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

[didit]

Ok new question. Was about to put all the parts into the cart and the choke fender would use, 022699, has 4H and 90mA, and 108 ohm as it's rating, but nowhere can I see any equivalent.

Hammond 194B is a direct substitute - https://www.hammfg.com/files/parts/pdf/194B.pdf.

..

[pompeiisneaks]

Yes, 25uf or 10uf, or 3.3uf, or 1uf or .68 makes a massive difference once you add up everything else. You would have to change the recipe a bit if you want to use 10uf. It simply will not sound the same as 25uf, with everything else the same.

Maybe I didn't ask the question correctly.

Will 10uF be a significant difference for tone for a guitar? It seems insignificant for bass, but may have some impact there, but I"m building a guitar amp.

~Phil

Phil , are those calculations correct? They seem too low frequency wise. At those frequencies there'd be no audible effect on a guitar signal.

From my experience one of the good things dumble did was reducing the bass by reducing the cathode caps from a 25uf to 5uf , this stops the amp falling apart at volume. I would say a 10uf would be a good thing for an amp that's going to be pushed into overdrive especially if using humbuckers.


M

I could double check on another online RC calculator, but yeah, this is for the 'normal' channel which I think is pretty close to the stock AB165 bassman values. I'm just thinking to do two things 1. save time/money and use what I have on hand and 2. make it more 'guitar friendly'

So I don't recall the calculator I found before, but here's a completely different one:

http://www.muzique.com/schem/filter.htm

I put in 1500 ohms and 25uF and I get 4.2 hz there as well. and 1500 and 10uF = 10.6hz.

Now that doesn't mean I'm not doing it completely wrong.

I did find this:

https://www.ampbooks.com/mobile/amplifi ... alculator/

and it shows a graph of the rolloff. it seems that with a 33k grid resistor, 100k anode and 1.5k cathode I get some rolloff starting around 100hz but it doesn't impact severely until sub 100...

but 25 shows a pretty similar rolloff that starts a bit lower around 100hz instead... not a major shift.

I even found a post on another forum by tubeswell about that same calculator.

Seems the general consensus is between 25 and 10 isn't a 'major' change to the low end other than making it a little less 'muddy' but that down to 5 or 2 gets a lot more bass 'out' which may not be what I'd want.

So... I think I'll use the 10uF's I have on hand for simplicity/speed. (and not needing to order only 2 caps from mouser too lol)

~Phil

[martin manning]

For the cathode resistor bypass you get a high-pass shelf response with a 5-6 dB cut, and the calculation is a bit more complex. See: http://www.valvewizard.co.uk/OtherStuff.html However, if you just do the 1/2πRC thing you will get a frequency that is roughly 1/3 of the way up the ramp, so ok for a relative comparison. Your 4.2 Hz and 10.6 Hz numbers are correct for 1/2πRC, and yes I think using the 10u caps will be fine.

[Charlie Wilson]

Phil, I think you are trying to do something with a lot of numbers and calculations(that's cool )that could easily be, and in my opinion, should be answered once you get the amp built. Just use the 10uf, then tack another across it and see what you like better. I think 10uf sounds and feels different than 25uf but lets see what you think.
CW

[norburybrook]

Martin /Charlie ,are those numbers the cut-off points at -inf and they slope upwards from there with a say 3db/octave shelf to say 80 or 120hz at zero.

Otherwise no one can hear 4 or 10 htz, they're subsonic frequencies but you can definitely hear the difference between a 25 uf and a 10 or 4.7 uf which wouldn't be possible if they were the high pass filter points.

Just trying to tie up theory and practice here.


M

[pompeiisneaks]

Sounds like that should work, and I've got extra 10uF's so I can do just that, build it with 10, and then for fun, temporarily tack solder another 10 in parallel to get the additional capacitance and see what happens.

Thanks,

~Phil

[pompeiisneaks]

Martin /Charlie ,are those numbers the cut-off points at -inf and they slope upwards from there with a say 3db/octave shelf to say 80 or 120hz at zero.

Otherwise no one can hear 4 or 10 htz, they're subsonic frequencies but you can definitely hear the difference between a 25 uf and a 10 or 4.7 uf which wouldn't be possible if they were the high pass filter points.

Just trying to tie up theory and practice here.


M

From that one link I gave from the ampbooks website it shows it as a db drop off on a curve as the frequency goes down etc. I really like that one, because you can see the shelf instead of just assuming it's 'off' at some point, which obviously it is not.

I'm attaching some screenshots of 10 VS 25.

10uF.png

25uF.png

~Phil

[norburybrook]

Martin /Charlie ,are those numbers the cut-off points at -inf and they slope upwards from there with a say 3db/octave shelf to say 80 or 120hz at zero.

Otherwise no one can hear 4 or 10 htz, they're subsonic frequencies but you can definitely hear the difference between a 25 uf and a 10 or 4.7 uf which wouldn't be possible if they were the high pass filter points.

Just trying to tie up theory and practice here.


M

From that one link I gave from the ampbooks website it shows it as a db drop off on a curve as the frequency goes down etc. I really like that one, because you can see the shelf instead of just assuming it's 'off' at some point, which obviously it is not.

I'm attaching some screenshots of 10 VS 25.

10uF.png
25uF.png
~Phil

that's exactly as I described and what I hear thankyou. when dealing with audio you mention the point at which the high pass filter starts not stops hence the confusion on my part.


it seems the slope is the same so the lower the -inf point the lower the actual high pass filer starts. In audio you chose the high pass filter point and use different slopes from -3db/octave which is gentle all the way to brickwall, which has no slope at all.


Edit: Actually Phil I think you're using the wrong terms from the graph. A high pass filer is a high pass filter and it's the starting point that's important from a sound point of view so perhaps if you look at the 1st graph it would have a 150hz point and the second a 100hz , it's the point where the slope starts to go down not where it ends that your ears hear.

Maybe this is a difference between an EE and an Audio engineer?

M

[pompeiisneaks]

Well, first I'm not an EE. I've probably got more experience as an audio engineer than an EE. I'm a systems/software engineer. I dabble in electronics and am learning.

I'm talking about a low pass filter here, not high pass. At first I was looking at the 'cutoff point' which is the 'bottom left spot' on those graphs but wasn't accounting for the gradual roll off, even though I knew it was there, it didn't click with me initially until I saw the graphs.

I would think the 'start' of the effect is where the graph shows the linear top part starting to roll down. In the 10uF range it seem to me to be at maybe 29 or 30dB and starts rolling down at maybe 120Hz but makes a significant break downwards at a little above 100hz. The 25uF one doesn't start downwards until maybe 105 hz or so and doesn't make the clear break downwards until sub 100hz. BUT I don't have enough ear training to know where that becomes seriously audible. I think like Charlie said it will be a good exercise to listen to something with 10uF and then add in the second cap in parallel to get 20.

I have attempted to learn to record my own music now for over 20 years starting on an old cheap ass yamaha 4 track with shit mics in the mid '90s and then worked for maybe a year at one point in a hotel doing audio visual stuff as well. I'm by far miles away from your skill level, but probably way better at that than I am at electronics. I've had a recording setup of my own for some 15+ odd years that was respectable, and I even recorded two different albums for two bands I was in over the years. Not sure you can call them amazing quality, but they're decent imho. Maybe some day I should share them with you to get a professional critique. I even had a full set of drum mic's at one point, that worked pretty well... mixing drums was HARD. At any rate, I digress.

~Phil

[norburybrook]

Phil a high pass filter is a low cut filter which is what we're looking at here. A low pass filter is a high CUT filter.


I think the misunderstanding from my point of view was using the point on the graph @4 or 10hz was confusing.


All good though , hope your amp works out well , and I'm sure the 10uf will be great. They'll probably be better if you're going to use a decent size 1 or 2x12. I think fender used 1.5k/25uF in those small , open backed combos that needed a little help in the bottom end.


M

[pompeiisneaks]

Phil a high pass filter is a low cut filter which is what we're looking at here. A low pass filter is a high CUT filter.


I think the misunderstanding from my point of view was using the point on the graph @4 or 10hz was confusing.


All good though , hope your amp works out well , and I'm sure the 10uf will be great. They'll probably be better if you're going to use a decent size 1 or 2x12. I think fender used 1.5k/25uF in those small , open backed combos that needed a little help in the bottom end.


M

Yeah my goal on this is to make it into a combo with an EVM12L in it... so I think bottom end won't be a problem.

D'oh that totally makes sense, just never put 2 and 2 together, that they're basically the same thing said a different way, when you pass high, that also by definition means you're cutting the lows. I probably knew that back in the day, but since I've done very little with audio in almost 10 years at this point, I'm getting bad...

~Phil

[pompeiisneaks]

Okay I've got some photo updates of the work I finally got done on this, this weekend. Layout seems to be working. I ended up with a 'doghouse style up top with that huge filter board.

BUT I've got an issue, maybe people have suggestions? One of the main boards screw holes happens to be right in the middle of the doghouse, so I can't install one or the other without some kind of 'trick' for that specific hole.

Do I just make a square hole and use cage nuts? Do I glue the screw in place somehow so that once it's 'there' it shouldnt slip too much? Or do I do my best to tighten it in place first, put the doghouse board in, then undo the screw, long enough to retighten it somewhat and add some locktite on it so it can't come undone even though it's a bit 'loosygoosy'?

The pic with the baord, the board is loose right now, just put it in for 'scale' purposes, but I've drilled all the holes it needs to mount.

I also have not yet (obviously) gotten to the tube socket holes and am awaiting an order of faceplates from indcom.graphics for the front and back panels so I'll do the drill holes based on that instead of my usual 'off by a bit' style

PXL_20210216_000127535.jpg

PXL_20210216_000124249.jpg

PXL_20210216_000039495.jpg

PXL_20210216_000029953.jpg

~Phil

[professormudd]

I think I understand your conundrum.

Kep nuts might help you tighten more without being able to hold the nut still by other means.

[sluckey]

I don't see the problem. DON'T USE ANY NUTS! Use these threaded standoffs to mount the board. Put locktite on the screws that fasten the standoffs to the chassis. No locktite on the board screws. Now remove the board but leave the standoffs fastened to the chassis.

Use self tapping machine screws or an actual tapping tool to tap the screw holes for fastening the doghouse to the chassis.

[didit]

Excellent progress Phil. As Steve suggests, why not use standoffs for your main board as there seems enough height inside?

Best .. Ian