Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on September 25, 2012, 07:24:19 am

Title: Rotary switch on OT secondary - How ?
Post by: kagliostro on September 25, 2012, 07:24:19 am

Is possible to use a rotary switch for the connection of the outputs of this OT ?

This are the stated connections

2 ohm = 3-7 / B-F

4 ohm = 2-6 / C-G

8 ohm = 3-8 / E-B

16 ohm = 2-8 / E-C

32 ohm = 1-5 / D-H   
 
128 ohm – 70v = 1-8 / E-D

I tried to understand something drawing a different schematic for each impedance
but also this didn’t helped me

I'm not able to understand how to connect a rotary switch

and also if the requested rotary switch will be a standard type or if a special unobtainable version will be required

K

Title: Re: Rotary switch on OT secondary - How ?
Post by: jjasilli on September 25, 2012, 10:34:50 am

I don't think a rotary SW is feasible.  You'd need a multistage SW; I'm not aware of any rotary SW's that can handle the power.  Usually they're rated only around 1/2 watt.  But there might be something out there. 

If you can live with 8Ω and 16Ω's, that's pretty simple, because they both tie the 2 Nero leads together.  Use either 2 output jacks, or a double pole SW to one jack for the outer leads.  You could use another double pole SW to "untie" the Nero leads and send them to:  i)  giall & giallo:  this would SW the 16Ω output to 4Ω;  or ii)  blu & blu:  this would SW the 8Ω output to 2Ω.

So, 2X double throw SW's (and maybe 2 jacks) could give you 2, 4, 8 & 16Ω's.  I think that would be the simplest solution for guitar, assuming you don't need the other output impedances.

Title: Re: Rotary switch on OT secondary - How ?
Post by: PRR on October 01, 2012, 01:12:01 am

This will work 2-4-8. There is a power-loss but inaudible.

Title: Re: Rotary switch on OT secondary - How ?
Post by: PRR on October 01, 2012, 01:18:50 am

This *may* work, inaudibly better; but if I or you get it wrong, the amp will be shorted.

Title: Re: Rotary switch on OT secondary - How ?
Post by: kagliostro on October 01, 2012, 09:47:08 am

Thanks Jjasilli & PRR

Jasilli

I think I've understand you (I'll try to draw a schematic as to see if I've understand correctly)

PRR

That is a very interesting solution, I hope I'm near to understand why you do that

About your second choice as to be sure about the connection and as to prevent to fry anything

Can I use a 50Hz voltage (from a transformer) on the primary and then measure output voltage on the secondary, connecting in parallel winding of one group to winding of the other group (as from your schematic) can I measure the output voltage and see if I've connected the winding in the right way ?

Thanks

K

Title: Re: Rotary switch on OT secondary - How ?
Post by: PRR on October 04, 2012, 11:38:26 pm

> Can I use a 50Hz voltage (from a transformer)

Through a series resistor!

This is a tube amp? Try 6VAC through 100r to the plate winding. When connected right, you find over 5VAC at primary and around 0.2VAC at speaker tap. If wrong, there's less than 1V at the primary and the other 5V is heating the resistor.

Title: Re: Rotary switch on OT secondary - How ?
Post by: kagliostro on October 05, 2012, 02:00:51 am

Many Thanks PRR

I knew you knew the solution to this problem   :grin:

Thanks again for your precious council  :thumbsup:

K