Need help.

[Mike_J]

I am using my center punch to mark the holes for my next attempt at making a nice 5f6a build. Got to the V3 plate wires to the PIs .1uF caps and the grid wires coming out of the other end of the .1uF cap to the power tube grids. I can adjust the length of either side. Is it more important for the V3 plate wires to be shorter or the grids to the power tubes to be shorter or does it make a difference?


Thanks
Mike

[Mike_J]

While I have no scientific reason for it I am going with the shorter grid wires. The distance from the preamp PI tube socket to the power tube sockets is fixed. Nothing can be done about it now. Chassis is same essentially as what was used on the original '59 Bassman. Only very small adjustments on the turret board can be made. May not make a difference but I am leaning towards its best to make the grid wires as short as possible although I don't know that it would make a difference either way.

[glass54]

Mike_J
Bear in mind that the signal levels at PI outputs are in the order of volts (RMS) not 10's millivolts like the input to V1, I suggest you have no real problem to deal with 
Kind regards
Mirek

[Mike_J]

Mike_J
Bear in mind that the signal levels at PI outputs are in the order of volts (RMS) not 10's millivolts like the input to V1, I suggest you have no real problem to deal with 
Kind regards
Mirek

Thank you for the response Mirek. More reason to try to try to keep the grid wires short to the power tubes at the expense of the PI outputs. Not a huge difference but I read somewhere that a small difference in the length of a grid wire can make a significant difference in the stability of the amp. I cured an oscillation in the amp when I first built it with 1.5K grid stopper resistors. Am hoping to be able to get by with the 47 ohm grid stoppers the reissue amp uses or less. Time will tell. Adding grid stopper resistors is certainly not the end of the world just not best case scenario.

[glass54]

Mike
Just for clarity. I was suggesting you should be able to ignore lead length either from PI or to gridstoppers on output tubes as you are staying with Leo's basic and proven design/layout. Originally Leo didn't use gridstoppers on 5881 outputs but I would. (Marshall copied Fender and likewise had no gridstoppers on outputs. I know that 2203 Marshall had them, 5k6?) I am aware that Leo use 1k5 gridstoppers for AA165 Bassman (with 6L6GC)
I'm not sure where your 47R gridstoppers value comes from, as I would use 1k5.

Have a look at AA864 Musings where Sluckey has helped out with layout of that project. https://el34world.com/Forum/index.php?topic=26289.0. You can compare lead lengths of PI and Output tube grids. Theres some really good info in that link and it may help you.

And by the way, I would use minimum 1Watt Metal Oxide ("MOX") resistors for Plate loads at PI and 470R 3Watt MOX resistors for Screens in Output tubes.
Kind regards
Mirek

[Mike_J]

Mike
Just for clarity. I was suggesting you should be able to ignore lead length either from PI or to gridstoppers on output tubes as you are staying with Leo's basic and proven design/layout. Originally Leo didn't use gridstoppers on 5881 outputs but I would. (Marshall copied Fender and likewise had no gridstoppers on outputs. I know that 2203 Marshall had them, 5k6?) I am aware that Leo use 1k5 gridstoppers for AA165 Bassman (with 6L6GC)
I'm not sure where your 47R gridstoppers value comes from, as I would use 1k5.

Have a look at AA864 Musings where Sluckey has helped out with layout of that project. https://el34world.com/Forum/index.php?topic=26289.0. You can compare lead lengths of PI and Output tube grids. Theres some really good info in that link and it may help you.

And by the way, I would use minimum 1Watt Metal Oxide ("MOX") resistors for Plate loads at PI and 470R 3Watt MOX resistors for Screens in Output tubes.
Kind regards
Mirek

You may have been involved in another post where I asked at what value would a gridstopper resistor have to be before it impacts the audible range. Don't remember the exact value but it was a lot higher than 1k5. I am unreasonably stubborn but have heard the same thing from enough people I respect that I won't be one bit concerned about using the 1k5 gridstoppers. The source for the 47 ohms was the schematic from the Bassman reissue amp from the 90s. Supposedly from some good sounding original although I doubt very much the original had 47 ohm gridstoppers.


I use two watt metal films everywhere except the entry to the bias circuit, the power rail and the screens. Use three watt metal oxides as you suggested. Reason I use two watt metal films is read during my Dumble phase high wattage metal films reduce the hiss factor. Have started using two watts MF for that reason. The resistors I use are of similar color and size compared to the one-half watt AB carbon comps that I now use for their nice leads if I need to make a connection in one of my amps.


Thanks
Mike

[glass54]

Hi Mike
As discussed I am a fan of MF and MOX resistors esp in demanding applications (Hi Volts) and your are right, there are noise figure benefits.
I have purchased reasonable large quantities of both, values as used in Fender Schematics therefore got the benefit of good prices.
As for the 59 Bassman (1990s Reissue), that's the first time I have noticed the 47R grid resistors. Did FMI fix Leo's design?
I'd say not, because if you look at nearby components in the same schematic, I would use 470R 3W MOX for Screen Resistors and min 1W MF (or 2W MOX) for R28, R29 Plate Loads for V3. And I'd stay with 1k5 grid resistors  . Lots of of our colleagues have built hundreds (thousands) of Fender style Power amps following this basic 1k5 grid resistor configuration.
Kind regards
Mirek

[pdf64]

Whilst MF are intended for low noise applications and so great for use anywhere in an amp where resistor noise might contaminate the signal path, I’m no sure the same is true for MO. They’re typically intended for power resilience type applications, noise wise they might not be much better than CF or even CC.
Of course anything is better than a CC that’s gone noisy, but that’s not really a valid comparison.

[glass54]

Hi pdf,
For a little more clarity, I would use MF in front end design to minimize self generated noise. However for Plate resistors (typically 100K in Fenders) I would be happy to use Metal Oxide if I didn't have some MF 1Watt on hand,  but not really necessary. MOX are more useful in the Power Supply nodes (B2, B3 etc, due to their robustness in possible voltage spikes but they cost more than MF.
"The metal oxide film has excellent noise characteristics, next to metal films the best. Unlike other films it does not run the risk of being oxidized at higher temperatures". (From EPCI European Passive Components Institute experts original articles: https://passive-components.eu/resistors-non-wirewound-resistors-metal-film-foil-and-metal-oxide/).
As my supplies of CCs dwindled (a couple decades ago), I purchased a full range of 0.4W MFs for general use and 1 Watt for Plate Resistors.
Now I also keep a range of MOX and 3W/5W Wirewounds for Power Supply resistors.
...and that's my excuse for MF, MOX and Wirewounds. Its a bit of a golden age as we have a choice of high performance resistors in small packages at very reasonable prices 
Kind regards
Mirek