Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Hellawatt on June 21, 2017, 08:59:52 pm

Title: What to do with unused 5V PT secondary wires?
Post by: Hellawatt on June 21, 2017, 08:59:52 pm: Hi. I am using an old Hammond organ PT from an ao-35 for a plexi 6V6 build. What is the best thing to do with the unused 5V secondary wires that went to the tube rectifier filament? Do I just isolate them or should they be connected to each other?
Title: Re: What to do with unused 5V PT secondary wires?
Post by: John on June 21, 2017, 09:11:53 pm: NOT connected, isolated. Heat shrink the end on each one, then tuck 'em away in the corner of the chassis somewhere.
Title: Re: What to do with unused 5V PT secondary wires?
Post by: Hellawatt on June 21, 2017, 10:01:08 pm: Got it. Thanks!

Just out of curiosity. What would happen if they were connected?
Title: Re: What to do with unused 5V PT secondary wires?
Post by: sluckey on June 21, 2017, 10:51:29 pm: Quote
What would happen if they were connected?
If lucky, you just blow a fuse every time you turn the power on. If not so lucky the insulation on the 5V wires would quickly melt followed by smoke from the power transformer.

I built a Plexi6V6 from a Hammond AO-43 and used a 5AR4 rectifier tube instead of silicon diodes.

http://sluckeyamps.com/6v6plexi/6v6plexi.htm (http://sluckeyamps.com/6v6plexi/6v6plexi.htm)
Title: Re: What to do with unused 5V PT secondary wires?
Post by: Leevi on June 22, 2017, 02:47:32 am: Quote
What is the best thing to do with the unused 5V secondary wire

I normally connect the unused 5V to pilot lamp. The 6.3v lamp will shine brightly even with 5v and lasts longer.

/Leevi
Title: Re: What to do with unused 5V PT secondary wires?
Post by: John on June 22, 2017, 09:44:09 am: Quote from: Leevi on June 22, 2017, 02:47:32 am
Quote
What is the best thing to do with the unused 5V secondary wire

I normally connect the unused 5V to pilot lamp. The 6.3v lamp will shine brightly even with 5v and lasts longer.

/Leevi

That's a great idea!
Title: Re: What to do with unused 5V PT secondary wires?
Post by: Leevi on June 22, 2017, 10:05:12 am: If you plan to have relays in the amp the 5V AC can be used as a source for DC voltage.
After rectifier you'll get about 7.5V DC which is suitable for 5V relays.
/Leevi
Title: Re: What to do with unused 5V PT secondary wires?
Post by: PRR on June 22, 2017, 01:16:53 pm: > What would happen if they were connected?

5V, "zero" Ohms, "infinite" current!! Smoke!

In a real PT, the Ohms won't be zero. Figure a PT can dump short-term about 10X its rated current. So say 30 Amps will flow, from a nominal 5V source. 150 Watts of heat inside the transformer. Assume a 100 Watt transformer, which was designed to throw-off 10 Watts of heat on the way to a 100W happy output. 15 times the design heat in the windings.

It is a smaller version of what happens if you short-out the 120V/230V from a wall outlet. BOOM.

That's why we fuse wall-outlets and transformers.
Title: Re: What to do with unused 5V PT secondary wires?
Post by: BrownIsound on March 27, 2023, 01:28:24 pm: Here’s another potential use for the 5V tap (I have not tried this myself).

http://www.ws.binghamton.edu/fowler/How%20To%20Use%205V%20Tap%20to%20Convert%20from%20115V%20Primary%20to%20120V%20Primary.pdf
Title: Re: What to do with unused 5V PT secondary wires?
Post by: dude on March 27, 2023, 02:25:01 pm: That AO-35 PT used a 5Y3. Not sure of the rating on that PT but I don't consider that PT a good choice for a fixed biased 6V6 Plexi (even with SS rectifier) you won't get the B+ needed for a real 6V6 Plexi. If you still want to use the AO-35 PT, use diodes and Cathode Bias instead of the 6V6's fixed bias, your B+ will just about right for a Cathode Biased 6V6 Plexi.

And I built a 6V6 Cathode Biased Plexi, nice amp, especially for Blues.