Vibroluxish build

[überfuzz]

The power tubes and the PT are at opposite sides of the chassis. Any suggestions on where I should route the leads between them? Same for NFB by the way.

I'm thinking, place the wires against the back of the chassis, but that's just me guessing...

[loudthud]

The power tubes and the PT are at opposite sides of the chassis. Any suggestions on where I should route the leads between them? Same for NFB by the way.

I'm thinking, place the wires against the back of the chassis, but that's just me guessing...

Does this mean that the input jacks are on the same end as the power transformer or the power tubes? Can you post a sketch or pic?

[überfuzz]

Hi thud!

Yes, the output tranny is sitting in the input end of the amplifier. I guess the undersigned of the original amp figured it would be more stabilised.

I've been messing with this amp before. The original amp had a pcb-board. Now I rebuild it with a new circuitry, with the only change to the hardware being the addition of a choke. I prefer to do this as I don't have any workshop where I can tinker at the moment.

I tried to place the signal path as far away from the output tranny as possible. The cathode side is quite close to the OT as I assumed that no noise would be introduced there. I attach a picture of the chassis (work in progress).

[überfuzz]

I trying to figure out how to wire the grounds in this amplifier. I've outlines a layout [pdf] for a ground scheme that I hope might work, but some pointers etc would be very appreciated. Please comment on this layout if there's some NONOs in the scheme. Also, I marked two alternative with red lines. As far as I understand the filter caps should go together with the preamp circuitry, but how should I connect that ground node to the lug?

groundscheme.pdf

[überfuzz]

When I fired up the PT I only saw 230 VAC. Fortunately there are other taps showing 320 VAC. After rewiring the PT I got ~375 VDC at B+. At this stage there is one thing that puzzles me. A short background: Between B+ and the first filter cap I placed a choke with roughly 105 Ohms DC resistance. Between the next pair a have a 2.2k resistor. The last resistor in the filter is 4.7k. Now, the thing is I get minor or now voltage discrepancy between the filter taps. I thought I'd see 10 to 15 V down steps at each filter tap. Is this due to the fact that I don't have any load, but a drain resistor from B+ to ground?

Edit, my multi meter isn't exactly top notch, hence the uncertainty in the readings.

[g1]

Yes, you need loads at each node, or at least at the last one (furthest from rectifier). You said you have a drain at B+ but it is separated from the first cap by a choke? Why not put the drain across the cap.

[überfuzz]

You said you have a drain at B+ but it is separated from the first cap by a choke? Why not put the drain across the cap.

The reservoir and filter caps are separated by a choke. Do you mean like a resistor placed there permanently to drain the caps when the amp is turned off?

[g1]

I'm not sure, I'm just going by what you are describing. A schematic would help. If you have a cap at the rectifier before the choke, that is normally where the bleed resistor(s) would be.
You called it a drain resistor, that to me means a bleeder for the caps. You refer to reservoir caps and filter caps, we are just using different terms. To me a reservoir cap, if it is the first cap at the recetifier, I call it the first filter cap.

[überfuzz]

I just fired up the power stage. I get funky readings when I plug power tubes.
Readings when there's no power tube inserted.

1. Jumper to 5
2. ~3 VAC
3. 378 VDC
4. -42 VDC
5. 378 VDC
6. jumper to 4
7. ~3VAC
8. Ground

Measurements when I plug power tubes.

1. Jumper to 5
2. ~3 VAC
3. 295 VDC
4. -35 VDC
5. 295 VDC
6. jumper to 4
7. ~3VAC
8. Ground

So, now I'm looking for errors or busted parts in the power section. Any suggestions on how to proceed would be very helpful. Can I draw any conclusions from the voltage drop seen when I plug different 6L6 tubes in the power section? The voltage drop differs slightly depending on the 6L6 tubes plugged.

[überfuzz]

Turns out I'm had a short to ground (one eyelet point to chassis).

I can't really test play it where it's sitting at the moment. I don't wanna disturb my family and neighbours, but everything seems all right. At the moment the only thing I think I have to tinker with is the reverb circuit. There's to much bass in the reverb signal.

I have a question regarding the 820 Ohm resistor in on the cathode side one the 2:d pre amp stage. When you folks only use one channel of a Fender pre amp, do you opt for a 1.5k Ohm resistor or do you keep the 820 Ohm resistor?

[g1]

When you folks only use one channel of a Fender pre amp, do you opt for a 1.5k Ohm resistor or do you keep the 820 Ohm resistor?

Not sure if you mean it is still wired up to 2 triodes or just one? If it still has the current of the 2 triodes through it, then I would leave the 820R there.

[überfuzz]

No they're separated now, or one is not there any more.

[g1]

Then you should probably go with the 1K5. If you find it negatively affects the tone, you can always go back. With the 820R I would expect less headroom/earlier clipping with the triode being biased hotter and lower voltage at the plate.

[überfuzz]

I did a short recording (in my flat) of the amplifier. I haven't set the tone stack, just gave it a tad bass and treble.


first test.mp3