How do you implement the bass/treble control? The negative feedback control can be an adjustable pot in the loop. But the bass treble I think would have to switch caps? I am interested in building a SA70 inspired preamp.
You are correct. It's switching cap values in the negative feedback loop. I always refer to this schematic, sorry the quality isn't the best. I snagged it from a different thread at some point... I think from EMRR.
In the last couple years, I've finally gotten comfortable building tube equipment and have slowly been working my way through simple tube preamp designs. Gates SA70 is a fairly easy build and sounds great. RCA BA-2C(B) is another one that I've really enjoyed building and sounds awesome. Gates SA-134 is a little more complex, but tons of gain. A couple others are the Western Electric 141-A, preamps from the RCA BC-6B and the Collins 6Q-1.
There are different revisions of the schematic in how the feedback is done, so the schematic is helpful. Are you switching C4? Trying to wrap my head around changing the cap in the feedback loop would change the frequency response of the amp. Opposite, I expect, a HPF in the feedback loop would be a boost in the lows for the amp. I guess building it up and experimenting would be the way to go.
I use a variety. Just sort of depends on what I have laying around or can snag off eBay for a decent price.
Is there any reason to use the 2.5w version over the 1/2 watt.?
Good question. If the unit was driving that much output you could go with the 2.5w version, but these preamps don't. I've used primarily the 1/2watt version which should give plenty of headroom from my understanding.
I used WSM 15k:600 in my LA2A build for the output and think they are really good also. It sounds great in that build anyway.
Are they basically a shielded O-1? I built several preamps with O-1 using O-17 shields and they sound great.
I honestly spent a lot of time on the forums here reading and reading and reading about whatever preamp I was working on. Most folks have run into common issues and posted about them, so I research and take notes before jumping into a new build. Then I’m usually following the schematic and building it close to the original design. For power supplies I tend to run old school with AC heaters/filaments and re-use the BA-2C power supply as a starting point.
O16 UTC are mumetal shield 150,250,600 input impedance to 50k output tube grid. +8dbm 30 to 20k response so it’s an O-1 with shield
None of these preamps can do a 1/2W. They can’t do 1/8W.
I just finished one channel of the sa70 circuit and read this post. The negative feedback and high and low tilt eq's peaked my interest.
Hi Nate,
Yes, that's a good recipe, that's how I build my Gates SA70 style amps too, with a small addition. I always have one position on the capacitor switch with no contact, so that the negative feedback is completely switched off. I like it, depending on the source.
