SE ODS

Post by **Colossal** » Sat Dec 01, 2012 2:52 am

I've had some Champ iron sitting around along with some nice NOS glass; one of those afternoon projects I've been meaning to get to when things slow down. But these SE D-style amps have me curious and we've seen and heard some cool builds from Honeydip and Mat. So I thought I might instead use my iron for a single ended D-style build. I would like to see if I can capture some horn-like, violin tone. Mat and HD ran lower plate voltages, but I am expecting about 340-350V with the PT on hand.

Here are the plans and parts:

Schematic
Ideas taken from ODS101, SE D-style amps by Mat and Honeydip, sketches by 10thTX

Preamp
V1a 220k/(3k3-5uF)
0.022uF coupling cap 6PS
Tweed Tone Stack (Volume/Tone)
220k/500pF treble peaker
V1b 150k/(2k2-1uF)
Switchable LNFB on V1b
0.022uF coupling cap
Clean Master Volume 1MA/100pF (labeled Level)

OD
220k/100kB OD entrance trimmer
V2a 68k grid, 220k||220pF/(3k3-5uF), 270pF anode to cathode shunt
V2b 150k grid, 150k||220pF/(2k2-1uF), 270pF anode to cathode shunt
0.0047uF -> 150k to Tweed Tone Stack (OD Level/Tone)

Controls
Preamp - Volume/Tone/Level
OD - Drive/Tone/Level
Power Amp - Morph (Triode/UL/Pentode), Master Volume (250kA/70pF)
OD on/off - front panel switch, rear footswitch via relay

Power Supply
330-0-330 PT 70mA
6.3VAC filament
5VAC rectifier (5Y3GT)
1 x EL84 cathode biased, adjustable with test jacks

Output Transformer
SE 8k primary
8R secondary
into a 1x12 with 8R EVM12L Classic

Choke
10H 200mA (CLC input filter, whole amp choked)
or 2 x 5H choke, one on B+ and one on ground side between first two smoothing caps (CLLC)

Tube Compliment
V1 - 12AX7
V2 - 12AX7
V3 - EL84
V4 - 5Y3GT

Chassis
12" x 8" x 2.5" aluminum

Options
1) Switchable EL84/6V6?
2) Morph control on the power tube. Interesting idea or skip it? I'm guessing the amp is going to sound best in pure pentode mode, but UL/Triode could be interesting.
3) Some kind of trimmer adjustable tone stack ground lift on CL1 for a PAB?
4) Passive effects loop between OD/Clean switch and Master Volume?

Thoughts? Comments?

Thanks.

LeftyStrat · Post by **LeftyStrat** » Sat Dec 01, 2012 3:18 am

Definitely sounds interesting. I'm going to pop some popcorn and watch this thread with interest.

M Fowler · Post by **M Fowler** » Sat Dec 01, 2012 4:23 am

LeftyStrat wrote:Definitely sounds interesting. I'm going to pop some popcorn and watch this thread with interest.

I'll bring the coca-cola's to enjoy with the popcorn.

Dave that sounds like a great build and I know your going to knock it out of the park

Mark

jelle · Post by **jelle** » Sat Dec 01, 2012 4:34 am

Sounds like a great little project. I did the morph control on one of Jon's EF86 builds and I liked it a lot, it very much tames the stage down into sweetness. So I would very much like to encourage you to do this, great idea!

Have fun!

Post by **Colossal** » Sat Dec 01, 2012 5:22 am

jelle wrote:Sounds like a great little project. I did the morph control on one of Jon's EF86 builds and I liked it a lot, it very much tames the stage down into sweetness. So I would very much like to encourage you to do this, great idea!

Have fun!

Very cool! Well, I was thinking of using the Morph control on the SE power section, so being able to operate the single EL84 in triode, UL, or pentode modes. but I was really questioning the utility of it. Practically, it would reduce a little power in triode mode and might have some interesting tonal variety, but I'm wondering if I won't always select for the complexity of full pentode mode. I thought a VVR to the power section only might be another option or just running into a small outboard variable load and light bulb compressor.

jelle · Post by **jelle** » Sat Dec 01, 2012 5:26 am

Exactly, why would it work differently there? I think it will work very well!

Post by **Colossal** » Sat Dec 01, 2012 5:28 am

jelle wrote:Exactly, why would it work differently there? I think it will work very well!

Ok right on, I wasn't sure if you meant try it in the preamp context. Very cool! I'll keep it in. It's only a 1M pot and seemed like something at least worth giving a shot, at the very least for academic interest.

jelle · Post by **jelle** » Sat Dec 01, 2012 5:32 am

I like academic interest. That's how we learn, right?!

Post by **Colossal** » Sat Dec 01, 2012 5:55 am

jelle wrote:I like academic interest. That's how we learn, right?!

Exactly.

I am thinking SE power supply hum and thought about trying a dual choke in the PS. Something like 5Y3GT -> C - L || L - C -> B+1. The whole PS would be choked. This would dump a little voltage while taking the noise floor way down. I want to get the voltages in the preamp right for the ODS101 sound and run the power tube around 340-350VDC.

212Mavguy · Post by **212Mavguy** » Sat Dec 01, 2012 8:11 am

I'd love to hear a SE topology with a bigger power section. I have a 2.5k primary 4/8/16 secondary SE OT, think it's rated for at least 150ma, in the basement, seller said 50 watts...hmmm. Pair of el34's, 6550's. It's like a Hammond 1600 series in size, big and heavy...It had been driven by 6550's by previous owner.

My idea was to use it to build a slave SE power amp and drive it from the unused right side channel coming out of my stereo out fx box, box hooked up through the 'lator equipped D-clone of choice. Hoping for stronger mix of even order harmonics in the sound of a slave amp than the typical PP power section. Just power supply, driver tubes, power tubes, volume. Haven't started on it, need to do more reading and learning prior to doing. I've never drawn up a schematic or layout...yet.

10thTx · Post by **10thTx** » Sat Dec 01, 2012 12:11 pm

How close is this to what you're talking about? (I took Mat's schematic and just edited it)

NOT sure what's going on between V2a to V2b ????

And I don't know what the morph control would look like?

I attached a SCH schematic that you can edit, hopefully.

With respect, 10thtx

greekie · Post by **greekie** » Sat Dec 01, 2012 7:15 pm

If you want to remove a lot of hum, my dad tought me an old 60/70s hifi-trick from when I was building hifi to support his studies:

You connect the cathode (minus pole) of the first filter cap to the cathode (pin 3) of the power tube. That way the power supply hum will be injected out of phase. It's also a tiny bit easier on the cap, voltage wise.

Tell me how it works out.

surfsup · Post by **surfsup** » Sat Dec 01, 2012 7:34 pm

I would think it better to set up a AC voltage divider off the B+ to the cathode of first stage equal to the inverse gain of the stage.

The method you describe may help, but could also make things worse? And also create a groundloop potential due to intermixing current from the poweramp stage and the preamp...

Post by **Colossal** » Sat Dec 01, 2012 9:34 pm

Going to use DC heaters throughout. Since it's SE, there is no common mode rejection. There's only 2A worth of filament current so easy to rectify and filter heavily.

Working on a schematic to post. Almost done.

surfsup · Post by **surfsup** » Sun Dec 02, 2012 1:37 am

Not talking about common mode rejection, talking about injecting hum anti-phase to cancel it at the input stage of the amp and/or at the powertube. For the powertube, if you have a gain of 10 lets say, put a 1000u from b+ to cathode and a 100u as cathode bypass to ground to inject 1/10th ripple into powertube cathode, which is then amplified by 10 by the powertube to unity value cancelling hum. Example assumes 100u PT bypass cap and A=10

The Amp Garage

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