ODS standalone preamp project

[bluemonk]

Hello. I want to build a Dumble standalone preamp and ask your help.
PS should be from back-to-back PTs: 1st external (230-12v, 4A) and 2nd inside the chassis (9-230v, 1A).
I am concerned by the relays supply because it couldn't come from a separate PT and I read about the noise problems related.
So I'm considering to use bare switches in place of relays (I had success to solve switching pops using bypassing resistors, in other projects).
Should it be a way to go? Any advice?
Thanks in advance!

P.S. I read the other thread but I wish just the Dumble design with no mods, mosfet etc (https://tubeamparchive.com/viewtopic.ph ... 4364fa3728)

[Structo]

Build ODS #124 which is extensively documented here.

[bluemonk]

Yes, #124 is my aim. I read a lot about it here!
My doubts are about no-relays option.
Thanks

[larsvictor]

You could use the 12VAC from the 1st PT for the relay ps. Bridge rectifier, LM 7812 and 2 caps. Something like this:

Victor

[bluemonk]

Thanks. Then I've to use the same 12VAC winding (it's a 6-0-6 indeed) for filaments' supply. What about the ground reference?
And also 12VAC have to go to 2nd PT to get high voltage for B+.
Is it ok?

[larsvictor]

Ok - It has a center tap. I would take the whole 12V secondary winding without the tap. For both the filaments and relays you may take 12,6 VDC.

There are voltage regulator-kits for less than 10 US$ on ebay (if you don´t want to built up the circuit on your own).

http://www.ebay.de/itm/LED-display-LM31 ... 541ccad502

Hope that helps.
Victor

[JazzGuitarGimp]

I would ground the ct, use two diodes in full-wave mode, then use a LDO 5V regulator and 5V relays.

[larsvictor]

I would ground the ct, use two diodes in full-wave mode

Does that work with the second transformer back to back for HV ?

[bluemonk]

Thanks for your answers. I attached a pic. Should that work good?

[JazzGuitarGimp]

You're close. I would make these two changes:

1. Remove the two 100R resistors, and instead, ground the ct. (Though your approach should work if you'd rather not run three wires to the preamp pedal)

2. This one is important: Your relay supply only needs two diodes to do the full wave rectification. If you decide to use a full wave bridge rectifier (4 diodes), DO NOT ground the minus output of the bridge - leave it floating (no connection). Otherwise, you will cause a short circuit from the relay B+ through the 100R resistors to ground, which will cause your power supply to draw more current than is necessary -or- if you implement my first suggestion (removing the 100R resistors), grounding the minus output of the bridge will cause a direct short from the relay B+ to ground, which will melt your mains-connected PT.

[bluemonk]

Thanks for your explanation. If I understand correctly the attached pic shows the way to go. Or also here the relays supply's minus hasn't to be grounded?
Please excuse my little knowledge.

[JazzGuitarGimp]

Thanks for your explanation. If I understand correctly the attached pic shows the way to go. Or also here the relays supply's minus hasn't to be grounded?
Please excuse my little knowledge.

Yes, you have it right. Use the CT ground for your relay supply return to ground.

This should give you about +7.5V for your relays, so you will want to use a low drop out +5V regulator for the relay supply, such as Texas Instruments LM2940T-5.0/LF08.

Cheers,
Lou

[JazzGuitarGimp]

I would ground the ct, use two diodes in full-wave mode

Does that work with the second transformer back to back for HV ?

Yes.

[larsvictor]

larsvictor wrote:
Quote:
I would ground the ct, use two diodes in full-wave mode


Does that work with the second transformer back to back for HV ?


Yes.

Ok, thank you, didn´t know that.

But now bluemonk has a step down from 230 VAC to 6 VAC. Feeding the second transformer with only 6 VAC will cause a lower output (~173VAC). The rectifier "raises" this to approx. + 241 VDC. This is not enough for B+5 in #124.

[JazzGuitarGimp]

larsvictor wrote:
Quote:
I would ground the ct, use two diodes in full-wave mode


Does that work with the second transformer back to back for HV ?


Yes.

Ok, thank you, didn´t know that.

But now bluemonk has a step down from 230 VAC to 6 VAC. Feeding the second transformer with only 6 VAC will cause a lower output (~173VAC). The rectifier "raises" this to approx. + 241 VDC. This is not enough for B+5 in #124.

Regardless of whether or not you connect the ct, you've still got 12VAC at the primary of the second transformer.