#124 Questions

[Bombacaototal]

Just a couple doubts:
1) Any reason not to use the same grounding point for GND3 and GND4 (pic attached)?
2) when using a MONO 3 tip (tip, tip shunt, sleeve) jack on the FX Loop return (all dumble amps I’ve seen use 3 tip on send and 2 tip on return) do I have to connect the mid lug of the return jack to the ground lug? Or do I take the 2nd/3rd gen FX Loop out approach where the mid lug is just left alone and not connected anywhere? I am thinking the latter

[erwin_ve]

1. You dont want big currents from the center tap of the PT interfere with smaller currents. Therefore seperate grounding points!
2. Use the #124, #102 of #183 lay-out for seperate in and out jacks.
2nd gen with only 1 jack for both in and out is wired different.

[Bombacaototal]

1. You dont want big currents from the center tap of the PT interfere with smaller currents. Therefore seperate grounding points!
2. Use the #124, #102 of #183 lay-out for seperate in and out jacks.
2nd gen with only 1 jack for both in and out is wired different.

Thanks Erwin, very helpful indeed regarding the grounding of the PT. I will make sure they are separate

As far as the FX Loop I’ve checked all available layouts but my doubt arises from the fact that while Dumble uses a 2 tip mono jack on the send, I have a 3 tip and hence not sure what to do with the middle lug of the jack (pic attached)

[Bombacaototal]

Yet another doubt on the grounding, any reason why the 20uF/500V are grounded separately from the 300uF/350V?

Could I have all of them at GND2 for example? Or best would be to have the 20uF at GND2 and the 300uF at the same grounding as the PT?

I am currently grounding them all together (green wire) and supply board is under the chassis like a Fender Twin

[Bombacaototal]

And I think this is my last question (for now): I am fighting space here so no terminal strips for connecting the tube grids, I connected the tips of the coaxial and the grid resistor and protected with heat shrink. My concern is that the cables of my supply board (from previous post) are very close to the unprotected (end of the coaxial and hence un shielded) connection with the grid.

Will this exacerbate the noise, and should I try to move them further apart or should it be alright (pic below)

[martin manning]

As far as the FX Loop I’ve checked all available layouts but my doubt arises from the fact that while Dumble uses a 2 tip mono jack on the send, I have a 3 tip and hence not sure what to do with the middle lug of the jack (pic attached)

Just ignore it. To be clear, those and really all 1/4” jacks have only one tip contact, the one that connects to the tip of the plug.

Yet another doubt on the grounding, any reason why the 20uF/500V are grounded separately from the 300uF/350V?

Could I have all of them at GND2 for example? Or best would be to have the 20uF at GND2 and the 300uF at the same grounding as the PT?

I am currently grounding them all together (green wire) and supply board is under the chassis like a Fender Twin

This goes against the advice given by Erwin above, as well as the typical Fender Twin ground scheme. Run a separate lead from the ground end of the stacked reservoir caps directly to the PT center tap ground. I would separate the screen filter ground as well, as Dumble did, and ground it with the power tube cathodes.

I would be concerned about the proximity of that grid wire and the power supply leads. I think you can fit a terminal strip attached to the socket mounting screw to hold the grid resistors and coax.

[pompeiisneaks]

The reason to separate those filter caps is that the big first ones are for the main power tubes, and will have a lot of noisy current flowing through them, and the others are for the less current driven sections 2 and 3 (screens and PI) and then again the next group of filtering is for the preamp. Dumble did this and many other do as well, to keep the main biggest current, the anodes of the power tubes on their own, and then the second biggest current draw, the screens and PI together but separate from both the Anodes of the power tubes and the rest of the preamp.

~Phil

[sluckey]

I like to use a standoff turret to support grid stoppers. This one has a base with 4-40 threads. Simply screws onto the socket screw.

[Bombacaototal]

Many thanks Martin, Phil and Sluckey

I will leave the extra FX loop tip on its own and will also separate the grounds of the filter caps.

Will also give a fair distance between the power supply leads and the grids! The idea of the terminal lug is great too but my resistors are too large and I am not sure I would have enough clearance

About the grounding of the PI cathode would you have it on the power transformer ground, power tube grounds or at the fx loop jack as per Dumble

Many thanks

[Bombacaototal]

So everything fixed except for the coaxial/grid resistors which I will see what I can do due to lack of space and size of those 60’s resistors

One grounding point is close to the volume and it is used for the grounding buss bar, input and FX Loop jacks (which have isolation washers) and preamp circuit. I also added the grounding of a cap can used for both preamp channels (I have a fender stack on my amp too) given I saw Dumble used this grounding point for the tonestack 20uF supply cap

I am running a 2 power tube amp and another grounding point has the 1ohm resistors for the power tubes, the COM of the bias testing point (grey), the grounding of the bias pot and the grounding of the 20uF filter caps ie screen grids (green - pic below). I will add the PI grounding here too (390R/1uF) - I cannot use the FX loop jack for the PI grounding like Dumble as they have isolation washers and are using the preamp ground.

The last grounding point has the rectifier board, 220uF caps and the PT ground.

Please let me know in case the bias pot grounding should be separate from the filter caps, I saw dumble does them separately from the filter caps using GND2 and GND3 respectively on #102, #124 but given I only have 2 power tubes, I only have GND2. On the 50W 2nd gen/3rd gen he grounds the bias pot on the rectifier board

[Bombacaototal]

Almost forgot, the heaters are on yet another ground (pic below) - and by the way I do not have relays. The ground just next to it has the large supply caps (purple) and the PT (red+yellow). I am still missing to wire the rectifier board which will go there too. Should I merge this 2 grounding points or is it fine as is?

[Bombacaototal]

I am assuming the gounding described above is fine...so moving on to another thing that occured to me.

Given I do not have the FET on my amp, would it be appropriate to add a 150K to ground off the connecting points of the V1 low plates (2x 100K) to simulate the effect of having the FET as far as the supply cap goes?

[norburybrook]

stick the 150k to ground at the end of your dropping string on the power board.



MC

[Bombacaototal]

stick the 150k to ground at the end of your dropping string on the power board.



MC

Thanks Marcus, will follow your advice. Just for my uinderstanding, usually the FET B+ would connect to the plates of the tonestack and then this connection point would go to the power supply board. Any reason to add the 150K straight into the supply board?

I think I have a Metal Film 1/2W (Dale) in 150K...but wondering if it would be safer to get a 2W?

[norburybrook]

because it's generally a lot easier as there's plenty of space there


it's actually coming 'from' the power board to V1 then on to the FET. All the B+ come from the power board and onto their respective valves.



MC