Dumble - help needed urgently!

[martin manning]

No problem having most things already done, I could see from your pictures that you had completed a lot of the wiring. Think of the steps I'm describing as identifying the group of connections and parts to double-check before applying power and testing that section of the circuit.

Yes, the routing of the wires and the location of the grounds is important. There is more than one way to do it, but it's a system and you want to keep it intact.

When you get power on next, measure the induced VAC on the OT secondary as I suggested above. Your OT is rotated 90 degrees from the usual Dumble build, so lets see if that is going to create any objectionable hum. Assessing that at this point will keep you from having to chase down the source later.

BTW, where are you located 'internationally?'

[Mousey Dung]

When you get power on next, measure the induced VAC on the OT secondary as I suggested above. Your OT is rotated 90 degrees from the usual Dumble build, so lets see if that is going to create any objectionable hum. Assessing that at this point will keep you from having to chase down the source later.

Love your systematic approach! After I posted that shot of the underside, I actually changed my mind about the 90-degree orientation and drilled some new holes to reposition it as it usually appears in other builds, realizing it was critical to the hum factor. Then I discovered that I'd positioned it badly and the main board couldn't be mounted, and I had to redrill four more holes to get it right. Thus my question #8 about the holey underside of the chassis near the OT.

BTW, where are you located 'internationally?'

I'm based in Northern Europe, get around quite a bit on tour with bands primarily comprising Americans.

[martin manning]

OK cool. BTW, I only suggested not soldering the 4-ohm tap because there is another wire (the negative FB) going on that lug. You can leave what you have. As I said, with the tubes out the current paths in the HV circuits are opened.

[Mousey Dung]

Okay, here's the test result sfter connecting PS board and powering on (standby switch off). The limiter bulb lit up, then dimmed to a low glow.

The voltages at the rectifier board were:
Relay: 6 V
B+ plate supply: 455 V
Bias supply: -59 V

The voltages at the seven nodes of the PS board were:
- Power amp V5-V8 plates: 454 V
- all other points: 0 V

On flipping on the standby switch, the limiter bulb lit up, then dimmed again. With the back-panel bias trimpots at their most negative point, the voltages at the various points were as follows:
- Relay: 6.8 V
- B+ plate supply: 446 V
- Bias supply: -59 V

- Power amp V5-V8 plates: 446 V
- Power amp V5-V8 screens: 446 V
- Clean V1 plates: 399 V
- OD V2 plates: 403 V
- Phase inverter V4 plates: 445 V
- Reverb rectifier V3b plate: 443 V
- Reverb driver Trans/V3a plate: 442 V

The power-tube plate and screen voltages:
V8 plate (pin 3): 445 V
V8 plate (pin 4): 8 V
V7 plate (pin 3): 445 V
V7 plate (pin 4): 8 V
V6 plate (pin 3): 447 V
V6 plate (pin 4): 9 V
V5 plate (pin 3): 445 V
V5 plate (pin 4): 13 V

Most negative point at wipers of both bias trimpots on back panel: -59 V

The voltage between the OT common (black wire) and the 16 Ohm (orange wire) was so low it showed up as 0.00 VAC on my digital multimeter (on which the lowest selector setting is 20 VAC).

Is that okay so far?

[Mousey Dung]

Shall I proceed to completing the wiring of the preamp board next?

[martin manning]

It all looks good except for the 8-13V on pins 4 of the power tubes (screen pins). That voltage should be essentially the same as on the power supply board at the screen node eyelet. You need to figure that out. Not much going on there, just a few bits of wire and the four 1k 5W screen grid resistors. Is the wire from the right side pair of screen resistors to the power supply board missing?

On the OT 0 VAC from 16-ohm tap to common, are the speaker jacks wired in at this point? If so they are shorting the output, and you will have to put a 1/4" plug into the main speaker jack to open its shorting contact, or just plug a speaker in there and listen.

Do you have the lavender and burgundy bias supply wires connected from the bias trim pots to the 220k resistors on the board, and the brown and the blue power tube grid wires connected from the other end of the 220k resistors to power tube pins 6? If so you can check pins 5 and 6 for the negative bias voltage.

[Mousey Dung]

It all looks good except for the 8-13V on pins 4 of the power tubes (screen pins). That voltage should be essentially the same as on the power supply board at the screen node eyelet. You need to figure that out. Not much going on there, just a few bits of wire and the four 1k 5W screen grid resistors. Is the wire from the right side pair of screen resistors to the power supply board missing?

Of course! I forgot to connect the last red wire going to the terminal lug above V6 and V7.
Now all the voltages at V5 to V8 are the same:
pin 3: 451 V
pin 4: 450 V

On the OT 0 VAC from 16-ohm tap to common, are the speaker jacks wired in at this point? If so they are shorting the output, and you will have to put a 1/4" plug into the main speaker jack to open its shorting contact, or just plug a speaker in there and listen.

Yes it is soldered. I will try it with a jack plugged in there. Don't have a 16-Ohm speaker to test it with.

Do you have the lavender and burgundy bias supply wires connected from the bias trim pots to the 220k resistors on the board

To the main preamp board? Yes.

, and the brown and the blue power tube grid wires connected from the other end of the 220k resistors to power tube pins 6?

Yes

If so you can check pins 5 and 6 for the negative bias voltage.

Will do that and report results shortly.

[Mousey Dung]

check pins 5 and 6 for the negative bias voltage.

Done. They are now showing as:
Pins 5 & 6 on V5 & V6: -57 V
Pins 5 & 6 on V7 & V8: -58 V

The voltage between the OT's black and orange leads still shows up as 0.0 VAC on my DMM.

[martin manning]

Great! You don't need a 16 ohm speaker, I picked that for measuring with the meter because it would be the most sensitive. Zero VAC on the OT is good- no hum!

On to the preamp board. Connect up the power supply and check voltages on the preamp tube plates (pins 1 and 6) to be sure it is getting through.

When all the other wiring is done, do the heater wiring from V8 to V1. You will want to twist the wires and elevate them above the centerline of the tube sockets. Use two different colors of wire and connect the same color to the same power tube pins and preamp tube pins. For example all pins 7 on the power tubes and all pins 9 on the preamp tubes are connected with the same color, meaning they are all on the same side of the filament winding. Like this: https://tubeamparchive.com/viewtopic.ph ... 903#318903

[Mousey Dung]

Great! You don't need a 16 ohm speaker, I picked that for measuring with the meter because it would be the most sensitive. Zero VAC on the OT is good- no hum!

I plugged in an 8-Ohm speaker. Dead silent!

On to the preamp board. Connect up the power supply and check voltages on the preamp tube plates (pins 1 and to be sure it is getting through.

Doing that now...

When all the other wiring is done, do the heater wiring from V8 to V1.

Not sure how I'm supposed to do this. Are you referring to connecting the 6.3V green wires from the PT to pins 2 & 7 of V8 and then stringing it along to the other tube sockets?
UPDATE: Ah, I see you updated your post. I understand. Thanks!

[martin manning]

Sorry, pins 1 and 6 are the plates on the preamp tubes.

[Mousey Dung]

On to the preamp board. Connect up the power supply and check voltages on the preamp tube plates (pins 1 and 6) to be sure it is getting through.

After connecting the preamp board, these are the voltages at the preamp tube plates:
V1 pin 1: 392 V
V1 pin 6: 393 V
V2 pin 1: 397 V
V2 pin 6: 394 V
V3 pin 1: 437 V
V3 pin 6: 443 V
V4 pin 1: 438 V
V4 pin 6: 440 V

[martin manning]

Great, looks like you are all set there.

If you have the relay supply connected to the main board (red wire from the rectifier board) and the three wires from the relay coils to the back panel switches, you should be able to operate those switches and hear or feel the relays pull in.

Got the filaments wired yet? Anything else left to wire up?

[Mousey Dung]

Got the filaments wired yet?

I'm trying to find the right wire for stringing the filaments. together. The kit came up quite short in this regard.

QUESTION #9:
Will wires with these specs suffice, since we're dealing with 6.3 V?
- Cable type: wire
- Diameter: 0.5 mm.
- Cross section: 0.2 sq. mm.
- Insulation: 1.4 mm., PVC
- Conductor: tin-plated copper
- Rated voltage: 60 VAC/DC,
- Test voltage: 900 V
- Temperature range: -15 to +65 degrees centigrade

Anything else left to wire up?

As far as I can tell, that's pretty much it for the amp section after the filament wires are all connected.

If you have the relay supply connected to the main board (red wire from the rectifier board) and the three wires from the relay coils to the back panel switches, you should be able to operate those switches and hear or feel the relays pull in.

The relay power is connected and chained to the bottom left pin of all three relay sockets. I will mount the relays and see if I can detect any relay triggers.

I might be wrong, but when I look at the diagram, to my untrained eyes, the PAB switch appears to go to the left relay, which then connects to the mid control.

QUESTION #10:
Are the back-panel PAB, Mid and OD switches all correctly routed in the diagram?

[Mousey Dung]

QUESTION #11:
What should I do next once the filament 6.3 V wires are twisted and strung along to all the tube sockets?