almost there
Hi all,
Haven't posted for a while, out of the country for 2 weeks 😎
I (re)built the amp following my last schematic I posted in here.
When switching on there is a huge 5 Hz resonance, making the meters that 'watch' the kathode current swing, and letting the woofers make big movements... B+ and B++ around 400 volt (can not measure well due to the resonance), gain seems all right now (connected source is amplified)
anyone any idea what can cause the 5 Hz problem ...?
Werner
Hi all,
Haven't posted for a while, out of the country for 2 weeks 😎
I (re)built the amp following my last schematic I posted in here.
When switching on there is a huge 5 Hz resonance, making the meters that 'watch' the kathode current swing, and letting the woofers make big movements... B+ and B++ around 400 volt (can not measure well due to the resonance), gain seems all right now (connected source is amplified)
anyone any idea what can cause the 5 Hz problem ...?
Werner
I remember reading about SRPP or MU follower that it could loose lots of gain if the load is wrong
I would pull the preamp tubes and see if it goes away. If not, disconnect the 47uF cap from B+ to cathode of the output tubes.
something with the peramp tubes
Disconnecting the mentioned C does not make any difference, but taking out the preamp tubes makes the resonance stop. The meters 'watching' the kathode of the KT88 hold at a nice 80 mA without moving anymore.
A bit strange is measuring the B+ is 380 volt while the B++ (that should be less) is 404 volt.
So the problem is connected to the (connection of the)preamp tubes.
Anyone ??
Thanks,
Werner
Disconnecting the mentioned C does not make any difference, but taking out the preamp tubes makes the resonance stop. The meters 'watching' the kathode of the KT88 hold at a nice 80 mA without moving anymore.
A bit strange is measuring the B+ is 380 volt while the B++ (that should be less) is 404 volt.
So the problem is connected to the (connection of the)preamp tubes.
Anyone ??
Thanks,
Werner
Some things to consider:
As you initially noted you're trying to get a more powerful amplifier than the 6B4G amps your harvested parts from, you're simply not going to get any significant amount of additional power in triode mode. All you've done is make an amplifier that dissipates twice the heat for perhaps another watt at higher distortion (than the 6B4G).
Second, a 5K load is too high for the KT-88. I did a KT-88 SE design almost 30 years ago in UL mode with a 2.5K load... output was ~12.5 watts before clipping.
As for your motor-boating, it's a design problem:
The DC-coupled cascade stages should not be driven from the same supply point. As it's DC coupled, when one stage increases current the next decreases and vica-versa. Using a small decoupling cap in the power supply just increases the likelihood of the motor-boating happening. Split the power supply to feed each stage and use larger decoupling caps.
As a side note, your output stage uses a combination of ultra-path and classic cathode bypass... you don't need both and there's no benefit from using both. Pick one.
Regards, KM
As you initially noted you're trying to get a more powerful amplifier than the 6B4G amps your harvested parts from, you're simply not going to get any significant amount of additional power in triode mode. All you've done is make an amplifier that dissipates twice the heat for perhaps another watt at higher distortion (than the 6B4G).
Second, a 5K load is too high for the KT-88. I did a KT-88 SE design almost 30 years ago in UL mode with a 2.5K load... output was ~12.5 watts before clipping.
As for your motor-boating, it's a design problem:
The DC-coupled cascade stages should not be driven from the same supply point. As it's DC coupled, when one stage increases current the next decreases and vica-versa. Using a small decoupling cap in the power supply just increases the likelihood of the motor-boating happening. Split the power supply to feed each stage and use larger decoupling caps.
As a side note, your output stage uses a combination of ultra-path and classic cathode bypass... you don't need both and there's no benefit from using both. Pick one.
Regards, KM
- Status
- Not open for further replies.