KT120 build. Some troubles

[Xander8280]

Similar to a 2204 but with KT120 for my pair. Ultra linear OT(Hammond 1650TA) HT voltage at 505v. The PT is a Hammond 373jx. Running the OT with load double that of the tap.

It sounds weak, I am driving it with 4 gain stages before the phase inverter.

The presence control does nothing(8ohm tap, 50k pot, and 680nF cap). When I turn my resonance control "down"(having the negative feedback go through a 0.001uF cap) the amp gets much much quieter and distorted. I am going to remove the pots(pres and res) from the NFB circuit tonight to see if it restores some volume. I was hoping to get at least 100W out of the pair of tubes as it is an amp bass/keys as well as guitar. I want the headroom and know fighting for those few extra watts won't get much noticeable volume but, I need headroom.

With 505V for my HT is it possible to achieve 80-110W?

Is it essential to use a 12AU7 or a driver stage?

Any help is great, I am very puzzled as to why my NFB chain is fighting me.

[Stevem]

Well that PT has tons of current to make a easy 120 + watts of rms power, but that 12 amps of heater current is massive for your needs, what the heck is your heater voltage running at?

Out of the PI your gonna need some 65 volts of peak drive signal to pump 100 watts of rms out of those two outputs.

[es345]

With 505V for my HT is it possible to achieve 80-110W?

Some comments to the OT:

First some background about this tube from my own experience.
The KT120 is very sensitive against cathode current overdrive. Heavy violation will kill this tube within hours. From the datasheet: Ik max=250mA

If you want to use the power stage in clean mode only:
Your OT should have an effective Raa of 2K7. This will give you about 100W and you are on the safe side regarding cathode current with normal drive. At 100% drive I would estimate about 230 mA cathode current.

If you want to go hard overdrive with the power stage (300% or nearly square signal):
I would choose an Raa not less 3K8 which will lead to about 80 W clean. On the other hand you will not violate I cathode max while overdriving. It will be around 230 mA during overdrive.

PS: some edits: I changed my comments from KT150 to KT120, but the message is still the same.

[Xander8280]

Well that PT has tons of current to make a easy 120 + watts of rms power, but that 12 amps of heater current is massive for your needs, what the heck is your heater voltage running at?

Out of the PI your gonna need some 65 volts of peak drive signal to pump 100 watts of rms out of those two outputs.

It is a stereo amp, single PT and a pair of 1650TA for the OTs. The heaters are a touch low if I only run one side. Is a 5.8v heater voltage going to hurt things or weaken output?

What's the best way to get 65v rms, is it a driver stage after the phase inverter? I wonder if I mimic the MESA d180/Bass 400 driver if I could get up there.

[Xander8280]

Would tacking a mosfet on each side of the PI output be enough to get more drive and keep things low impedance wise? I have room for a twin triode to do the job.


idea from this link

http://www.geofex.com/Article_Folders/m ... tfolly.htm

Thanks for the help so far guys!

[Stevem]

I have always concided 6.1 to be the minimum heater voltage at least for preamp tubes if you want to get the amplification factor that your component value used should net you.

[martin manning]

I have always concided 6.1 to be the minimum heater voltage at least for preamp tubes if you want to get the amplification factor that your component value used should net you.

Data sheets say 6.3V +/- 10%, so 5.7 to 6.9 is the acceptable range.

[ToneMerc]

With 505V for my HT is it possible to achieve 80-110W?

Some comments to the OT:

First some background about this tube from my own experience.
The KT120 is very sensitive against cathode current overdrive. Heavy violation will kill this tube within hours. From the datasheet: Ik max=250mA

If you want to use the power stage in clean mode only:
Your OT should have an effective Raa of 2K7. This will give you about 100W and you are on the safe side regarding cathode current with normal drive. At 100% drive I would estimate about 230 mA cathode current.

If you want to go hard overdrive with the power stage (300% or nearly square signal):
I would choose an Raa not less 3K8 which will lead to about 80 W clean. On the other hand you will not violate I cathode max while overdriving. It will be around 230 mA during overdrive.

PS: some edits: I changed my comments from KT150 to KT120, but the message is still the same.

I set my KT120 pair up at 580V-plates / 290-screens with a 4K OT, I was hoping around 85W from the pair.

TM

[es345]

You should be able to get about 100W. Please see the result of my rough design aid estimation. Due to the low Ug2 I expect a relatively high saturation voltage of the plate (100V) which limits the output power.

[ToneMerc]

You should be able to get about 100W. Please see the result of my rough design aid estimation. Due to the low Ug2 I expect a relatively high saturation voltage of the plate (100V) which limits the output power.

Thanks es3245, can you run it at 560/280 as well?

TM

[es345]

Here you are. I didnt change the saturation voltage, I don't expect a big change due to Ug2 =280V. Please keep in mind that this is a design aid estimation only. But it gives a good indication what will happen.

[ToneMerc]

Thanks es345 and sorry Xander for jacking your thread.

TM

[Xander8280]

Thanks es345 and sorry Xander for jacking your thread.

TM

No no, it is okay!
I am wondering if the Ultra Linear setup is holding me back! I thought about having a dpdt switch for the screens to go to some dc supply.

I'm curious about your set up, what are you using for a phase inverter?

[ToneMerc]

I'm curious about your set up, what are you using for a phase inverter?

More or less this PI topology; 12AU7 or 12AT7 PI and 12BH7 driver. The power supply is voltage doubler type and OT is a custom big honking 4K.

https://tubeamparchive.com/download/file.php?id=24372

TM