SF Vibrolux "blackfacing"

[JAM]

Thanks JAM. You're right, I am confusing neagtive bias voltage with current.

I have also done a little research, and apparently that 15k resistor should have stayed in. The supply resistor next to the diode is a 1k on my amp, whereas on the BF circuit it is a .470 unit. So the larger 1k there plus the larger 22k on the bias pot...that explains that.

So I can just permanently install these 1ohm resistors you speak of between pin 8 and ground, and measure across them to see a voltage equivalent to my current? Do they need to be a certain wattage rating, or will 1/2w work? Thanks for helping me out.

Yes, you can permanently install the 1ohm resistors. I used 2 watt (mouser #71-CPF2-F-1). Overkill for current, but very stable.

[boobtube21]

Well, it works. Yay! It sounds....well, I won't lie to you, it sounds the same to my ears! Maybe I'll hear a difference if I crank it up, but it's not a good time for that right now.

I had to use two 2.2 ohm resistors in parallel, and all I could get today was 5% tolerance, but wired together they came out to 1.1, which I figure is within 1% tolerance for a 1ohm.

I went through a couple different tubes ( I have a new set for the Twin that haven't been used yet) before I could get a set that was matched closely enough. Perfect Pair schmerfect pair, they're all labeled '20', but with one pair I got 31mv out of one, and 46 out of the other. I ended up with one at 46.1mv, and the other at 45.2. I'm happy with that.

Thanks for the help everybody, JAM, Kap'n, NT, cdw, I really appreciate it.

Stay tuned for part II...phase inverter.

[cdw2000]

1.1 ohm is fine. Just understand that 1 ohm is used so you can read mV and translate directly into mW.

Assuming you have no electrical background:

According to ohm's law E (voltage) = I (current) x R (resistance).

When R = 1 ohm, E = I

So you now have E = I x 1.1, therefore to dial in a certain current you need to multiply it by 1.1 to get the voltage you should adjust to.

If you measure voltage and want to know the current, divide the voltage by 1.1.

To be really accurate, measure the actual resistance and use that.

[boobtube21]

Thanks for the tip cdw. I actually couldn't get the current any higher than ~45ma, and 70% was 48.3 or something like that so I figured it's fine where it is.

Maybe I'll switch out that 22k resisitor for the stock 15k, and I might be able to get up to 70%.

[boobtube21]

HOLY SHIT.

Ok, I just demo'd the amp nice and loud (lunchtime, neighbors aren't home) and now I can definitely hear a difference. A big one, too. The signal is clearer, more present, and less saggy.

That and I can actually use my EQ now. Before the bass had to be on 0 or 1 or it was too boomy, and the treble had to be up at 8 or 9, but it was a little icepicky. Now I've got the bass at 3 1/2, treble at 7.

This thing sounds phenomenal now. It just needs a touch more gain, then I think I'm gonna leave it alone.

[boobtube21]

Well, I replaced a couple of resistors on the phase inverter circuit this afternoon. Literally 2 resistors, and it gave me a lot more gain. I only had a minute to see what it sounded like at full volume, I didn't want to piss the neighbors off.

There's also a cap I can change out from .01 to the BF spec .001 but that's supposed to choke some bass out, which I'm not sure if I want to do that at this point.

I understand the speakers that came in BF amps were a little darker, so when they went with brighter speakers in the SF they boosted that cap value to .01 to let a little more bass through. So unless I change the speakers out for darker ones I'm going to leave that cap in. I bought 25 of them from Mouser in case I change my mind though.

I'm going to get some pics together and post what I've done here. Anyone out there with a SF amp should definitely at least consider trying this out...that is if you're comfortable playing with electricity. If not, maybe have your tech change a couple of things for you to see if you like it. At least have him fix the bias circuit for God's sake, the "matching" version is a joke.

Thanks again for the help everyone. As long as this thing doesn't catch on fire I'm a VERY happy camper.

[boobtube21]

Okay, here's a couple of phase inverter drawings. The first one is the SF model Vibrolux I have, the AA270. I highlighted the resistors and and cap that can be changed.



Now here is the BF Vibrolux drawing, AA964. Again the same resistors and cap are highlighted, and you can see the different values written next to them. Change them to these values and that's it.



Again that .01 cap is somehow related to bass response. I haven't tried changing it out because I like how it sounds so far, but I might just out of curiosity. Switching out the 47k resistors for the 82k and 100k gives you some more drive.

Okay, time to go to bed.

[JAM]

I dont like the .001 going into the PI personally. This is a good place to experiment with though.

[boobtube21]

Well, I just tried it (the .001 cap), and it works for me. It didn't make as much of a difference as I expected, but it's different anyhow, not as midrangey. Before it was kind of Vox-y, NTTAWWT.

Now I'm seeing a resistor in the power section that could be changed out, I might change that one out too.

I can't believe what a difference 4 resistors and a capacitor make. It's a completely different amp now from when I started. It's a freakin' monster.