Hello from a new member

[stw307]

JCM 84
is a parts-at-hand project, JCM style preamp to SE EL84

After repairing and modding a few amps, I decided to do my own build. Having some education and some experience, I did not order a kit, but went the other way. The deep end, designing from scratch, felt too heavy a job, at least right now. So I jumped in the middle of the pool, studied countless verified designs, copied and modded some of them to suit my stockpile of spares. Many books were read, but special thanks go to EL34world and RobRobinette sites for bringing all these theories to life with understandable, plain English.
Now, to the project itself: The goal is to build a "simple", moddable / modded 2-tube preamp with parts at hand. This baby's preamp is going to be a testbed / work horse for those other, bigger amps to come.

Parts at hand
OT_tr SE EL84 from BH5H
PWR_tr capable of at least four tubes. Supposedly 5th heater would overload?
Hammond 4H high-R coil
Selection of tubes
Stuff'n'sh*t

Power amp
Power amp section is a shameless copy of a Blackheart, for obvious reason, the OT_tr. And BH5H has a pentode / triode switch! I dont fully understand the operating principles of the switch, I read somewhere that it is not a true triode / pentode? That might confuse a bit?

Preamp
Something I found online. Modified. Single channel JCM style higain.

Power supply
PWR_tr 280-0-280 to rectifiers
6.3VAC to artificial 0 and heaters/LEDs
5VAC to vacuum rectifier tube

PWR section
PWR_tr 280-0-280 to mil-spec 5C4M or 4x 1N4007 w/ load distribution.
Rect_tube 5C4M backed up with 2 diodes for safety and longevity.
pwr_ON+stb_OFF bleeds through diodes & R2W to fill the res caps.
So, standby switch is AC30 style, because 5C4M might not like the heavy load of all these capacitors when lit up.

NFB section flew over my head. Found some online info though. According to Excels and others experience = NFB is trial and error. Feedback to V1A or V2A cathode?
Please look at the schema and report the bugs. Some of those are under investigations already, of course. I can already see some of the bugs myself. And I am aware, that if I flip all the switches to highest gain, this thing might become an oscillator. There is a plan B... I'd like to hear from you guys, who have built similar things. Thanks!

- Mike

Edit. Filament grounding and diode layout fixed

[Latole]

Welcome to the forum.

[stw307]

Welcome to the forum.

Thank you! And thanks for better embedding of the pdf. I'll try to do that myself the next time. Supposedly it takes a few tries, though...

[stw307]

JCM 84
is a parts-at-hand project, JCM style preamp to SE EL84

modded 2-tube preamp with parts at hand.

Power amp
Power amp section is a shameless copy of a Blackheart, for obvious reason, the OT_tr.

Power supply
PWR_tr 280-0-280 to rectifiers

NFB section flew over my head. Found some online info though. According to Excels and others experience = NFB is trial and error. Feedback to V1A or V2A cathode?
Please look at the schema and report the bugs. Some of those are under investigations already, of course. I can already see some of the bugs myself. And I am aware, that if I flip all the switches to highest gain, this thing might become an oscillator. There is a plan B... I'd like to hear from you guys, who have built similar things. Thanks!

*two-tube preamp = 3-tube amp with EL84
*PWR TR 275-0-275

Power section schema of a BH5H calls for 103 1KV condensators in the rectifier section. I found the original board, it has 4x 630V in it. I went with the schematics, one kilovolt.

Studying NFB, I think I'll breadboard a pot and scope the whole amps signal path. Scope saves me from a lot of trials. Hopefully.

[stw307]

This is my RF shield.

Looking good from 5ft away...

The beginning of a journey!


Wiring shall begin with the basic functions. All of the switches and mods will be added up later on. Any comments on this approach?
I'm wondering, if the depth adjustment would have any audible/remarkable function on a little amp like this. Maybe I'll just dump it?
Any comments about the negative feedback? For me, it is the big mystery in all of this. Supposedly it is on the right side now, and not making positive feedback.

- Mike

[bmccowan]

Welcome!
I think you need a few more holes in that chassis.
I concur with not using a "kit." I started the same way and when you get a good sounding amp it is satisfying.
But, you state "The goal is to build a simple" Forgive me, but I am not seeing simple in that schematic. SE amps are already nosier than PP, and when you add a lot of features and wires, you risk producing a noise machine.
The best SE amps are simple IMO - it does not need to be as simple as a Champ, but..
And you ask, "All of the switches and mods will be added up later on. Any comments on this approach?" Having learned the hard way, I suggest upping the time spent planning and drawing, keeping the soldering iron at room temp until you have a solid plan. Folks here will gladly help with schematics and layouts. The projects that were built with little planning are the ones that end up in lengthy trouble shooting, more chopsticking than seen at a Sushi bar, and often a complete do-over.
Anyways - that's my 2 cents. Regards.

[stw307]

Welcome!
I think you need a few more holes in that chassis.
I concur with not using a "kit." I started the same way and when you get a good sounding amp it is satisfying.
But, you state "The goal is to build a simple" Forgive me, but I am not seeing simple in that schematic. SE amps are already nosier than PP, and when you add a lot of features and wires, you risk producing a noise machine.
The best SE amps are simple IMO - it does not need to be as simple as a Champ, but..
And you ask, "All of the switches and mods will be added up later on. Any comments on this approach?" Having learned the hard way, I suggest upping the time spent planning and drawing, keeping the soldering iron at room temp until you have a solid plan. Folks here will gladly help with schematics and layouts. The projects that were built with little planning are the ones that end up in lengthy trouble shooting, more chopsticking than seen at a Sushi bar, and often a complete do-over.
Anyways - that's my 2 cents. Regards.

Thank you sir, very good advice & opinions.
Yes, I agree, we're not talking about an amp simple as a Champ, but still, we're using less than a half-foot of a turretboard. So it is on a simpler side of an amp, so to speak. And about all of those features; the build is going on ~halfway, I'll just leave all "extras" off 'til I have an amp without buzz and hum. Just because of the point you brought up, I like sushi but not the chopsticking...
To put all of this another way, what I have is a Blackheart with preamp replaced. That's simple. What I'll have later on, we'll see about that.

I am fully prepared for a few complete tear & repair & do-overs. After all, this is the first one. One has to be humble.

-Mike

* edit. I presume adding mods to a functional amp is far easier than assembling a pile of cords and switches and trying to guess which one is the problem.

[bmccowan]

I presume adding mods to a functional amp is far easier than assembling a pile of cords and switches and trying to guess which one is the problem.

Oh yeah! And made much easier with sufficient planning and drawing. I recall Sluckey saying that he spends more time drawing/tweaking a schematic and a layout for each project than he spends physically building. I have often been guilty of shortcutting the planning/drawing stage and ended up regretting it - modding chassis and/or turret board to make room for something forgotten. Since you are breadboarding, you are likely already doing this, but a schematic of your simple base amp, and an intended layout of both base and full-featured amp would be helpful IMO. Regardless - have a blast!

[stw307]

I presume adding mods to a functional amp is far easier than assembling a pile of cords and switches and trying to guess which one is the problem.

Since you are breadboarding, you are likely already doing this, but a schematic of your simple base amp, and an intended layout of both base and full-featured amp would be helpful IMO. Regardless - have a blast!

I do have a layout schema of a full featured amp. It is drawn by hand, and it is certainly NOT something I'd upload. Messy s**t.
I made some controversial choices concerning layout and lead dress. We'll come back to that later on.

What I have on my hands today, is a snakepit and endless confidence on my skills. 
Hopefully I'll get a photo or two tomorrow, so you all can analyze the layout & other stuff.
Inner guts are at least interesting... 

[bmccowan]

I find both ExpressSCH and DIYLayout to be easy to use and even fun. My hand drawn schematics and layouts look like they belong on the refrigerator of a family with a two-year old.

[stw307]

I made two mistakes with the wiring.
I used too thin a wire in the power amp section. I woke up looking at the drawings, wait what, 5W resistor and 22awg wire!? 
And maybe I should've used solid core thru the whole amp. Would've been easier to keep the wires straight & tidy. What kind of wire do others use?

I might've shot myself in the leg with that can cap hole. But hey, there was plenty of empty plank between diodes and power amp... It was easier to solder this way.
Photo attached. And schematics which shows just the parts installed as of today.

Grounding:
AC mains and heater artificial tap to GND_1
Power amp and power caps star grounded to GND_2
Daisychain preamp-tonestack-input jack-GND_3

And now back to soldering. Gotta replace those wires with sturdier ones. First tear & repair before even connecting to mains... 

[Willabe]

I made two mistakes with the wiring. I used too thin a wire in the power amp section. I woke up looking at the drawings, wait what, 5W resistor and 22awg wire!?

No, that 22awg is probably fine.

Big amps like Fender Twins and Marshall 100's used 22awg for the signal wire. This amp is a little PP EL84?

The heaters for the big power tubes need a larger awg. Fender used 18awg. The little preamp tube heaters are fine with 22awg. The PT choke and OT fly leads are heaver than 22awg.   

[stw307]

No, that 22awg is probably fine.

Big amps like Fender Twins and Marshall 100's used 22awg for the signal wire. This amp is a little PP EL84?

The heaters for the big power tubes need a larger awg. Fender used 18awg. The little preamp tube heaters are fine with 22awg. The PT choke and OT fly leads are heaver than 22awg.
[/quote]

All of the heaters are already 20awg. The mistake was to use 22awg in everything else. Even in B+ to power switch...
Now EL84 connections (not signal) and most of the HV are replaced with 18-20awg and 1,5mm2.

Thanks for the info! PT, choke and OT are as is, all of the original wires were long enough to reach their destinations.

- Mike

[Willabe]

.... The mistake was to use 22awg in everything else. Even in B+ to power switch...
Now EL84 connections (not signal) and most of the HV are replaced with 18-20awg and 1,5mm2.

No, you didn't need to do that.

Fender used 22awg for their power and standby switches. They used 22awg for ALL the B+ dcv. They used 22awg for ALL the tube socket connections except for the OT plate fly leads.

And that was with Fenders biggest amps, Marshall, Vox, Ampeg, Gibson, all of them. Your amp is just a little SE EL84 5w amp with 2 little preamp tubes. And EL84's take lower dcv than most other power tubes.   

That's why I told you about only the power tube heaters and PT/choke/OT fly leads. Everything else was ALL 22AWG.

You run into problems with heavier gauge wire. Best not to use it if you don't need to. It's harder to work with all the way around, harder to get in place to solder, it's harder to get through the tube socket pins openings, it's harder to get to stay in place after it's soldered up, it's harder to rout through the amp.   

You might need to do more research/reading/studying before you go further. 

[Willabe]

And made much easier with sufficient planning and drawing. I recall Sluckey saying that he spends more time drawing/tweaking a schematic and a layout for each project than he spends physically building.

This, to me, is the key.

Any build I did, I spent 5 to 10 times the amount of time with the layout drawing and schematic drawing then the actual build. That's no exaggeration.

To many guys just want to get to the soldering and I understand that, but your gonna mess it up going to fast with the layout.

Much better to know for sure it's all going to fit and not have parasitic problems. 

[stw307]

.... The mistake was to use 22awg in everything else. Even in B+ to power switch...
Now EL84 connections (not signal) and most of the HV are replaced with 18-20awg and 1,5mm2.

Fender used 22awg for their power and standby switches. They used 22awg for ALL the B+ dcv. They used 22awg for ALL the tube socket connections except for the OT plate fly leads.

Oh, that's very interesting indeed. Should've asked first. But I already changed those wires.  So, nothing dangerous but potential noise sources.
Damn. I feel stupid now. But on the other hand, that's good, better ask than build + blow up.

I'll upload some photos. Gotta fix that broken heater wire.
Is there anything I should do about this build, before I connect it to mains next weekend? First through bulb v-dropper w/o tubes of course. Then w/o bulb, and I'll meter / scope the whole thing thorough. Everything is now double-checked, I'll check 1-2 times more before powering up.

Thank you all, this info is priceless!

[stw307]

Some photos.

[bmccowan]

Damn. I feel stupid now.

No, not stupid. Stubborn I could go with.
I still suggest that you draw that out and post. Willabe is one of the experts - I'm not - and he would go over the layout with you.
But, I suspect you are not going to do that, so I suggest you go here:https://el34world.com/Forum/index.php?topic=2376.0
Following a known procedure will help you avoid various calamities.

[Willabe]

The wires coming out of the PT look really close to the rectifier tube.

You got big trouble if those wires get so hot the insulation melts and they short out. 

What ever company made those transformers, they should have run the fly leads out of the very bottom of the end bell cover, to protect them. I've never seen a company do it like that before.

It's not correct and they should have known better. 

[stw307]

What ever company made those transformers, they should have run the fly leads out of the very bottom of the end bell cover, to protect them. I've never seen a company do it like that before.

It's not correct and they should have known better.

Made in China. Didn't find any info online, but it is Blackheart original. Thanks, I'll do something about it.
Looks as good as Bugera's PT molex back in the day... 

With all the concerns you guys brought up, I'm pretty sure this thing will hum and buzz like hell as is. I'm also confident, that all the connections are safe, sure and sound. Every solder is tested as solid, one could lift up the amp with one wire. Supposedly everything is also in the right place... So, I don't see any real reason not to power up this little amp. If any of you see a reason, pls address the problem.

I'll draw the layout again with DIYLayout or similar. I now see, that this is essential.

- Mike

[HotBluePlates]

The wires coming out of the PT look really close to the rectifier tube.
...
What ever company made those transformers, they should have run the fly leads out of the very bottom of the end bell cover, to protect them. ...

The PT and choke could be rotated 90º to move the wires out of danger.

... With all the concerns you guys brought up, I'm pretty sure this thing will hum and buzz like hell as is. ...

I'm not "confident it will buzz & hum."  Might sound just fine.

There's a guy on YouTube who poo-poos basically everyone's amp.  To hear him tell it, no one makes anything properly.  And yet, nearly all of the amps he maligns sound & perform just fine for most users.   

[stw307]

The wires coming out of the PT look really close to the rectifier tube.
...
What ever company made those transformers, they should have run the fly leads out of the very bottom of the end bell cover, to protect them. ...

The PT and choke could be rotated 90º to move the wires out of danger.

... With all the concerns you guys brought up, I'm pretty sure this thing will hum and buzz like hell as is. ...

I'm not "confident it will buzz & hum."  Might sound just fine.

There's a guy on YouTube who poo-poos basically everyone's amp.  To hear him tell it, no one makes anything properly.  And yet, nearly all of the amps he maligns sound & perform just fine for most users.   

Rotating, well that was too obvious to pop into my brain. Oh darn... I'll have a laugh on myself 

About the hum: Expect the worst to avoid the disappointment. If it works, it works, great!
Making things wrong or a tad off the par is a part of the learning curve. You learn the "what and why" kind of stuff. If one has a proper schema and a proper layout at hand and one knows how to solder that together, what does one learn?

[bmccowan]

If one has a proper schema and a proper layout at hand and one knows how to solder that together, what does one learn?

Oh trust me - speaking from experience, one can still make plenty of mistakes even with a proper schematic and layout in hand. But I understand. Someone recently asked me how I learned to do carpentry. One mistake at a time was my answer.
You have a SS rectifier in addition to tube, right? So you can first test without the rectifier tube.
HBP - I think I know who that YouTuber is. If so, he apologizes profusely as he slams everybody's work. It's rather annoying. 

[bmccowan]

Hey STW. Did you fire her up? If so, I hope it went smoothly.

[stw307]

Hey STW. Did you fire her up? If so, I hope it went smoothly.

Got myself home just an hour ago, so no. Thanks for asking, though!
Found my trip switch, but no bulbs. Only halogens. I don't like them. It's been a while since I last needed a voltage dropper. Variac would be good...
I'll come back to this topic before Monday.

- Mike

[bmccowan]

Thanks for the update. Again, hope it goes well. And yes, a variac is a nice tool to have.

[stw307]

Hey STW. Did you fire her up? If so, I hope it went smoothly.

Took me half an hour to assemble that voltage dropper. Didn't find a regular bulb though, that one is a 35 watt halogen. I think it does the job the way it's supposed to do.

Thanks for the tip Willabe, tubes get hot! I took apart the bells and turned the whole PT upside down. Awkwardly the result is, that the 230AC leads lean towards the rectifying coil. 

[stw307]

Check, check and check it one more time!
Found one diode wired in reverse. One resistor was missing. Heater voltages 6.3 and 5 were vice versa, I must've read those color codes wrong.
An hour or so of soldering and re-checking, now it was time to flip the switch. After several checks I removed the voltage dropper and installed the tubes. One 12AX7 was flashing its heaters when powered up. I checked that tube socket one more time, no problems. The tube was just dead. No wonder, it was at least 20 years old and heavily used. After replacing it with a properly working unit, all went as good as things could go.

No hum, no pops, nothing wrong at all. Only "normal" low hum if you max out every pot.
This amplifier has only one problem. That Hammond 157L resists too much, Vin ~350V Vout ~280V. As a result, performance is poor. I suppose that choke is from a bigger amp. Also, it was a 14H unit, not 4H... I misread the label. I'll replace it with a resistor. Or another Hammond, which I don't have on shelf.
I really can not say anything about the sound yet, as I suppose those tubes are strangled close to death. As an example, TMB tone stack kills almost all of the signal when minimized. On the opposite, everything maxed, I can still put my ear close to speakers. Two 12" Fenders, btw. That EL84 should deliver 3-7W, that would be hell of a noise in my livingroom in my experience.

[stw307]

Check, check and check it one more time!

This amplifier has only one problem. That Hammond 157L resists too much, Vin ~350V Vout ~280V. As a result, performance is poor. I suppose that choke is from a bigger amp. Also, it was a 14H unit, not 4H... I misread the label. I'll replace it with a resistor.

That got even worse. Voltage drop over 1K/5W is 100V, which equals current draw of 100mA. So V-drop of 100V * 0,1A = 10W. One could fry bacon over the resistor. I have to figure this out. Anyhow, that was a mistake to use a 5W unit, even for a brief test.

Edit. Same calculations with the choke on, V-drop 70V / r429 = 163mA = 11,4W which is a bit over the rated 150mA. Something here is sucking as much amperes as you can throw at it. Next weekend I'll meter the whole thing once again with the tubes out. The power tube is suspicious, it is also old and abused... I would ask for any other ideas, but the fault could be literally anywhere.

[tubeswell]

There's a guy on YouTube who poo-poos basically everyone's amp.  To hear him tell it, no one makes anything properly.

Got a link? (I'm in the mood to chuckle)

[stw307]

Check, check and check it one more time!

This amplifier has only one problem. That Hammond 157L resists too much, Vin ~350V Vout ~280V. As a result, performance is poor. I suppose that choke is from a bigger amp. Also, it was a 14H unit, not 4H... I misread the label. I'll replace it with a resistor.

That got even worse. Voltage drop over 1K/5W is 100V, which equals current draw of 100mA. So V-drop of 100V * 0,1A = 10W. One could fry bacon over the resistor. I have to figure this out. Anyhow, that was a mistake to use a 5W unit, even for a brief test.

Edit. Same calculations with the choke on, V-drop 70V / r429 = 163mA = 11,4W which is a bit over the rated 150mA. Something here is sucking as much amperes as you can throw at it. Next weekend I'll meter the whole thing once again with the tubes out. The power tube is suspicious, it is also old and abused... I would ask for any other ideas, but the fault could be literally anywhere.

Without tubes everything is fine. Multimeter shows good voltages everywhere. Oscilloscope, no big surprises. Very very good, dead flat DC at preamp B+. Plug the tubes in, and power up. Voltages raise to healthy levels everywhere (~310V plate voltage) but everything starts to drop after a second. Symptoms are the same with or without preamp tubes, with each and every EL84 I took to test. As I see and meter it, EL84 has a quiescent wattage of 11W, very close to its limits. IMO, next logical move would be disassembling power amp section and triple check everything. Any other suggestions would be highly appreciated?

[bmccowan]

Some things to consider before you tear it apart.
You mention no tubes and all tubes in. Did you try installing one tube at a time and testing? Did you follow the startup procedure Doug has posted? Or another known process? Typically, it's no tubes - test, add the rectifier - test, add the power tube(s) - test, add preamp tubes one at a time testing after each one.
You mentioned using a 35W halogen lamp as your current limiter - I'm not sure if that is enough wattage. Not sure, as I use a variac.
You have all used tubes? No fresh tubes to try?
Longshot - but I guess they all are - you might want to disconnect the bypass capacitors.

 

[stw307]

Some things to consider before you tear it apart.
You mention no tubes and all tubes in. Did you try installing one tube at a time and testing? Did you follow the startup procedure Doug has posted? Or another known process? Typically, it's no tubes - test, add the rectifier - test, add the power tube(s) - test, add preamp tubes one at a time testing after each one.
You mentioned using a 35W halogen lamp as your current limiter - I'm not sure if that is enough wattage. Not sure, as I use a variac.
You have all used tubes? No fresh tubes to try?
Longshot - but I guess they all are - you might want to disconnect the bypass capacitors.

Had to take a two week trip abroad. Haven't had the time to examine. With the halogen lamp everything was fine as I followed the procedures. Another fast check with full voltage points towards power amp. The tubes are not the problem, I have a good bunch of them and I've tried many. None are redplating or anything. So maybe that guess of yours is not so long of a shot after all. All caps and the output transformer are my next target. Maybe I'll begin with the power tube's bypass cap. Thanks.

[stw307]

Things advance slowly.

I got the thing up and running. With full voltages on, it measures over 10 watts of idle dissipation. No wonder those guts get warm. I was curious enough to play it through one of my 4x12" cabs and damn that little amp was loud!

Idle voltage over EL84's cathode resistor was ~25V. Any playing, it sagged to ~20V. Loud playing cut the voltage near zero. I'll replace both the cer.res and the cap some day.

[AlNewman]

I didn't read the whole thread, but your schematic says 250R for the cathode resistor, and you're dropping 25V across it?   And that's a single EL84 output section?

[stw307]

I didn't read the whole thread, but your schematic says 250R for the cathode resistor, and you're dropping 25V across it?   And that's a single EL84 output section?

Well yes, but not on purpose, that's the problem I've been chasing around. It is a SE EL84, output @ ~5W by design. These JJs can take tremendous loads of raw abuse! I took some 25 watts out of a single tube. Only for a minute, but still...

I also found the cause of this fault. It is a very obvious beginners mistake in the power tube section of the schema. Now it is corrected & soldered, time to test.

[stw307]

This thing is fully working, thank you guys for all the tips. No wonder the problem was a nightmare to trace, because it was in the schematics.
I'll post the corrected one some day, so you can take look at the old one. We'll see if anyone spots the mistake.
Of course there was a bunch of minor issues to fix, too. All of these were my own fault as usual 

And finally some reasonable readings:
Tube rect: Ua 270V, Ik 33mA = 8,91W idle dissipation
SS rect: Ua 307V, Ik 39mA = 11,973W (so it is better to use JJs or some other tube with at least 12W specs)

Now back to soldering, it still misses a pilot light and some other things.

[stw307]

If you look at the first post and the schematic, EL84 was missing a grid resistor. One 220k from grid to ground fixed this problem. I've been playing many hours with this amp, it performs very well. No need to dime the master even when playing with a band. Looks funny over a full stack, though