Need help with building the right amp ( update)

[chikov]

Thank you John for help, i can't wait for my parts order arrive from Tubesandmore. It is interesting that in those Fender schematics where they do not use chokes, they use 2.5k resistors or higher, but the chokes range 125 D.C.R. (5G9) to 150 ohms. Of couse chokes have impedance, not just resistance and i am starting to get that. I just saw youtube video where the guy demonstrates on his occilscope that chokes flter AC ripple much better. I definitely will be using one of my chokes and 470 ohms 1W resistors in the screens like in Fender 5G9.

[chikov]

I decided to follow closely to this modified schematic, but as I began populating my chassis with parts, I ran into 2 questions...May be of you you could help me. On this schematic you can see the two 100 Ohm resistors in line of the 6.3 V filament circuit. What are they there for? Is it important? I use separate transformer for my filament power supply...
Second question, I am installing 155Ohms choke instead of 1k resistor in front of the B+ screen supply - will it work ok in this schematic? Should I install 430 Ohms resistors on each screen B+ supply as well?
Thank you...

https://tubeamparchive.com/download/file.php?id=28211

[martin manning]

The 100 ohm resistors are poorly drawn in that schematic. They do not go in line with the filament leads, they go from each side of the filament winding to ground. This technique is used if there is no actual center tap on the filament winding. If your filament transformer has a center tap, use that and omit the 100 ohm resistors.

Replacing the 1k with a choke should work fine. I would use screen resistors on the sockets either way, and 470 ohms is a typical value for those, but you can go larger. Measure screen voltage when you get it running. It should be at or just below the plate voltage.

[chikov]

The 100 ohm resistors are poorly drawn in that schematic. They do not go in line with the filament leads, they go from each side of the filament winding to ground. This technique is used if there is no actual center tap on the filament winding. If your filament transformer has a center tap, use that and omit the 100 ohm resistors.

Replacing the 1k with a choke should work fine. I would use screen resistors on the sockets either way, and 470 ohms is a typical value for those, but you can go larger. Measure screen voltage when you get it running. It should be at or just below the plate voltage.

Thank you for your answer - does not get better than that!
PS, I do have the center tap, so I am good to go.

[chikov]

I just ran into another dilemma... I decided to install the "totem pole" cap stuck before the stand by switch. does it mean I do not need the first 20 uf cap that is connected to the centertap of the output transformer's primary?

[chikov]

I guess, I found the answer myself:
http://www.thevintagesound.com/ffg/sche ... _schem.gif

[martin manning]

Yes. Slide the 20 uF cap (to be replaced by a stack of two 40 uF) at point A over closer to the rectifier and put the standby switch between the cap and point A. With a choke after the standby, the cap at point B will see a pretty high voltage spike when you switch the standby to play, so it would be a good idea to replace that with a stack of two 40 uF too, depending on how high your B+ voltage is. Might be better to just leave the standby switch out and let the rectifier filament slow-start the power supply as it warms up.

[chikov]

Yes. Slide the 20 uF cap (to be replaced by a stack of two 40 uF) at point A over closer to the rectifier and put the standby switch between the cap and point A. With a choke after the standby, the cap at point B will see a pretty high voltage spike when you switch the standby to play, so it would be a good idea to replace that with a stack of two 40 uF too, depending on how high your B+ voltage is. Might be better to just leave the standby switch out and let the rectifier filament slow-start the power supply as it warms up.

Thank you, will do. One more small question for tonight I bought two 220K one watt carbon composition resistors for the power tubes grid bias. but when I tested them with my tester they were about 190kOhms. I heard their value has to be precise... Can I still use them, or should I get carbon film ones that are EXACTLY 220K each?

[martin manning]

It's important for the two resistors to be matched, but the exact value isn't too critical. If they are both at 190k and within a few percent of each other then they are ok to use. Half-watt resistors would actually be sufficient there.

[johnnyreece]

Sorry for missing a lot of this discussion...Spring Break kept me busy. Just to agree with Martin, yes, I did use screen resistors on my build. Happy building!

[chikov]

Sorry for missing a lot of this discussion...Spring Break kept me busy. Just to agree with Martin, yes, I did use screen resistors on my build. Happy building!

Thank you. I already soldered in my Power Supply and starting on my Preamp and tone controls:
[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/IMG_3574_zpsjcaykmf9.jpg[/img]
The transformers are not attached yet, I just sat them on top to see how crazy it is going to look...

The inside of the 60 year old chassis does not look too pretty, but I try to watch for ground loops and keeping the preamp away from the power supply parts.[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/d85eb364-e5a8-495c-9367-74321c28cdf8_zpsjrkhrc07.jpg[/img]

[martin manning]

You said your transformers are not yet attached to the chassis, but the orientation you have them in is likely to allow magnetic coupling of the PT to the OT. You should rotate the OT 90 degrees and place them on the same center line to minimize that. With power applied to the PT you can listen to or measure the AC voltage on the OT secondary to find the best location for it.

[chikov]

You said your transformers are not yet attached to the chassis, but the orientation you have them in is likely to allow magnetic coupling of the PT to the OT. You should rotate the OT 90 degrees and place them on the same center line to minimize that. With power applied to the PT you can listen to or measure the AC voltage on the OT secondary to find the best location for it.

Thank you, Martin, for this advice. I forgot all about it! And, I did not know this cool trick about measuring the ac voltage on the scondary of the OT with the power on the PT. i mght have a coule of more questions on the fixed bias circuit. They seem a little dfferent on each schematic... I am not sure which one to use...

[chikov]

https://tubeamparchive.com/download/file.php?id=28211

I am getting close to the completion, but I just realized that I might not have the two 10uf caps that go on the fixed bias power supply. may I use the two of 20 uf 150V instead?

[martin manning]

Yes, should be fine.