SLO clone elevated heaters + regulated supply

[frusciante89]

Hello boys and girls!
As you know, I have done several PCBs in the past, in particular hot rod, marshall and fender style boards.
I got a lot of requests for a SLO style board, so I took some time last week to prototype a board in Eagle.
I’d love to pick your brains on some improvements I want to add with respect to the original one.
Original slo has elevated heaters, LDRs for switching but doesn't use DC for heaters.
I'd love to keep these features on the board, but at the same time add relay switching and DC heater supply.
Basically, this means using the 6.3VAC tap from the PT to get a 12VDC rectified supply.
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I sketched out how the DC supply will work. I'm using a voltage doubler scheme from the 6VAC CT tap, going into a diode bridge (GBU8B or similar). Voltage is then doubled as shown and regulated through a LM7812. The 12VDC is then going to the relay circuit and to V1, V2, V3.
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Notes and doubts:
- I'm doubling the voltage due to the fact that the LM7812 has a max current rating of 1.5A which works fine for powering 3x12AX7 @12VDC (150mA each). If I don't double it and use a LM7806 (max current output of 1A), I can't drive the 3x12AX7 (which will require 300mA each @6VDC). Does this make sense?
- I'm still using an elevated CT as in the original SLO. Is this necessary correct given I've added some extra bits of circuitry? I'm particularly worried about the LM7812
- I'm only using a single LM7812 for regulation, even though most of the voltage doubling rectification circuits I've found online use a LM7812 for the positive leg and a LM7912 for the negative leg. I believe my circuit is right, but would like to get your opinion as well
- I've added the classic flyback diode and a very simple RC snubber to the relay coils to minimise popping when switching channels

Does this make sense?
Let me know your thoughts,
Cheers from London,
Andrea

[mhuss]

I wouldn't trust the stacked caps to balance the voltage, add parallel resistors to force an even voltage split.

The floating foot switch voltage might be an issue if the user connected it to a foots switch controller vs. a simple SPST switch, i.e., a situation where the controller assumes sleeve = ground.

[LOUDthud]

That's not a Voltage Doubler, it's simply a full wave Bridge with about +/- 4V output with reference to the CT.

Even if you used a Voltage Doubler, you'll find that 7812 needs too much overhead Voltage and there will be ripple spikes on the regulators output. You might be able to get away with using a LDR (Low Dropout Regulator).

[LOUDthud]

A quick simulation shows a minimum Voltage for the regulator of 13.6V which isn't enough to keep the regulator working at 12V.

SLO_sim_1.GIF

Edit: OK, I dug out my National Data book from 1980. According to the data sheet the dropout Voltage at 500mA and 50 degrees C is 1.5V input to output. At 1 Amp, it's about 1.8V.

This is a Voltage Doubler that uses a pre packaged Bridge Rectifier. Note that two of the diodes just provide reverse Voltage protection for the two caps.

V_dub_2.GIF

[frusciante89]

Hey guys! It took the good part of this week's evenings, but I finally completed the PCB
It supports both LDRs and Relay switching, plus elevated heaters and V1/V2 DC supply. All in a 400x78mm footprint.
Attaching layouts and schematics, would love to get your opinion guys! Still haven't fully debugged everything, so there might still be some typos!
Best from London

RELAYs version:






LDRs version: