I did a very rough-and-ready comparison of some plate chokes today, using 3 tracks from opera, orchestral and popular vocal. The Lundahl and Hammond are interstages so just used the primaries. My first observation is that they were all satisfactory and pretty close in sound quality. Preamp was 2P29L running at 13mA. My tentative rank order was:
1. NP amorphous core 136H, 15mA - sweetest treble, smoothest
2. 2 x Hammond 126C in series, 212H, 15mA - clearest, most detail
3. No-name ?Chinese choke 50H ?40mA - clear and quite smooth
4. Lundahl LL1660/18mA - clear and quite smooth
I have less than an hour on my larger NP plate choke, so needs more burn in but already I can tell it's clean and detailed.
Conclusions? Amorphous was the sweetest sound here, but I don't have frequency or distortion testing equipment so I'm afraid this is listening only, and as said, results were close and all were satisfactory. I hope to do some testing against Ale's gyrator in the coming weeks.
Sure, I can. I can order more boards and sell as kits, or boards only, or complete soldered boards. But I am a design engineer, not a soldering pro, so may be you can do it better.
The board also contains an anode load resistor for the previous stage and a coupling cap, I am not sure which values would you like.
If there is an interest, I can run some production for a group DIY. In my Gubernator-71 thanks to such a driver I am getting 45W per channel. I plan also to use this boards for hybrid phone amps, as an output stage.
Ok, I will post in a "wendor bazaar" as soon as ready!
PM me please so I will remember who asked.
I would also be interested in that. The gyrator has too much gain for my system (power amp is 250W @ 1V input!
)
New AVC
Just connected the new AVC on the output. All i can say is, wow!
Took this preamp to new levels. I can definitely hear a distinct difference between a resitive input volume control and an AVC volume on the output. The 01a has plenty of current to drive the AVC despite the concerns. With volume around 50%, output impedance is around 600ohms which drives cables and following preamp beautifully. I’ll run some sweeps in the following weeks along with my new/old HP8903 i picked up last month. Think of an AVC like a step down IT transformer driving grid current on a transmitting tube. I think my GM70 amps like to see this impedance.
See post 291 for some interior pics prior to the AVC
Just connected the new AVC on the output. All i can say is, wow!
Took this preamp to new levels. I can definitely hear a distinct difference between a resitive input volume control and an AVC volume on the output. The 01a has plenty of current to drive the AVC despite the concerns. With volume around 50%, output impedance is around 600ohms which drives cables and following preamp beautifully. I’ll run some sweeps in the following weeks along with my new/old HP8903 i picked up last month. Think of an AVC like a step down IT transformer driving grid current on a transmitting tube. I think my GM70 amps like to see this impedance.
See post 291 for some interior pics prior to the AVC
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Last edited:
I don't have a complete schematic except for some simulations I did in LTSpice per attached.
Design is simple, tube rectifier to choke loaded power supply to a voltage regulator, coleman supplies, filament bias, plate choke.
Chassis has a divider with ultra perm mu metal to provide additional shielding.
Also using a soft start delay for warm up, I also use an octal to 4pin adapter to try different rectifiers like the 83.
The preamp is dead quiet
Design is simple, tube rectifier to choke loaded power supply to a voltage regulator, coleman supplies, filament bias, plate choke.
Chassis has a divider with ultra perm mu metal to provide additional shielding.
Also using a soft start delay for warm up, I also use an octal to 4pin adapter to try different rectifiers like the 83.
The preamp is dead quiet
Attachments
Absolutely not! It was somewhat with a resistive input volume control but not with the AVC. I have thrown just about every genre at this preamp and it performs beautifully. I was using an Audio Research SP6 prior to building this pre-amp. I expected a subtle difference, however in the first 5 seconds it was obvious this preamp is much better than the SP6. Not sure if its the Z or a combination of the 01A DHT and the AVC.
Remind me what plate choke you are using? Is this one of Dave Slagles as well? Plate chokes are my personal choice, and as I was saying earlier in the thread I'm really liking the amorphous core ones from NP Acoustics in Vietnam. I think this is a good place for amorphous. Some prefer other choices for the OPT.