Silvertone 1485 output transformers

[hossymandias]

Hello again everyone.

I'm doing some repairs on a beat up old 1485 head my friend has. I've replaced drifted resistors, fixed some wiring issues, and cleaned everything out. I'd like to try it...but I realized I don't have a cabinet that is fit for it!

Someone modded the output so that each transformer has its own output jack. There are red/yellow wires going to each jack, which I've read are the 2 ohm taps.

Tell me if I'm wrong, but I would need to find a 2 ohm speaker cabinet before I can even tell if this thing works! Can I rewire the transformer taps to make an 8 ohm output? Can I do anything relatively simple to make this thing a practical head without buying an original cabinet (which was wired for two sets of three Jensen speakers)?

Thanks

[LOUDthud]

AFAIK the taps on those transformers are 4 and 2.66 ohms.

[hossymandias]

So if I found the 4 ohm taps I could rewire for more usable outputs. Is there a way to tell whether I have the 2.66 or 4 ohm taps? Can I measure this with a multimeter?

[Phil_S]

it is all explained here. Find the turns ratio. Calculate the impedance ratio.
http://www.radioremembered.org/outimp.htm
http://www.regiscoyne.com/tech/unknownOT/

[hossymandias]

Thanks Phil_S

When I get the 4 ohm taps wired to the output, will I need to change anything else as far as the amp goes (tubes, bias, etc). Or can I just rock?

[Stevem]

No, you can just rock on at that point, but read on!
These OTs are very prone to fail if a output tubes shorts out and or red plates.
To limit this crap get two inline fuse holders from radio shack or where ever and wire one each in series with each OTs center tap lead and install a 1/4 amp slow blow fuse in each holder.
also note that these amps are most times at there limit with having enough bias voltage to keep a good strong set of tubes in check
!
The mod of installing a 1 ohm 2 to 3 watt 1% resistor in series with the pin 8 wire to ground will let you measure the idle current directly with just placing a meter set for dc volts across that added resistor.
I do not like to see more than .042 ma at idle due to the quality of these light weight OTs in these amps.

If you are above .042 ma and changing out the bias resistor will not get ya more negative voltage than its time to convert the bias supply to a voltage doubling type and install a 10 to 20 turn ceramic trimmer to adjust it.

Also if the bias caps have not been changed yet and are original its time to do that, and stepping up to caps with 50 mfd more may get you another 1/2 volt or so to play with and not have to go the doubler route!

[Stevem]

WAIT!!!! I think I am totally off base with the way this amp (1485) gets it output stage baised, its not like the 1484 which is what I was thinking with my post to you!
Let me check this out and or may be someone before me can back my brain fade up with the info check out!

[martin manning]

1485 schematic http://www.silvertoneworld.net/amplifie ... /1485.html shows three 8 ohm speakers on each OT secondary, so 2.7 ohm, and no other taps.

The bias comes from its own winding using just a diode and 100u cap. No resistors to adjust. Converting to a doubler supply and adding a pot is a good idea.

[Stevem]

Yup, your right Martin, but there is one of 1960s silvertone buggers that is biased strange!

Hossey, you should check out the thread and the 1484 Silvertone on here and may be move the effects to the other channel which cuts down on a lot of hum that these amps have when run near wide open and even idle.

[hossymandias]

What a wealth of information and advice you all are.

I will be willing to change it to the doubling supply if my friend is willing to pay for the work!

Here is my other problem: because it has two 2.66 ohm outputs (and needless to say I don't have any speaker cabinets or wiring possibilities to create a matched cabinet) I haven't been able to test the cabinet.

Before he gave it to me, my buddy told me he plugged it in (I assume only one of the ot outputs) and a blue spark shot out. Could this have been an ot frying, or something else.

Anyway, my point is at what point do I replace the ot's? I don't want to put a bunch of work into modding it and then find out that the ot's need to be replaced, and have to tell my friend he has to drop another $100 on it.

Thanks again for all the advice and information, I have learned so much on this forum!

[Stevem]

A tube output stage with a OT should never be run without a load on it, as then the OT acts like a automotive spark coil with no place for the high voltage spark to go but to ark thru the windings!
A good way to prevent this from possibly taking place is to install a 250 ohm 7 watt resistor across each OTs secondary.