Simple SE 845 Class A Stereo Amp

[THD+N]

To help promote transmitting tube amp builds, I am posting this in a stand alone thread because it may have been lost in the below thread on driver circuits.

Transmitting Tube Driver Circuits - 211 / 845

@AlexHempel is designing an amp around a 845 and I wanted to explore different opinions on driver circuits. The challenge with driving the 845 is the voltage swing for Class A1 operation not to mention A2 operation. The Miller capacitance is not any worse than a 2A3 or 300B so it's strictly a...

audiokarma.org

The Simple SE 845 circuit is intentionally designed to be simple, yet drive the 845 to full Class A1 operation without a complex expensive circuit. The power supply is more complex than the actual amp circuit, but it is fairly standardized and uses commonly available (inexpensive) components. It is also a fixed-bias design, which is something I do not usually do, preferring cathode-bias in almost all of my SE amps. Using fixed-bias helps preserve some of the plate voltage since we need about 105VDC of bias. This voltage can now be used for plate swing and turned into useful output power while keeping the main B+ under 1000VDC.

A Music Angel XDSE845 was used as the test mule for this amp circuit and power supply.

The main power transformer selected is an Edcor model XPWR314. Note that I have not actually bought or used this transformer, but based on its specs, should perform well for a stereo amp version with no problems. The XPWR314 power supply sections of the amp were confirmed with Duncan's Power Supply Design software. Additional power transformers will also need to be purchased for the various stages.

The biggest expense will be a good output transformer. Although not as popular as 2A3 or 300B OPT's, good 845 OPT are limited, but available. Be prepared to spend between $600~$1200USD for a good pair. Lundahl and Hashimoto come to mind, but there are others available as well. Be sure and choose a 10k primary impedance.

Resistors Rp1 & Rp2 may need to be increased/decreased depending on the final B+. You want this voltage to be between 860~880VDC. Main B+ voltages (A & B) are nominal values and may vary somewhat from the posted schematic. These voltages are easily measured with a standard meter since most all digital meters can easily handle 1000VDC.

Resistors Rf1 & Rf2 may also need to be included if your 845 filament voltage is a bit high. increase or decrease these resistors in 0.1 Ohm increments until you reach a nominal 10VDC output.





Full Power, 1kHz, 5% THD+N, 8 Ohm


20kHz Full Power, 8 Ohm


1kHz, 1W, 8 Ohm


40Hz, 1W, 8 Ohm


20kHz, 1W, 8 Ohm


Frequency Response Sweep (Teal), Distortion (Yellow), 1W, 8 Ohm


40Hz, Full Power, 8 Ohm


Power Supply

 

[zackthedog]

How nice, thank you!

 

[StepheK]

Very nice, thanks for sharing

 

[THD+N]

Thank you for thanking me.

I was listening more to this amp this weekend and although the amp circuit is deceptively simple, it sounds extremely refined and very musical. It is dead quiet using a brute force DC power supply on the 845 filaments. The magic is the 150R "humdinger" pot. This cancels the residual AC component on the 845 filaments and it is very effective.

I'm struggling with the reality that this amp (minus the additional power) sounds as good or better than my 211/845 Class A2 mono block amps that costs considerably more to make.

I'm working on a modified bias arrangement to be able to use 211 or 845 in the SE 845 amp.

 

[StepheK]

Thank you for thanking me.

I was listening more to this amp this weekend and although the amp circuit is deceptively simple, it sounds extremely refined and very musical. It is dead quiet using a brute force DC power supply on the 845 filaments. The magic is the 150R "humdinger" pot. This cancels the residual AC component on the 845 filaments and it is very effective.

I'm struggling with the reality that this amp (minus the additional power) sounds as good or better than my 211/845 Class A2 mono block amps that costs considerably more to make.

I'm working on a modified bias arrangement to be able to use 211 or 845 in the SE 845 amp.

I can see myself building this. I love simple, low parts count tube amps like this and I'm sure it sounds nice!

 

[TriodeLuvr]

I'm struggling with the reality that this amp (minus the additional power) sounds as good or better than my 211/845 Class A2 mono block amps that costs considerably more to make.

The A2 design should pay off when the amplifier is pushed to higher power levels. The simpler capacitor coupled design will experience bias shift and recovery issues that the A2 amplifier won't.

I'm working on a modified bias arrangement to be able to use 211 or 845 in the SE 845 amp.

That's a tall task if the goal is A2 for both.

Jack

 

[THD+N]

The A2 portion does indeed pay off if you need the extra power, but if you just need 15W or less, this A1 design is very musical barring the selection of OPT.

 

[THD+N]

The A2 design should pay off when the amplifier is pushed to higher power levels. The simpler capacitor coupled design will experience bias shift and recovery issues that the A2 amplifier won't.

That's a tall task if the goal is A2 for both.

Jack

No A2, just A1.

 

[Primotubes]

Many thanks for the circuit for this amp. I am contemplating a transmitting tube amp and one decision is powering the filaments. I have read that some builder prefer using 10 vac with hum pot because the sound is perhaps more dynamic, while other prefer 10vdc which has very low hum level and a different sound quality from the ac filament and also you have the issue causing the filament to wear out sooner. I have also read others using a 10v switch mode power supply. Any comments on why you chose the 10vdc option?

 

[THD+N]

Many thanks for the circuit for this amp. I am contemplating a transmitting tube amp and one decision is powering the filaments. I have read that some builder prefer using 10 vac with hum pot because the sound is perhaps more dynamic, while other prefer 10vdc which has very low hum level and a different sound quality from the ac filament and also you have the issue causing the filament to wear out sooner. I have also read others using a 10v switch mode power supply. Any comments on why you chose the 10vdc option?

The Music Angel amp circuit was a (845) DC filament design and worked (and sounds) very well so I carried over the circuit to the Simple version. AC filaments on transmitting tubes is not really an option (IMO) because you will have hum regardless of the hum pot setting.

I have not used SMPS so I can't comment on that, but others here on the forum have (@TriodeLuvr).

On my other 211/845 amp, I have used a 317 based regulator for the 845 filaments and it is performing/sounding great.

 

[TriodeLuvr]

The A2 portion does indeed pay off if you need the extra power, but if you just need 15W or less, this A1 design is very musical barring the selection of OPT.

Not to belabor this point, but it's really about more than creating additional power. The dynamic range of music requires significant headroom, meaning that only a small fraction of the RMS rating is available for the average power level. Of course, given how the low frequency saturation characteristic of the OPTs creates another bottleneck, it might be that none of this really matters. Maybe a power stage designed for 15W (excluding the OPT) is as good as it gets.

Jack

 

[THD+N]

Noted your comments Jack and you are right to a point, but even a 2A3 amp sounds great with only 3.5W at hand. This design allows that magic (and more) to shine at even higher power levels. IMO, high efficiency loudspeakers are a must for these type of amps.

 

[TriodeLuvr]

Noted your comments Jack and you are right to a point, but even a 2A3 amp sounds great with only 3.5W at hand. This design allows that magic (and more) to shine at even higher power levels. IMO, high efficiency loudspeakers are a must for these type of amps.

The problem being that most speakers lose efficiency at the same low frequencies where the SET transformer limits output power. It's a double whammy. Understand, I'm backtracking on the point regarding the benefits of A2, and I completely understand your statement that "I'm struggling with the reality that this amp (minus the additional power) sounds as good or better than my 211/845 Class A2 mono block amps that costs considerably more to make." It won't matter much that there might be grid blockage on peaks at 25Hz or 30Hz if the OPT is saturating anyway. And of course, the type of music and the listening level are big factors in all this. My own speakers are about 95dB and flat to nearly 20Hz. With these, 15 watts is enough for most situations. I'd love to have speakers at 105dB that extend to an honest 25Hz or so, but that would require full size horns across the ceiling. My better half has already said no.

Jack

 

[Primotubes]

The Music Angel amp circuit was a (845) DC filament design and worked (and sounds) very well so I carried over the circuit to the Simple version. AC filaments on transmitting tubes is not really an option (IMO) because you will have hum regardless of the hum pot setting.

I have not used SMPS so I can't comment on that, but others here on the forum have (@TriodeLuvr).

On my other 211/845 amp, I have used a 317 based regulator for the 845 filaments and it is performing/sounding great.

Do you know if there is a commercially available unit that can supply at least 10 amps at 10vdc that is in a form that would be able to install under chassis? I try to make a power supply chassis with an umbilical to the amplifier chassis trying to keep the AC nasties away from the DCV.

 

[TriodeLuvr]

Do you know if there is a commercially available unit that can supply at least 10 amps at 10vdc that is in a form that would be able to install under chassis? I try to make a power supply chassis with an umbilical to the amplifier chassis trying to keep the AC nasties away from the DCV.

An umbilical won't remove all the noise from the output of a standard SMPS. The problem is that they're pulse-width modulated, and that produces a mish-mash of frequencies that extend into the audio range. A good LC network will be required to clean this up.

I have not used SMPS so I can't comment on that, but others here on the forum have (@TriodeLuvr).

Well, the supply I used wasn't DC, and it wasn't regulated, so not pulse-width modulated. It was a modified halogen lighting unit that oscillates at a fixed frequency well above the audio band. The output is an ultrasonic square wave that heats the filament. This eliminates hum, but retains what I believe are the superior audio qualities of AC heating. Definitely not something you can buy off the shelf. The description of what I built is in a thread that starts here:

Ultrasonic Heater Supply (at HiFi Haven)

This is the finished project. I built two of these, one for each monoblock. They convert 120VAC to 10V RMS equivalent at 80kHz, good for heating one 211 or 845 each.



Jack

 

[THD+N]

Simple SE 845 (doubles as a space heater) playing in low light (the best way) conditions.

 

[THD+N]

Do you know if there is a commercially available unit that can supply at least 10 amps at 10vdc that is in a form that would be able to install under chassis? I try to make a power supply chassis with an umbilical to the amplifier chassis trying to keep the AC nasties away from the DCV.

The AC & DC voltages play much nicer together than most people think. This compact chassis is very tight and it is extremely quiet. No need for an umbilical in my opinion.

A good (thought out) layout will eliminate most potential coupling issues.

 

[StepheK]

The problem being that most speakers lose efficiency at the same low frequencies where the SET transformer limits output power. It's a double whammy. Understand, I'm backtracking on the point regarding the benefits of A2, and I completely understand your statement that "I'm struggling with the reality that this amp (minus the additional power) sounds as good or better than my 211/845 Class A2 mono block amps that costs considerably more to make." It won't matter much that there might be grid blockage on peaks at 25Hz or 30Hz if the OPT is saturating anyway. And of course, the type of music and the listening level are big factors in all this. My own speakers are about 95dB and flat to nearly 20Hz. With these, 15 watts is enough for most situations. I'd love to have speakers at 105dB that extend to an honest 25Hz or so, but that would require full size horns across the ceiling. My better half has already said no.

Jack

Interesting, so all these SE amps people build and listen to really aren't usable for Hi Fi, even with 95db+ efficiency speakers? I can see that of course there is no other way to have the 20-40hz range, which we all now know contains most of the music, without speakers and the main amp making full power there. Darn I have been focusing on the wrong thing, I clearly was wrong assuming the 40-10K range is where most music exists.

Thanks for letting us all know that we need to replace all of our gear, speakers and amplifiers! I'll get right on that...

 

[TriodeLuvr]

Thanks for letting us all know that we need to replace all of our gear, speakers and amplifiers! I'll get right on that.

I don't understand this sarcasm. Are you offended by the laws of physics?

Jack

 

[THD+N]

An umbilical won't remove all the noise from the output of a standard SMPS. The problem is that they're pulse-width modulated, and that produces a mish-mash of frequencies that extend into the audio range. A good LC network will be required to clean this up.

Well, the supply I used wasn't DC, and it wasn't regulated, so not pulse-width modulated. It was a modified halogen lighting unit that oscillates at a fixed frequency well above the audio band. The output is an ultrasonic square wave that heats the filament. This eliminates hum, but retains what I believe are the superior audio qualities of AC heating. Definitely not something you can buy off the shelf. The description of what I built is in a thread that starts here:

Ultrasonic Heater Supply (at HiFi Haven)

This is the finished project. I built two of these, one for each monoblock. They convert 120VAC to 10V RMS equivalent at 80kHz, good for heating one 211 or 845 each.



Jack

I still mulling over if DC really effects the final sound of a DHT. AC in a 2A3 amp sounds good but it is still modulating the music signal albeit at a low level. When I compare my 2A3 amp with AC filament to a 2A3 amp with DC filament, I can't really say I hear a difference except for the lack of "hum".

I'm not deaf by any means and have good hearing although I do have a hard time hearing a lot of what people say they can hear.