Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: ilyaa on February 08, 2015, 02:56:50 PM

Title: magnatone ancient amp
Post by: ilyaa 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
Title: Re: magnatone ancient amp
Post by: J M Fahey 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:
Title: Re: magnatone ancient amp
Post by: ilyaa 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?
Title: Re: magnatone ancient amp
Post by: Roly 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?)
Title: Re: magnatone ancient amp
Post by: ilyaa 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?
Title: Re: magnatone ancient amp
Post by: Roly on February 20, 2015, 04:44:00 PM
Quote from: ilyaanot 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 (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.
Title: Re: magnatone ancient amp
Post by: ilyaa 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....
Title: Re: magnatone ancient amp
Post by: Roly on February 20, 2015, 09:28:22 PM
Quote from: ilyaayou 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: ilyaawould 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:
Title: Re: magnatone ancient amp
Post by: ilyaa 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)
Title: Re: magnatone ancient amp
Post by: Roly 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.
Title: Re: magnatone ancient amp
Post by: ilyaa 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?
Title: Re: magnatone ancient amp
Post by: g1 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.
Title: Re: magnatone ancient amp
Post by: Roly 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.
Title: Re: magnatone ancient amp
Post by: ilyaa on February 24, 2015, 09:04:00 PM
QuoteThe 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!
Title: Re: magnatone ancient amp
Post by: Roly 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.
Title: Re: magnatone ancient amp
Post by: ilyaa on March 02, 2015, 04:32:31 PM
alright seems good -

one question: the cathode resistor is unbypassed - its at about 30V. i get a great signal swing from the PI, but the power tubes are only putting 13 watts or so into 4 ohms. im probably going to replace them (they are quite old, too), but am wondering about the unbypassed cathode resistor. does that seem odd for a push-pull 50 watt-ish design?
Title: Re: magnatone ancient amp
Post by: Roly on March 02, 2015, 07:05:22 PM
Quote from: ilyaaam wondering about the unbypassed cathode resistor. does that seem odd for a push-pull 50 watt-ish design?

Yes, it does.  No signs there was one in there?  The few Magnatone ccts I looked at had one.  That would introduce quite a bit of local NFB to the output stage.  It is generally much more common for the cathode bias resistor to be bypassed with an electo of suitable ripple current rating.  Lash something in and give it a burl - you can always take it out again.

I'll just remark that the 40 to 50/60 watt step is generally the break point where designers switch from cathode to fixed bias, not that there is any hard and fast rule about it, but generally with all things considered...
Title: Re: magnatone ancient amp
Post by: ilyaa on March 04, 2015, 03:49:15 AM
cool!

put in a 47 uF cap and got more clean power as well as a squar-er full power overdrive (at least on the scope) - and more power overall, actually.

we'll see what happens when i put new tubes in -
Title: Re: magnatone ancient amp
Post by: Roly on March 04, 2015, 04:18:34 PM
{...the cap will explode?  :o }


Just off the top of my head 47uF is a bit light for a 50W output stage.  I'd expect something more like 220uF to 470uF to have sufficient ripple current rating.
Title: Re: magnatone ancient amp
Post by: ilyaa on March 04, 2015, 06:04:21 PM
really??

i calculated for a 40 Hz-ish cut-off - transconductance of 6BG6 is about 8mA/V - the cathode resistor is 250 ohms. 47 uF seems to be just right for that cut-off.

i didnt realize i was worried about ripple current there....and i thought a bigger cap would have created bias excursion problems.

am i missing something?
Title: Re: magnatone ancient amp
Post by: Roly on March 05, 2015, 03:19:56 PM
If anything a bigger cap will tend to hold the bias steadier than a small one (think: a one farad cap is like a battery).

As with the filter caps in a s.s. amp power supply we can calculate a value from the required ripple and current draw, but the available caps may not align with both the required value and ripple current rating, and it is common that we have to use a higher value to get sufficient ripple current rating.

In the case of cathode bias we want an RC time constant that is long enough to satisfy the lowest frequency we want to reproduce, say 40Hz, but again the ripple current rating of the cap that will give us that frequency will typically be too low to handle the 200-300mA expected (in a 50-60W amp).

As we often have to do, where we have different constraints we have to use the most conservative (e.g. transistor voltage, current, and power ratings).
Title: Re: magnatone ancient amp
Post by: ilyaa on March 05, 2015, 03:33:29 PM
what about this:

http://www.davidsonamp.com/sf/images/twin5c8.gif

isnt that bypass cap too small, according to what youre saying?
Title: Re: magnatone ancient amp
Post by: Roly on March 05, 2015, 04:04:23 PM
That's a very old example and when that was built you could get 25uF caps the size of a C-battery and they had ripple current ratings known as "enuf".  You will be hard pressed to find a 25uF cap these days with that sort of ripple capacity - a modern 25/25 might be 5mm dia and 10mm long.

If we dig a bit, the ripple current limit is a thermal internal heating limit.  The ripple current flows through the cap and causes heating by passing through the internal loss resistance.  With a tiny capacitor it has very little surface area to get rid of this heat and if you over do it the electrolyte will boil and the cap explode.  Older caps were lossier than modern ones, but they also had a lot more surface area to get rid of the heat.
Title: Re: magnatone ancient amp
Post by: ilyaa on March 05, 2015, 08:50:35 PM
hmmm i see i see -  good to know i was totally unaware of those constraints!

would a 220 uF 50V cap work better? its still small physically.....

where could i find the ripple current limit or calculate it for a modern cap?
Title: Re: magnatone ancient amp
Post by: Roly on March 06, 2015, 10:17:17 AM
Quote from: ilyaawould a 220 uF 50V cap work better? its still small physically.....

where could i find the ripple current limit or calculate it for a modern cap?

Try looking here (http://lmgtfy.com/?q=electrolytic%20capacitor%20ripple%20current).
Title: Re: magnatone ancient amp
Post by: ilyaa on March 06, 2015, 03:12:05 PM
hm i see very funny serves me right

hard to figure out all the parameters for a cap i already have - but i looked at all the similar ones (size/temperature/voltage rating) and my 220uF guy should be alright. the 47uF would not have been!
Title: Re: magnatone ancient amp
Post by: Roly on March 07, 2015, 12:34:21 AM
A conservative rule of thumb might be "1mA per uF".
Title: Re: magnatone ancient amp
Post by: ilyaa on March 10, 2015, 04:48:11 PM
hm now you have me all paranoid about ripple current -

would a physically large, high voltage 47uF have a high ripple tolerance generally? or does it really depend cap to cap and better to just err on the side of caution?

i attached a datasheet for one i have - a 47uF / 350V - its 16x20 mm - but there is no ripple current on the datasheet (in general ive found that parameter hard to find......)

PS its 30V over a 250 resistor at idle so 120 mA of idle current....
Title: Re: magnatone ancient amp
Post by: Roly on March 11, 2015, 12:37:13 AM
Nah, look, you just want a reasonably chunky cap.  I wouldn't go above about 50/63V, but anything physically bigger than the required micro 47uF, and maybe low ESR if it's to hand, but a physically big cap that means it will most likely be 100uF-470uF.

You don't have to be paranoid, but many people ignore it and come unstuck, so just try to keep it in mind with any electro cap, value, voltage, ERS/ripple current.

This Jaycar page gives the ripple current ratings of sundry electros;
http://www.jaycar.com.au/search?q=electrolytic%3Arelevance&page=1 (http://www.jaycar.com.au/search?q=electrolytic%3Arelevance&page=1)

e.g.
QuoteValue: 47uF
Working Voltage: 25V
Lead Spacing: 2mm
Size dia x H: 5 x 11mm
Ripple Current mA: 100
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount
105 degrees but only 100mA, however there are quite a few other options depending on the highest expected cathode voltage;

Quote
Value: 100uF
Working Voltage: 63V
Lead Spacing: 5mm
Size dia x H: 10 x 12mm
Ripple Current mA: 280
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount
-
Value: 220uF
Working Voltage: 25V
Lead Spacing: 3.5mm
Size dia x H: 8 x 12mm
Ripple Current mA: 320
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount
-
Value: 220uF
Working Voltage: 50V
Lead Spacing: 5mm
Size dia x H: 10 x 17mm
Ripple Current mA: 430
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount

...all under a dollar.

You can use this as a rule-of-thumb guide because other manufacturers will be trying to meet similar specs (fraud aside).
Title: Re: magnatone ancient amp
Post by: ilyaa on March 15, 2015, 04:10:29 PM
cool, thanks Roly -

i think this amps good to go - louder sounding and all cleaned up -