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Author Topic: magnatone ancient amp  (Read 9595 times)

ilyaa

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magnatone ancient amp
« on: February 08, 2015, 02:56:50 PM »
another one from a friend -

works - speakers are shot (cone-wise) but working - while he's getting those fixed up or getting new ones im trying to see if anything inside needs work

its all pretty old and worn/rusted (see pics) - i think the amp's from 1948!!

its got those cool tubes with the plate tap on top!

two primary concerns (as youll see in the pics):

1) someone put in a new power cable and just thought to leave the safety ground hanging!!!

2) the leads coming out of the OT are kind of weird - each side of the winding comes out as a two, unstranded, wound together, exposed copper wires....these have some green oxidation on the ends right before the terminal where they hook up to the speaker wires - thoughts? i tried to look up this OT - triad 1746 - but not much luck....whats up with these exposed leads? thicker for wattage handling? kind of a small OT, too....anyway, just curious if i should clean them/cut off the green part/replace the OT.....

http://dropcanvas.com/#7SH56kJ26C2G6C
« Last Edit: February 08, 2015, 04:55:59 PM by ilyaa »

J M Fahey

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Re: magnatone ancient amp
« Reply #1 on: February 08, 2015, 11:19:40 PM »
Those are the enamelled copper wires actually used to wind the transformer .

Instead of scratching/burning enamel at the ends, twisting and soldering to stranded hookup wire pigtails, they are simply taken out, usually inside some varnished cotton or plastic "spaghetti" tube, and soldered outside straight to the proper terminals.

Very common in transformers which use somewhat heavy wire, such as filaments.

Wires are double because they probably were short of the proper gauge and used a lighter one.

Since enamel is burnt (in those days with a small alcohol burner)  I think copper might corrode somewhat after mere 66 years  :o

Leave original transformer there if it works.

For peace of mind  ::)  you can lightly wipe the greenish area with a moist cotton ball, let dry and cover with transparent nail enamel or something.

Then it will be fine for some extra 1000 years  :lmao:

ilyaa

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Re: magnatone ancient amp
« Reply #2 on: February 19, 2015, 07:46:30 PM »
the amp is all good -

tried it out with a 4 ohm bass cab i have and it sounds great!!

issue is:

the original speakers measured about 4 ohms each and were hooked up in parallel - so im thinking maybe the OT is looking for a 2ohm load. but i have no way of knowing that the speakers were wired right. before i tell my buddy what speakers to buy, i was wondering if you guys know of a good way to determine output impedance for an unknown transformer?

Roly

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Re: magnatone ancient amp
« Reply #3 on: February 20, 2015, 08:18:54 AM »
With an amp of this vintage it's a pretty fair bet (guess) that it was 8 ohm output to a parallel pair of 16 ohm speakers.  8 ohms was the most common in those days.


To confirm you need to get a bit techy.  You energise the OPT backwards using the heater line (or external source of 6-12VAC, with the speakers disconnected and the OP valves pulled).

You carefully measure the actual voltage at the OPT secondary and across the primary (careful! - 100+VAC), anode to anode.  This gives you the voltage and turns ratio, typically between 20:1 and 30:1.

The square of this ratio is the impedance ratio, typically between 400:1 and 900:1.

Multiplied by 8, does this give you a reasonable plate-to-plate impedance for the OP valves used?  It will be somewhere in the range 3k to 10k, most likely somewhere in the general vicinity of 4k; e.g. impedance ratio 600:1 times 8 ohms gives 4.8k anode-to-anode.

This is a bit rubbery because these OPT's were not much chop at power mains frequencies, but it should put you in the ballpark close enough to tell if it is 4, 8 or 16 ohms - but my money is on 8 ohms and I'll give you odds it isn't 2 ohms.


There are about 20 Magnatone amp circuits here;

http://chasingtone.com/schematicheaven/bargainbin.html

... but I'm not going through them for you, you'll have to work out which is the closest to what you've got.

The few I've looked at specify an 8 ohm output, as expected.

So you can't find a model number anywhere?

What OP valves does it use?  (and what's the valve line up?)
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: magnatone ancient amp
« Reply #4 on: February 20, 2015, 03:54:16 PM »
its a magnatone M-198-8D

cant find a schematic for that specific one anywhere...

it uses a pair of 6BG6's for the OP

not sure what you mean by lineup - push-pull

but in the rest of the amp,
the valve lineup is: 6SJ7(3) 6SL7 6N7 6BG6(2) 5U4

a few questions about the test:

1) its okay to turn on the amp without a load if the OP valves are pulled?
2) i just hook up the two heater wires to the two secondary leads of the OPT?
3) doing this with everything else turned on in the amp is OK? the pulled OP valves isolate the output stage from the rest of the amp, presumably?
4) measuring across the primary do i measure form one end to the center tap? or across both ends?
5) okay to measure with a good DVM or should i use a scope?
« Last Edit: February 20, 2015, 03:55:28 PM by ilyaa »

Roly

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Re: magnatone ancient amp
« Reply #5 on: February 20, 2015, 04:44:00 PM »
Quote from: ilyaa
not sure what you mean by lineup - push-pull

Valve types used (generally from input to output).
The 6BG6 is very "off Broadway" in my experience, don't think I've encountered that one before; ratings look similar to a 6L6.

http://tdsl.duncanamps.com/show.php?des=6BG6

Quote
Class     Va  Vg2 Vg1    Ia         Ig2        Rk Zout  Pout THD    
AB1 P/P 450 400 -37.0 116-210 5.6-22.0 -- 5,600 55.0 1.8


So we are expecting about 5k6 plate-to-plate.

{Assuming 8 ohm output...

5600/8 = 700:1 impedance ratio

700^0.5 = 26.5 voltage/turns ratio.  (square root)

For 6.3VAC...

6.3 * 26.5 = 166.9VAC plate to plate.

It may be significantly different at 50/60Hz, but should be within 2:1 either way.}


1) Yes

2) Yes, but be careful of the grounding on the heater circuit and the OPT secondary - it may be one wire simple, or you may need to also lift the OPT secondary grounding, or it may be easier to drive it from an external 6-12V transformer (depending on what you find).

3) Yes and yes, just don't forget that the HT/B+ will be active, and lacking the output stage may stay charged for quite some time after switch-off.

4) from anode to anode across the whole winding (easy to stick your meter probes in the OP valve holder holes).

5) Use your meter.  In general you should be very cautious about probing the anodes of an output stage under drive because the peak-to-peak AC voltage may be up to double the HT/B+ supply voltage, and this may embarrass some test instruments such as CRO's with a 500V limit.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: magnatone ancient amp
« Reply #6 on: February 20, 2015, 05:00:05 PM »
ya the 6BG6 has the plate tap on the top of the tube -

Quote
2) Yes, but be careful of the grounding on the heater circuit and the OPT secondary - it may be one wire simple, or you may need to also lift the OPT secondary grounding, or it may be easier to drive it from an external 6-12V transformer (depending on what you find).

you mean be careful that the OPT doesnt have a separate wire running to ground, aside from the two wires feeding the load?

would it be safe to drive it from the PT from another amp, that way i dont have to turn the magnatone on at all? i dont have any random little transformers laying around to give me 6VAC....

Roly

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Re: magnatone ancient amp
« Reply #7 on: February 20, 2015, 09:28:22 PM »
Quote from: ilyaa
you mean be careful that the OPT doesnt have a separate wire running to ground, aside from the two wires feeding the load?

Normally the heater supply has one side or the mid point grounded, similarly the secondary of the OPT often has one side grounded (particularly if the amp has NFB).  If you get it wrong you will put a short across the heater supply, so just be aware and careful, and lift the ground on the OPT secondary if required - this is also true with a driving amp which will be co-grounded via the mains safety earth).

Quote from: ilyaa
would it be safe to drive it from the PT from another amp, that way i dont have to turn the magnatone on at all?

Yes (tho if the other amp is a valve amp it will still needs a load, but its speakers will do; the tranny under test doesn't represent much of a load in itself, being o/c on the valve side).  This also means that you can use a more reasonable frequency.  Check the waveform between the trannies with your CRO to be sure it is reasonably sine.  You can do this at reduced voltage and measure with your CRO if that is more accurate, but remember we are talking ball park figures here to determine 4, 8 or 16 ohms, not 0.1% accuracy.  +/-20% is generally fine.  (but don't assume your drive voltage, always measure it)

I'm going to put a dollar on 5k6p-p to 8 ohms.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: magnatone ancient amp
« Reply #8 on: February 20, 2015, 09:49:35 PM »
wait, so are you saying to feed the magnatone OPT with the the OUTPUT from another amp? just make sure its at a low-ish voltage?

(i was thinking just to use the heater taps from another amp to avoid any possible grounding mishaps - the magnatone would be unplugged from everything - safety earth included)
« Last Edit: February 21, 2015, 12:24:02 AM by ilyaa »

Roly

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Re: magnatone ancient amp
« Reply #9 on: February 21, 2015, 12:59:11 AM »
Ah, I misunderstood your intention.

Well you can go either way, but sure, snarfle 6.3VAC from the heater line of another amp if you have one to hand, and as you say, if the Magnatone is otherwise disconnected from everything you shouldn't have any problems.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: magnatone ancient amp
« Reply #10 on: February 24, 2015, 12:55:31 PM »
alright i did the test:

on the secondary i put in ~6.8VAC

on the primary i got ~288VAC

so thats about 42:1 ratio

42^2 = 1764

if we take the 5.6kplate to plate

that gives me 5600/1764 = ~3

so were looking at 4 ohms then, huh?

i found a PT at a swap meet on sunday and used that for the input voltage, so its 50/60 Hz - my results seems okay?

g1

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Re: magnatone ancient amp
« Reply #11 on: February 24, 2015, 03:19:01 PM »
  Your math seems ok.  This amp had 2 x 12" speakers so they were probably 8 ohm wired parallel for 4 ohm load.

Roly

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Re: magnatone ancient amp
« Reply #12 on: February 24, 2015, 08:17:43 PM »
Yeah that looks reasonable.

The only reservation I have is that all the Maganatone circuits I looked at showed "8 ohm" outputs, so I'm wondering if that's the original OPT fitted, or maybe blown and changed some time in its long history.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: magnatone ancient amp
« Reply #13 on: February 24, 2015, 09:04:00 PM »
Quote
The only reservation I have is that all the Maganatone circuits I looked at showed "8 ohm" outputs, so I'm wondering if that's the original OPT fitted, or maybe blown and changed some time in its long history.

hm hard to say, i guess. a very long history.....

but either way probably a safe bet to get two 8 ohms speaks in parallel, right? i did try this through a 4 ohm cab and it sounded great!

Roly

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Re: magnatone ancient amp
« Reply #14 on: February 26, 2015, 12:10:26 AM »
All you can do is give it a long flog looking for any signs of distress, smells, smoke, red-plating, (which is where a dummy load is handy) and if all seems well, call it cured.
If you say theory and practice don't agree you haven't applied enough theory.