KT88 O/P tranny

[ernie_jr]

Will KT 88's be able to pull a 3800 ohm load at 530 volts on the plates?   I have Hammond O/P that is rates 60 watts at 3800 Ohms
Thanks,
Ernie

[jojokeo]

Not exactly enough info Ernie. However in fixed bias pri Z is in the ballpark but OT power rating not enough, cathode bias pri Z is not good even mis-matching pri z w/ speaker load but ballpark may be acceptable with tone and output power losses? Hard to say on that one until built & tested?

[ernie_jr]

Fixed bias and I wont be running it full out
Thanks,
Ernie

[shooter]

I wont be running it full out

you probably won't be able to even if you wanted at 3.5k, but 3.5k at 8ohms, should be 7k at 4ohms, then you'd be close enough from the *old* data sheet I have

[HotBluePlates]

Will KT 88's be able to pull a 3800 ohm load at 530 volts on the plates?   I have Hammond O/P that is rates 60 watts at 3800 Ohms ...

In class AB, the load to one side will be Primary Z / 4, or 950Ω for your OT.

Page 7 of the KT88 data sheet shows a knee for G2=200v at either Plate=50v, 260mA or Plate=40v, 250mA. 950Ω * 260mA = 247v + 50v (across the plate) = ~300vdc. You're best adding some additional plate voltage to allow for sag, and if cathode biased you should add the bias voltage to this figure. Power output will be ~32w.

Page 7 has a separate graph for G2=300v, with a knee at (an estimated) Plate=80v, 400mA. 950Ω * 400mA = 460v, plus anticipated sag (with an average current of ~255mA just for the KT88 plates). Power output will be ~76w, so bass response/bandwidth will probably suffer a bit.

Or you could split the difference. Say 500v plate, 250v screen and call it a day?

[ernie_jr]

the amp is basically and Ampeg b 25
Ernie

[HotBluePlates]

That amp would need a significantly-different power supply to provide KT88's a screen voltage which makes sense with that load.

[DummyLoad]

Will KT 88's be able to pull a 3800 ohm load at 530 volts on the plates?   I have Hammond O/P that is rates 60 watts at 3800 Ohms
Thanks,
Ernie

the only hammond 60W OT i can find are 4.3K or 6.6K. is this OT from a hammond organ?

btw, it'll work. not optimally, but it will. you can run the screens at near B+, however, you'll need to increase (-)biasV significantly and grid drive requirements will increase more or less proportionally. i'd use 1K on screens perhaps even 1.5K.

--pete
 

[ernie_jr]

It is an old hammond, not even a UL version I got off Ebay for $40.  I can still find the specs when I google it.  It has alot of iron for a 60 watt transformer. The power supply I am using is straight out of a SUNN 200s with Ampeg b25 pre-amp and phase inverter.
Thanks,
Ernie

[HotBluePlates]

It is an old hammond, not even a UL version ... The power supply I am using is straight out of a SUNN 200s ...

You will make a screen node, right? The 200s had a UL OT.

Dummy load is right that you could have KT88 screen voltage up near plate voltage, but as he says, you'll need a much bigger bias voltage (than using 250v screen) and consequently a much bigger drive voltage from your phase inverter.

[ernie_jr]

Have 1000 ohm 5 watt screen resistors and I can get as high as -64 volts for the bias,
How did Ampeg get away with voltages like this on the 7027's?
thanks
ernie

[HotBluePlates]

I meant that since the 200s is UL, there is no screen node in the power supply. You'll need to add a screen supply node, at which point you can consider what d.c. voltage should be at that node.

[ernie_jr]

Thanks, I Know, will use taps as fender dose, When I said SUNN, I meant 400 volt P/T, 1.5 Henry choke, just using filter caps of 110, 50, 40, 40mfd for the 4 nodes.
Thanks for all the advice,
Ernie

[DummyLoad]

Have 1000 ohm 5 watt screen resistors and I can get as high as -64 volts for the bias,
How did Ampeg get away with voltages like this on the 7027's?
thanks
ernie

600VDC is max plate voltage and 500VDC is max g2 as stated in RCA datasheet. apply higher (-)biasV to compensate for higher g2 voltage. there is a condition for 2 tubes @ 540V B+ and 400V g2 that makes 75W into 6.5K. if driving with 2 pair into 3.25K load, you theoretically have a 150W amp. ampeg V4/V22 run 2 pair of 7027 into 2.9Ka-a Z load and ampeg states that it makes 100W. it would seem that with 2 pair of 7027 it should be 150W, however, the V4 manual states that it can be as much as 30% higher than the figure printed on the back of the chassis.


ampeg V4 manual


RCA 7027-A datasheet


--pete