I built this last weekend and am very pleased with the sound.
I had initially used a paralleled 6N1P as the driver but had found it a bit sugary sounding for my liking.
The only other valves in my box that were really suitable were a pair of Mullard ECC81 (12AT7).
Parallelled up and run without cathode bypass caps they have proven to be excellent sounding drivers for the amp. It only puts out around one watt but my 100dB open baffles have no trouble making the windows rattle.
The 10K output loading might raise a few eyebrows but it sounds excellent at that load; better than at 5K, which would be the obvious choice.
The 46 seems to be a great tube; not powerful but beautiful-sounding nonetheless. The amp is giving my PX25 DRD some serious thinking to do.
Has anyone else made anything out of a 46. There does not seem to be much DIY action anywhere with this tube.
Steve
An externally hosted image should be here but it was not working when we last tested it.
I had initially used a paralleled 6N1P as the driver but had found it a bit sugary sounding for my liking.
The only other valves in my box that were really suitable were a pair of Mullard ECC81 (12AT7).
Parallelled up and run without cathode bypass caps they have proven to be excellent sounding drivers for the amp. It only puts out around one watt but my 100dB open baffles have no trouble making the windows rattle.
The 10K output loading might raise a few eyebrows but it sounds excellent at that load; better than at 5K, which would be the obvious choice.
The 46 seems to be a great tube; not powerful but beautiful-sounding nonetheless. The amp is giving my PX25 DRD some serious thinking to do.
An externally hosted image should be here but it was not working when we last tested it.
Has anyone else made anything out of a 46. There does not seem to be much DIY action anywhere with this tube.
Steve
dsavitsk said:
Yeah I too was a bit dubious about the raised Ra caused by not bypassing the driver and the problem of not enough henries in the choke that it would expose. The amp is a work in progress as it was built from bits I had spare.
It does sound very good despite all that.
I have some caps to bypass the driver and will stick them in there.
OTOH the bias is just about ideal for an LED...decisions decisions
Steve
Consider more capacitance across the output cathode resistor. While it's true the corner frequency as-is will be very low, the impedance 20uF presents from cathode to ground won't be at low frequencies. An impedance magnitude in the range of 400 ohms at 20 Hz if my quick draw calcs are right. Monitor the voltage across the cap with scope and play something with strong low end content.
rdf said:
Hi RDF
I have increased the bypass cap to 47uF and have noticed a bit more control at the bottom end. Not night and day but useful. IME with these direct coupled amps you have a bit of a balancing act to perform, due to the fact that you need a higher voltage cap than you would with cap-coupling, due of course to the elevated cathode voltage. This has a knock-on effect in terms of the physical size of the cap, influencing, the sound as well as the actual capacitance presented at the cathode.
With previous DC coupled amps I have built using 45s and 2A3s I have usually arranged things so I don't have to go any higher than 47uF.
Strange thing is 47uF seems to be a good value, no matter what valve I have used.
Steve
Hi Guys,
Well, over the course of this week I have been playing with various ideas for the amp.
It has evolved a fair bit and now sports a triode strapped C3g as the input/driver valve.
Afer bypassing the cathode of the 12AT7 I thought we were getting a little too close to
slewing distortion for comfort. The -3dB point was 24kHz with a 25K source impedance.
I found a long forgotten pair of C3g pentodes at the bottom of my tube box.
Looking at the triode curves, it seemed ideally suited to the voltages I had going on in the
amp.
Here's the new schematic:
The 12AT7 makes a good driver but the C3g as triode makes a better one.
The amp sounds pretty beefy, with nice detail and an unfatiguing presentation
Here it is in situ driving my 100dB efficient open baffles.
Steve
Well, over the course of this week I have been playing with various ideas for the amp.
It has evolved a fair bit and now sports a triode strapped C3g as the input/driver valve.
Afer bypassing the cathode of the 12AT7 I thought we were getting a little too close to
slewing distortion for comfort. The -3dB point was 24kHz with a 25K source impedance.
I found a long forgotten pair of C3g pentodes at the bottom of my tube box.
Looking at the triode curves, it seemed ideally suited to the voltages I had going on in the
amp.
Here's the new schematic:
An externally hosted image should be here but it was not working when we last tested it.
The 12AT7 makes a good driver but the C3g as triode makes a better one.
The amp sounds pretty beefy, with nice detail and an unfatiguing presentation
Here it is in situ driving my 100dB efficient open baffles.
An externally hosted image should be here but it was not working when we last tested it.
Steve
Sheldon said:
OOPS! Thanks for pointing that out!
The PSU was adapted from one I had been using years ago with a 2A3 Loftin White amp. It was built from someone else's circuit. Not surprisingly, the resistor used to get hot.
The bleeder is now a 220K resistor. I'd better alter the schematic.
Steve
Schematic as it should be.
Steve
An externally hosted image should be here but it was not working when we last tested it.
Steve
Sorry for the delay. Attached is my 50. I know I'm using a 1K cathode resistor on the 50 and B+ is 400V. I'm certain the rest is correct. All globe. Note the output - I'm using a magnequest robin hood for the 50 load and cap couple the output to an array of 8 800ohm vintage philips drivers. I have to make a quick wiring change to 8ohm when visiting friends but it's easy. Filaments are current regulated per Jim de Kort's (Mr. VT52) examples for the 46 (not sure if it's truely required here but I was conservative) and 50.korneluk said:
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