Dumble Overdrive Special #124 build video series

[pompeiisneaks]

I don't right now. I've checked everything I can think of. As mentioned I can see the output from the PI being perfect with a sine wave and a scope. I see it hitting the grids of each tube, I see the output on the speaker, but it's definitely super weak. It's also a 4 ohm load so for that math I'd get:

7*7/4 = 12.25 watts, but wait I have an 8 ohm speaker on it, so I guess that's more right, even though it's a 4 ohm tap. (I don't have a 4 ohm speaker cab around).

I guess I should write down all my voltages, I checked them and they're consistent across the board. The bias is set well, I think the negative voltage was like -45VDC and it was around 48mA plate diss avg per tube. So it seems to me to be conducting well, the tubes are all lit up, input is good and strong. I know if one or some of the tubes were bad I'd see weird voltages on those tubes. I don't. It just seems like the output is super weak, and that touch sensitivity of the transformer seems odd as well. Let me get voltages exactly for the power tubes and come back.

~Phil

[sluckey]

If you have an 8Ω load on a 4Ω secondary tap (ie, you doubled the load resistance), then the reflected plate impedance will also be doubled. Is that OK for the tubes you are using? What tubes and what OT do you have?

[erwin_ve]

Hi Phil, that's too bad after so much invested hours. As for the fet: the 8k2 at the source is for biasing the fet. It's basically looking like a cathode bias from a 12ax triode, the limits for the fet are specified in Martin's Fet bias procedure.

I can't give any clues for the output malfuctioning. What does it look like on the scope(at Speakerjack)? Maybe it can give a clue. 6-7vac is a very low output for a 100w amp

[pompeiisneaks]

If you have an 8Ω load on a 4Ω secondary tap (ie, you doubled the load resistance), then the reflected plate impedance will also be doubled. Is that OK for the tubes you are using? What tubes and what OT do you have?

It's the stock Fender bassman 100 transformer, I'd have to look at the codes. I've got 4 JJ 6L6GT's in the amp.

~Phil

[pompeiisneaks]

Hi Phil, that's too bad after so much invested hours. As for the fet: the 8k2 at the source is for biasing the fet. It's basically looking like a cathode bias from a 12ax triode, the limits for the fet are specified in Martin's Fet bias procedure.

I can't give any clues for the output malfuctioning. What does it look like on the scope(at Speakerjack)? Maybe it can give a clue. 6-7vac is a very low output for a 100w amp

Cool, thanks, I may try putting in a variable 10k pot then from there to ground and see if I can adjust it.

Edit: and I'll get some screen captures from the scope on the PI Output and the speaker jack.

the NFB done wrong couldn't do this could it?

~Phil

[sluckey]

It's the stock Fender bassman 100 transformer, I'd have to look at the codes. I've got 4 JJ 6L6GT's in the amp.

Did you verify it was good before you removed the bassman circuit?

[pompeiisneaks]

yes I have a video showing me playing through it I was trying to 'fix' it but it worked fine and needed no fixing.

Here's the data that I got from testing it today:
(V1-V4 are just my power tube naming for the test. The 3.2VAC was one half to ground, so it should be about 6.4 between the two)

v1
1: -41.07
2: 3.212VAC 0VDC
3: 419.2
4: 422.2
5: -41.08
6: 423.3
7: 3.216VAC 0VDC
8: 0
V2
1: -41.02
2: 3.218VAC 0VDC
3: 419.2
4: 421.9
5: -41.08
6: 423.0
7: 3.223VAC 0VDC
8: 0
V3
1: -41.94
2: 3.242VAC 0VDC
3: 420.2
4: 420.1
5: -42.06
6: 423.4
7: 3.226VAC 0VDC
8: 0
V4
1: -41.98
2: 3.253VAC 0VDC
3: 419.7
4: 421.9
5: -42.01
6: 423.7
7: 3.241VAC 0VDC
8: 0

Here's a screenshot of one half of the output at the PI (yellow) and the speaker jack, it's actually even lower:

SignalOutput.png

[pompeiisneaks]

Hit submit too fast and then internet went out lol.

so to continue, I've got the sockets wired with 470 3W resistors to the screens, jumped from pin 6. I see the dumbles have 5.1k instead. It does seem odd that the screens are higher than the anode. that definitely makes me think the screens may be stealing the signal. I may ahve to double check I didn't put the screens to the wrong place? At any rate, I may need some 5.1k to drop that better no?

~Phil

[erwin_ve]

Phil are you're pins not mixed up with another kind of tube?
The 5k1 are grid resistor s not screens!
The induction in the choke van give a slightly higher voltage at B2

[sluckey]

Speaker jacks wired correctly?

[pompeiisneaks]

Let me make sure I'm explaining that right< i may have mixed up screens and grid.

there's a 1.5k grid resistor on pin 5, jumped from pin 1, (5 is screen/input with negative bias that's showing correctly, and 1 shows that as well, as it's the jumper point)

Theres a 470 ohm resistor on the screen that's going into pin 4 jumped from pin 6 (6 is a tad higher due to the drop over the resistor).

The jacks seem right to me, but I'll take a pic and upload it so you can see.

~Phil

[pompeiisneaks]

Okay here are pics of the OT number, and the Speaker Jacks

OTinverted.jpg

SpeakerJacks.jpg

The purple wire coming away from the jack is the NFB to the board.

~Phil

[pompeiisneaks]

OMG

I didn't see this until I just blew up my own picture

See the udpated one. I just found a blob of solder that seems to have bridged as a crappy resistor to ground, see where I circled in this pic.

InkedSpeakerJacks_LI.jpg

I removed that and turned it on and got the typical REALLY LOUD "your NFB is PFB" so I'll swap those and see.

~Phil

[erwin_ve]

OMG

I didn't see this until I just blew up my own picture

See the udpated one. I just found a blob of solder that seems to have bridged as a crappy resistor to ground, see where I circled in this pic.

InkedSpeakerJacks_LI.jpg

I removed that and turned it on and got the typical REALLY LOUD "your NFB is PFB" so I'll swap those and see.

~Phil

you must be so glad that the OT is fine!

[pompeiisneaks]

yes I am, I swapped the PI leads and bam, sounds outstanding, I had to have the MV down to like 3 to be able to play now (my daughter works the night shift so she's sleeping)

It's up and working. The only thing now is that the drive seems pretty gnarly. I'll have to play with it more later when she's up!

~Phil