5F6A Build

[vampwizzard]

Long story short: did a songwriter contest, met an artist from nashville, got on the topic of building amps. The night before I let a local blues guy play that AO-39 conversion and they loved it. New friend asked about getting an amp with a better blend between fender and marshall. So, had him try a JTM45. Loved it.

I built out a 5F6A hoffman board on a mojotone clone chassis but inverted mounting. For the most part everything went together smoothly. Im up to the filter caps.

Frenchie Films on youtube

&index=6
built a 5F6A clone and mentioned "Doug's old trick" for mounting 5F6A filter caps inside the chassis. From his video, it looks like he just mounted them directly on the board at their respective B+ nodes and tied the preamp grounds to the preamp ground bus bar running along the pots. Definitely dont want to do that for the first node of paralleled capacitors.

Main question i suppose is where can i find documentation on this secret Doug method for P2P-ing the filter caps? Where would I ground the power section parallel cap? PT ground?

[davidwpack]

I made a small power ground buss you can see in I think the 2nd picture here.http://el34world.com/Forum/index.php?topic=24454.0

[sluckey]

All this info came from the "Tube Amp Library" link at the bottom of this page.

Doug mentions how he likes to mount filter caps in his common hookups instructional pdf. Lower right corner of page. Very last sentence...

     https://el34world.com/Hoffman/files/CommonHookups.pdf

Doug shows his method of mounting filter caps in this pic... (You will have to adapt for your narrow tweed style chassis.)

[vampwizzard]

Solid. Thanks Sluckey. The youtube thumbnail image actually shows how he did the first cap... it very much looks like its ground was soldered to a wire and run to the PT ground. Is that a fair assumption?

[vampwizzard]

I made a small power ground buss you can see in I think the 2nd picture here.http://el34world.com/Forum/index.php?topic=24454.0

that's really helpful thank you. sorry i missed it. great looking build there too!

[sluckey]

it very much looks like its ground was soldered to a wire and run to the PT ground. Is that a fair assumption?

That's correct. Doug would never do that though.

[vampwizzard]

well. they went in. im the A node filter caps, output jack, and input shielded cables away from testing this thing. The fender style tweed chassis wouldve made this a little easier. Not sure how they crammed all this in the JTM.

I wasnt sure if i would have to pull the preamp ground bus off the pots to let it float above the chassis. I'll clean up those cap leads after confirming the layout isnt competely Effed.

[davidwpack]

Looks nice! You'll want to keep your PA buss separate from your preamp buss though. I'm looking on a phone but it looks like you've got them all on 1 buss. Probably have a lot of noise. IOW I would take those 1st two caps off of the pot's ground buss and run them somewhere near or on your PT.

[vampwizzard]

davidwpack: Ive got nodes D C and B on the preamp, node A will go direct to the PT ground.

[davidwpack]

Oh ok. It's kinda hard to see from my phone. Looks really good from what I can see though.

[vampwizzard]

alright. got the rest of it wired up and got ready for testing. Initially, I forgot to attach the bias tap to the board. Since my PT doesn't have a bias winding, I jumped off pin 6 of my rectifier and attached to its location per Doug's 5F6A layout. With the standby switch on (cutting off B+) the resistor started to overheat and sizzle. Solder connection is good, looks like it was burning up the 1K/3W resistor.

This isnt normal, obviously, but what could be causing this? What input voltage is the bias circuit expecting? if its expecting 50V do I replace the 1K/3W with a 100K~250K 3W like the marshall HT derivation circuit?

PT is out of the AO-29. 320-0-320.

Lastly, do I go play poker at cherokee?

[sluckey]

burning up the 1K/3W resistor

You need to change that resistor to about 100K if you get the bias AC from pin 6 of the rectifier tube.

[vampwizzard]

just edited my initial response as your reply came through.

So much for having this thing done this weekend.

100K? or should I go with 250K? Any other considerations if im using that AO-29 "I can power 4 million tubes" PT?

[pdf64]

The bias supply caps will probably have suffered significant overvoltage? The pot and other resistors are likely to have taken a beating too. Best to replace it all.

[vampwizzard]

PDF: I had the standby switch off, no B+ to the filter caps.

[sluckey]

PDF is talking about the components in the bias supply. The standby switch has no effect on those.

[vampwizzard]

Ah! My apologies. Will do!

[vampwizzard]

Update: Got my replacement parts in and soldered them into the circuit.

B+ at node A is 423VDC, so serviceable.

V3 is showing some odd voltages.. only 218 at pin 1 vs 368 on pin 1 of V1. I suspect ive got a cold solder joint or two. There is no output.

Voltage chart:
V1
1: 368
2: 0
3: 361
6: 368
7: 0
8: 361
Heaters: 6.2VAC

V2
1: 368
2: 0
3: 361
6: 404
7: 368
8: 397
Heaters: 6.2VAC

V3
1: 218
2: 4
3: 35
6: 373
7: 11
8: 35
Heaters: 6.2VAC

V4
1,8: 0
3: 421
4: 422
5: -37
6: -36
2,7: 6.2VAC

V5 was similar. After measuring voltages on V4 and not getting my typical noise/static through the speaker I turned everything back off. I did add a NFB pot like the october amp.. tied it in at the 27K resistor where the feedback tag is on Doug's board.

Do the voltages suggest something very out of whack? Im thinking ive got a bad solder joint tied to pin 1 on V3.. any thoughts?

The board I used: https://el34world.com/Hoffman/files/Hoffman_5F6A.pdf
The NFB pot schematic: http://www.ax84.com/static/october/AX84_October_Stage_090501.pdf

[sluckey]

The cathode voltages for V1 and V2 indicate that the cathodes have no path to ground. Did you forget to connect that on board bus for those two tubes to chassis?

[vampwizzard]

The cathode voltages for V1 and V2 indicate that the cathodes have no path to ground. Did you forget to connect that on board bus for those two tubes to chassis?

Yup. That was stupid.

After correction:

V1
1: 124
2: 0
3: 1.6
6: 151
7: 0
8: 2.4

V2
1: 168
2: 0
3: 1.1
6: 309
7: 168
8:176

V3
1: 209
2: 3
3: 30
6: 343
7: 10
8: 30

That being said.. my PT has a center tap for the heater winding so I lifted my heater to ground connection off the board.. going to guess i need to ground that portion of the circuit and remove the resistors instead. Am I correct in that assumption?

[sluckey]

I would just connect the heater CT to chassis at the same point the Red/Yel HT CT is connected. And not connect anything to those 100Ω resistors on the board. Makes for neater wiring IMO.

[vampwizzard]

She lives! Had the most negative bias voltage (approximately -50VDC) and did a sanity check. My test speaker cab had a messed up jack.. went to a back up rig and it makes noise.

Ill be getting rid of 100ohm artificial tap resistors. Turning the NFB knob all the way in one direction starts a self oscillation thats unusable.. I may just remove the knob if I cant get that under control.

Thanks again folks! hope to get some soundbytes when its fully tweaked.

[Tony Bones]

Turning the NFB knob all the way in one direction starts a self oscillation thats unusable.. I may just remove the knob if I cant get that under control.

First to suggest reversing the phase of the NFB. Either swap the OT leads to the plates OR swap the grid leads.

[vampwizzard]

Ill give that a try, Bones. Ill also verify that its a polarity problem.. not sure if its removing NFB or adding in more NFB thats causing that particular issue.

[Tony Bones]

not sure if its removing NFB or adding in more NFB thats causing that particular issue.

It's likely that right now the phase is such that the feedback is positive instead of negative. Adding more eventually causes squealing. After you make the change the feedback will be negative like it's supposed to be and won't cause squealing.

It's a very common problem with new builds, maybe as much as 50% of them. 

[vampwizzard]

not sure if its removing NFB or adding in more NFB thats causing that particular issue.

It's likely that right now the phase is such that the feedback is positive instead of negative. Adding more eventually causes squealing. After you make the change the feedback will be negative like it's supposed to be and won't cause squealing.

It's a very common problem with new builds, maybe as much as 50% of them. 

60% of the time it works.. everytime! LMAO

[sluckey]

  That reminds me... I still need to trim my OT primary plate leads and permanently solder them to the tube sockets for my last project. I just happened to get the phase correct the first time and forgot about the leads.

I always leave the OT plate leads full length and just tack solder to the sockets because I know it's a 50/50 coin toss of getting the phase correct. Only when I know the NFB is phased correctly will I cut those leads to length. I hate getting it wrong and having to use my wire stretchers. Thanks for the reminder. Look at the brown and blue wires going to the brown tube sockets in this pic...

     http://sluckeyamps.com/phoenix/p12_big.jpg

[vampwizzard]

I had read somewhere in a build thread that its possible to screw up polarity on PP amps so I make both leads long enough to reach either tube socket. On this one the OT wire run to the output tubes ran directly between the tube sockets so it was easy to keep them the same length. Ill post the results later on.

[vampwizzard]

swapped blue and tan wires to my output tubes and all is solved. NFB knob doesnt do much at bedroom levels and im not cranking this damn thing in the house.

For those who read this later: PT is out of non field coil hammond AO-29. OT is a Hammond 1760J only tapped to the 8ohm output. Hoffman 5F6A kit and I added a NFB pot. Derived bias voltage off the high tap like older marshalls, replacing the first bias power dropping resistor from the kit with a 220K.

Tubes are 12AY7 for V1, 12AX7 for the second stage and PI. 6L6GC's. I'll likely swap a 12AX7 into V1 to hit the front end a little harder. 45Watts of glory.

Thank you all for the help! Eventually ill owe Sluckey a keg worth of beer. thank you Bones and PDF.. drinks are owed y'alls way as well. 

[vampwizzard]

This amp was finished and delivered to a very happy recipient in nashville this past weekend.

Jumping the channels made this thing a glorious beast of overdrive and delight. best of all, that teal spitfire is back in my hands. 

The chassis was a NIGHTMARE because the flanges were inverted and I had no presence of mind to order cage nuts. I used #10-32 screws to mount and had a little success using self clinching nuts but couldnt get them pressed fully into the #10 holes like they shouldve been. If you source chassis from the Arkansas ebay guy, be careful and get mounting flanges that go out like the vintage units.