DG300b Build Thread

I built my DG300B to power speakers, so I chose to optimize the chassis for size at the expense of the possibility of a little hum. I have built two of these amps. One has no hum at all. The other has a very slight hum (about 1 mV) that appears to be from mechanical coupling (!) from the power transformer into the 300B tubes.

It may be worthwhile to isolate the power transformer mechanically from the chassis. Vibration dampening mounts can be found at McMaster-Carr.

Some Classic-Tone transformers hum. Some don't. Classic-Tone says it's not their issue to fix. Should you have issue with hum, it's most likely due to a DC component on your mains voltage. A simple DC blocker can be used to address this.

As long as the power transformer isn't physically vibrating/humming, the DG300B is quiet. Unfortunately the Classic-Tone transformer I specify is the only readily available power transformer that'll work in the DG300B. Those who wish to get a custom transformer wound (which could perform better than the Classic-Tone) should consider Toroidy. I have no personal experience with them, but I hear they do solid work.

I love perf plate. I use it for the bottom plate in my DG300B amps. The key is to use the perforated holes to the extent possible as punching additional holes, off grid, in the perf plate can be a royal pain.

I'd still wait until I had the components on hand before ordering the chassis, though. Also note that Jack/Electra-Print doesn't publish mechanical drawings.

Tom

 

I was thinking more about hum fields on the surface of the aluminum. Sometimes you see laminations used to guide those fields away from the output. But I think hum field levels can be measured to determine distance and angle for core alignment, etc...

 

Small update, put on bridge rectifiers and sinks.


Still waiting on 9p tube sockets and those opts. Called up to get a status update and discovered my order got lost. Now I'll be getting partial (secondary only) silver stranded windings.

Collection of parts. Have a pair of the default Solens coupling caps, some boutiques to try... Mundorf aluminum oil, clarity MR.
Some spade connectors and taps
The IEC inlet and fuse box

Finally some tube sockets that work

Some OPT's arrived, too bad they are the wrong ones. They will be going back to Nevada.

 

@tomchr

Can you make a DG 2A3 with this kit?

 

The driver board and regulators can easily be configured for 400, 250, and 275v B+.

Yes 300b, 45, 2a3 are easy changes.
Driver tubes can be changed as well.

The default classictone power transformer is quite flexible as well. It is configurable from 300-400+

Block diagram of the overall amp

What that really looks like (not mine)


This chassis has waited almost ten years to be used...see the dust


Time to start contemplating floor plan

 

Started the top plate design process. I haven't accounted for cooling vents and transformers yet. Still not sure whether I want to mount them under our on top. If I mount on top, I would consider buying a new 2u enclosure. Thinking grey metal powder coated or bead blasted finish. This isn't going to be cheap.

I didn't want to buy the really overpriced WBT tap plates, so im going to CNC them into my fully integrated topplate. The left middle section has provisions for extra heatsinking on the B+ regulator.

 

i could put this somewhere else, but the projects are related. The flatpacks come in a nice relatively small but dense package. This is my first time working with wood so bare with me. It's the ancient design BK-20.



I went back and sealed everything with silicon filled all gaps, it's not as bad as it looks. I definitely should've used less glue followed by a second round of clear sealant. The second cabinet is much faster and better now that i know what im doing.




Looking for some audiophile sand to fill the permanent cavities with. To finish this, i think i will trim route the edges and finish it matte black. Waiting for the tax bill to determine whether i will buy t500 super tweeters this year.
Bought an assortment of parts to make PEQ and Crossover for the tweeters that i don't have yet. Hopefully i can find the right values with these and then go find some nice caps to put in there perma.

 

Don't forget to solder (or epoxy) the pins on the PCB mounted heat sinks.

Looks like the speaker flatpack came together nicely.

Tom

 

Glued the heatsink pegs to the board with a drop of thin super glue. That way I can easily burn it off with the iron if I need to remove it.

Small updates, crazy life events, and been working 15 hour days.

Did the CNC two ways. One with the power trafo horizontal and vertical. Still need to drill holes for the wires to go through. I might switch the inputs and outputs so the inputs are father away from the AC in and power trafo. Vertical is more traditional layout and more people seem to like that.

CNC is quite expensive given the size, material, milling on both sides, and edge treatment.




Sealed up everything with silicon caulk. Plan to round the corners with a trim router once I join the halves. Will probably paint black, steel wool, matte black finish.

 

I take it that you don't plan on running the fe208ez full range?

 

Great point. I intend on starting full range fe208ez. If I think it needs supa tweeters then I'll pick them up.

Would be nice to save two grand that I can put towards a better preamp or build a second power amp.

 

You should keep the crossover external. That way it's easy to switch between full range and adding the tweeter later.

 

Crossover will be external. Will probably do it turret board p2p style. I might even integrate the PEQ portion into the amp itself since i have quite a bit of extra space in the amp enclosure (between the output transformer and the speaker taps). I have a bunch of new tools that i need to learn how to use - measurement mic and REW.

I think this is the final iteration subject to verifying nuts, bolts, thread pitch, counter sink angles, bolts, grommets, isolation bushings. This is going to be expensive, but i want my builds to look like i put some effort into it. Might cut a few more cooling vents on the sides. It's pretty much a matter of whipping out the checkbook now for this to get made.

I am going to have a few hundred in this top plate, a 3U pessante frame is actually really cheap...Side plates are like $7 each and then the bottom plate is like $15. If i want to change to wood later, it wont be an issue.



Update: Fully QC/Verified all dimensions, thread pitches, hardware, and ventilation. This is it.
Moved the PSU board closed to the rear directly under the power transformer and away from the tubes.

 

Some updates...ordered the nuts and bolts today as well. As you can see i had to cut some costs to keep this within the specified budget yet i still desired to keep it professionally made looking. I took design cues from EC and DNA amps.

 

It's about that time where you're pretty confident that everything functions as intended. I had a few miscalculations with the CNC, like the power transformer wires don't all fit through a 3/8" hole so i'm gonna have to ream and deburr it. We're very close...as soon as i get through this work week i'll finish this thing cause i'm dying for the next project.

The power transformer really hums. I hope the sorbothane isolation washers i implemented in this are good enough to prevent microphonics. I have a feeling that genalex might be similar to EH 300b.

When the speakers are done, im going to put this head to head against the EC 2a3 MKIV. It's going to be an interesting showdown.

Verified function of B+ HV Regulator 400v, filament regulators for the output tubes and driver tubes.




I got a pretty cool project coming up. I'm gonna play with some vintage stuff.

 

You're getting damn close, and if it's done and if you're around the weekend of the LA Audio Show and want to demo it, I'd love to hear it.

BTW, was there no way to mount those trim pots on the other side of the board and give access through the top plate? If it's not too late, something to think about, as long as you're reaming.

 

I actually cnc two holes in the top plate for the trim pots assuming I mounted them on the other side. Would not be difficult to flip them around. I would still have to access the underside of the PCB to take the bias readings from the jumper pins.

I didn't think I would be adjusting the bias very much which is why I just left it this way.

@tomchr would there any sq improvement to be gained from using two independent filregs on the driver tubes rather than sharing a single 6.3v? The output tubes have dedicated filregs. I think it is even made a point on the zds that every tube has it's own transformer + crc?...not as fancy but still independent and clean sounding.

 

Yeah many amps have the holes for DMM probes as well. The last one I played with was a Manley, and it's super convenient to be able to measure and adjust through the top plate. It'll drift over time, and you want to be tube rolling friendly, but it might also depend on how OCD you are. I say do it once; do it right. I was thinking of wiring in meters that'd be installed in the top plate, myself, and maybe switched. :-D

 

I'm a bit of a tube roller, so I exposed the bias adjustment on the top, and mounted DMM plugs on the back. Add a nice analog meter, and I have a very simple way to tweak bias.

Also worth adding for completeness - amp circuits / tubes can change a bit sonically depending on the bias you run them at. Further, mesh tubes tend to last longer if you turn the bias down a bit. Taken together, it's worthwhile to expose bias adjustment (IMO).

[EDIT] @bazelio - yup!