How to tell different ohm taps in an output transformer?

[slidincharlie (Carlo P)]

I am trying to refurbish and put to working condition an old Guild 99-J amp that I bought off ebay.
This amp has got two speaker jacks, each connected to a different wire from the output transformer. I suppose that one is for the stock 8 ohm speaker, and the other for an external cabinet
How do I tell which one is the 8 ohm jack?

[stokes]

Most amps that have a main skr jack and an external spkr jack will have a shorting jack for the main jack and the ext.spkr jack is wired parallel and usually uses a non-shorting jack.If as you say there are two wires coming out of the OT,you can check the resistance between the common or usually black lead and each of the other two "hot" leads.The readings will be very close,but if for instance you have an 8ohm and 4ohm tap the 8ohm will be slightly higher than the 4 ohm.We are talking resistance of like 1.2 ohms as compared to .8 ohms.You can also check the actual primary impedance by putting a .5 vac signal on the secondary and measuring the volts at the primary from end to end,dont connect the center tap.Lets say you get 15 volts you then divide that by .5 and you get 30 so your turns ratio is 30 to 1.Now square that 30 and you get 900.Multiply 900 times the expected spkr impedance,say 8ohms you get 7200.A pair of 6v6's runs about 6600ohms plate impedance so it would be safe to use an 8ohm speaker with that tap.If you have a pair of 6L6's you want to see a primary impedance of a round 4500ohms for a pair.Hope this helps.

[slidincharlie (Carlo P)]

stokes,
neither jack is shorted. Both are plain two-lug jacks. They share a common ground (black wire from the OT), and have a different wire from the OT going to each hot lug.
The resistance between the hot lug and the common ground is 1 ohm in one jack and .8 ohm in the other. Is this enough to say that the "1 ohm jack" wants an 8 ohm speaker, while the ".8 ohm jack" calls for a 4 ohm speaker?
I don't have a way to put a .5 V signal on the secondary, so I can't run your second test.
The only thing I know for sure is that the schematic says "8 ohm speaker" (but it shows only ONE jack), and the current speaker (probably stock) is 8 ohm.

[Satamax]

http://www.radioremembered.org/outimp.htm

[stokes]

I guess it is safe to assume that the one that measures 1ohm is an 8ohm tap and the .8ohm is a 4ohm tap.

[Amp Kat]

Is there maybe a switch around that jack for 4 to 8 maybe ? What are the colors ?

[slidincharlie (Carlo P)]

http://www.radioremembered.org/outimp.htm

Thanks Satamax,
I'll check that piece of text.

[slidincharlie (Carlo P)]

KB,
there isn't any switch.
In the photo below you can see a detail of the jacks.
One wire going to a hot lug is yellowish, the other brownish (they are very old).

[stokes]

Carlo,if you dont have a variac,as is shown in that article to inject a voltage into the tranny for the test I described a signal generator works.I use a 1k signal of 1/2 volt for the test.

[slidincharlie (Carlo P)]

I can generate a signal with a computer signal generator . Should it work?

[Satamax]

Carlo, i would use a small transformer, liek 24V at 50HZ, it's good enough. May be one from a telefonino adapter would work too.

[Enzo]

I KNOW you have 6VAC in your shop, the heater winding of this very amp would provide it. Use that with a couple clip wires. Or from ANY power tranny you have laying around. The voltage is not critical, doesn't need any current, all we are doing it seeing if the suspect transformer wiorks.

[slidincharlie (Carlo P)]

This is not clear...

I'm a bit lost...
Should I apply voltage (from the 6.3V heaters of another amp's power tranny, or even from a mobile telephone charger) to either secondary of the Guild's output tranny?
BTW satamax, how do I hook the charger's plug to a tranny's secondary?)
The amp under test must be shut off, I suppose?
Sorry for being so ignorant...

[Satamax]

Ciao Carlo.

Well, you can use any AC voltage source for the test. If you have a small voltage aply it to the secondary, and with a high voltage, providing you know how to work safely with it, you apply it to the primary. Myself i would unsolder all the primary and secondary leads to do the test. An use screw connectors. (dominoes) To connect and protect the leads. Like the one in the picture http://satamax.free.fr/shuntbias.jpg Plus iirc the charger might have a rectifier in it. Or switching circuit. So the best way would be to use an old charger and dismantle it. Or the 6.3 vac is good. Or even 240V from the wall if you feel safe with it. Or your variac if you have one. Any source of AC. Obviously the amp must be shut off.

Bye.

Max.

[slidincharlie (Carlo P)]

Tube plate impedance value

stokes,
I am finally in the process of identifying the load required by the speaker jacks in this Guild amp, as well as by two other amps (a 1472 and a 1482 Silvertones that have two separate taps on their output tranny).
I have also found the technique described here by you, satamax and enzo in the Gerald Weber's dvd, that definitely helped me to understand how the test is practically done.

Now I have a doubt about the plate impedance value of 6V6 and 6L6 tubes.
You mentioned 6600 ohms for a pair of 6V6 and 4500 ohms for a pair of 6L6.
I have found different values in my tube data sheets for class AB1 push-pull operation, that is:

- 8000 ohms for a pair of 6V6GT (General Electric datasheet, Duncan Amps TDSL datasheet);
- 5600 ohms for a pair of 6L6GC (Svetlana datasheet, Duncan Amps TDSL datasheet).

Which one is correct?