I’ve been on some business travel so haven’t had much time to work on stuff, however I did get a set of gyrator boards for a friend and a customer:
- BF862 configured for 4P1L preamp
- 2SK170 configured for 01a preamp
4P1L preamp with BF862 gyrator
Many have asked me about this preamp with gyrator load. Here is the latest implementation which I preferred most in terms of sound. The mu resistor is 470Ω which is a nice compromise between BF862 transconductance and distortion. I adjusted it on test. I use a 100nF for C1 so R6 is 10MΩ. R4 can be either 300KΩ, 330KΩ or even 390KΩ. Difference would be only on the voltage range for the CCS. I found running it at 25mA to be perfectly fine, some BF862 can even do J310. I prefer this SMD compared to the J310. It performs much better even at high frequency:
Back to the 01a Preamp
The finished boards looked like this:
Here is a quick measurement with a lovely UV201a (circa 1922-1924) with brass base:
The FR is not as great as the BF862 with 4P1L, however it reaches out to 100kHz which is fantastic compared to other preamps using different loads.
Here is the harmonic profile for a 2Vrms signal showing the low distortion and the cadence of harmonics characteristic of this valve:
Hi Ale,
I had the same experience comparing the BF and the J310.
I definitely prefer the SMD. I had some issues with the stability of the gyrator but figured out that it was related to not grounding the heatsink.
Best,
Radu
Hi Radu, yes with the BF862 you get an HF amplifier! Precaution needs to be taken to avoid oscillation due to stray capacitances, etc.
Just place a voltmeter on the anode and you get it going 🙂
Cheers
Ale
I’ve just built a line stage with the 01A board shown above. I used a choke input PSU – 250-0-250v AC secondary transformer, EZ80 rectifier and 0.1uf first cap into a couple of Hammond 155J chokes (15H, 30mA), so (C)LCLC. All polypropylene PSU caps – standard motor runs at 40uF. I needed a 15K bleeder resistor for critical inductance, and used 2x Welwyn W23 33K in parallel. The 01As just add up to 6 or 7mA so the current needs to be topped up. Filament bias supply was a 12+12v toroid at 25VA into Schottkys and 10,000uF caps giving 15v with a 5.6R Welwyn W21 dropper resistor.
As for the sound – what can I say, it’s the best line stage I’ve ever heard, and replaces my various 26 and 4P1L preamps with plate chokes and Lundahl interstages, though the 01A into LL1635/5mA runs it very close. I do urge anyone to build this – you won’t be disappointed, and it’s a surprisingly economical build for the quality you get. I’m building another!
HI Andy,
I’m glad you like it and am keen to wonder around to listen to the various setups. There are plenty of other implementations under way so would be keen to hear somebody else’s opinion soon!
Cheers
Ale