SE Tube Amplifier : Project Preview ...

[happybear]

This SE tube amplifier project is currently a work-in-process.

The PT came from a Westinghouse phono console that supported (2) 6V6, a few 12AX7's and a 5Y3 rectifier.

On order are (2) NOS AES PT-31 SE 5K/8 OPT's.

No final decision has been made regarding the signal circuit on this project.

A no-brainer would be building a Magnavox DG SE-1 6BQ5 clone.

Perhaps AK Tube forum participants could provide other circuit suggestions or ideas?

happybear

 

[StepheK]

A 6BM8 based amp is another possible idea?

 

[unga-unga]

I like to sandbox an amp before constructing it - keep in mind that if your first pass with the circuit is building it "final," you may have some work to do with solder braid. The number one change that tends to need made is the value of the resistors on the b+. You'll choose a design that expects a given voltage at each plate, but you'll be using an x-fo that's on hand and somewhat different. The operating points need to be matched, not the resistor values or other ps components values.

The world is your oyster with those opt's. Keep in mind that you can load 16ohm speakers and get a 10k primary, if you want it. I always feel motivated to build around output tubes that are new to me. Anything new is exciting. If this will be one of your first few builds, I would go after a design that 1) simple and 2) has a strong positive consensus around it, and alot of discussion of the design lying around on the various forums.

You could try the Bugle 45 amp it's very nice sounding, and a simple circuit.

I run 2a3's and 6a5g's on 5k opt's. Let's see if we can find a design that actually calls for one... Most call for 3.5 or 3k, I think you get better headroom and bass on a 5k. But that's just by ear.

Edit: I now see that the sockets are mounted, and all miniature 9-pin which I haven't really explored in the output position, so I'll just take a step back here, and someone will chime in with a great recommendation based on experience.

 

[trinhsman]

This SE tube amplifier project is currently a work-in-process.

The PT came from a Westinghouse phono console that supported (2) 6V6, a few 12AX7's and a 5Y3 rectifier.

On order are (2) NOS AES PT-31 SE 5K/8 OPT's.

No final decision has been made regarding the signal circuit on this project.

A no-brainer would be building a Magnavox DG SE-1 6BQ5 clone.

Perhaps AK Tube forum participants could provide other circuit suggestions or ideas?

happybearView attachment 2476624

A reach out to AK member Gordonw might help. IMHO, he is a master at tube amps.

 

[happybear]

A 6BM8 based amp is another possible idea?

Noticed your SE 6BM8/ECL82 project. Need to review the video series on YouTube.

There is a SE ECL82 project on page 121 of the Mullard "Tube Circuits For Audio Amplifiers" book, published THD vs. power curves on that project are just OK, not outstanding.

Unfortunately, there are no 6BM8/ECL82 in the tube inventory at the moment. Other 9-pin finals to consider might be 12B4A.

happybear

 

[mcheinstein]

DG-SE1 is well tested, and has Dave's master touch on it (I think he even used the same AES OPTs). I built one and it's quite nice, upgraded to Edcors from the AES... I measured a bandwidth improvement in low freqs (saw pretty steady roll off after 80-100 hz). After Edcor upgrade, still not as nice as my 2a3 amp, but nice enough to keep powering the Cornwalls in the dining room. Pretty great at the price point. I know you already said obvious choice, and I second that especially with the OPTs you have. If you want to consider some of the more esoteric DHT builds etc, I'd invest in more expensive iron first.

 

[happybear]

DG-SE1 is well tested, and has Dave's master touch on it (I think he even used the same AES OPTs). I built one and it's quite nice, upgraded to Edcors from the AES... I measured a bandwidth improvement in low freqs (saw pretty steady roll off after 80-100 hz). After Edcor upgrade, still not as nice as my 2a3 amp, but nice enough to keep powering the Cornwalls in the dining room. Pretty great at the price point. I know you already said obvious choice, and I second that especially with the OPTs you have. If you want to consider some of the more esoteric DHT builds etc, I'd invest in more expensive iron first.

The chassis already has (3) 9-pin sockets installed and there are tons of ECC83/12AX7 and a handful of 6BQ5 types in the inventory.

For the money, AES PT-31's are not bad OPT's. Going with a proven design makes things a lot easier on the brain. Adjust the power supply a bit, pretty easy to do.

happybear

 

[JohnT123]

Go with your gut happybear. There's a ton of options and each has it's own pro/con issues. IF you go with the D.G. amp you'll have lots of information to help you debug it should the need arise.

 

[happybear]

The Magnavox DG SE-1 6BQ5 circuit was selected and signal circuit wiring is complete.

B+ is about 15VDC higher than the original design, using SS UF4007 rectifiers with snubbers.

This required an adjustment to the 6BQ5 cathode bias resistor from 120 to 150 ohms or the bias current was too high @ almost 60mADC.

Power supply filtering is : 20uF -> 7H 58 ohm -> 100uF -> 1250 ohm -> 100uF//200K ohm bleeder.

Testing with scope and PC spectrum/distortion analyzer to follow.

happybear

.

 

[JohnT123]

Looking good!

 

[happybear]

Square waves @ 1KHz & 10KHz.

OPT secondary loaded with 8 ohm 20 watt non-inductive power resistors.

Bandwidth on scope : -3dB Low @ 25Hz , -3dB High @ 65KHz

 

[analog addict]

Do something different. Use 6GK6s instead of 6BQ5s. Same tube essentially, different pinout, a $hitload cheaper for good old stock tubes

 

[happybear]

THD% vs. Power Output measurements taken with ARTA.

THD% (RT Channel) :1W = .54%, 2W = .87%, 3W = 1.35%
THD% (LT Channel) :1W = .58%, 2W = .88%, 3W = 1.26%

Notes: driver tube is Telefunken ECC83/12AX7.

 

[Kidmoe]

Could always plug a 5AR4 in to lower that b+ voltage down while getting benefit of slow voltage ramp up.

 

[happybear]

Finished SE 6BQ5 amplifier photo. Power supply filter choke was mounted to side of wooden chassis.

Maybe tomorrow there will be time for subjective listening.

 

[happybear]

The amplifier is now playing Qobuz streaming audio on Altec 2-way horn speakers ... happybear has happy ears!

The cost vs. performance ratio is very favorable using console amp iron and the AES PT-31 OPT's.

 

[JohnT123]

SE tube amps and high efficiency speakers are a classic combination. It sounds better when you actually built the amp.

Enjoy!

 

[happybear]

Schematics: signal circuit and power supply filter (PT & rectifier not shown).

Some of the component values deviate from the DG SE-1 schematic for various reasons.