Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on January 31, 2019, 02:39:14 pm

Title: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on January 31, 2019, 02:39:14 pm
The output transformer of the Silvertone 1482 I am restoring has a 4 ohm and 8 ohm tap.


I would like to add an external speaker jack that would run the internal and external speakers in parallel off the 4 ohm tap, but run the internal speaker off the 8 ohm tap when the external speaker is not connected. Is it possible to do this with just one jack? I have and amp that disables the internal speaker when I plug an cable in to the external speaker jack but I can't think of how to make both speakers run in parallel AND switch between the leads.



If not, I can always cheat and simply add 3 speaker jacks. One for 8 ohm and two connected in parallel for 4 ohm. Just trying to be clever.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: Papa Jim on January 31, 2019, 04:50:50 pm
There are lots of jacks available with normally closed, or normally open switches. You can simply connect the sleeve of the jack to the - of the internal speaker, and the  tip of the jack to the + of the internal speaker. Then use the proper switch on the jack you select to do nothing but break the 8 ohm tap to the internal and connect the 4 ohm tap to it when the plug is installed while the speakers are in parallel.

Or one jack wired to the internal speaker with a separate single throw double throw switch that switches which tap goes to the tip. If it is switched wrong no danger I don't believe if a 4 ohm tap inadvertently gets connected to your internal 8 ohm speaker. If so others please chime in.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: jjasilli on January 31, 2019, 05:16:41 pm
Checkout the schematic of the Fender Hot Rod Deluxe.  It does exactly what you want.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 21, 2019, 08:08:06 am
Quote from: jjasilli on January 31, 2019, 05:16:41 pm
Checkout the schematic of the Fender Hot Rod Deluxe.  It does exactly what you want.
I moved, got a new job, death in the family, kids went to college, etc. Its been a busy 6 months but I'm finally going to wire this thing up so I'm resurrecting this thread.


Attached is the schematic for the FHRD. I'm confused by the connections. It looks like one jack has 4 connectors and one has 3. Can someone help me identify the parts I will need to buy to implement this?
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 21, 2019, 08:20:35 am
I think I found them here (https://www.amprepairparts.com/fenderhrdlx.htm). Parts J-12A and J-13A. Now I just need to decipher which connection goes where.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 21, 2019, 09:44:05 am
Anyone know what the wire labeled Gray in the diagram above is? NFB Loop maybe? I don't think the 1482 has any NFB.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: sluckey on August 21, 2019, 09:54:30 am
Yes, NFB loop.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 21, 2019, 08:07:15 pm
Now that I have the jacks on their way... where do I mount them? There isn't really much room on the back panel and the space that is available has the model number, etc that I would like to keep.

There is some space on the top/side (whatever you call it) near the power tubes. This would allow me to plug in the speaker more easily... I may have to re-route some wires on the inside of the chassis, but there isn't much going on there. Should I be worried about introducing hum?

Another thought I had was making a "chassis extension." Basically a small sheet of metal that I would drill holes into... one for each jack and then another that I would use to hold on the extension by threading the Footswitch jack through and securing with the nut. Maybe I can use two holes for the footswitch and the pilot light so it will be a little more reinforced. The problem with this plan (if it is a problem) is the output transformer wires would be on the outside of the chassis running past the power tubes.


Your advice and suggestions are welcomed and encouraged.



(https://blissamps.weebly.com/uploads/7/2/4/8/72487915/img-4525_orig.jpg)
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: Tony Bones on August 21, 2019, 09:03:58 pm
Would they fit around the corner from the model number? On the same surface that the tubes and transformers are mounted.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dude on August 21, 2019, 10:35:09 pm
Your idea of hanging a piece of metal  off the foot jack seems like a good method, won’t butcher the amp and easy to do. Use a piece of metal thick enough to support the two shorting spk jacks. That schematic would give you one 8 ohm spk on main jack on 8 ohm tap and when using another 8 ohm spk the 4 ohm tap would be used.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: pdf64 on August 22, 2019, 01:55:43 am
The pictured amp looks to be in dire need of a safety ground for its chassis 
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 22, 2019, 08:14:01 am
Quote from: pdf64 on August 22, 2019, 01:55:43 am
The pictured amp looks to be in dire need of a safety ground for its chassis
That's a before picture. I have since upgraded to a grounded cord.
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 22, 2019, 08:26:22 am
Quote from: dude on August 21, 2019, 10:35:09 pm
Your idea of hanging a piece of metal  off the foot jack seems like a good method, won’t butcher the amp and easy to do. Use a piece of metal thick enough to support the two shorting spk jacks.
Another challenge with the "outrigger" that I have found is there is not a lot of thread left on the footswitch jack. So, using a thicker piece of metal will not allow me to screw on the nut. I could get a longer threaded jack.


Another thought I'm having... what if I mounted a jack plate on the rear panel (I'm making a custom cabinet anyway). It might be weird to have the combo speaker wire run to the outside to plug in and it would make working on the amp more challenging... but it might work.


(https://www.parts-express.com/Data/Default/Images/Catalog/Original/260-287_HR_0.jpg)
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: dbishopbliss on August 22, 2019, 03:23:46 pm
I just thought of a simpler solution (I think it should work). Rather than having two jacks, I will only have the Switchcraft 13A (Tip/Sleeve with transfer switching) jack installed into the back panel of the cabinet. Then I will make the following connections:
This way, when no external speaker is plugged in the 8 Ohm connection will go to the internal speaker. When I plug in an external speaker, the open shunt connection will be made and the shunt connection will be broken thereby connecting both speakers in parallel to the 4 Ohm tap.

The only drawback (which I don't think is an issue) is the internal speaker will always be connected. I don't really plan on running this amp as a "head" and only wanted to have an extension speaker to add a little variety.


Please let me know if there are any issues with doing this.



I will run the 8 ohm OT wire to the Shunt tab
Title: Re: Can you switch between output transformer taps with a speaker jack?
Post by: shooter on August 22, 2019, 03:46:02 pm
Quote
•Internal Speaker + to Tip tab of the Jack

don't see a problem except the "normal" problem with any head unit, IF the tip
sticks open with no speaker plugged in.  low probability chance anyway