Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: tmknight on September 04, 2021, 04:03:52 pm

Title: Power SS circuit from AC Bias tap
Post by: tmknight on September 04, 2021, 04:03:52 pm

I've been racking my brain over this, researching, trial and all error.  Finally posting for assistance.

The problem:  I want to create a SS circuit to drive a spring reverb tank.
The (proposed) solution: Leverage the 25VAC Bias tap from the power transformer.

I've created a circuit inspired by RR (https://robrobinette.com/Compact_Reverb.htm#Solid_State_Spring_Reverb), however, I either get almost no output or too much and end up popping capacitors.

Attached is a depiction of the 25VAC Bias tap of my transformer as well as the proposed circuit a-la LTSpice.

Should I attach both the 25V and the 0V to the circuit or should I use only the 25V and go to ground?  I've done both and neither achieves what LTSpice generates.

What am I missing? I'm about to hack and gerry-rig an old wall-wart and be done with it...

TIA

Title: Re: Power SS circuit from AC Bias tap
Post by: PRR on September 04, 2021, 09:07:47 pm

Welcome.

25V AC is an odd value for a bias tap; are you sure?

Bias *tap* suggests this winding has other connections. Very often it is a tap on the 350VAC winding.

Show the WHOLE circuit, as-built.

Title: Re: Power SS circuit from AC Bias tap
Post by: tmknight on September 05, 2021, 09:04:58 am

Thanks for the reply.  Full schematic attached.

Title: Re: Power SS circuit from AC Bias tap
Post by: sluckey on September 05, 2021, 09:17:08 am

That's not a schematic.

Title: Re: Power SS circuit from AC Bias tap
Post by: tmknight on September 05, 2021, 09:33:52 am

Doh!  Fixed...

Title: Re: Power SS circuit from AC Bias tap
Post by: sluckey on September 05, 2021, 09:50:53 am

You can't use the PT bias winding to develop the +16v and -16v you need because your circuit requires one lead of the bias winding to be connected to ground, but the voltage doubler bias supply of that amp requires that both leads of the bias winding must be ungrounded. Sorry, but you will need a separate transformer for your circuit.

Title: Re: Power SS circuit from AC Bias tap
Post by: tmknight on September 05, 2021, 09:56:20 am

Thanks for the reply; that makes sense based on my results.  Have been thinking about the use of an HVAC 24V transformer (http://media.diversitech.com/doc/DOC132907.pdf), thoughts?

Title: Re: Power SS circuit from AC Bias tap
Post by: sluckey on September 05, 2021, 10:52:00 am

They're cheap and plentiful. I've use a lot of them for low voltage power supplies. Be sure to get a 120:24 if you are in the states. One more thought... Drop that zener regulator circuit and use a 7815 and a 7915. Cheaper, simpler circuit, and much better regulation.

Title: Re: Power SS circuit from AC Bias tap
Post by: tmknight on September 05, 2021, 10:55:12 am

Excellent input, sluckey; cheers for your insight!

Title: Re: Power SS circuit from AC Bias tap
Post by: sluckey on September 05, 2021, 11:05:26 am

No cheers yet! The circuit I'm thinking of requires a center tap on the 24V secondary. Might be hard to find in a HVAC control transformer. Here are some good low voltage PTs with center tap on secondary that would be great for a bipolar supply...

     https://www.tubesandmore.com/products/transformer-hammond-low-voltage-filament-open-36-vct

Title: Re: Power SS circuit from AC Bias tap
Post by: PRR on September 05, 2021, 01:36:08 pm

Regulators. Center taps. For one lousy opamp?

This will work.

Title: Re: Power SS circuit from AC Bias tap
Post by: tmknight on September 05, 2021, 02:26:32 pm

Thanks, PRR.  The challenge I have is a lower AC source.  Seems my existing bias tap is unsuitable.  Unless there is potential to parasite another post-rectified source?  Though, I'm betting the voltage will be variable based on amp load.  Just the same, see attached...

Or maybe combine the two 6.3V taps?

Title: Re: Power SS circuit from AC Bias tap
Post by: PRR on September 05, 2021, 03:39:39 pm

> combine the two 6.3V taps?

Is it really two taps? Or sloppy drawing?

IAC the heater winding has to float to null heater hum.

I see a way to "triple" three outputs but I don't want to draw it, or expect anybody to build it error-free. And as you say, the more complicated the "parasite" the more chance of upsetting the bias on some costly power tubes or burning-up an OT.