#40 resistor choice

[fred.violleau]

Hey guys,

I am on my way to build a clone of #40, (2nd gen I think).

I have invested in some old resistors a little while ago.
Despite my efforts to find most of the desired ones, I still miss a few values and that's where I would like to leverage the community's knowledge :

All the plates resistors are listed as Dale RN65 or MEPCO MF6C. I could not find any of those, but got my hands on MEPCO RLR 20C1003GS, would these be a good replacement part?.

#40 - 16.jpeg

I also see that the 820R NFB resistor is a lot bigger. Is it rated at 1 watt or 2 watts ? rather than 1/2 a watt ?
Same for the 1Meg and the 100k, what is the rating of those ? 1 Watt? I read on this forum that using higher rating for the plates and cathode bypass resistors would decrease hiss noise and would darken the sound a bit. What is your take on this ? Does anyone knows what manufacturer is that huge 1 meg resistor on the output of the overdrive?

#40 - 20.jpeg

Are some of those grid resistors carbon comp? I have a hard time identifying the type for V1B, V2A and V2b that kind of look like carbon comp to me...

Thanks again for you tremendous help and knowledge,

Fred.

[erwin_ve]

Hi Fred,

The 820R is a 2w.
Most brown resistors are Piher carbon films, btw Beyschlag looks the same, dont know if it is similar. A carbon comp typically has no bulbed edges.
The huge resistor on the v2b overdrive exit looks like a piher; like this one; https://radio-antigua.com/vintageoldpih ... anguage=en

[Charlie Wilson]

820 ohm is a 1w Piher and OD out is a EKC molded 1 meg metal film.
CW

[erwin_ve]

820 ohm is a 1w Piher and OD out is a EKC molded 1 meg metal film.
CW

Ignore my post, CW knows

[fred.violleau]

Thanks erwin_we and Charlie Wilson for your valuable feedback!

A quick question about the PI trimmer we see on later models, I really enjoy these to fine tune the mids and harmonics, would it be a good addition to this circuit?

Fred.

[erwin_ve]

Thanks erwin_we and Charlie Wilson for your valuable feedback!

A quick question about the PI trimmer we see on later models, I really enjoy these to fine tune the mids and harmonics, would it be a good addition to this circuit?

Fred.

IMO yes!

[fred.violleau]

@Erwin_we, thanks, added the PI trimmer to my layout.

Another question about the choke used on #40
It is documented at 4H and 90mA DC.
If I was to use a 4H, 50 mA one, would it still fit the duty or would it change the feel of the amp?

EDIT
Reading on aiken's website about choke sizing : https://www.aikenamps.com/index.php/chokes-explained
"If, on the other hand, you are selecting a choke for a capacitor input supply (such as the typical Marshall or Fender design), then the requirements are relaxed quite a bit. The purpose of the choke in these type supplies is not for filtering and voltage regulation, but just for filtering the DC supply to the screen grids of the output tubes and the preamp section. The screens typically take around 5-10mA each, and the preamp tubes draw about 1-2mA or so (for the typical 12AX7; 12AT7's are usually biased for around ten times that). This means that you can get by with a much smaller choke, and, in addition, the preamp supply current doesn't vary that much, so you can get by with a higher DC resistance, which means smaller wire can be used to wind the choke, which means higher inductance for a given size core. Just add up the current requirements of the screens and preamp tubes, and add a bit more for margin. For a 50W amp, a typical value might be 50-60mA."

It looks like I am on the safe side with a 50mA choke, that leaves feedback based on the feel difference between a 50mA or 90ma choke

Fred.

[fred.violleau]

Moving in slowly, waiting for some missing parts.

PXL_20211116_225257047.jpg

Fred.

[Charlie Wilson]

Notice on the 2nd and 3rd Gen amps, Dumble orientated the cap outer foil to the pots. I don't know if you are going to like those rn60 100ks. PM me your address and we'll get you some plate resistors.
CW

[fred.violleau]

Hey Charlie, very interesting detail on the outer cap foil orientation,. Much appreciated, I was not aware of that. Doesn't it goes against the rule to get the quietest amp? I read that Erwin enjoyed the mid cap the opposite way. So on this build every outer foil cap is towards the pots or only v1 and V2, and PI stays the same?

Fred.

[Charlie Wilson]

Hey Fred, to be honest with you, I hear no difference in noise level with cap orientation. I do however hear a tonal difference. To give an example. I was working on an early seventies Fender Showman reverb with the marron/brown Paktron capacitors. Now in those amps, for some reason, they decided to have all the coupling caps orientated towards the pots. The opposite of the sixties amps with the blue molded. I decided to "fix" the amp and orientate the caps towards the tubes(except for PI) and the amp sounded horrible. I think those particular caps don't have a good sounding midrange, so if you push the mids by having the caps going towards the tubes they sound bad. Nice orange drop caps can sound good either way but sound different. Maybe Dumble was trying to scoop the mids a bit and push the top end by having the caps go towards the pots. So in your amp, I would try having all of the caps going towards the pots including the PI, which are probably going that way already. Having said all of that, there is no right or wrong. You may like it better the more traditional way towards the tubes. This is where the "art" of amp building comes in. Play around with it and find what you like. Do this before you start gluing the caps to the board. Got your PM and will send you some stuff to play with.
CW

[fred.violleau]

Hey Charlie,

Again great insights here, thanks for sharing. Since I started my Dumble journey, I tend to look for fuller and meaty mids. I realised I was battling a lot with the guitar (and loosing my fingers )with more scooped amps like traditional fenders. These are fantastic for chimey clean sounds, but then I start to fight on the OD side. I will build the amp with all the outer foils opposite as what I am used to doing and see (hear) where it takes me.

Speaking of pushing the highs, I see this version of the preamp use two mica caps. I used these on many amps and always found the highs to be harsh compared to ceramic quality ones. I have ordered a mica to see if I like it but I have a Vishay X5F as a backup It feels like a relic from the fendery design.

Fred.

[talbany]

IMO You can sometimes get away with using a good Mica in a 2nd gen since those amps were loaded with Pihers & AB's.Dumble switched over to the Ceramics when he went with the Q-lines in the later 4th generations. (there may be a few exceptions as the case in 102)
I would try both
Tony

[fred.violleau]

Finally received my beloved faceplates from Steve, once again, great job! Let the drilling begin!

PXL_20211229_161529505.jpg

Covered the headshell with some tweed in the meantime. Will add a few coats of shellac to give it that aged look.

PXL_20211229_162122739.jpg

Happy holidays!

Fred

[bepone]

only my 2, i found pihers too dark sounding, will be interesting how this amp will behave..i quit using brown pihers because of that and also of too big price for not desired effect. although i have wide colections still somewhere