Dumble Modded Bassman Layout/Schematic

[pompeiisneaks]

I'm thinking of this being my next scratch build.

I've done a layout from the schematic and pictures I found in this thread:

https://ampgarage.com/forum/viewtopic.php?f=5&t=20544

And here's the Layout I have. The schematic I based it on is the one by Aaron on Pg 3.

I don't know if it's correct or not. I did have one area I've put a comment on, because I can't, for the life of me, see where the cathode follower 100k to ground is on the real amp, so I set it to ground right at the tube. Is that smart? Don't know. Hoping someone has a better suggestion.

Some other comments, confusion. In the actual amp, there's an odd wire coming up through the board to an eyelet that jumpers to another eyelet. I don't see that connecting to ANYTHING that makes sense to me.

I'm also not 100% sure where I should bring in the B+ 1-5 nodes on this exaclty, as I've left those blank, I'm assuming they come in from the dog house etc, but that should be pretty obvious once I sort out the rest, so I've removed a bit of the clutter of that for now.

Let me know if anyone sees any errors/ problems

another oddity is the schematic had a few resistors like "45k" and the like that I replace with the more common 47k or similar values, or 0.047uF for 0.05 and 0.022 for 0.2 etc. but not sure if that's appropriate?

Also I made all resistors MF for this layout, but people should likely look at the pictures to see where the CF and CC's were used etc.

This should at least give the map of values for people.

Please let me know if you see any issues.

I'm hoping to start collecting parts to build it sooner than later.

It's going to be a multi month project, because I've got a limited monthly budget and I'll be buying the parts as I go.

I'll add a power filter board later too, but that stuffs the easiest, this part is the more complex stuff I need more eyes on for now.

As this post goes, I'll update any corrections/errors etc here and make sure it stays updated.

DumbleModdedBassmanLayout.png

MainSchem.png

Bias-PSU.png

Edit1: fixed the Bass Pot and used 470k resistors (I don't have the 5 banders on the visio shapes, I could be less lazy possibly and make some )
Edit2: Fixed 85/100k first stage to 110/120k and have updated resitor types/colors to reflect CF/MF/CC and fixed a few other errors like missing second volume post channel 1, messed up cathode follower wiring, and added B+ node indications. Also fixed missing 100k negative feedback resistor into the tail of the LTPI
Edit3: Fixed the board settings of the 100k CF on board and the 820 cathode of first half to the socket. Fixed the 110k to 150k for the anode of V1A.
Edit4: Fixed the Deep Switch, fixed a few other missing things, adding in schematic images.
Edit5: fixed power supply error in schematic
Edit6: fixed error on schematic related to rock/jazz switch
Edit7: updating PSU filtering from 20uF per node to 50uF per a common dumble and marshall topology
Edit8: fixed missing 100k NFB resistor.
Edit9: added BOM for what is on the current schematic.
Edit10: fixed error on BOM and fixed missing named 1.8k resistor on the normal channel tone stack
Edit11: Fixed an error in the layout for the normal channel, didn't match schematic.
Cheers,

~Phil

[alkuz1961]

...another oddity is the schematic had a few resistors like "45k" and the like that I replace with the more common 47k or similar values, or 0.047uF for 0.05 and 0.022 for 0.2 etc. but not sure if that's appropriate?

~Phil

Hi Phil,
I think there is some inaccuracy in the schematic. If you look closely at the picture, it is obvious that these are resistors with five strips and their value is 475k

0 HAD Bassman photo-A.jpg

[pompeiisneaks]

Oh cool okay, I can update that for sure. Looking for any other changes as well, that does seem like some pretty significant attenuation on those signals, two inline between V1A/V1B and one to ground between V1B and V2A.

Thanks!

~Phil

[Colossal]

Nice start on your layout. Is that bass pot really 2MA?

[10thTx]

Is this the schematic you are working off of?

With respect, 10thtx

Phil, have you ever built the ManZamp https://ampgarage.com/forum/viewtopic.p ... &start=450
The YouTube clips of that amp are very close to my favorite of all the Dumble amps

[beasleybodyshop]

Can confirm the 45K resistors are actually 470/475K. I've built this mod into several bassmans with good effect. It's basically a modded JCM800 in the bass channel. Aside from the 470K resistor sub it's a fun circuit! Very meaty overdrive

[pompeiisneaks]

Nice start on your layout. Is that bass pot really 2MA?

Oh woops no that's 1MA

I'll fix that.

[pompeiisneaks]

Is this the schematic you are working off of?

With respect, 10thtx

Phil, have you ever built the ManZamp https://ampgarage.com/forum/viewtopic.p ... &start=450
The YouTube clips of that amp are very close to my favorite of all the Dumble amps

I believe that's the right one yes. Unless there are two versions of aaron's schematic in that thread and I missed one.

I did see yours earlier in the thread but assumed (possibly incorrectly) that the latest in the thread might be the 'greatest'.

I've not heard the manzamp no, I will take a peek.

~Phil

[pompeiisneaks]

Just looked over the manzamp thread and sadly a lot of that history is dead because photobucket sucks... so there's no longer a usable layout... unless ic-racer comes back and finds out why they nuked his images.

At any rate, I'll still stick to this one, since I've spent so much time making a layout etc

~Phil

[Colossal]

Just looked over the manzamp thread and sadly a lot of that history is dead because photobucket sucks... so there's no longer a usable layout... unless ic-racer comes back and finds out why they nuked his images.

At any rate, I'll still stick to this one, since I've spent so much time making a layout etc

~Phil

Photobucket is beat

Here is what I have from ic-racer's thread. No layout though.

Manzamp.jpg

[alkuz1961]

What can you say about the value of the first plate resistor? If you decipher the value of the color bands it looks like 80 kOhm. But I have never seen such a value. Could it be that this resistor has another band, that is hard to notice in this photo with low resolution?

HAD Bassman photo 1234.jpg

[pompeiisneaks]

Yeah I'm not sure... it's too blurry to say for sure, but per this picture I've got as well ( I think it's the same one) I've marked the 4 bands I 'can' see.

The 5 band ones you've shown do have precision versions that can go down to specifics like 80.5 k etc.

BUT if I play with those on a 5 band resistor I come up with really bad options...

Example: From the picture if I use it as I think, it's 4 band 80k, but that's not a normal resistor (doesn't mean he didn't have access to some older weirder values).

But if you try replacing some of those with different positional values it gets really weird.

if I try grey, black, orange, something red, then I might be seeing a greenish or blueish color then it's eithr 80.3 M or 803 M or 803G if I go violet all non good values for a plate resistor

If i try grey something black orange red, then I get all values in the 800k range. also not good. Therefore I think i can conclude it makes more sense that it's 80k and just some rarer precision resistor. If you desperately wanted 80k, you could buy 82k resistors at 10% tolerance you could measure quite a few and with up to 8200 ohms variance likely find one that's 2k low.

Seems to make sense to me since the common values there are between 80 and 100 for typical fender/marshall esque stages. (usually it's only the PI that has the 82k and 100k but that's also up for load line play time I'm sure).

~Phil

[alkuz1961]

The location of the bands on the body of this resistor is more like 5 bands. This Board also has MF resistors with 4 bands and these bands are arranged differently. A brown color on a blue background looks like gray. In this case, it can be brown/brown/black/orange/red, which corresponds to 110 kOhm. And this is a more real value than 80 kOhm
By the way, I use the same image, just slightly increased the resolution using photoshop

As an example, this resistor most likely has a value of 100 kOhm, but the brown stripe looks like a gray. This may be misleading.

2020-09-20_21-56-48.jpg

[Charlie Wilson]

I'm seeing two types of metal film resistors in that photo. NTE metal film(4 band) and Roderstein Mk3(6 band). The fifth brown(temp) band is almost completely washed out on all of the resistors in the photo. This means there is a double band before the black band on the resistor in question. I see either a double brown or double grey and I don't find it very likely that it is 88k. So I'm voting for 110k. The carbon film 120k is a Q-Line and the 100k carbon films look like Radio Shack. This also confirms to me that the channel isolation resistors in the Bandmaster UP are 475k. Phil that wire to nowhere looks like it is the brown wire that comes from the pots through the hole under the board and to the jumper on the top of the board. The brown wire floating above the board with the yellow wire connects to the jumper and goes all the way down to the channel isolating 470k resistors. The cathode follower 100k appears to have a white wire on one end that goes down into the wire bundle that goes to the filter caps. Finally, check that switch with all the capacitors and 100k resistor on the front end. It appears to wire to the volume pot not the treble. Also notice the funky way of rolling off some high end at the first input. A 100pf or 10pf ceramic disc going from input to the shield of the coaxial. Phil I'd really enjoy hanging out with you on a live stream. I promise, I'll wear a virtual face mask.
CW

[pompeiisneaks]

I'm seeing two types of metal film resistors in that photo. NTE metal film(4 band) and Roderstein Mk3(6 band). The fifth brown(temp) band is almost completely washed out on all of the resistors in the photo. This means there is a double band before the black band on the resistor in question. I see either a double brown or double grey and I don't find it very likely that it is 88k. So I'm voting for 110k. The carbon film 120k is a Q-Line and the 100k carbon films look like Radio Shack. This also confirms to me that the channel isolation resistors in the Bandmaster UP are 475k. Phil that wire to nowhere looks like it is the brown wire that comes from the pots through the hole under the board and to the jumper on the top of the board. The brown wire floating above the board with the yellow wire connects to the jumper and goes all the way down to the channel isolating 470k resistors.
CW

Cool, thanks!

So basically my layout's yellowish looking sire that goes from the output of the Vol pot is right?

I also just realized I never finished the wiring of that pot!

I can definitely update that to a 110k resistor, it seems to fit more with the normal values of a 12AX7 preamp (non PI) tube that dumble used. I do now note the red band, and that does mean that resistor is 120k not 100k like I have on the Aaron's schematic and my layout. I'll have to update my layout and I really need to update the schematic, but I don't have aaron's original so I guess I'll redraw it from scratch once we finalize the component values.

Thanks!

Charlie, up for a livestream of me building it again

~Phil