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Small power all russian tubes single ended amp

I recently finished a nice little single ended amplifier.

Well, 'finished' is a bit relative; the wood parts need a second full coat of lacquer and a transformer cover for the OT's needs to be made, but the electronics are finished!

This is a bit of a "scrapbox amp", to be used as a second amp for my girlfriend. The aluminum chassis' were originally drilled in 2011 and have since been used in a couple other amps. The space inside is very tight, but one of the design goals was small size, so I decided to go for it.

Anyway, the input stage is 6S45P with fixed grid bias cap coupled to a source follower, which is direct coupled (and providing negative grid bias) to 6E5P triode strapped. All tubes have gyrator plate loads.

OT is parafeed Hammond 125C (10k : 4 ohm) via a MBGO 4µF film cap.

Coupling caps are also MBGO film caps. The 1µF value is too big, but I had those at hand so I decided to use them. Gyrator caps are Russian K73-17 film.

6S45P is very eager to oscillate, so I put 47 ohm resistor from each of the cathode pins to ground, which in parallel make about 12 ohms. I also put 100 ohm grid stopper to each grid pin, and 100 ohm stopper to the anode pin.

6E5P is also very eager to oscillate; I put 10k resistor between g2 and anode, and a 100 ohm anode stopper, a 100 ohm grid stopper and 100 ohms from both cathode pins to ground, which in parallel makes about 50 ohms between cathode and ground.

The 10k triode strapping resistor and 50 ohms cathode resistance decrease the transconductance of the 6E5P a lot, but it has a great amount of it to begin with, so there's plenty left.

PSU has 3 of 100 + 100 µF JJ caps. I parallel the sections, so it's three 200µF filtering caps. This is quite pointless, since I always use a simple 2 FET reg anyway, but the caps were there (I didn't use regs back in 2011), so I just used them. On the plus side, there's no B+ hum at all.

B- is also regulated with a simple 2 FET reg. Heaters are AC.

It's small power, but it easily fills my girlfriend's living room with sound, with good headroom. Speakers are unknown, but the sensitivity is "about the same" as my 87dB/W speakers.

While not as clean as my balanced amps, it's a nice little amplifier. Bass extends low and has good power, and dynamics are very good; the source followers driving the grids really improve transient response very much. The amp has a lot of kick in it, even at very low volumes!

All the components are very cheap, so for any curious builders there might be it's not a big investment to try out. But do get a bigger chassis...
 

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needtubes

Member
2003-05-24 9:23 pm
I am curious why you chose to use the extra source follower, given that you already had a MOSFET mu-follower on the 6C45. Sure, it sets the negative grid bias of the 6E5P and is DC-coupled to the 6E5P grid, but the first stage is AC-coupled to the source follower (so, we still have a cap in the signal path) and the source follower requires an extra bias supply of its own, making the power supply more complicated.
 
Eliminating coupling capacitors is not my goal. If it were, I wouldn't have one at the input; instead I'd have LED at the 6S45P cathode. As it is, I prefer fixed grid bias to LED bias by a small margin.

However, it is not insignificant where the coupling cap is located. When you compare a capacitors subjective auditive performance

1) before a source follower gate
2) before a high pulse capability high(ish) mu tube grid,

it has become very clear to me that scenario 1 makes the cap very transparent, whereas scenario 2 let's you hear the cap.

The point of the source follower is to be directly coupled to the grid; to achieve this, with a single sensible B+, this is the only (or, easiest) way to achieve it.

Providing a regulated B- of 19 volts (which is also used for the input stage grid bias) is a simpler task than going direct coupled.

Also providing the couple of volts positive bias for the follower is a matter of having one trimmer pot. It is actually in series with the trimmer pot that provides the voltage reference for the plate loads; R14 & R15 are actually a trimmer pot.
 
6P41S must've given some real power out! I've got 24 pieces of 6P36S waiting for use. The heater power is a bit of a headache though, 8A for a balanced amp...

This amp and all my amps basically are inspired by Wavebourn's 4P1L SE amp, so thank you!

You are welcome! Yes, 6P36S would be better, I just had 6P41S in my pile, but the sound was much better than I expected. My jaw had dropped!
 
hello all ,

me old build based to 6N2P and 6P41S ...

the max power to good sound ( and use long life 6P41S ) is 6W to 1 Khz .

i link the plan , the curve in oscillo and final build to décember 2014 .
 

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  • The Small USSR - 6P41S - 6N2P.JPG
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