Thats in theory, i spoke with Dave Slagle at Intact Audio prior to making the move with the AVC. He helped me run a couple simulations with spice and i decided to bite the bullet.
After installation, i ran some sweeps and the pre amp performed better than expected. Distortion and FFT plots looked great but the quality of sound was incredible coupled with Ale’s gyrator and the 01a. I had been using a Audio Research SP6 and the 01a with the AVC output smoked the SP6.
I not only compared my SP6, SS Threshold and the 01A in my system but also compared the FFT, freq response and distortion plots. Dave’s AVC’s did not saturate as you might of expected. If you're ever in the N Fla area come listen for yourself, i’ll connect it to my 2 distortion analyzers if you want to see for yourself.
My Sowter TVCs are rated to 7v input, so at the moment I have set the input pot to reduce the voltage. I may make a fixed voltage divider using bulk foils at a later date. But being lazy, I use the 32 bit digital volume control on my Benchmark DAC3 HGC for general use.
I also have a #26 pre with output transformer, wired for minimum gain and OP impedance, and that has enough output for my power amp.
PSU schematic for 01a pre (with filament SiC bias)
Hi everyone!
What's your latest/best PSU for the 01a preamp?
I'm about to start getting my hands on all the parts for this pre and I hope that some of you (and hopefully Mr. Moglia himself) would share their PSU schematics with me because I wasn't able to find out what PSU design is the current best/ recommended one.
Best regards
Brzzl
Hi everyone!
What's your latest/best PSU for the 01a preamp?
I'm about to start getting my hands on all the parts for this pre and I hope that some of you (and hopefully Mr. Moglia himself) would share their PSU schematics with me because I wasn't able to find out what PSU design is the current best/ recommended one.
Best regards
Brzzl
It's easy to work out a B+ supply and there are plenty of variations. But if you use choke input, which is nice, you have to allow for critical inductance which may need a bleeder resister to ground depending on the size of your choke.
The filament supply is critical, so don't ignore it. I use choke input and then a Rod Coleman reg. A hammond 158T is good for choke input - 1H at 300mA. I use SIC diode bias rather than filament bias for the 01A, and so does Ale Moglia who I took the idea from. Here's a rough idea:
The filament supply is critical, so don't ignore it. I use choke input and then a Rod Coleman reg. A hammond 158T is good for choke input - 1H at 300mA. I use SIC diode bias rather than filament bias for the 01A, and so does Ale Moglia who I took the idea from. Here's a rough idea:
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Thanks for your response with the filament supply, but what about the B+ supply feeding the gyrator? I know that, as you mentioned, it is not hard to design one but I'd like to hear what you or other people who have built this preamp are using/ what worked best.
I used a 21st century Maida regulator so I could dial in a desired power supply voltage.
Entire left side of pic is just my power supplies for the 01A
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Looks like putting an 01A into the audio path creates far too many problems. The 264 & 864 are built for low microphonics. A better starting point.
I don't know the 264 or 864. They do have more filament current which is generally a good sign for lack of microphonics. But this puts you squarely in 26 territory - a much more common tube, plus you can put 7mA through it. The 01A doesn't suit all uses - I don't use it because of its low current. But it's thoriated and it sounds wonderful in the right context.
The father of one of my school pals when I was about 12 yrs old was the projectionist at a local movie theater (1944). So he had quite a bit of WE stuff which included some 864s. So one of those made its way to me. I scrounged a #6 cell, they were a very large dry cell used by the Bell Companies. They are still available last time I looked.
Carefully pulled a lot of wire out of a discarded receiver power transformer. With a few more junk parts I had enough to build a BC band regen with the 864, it worked very well into an old set of headphones. The antenna was a wire strung out my bedroom window to the chicken coop.
864s were fairly common at that time. The 264 is the non-WE equivalent, don't know how that happened. I have one in my stash, always thought I'd build another receiver based on that.
Carefully pulled a lot of wire out of a discarded receiver power transformer. With a few more junk parts I had enough to build a BC band regen with the 864, it worked very well into an old set of headphones. The antenna was a wire strung out my bedroom window to the chicken coop.
864s were fairly common at that time. The 264 is the non-WE equivalent, don't know how that happened. I have one in my stash, always thought I'd build another receiver based on that.
Another CX-301A amplifier is born. I had been using a 4P1L to drive an F4 amp for many years, similar to Ale's Siberian Gen 4 but with a K&K audio shunt reg feeding CCS plate loads. Loved that amp, but the 01A builds had me intrigued ...with covid lockdown and "working from home" there was no time like the present to experiment.
Recipe is almost identical to Ale's 01A, Salas SSHV2 regulators feeding into Gyrators with a BF862 in the lower position, cathode biased with Coleman regulators.
Colour me impressed, vs the 4P1L it initially sounded softer, but having spent a few days with the 01A i have come to appreciate how nicely balanced it is from top to bottom, with a lovely sweet mid range making vocals and acoustic music just sound great. This amp is definitely a keeper so i have to start planning a permanent enclosure to build this thing for real.
Initial testing was a mixed bag, i hadn't used RMAA or my Millett interface in a long time so connecting it up was a bit of a faff. Long story short, i am seeing a little more low end roll off than i was expecting, but cant say for sure how accurate my testing setup was. I am not using the source follower setup, just a .22uf cap on the output so i may just need to increase its value.
Having just read through this thread i am also going to look into the SIC diodes for the biasing as i have always used LED biasing for pre amp stages, and measured significant improvements in distortion and frequency response doing so.
Lessons learnt: These tubes (CX-301A) are fragile, i bought a cheap set of five tested tubes from eBay, by the time they got here from the US two had open filaments, and i managed to kill another one during testing. If you are building an amp, don't start with a pristine matched pair, buy some cheap sacrificial ones just in case.
Thanks again Ale for publishing the design, and everyone else for the goldmine of information here. If you have any tweaks or suggestions since you built your please shout...
Cheers
Recipe is almost identical to Ale's 01A, Salas SSHV2 regulators feeding into Gyrators with a BF862 in the lower position, cathode biased with Coleman regulators.
Colour me impressed, vs the 4P1L it initially sounded softer, but having spent a few days with the 01A i have come to appreciate how nicely balanced it is from top to bottom, with a lovely sweet mid range making vocals and acoustic music just sound great. This amp is definitely a keeper so i have to start planning a permanent enclosure to build this thing for real.
Initial testing was a mixed bag, i hadn't used RMAA or my Millett interface in a long time so connecting it up was a bit of a faff. Long story short, i am seeing a little more low end roll off than i was expecting, but cant say for sure how accurate my testing setup was. I am not using the source follower setup, just a .22uf cap on the output so i may just need to increase its value.
Having just read through this thread i am also going to look into the SIC diodes for the biasing as i have always used LED biasing for pre amp stages, and measured significant improvements in distortion and frequency response doing so.
Lessons learnt: These tubes (CX-301A) are fragile, i bought a cheap set of five tested tubes from eBay, by the time they got here from the US two had open filaments, and i managed to kill another one during testing. If you are building an amp, don't start with a pristine matched pair, buy some cheap sacrificial ones just in case.
Thanks again Ale for publishing the design, and everyone else for the goldmine of information here. If you have any tweaks or suggestions since you built your please shout...
Cheers
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Great job Mark, looking really nice.
You're right, the 01a (as most of the similar DHTs of the time) are fragile devices. think about how old they are
I find that the hybrid mu-follower provides the most clearer sound out of all configurations in a preamp. In my opinion, the SiC diode bias is more detailed and clear for the 01a and some other preamps, but is not the perfect answer for every bias situation of course. There are multiple factors which influence the sound as you well know. In some cases even it may expose deficiencies from other parts of the system.
Cheers
Ale
You're right, the 01a (as most of the similar DHTs of the time) are fragile devices. think about how old they are
I find that the hybrid mu-follower provides the most clearer sound out of all configurations in a preamp. In my opinion, the SiC diode bias is more detailed and clear for the 01a and some other preamps, but is not the perfect answer for every bias situation of course. There are multiple factors which influence the sound as you well know. In some cases even it may expose deficiencies from other parts of the system.
Cheers
Ale
And if you're interested in digging out more around DHT preamps and measures of SiC diodes, etc. have a look at this:
ETF.18 DHT Preamps Lecture – Bartola(R) Valves
for bedtime reading
ETF.18 DHT Preamps Lecture – Bartola(R) Valves
for bedtime reading
I have to say that when I heard Ale's 01A with the SIC bias it sounded great - very clear as he says. It was really nice. I've been using SIC bias with various other tubes and had less success in my terms - detailed for sure but I'm always after the last ounce of smoothness. I haven't tried the SIC diodes with filament bias, though, and this may be different. As Ale says, it's all in the implementation. It certainly seemed to suit the 01A from what I heard.