The new dumble modded bassman build video series... begins!

[professormudd]

Too bad there is no ctrl+z or ctrl+x/ctrl+v when doing this kind of work.

[pompeiisneaks]

Hahah right?

I seem to always find a way to do some form of amp dislexia on every build... One of my builds I put the Transformers on the wrong side of the amp, and had to really fight to get the amp not to have noise with the most sensitive preamp components next to the power supply. I eventually went back and rebuilt the board myself the 'right' way That was my Gibson EH185 build.

It sounded great after.

~Phil

[pompeiisneaks]

As an exciting side note my EVM12L arrived today that I'll put into the combo! Gotta fix the chassis still, but should have time this saturday to do that.

~Phil

[norburybrook]

As an exciting side note my EVM12L arrived today that I'll put into the combo! Gotta fix the chassis still, but should have time this saturday to do that.

~Phil

EVM12 in a combo.....brave man if you're carrying it around yourself



M

[pompeiisneaks]

Agreed, but I'll be keeping it here at home most likely, so it's not going to be moved much if ever

Unless someone buys it, then it's their problem lol

~Phil

[pompeiisneaks]

So I'd been mulling some things on this build, it's getting closer btw, I'm about to start final wiring before I drop the board in....

But I had a thought. I didn't include an effects loop on purpose because I wanted to keep it pretty stock to the original.

BUT I had another thought, that at first failed but let me explain.

There are two channels, a clean and a drive one. so why not put an A-B pedal in front of the amp and send guitar to drive pedals, then to A-B then A to the drive input, b to mod effects like reverb, delay etc, then to the normal channel input, and use it like a drive/dry clean/wet mixer.... EXCEPT that the two are out of phase... so could I take the output from the normal channel and instead of inserting it at the 470k mixer put the output from either the 470k to the grid of the second have of the PI, near the NFB? Or would that cause issues? I'm not sure if the PI having NFB on the cathode of the second side would impact that type of use case or not?

See screenshot, I see 1 or 2 as options here... 1 I cut at the first red line, then send the green wire off to the grid of the second tube, or 2, I cut before the junction of the mixers and send a blue wire to the same, or do I put either/both at the pre coupling cap? This stage already has one, so I didn't see a need for two coupling caps in a row there?. Not sure if either is optimal (wire color doesn't really matter, just was using to show two options of routing? )

A+Bmodes.png

~Phil

[pompeiisneaks]

Nobody has any thoughts? Was my question too poorly phrased?

~Phil

[dbharris]

Phil, I am not positive but I don't think you can use the other input of the PI when GNFB is also used in the circuit. The feedback from the output would be in phase with the signal on the same side of the PI and would be positive feedback.

That was my conclusion anyways when I was adding GNFB to an 18W Marshall style amp with 2 preamps. I moved the second channel from the input of the PI where I applied GNFB to the other side of the PI and added mixing resistors. No more channel jumping since the preamps are out of phase with each other though.

-Dan

[pompeiisneaks]

that's what I was wondering/worried about... damn. Oh well, back to the stock build for now. If I ever decide to add an effects loop, I'll sort that out later.

~Phil

[pompeiisneaks]

Finally got some work done, and uploaded a new video. Just need to check it all, test fire it up and see how it sounds:



~Phil

[pompeiisneaks]

Ok you all get the scoop first.

I've fired it up the drive channel seems to be working a treat! If I dial the drive level down low it's nice and clean, with a touch of drive if you dig in. If I dial it up to max it growls hard. There's a bit of a buzz I nee to sort out. I think its 120hz, but I'm first trying to sort out the normal channel.

I get super weak output from that channel. I've troubleshot a bit, and so far here's what I see:

The amplification from a guitar signal measuring ac on my DMM shows hundreds of mV going in and after the first gain stage (V3B on the schematic) I seem to have about 1V or so, or very close at the output side.

The next part, (V3A on the schematic) which is the recovery stage post tone stack, seems to be the problem. For whatever reason, that side seem to not be conducting at all...

The anode voltages on other tubes are sub 300VDC, even the other half that seems to be working. This one, on the other hand seems to not be conducting and the anode voltage is sitting around 375VDC.

I've tried swapping tubes with the first preamp tube that previously worked fine, and get the same behavior. The output in AC volts post that stage is in the noise level of maybe 20-40 mVDC. I have double and triple checked the connections, and see no problems with continuity, I see 1.5k from the cathodes to ground on both halves of the tube.

The ONLY thing I can think of right now would be that since the cathode follower of the other channel shares that b+ node could 'maybe' be causing a problem? Not sure? could it be sending it's signal into the anode of the other side somehow? there's no coupling cap between the two...

One other thing that may be related is that I do note I have a 1M resistor to ground on the input and looking at the latest schematic (at bottom) I have 100k instead of 1M on that input.... is that a typo on my schematic or my error? It doesn't seem to me it would cause the problem we're seeing as the rest of that gain stage looks like stock fender to me (and isn't showing any obvious problems?)

I've also checked the grid of the second stages input and it still basically is seeing that almost 1V output from the first stage...

I've gotten my DMM on continuity mode on the pins in the socket to make sure none are shorting to ground or to other pins etc as well... don't see anything there.

Thoughts on what to test next? Why would this stage not conduct through that tube?




I can't wait to demo the drive channel on this thing, it sounds so good through the EVM12L
~Phil

[Aaron]

Hi Phil,

The drive channel was a favourite on mine as well. I also did the Jose Arredondo diode mod on mine with a push/pull pot.

On the schematic the bright caps on the volume pots are drawn wrong, should be on pins 1-2 not 1-3.
Regarding your no signal, what are the voltages on V3B? Maybe post a pic, a fresh set of eyes might be needed.

Aaron

[pompeiisneaks]

Here are some pics...

I didn't write down the voltage on V3B but it was normal ish like 304 above the anode resistor and 240 or 220 or so at the bottom of the anode resistor... I'll double check here in a few. The bright cap may be wrong on the schematic, but wouldn't that just dump some high end to ground and the rest would end up being there? It's also likely an error in my schematic... In the photo's you can see the bright switch wiring has a lead from the middle lug of the switch (purple) to the middle lug on the pot...

Also of note that resistor on the bass pot goes to the ground buss, it should be visible there on the other pic near the input (it's the black wire)

PXL_20210707_234230163.MP.jpg

PXL_20210707_234232820.jpg

PXL_20210707_234241182.MP.jpg

PXL_20210707_234244098.MP.jpg

~Phil

[pompeiisneaks]

Voltages on V3B 341VDC at top of anode resistor 229 VDC at bottom of anode resistor near pin
V3A top of anode 374VDC at top of anode resistor, 369VDC at bottom near pin,

Thus why it seems to be not conducting for some reason, I can't gather...

~Phil

[Aaron]

Whats the voltage on the cathode of V3B?