Shuguang SG-845BS amplifiers schematic
A friend brought me two of these amplifiers (minus tubes) to get running.
The first thing I did was drew a schematic (attached) as there seemed to be very little technical information regarding these amplifiers on the net.
I purchased tubes from eBay 2x(6SL7/300B/845), replaced the 450V filter caps with the biggest 550V ones I could find, added a second relay contact to switch ALL the HV during filament warmup and both amps put out a clean 25W into 8 ohms. Not my favorite amp, but included my schematic here, as there seem to be quite a few similar amps out there.
Mike
A friend brought me two of these amplifiers (minus tubes) to get running.
The first thing I did was drew a schematic (attached) as there seemed to be very little technical information regarding these amplifiers on the net.
I purchased tubes from eBay 2x(6SL7/300B/845), replaced the 450V filter caps with the biggest 550V ones I could find, added a second relay contact to switch ALL the HV during filament warmup and both amps put out a clean 25W into 8 ohms. Not my favorite amp, but included my schematic here, as there seem to be quite a few similar amps out there.
Mike
That 10uF has wrong polarity.
I wonder what happens when the amplifier is switched on and V3 is cold, not conducting. PS choke pushes the supply to 500V.... hmm.
This in the light of someone copying the circuit and replacing the 6Vac with modern filament supplies which in turn might be ignited before switching on the PS...
I wonder what happens when the amplifier is switched on and V3 is cold, not conducting. PS choke pushes the supply to 500V.... hmm.
This in the light of someone copying the circuit and replacing the 6Vac with modern filament supplies which in turn might be ignited before switching on the PS...
Attachments
Last edited:
I've built many 300B circuits. For an integrated amp (no line preamp in front of it) you can't go wrong with the JC Morrison / Fi Primer 300B in the original post. It has about a 0.25V input sensitive which is perfect for playing a CD player, phone/tablet, or phono preamp directly. It was a wonderful sounding circuit.
My first amp was the Tubelab SE 300B. I found that amp to be really designed for 45 and 2A3 operation. It made an incredible amount of heat for running the voltages necessary for 300B. It's a very good sounding amp, but to my ears tube amps without MOSFETs or other chip devices sound so much better.
You also can't go wrong with Joe Roberts' abridged Western Electric 91A, published in Sound Practices magazine during the 1990s. I breadboarded it using 6SJ7 as input tube and it was fantastic. I loved it so much that I'm now developing my own WE91A from the original circuit, using the 6C6 input tube as per the 1938 schematic.
My first amp was the Tubelab SE 300B. I found that amp to be really designed for 45 and 2A3 operation. It made an incredible amount of heat for running the voltages necessary for 300B. It's a very good sounding amp, but to my ears tube amps without MOSFETs or other chip devices sound so much better.
You also can't go wrong with Joe Roberts' abridged Western Electric 91A, published in Sound Practices magazine during the 1990s. I breadboarded it using 6SJ7 as input tube and it was fantastic. I loved it so much that I'm now developing my own WE91A from the original circuit, using the 6C6 input tube as per the 1938 schematic.
QQ - Did you ever build this amp, and if so do you have a schematic?
---------------
Ah - just found the thread. Sorry
Last edited:
how is the building?
Hi Folks,
I already have a pair of this output transformers:
Primary: 2,5k or 3,4k
18W
I would like to build a 300b stereo amplifier or two mono amplifiers for my HiFI.
For the moment I have a SE PCL 86 amplifier with 4watts, which is actually enough for my speakers, but I hope to achieve 2x8watts with the 300b's.
There are a lot of schematics:
I would like to know which one fits my output transformer best - or if there are other schematics - feel free to post them here
What would also be nice: If the amp wouldn't need a preamp - I thought about an aikido preamp to drive the 300b but the schematics from tubecad require different primary impedances.
Have a nice day!
Felix
I've built a few 300b amps recently that I've been very happy with. My strategy is quite simple:
1. 2 stage amp. First stage preferably a triode or pentode in triode.
2. Plate choke with high inductance (150H) on the driver. Reasons for this:
a) I prefer it to an active load, smoother sound
b) A resistor load can't cope with higher currents unless the tube has very low Rp which is unlikely with a high gain driver tube. Exceptions like 6e5/6e6
c) With a Russian teflon FT-2/FT-3 coupling cap I prefer the sound to an interstage.
3. Now we have a high inductance plate choke we can use an unbypassed cathode resistor up to around 220R. This allows for some flexibility with tube and operating point.
To complete this design use a choke like Lundahl LL1668/10mA or LL1668/15mA. Then try out a variety of drivers in triode, like D3a, C3g, 5A/152, EF810, E280F, EC810, 6e6P, 6e5P, EF184, E180F, EC86, 6S3P, 6C45P...... plenty to try.
This plate choke circuit gives you great flexibility. Attached an example with EC86 driver, but any of the above will work.
1. 2 stage amp. First stage preferably a triode or pentode in triode.
2. Plate choke with high inductance (150H) on the driver. Reasons for this:
a) I prefer it to an active load, smoother sound
b) A resistor load can't cope with higher currents unless the tube has very low Rp which is unlikely with a high gain driver tube. Exceptions like 6e5/6e6
c) With a Russian teflon FT-2/FT-3 coupling cap I prefer the sound to an interstage.
3. Now we have a high inductance plate choke we can use an unbypassed cathode resistor up to around 220R. This allows for some flexibility with tube and operating point.
To complete this design use a choke like Lundahl LL1668/10mA or LL1668/15mA. Then try out a variety of drivers in triode, like D3a, C3g, 5A/152, EF810, E280F, EC810, 6e6P, 6e5P, EF184, E180F, EC86, 6S3P, 6C45P...... plenty to try.
This plate choke circuit gives you great flexibility. Attached an example with EC86 driver, but any of the above will work.
Attachments
Hi Andy,
totally agree with your strategy, but never tested the russian caps for coupling, right now I prefer the Mifex KPCU for their smooth, fast / live and uncolored sound.
Everything else is (with D3a drivers) in use in concepts with EML 300B Mesh, EML 320B and EML 1605 power tubes.
I need to test to skip the cathode cap of the D3a, sounds like a good plan, thanks for that hint - I forgot that option. Gain is more than enough with the high output voltage of modern DACs.
BR,
Bernhard
totally agree with your strategy, but never tested the russian caps for coupling, right now I prefer the Mifex KPCU for their smooth, fast / live and uncolored sound.
Everything else is (with D3a drivers) in use in concepts with EML 300B Mesh, EML 320B and EML 1605 power tubes.
I need to test to skip the cathode cap of the D3a, sounds like a good plan, thanks for that hint - I forgot that option. Gain is more than enough with the high output voltage of modern DACs.
BR,
Bernhard
Of course you can.
In my case I've had a few Lundahl interstages like the LL1660 and I preferred the sound just using the primary as a plate choke with a FT-2 teflon cap. My preference based on what I've tried is for a plate choke. Cheaper as well as a sound that I like.
I haven't tried any exotic interstages like Tango, but in the case of Lundahl I would certainly go for a plate choke like the LL1668. In my case I use an amorphous plate choke from NP Acoustics which I'm very happy with.
I have 2 kinds of NP Acoustics amorphous plate chokes, a small pair and a large pair. The smaller ones are 136H/15mA and the larger ones are 156H/40mA. The larger ones are a little more detailed and dynamic, and the smaller ones are very sweet. Both good. I haven't compared them directly to LL1668.
NP Acoustic - NP Hi-End Audio
They're in Vietnam. Quite active on Facebook too. Very nice people.
Phương Nguyễn Phone: 01.668.668.710
Address: 773 Nguyen Khoai, Hoang Mai, Ha Noi, Viet Nam
NP Acoustic - NP Hi-End Audio
They're in Vietnam. Quite active on Facebook too. Very nice people.
Phương Nguyễn Phone: 01.668.668.710
Address: 773 Nguyen Khoai, Hoang Mai, Ha Noi, Viet Nam
Last edited: