Charlie Wilson wrote: ↑Tue Mar 16, 2021 6:39 pm
Tony, in a totally unrelated, derail, I think that amp the Russian guy in the video is playing through sounds great! Check out the two added resistors in the OD section that are in the gut shot in the video. Any ideas what those do? They go from the OD out to the two leads of the bypass caps.
CW
Yeah..I Thought so too! especially the clean side has some throat!!..I have seen that mod done in other amps going through some old pics last night. It's hard for me to see any values?..After work I'll dig those out and take a closer look and get back..Unless someone else has a shot and knows off hand?
(looks to be some kind of feedback loop at first guess)
Brown Dales and MEB's Sweet!
Tony
maybe parallel the cathode resistors to make the OD a bit gainier?
Possibly however one of those cathodes would be out of phase with the other? which would be negative feedback and help clean it up.
" The psychics on my bench is the same as Dumble'"
Charlie Wilson wrote: ↑Tue Mar 16, 2021 5:47 pm
Someone once said that Dumble would just cut off the goop components and all, and reinstall, if he had to fix something. I was incorrect about the wattage rating of the LCA0411, they are .55w.http://pdf.datasheetcatalog.com/datashe ... 8260_1.pdf. The other thing about the goop, I'm pretty sure that if he saw signs of the goop being removed you would not get your amp serviced ever again by him.
CW
Many thanks Charlie, there's been a lot to process today but this datasheet is Gold! I can use it to start scouting for parts with a similar spec now. Coming alongside all the other nuggets coming this way, I see what a labor-intensive process it is going to be. What fun
martin manning wrote: ↑Tue Mar 16, 2021 12:23 pm
Relatively massive components standing on wire leads (Orange Drop caps, say) are subject to resonant vibration and fatigue failure of the leads. There could also be some audible effect when vibrating components are spaced closely enough for some coupling to occur. Applying a bit of silicone can prevent this movement, and you can see that HAD has done that in some instances. Covering the whole preamp with it is unnecessary, though, and makes servicing the amp more difficult. I'm standing up and saying I believe it's for IP protection.
I completely agree!
As an amp manufacturer I would want to be able to have access to most of my preamp without having to spend several hours scraping goop from the thing.Especially in Dumbles case, being a custom amp designer to be able to go in and quickly modify or repair something or in the case of 124 upgraded from a Low plate classic to a Skyliner you would want that flexibility and there would be a good reason for gooping the entire thing..A dab of silicone can always be applied to the board to secure leads secure caps and prevent vibration, moisture from entering the cap through the lead extensions is recommended but not the whole thing.. Given the Precision resistors Dumble used in key spots and Flameproof Q-lines I would think under normal operating conditions heat would not be a big factor.
(For those that are interested? he uses what's called Permatex high dielectric RTV. For grins I gooped one similar to how Dumble does his.There was a slight shift in tone I would say would be similar to going from a maple neck guitar to a Rosewood was kind of what it reminded me of.This was back when I was really into Dumble amps )
BTW.My theory is that he was not only protecting the design but also what type of parts he used like Precision metal films on plates,cathodes..imo is a rather large part of the Dumble sound..Because anyone with a meter and cap tester and a little knowledge of guitar amp design can determine RP/RK values along with a few cap values where it's gooped.Again it's just my theory
Tony
Fascinating, "a slight shift in tone". I mean, I am only speculating it could alter the performance of the amp, but you actually observed it. Awesome! Taking everything into consideration and considering convection as well, I was thinking of a little experiment. Just to take 2 same value resistors +/-1% coat one in silicon and the other naked then throw them in the oven on a slow bake for 30 minutes or so. If they come out with the same degree of variation it proves the thermal isolation doesn't work, if they do, it says there's more to it... If I don't burn the kitchen down - don't worry, my wife already thinks I'm strange... Either way, it could answer this question positively. I suppose Permatex would be the silicon of choice for the discerning Dumble amp builder. Not you of course, you've grown out of them these days However, for anyone else that reads this, it's probably worth noting that this Permatex doesn't contain acetic acid (the smelly stuff in ordinary household silicon gunk) in fact, it appears to be something like an anaerobic gasket sealant. So, as my mum used to say, avoid the smelly stuff!
Charlie Wilson wrote: ↑Tue Mar 16, 2021 10:13 pm
Use the sensor safe Permatex.
CW
Good call. And in a completely unrelated derailment of my own, I use acetic acid to sharpen my files. Even brand new files can be made sharper by bathing them in it for longer or shorter depending on the number of teeth per inch. It will dissolve the teeth of a needle file clean off if you leave it in too long.
Charlie Wilson wrote: ↑Tue Mar 16, 2021 6:39 pm
Tony, in a totally unrelated, derail, I think that amp the Russian guy in the video is playing through sounds great! Check out the two added resistors in the OD section that are in the gut shot in the video. Any ideas what those do? They go from the OD out to the two leads of the bypass caps or maybe parallel the cathode resistors to make the OD a bit gainier?
CW
C.W
Here is the pic I was thinking about and it looks to be the same amp #38 It's hard to make out the values however If these are a high meg it looks as though he is grabbing signal there and feeding it back at the OD exit point. Since he is getting it from both cathodes one would be in phase and one would be out of phase since the output from the anode is inverted and from the cathode it is not..My guess here is that the effects from this circuit would be some kind of smoothing (or compression) circuit to the OD exit junction.It would be nice to know the value of these resistors and see how it works
Anyone else have a clue?..Bring it
Dumble-#40-(1).jpg
Tony
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" The psychics on my bench is the same as Dumble'"
talbany wrote: ↑Wed Mar 17, 2021 2:07 am
Here is the pic I was thinking about and it looks to be the same amp #38 It's hard to make out the values however If these are a high meg it looks as though he is grabbing signal there and feeding it back at the OD exit point. Since he is getting it from both cathodes one would be in phase and one would be out of phase since the output from the anode is inverted and from the cathode it is not..My guess here is that the effects from this circuit would be some kind of smoothing (or compression) circuit to the OD exit junction.It would be nice to know the value of these resistors and see how it works
Anyone else have a clue?..Bring it
Dumble-#40-(1).jpg
Tony
You might ask the person that took the picture to kindly go back and snap some better close-ups?
Suggest if it's feedback, and certainly appears so, then the signal is being picked off the OD output and inserted back into the cathodes of both halves of the OD circuit. To work those must be large value
resistors in the range of 22M.
Thanks Tyler
It looks as though the shots you have he's changed the value of cathode resistors to (530.17 ohms) 820/1.5k
BUT? he also changed the value of the plate resistors as well to 320k 100k+220k..If you take a look at #38 it appears he did the same to those plates as well..He's changed both RP and RK values..Someone want to do the math?
BTW.I think it's the same amp #38..
Tyler you got any more
Tony
Last edited by talbany on Wed Mar 17, 2021 4:08 pm, edited 5 times in total.
" The psychics on my bench is the same as Dumble'"
I've been following the recent posts about the OD smoothing mod of the #38/#40 with interest. I want to offer Charlie a warm public thank you for bringing this up and to reassure all of you that I am totally ok with the "derail". I confess, it's all a bit above my head yet but here is what must be some of the finest minds in the amp community coming together to explore the potential of this great amp. I don't own the internet and no one here is misappropriating or micromanaging anything. I see it all as pertinent, and deeply relevant to the topic under discussion so please, relax everyone, and just go for it!
If I may ask someone to post the link to the video that would be great because, truth be told, I'm busy working on the next iteration of the first stage at the moment and I couldn't immediately find it myself. Also, if anyone would like to include a schematic or anything you think might be relevant I can include it along with a brief description in the first post of the topic where future readers can easily locate it. If you PM me with that information, that might be the easier way to do it.
Sweet!
)
However, for anyone else that reads this, it's probably worth noting that this Permatex doesn't contain acetic acid (the smelly stuff in ordinary household silicon gunk) in fact, it appears to be something like an anaerobic gasket sealant. So, as my mum used to say, avoid the smelly stuff!
