[thinktreasure]

Is there a valid sub for 25A6G?
The chassis I'm working on has a 25L6GT in place of it.

My sub books don't indicate this is a valid sub.

The chassis is a 37-611 connected to a AK speaker with a resistance of 500 ohms. The schematic calls out a 200 ohm speaker load on the output.


Steve

[Bob Andersen]

The RCA tube sub. guide for emergency services lists the 25L6 as a sub for a 25A6. Filament power is the same in both but looks like the 25L6 has more gain.

DC resistance isn't the same as impedance so you can't really say is it's mismatched or not.

http://radiorestorer.com/rcawartubes.PDF

[thinktreasure]

The RCA tube sub. guide for emergency services lists the 25L6 as a sub for a 25A6.  Filament power is the same in both but looks like the 25L6 has more gain.

DC resistance isn't the same as impedance so you can't really say is it's mismatched or not.

http://radiorestorer.com/rcawartubes.PDF

Thanks for the substitution information, I have never seen that.
On the impedance, and the loading on the audio output, when Philco puts 200 ohm's for the speaker coil in their schematic this is not the static resistance of the coil?
What i was questioning was the AK speaker operating on the Philco chassis.


Step 1 will be to get the correct tube it was designed to operate with.

When measuring the cathode bias on the tube this is taken from the B- gnd? at the main filter.
The cathode voltage should be negative, correct?

I have the radio playing and do intend to replace the usual things, caps, out of tolerance resistors, however when the volume is increased it distorts. The signal remains clean at the volume control, and the tube has been tested.


Steve

[Bob Andersen]

Yes, but you can have two coils with different static (DC) resistance that have the same impedance. For example one may use a larger gauge wire which would result in lower DC resistance but have the same impedance.

[thinktreasure]

Yes, but you can have two coils with different static (DC) resistance that have the same impedance.  For example one may use a larger gauge wire which would result in lower DC resistance but have the same impedance.

Ok Bob, let me see if I can flush this out...

The 25A6G pentode  impedance at 400hz is 4000 ohms 
The original speaker matching transformer with a 1 ohm voice coil primary is 200 ohms (per Philco)
Divide 4000/1.25 and get 3200 or 3200:1
Take the sq rt of 3200= 56.6 and the output transformer for the original speaker should have a transformer with a 60:1 turns ratio
Since Philco calls out 200ohms  200/60 =approx 3-4ohms Or the original transformer would be 4ohms / 200 ohms

If I wanted to use an 8ohm speaker

4000/8.25 =484 sqrt= 22  or  a transformer with  about a 20:1 ratio  10/200

Does this seem right?

The speaker the chassis is connected  to now has a voice coil of about 1.2 ohms and a 500 ohm primary with a secondary of 1/2 ohm
So for the impedance to be right the secondary  should be more like 10 ohms?
So this speaker they connected seems to be not the right choice.

I have no idea how to calculate the needed choke value to replace the field coil.

To make this a whole lot easier I'm hoping to find the real speaker.

[codefox1]

Just about any tube output t
ransformer will work OK, they are not so fussy. 2500 ohms to 3.2 is a familiar one.

[Radioroslyn]

If you have any spares from a set with a 50L6,50B5,or50C5 or the 35v versions will provide a good match. The FC /choke isn't very critical. A choke of a few henrys will work fine and if a do have a bit of 60cy hum you can up the mfds of the filters. On later model AC/DC sets with a PM spkr the choke is replaced with a resistor of 1500 ohms at 2 or 3 wts.
Condensers are upped to 47mfd or so.

Terry