50w Hot Rod Plexi AFD #39

[M Fowler]

I notice the Mojotone JCM800 50w transformers are listed as 720vct and the CEdist & Magnetic Components transformers both have 690vct slight different in plate voltage.

I have been building my Hot Rod Plexi's with whomever has parts in stock, it's been a waiting game with vendors over the past 2 months.

Mark

[joeboo88]

Holy smoke Mark! I hope everything turns out ok for the family

I just had my 2 grandkids fly off to Ottawa for 2 weeks, they stayed with me for a week and I'm also worn out

my trusty soldering iron dies so i had to order another, plus family here I didnt get much done in the past 2 weeks.

But yesterday, i managed to get this Fowler #39 80% finished, hopefully will be done by the end of the week, and will post a picture of it:
It wont be as pretty as yours because of the small chassis but whatever.

again hope the family comes through good, we're pulling for ya
Joe

[M Fowler]

Thanks Joe, looking forward to photos of your build.

Mark

[dorrisant]

Positive vibes to you and your family. Makes me wonder how fast you would build without any distraction. Most guys wouldn't try to build four amps in a year, lol!

[M Fowler]

Thanks Tony.

The part I enjoy the most is taking a blank chassis marking the faceplate component holes, centering, center punch, then drill. The IEC is done with drill and file.
Then I use a 18.6mm hole punch for the Belton 9 pin preamp tube sockets. The octals are 30mm again Belton.

I have layout measurement sheet for socket holes and transformer placement. I'm on my 4th board change and plan to make a 5th change on next board. Might finalize the design then.

The second most favorite thing in amplifier building is firing the bitch up and playing.

Mark

[joeboo88]

Mark: # 4 looks awesome, you have really refined your process. Also your work bench looks like its supposed to

[M Fowler]

Thanks Joe, that organized bench is kicking my ass, too tall and no leg room since I put drawers underneath the bench. No choice but to put another top on it, my back can't take standing like I have been.

Mark

[joeboo88]

Hiya Mark. Ok your layout/schematic work fine.
on the plates I get 367 and screens 366. I have the 5881’s biased at 39.0

This is an aggressive little guy..... however if I turn the MV and gain up, the amp farts out, like its being overloaded or something
I did a quick check and all voltages on the pre amp looks good.
I replaced some tubes but still have some checking to do... will get back to you and with a picture but have to take the wife out for dinner....peace, Joe

[M Fowler]

Some guys use 100k/100k rather then 220k/220k grids for the 6V6 tubes and 5k6 grids on each tube.

With today's JJ 6V6 I use 220k/220k.

What power amp design did you attach this AFD #39 preamp to?

My layout is basically a Marshall JCM800 2204 with axial caps rather then can caps.

Also I run a 5751 in V1 because this amp has a lot of gain.

Mark

[joeboo88]

Hey Mark: I used what you have. I figured I would start with your layout used and fix if needed.
I used a 50 + 50 UF cap can and a 40/20/20/20 cap can.
I tried the JJ6v6 and Some good 5881's because I used a 50 watt 7.3K OT ( its what I had left ) My PT is a 275 275 ( Hammond 270 HX )
I didn't use EL34's because the voltages are low - 375 on the plates, and the OT's impedance

I didnt do too much troubleshooting yet as I was busy this past weekend


when I get home from work I'll take a picture and post here

[joeboo88]

Hey Mark sorry to clog up this thread. I have never had a bad OT before until now.
I have no idea how it’s bad, I had it in another amp but it’s sat for a year.
After playing the this amp with the master on one and volume up it sounds glorious. As soon as I turn the master past 2
I get splatter farty sound.
I put in a 20 watt OT that I have saved for a hiwatt build that I’m half done.
The amp sounds great, love the cleans I get..and the AC/DC I get everywhere else.. But I have to find another OT and maybe awhile, Canadian dollar is way down.

Thanks Mark for brining this build to us...much appreciated..
Joe

[M Fowler]

Joe I hope you can find a suitable OT even a 4k Fender 40 to 50w would work.

The circuit is a killer sounding high gain monster.

[pdf64]

... with the master on one and volume up it sounds glorious. As soon as I turn the master past 2
I get splatter farty sound...

Maybe it's oscillating; are you able to scope it?
Have you tried flipping the OT polarity?
Do 1nF caps across the LTP plates and across the last stage's plate resistor help?

Aren't Hammond transformers available at a reasonable cost in Canada?

Best wishes with the health issues Mark

[joeboo88]

Thanks pdf64 and Mark.
I have no means to scope it. the first thing I did was flip the OT leads. For whatever reason when I heard the sound , I thought It was either the choke or first filter caps or the OT ( I used used parts that I had)
I Started with the hardest and tried a temp OT that I have for another build, and that solved the problem. Can Crank the amp and its fine with good voltages
I may have a line on a 50 watt Hammond OT that is 3.2K - but then I have to use EL34's, and my PT ( 275 0 275 ) would only have 350ish on the plates for EL34's
Maybe I'll try that and see how it sounds.
The Bank of Canada ( my wife ) is the other problem..... like a dope I said I wouldn't buy any more stuff this year, Hence why I'm using a bunch of used parts.....

[M Fowler]

So you need a 6V6 OT around 7K and 25 watt?

The Hammond 50w would be over kill but the 3k2 primary is not a problem just use whatever secondary impedance needed to get higher load.