6AQ5 jcm800 lite

[slowferrari]

Okay, build thread. I'm still new here, so let me know (gently) if I'm making a fool of myself. I'm attempting something. It's a point to point jcm800-based three knob amp, with a 6AQ5 output tubes.

I took a week off work, ostensibly to build this amp. I spent a whole day drilling out an 11x7x2 bud chassis, only to get my terminal strips mounted and realize I had overlooked clearance issues freaking EVERYWHERE. This morning, I started running heater wires and decided to put the chassis on the pile and start over.

This time I started with a flat piece of aluminum, the top panel from a 2U rack enclosure in a former life. It was only supposed to be a head, but I've got it laid out like a combo chassis now (so, I guess I'm building another cabinet), because it seemed to work out better in terms of space.

I've got more terminal lugs than I need and I'm switching to lighter gauge wire throughout, mainly because I'm struggling with the mini-7 sockets.

Wiring commences tomorrow.

[TitaniumValhalla]

Cool project! Looking forward to seeing how it turns out.

[slowferrari]

Got the PT terminal block connected, ran the heater wires, and got most of the way through wiring the mains today.

Notes for anyone interested, again, tell me if I'm doing something stupid.

Didn't have the correct colors of most wires, hopefully it won't cause too much confusion if (when) I have to go back in someday.

I'm running 22ga solid core for the heater wires (except from terminal block to the balance resistors, the 20ga stranded I'd planned to use everywhere still worked for that). It became immediately apparent that the mini 7 sockets were too small for the 20ga, and I checked and double checked the current rating of 22ga solid core, and it should be okay at 1.8A of heater current. It makes me a little antsy, but it'll be safe enough, right? The artificial center tap isn't referenced anywhere yet. I'm planning to just tie it to ground initially, I can always change it if I get too much hum, I suspect there will be some.

I should've waited until I had run the heater wires to install the rearmost terminal strip, but I'm not trying to make it pretty, I just want it to work properly, so it's okay if they look a little janky, I'll tidy it up as best I can't towards the end.

Also, I used ring connectors on all the screw connectors and I am sick of soldering ring terminals...

More to follow.

[bmccowan]

Didn't have the correct colors of most wires, hopefully it won't cause too much confusion if (when) I have to go back in someday.

I think that's ok, but I suggest that you be consistent in your wire color use. Keep the wires carrying high DC voltage one color. Keep the signal carrying wires one color, etc.
But you also ask for overall comments and I think you are making things more difficult than they should be. Maybe I am unaware of advantages, but screw terminal lugs are a pain. Your project would be a lot easier if you follow something like the JCM800-6V6 that is on this page:https://robrobinette.com/RR2104_Master_Volume_Micro.htm The posted schematic and layout are for 6V6s, but can be adjusted for 6AQ5s. Or maybe there is similar documentation for 6AQ5s? I see some have built that amp.
Do you have a schematic and layout for this project? Horse and cart mixup may continue to bite you.
 

[slowferrari]

Horse and cart mixup may continue to bite you.

So, yeah.

You're absolutely right, this would be easier if I could follow Rob's project. 6AQ5s are equivalent to 6v6s, so I'm cribbing the majority of this project from his page.

The major reason I can't follow it completely is financial. I was able to scrape together enough money to buy all the parts I didn't already have on hand, but I really couldn't afford to invest in new transformers, choke, turret board, and a bunch of other parts when I had a lot of stuff around that *should* work.

You're right that I need to share my schematic and layout if I want helpful feedback.

So far, I haven't actually built any of the circuit, but I'm now at that point, so I'll post my notes. They're all handwritten at this point because I really don't like working on a computer in my free time, despite the fact that I could've saved some headaches with a couple of CAD drawings, just like I could avoid some future headaches with, say, SPICE models.

Oh, and the screw terminals. I really wanted to be able to pull this transformer in the future if I want to. I'm just feeling some fear of commitment because I worry this won't work out.

[slowferrari]

There's a lot to unpack here, so bear with me.

Apologies again for the handwritten stuff. But here's what I'm working with, with some notes.

Almost everything is Robinette's JCM800 6V6, but with some blanks filled in courtesy of Merlin Blencow's books.

Trying to run through this in order:

V1a cathode bypass cap was going to be switchable, and I'm still not positive 680n is the right value, so it'll be tacked in until I can do some by-ear testing.

Most of the coupling caps have been changed to 10n, because the difference should be negligible, and I have a lot more 10n's than 22n's.

Left the bright caps all in place for now.

The tone control is from Blencowe, the switch moves the hpf corner for some mid range options (at 220p, the curve looks a lot like the stock jcm with mids at zero, the switch provides options for "about flat" and "boosted all to hell") without doing a three knob tone stack. This was simplified because I wanted fewer knobs and fewer parts.

The phase inverter has feedback, but I suspect this may need tuned to work best with the output tubes. I also left out the presence control entirely. If I decide I need it, I've reserved a little space on the back panel to tack one on later.

I expect the power tube grid stoppers to be about 5k, but I was also planning to experiment with that. I might try as low as 1k5 or go higher depending on how the thing sounds.

The power supply is based around this antek transformer:

https://www.antekinc.com/as-05t240-50va-240v-transformer/

I had it in a cathode biased amp previously, which worked, but I honestly didn't like the thing all that much, so I decided to try using it for this and order a new one if I decide to revisit the other one.

The 1k 5W resistor replaces the choke (again, I didn't want to buy any new iron for this), a la Princeton Reverb. I also ordered a 4k7 5W in case I found any reason to drop the screen voltage.

The bias circuit is Blencowe again. Since I don't have a center tap on my HT winding, I'm giving this a try. I copied the circuit directly from here:
http://www.valvewizard.co.uk/bias.html

Also, the indicator is a 12VDC panel lamp which I normally use for pedals. It's a lot more compact than a bayonet bulb holder, and I have literally hundreds of them.

I think that's it. [be gentle]

[bmccowan]

Nicely drawn. I used to love architectural drafting and hated early CAD - what a damn learning curve. But I find using ExpressSCH pretty easy and quick. I understand the sentiment of using what you have. I would still nix the screw terminals, but maybe that's just me. Since you are following a trusted schematic, and some Merlin B bits, it should work out fine.
 

[slowferrari]

I'm a fan of diptrace for transistor and ic stuff, but I only use it for routing boards based on schematics. My plan here is to get it up and running, then document the final build with a proper schematic and wiring diagram which I'll probably do in DIY Layout Creator.

That way I can print a copy of each and stow them inside the cabinet for future reference.

[PRR]

Preamp: upper input puts 200VDC on the guitar cord, which may be bad for the guitar or you.

Power amp: the 16/8/4 taps should be the other way on the winding. "4" gets fewer turns than "16" etc. Just a mis-drawing, not a design flaw.

Hand-drawn of this quality does NOT have to be apologetic. There's keystone and glare but I've seen heaps of "Computer Drawing" that is much less clear. (Drunken mouse syndrome.)

[shooter]

I learn't here Vc gets replaced with B+
Nice drafting, I used to do PC boards in the wire-wrap days, I had my 64-color pack of drawing pencils.

[sluckey]

Preamp: upper input puts 200VDC on the guitar cord

Easy fix...

[slowferrari]

Easy fix...

So, I can say with confidence that I knew where that decoupling cap was supposed to go, but I definitely drew it with a SPICY input.

[PRR]

It is a coupling cap, not decoupling. And while easily fixed, the fact it was not caught on the drafting board suggests a blind-spot in the checking department.

[slowferrari]

Update: flipped the switch today. Continuity tested the entire signal path and power supply beforehand. Power up indicates no shorts, we have a negative bias voltage, and 350VDC HT with no load. Considering I started this over a month ago, I am relatively elated.

Pausing to post and see if anyone wants to point out glaring mistakes before I do something like fry an OT.

[slowferrari]

Update: got impatient and ultimately decided no news was good news. Installed tubes and fired it up.

I'm getting about 275V on the 6AQ5 plates, 260 on the grids. Preamp voltages are a bit low, going down to about 165V at V1. I appear to have sacrificed some of the gain I'd have gotten if it was running at higher voltages, but it's still good for a fair amount of preamp distortion.

Power tubes are also  a bit off mismatched. I biased it pretty cold initially, and with 8.25W on one tube, the other is at about 6.3, so I think I'm going to dig up some different 6AQ5s before I run the thing any hotter/push the master volume at all.

Regardless, I'm calling this a win.

[AmberB]

You could raise the voltage in your preamp a bit by using a 10K resistor in place of the 22K resistor in the power supply filter section...

[tubenit]

Preamp voltages are a bit low, going down to about 165V at V1

I have found I typically like preamp tube plate voltages around 160v give or take some.  I've found I have liked the tone with plates anywhere from 145v to around 180v on 12AX7's.


With respect, Tubenit