Matchless Hot Box question: proximity of tubes to PT?

[MikeH]

I'm currently working on a hot box build, and my current layout has the tubes nestled next to the power transformer.  I know that in high voltage tube amps there's some sort of issue with this, but is that just with output transformers?  Does that apply in this design?

[bancika]

It's not a great idea to have anything that carries the signal close to the power transformer. If you have enough space try to move it as far away as possible, on the output side of the circuit (input stages are more prone to noise). Try to shield the transformer and use tube shields...
just my .02$

[MikeH]

Cool- how would one shield the transformer.  The more I think about it, there's really no place inside my enclosure to put the PT that's 'away' from signal path.  Pretty tight in there

[MikeH]

Hey banicka, I spent a good chunk of the afternoon reading your build report on your hotbox variant.  One big question: It looks like you're running your heaters on DC- is that right?  If so, I didn't realize you could do that.  It's handy that way though, because you can use LED indicators.

[bancika]

you can wire led on AC but it will blink 60 times a second, bit annoying
anyways, running DC heaters isn't easy but pays off if you want really quiet build.
First you need to have few extra volts, if you want to go with 6v you need 9vac or more on the transformer. Then rectify with bridge rectifier, filter with 2200uF or better, regulate with 7806 (heat sink it), filter with another 2200uF parallel with smaller film cap (22nF or so). That should do the trick...

and to shield the transformer just put a piece of metal in between and make sure it has contact with chassis...it's not ideal but better than nothing.

[MikeH]

Ah I see.  Not an option for me as my PT has 6v secondaries.  I actually got the exact PT that matchless used.  Some small parts dealer near by me bought up a bunch of them when matchless stopped making the hotbox.

[petemoore]

  Although it's neater to finish the chassis before the amp, you could use a grounded plate of shielding and wave it where you think it'll shield to see if whatever noise source you have is actually alterable with shielding between the PT / Tube relationship.
  I've seen links with humungous amps packed with parts like a case of cola on a petite chassis, everything elbow to elbow.
  I was faced with finding a chassis more like 19'' long, or build a 5e3 in an 11'' long chassis. Starting with long leads on the PT, waving it around to find the angle-position that tunes out the noise, the OT ended up on the back panel for space/positions sake, it's pretty non-noisy, like my second 5e3 build with normal 19'' large chassis.
  A 4xAA battery pack to get an easy alternate DC6v, they don't last too long, but allow the heaters to be purely DC'd for assessing how well the power supply system for the heaters does as far as noise.

[bancika]

I recommend doing DC elevated heaters with faux center tap, that will help with heater noise greatly...

[MikeH]

I recommend doing DC elevated heaters with faux center tap, that will help with heater noise greatly...

What if my secondary has a center tap already?

[bancika]

you don't need virtual tap then just connect positive DC reference to heater CT and you're all set. 50-100V should do the trick.

[frequencycentral]

@ MikeH: I hate it when people I 'recognise' change their avatars.   

[MikeH]

@ MikeH: I hate it when people I 'recognise' change their avatars.   

I know!  I hadn't planned on changing it, but my profile picture 'broke' and I couldn't find original to reupload.  Sorry bud.