Project BFC

[kdmay]

I have just realised this is my first post here - long time lurker and heavy reader, but no contributions. Hopefully this will change that.

I am about to embark on my next build. I've built a handful of amps (a few plexi's and SLOs).

Next on the list is a Big Fat Clean, hence Project BFC. I played a Blackface Deluxe Reverb and a Two Rock Custom Signature v3 and loved the cleans in both. I especially loved the Two Rock's big round tone and the ability to bypass the tone stack with a footswitch. I didn't think much of the OD channel, but I have plenty of amps for dirt, so this is primarily a clean/mild break up build. The kind of amp you'd pick up a tele and want to play through.

Here is what I have to play with:

- a relatively small chassis 335mm x 190mm (3 preamp tubes, 2 power tubes, 1 tube rectifier or filter can hole).
- a set of AC30 iron
- holes for 4 pots on the front
- enough space for 6 holes on the back (so potentially more pots if required)

Here is a shot of the chassis. The first one is back when I was going to build another (smaller) plexi style amp.

[IMG:800:480]http://i641.photobucket.com/albums/uu13 ... d/file.jpg[/img]

[IMG:800:532]http://i641.photobucket.com/albums/uu13 ... file-2.jpg[/img]

[IMG:800:532]http://i641.photobucket.com/albums/uu13 ... file-1.jpg[/img]

I only need 1 channel, so was thinking of potentially building an ODS clean with the PAB and midboost/bright switches. Power section would be switchable 6v6 (mismatched OT secondaries) and 6L6s. Tube rectified for a bit of sag and compression.

But this would leave one unused preamp tube - I'm not sure I could do this! What about using the spare tube as a post-PI driver (like the SSS). Or as a footswitchable extra gain stage?

Anyway, long first post, but seeking thoughts and opinions on what I should build.

Thanks!!

[xtian]

Welcome, and please post your location in your profile (I noticed you posted metric measurements and got curious).

[LeftyStrat]

I might consider using the extra tube for a Dumbleator/fx send.

Though your idea of doing the cathode follower after the PI is interesting, I've always wanted to try that, especially if you're going for 'Big and Clean.'

As I understand it, the post-pi CF reduces blocking distortion, giving you more clean headroom. You'd need some extra voltage from your bias supply.

[kdmay]

OK - location updated. I am in Melbourne, Australia.

Using the extra tube for an FX loop is a good idea. Another idea is to incorporate the OD side of the ODS but, given I don't have enough room for an extra two pots, include a on/off/on switch that gives me 3 gain presets (and also attenuates to compensate for volume). Kinda like a fixed boost.

Although the post-PI CF sounds interesting, I've never tried it, so it's somewhat of an uncharted territory and I don't have a verified schematic to guide me (are you aware of any?).

I've never played an ODS - are the cleans "BFC" stock? If they sound anything like the Two Rock Custom Reverb Signature v3 I played, I'd be happy I think. And based on the unverified schematic I've seen of this (although not sure it's same version), it looks pretty close to the ODS.

Any other ideas for a Big Fat Clean?

Cheers

[LeftyStrat]

You can look at the SVT schematic.

And Randal Aiken gives a pretty good description of the method in his "What is Blocking Distortion" article:

"Add a DC-coupled cathode follower between the phase inverter and the grid of the output tubes, with the cathode follower cathode resistor returned to a high negative voltage, and the grid bias applied to the grid of the cathode follower. This effectively isolates the output tube grid circuit from the phase inverter and its associated AC coupling, and provides a very low impedance source for the output stage. This will prevent the output stage from going into grid clamp, and will eliminate the long time constant of the AC coupling. This method has the unfortunate side effect of requiring an extra tube and completely ruining the value of your vintage amp, so it is best used only on new designs, but is highly recommended. You will also get more power out of the output stage because it is now running in class AB2 or class A2 (the "2" suffix indicates grid current flows for a portion of the cycle)."

[tubeswell]

Any other ideas for a Big Fat Clean?

Cheers

Tweed Bassman or 5E8A Twin (incl 12AY7s in pre-amp), with warm-biased (~1V) pre-amp stages that are fully bypassed. I see you have biggish OT iron already.

[LeftyStrat]

Here's a schematic using the 12bh7 post-pi cathode follower. Heathkit w6a.

[kdmay]

Thanks for the suggestions. I definitely want to do a Twin / Twin Reverb at some point, but I'd probably do that in a combo.

I am now thinking of just building the Dumble / Two Rock clean from the schematic I have. I was going to incorporate the OD also, given the extra tube, but now I am thinking I might venture into the realm of reverb.

A few questions as I've never done this before. Why are reverb drivers (I think this is what you call them) paralleled triodes? Is this because they need to be 12AT7s? I had an idea that I could use a 12AT7 in my spare socket and use one side for the driver and the other side for the recovery. But does the recovery need to be a 12AX7 (i.e. not enough gain in the 12AT7)?

Failing that, I could add another tube and use one 12AT7 for the reverb driver and one 12AX7 for the recovery driver and FX return driver. (At this stage I wasn't planning on implementing an FX loop).

It's a lot to cram into a small chassis though!! I'm 75% through a layout (albeit with the OD channel) and there's lots of parts for what started out as a "simple" build.