Building 5F1 Champ Style Amp

[ric]

Thanks for the answer g_one. Htr CT to cathode of output tube(s).

[g1]

I was just repeating what Bruce said .

[DrGonz78]

So I am now at the point where I am triple checking everything. I am attaching the mojotone wiring diagram as a reference. Pin 8 is the cathode pin on the 6v6 output tube and on the diagram it shows a line connecting it to pin 1 too. It says on the pin layout of a 6v6 that pin one is shell, what does that mean. If it is 6v6gt then it is none/NC. I have a 6v6 so I need pin 8 connected to pin 1 right? What is this doing exactly?

So pin 8 is where I can connect the green/yellow CT right into the cathode?

I have seen many examples of ways to hook up the mains power... Is this diagram a perferred way or is there a better way? I have nearly everything connected but just wanted to ask these last few questions. Great learning experience building your first amp and already thinking how much easier it will be next time! lol.

BTW I have applied the earlier advice to my build already, so the wiring diagram is not exactly what I have now. But as far as the line voltage I have one black primary wire from PT going to fuse. I have the other primary black wire going to the switch. Right now I am deciding if neutral goes to the other side of the fuse or connect it to the other side of the switch. The example in the attached diagram has hot running to the fuse and the nuetral connected to the switch. I think that is the way to go.

[Jazz P Bass]

Pin 1 of the 6V6 socket is mearly used as a convenience.

A place to tie two wires together.

As to the power wiring, I have always switched the black lead.

In that way, off is off, you are not simply breaking a working circuit.

[g1]

The original 6V6 was a metal tube. The caising or "shell" was connected to pin1. If yours is glass, consider it a GT, pin1 is not used.
For your AC wiring, with modern 3 prong grounded chassis the fuse and switch are both in the hot (black) side. White (neutral) connects direct to PT primary. AC cord black goes to end pin of fuse holder. Fuse holder side pin connects to power switch. Other side of power switch connects to PT primary.

[DrGonz78]

Great that is what I was thinking about the wiring of the AC line. I know it's basic and I guess I knew the answer, still never hurts to ask. Also, one last question involves the unused secondary taps on the Hammond 125DSE OT. Do I just snip them down and insulate the ends? I would think that is what I need to do but just figured I should ask. This has been a fun project and really glad everyone has shared their knowledgable experience with me. Thanks!

[g1]

Yes, snip, insulate, and secure the unused windings. But consider if you may ever use it elsewhere and snip the length accordingly.

[DrGonz78]

Well I wish I could report a successful build but this is not the case yet. I have a problem. I am getting 460vDC for my B+ voltage which is way too high. This is with the 5y3 connected to observe my rectified voltage to the 475 main filter caps. Right now between the 10k resistor between the other filter caps there is no drop in voltage. Probably not the best idea but I plugged in the 6v6 & 12ax7. Voltage did drop to 450 or so but after about 20 seconds it started to squeal. To recap I am using a classictone 40-18019 PT and Hammond 125dse OT. I have an 8 ohm Mod speaker in the cab. On the Mojotone layout it has the red primary wire of the OT going to the positive side of the main filter cap. The Blue primary (OT) goes to pin 3 (plate). I have attached both the Mojotone OT data sheet and the Hammond 125DSE sheet to the post. In my amp the Brown wire is connected to positive side of the main filter cap and Blue primary is attached to pin 3. I selected the yellow secondary wire on the Hammond and that is connected to the tip of my output jack. The output jack has yellow secondary and the signal wire from the board connected to the tip. I have the black secondary connected to the ground lug on the output jack.

So perhaps I have HV red transformer wires backwards? At this point I am a bit confused as I have everything connected up exactly the way it shows on the schematic/layout. All the wires/components have been checked over at least 5 times now. Not sure if my impedance on my OT is wrong and I should use Green wire? Just confused how my B+ could be that high out of spec. Any ideas what to check would be greatly appreciated, thanks.

[DrGonz78]

After reading a bit about newer 5Y3 rectifiers, I understand better now that the B+ can be a bit higher than the NOS ones. Pin 8 on the 6v6 has the expected 470ohm reading from the bias resistor and seems to be connected correctly to the cathode. Still without tubes the voltage test points did not differ and perhaps with tubes connected those test points would be there in full circuit. I got a bit freaked by the squeal that I forgot to test voltage test points with the tubes connected. So I will have to check that again tomorrow.

Edit: Just one more question here... I chose to use the yellow secondary wire on the OT at 8ohm @5000ohms primary. Would my B+ voltage go down by changing that secondary wire to green at 8ohm @1000ohms primary? Just a thought but not sure if it makes any sense?

Edit: Just had to say when I typed 1000ohms I meant... 10k ohms not 1k like I had typed...

[ric]

Hi,Doc. Sorry you are having difficulties. I am not a tech, so this is just the thoughts of a hobbyist. OT secondary is usually grn:8 ohm, yel:4 ohm, blk: common. That looks like what your sheet shows. PT secondary reds are each end of a winding and should be reversible. The grounding point of their CT (traditionally red/yel) is important. It, as I understand, should be at same place as the ground for the filter cap that supplies the node feeding HV to the OT. As far as the "signal" wire that attaches to the spkr load jack tip, I imagine that to be neg feed back? If for some reason it is injected back into the signal where the phase is the same, that is positive feed back which would squeal. Well I'm sure someone qualified to advise you will answer soon.

[g1]

Try reversing the OT primary wires (blue & brown).

[DrGonz78]

Well I just reversed the OT primary wires brown and blue. Also I changed the yellow secondary for the green wire just because I think that is the one I wanted anyway. So.... It works great!!! Did some voltage test point readings and I am getting 370vDC on the plate. Test points are all within the +/-10% margins except for one, it is pin 1 of the 12ax7 tube. Value on Mojotone says 165vDC and I am getting 184vDC which is about 11.5% higher. Still my PT is 325-0-325 and their build uses 290-0-290 as I recall. In addition my voltage readings are not with the amp on for long time either, so that may have something to do with it too.

It sounds great so that is a great improvement from a loud constant squeal. I want to thank everyone here for their support and help on this build, it is my first. Definitely won't be the last either!!

Edit: Also I forgot I just bought a great Variac with ammeter. So I got to try that out too and I should have been using that the first time I plugged in the amp.

[ric]

Congratulations Doc. G_one to the rescue. Thought that might happen. In my limited experience 165-175 V on first plate gives the "brown" sound. 185-195 gives the Fender-y brite sound and 200+ is best saved for later stages! So she oughta sound like a champ but be ready to drive if pushed. Glad it worked out. I have a stripped and painted combo chassis, but figured I'd just toast the tiny OT like I did on an SE el84 build. If you feel like posting a "tone" report on that 6v6 SE OT, I for one would like to read it. Adios.