Choosing tube types based on mod iron specs

[K8DI]

So I’ve acquired a mod transformer. I bought it for dirt cheap without a plan. I’d like some ideas and suggestions on tube choice. The primary is rated as 5700 ohms. There’s quite a few audio tubes that can play nice with that. The secondary is 2600 ohms.

First, looking at mod transformer impedance, how should that relate to final plate load?
Second, given that impedance, what tube choices are close?

The transformer is a potted UTC  unit rated for 2500vrms hipot, 250mA secondary. It’s rated for 70 watts, but that feels conservative given the weight/size. I don’t know what it came out of. It came alongside a filament transformer with two 6.3v and two 2.5v windings, all four at 10 amps, so my guess is it had mercury rectifiers for the HV.  The plate transformer that came along with it is 1500v, ct, 325mA. Maybe one of you recognize that combo and can say what it was built with, but, barring that, what would you build with it??

Ed

[n8fvj]

I would use a pair of KT88 at 575 volts plate, 300 volts screen and -35 volts fixed bias. 80-90 watts is available.

[K8DI]

Googling around I found the mod transformer model number may be from a National Co. VHF AM transmitter for airband use, p/p 6F6 driving p/p 809 in class B modulating a pair of 4-65A. B+ of 600v, and from other parts listed in that twenty year old ad, there’s more iron to find.  I’ll head back to the seller’s next week to see what else I can find — interstage  audio and four chokes seem to be possible.  

I don’t have a model number for the TX. If anyone has an idea I’d be happy to hear it.

I’m not at all married to those tubes. 809’s are expensive and have pretty low plate dissipation, 4-65As would do better with significantly higher B+, which 809s won’t tolerate, etc.

If I keep the HV down, my go to audio tubes will be 6550 or 8417, because I have several of each in my stash, but I’m still mostly looking for a translation from mod secondary impedance to final tube choices…

Ed

[n8fvj]

8417 would work about same as KT-88. 100 watts audio output.

[DMOD]

So I’ve acquired a mod transformer. I bought it for dirt cheap without a plan. I’d like some ideas and suggestions on tube choice. The primary is rated as 5700 ohms. There’s quite a few audio tubes that can play nice with that. The secondary is 2600 ohms.

Does the primary have a centertap at 2850 ohms?

[K8DI]

So I’ve acquired a mod transformer. I bought it for dirt cheap without a plan. I’d like some ideas and suggestions on tube choice. The primary is rated as 5700 ohms. There’s quite a few audio tubes that can play nice with that. The secondary is 2600 ohms.

Does the primary have a centertap at 2850 ohms?

The primary is tapped, the tap is labeled CT, although the diagram on the can doesn’t give an impedance at the tap, and I haven’t measured it.  It appears to be the typical setup, push pull modulator.

Ed

[DMOD]

The closest tube I could find, as of now, would be a 6146W Class AB2 modulator as per the schematic below (from tube specs):

This could potentially modulate a 220Watt RF deck to 100%.
 

[n8fvj]

6146B is a little light on plate dissipation for 100 watts audio compared to 8417 or KT88. Use Sylvania 6146B known to have best plate dissipation.

[DMOD]

The 6550 might work but it seems the big unknown is the current carrying capability of the transformer's primary.

[KD6VXI]

There is a KT120 out.  Little more Pdiss.

I've never used it, but I do love me the 6550 and KT88!

--Shane
WP2ASS / ex KD6VXI