Primary OT impedance for a pair of 6550's

[LooseChange]

Trying to match up some transformers for a pair of 6550's.
What should the primary impedance be?
Thanks!

[VMS]

Datasheets say 3,500 or 5000 Ohms:

http://www.drtube.com/datasheets/6550c-sed2001.pdf

http://www.drtube.com/datasheets/6550a-ge1972.pdf

http://www.nj7p.org/Tube4.php?tube=6550

[LooseChange]

I looked at those. But how do I know what is best?
500v on the plates 490v screens.

[VMS]

Amp with most power WINS!   

I don't Know, hopefully PRR will chime in.

How much current does your PT give?

Also I think the max screen voltage for 6550 is 425V in pentode mode.

[HotBluePlates]

... But how do I know what is best?
500v on the plates 490v screens.

Are those your known supply voltages? Do you know what current capacity you have available (from your B+ supply)?

[LooseChange]

Not sure but it's capable of handling a Twin Reverb. That okay?

[bobmegantz]

Here's one way to think about it:

1. OT primary impedance is 3500 ohms p-p, so each plate see 1/4 of that, or 875 ohms.
2. So, each tube sees peak power of (500Vx500V/875 ohms), or 285W peak, which is 142W avg.
3. Each tube handles half of the waveform (at high power outputs), so the total output power is 142W avg.

[VMS]

Hey bobmegantz, can this be done also in reverse?

3. Twin Reverb PT, so it is 100W avg.
2. (500Vx500V)/200W peak = 1250 ohms
1. OT primary impedance is therefore 4x1250 = 5000 ohms

[andrew_k]

Have you found a new production 6550 capable of handling 490V on the screens?

[DummyLoad]

anything 4K-5K will work with around 500V B+. watch 1/2 primary current rating of whatever you decide to use - the two OT below are well within current ratings for 2 x 6550 KT88 and both are 100W iron.

triode electronics 431a is 4K2 - designed for UL w/ 6550/KT88 will work in pentode mode - tune Ibias to keep each tubes idle @ 70% of max Pdiss.

hammond PT1650R pentode or UL mode - it's 5K.

--DL

[LooseChange]

Thanks for all the great info.
I never thought that 490v on the screens was a big deal. Any thoughts on thaat?

[FYL]

Sovtek-sourced 6550x are OK in big bad amps provided that you use suitable screen resistors and bias feed resistors - say 82K max.

[triode]

Thanks for all the great info.
I never thought that 490v on the screens was a big deal. Any thoughts on thaat?

It depends. I know I have read  on datasheets that the max is around 440V...

http://www.triodeel.com/6550ap2.gif

450V if UL. I have seen people do more. I have also seen those same
people complain about how 6550s "suck", or that they are "unstable". Could be a
correlation there... I have built a few amps with 6550s, but I regulate the screens down
to under 450V.  Why not start with something from the data sheets?

http://www.triodeel.com/6550ap3.gif

[jjasilli]

Thanks for all the great info.
I never thought that 490v on the screens was a big deal. Any thoughts on thaat?

Lots of old posts. Problem:  plates take 600V, but screens max out at 440V, and are "typically" run at only 300V per the spec sheets.  There's no good way to supply 600V to the screens plates and then make the required voltage drop to the screens.  I think there are 2 solutions:

a) sacrifice wattage, and supply a plate voltage closer to 440 (or less) so you can drop down to a proper screen voltage;

b) use a separate B+ supply to the screens:  UL OT (as mentioned); or a multi-tap secondary PT.  The later latter is the approach used by Stromberg Carlson in my APH-1100 PA power amps:  2X 6550's @ 100 Watts - 600V plates; 300V screens with separate high voltage PT windings.

[rzenc]

500v on the plates 490v screens.

I have an amp running a pair of 6550 with 480V @ plates and 473V @ G2 with primary impedance of 3K5. Biased @ -43V it eats up to 100mA's on each cathode. Full blast reads up to 297mA's on each cathode. Don't know if it's optimum conditions but sure gives very nice clean sounds, big and fat. And very loud too

Have you found a new production 6550 capable of handling 490V on the screens?

I have used "C" 6550's and they are handling the it. On the mentioned amp, I have a triode/pentode switch and the big bottles aren't breaking a sweat.

With Respect
Rzenc

[bobmegantz]

Hey bobmegantz, can this be done also in reverse?

3. Twin Reverb PT, so it is 100W avg.
2. (500Vx500V)/200W peak = 1250 ohms
1. OT primary impedance is therefore 4x1250 = 5000 ohms


you bet, but you can also measure the turns ratio pretty easily (using an oscillator) and calculate the primary impedance.

[DummyLoad]

twin reverb OT is 2K.  you don't think that 5K is kind of a stiff load for 4 6L6GC?

--DL

[VMS]

I was talking about Twin Reverb PT.

LC said that the power transformer is capable of handling a Twin Reverb, so I assumed that you can build a 100 Watt amp with that.