High Power Tweed Twin PT

[mad_dog]

I've had the thought of using a High Power Tweed Twin (5F8a) PT, so that you can use the 5v rectifier tap to power the relays, compared with a Blackface PT that didn't use a tube recto hence no 5v taps.

The questions I have are 1. The Heyboer PT that Mojo sells has no CT on the filaments, is there any advantage or disadvantage to using the 2 100R resistors to ground rather than a CT grounded???

And other than space requirements is there any advantages to using the 5v tap rather than a separate 6.3v PT???

[ChrisM]

The questions I have are 1. The Heyboer PT that Mojo sells has no CT on the filaments, is there any advantage or disadvantage to using the 2 100R resistors to ground rather than a CT grounded???

Well the BF iron HAD used generally didn't have a CT either. He created one using two 120R resistors.

And other than space requirements is there any advantages to using the 5v tap rather than a separate 6.3v PT???

Convenience

[llemtt]

is there any advantage or disadvantage to using the 2 100R resistors to ground rather than a CT grounded???

actually no, but +/-2 on components count

is there any advantages to using the 5v tap rather than a separate 6.3v PT???

I can't see any

cheers
teo

[mad_dog]

Actually the more i read into this, the 2 resistors to ground has some advantages that they can be more closely matched than the CT. I guess I just threw the question out the as the last few builds have all had a CT.

[talbany]

is there any advantage or disadvantage to using the 2 100R resistors to ground rather than a CT grounded??

I like to use the 2 100 ohm-rs for several reasons.. the main one being a Heater cathode short or any other heater related shorts as this takes out the resistors instead of the transformer (In theory).. The other is like mad dog says better balancing although I have never had any issues with hum using a CT..
Tony

[David Root]

I never had any hum issues with either CT or 2x100 ohm resistors, sooo... why is there a hum balancing pot in the original SVT, for example? They're usually burned out when I've seen them anyway, in that amp.

[mojotom]

From my (short) study of Audio Note Hifi tube system they seems to prefer using artificial CT for hum reduction. I recently rewired my Express and will try using 100R, maybe it will reduce hum. Using tantal 1% resistors allow for a more precise CT so less hum in return said one of theirs techs.

From another point of view it's a nice fuse in case power tube short. Ten years back when I bought my old time favorite Ampeg V4 it was humming like crazy, went to a tech, spend a lot but the hum was still there. I opened the amp and thanks to Weber book change the 100R pot and voila no more hum. If one tube short the pot take the blame, not the transformer, clever design.

I didn't use the CT on my 124 either and after a few hour one 6l6 wingled C got a short, the resistors got burnt (I was very anxious when it happened) but change them and the tube and nothing else is screwed and the amp was back on track.

[Bob-I]

I recently rewired my Express and will try using 100R, maybe it will reduce hum. Using tantal 1% resistors allow for a more precise CT so less hum in return said one of theirs techs.

I usually just match the resistors with a DMM. Since we're going for balance, not precision resistance this seems to work just fine, I've heard a difference when the resistors are closely matched.