ODS 102 with 2x KT88s and built-in Dumbleator

[Stephen1966]

After doing a bunch more suggested reading on the Loop, Stephen, I lifted your circuit from your 183 build. In the interest of not drilling too many more holes or installing parts that I think won't get used, I removed the the line/instrument switch as well as the jacks for the passive loop. I'm thinking I will probably put the Series/parallel switch and the Loop IN/Out switch on the back panel with the send and return jacks, but put the three control pots on the front panel.
May 13.jpg

Next I will take look at the FET input and relays switching circuits.

That's cool. You aren't likely to need the 68pf cap on the Master though.

[bcmatt]

... You aren't likely to need the 68pf cap on the Master though.

Good to know. Thanks. I'm out of the loop on any discussion on that one. I see most with the 68pF, one with 15pF, and some with none. I guess with bright switch in the loop, and a presence control later, it becomes pretty irrelevant?

[Stephen1966]

... You aren't likely to need the 68pf cap on the Master though.

Good to know. Thanks. I'm out of the loop on any discussion on that one. I see most with the 68pF, one with 15pF, and some with none. I guess with bright switch in the loop, and a presence control later, it becomes pretty irrelevant?

Yeah the cap on the Master, I'm not going to open that can of worms but trust me, I didn't use one and it doesn't need one. I also tend to agree with Martin, the switched bright cap on the DRIVE pot is not consequential. The 270pf on the GAIN pot however, is.

The PRESENCE control is very responsive in my builds. I would not omit that.

What was your thinking with the power MOSFET for the B+ branch feeding your reverb and FX loop? To get the magic 30V on the cathode your "B+5" voltage should be in the order of 260V on the plate of the driver and 360V on "B+4". With these values, the 30V is all but certain.

With the MOSFET feeding (regulating) your screens is the bias point for your screens a fixed ratio of the B+? Sitting right after the choke and where the branch splits three-ways (to the screens; to the rvb and fx; to PI and preamp) just curious what your design goal(s) were here.

[bcmatt]

What was your thinking with the power MOSFET for the B+ branch feeding your reverb and FX loop? To get the magic 30V on the cathode your "B+5" voltage should be in the order of 260V on the plate of the driver and 360V on "B+4". With these values, the 30V is all but certain.

That MOSFET is a VVR for the Screen control for the KT88s so that I can dial in about 65% of the Plate Voltage. That's already currently implemented and working well in this YGL-3a. Martin Manning helped me implement it here:
https://ampgarage.com/forum/viewtopic.php?f=6&t=35434

I do need to plan out the filtering a little better still.
I forgot the supply for the D-Later earlier but added it now.

May 13B.jpg

Right now I'm looking for that FET level knob mod with relay switching that Martin had drawn somewhere... and trying to figure out what transformer works well for the relay power...

Whoops! I see a mistake in the VVR drawing! I forgot to add in the 1Meg pot!

[martin manning]

Right now I'm looking for that FET level knob mod with relay switching that Martin had drawn somewhere... and trying to figure out what transformer works well for the relay power...

There's info on these in the ODS boards .pdf here https://ampgarage.com/forum/viewtopic.php?f=5&t=29630

[Stephen1966]

What was your thinking with the power MOSFET for the B+ branch feeding your reverb and FX loop? To get the magic 30V on the cathode your "B+5" voltage should be in the order of 260V on the plate of the driver and 360V on "B+4". With these values, the 30V is all but certain.

That MOSFET is a VVR for the Screen control for the KT88s so that I can dial in about 65% of the Plate Voltage. That's already currently implemented and working well in this YGL-3a. Martin Manning helped me implement it here:
https://ampgarage.com/forum/viewtopic.php?f=6&t=35434

I do need to plan out the filtering a little better still.
I forgot the supply for the D-Later earlier but added it now.
May 13B.jpg

Right now I'm looking for that FET level knob mod with relay switching that Martin had drawn somewhere... and trying to figure out what transformer works well for the relay power...

Whoops! I see a mistake in the VVR drawing! I forgot to add in the 1Meg pot!

Cool, I will have a read of that...

Again, you can look at my Delta schematic, it has a level pot and relay switching and if you want a good relay transformer, the Triad F-105Z is a good option providing up to 1A. Check dimensions though...

F_105Z-780774-1.pdf

[Stephen1966]

Whoops! I see a mistake in the VVR drawing! I forgot to add in the 1Meg pot!

I though you were going for fixed bias. Is there any reason you need this to be adjustable?

[martin manning]

... and trying to figure out what transformer works well for the relay power...

Raoul Duke posted a data sheet for this small Triad for relay power using a FWB: https://www.mouser.com/ProductDetail/Tr ... 4ZQw%3D%3D
This Hammond is also a possibility for FWB: https://www.mouser.com/ProductDetail/Ha ... Gc8w%3D%3D

[bcmatt]

Right now I'm looking for that FET level knob mod with relay switching that Martin had drawn somewhere... and trying to figure out what transformer works well for the relay power...

There's info on these in the ODS boards .pdf here https://ampgarage.com/forum/viewtopic.php?f=5&t=29630

Whoa! Thanks Martin! This has what I was looking for and much much more. Lots to sort through here.
Am I seeing right that I may be able to take my heater supply and use it to feed the power for the relay switching?
Or is that generally more trouble than using one of these 12V transformers?

[bcmatt]

Whoops! I see a mistake in the VVR drawing! I forgot to add in the 1Meg pot!

I though you were going for fixed bias. Is there any reason you need this to be adjustable?

Probably not. I think I could just measure my current values and put resisters in instead. I think it was more for experimental purposes at first. Although, the 1meg trimpot is already installed in there and I don't necessarily have need of it.

20240513_112212.jpg

[bcmatt]

Even though I have much more modding to do to the schematic, especially regarding the relay situation, my OCD won't let me not post the current schematic I am working on that reflects the Screen Supply VVR:

May 13 with Traynor Filtering.png

[martin manning]

Am I seeing right that I may be able to take my heater supply and use it to feed the power for the relay switching?
Or is that generally more trouble than using one of these 12V transformers?

I think it's simpler to use a separate transformer for the relays. You can use 12VAC and a bridge, or 6VAC with a doubler.

[Stephen1966]

Whoops! I see a mistake in the VVR drawing! I forgot to add in the 1Meg pot!

I though you were going for fixed bias. Is there any reason you need this to be adjustable?

Probably not. I think I could just measure my current values and put resisters in instead. I think it was more for experimental purposes at first. Although, the 1meg trimpot is already installed in there and I don't necessarily have need of it.
20240513_112212.jpg

Or, you might use a switch with resistors, that way you have different levels of regulation for KT88s or EL34s. A potential failure point as I see it is the power rating of Piher trimmers. A cermet type would be better but a higher rated switch is just easier to source and install.

[bcmatt]

Or, you might use a switch with resistors, that way you have different levels of regulation for KT88s or EL34s. A potential failure point as I see it is the power rating of Piher trimmers. A cermet type would be better but a higher rated switch is just easier to source and install.

Oh yeah. I guess I don't want that pot to fail. I think I'll just put the resisters in.
I think we considered the switch before, but decided that resoldering a couple wires to bypass the VVR circuit would be better since the lid should be opened to re-bias if EL-34s or KT-88s swapped in anyways. I wouldn't want the switch getting flipped by accident. I guess if the switch was internal it would be fine... then tube swaps could be done without needing a soldering iron... but what switch could be internally mounted while being robust enough to not worry about?

[bcmatt]

So, I'm just looking at the FET circuit and the Relay power supply.
If I use the Triad F-216X 12VAC supply, it looks like I use a FWB rectifier. I need a TO220 Regulator? Any recommendations? It just needs to regulate that 12VDC?

May 13 withRelays.jpg