 250ohm/5w and with it disconnected I can't get a reading, there is no signs of heating, so i'll take it as OPEN.

A close look at the 6AQ5 has some black coating on the inside of the glass, ummmmmmmmmm

Where to go from here???

[Toxophilite]

Cute amp!



To clarify . Is the amp working? Your first post would seem to suggest it is


Personally I have no problem with ditching old caps and making amps safe (3 prong plugs and fuses) if it's going to be an amp you actually use, unless it has huge amounts of collector value I guess.

I don't think that black coating necessarily implies a bad tube. especially if the amp is working. There are many tubes with smoked glass coatings


Still, cute little amp! what fun

[DummyLoad]

OK, There may be some issues

The bypass resistor is 250ohm/5w and with it disconnected I can't get a reading, there is no signs of heating, so i'll take it as OPEN.

A close look at the 6AQ5 has some black coating on the inside of the glass, ummmmmmmmmm

Where to go from here???

new caps, new power cord, new cathode resistor, new 6AQ5/6005. play amp. grin like a cheshire cat. 

 

--pete

[birt]

if that tube works fine, it is fine.


i think if that cathode resistor was open the amp wouldn't have worked.

[kagliostro]

Ciao TIMBO

Nice Amp !

I'll follow your restoration with interest, I'm curious about the sound with those two in series pentodes on the preamp


---


BTW wich is the AC voltage at pin 6 and 1 of the 6x4 tube ?


Franco

[DummyLoad]

Ciao TIMBO

Nice Amp !

I'll follow your restoration with interest, I'm curious about the sound with those two in series pentodes on the preamp.


Franco

the first 6AU6 is wired as a triode - similar to 6C4 with slightly higher AV.


--pete

[kagliostro]

Thanks Pete

May be it worth to give a try to the Merlin's Morph mod as to push less or more V2




Franco

[TIMBO]

Thanks guys, The local guys have given the green light to replace the suspect partz.
And I agree with DL on CAT thing.

I have partz to replace resistors, coupling caps and Ecaps.

The power cord, switch and fuse....................
I think an internal fuse mounted on a tagstrip.
And I'm thinking an inline switch.
http://ecx.images-amazon.com/images/I/410tiIKps2L._SY300_.jpg

[kagliostro]

So you can preserve the original aspect of the amp, good idea

Franco


p.s.: Please TIMBO, when you have the occasion, can you measure the AC at the imput of the rectifier ? Thanks

[TIMBO]

Hi K, Each leg measures 254v.

[kagliostro]

Thanks TIMBO

Franco