Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 19, 2013, 02:41:19 pm

Title: TA and TU for pcl86 amp question
Post by: kagliostro on March 19, 2013, 02:41:19 pm

The schematic is this

(http://licho.rowerpower.org/gitara/practic2_lite.gif)

and the transformers are those of this stereo amp

(http://s8.imagestime.com/out.php/i838089_KitHiFi02.jpg)

(http://s8.imagestime.com/out.php/i838088_KitHiFi01.jpg)

TA 230VAC 100mA - 13.5V 1A

TU 5k - 4 and 8 ohm

the rectified 230VAC gives near 320V, the schematic say B+ 280v and the pcl86 datasheet say max 250V plate (300V for the ecl86)

http://www.r-type.org/pdfs/pcl86.pdf (http://www.r-type.org/pdfs/pcl86.pdf)

http://www.nj7p.info/Common/Tube/SQL/Tube_query.php?Type=6GW8 (http://www.nj7p.info/Common/Tube/SQL/Tube_query.php?Type=6GW8)

However I noted there is a big difference in cathode resistor value, the guitar amp 170R the HiFi amp 330R

How you can get to square the circle ?

Quote

EDIT: I forgot to say that in the HiFi stereo amp there was also a 5H 100mA 41.2ohm choke who is disposable for the guitar amp

Thanks

K

Title: Re: TA and TU for pcl86 amp question
Post by: PRR on March 19, 2013, 06:38:05 pm

> 320V ... 280v

From 280V to 320V, for the same dissipation, you must use a 1.3 times higher resistor.

The tube levers the cathode resistor to make an equivalent resistor.

So it makes sense to find a larger resistor. 330/180= 1.8 times higher does seem strange.

> datasheet say max 250V plate

Is the tube inexpensive? Can you change it yourself? Then don't worry too much.

Build it, use the larger cathode resistor, measure dissipation, and adjust.

> How you can get to square the circle ?

http://rationalwiki.org/wiki/Squaring_the_circle (http://rationalwiki.org/wiki/Squaring_the_circle)

Title: Re: TA and TU for pcl86 amp question
Post by: kagliostro on March 20, 2013, 12:42:52 am

Hi PRR

Many Thanks

Ok I can follow you about the change in value of the resistor because of the higher B+ voltage

where I'm still a bit confused is in relation with the OT primary impedance

I've seen schematics that say to use a 7k OT but the HiFi amp moreover the 322V B+ uses a 5k OT

in which manner this has effects on the circuit with the higher B+

---

As there is disposable (from the HiFi amp) the choke (5H 100mA 41.2ohm) I think this will help on tamer a bit the B+, there is math that can help to predict which will be an expected voltage drop ?

Or better, can I use the same math used for a 41.2ohm resistor ?


Thanks Again

K

one other "squared circle" :grin:

http://en.wikipedia.org/wiki/Vitruvian_Man (http://en.wikipedia.org/wiki/Vitruvian_Man)


(http://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Da_Vinci_Vitruve_Luc_Viatour.jpg/300px-Da_Vinci_Vitruve_Luc_Viatour.jpg)

Title: Re: TA and TU for pcl86 amp question
Post by: PRR on March 20, 2013, 08:16:52 pm

> can I use the same math used for a 41.2ohm resistor ?

Yes.

A good guess for load impedance is V/I of the tube. For 320V supply, 12W dissipation, about 300V across the tube, 12W/300V= 0.040A max current, and 300V/0.040A= 7,500 ohms. +/-30% is all the same.

Title: Re: TA and TU for pcl86 amp question
Post by: kagliostro on March 21, 2013, 01:16:57 am

Many Thaks Sir (http://www.diyitalia.eu/forum/images/smilies/salutations-489.gif)

K