Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 05, 2020, 11:43:31 am

Title: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: kagliostro on March 05, 2020, 11:43:31 am

I've a doubt

If a new transformer is planned without High Voltage CT, is correct to plan an itake for the Bias PS or it must be used a separate winding for it

(wanting to exclude the solution of taking voltage from one of the two high voltage branches with a capacitor)

I'm not sure about, because I remember something about problems with an intake, but don't remember well


Looking here

https://sound-au.com/valves/design2.html (https://sound-au.com/valves/design2.html)

seems that the correct way is a separate winding for the bias supply

Thanks

Franco

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: sluckey on March 05, 2020, 12:39:30 pm

Look in my amp scrapbook.

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: kagliostro on March 05, 2020, 12:57:47 pm

Ciao Steve

I remember your scrapbook bias page

what I'm not sure about is about the following circuits

is the separate winding the only correct one ?

(https://i.imgur.com/y4BuM2v.jpg)


Thanks

Franco

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: sluckey on March 05, 2020, 01:12:28 pm

Use a completely separate winding

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: 2deaf on March 05, 2020, 01:17:16 pm

The bottom one with the 32V tap in the 250V winding places a direct short through the diodes on the secondary winding.

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: kagliostro on March 05, 2020, 04:29:05 pm

OK Steve, Thanks

Thanks also to you 2deaf

Franco

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: tubeswell on March 05, 2020, 05:35:53 pm

If you haven't got a separate winding for the bias supply and you have a FW bridge HT winding, then you can make a capacitor coupled bias supply from the HT winding. (See attachment from Merlin's site)


Otherwise get a PT with a seperate bias supply winding

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: kagliostro on March 06, 2020, 02:31:12 am

Ciao Tubeswell

Thanks for the answer

I know that solution, I was only wondering which will be the correct option planning a new PT

I remembered something about the (bad) choiche if the bias will be from an intake on the HV winding (a thread I've read somewere a lot of time ago), but didn't remembered enough to be sure about that

So, the correct way, with no CT HV winding, is to have a separate winding for the bias (or the way you pointed to)

Franco

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: pdf64 on March 06, 2020, 03:37:27 am

Quote from: kagliostro on March 06, 2020, 02:31:12 am

Ciao Tubeswell

Thanks for the answer

I know that solution, I was only wondering which will be the correct option planning a new PT

I remembered something about the (bad) choiche if the bias will be from an intake on the HV winding (a thread I've read somewere a lot of time ago), but didn't remembered enough to be sure about that...

For our purposes, I can’t see a rational argument to consider that the ‘separate bias winding’ arrangement is correct, and the cap isolated method isn’t.
If the amp is to be used in both 50Hz and 60Hz regions, then a separate bias winding is preferable, as ideally, with the cap isolation method, a bias check should be done in both regions, as the different supply frequency will alter the ratio between the HT and bias supply magnitudes. Obviously that’s over and beyond any necessary supply voltage adjustments.
But Marshall are happy use the cap isolated method without recommending such a check.
And the % of their tube amps that get used across regions is probably much higher than those made by DIYers etc such as ourselves  :icon_biggrin:

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: kagliostro on March 06, 2020, 05:10:47 am

Ciao Pdf64

Quote

I can’t see a rational argument to consider that the ‘separate bias winding’ arrangement is correct, and the cap isolated method isn’t.

I too

my concern was only about no CT HV winding, with intake for bias or separate winding for bias


like here


(https://i.imgur.com/y4BuM2v.jpg)

Franco

Title: Re: Power Transformer without CT - Bias intake ?? (Don't remember well ....)
Post by: pdf64 on March 06, 2020, 07:13:05 am

Ok! :thumbsup:
Where did you find that schematic image? The 2nd one needs deleting from the face of the earth, some noob might try it and smoke their PT  :rolleyes: