6aq5 SE OT

[jeff]

 I recently scavenged some parts from an old TV. The audio amp was a 6AQ5 SE and I thought I'd use the parts to make a small amp.

 The 6AQ5 is a 12 watt tube with a max of 250V. The datasheet says something along the lines of it having the same characteristics as a 6V6 within its ratings and shows an example of a 5K load with 250V.

The speaker measures 3 ohm resistance so I assume it's about a 4 ohm impedance. I decided to check the impedance ratio of the OT and was expecting to see about 5K:4ohm but to my suprize it was 17K:4ohm. (I'm quite sure I did this correctly and tested a known transformer with the same method and the numbers made sence)

I just took the parts and don't have the schematic but under what conditions would you use a 17K output transformer(with a 6AQ5)?

My guess is maybe they didn't go for the max output and used a bigger transformer but biased it colder? I don't know?

[FYL]

I just took the parts and don't have the schematic but under what conditions would you use a 17K output transformer?

I wouldn't. Any OT with more than 10K on the primary and a low Z load on the secondary is a nightmare to wind properly...

[HotBluePlates]

I recently scavenged some parts from an old TV. I just took the parts and don't have the schematic but under what conditions would you use a 17K output transformer(with a 6AQ5)?

The same people, in the same tube manual, will tell you the 6V6GT is a 315v maximum tube (G.E. 6AQ5, G.E. 6V6GT). So we should know we can bend the rules a bit. Call a 6AQ5 a "6V6 in another bottle".

17k seems awful high... Are you getting about 32.6:1 as your turns ratio? If so, a 2 ohm load lands you more like 8500 ohms, which would be dead-typical.

[jeff]

17k seems awful high... Are you getting about 32.6:1 as your turns ratio? If so, a 2 ohm load lands you more like 8500 ohms, which would be dead-typical.

Did you mean 8 instead of 2
 
Wouldn't a turns ratio of 32.6:1 give you a 4251:4ohn or 2125:2 ohm?

It took 6.5 v in primary to get .1 volt out (6.5/.1=65(turn ratio) , 65²=4225(imp ratio) 4225X4=16900)

[jojokeo]

No HBP is correct - w/ a 2ohm load you're lowering the primary Z by half which turns your 17k into 8.5k seen/reflected. IF you raised the load/output Z from 4ohms to 8ohms you then would be increasing the primary Z by double to 34k.

[HotBluePlates]

Oops! Yes, I meant with your present measurement, if you had a 2 ohm load, you'd land at about an 8k primary.

65:1 voltage ratio -> 65² * 2 = 8,450 ohms. But I messed up the math for how to arrive at numbers we want.

9k:4 ohms = 2,250, and the square-root of that is 47.4:1

You got 6.5v:0.1v, or 65:1. Is you meter really good at resolving 0.1vac? If the a.c. on the secondary was really very slightly higher, you might land nearer a workable primary impedance (for example, if the 0.1v was really 0.14v; then your primary is closer to 8k with a 4 ohm load).

Try applying the 6.5vac (presumably from a filament transformer or winding) to the secondary. Before you do that, set you meter for higher voltage, and attach the leads with clips to the primary. 6.5v * 65 = 422v. To get 8k:4 impedance ratio (2000:1), you need a turns ratio of 44.7:1. For 6.5vac applied to the secondary, you should see something like 6.5v * 44.7 = ~290v.

Your meter will probably have an easier time resolving the higher voltage more accurately.

[HotBluePlates]

No HBP is correct - w/ a 2ohm load you're lowering the primary Z by half which turns your 17k into 8.5k seen/reflected. IF you raised the load/output Z from 4ohms to 8ohms you then would be increasing the primary Z by double to 34k.

Nah, he's right. I boogered the turns ratio. The impedance, if halved on the secondary, will be halved on the primary. But the turns ratio shouldn't be halved, it should be lowered by the square-root of 2.

In any event, something in the neighborhood of 44-47:1 will arrive at a good primary impedance.

[jojokeo]

I didn't read the math parts, I just stated the basic load vs primary Z understanding principle. I don't get wrapped up in turns ratios, just give the tranny's ratings is all. But I understand he's trying to calulate an unkown tranny's value so...