Dear all,
i am being asked to build a pp amp using the KT150's
monoblocks, in pentode mode...class AB1...
can you recommend anode loads and B+?
i am being asked to build a pp amp using the KT150's
monoblocks, in pentode mode...class AB1...
can you recommend anode loads and B+?
It is going to be very easy 🙂 1% thd at 80watts possible 6H30 drivers or 6SN7 in cathode followers.
B+ anything from 400 to 700 I would say. Depends on the bias and transformer impedance.
The important is to get a 'smooth' transition to class AB and there are no math for this... Bias 70 to 125ma, with fix bias and a THD meter it should be easy to find this best ratio.
B+ anything from 400 to 700 I would say. Depends on the bias and transformer impedance.
The important is to get a 'smooth' transition to class AB and there are no math for this... Bias 70 to 125ma, with fix bias and a THD meter it should be easy to find this best ratio.
Ok, you need too have a low impedance power supply. 66ma is on the very low side, the tubes will operate most of the time in class AB which is alright providing they have enough power.
In class AB the tube can tolerate 250ma of current peak, make sure your power supply can provide at least 175mA per /600V side without (as little as possible) voltage drop in order to feed correctly on demand.
You will need good diodes to rectify this current, forget noisy tube rectifiers 🙂 power diodes on heat-sinks.
A good power transformer would be 900vCT at 700ma + 8 Amperes for power tube heaters + small transformer for pre-amp tubes heaters.
In class AB the tube can tolerate 250ma of current peak, make sure your power supply can provide at least 175mA per /600V side without (as little as possible) voltage drop in order to feed correctly on demand.
You will need good diodes to rectify this current, forget noisy tube rectifiers 🙂 power diodes on heat-sinks.
A good power transformer would be 900vCT at 700ma + 8 Amperes for power tube heaters + small transformer for pre-amp tubes heaters.
One option is 600V plate and 290V to 300V screen with approximately a 5KΩ primary. Should deliver upwards of 75 watts at the speaker terminals if the power supply and output transformer are hefty enough.
That's very tame for the KT150, es345 has shown that the "typical" Class AB1 output from a pair of KT150's is 150W~200W. May be you are aiming for Class A PP?
here you can find some info:
Fab Four: 4 x KT150 monoblock
the OT I use have a 5 ohm single out.
In this stuff I used 19:1
Vdc 520 v, Ia = 40-50 mA each KT150
I found for other schematic similar to this but one pair in AB a ratio 28:1 a good solution.
With 8 ohm you get 6200 ohm a-a
Walter
Fab Four: 4 x KT150 monoblock
the OT I use have a 5 ohm single out.
In this stuff I used 19:1
Vdc 520 v, Ia = 40-50 mA each KT150
I found for other schematic similar to this but one pair in AB a ratio 28:1 a good solution.
With 8 ohm you get 6200 ohm a-a
Walter
Thank you for reminding us of that thread - a very well-made amp, do you remember what the KT150's were biased at?
Normally I use 45 mA each one. At 30 mA the performance are also good and the dissipation is very conservative
I check also 60 mA, 30 watt of dissipation each KT50 with a good results specially because it runs in full class A until 20w (little bit more)
Walter
I check also 60 mA, 30 watt of dissipation each KT50 with a good results specially because it runs in full class A until 20w (little bit more)
Walter
Thanks, don't know how I missed it in post #10... anyway, since they are running really conservatively, so why not just use the KT88's?
Yes, kt88 will sound better, cost less, especially heater current is very reduced... they are super powerful in the 50Watts range, actually can attain less thd than Kt120 and 150 at this wattage if properly implemented. after 60 watts the distortion climbs fast.
The KT150 allow me to use >500 Vdc in safe way
It also is bigger as internal components and glass so the warm can be dissiapted better.
About the saound it is close to KT88
In my opinion
Walter
It also is bigger as internal components and glass so the warm can be dissiapted better.
About the saound it is close to KT88
In my opinion
Walter
I'm using KT120 in a textbook example of the Williamson. I use 560V B+, idle at 50ma, screens at 300V or so. I use 6A diodes for the rectifier, and about 2000uF in the supply. You might consider running in UL. You get less distortion, the same output power, and no need for the screen supply or the 25ma or so of extra current.
This is the best datasheet for the KT88 that I've seen. https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf
This is the best datasheet for the KT88 that I've seen. https://frank.pocnet.net/sheets/084/k/KT88_GEC.pdf
Personally I like the pentode/tetrode connection, it is more interesting to play ( and a little bit difficult)
Walter
Walter
i drew a load line and came pretty close to that load, thanks...
i want those tubes to last....anyway i do not have too faith about manufacturer's data so i would rather err on the side of caution...
yes, Walter, i too have good experience with pentode mode of operation,
more authoritative dynamics, but of course this comes at the cost of a beefy power traffo and psu caps...
i will be using a big big power traffo, a hefty opt, and generous psu filtering..
and G2 will be mosfet regulated at around 250 volts...
the octal sockets is limiting imho, arc over between pin3 and pin 2, i have seen it on an el34 with B+ of 440 volts...i guess that a shrink tube sleeve over pin 3 might help...
if you design for a 1/2 power to cope for missing understandings of class AB and grid current ok maybe.
still keep in mind that the dissipation in AB1 is a lot higher than the bias point and you cannot control this without regulators and sensors of grid bias and current. If the power supply can give the power the tube will take all it can, if it is not wired or bias properly it will exceed the maximum parameters and cause premature failures. One very obvious premature cause of failure is drifting bias, parallel tubes, different sections/ types of tubes, uncontrolled class AB or B bias and grid power dissipation, shut down issues with bias, heaters shocks /under/over power.
of course, and there is a real possibility that tube ratings can be exceeded and that is what i am wanting to avoid...
i hope Miles Prower chimes in...
i hope Miles Prower chimes in...
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