Once and a while we may come across components that just stay put in our system for years and years. Not because they are the very best sounding gear around, but because the overall performance, appearance and not least daily handling makes you feel good. I was seriously thinking of building a PSE from 300B tubes two years ago, but would it really sound significantly better? Probably not much. I also cherish a product where money seems to have been well spent. All transformers here are wound for handling also the 6C33 tubes delivering some 50 watts, thus fairly oversized. The stainless steel chassis doesn't easily get scratched and it's easy to clean and so are the acrylic side panels. Changing speakers and amps is something I do regularly and I need gear that's easy to handle and doesn't take cotton gloves every time I rearrange my set-up. The same goes for my Garrard 401 turntable, which makes me feel good by looks and handling - and not least sound.

I had Vladimir Bazelkov adding 4 ohms speaker taps and a switch for 500 mV and 1000 mV input sensitivity. These mono-blocks run by Transcendent GG line stage at 500 mV input sensitivity made an excellent match, significantly better compared to my previous Audio Research SP16L line stage - until I tried the 5687/7119/6N6P w.o.t. line stage. This line stage, finally finished summer 2009, lifted the sound quality even further and these two components now form the core of my system, supplemented by a Transcendent RIAA + SAC MC trafos.

All tubes for the AM mono-blocks are readily available - and at low cost. I have a range of 6SL7 and 6SN7 tubes but can't hear noticeable differences. I recently bought five 6AS7 out-put tubes from China (eBay - google 6AS7) and they sound as good as the Russian tubes originally supplied. 8 $/ea.... doesn't come much cheaper than this. A seriously oversized output transformer may be part of the good sound and potent bass these amps can deliver. I've had a number of visitors in for comparing my 300B and Audio Mirror amps and they always prefer the Audio Mirrors and I'm sure the AMs win due to the 20 watts compared to the 8 watts from the 300B. There's more punch and drama from the AMs, where the 300Bs deliver that special 300B sound that becomes addictive.

(BACK TO TOP)

The schematics shown here is without any guarantee and I'll see if I can work out the power supply section, which is bit more tricky as it's partly a PCB set-up. So please do not ask how it looks.

The mains trafo has two taps for filament supply. 12.6 V for the 6AS7 out-put tube coupled in series and 6.3 volts for the input/driver tubes. I made these into DC to reduce noise, which really didn't help much I must say. Not that hum is a problem here, but we can always try to improve things if possible. The 100 ohm resistor from V2/pin8 to ground was removed. All filament supplies' negative was connected to ground via 100 ohms.

Input phono sockets' ground connection were insulated from chassis and power supply's central ground was connected to chassis via 10 ohms resistor.

The HT rails are delayed 90 sec. after turn-on.

The mains trafo also has a 100 volt tab (for negative bias) not used in the 6AS7 version, which points to the fact that the 6C33 (2 x 40 watts) version of this amp uses the same mains transformer and output transformer as this 20 watt version.

I feel certain the output transformer has 600 ohms primary winding. Output taps for 4 and 8 ohms. I actually never use the 4 ohms taps.

Future tweaks:
New teflon sockets for all tubes and complete rewiring of amp.
Kiwame resistors for signal paths.
Move on-off switch to rear of amps (having 230 volt right next to the input stage is not ideal)
Further coupling cap tests, VCaps?, Mundorf silver/gold/oil?, tin/teflon? Hard to tell where it ends. Trying some "no-cost" Russian silver/micas has made me wonder whether I really need to spend 300 USD on super caps...

Power supply schematics - including my mods (DC filament supply).
As can be seen the power supply is hugely overdimensioned, e.g. 230 watts for a 20 watts amplifier!

Having received the Obbligato (SCR) tin-foil/teflon caps I quickly inserted these and even out-of-the-box they immediately made it clear that we here have an exceptionally good product. What they do in comparison to the silver/micas is clearing up the treble range. I never really could decide what I should say about the treble performance of the silver/micas but they do smear treble detail a tiny bit in comparison with the SCRs, that is, when they have to handle complex signals like a full symphonic orchestra. With less complex music, the difference is hard to tell.

I have been using the Mundorf silver/gold/PP in many applications and I like them. The level of transparency is excellent, and except for at slight emphasis on the two top octaves, which in certain applications can make them sound a bit hard, I couldn't ask for more.
The VCaps I only know from my 300B amp and when I use the VCaps I miss the Mundorfs and visa-versa. The VCaps are smooth and detailed and do not have the Mundorf silver/gold/PP treble emphasis. The VCaps are simply more right, I think.

The final choice was the SCR tinfoil/teflons as can be seen below.