300 - 500 ohm speakers

[Peter212]

Something that just dawned to me after reading few articles on OTL amps.
Back in the days while living in Europe, I came into possession of an old mid 60s all tube b/w TV set that caught on fire. I was able to salvage a lots of parts, including two unusual speakers that measured 300 and 500 ohms respectively. Although quite uncommon. I said heck with that not giving it a second thought. Then, while going through some tubes in.my collection, I came across PCL82, a triode/pentode. It turns out that this particular tube was used as an audio amp in the 60s tv sets. My guess is triode as a preamp and pentode as a power amp. Is it possible that these high ohm speakers were directly connected to the plate skipping the output transformer?
Thx

[kagliostro]

To me NO

Datasheet say 3.9K load @ 100V B+

Franco

[Latole]

Is it possible that these high ohm speakers were directly connected to the plate skipping the output transformer?
Thx

No.

It is not a matter of ohms.

In simple terms, the OT converts the output tube or tubes' high-voltage, low-current output signal to a low-voltage, high-current signal, which also makes the tubes' high-impedance output into a low-impedance signal that can drive a speaker.

[kagliostro]

I salvaged from a military apparatus a small amp with 6AQ5 tube that has an OT with 600ohm secondary, if you do a search you can find headphones with such an impedance

As to use your speakers without an OT using PCL82, assuming you connect the two speakers in series for an 800ohm load, you must put 7 x PCL82 (the pentode part) in PSE and use a B+ of 200V, to use less tubes you must use 100V or 170V B+ and 5 x PCL82 tubes in PSE (but here the impedence match is less accurate)

Also there is to consider if those speakers can afford the power

1 x pcl82 @ 100v B+ will result in near 1W of power (so 5 x 1 = around 5W total power)

1 x pcl82 @ 170v / 200v B+ will result in near 3.3/3.5W of power (so 7 x 3.4 = around 24W total power)

I think it don't worth the effort

Franco

[PRR]

There was a European TV audio tube with a suggestion for "single ended push-pull" (2 sections) which drove ~~500r without a transformer.

The extra section was about as expensive as an OT (deep bass was not expected). Over the years, these high-Z (fine wire) voice coils tend to fail from corrosion.