OPT Primary Impedance Matching to Power stage?

[Stavee]

I'm new to tube amps, and trying to learn a lot real fast. The set up I'm wanting to do is two KT-88's in PPUL fixed bias. Going to run them at 460v B+ for the neighborhood of 70W. Don't want to burn them out too soon seeing as they were quite expensive. So on to my questions which will reveal how green I am.......

1) The G.E.C. info sheet for this operating condition gives Rl(a-a) as 4K Ohms. Is this the primary impedance I'm looking for in the OPT and NOT the Zout?(6.5K)

2) Edcor has a 100W 4.2K primary impedance OPT. Does the Rl(a-a) have to match the OPT primary exactly?
And what effect might a slight miss match have?
B) Edcor also has a 60W 4K Ohm primary OPT, but I wanted to leave a little room on the watage rating to stay away from any core saturation

 

[Buck Rogers]

It would be very hard to measure the difference.

This will work fine.

 

[gadget73]

yeah, 4000 vs 4200 plate to plate is not enough to matter. The speakers will likely swing the impedance much more than 200 ohms anyway

 

[TriodeLuvr]

FWIW, 70W is the output power at the anodes. You will likely see closer to 50W at the output of the transformer.

Jack

 

[kward]

Anywhere between 4K and 5k will be fine for KT 88 tubes in UL mode @ 460V plate. You’ll get a little more power at 4K vs 5k.

Under those conditions you should expect right close to 50 Watts output at the speaker terminals.

In order to get 70 watts at the speaker terminals you’d need about 500v to 515v delivered to the center taps of the output transformers. And to get that wattage with both channels simultaneously driven (assuming a stereo chassis build where both channels share the same power supply) you’d need either a very stiff supply or a regulated supply (a regulated supply is a little difficult to do correctly).

50 watts is still very decent though and not that difficult to pull off with a simple non-regulated capacitor/choke/capacitor filtered supply using diode rectification.

 

[Stavee]

Anywhere between 4K and 5k will be fine for KT 88 tubes in UL mode @ 460V plate. You’ll get a little more power at 4K vs 5k.

Under those conditions you should expect right close to 50 Watts output at the speaker terminals.

In order to get 70 watts at the speaker terminals you’d need about 500v to 515v delivered to the center taps of the output transformers. And to get that wattage with both channels simultaneously driven (assuming a stereo chassis build where both channels share the same power supply) you’d need either a very stiff supply or a regulated supply (a regulated supply is a little difficult to do correctly).

50 watts is still very decent though and not that difficult to pull off with a simple non-regulated capacitor/choke/capacitor filtered supply using diode rectification.

Is 500v (300 below max Va) in a safe range to NOT burn the tube up too soon?

TIA

 

[GordonW]

Dynaco got 60 watts from 480V B+ into KT88s at 4300 ohms UL, in the Mk III amplifier. That's pretty typical for KT88s in UL.

500V B+ on a 6550/KT88 should be just fine. Done all the time.

Regards,
Gordon.

 

[Stavee]

Dynaco got 60 watts from 480V B+ into KT88s at 4300 ohms UL, in the Mk III amplifier. That's pretty typical for KT88s in UL.

500V B+ on a 6550/KT88 should be just fine. Done all the time.

Regards,
Gordon.

thanks Gordon!

 

[spantou01]

Is 500v (300 below max Va) in a safe range to NOT burn the tube up too soon?

TIA

It is more a case of current (and resulting plate & screen dissipation) than plate voltage that “burns up” the tube. So, be aware of your bias settings and currents through the KT88s.

 

[GordonW]

Yes, with 500V B+, you probably don't want to go over about 65ma plate current. That's 32.5 watts plate dissipation.

Dynaco used about 70ma cathode current per tube in the Mk III. That's for the plate and screen combined. I'd back off on that a bit- maybe 68ma or so idle cathode current, with 500V.

Regards,
Gordon.