ANUBIS - Another Tube-Transistor Hybrid Amplifier - diyAudio
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Old 10th July 2014, 08:35 AM   #1
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Default ANUBIS - Another Tube-Transistor Hybrid Amplifier

edit 20-Jul-2014.

See post #38 for ANUBIS final schematic and the power supply design.

Hi,

Just finished building this but i haven't flipped the power switch ON.
The reason being that i'm not sure if this contraption i designed would work. I have personally gone through the schematic many times and can't see anything wrong with it.. but i can very well be wrong.
If anyone can check the schematic for any fatal error before i end up with burned transistors, it would be great. So.. here's Another Tube Transistor Hybrid Amp.. or ANUBIS for short.
Click the image to open in full size.

The basic idea is simple: take a working solid state power amplifier design and replace the input stage with tube. The topology is very common: Long Tailed Pair --> Class A VAS --> Compound Pair output. Have Rod Elliot's P3A PCB lying around? With some minor modification and creativity, you can use it for ANUBIS. For fellow Indonesians, our ubiquitous "150-300W OCL Amplifier" PCBs can also be used. I have also attached the Power Supply Design i use.

These are the stuff that i'm still not sure about ANUBIS:
1. VAS Class A stage, especially whether R8 and R9 values are correct. I tried to aim at 10-11mA of idle collector current. Can anyone show me the correct calculation for R8 and R9?
2. Will there be problem at cold starts? I mean when first turning on and the tube has not warmed up (and conducted) yet. I use DC protection for the speaker so temporary high offset is ok. I'm just not sure if the transistors will survive.
3. Will there too much voltage offset at the output?

note: i've created similar thread in the Tube forum but decided since i'm basing this off a working SS amp, it should be here. I will ask mods to delete my thread there.
Attached Images
File Type: jpg 6N23P_anubis.jpg (118.6 KB, 421 views)
File Type: jpg V doubler.jpg (29.1 KB, 415 views)
File Type: png Hybrid_OCL150w_v8.png (33.2 KB, 413 views)

Last edited by ballpencil; 20th July 2014 at 10:39 AM.
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Old 10th July 2014, 11:53 AM   #2
Bigun is offline Bigun  Canada
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Great - I love to see the interest in hybrids these days. Just a very quick look - careful about power dissipation and voltage rating for the VAS device since it hangs off the B+ rail and will swing as far negative as the signal itself. You might be able to reduce your B+ as these tubes are usually known to be good for lower voltages.

Also be careful of turn-on/off behaviour as tubes warm up.
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Old 10th July 2014, 07:03 PM   #3
indra1 is offline indra1  Indonesia
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- Emitter of Q2 should connect to collector of Q4 and emitter of Q3 should connect to collector of Q5 for stability, not to output
- Q7 should be bolted on the heatsink along with Q2-Q5 for thermal compensation
- There should be a 100-220pF capacitor from base to Collector of Q3 to prevent oscillation
- A 1A diode in series with ~10 R should connect collector of Q1 to protect SS section during power on/off and input section malfunction
- Heater of V1 should be on 10-30s before B+ and SS supply
- A slow start B+ supply is recommended
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Old 10th July 2014, 09:13 PM   #4
hbc is offline hbc  United Kingdom
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I found if you connect some npn transistors in parallel with the tubes they work straight away, and when the tube warms up it turns the transistors off and takes over. It is still messy but not as bad as output hitting the rail till the tube warms up. Ymmv.
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Old 11th July 2014, 12:32 AM   #5
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Revised schematic attached based on some input from Indra and recalculated R8, R9 for correct Iq on Q1 (11mA).

Quote:
A 1A diode in series with ~10 R should connect collector of Q1 to protect SS section during power on/off and input section malfunction
Care to explain how this would help? Assuming the malfunction being the tube failed to conduct (similar situation to when the amp is turned on and tube is warming up), this would mean no current will be flowing through R4 and Q1 would remain turned off. Subsequently, base of Vbe multiplier will be pulled to negative rail and finally Q3+Q5 will overheat (i have DC protection to prevent speaker damage). Which part do the diode & resistor prevent?
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Old 11th July 2014, 12:45 AM   #6
indra1 is offline indra1  Indonesia
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Another fault mode that can cause voltage of collector of Q1 to be at >35V, could be due to an open D3/D4.
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Old 11th July 2014, 05:04 AM   #7
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Perhaps a reverse biased diode between Q2 base and the +33V rail in case Q1 tries to pull up to the 120V rail.
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Old 11th July 2014, 05:23 AM   #8
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Quote:
Originally Posted by jerluwoo View Post
Perhaps a reverse biased diode between Q2 base and the +33V rail in case Q1 tries to pull up to the 120V rail.
Good suggestion. I was relying on the hearing method for this one. That is, stop turning that volume pot when you start to hear clipping but this is not really idiot proof.
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Old 11th July 2014, 06:00 AM   #9
indra1 is offline indra1  Indonesia
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Q2 base is Q1 collector. Better put some resistance in series like ~10R to dissipate the fault current. Add 50mA fuse each channel on B+ line.
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Old 11th July 2014, 08:38 AM   #10
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Indra 1
Q7 is NOT supposed to be on the main heat sink This is a CFP amplifier Q7 will have to monitor temperature on the drivers and these will work better on a common heat sink separated from the output
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