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.
.
.
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.
.
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
![Very Happy :D](./images/smilies/icon_biggrin.gif)
- 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
![Image](https://i.ibb.co/ctMLwxp/Screenshot-2022-10-17-at-12-50-46.png)