I've noticed a couple of threads on here about flea power PP amp projects. I recently built a flea power SET so I thought I'd post about it in case anyone is interested.
I rarely post here because so much of the discussion is way over my head. I'm not very technically oriented and I'm pretty ignorant about theory so please go easy.
This project was inspired by an eBay auction that I saw several years ago. The seller (Spiffydoodl) had taken one of the small Magnavox SE 6BQ5 amps and converted the outputs to 12BH7s, running the two halves of this dual triode in parallel. It's kind of like a mini 2A3 except it's not directly heated. He claimed it sounded better because the lower power output put less stress on the tiny OTs and allowed for more headroom. He claimed the power output is similar to a 45 but I think that's a bit optimistic. It looked interesting so I took a screen shot of the amp and filed it away.
Magnavox made a couple versions of this amp, the 8300 series and the 8600 series. The main difference is that the 8300 uses a smaller PT because it amplified a turntable only while the 8600 used a more robust PT to power a tuner in addition to the TT.
Recently I contacted the seller and asked him for some details. He had never drawn a schematic so I drew one based on his description and started a thread about it over at AK.
Magnavox SE 8300 or 8600 - Conversion to 12BH7 Outputs | Audiokarma Home Audio Stereo Discussion Forums
Dave Gillespie, one of the resident experts over there, critiqued the original circuit and gave me lots of valuable advice. I'm pretty much a copy and paste builder, not a designer.
Dave has a few monster threads dealing with different Magnavox console amps including one on this amp. He replaces the OTs, which have a single 4 ohm secondary, so that the amp can be used with more common 8 ohm speakers and makes numerous other changes. Apparently a lot of people have built his modded version with excellent results.
To me, the beauty of this design is that the all the original transformers are used so it retains more of its Magnavox heritage. Calculations indicated that the ideal load for the 12BH7 wired in parallel is 10,750 ohms. The primary of the stock OT is 5.7k with a 4 ohm secondary so, using an 8 ohm speaker results in a primary of 11.4k. A good match for the tube.
The amp also uses the stock 6CA4 rectifier and 6EU7 input tube. At some point I may rewire the socket to allow use of 12AX7s or other tubes with the same pinout. I may even try using all 12BH7s but the Mullard 6EU7 that's in there sounds great.
Parts used are just what I had on hand, nothing remotely "boutique". I did experiment with some different coupling caps and ended up using some of the Russian FT-2 Teflons. I had bought some of these a while back but hadn't used them in a project yet. I compared them to 8 or 10 other caps of various construction (PIO, metalized polyester, polypropylene film and foil, silver mica) and I really liked their clarity. I also liked the Russian KSG-2 Silver Mica a lot but the FT-2s were easier to mount.
Here's some before and after pics and the schematic.
I rarely post here because so much of the discussion is way over my head. I'm not very technically oriented and I'm pretty ignorant about theory so please go easy.
This project was inspired by an eBay auction that I saw several years ago. The seller (Spiffydoodl) had taken one of the small Magnavox SE 6BQ5 amps and converted the outputs to 12BH7s, running the two halves of this dual triode in parallel. It's kind of like a mini 2A3 except it's not directly heated. He claimed it sounded better because the lower power output put less stress on the tiny OTs and allowed for more headroom. He claimed the power output is similar to a 45 but I think that's a bit optimistic. It looked interesting so I took a screen shot of the amp and filed it away.
Magnavox made a couple versions of this amp, the 8300 series and the 8600 series. The main difference is that the 8300 uses a smaller PT because it amplified a turntable only while the 8600 used a more robust PT to power a tuner in addition to the TT.
Recently I contacted the seller and asked him for some details. He had never drawn a schematic so I drew one based on his description and started a thread about it over at AK.
Magnavox SE 8300 or 8600 - Conversion to 12BH7 Outputs | Audiokarma Home Audio Stereo Discussion Forums
Dave Gillespie, one of the resident experts over there, critiqued the original circuit and gave me lots of valuable advice. I'm pretty much a copy and paste builder, not a designer.
Dave has a few monster threads dealing with different Magnavox console amps including one on this amp. He replaces the OTs, which have a single 4 ohm secondary, so that the amp can be used with more common 8 ohm speakers and makes numerous other changes. Apparently a lot of people have built his modded version with excellent results.
To me, the beauty of this design is that the all the original transformers are used so it retains more of its Magnavox heritage. Calculations indicated that the ideal load for the 12BH7 wired in parallel is 10,750 ohms. The primary of the stock OT is 5.7k with a 4 ohm secondary so, using an 8 ohm speaker results in a primary of 11.4k. A good match for the tube.
The amp also uses the stock 6CA4 rectifier and 6EU7 input tube. At some point I may rewire the socket to allow use of 12AX7s or other tubes with the same pinout. I may even try using all 12BH7s but the Mullard 6EU7 that's in there sounds great.
Parts used are just what I had on hand, nothing remotely "boutique". I did experiment with some different coupling caps and ended up using some of the Russian FT-2 Teflons. I had bought some of these a while back but hadn't used them in a project yet. I compared them to 8 or 10 other caps of various construction (PIO, metalized polyester, polypropylene film and foil, silver mica) and I really liked their clarity. I also liked the Russian KSG-2 Silver Mica a lot but the FT-2s were easier to mount.
Here's some before and after pics and the schematic.
Attachments
Cool project! I stuck a choke loaded 12BH7 driver in an amp and was really impressed with the sound compared to resistor loaded 12XXX variants I've used in the past.
I think the 12BH7 is used PP in some 1W guitar amps as well (Blackheart, IIRC).
I think the 12BH7 is used PP in some 1W guitar amps as well (Blackheart, IIRC).
I've seen the guitar amp schematics too and also a couple of stereo schematics using 12BH7s as power tubes. One was a SE design that seems to be primarily for use as a headphone amp. Another is a PP design by the famous Japanese designer, Shishido.
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