Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on April 02, 2016, 03:57:57 pm

Title: PT Question
Post by: kagliostro on April 02, 2016, 03:57:57 pm

Today at a flea market there was a PT (a sealed PT) that had the CT of the High Voltage connected (internally) with one of the two windings of the 6.3v

externally similar to this

(http://i.imgur.com/mggAnPd.jpg)

with a connection like this

(http://i.imgur.com/ZreAMxI.jpg)

I suspected this wasn't a good feature and I don't purcased the PT

Was I wrong ?

Franco

Title: Re: PT Question
Post by: DummyLoad on April 02, 2016, 04:37:35 pm

for a single ended class a amp probably would have been fine. you ground the CT/ heater lead. look at champ schema/layout...

--pete

Title: Re: PT Question
Post by: sluckey on April 02, 2016, 04:46:55 pm

I would not buy it. You'll be forced to ground one side of the filament circuit. That's old school thinking. There are better ways to reference the filament to ground for hum reduction.

Title: Re: PT Question
Post by: darryl on April 02, 2016, 09:42:08 pm

If it has two 6.3 volt windings, and only one is connected to the HT centre tap, then the independent 6.3 volt winding could be used for the preamp valves, with the usual hum reduction techniques. The other 6.3 volt winding which has to be grounded on one side could be used for the output valve(s) where heater hum is not an issue.

If the price was right, it might be worth the challenge . . .  :wink:

Title: Re: PT Question
Post by: kagliostro on April 03, 2016, 04:45:03 am

Thanks Friends

I've think the same as Steve and so my decision

effectively there were two 6.3v windings but also more High Voltage windings and CT where connected to both 6.3v

Thanks

Franco

Title: Re: PT Question
Post by: shooter on April 03, 2016, 10:42:41 am

As an academic question, could you use the 6.3 WITHOUT a CT ground reference, and a FWB for the HV, using the bridge ground NOT the tranny CT?

Title: Re: PT Question
Post by: kagliostro on April 03, 2016, 11:46:38 am

Assuming to use a FWB you'll se the High Voltage doubled if you connect the minus of the bridge to ground

and give a look to this old Philips PA amp PS circuit


(http://el34world.com/Forum/index.php?action=dlattach;topic=11999.0;attach=24768;image)


(http://el34world.com/Forum/index.php?action=dlattach;topic=11999.0;attach=24770;image)


as you can see you'll have a voltage on the CT (1/2 of the whole AC voltage) and I don't think this will be positive if connected to one leg of the heater winding also if you keep the heater floating, this positive voltage will not be on a CT but on a side

I'm not sure what will happen but I don't think about in a positive way

Franco

Title: Re: PT Question
Post by: shooter on April 03, 2016, 03:39:47 pm

Quote

1/2 of the whole AC voltage

Thanks Franco, after I posted, came to the same conclusion probably not good for filaments at 1/2 HV potential.

Title: Re: PT Question
Post by: jjasilli on April 03, 2016, 05:21:19 pm

+1 to dummyload.  Or use a separate filament tranny.  Depends on price and subjective preferences.

Title: Re: PT Question
Post by: trobbins on April 03, 2016, 09:01:02 pm

K, was the PT fully sealed (eg. oil filled with ceramic bushings for terminals)?

If the internal wiring can be accessed then you could disconnect yourself - I recall doing that on one PT. 

Title: Re: PT Question
Post by: kagliostro on April 04, 2016, 01:18:13 am

I didn't purcased the PT and .... yes it was completely sealed

I don't know if inside the box there was oil or a plastic resin

Franco