6V6 Brown Super Build

Hello Everyone,
Here is my latest covid project. I am building a 6G4A Brown Super with a 6V6 power section and the normal channel modified into a 6G3 Deluxe bright channel.

I arrived at this mashup due to parts I have from a salvaged Hammond S6 chord organ which is 6V6 push pull with a 4ohm OT. The PT is huge and will be able to support the tube complement and puts out a B+ of about 350v. I am reusing the chassis, transformers, tube sockets, some boards and tag strips, baffle board, and most of the wood from the organ.

Stripping it down is easy, but cleaning up the tube sockets and the boards is kind of time consuming. Reusing the wood for the cabinet is probably a waste of time. I should have just made a pine cab for it, we'll see how the refinishing goes.

This is my most ambitious build to date and I have had to learn DIYLC layout program and Inkskape. I have not decided on the front panel color scheme yet.

Here is what I started with.


Stripped down and cut to size.


Preamp and first stage of the trem boards populated and turret board for the second stage of the trem phase inverter and power section. Front control panel attached. I'm going to use 5 of the tube sockets between the board for the preamp and trem circuit and use the other socket for the phase inverter. I'll remove the unused sockets after I figure out my wiring layout.

The power tubes are located on either side of the OT.


And assembled cabinet with chassis and front panel mockup.


Take Care
John

 

The OT is kind of small and is probably the weak link, but I did use the same set up to build a 5E3 circuit that worked out good. Plenty loud with the 2 10" speakers. From what I researched the non-electrolitic capacitors in this era of Hammond are supposed to last forever. I saved all of them when I tore it down, and the few that I tested with ESR meter were good, but since they are cheap anyway I bought new ones for this build.

The PT is huge and it is mounted in the organ cab at the bottom of the chassis along with the 5U4GB rectifier. Usually I do my builds as heads, but with these S6 builds I am doing 2 x 10" combos, leaving the PT in the original position and running an umbilical wire harness to the upper chassis just like Hammond did.

These tube Hammond's are really things of beauty. They are so well put together and you can tell that they were wired up by true craftsmen/craftswomen. I felt kind of bad tearing them down, but they were non operational and my understanding is that service and tuning them is not really worth the effort.

Later
John

 

Got a few hours in this afternoon. Stripped and rough sanded the cabinet. The wood is better than I thought it would be, I think it will stain up nicely.

I'm thinking of painting the wood trim pieces in the front of the cabinet black and staining the top and sides. I'm going to do some test strips on the cut offs to see how it looks.

I usually do the amp build first then the cab. I decided to do them together this time so when I'm done with the build I can install it right away into the cab.

 

Thanks Rolling. The way the speaker baffle is executed by hammond lends itself to cutting it down and reinstalling the original side panels, the hard part is stripping out the excess unused wood that is jointed, glued and nailed to the side pieces without cracking the wood. I spent a lot of time messing with the top and trim pieces and ended up with a seam in the middle of the top, but it allowed me to match the corner radius. A good table saw would have made my life easier.

Spent today building my bias board, doing layout and prepping the chassis. And more work on the cab.

Chassis getting closer, unused tube sockets removed, tag strips and tube sockets cleaned up, and most of layout done.



Cab almost ready to stain and paint, colors picked. I'm going to paint the front trim semigloss black and stain the rest of the wood.

 

I Everyone,
Been doing some work on the amp. I had a minor setback with the cabinet, I got it finished sanded and stained and it looked good. This morning I put a coat of cheap poly on it and it dried with a milky color and totally obscured the stain. I think it is because the poly was old and a satin finish. I will need to have to fix that.

Here is the front panel mocked up.



Gut shot. Got my preamp ground buss in, boards in place, most of the power supply and tube sockets wiring. Started on wiring up the pots.

 

Been working on the amp on and off as time permits. Just about ready to fire it up. I'm going to check everything with fresh eyes the do the standard startup procedure, with a light bulb limiter. Since this is such a complex amp I think I'll get the 6G3 channel working properly, then hook up the 6G4 preamp and get it up and running, then add the trem circuit in. I also will implement the Master volume after I make sure everything is good.

This is my first attempt at doing a switchable gain stage/boost, which I am using on the 6G3 channel. I am using a Hoffman relay and power supply for the channel switching.

I am struggling to get a decent poly coat on my cabinet. I think my wood prep was not too good. 4 coats of poly with sanding with 220 grit in between. Last coat still drying, We'll see.

Upper chassis wired up. I left the first channel gain stage wired up temporarily until I hear how it sounds. I ended up with wires running all over the place, I hope it's not an issue. I didn't do myself any favors with the unusual layout. I ended up with short runs to the tube sockets, but longer runs to the controls and some of the B+.



Here is the power transformer and rectifier that will sit in the bottom of the chassis. Grounded 3 wire chord and fuse added.

 

Went through the amp this morning, I found a few missed connections and one cap was missing, got that sorted.

Plugged it in with no rectifier with light bulb limiter, all my heater wiring voltages are spot on, pilot works, power to relay switcher spot on. Bias voltage was all wonky. Found my mistake, this amp does not have a bias supply so I am doing a hi voltage tap. Well I tied into the B+ voltage instead of the high voltage at the rectifier. Poor reading comprehension and not paying attention, but I figured it out and corrected it. Perfect bias supply now. One thing I have learned is to get the bias supply voltage correct before moving on.

Plugged in rectifier, B+ in all the right places. Plugged in the tubes with a speaker attached.

The good news is that my B+ to my power supply is spot on for a 5G9 which is my model for the power section. Adjusted the bias to get power tube dissipation at 24.8mA and 24.5mA at 370V, 65% plate dissipation.

The Hoffman relay switch works as designed with panel switch and foot switch.

The bad news, strange voltage readings at the preamp and phase inverter tubes. Either too high or too low. I only have the 6G3 channel hooked up for now. It has very low volume and is distorted, no clean at all.

I am going to double check my schematic and layout to look for mistakes, then start tracing things out on the amp. I probably missed some components when mixing and matching the different schematics and layouts. Let the hair pulling begin, ha ha.

Later
John

 

Ok, I have most of the voltage issues sorted and found some errors in my blending of the two different preamp sides, which I fixed.

Biggest issue I see now is high voltages on the phase inverter plate and low voltage on the cathode. I should be getting around 220 volts at the plates and they actually close to 320. The cathodes are about 5 volts and should be 27.

I have everything hooked up and both channels are way too quiet and have lots of distortion, both about the same. The trem works but is also very quiet.

There is a 82K and a 100K resistor that feed the plates of the inverter tube, both tested good. I tried 100K and 120K with jumpers to see if it made a difference and it did not, still high voltage. Checked all resistor values with a meter and they are all correct. Checked the tube socket for shorts etc. none found. Switched tubes around, no difference.

Any ideas. Everything else is basically spot on. All heater voltages good.

Voltage chart



Updated schematic with pertinent voltages

 

Update, figured out the phase inverter voltage issue, sort of. I had a 470K bias resister instead of 470. I don't know how many times I checked this. My auto ranging meter tends to select the Megaohm and read as a decimal for lower ohms. Confused my little brain.

Now all of my voltages have dropped and my B+ is down to 350v. I need to reset my bias and do a new voltage chart.

I am now getting a loud thumping that does not seem to be coming from the tremolo circuit. Fairly normal guitar sounds now.

I will spend some more time this PM, I need to walk away for now.

Lesson learned, on a new build it's usually operator error not the components.

 

Thanks Kingfan. My usual issue is with capacitor values. The pF uF nF thing trips me up for some reason.

The great thing about trouble shooting is that I always learn something. Today it was a better understanding of the LTP and what the high voltage on the tube meant.