I broke my amp

[dwinstonwood]

 
Hey all,
Well, I thought I could modify the AA864 amp I recently built, but I broke it.

I've attached the schematic that sluckey worked on and finalized for me.

So, I decided to do away with the gain control and replace it with a 220K/220K voltage divider as shown on the Fender schematic (link below).

I figured I could convert the gain pot into a midrange pot. I know it's not the right value, but I thought I could at least wire it up and try it out. It should be a 10K, or 25K, not a 500K.

I followed Hoffman's and sluckey's AB763 schematics and I can't see where I made the mistake(s).

Also, I'm not sure where or how to ground the second 220K resistor in the voltage divider. I tried running it to the buss that grounds the pots and I just get really loud hum and scratchy static. So, I tried grounding it to the power ground and got the same results.

If I fire it up without the second resistor the amp is quiet, but no signal is reaching the speaker. That might be because of a tone stack wiring error.

Here are two photos. I can upload more. The pot order is: volume, treble, bass. middle.

Thanks!

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa864_schematic.pdf

[dwinstonwood]

Here's a diagram of how the tonestack is wired. If there's a mistake I can't find it.

[centervolume]

The order of the controls appears off; isn’t volume next to the inputs ?

Or perhaps your drawing is from the perspective of the front of the amp not from the rear (as you photographs are)

[dwinstonwood]

Thanks centervolume,
My drawing is looking at the backs of the pots (inside the chassis).
From left to right on the front (outside) of the amp is: input, volume, treble, bass, mid.
It's a confusing - to me - chassis layout, like a mirror image of a Marshall.

[shooter]

a wise tech once told me, "if it worked when you showed up and it don't when you want to leave, it's always the last thing YOU did" 

[dwinstonwood]

Yep, shooter.
It actually sounded great and was very quiet 12hrs ago.
I'm just going to put it back like it was.
Thanks!

[sluckey]

Here you go. Keep in mind... That tone stack will not be happy with a 500K MID pot. You can just put a 22K resistor between the wiper and the ground lug until you get the correct pot.

EDIT... I would not use the .001µF cap in the voltage divider. That was all about bass guitar in the orig Bassman.

[dwinstonwood]

Wow! Thanks sluckey.

OK, I'll wire that up tomorrow with fresh eyes.
Grounding the second half of the voltage divider on the board is the ticket, I'm sure. Yeah, I was going to leave that .001uF off; I figured it just drained off highs.
And, I've already order a 10K-A pot.

While I'm back inside this thing I'd also like to add those bias tip jacks shown on your layout. It looks like Hoffman sells them.

Happy New Year sluckey, and thanks again!

[dwinstonwood]

I just got an email tonight from the cabinetmaker I ordered from. My cabinet should arrive from TRM Guitar Cabs on Monday.
It's a custom designed 12" combo. It will be taller than a Fender combo, and a bit more narrow... sort of like a Matchless Avalon.
Also, I've designed a faceplate in Inkscape. I bought some two-layer laserable acrylic sheet. When I get back to work next week I'll cut it on out laser cutter.
It will be a cool amp!

[sluckey]

I was just looking at your pics. It would be neater to route that second 220K under those two Xicon caps. It's a short, straight, diagonal path to the ground lug. I'd probably put insulation on the resistor leads if they get too close to other stuff.

[Glenn]

Dwinstonwood, did you not like the gain control, from when you first built this?

Just curious because I’m contemplating building this amp.

[dwinstonwood]

Hey Glen,
The gain control was fine. It worked well, nothing wrong with it.
But, I think that in order to really get the best use out of it - without pedals - the amp needs to be turned up louder than is reasonable for me at home (I'm not a working musician).

I would totally recommend building this amp. It sounds very good. With the high-ish plate voltages I ended up with (468V) and the diode rectification it sounds tight, fast and strong to my ears. Sluckey's layout and board design make it a straightforward and relatively easy build -  so, it's a quiet, stable amp. Definitely follow Hoffman's 6V6 Plexi sequence, because it is a small chassis and is best built in "layers," so to speak:
https://el34world.com/projects/Plexi6V6.htm (read from bottom of page to top)

I hope you give this project a try. Thanks!

[dwinstonwood]

Well, sluckey, I followed your layout and instructions, but there was still no guitar signal to the speaker, just the same loud hum and hissing. Maybe it's the 500K pot, or maybe the third triode is doing something weird.

I wired it back the way it was with the gain pot. Fortunately, I didn't damage anything because it's now as quiet as ever. Whew!

I did leave the one 220K resistor on the board (the one in my photo above). I ran out of 470K's - the part you suggested. I'll probably try a 470K when I get more.

The only other difference from the original schematic is that I used an 0.0022uF cap leading into the PI because I didn't have an 0.001uF. It probably sounds better, anyway.

Lesson learned: I should just build your AB763 Lite with 6L6's if that's what I really want.

Glen, I think you should build one here on the forum! It's a very good amp.

So, thanks again! It's now time to go eat some more of the Virginia country ham I baked yesterday... cloves and brown sugar crust, and sliced paper thin.

[sluckey]

Bet you didn't put a 22K resistor across the pot like I suggested.

Look, you're doing two totally unrelated mods, so do the voltage divider mod and check it out. Then wait for the proper MID pot to do that mod.

[Mike_J]

I just got an email tonight from the cabinetmaker I ordered from. My cabinet should arrive from TRM Guitar Cabs on Monday.
It's a custom designed 12" combo. It will be taller than a Fender combo, and a bit more narrow... sort of like a Matchless Avalon.
Also, I've designed a faceplate in Inkscape. I bought some two-layer laserable acrylic sheet. When I get back to work next week I'll cut it on out laser cutter.
It will be a cool amp!

Can you elaborate on the two-layer laserable acrylic sheet and what size it comes in? I have used Inkscape for a number of years but have used a local print shop to print it on a vinyl sheet covered in a lamination. He can do as wide as 30" which isn't always needed but sometimes comes in handy. Don't really understand the laser cutter either. My guess is the acrylic has decent thickness to it. Probably better than average.

[dwinstonwood]

Mike, this business was recommended to me by the director of the maker workshop at the university where I work:
https://www.inventables.com/categories/materials/acrylic?utf8=✓&selected_filters%5BEffect%5D%5B%5D=2+color

The prices are reasonable, but shipping was more that I like to pay for just one item.

I bought a 12"x8" sheet of the gold/black:
https://www.inventables.com/technologies/brushed-gold-on-black-laserable-acrylic-sheet

I can get both the front and back panels for this amp out of that one sheet. (Hoffman's Stout chassis is 12"x3") They sell up to 12"x24" sheets, plenty wide for most amps. The last faceplate I had made at a local engraving shop - for a Vox AC4 build - was $37!

[dwinstonwood]

Sluckey, somewhere I clearly wired something up wrong. I went over it several times, but couldn't find the error. I did add the 22K between the middle wiper lug and the lug connected to the ground buss. And, I put insulation on the 220K resistor leads and routed it under the caps.

I even went so far as to remove the wires from the pots, and clean off all the old solder with braid before starting over. I'm guessing I missed an important ground connection somewhere, or maybe somehow allowed DC onto a pot.

I can see and comprehend the wiring of tube sockets and transformers, but pots and tone stack connections continue to baffle me. Everything I read talks about frequency cutoff, HP and LP filters, etc., but doesn't seem to explain how things need to be physically wired and why.

Thanks again. I have a new project I'm brewing up that could either be pretty cool or a total disaster. I'm considering the AA864 a success, and I'm going to finish drawing up this new "hybrid" creation while I'm still on vacation.

[dwinstonwood]

...do the voltage divider mod and check it out. Then wait for the proper MID pot to do that mod.

That sounds good. If the voltage divider doesn't add any hum or hissing, then I'll leave it and wait for the pot to get here.

[dwinstonwood]

I added the second 220K resistor back into the circuit to complete the voltage divider, and the amp sounds great. Very quiet, and surprisingly, turning up the gain knob seems to sound better than before, not as harsh or cold sounding. Maybe because the overall volume is slightly lower. I don't know.

I'm going to leave it like this for now - with the gain control, and play the amp in its new cabinet (which is arriving Monday). Then, I'll get a truer impression of how it sounds.

I have no idea what was causing the hum and hissing. I guess it was using the wrong pot value.

Thanks!

[sluckey]

I added the second 220K resistor back into the circuit to complete the voltage divider, and the amp sounds great. Very quiet, and surprisingly, turning up the gain knob seems to sound better than before, not as harsh or cold sounding. Maybe because the overall volume is slightly lower. I don't know.

Say what!!! You're supposed to remove the gain pot when you put the voltage divider in. The voltage divider resistors simulate the gain pot set half way up.

[dwinstonwood]

Oh. That does makes sense, now that I think about it.

Well, as long as it's not hurting anything, I like the way it sounds.

But, at least I know that the noise issues were caused by the 500K pot. When the 10K pot arrives I'll be starting with a working amp.

[shooter]

I'll be starting with a working amp.

again 

[dwinstonwood]

shooter this might end up being a "sandbox" amp... hands-on learning through mistakes.
But, it's a good thing sluckey had me add the 220K 3W bleeder resistor on node A, as much as I've been poking around in this amp.

[glass54]

Hi dwinstonwood.
On a completely different topic but relating to this build, be careful with your solder joints. We all chop/change components to optimise/voice our amps, but be careful to leave nice silver well flowed joints, because you may get caught and create your own fault with dry powdery joints.
Please see resister/lead on turret (circled). I can only assume a less than perfect joint in this photo, while surrounding joints are very good.
Just trying to be constructively critical, so please forgive me. 
Kind Regards
Mirek

[dwinstonwood]

Thanks Mirek!
That was bothering me, too.
I redid the connections on that turret, and I even replaced some other wires where I had scorched the insulation.

[sluckey]

Since you like the sound with the voltage divider and the gain pot, do you intend to keep the gain pot and simply add the MID pot?

[dwinstonwood]

Well... there are three extra holes in the front of the Stout chassis. I hadn't though about having both, but why not!

That sounds like the way to go.

[shooter]

shooter this might end up being a "sandbox" amp.

It's a nice build, and i'm actually here cheering you on, in a dave sorta way


aside, I needed a password reset, talking with the rep, she needed my user name, told her sandboxer, i got a laugh, said that made her day

[dwinstonwood]

I had to re-order and rotate this drawing so that it depicts the actual setup in my chassis.

Looking at the backs of the pots - from inside the chassis - does this wiring look correct?

Thanks.

[sluckey]

This is right and it's also how you have it wired in the chassis.

[dwinstonwood]

Thanks!

Dang, and I see how I made the mistake. When I rotated the pots in the drawing 180 degrees I didn't catch the fact that the two treble pot wires needed to swap positions to stay on the correct pot eyelets!

[sluckey]

Why are you messing with that Hoffman layout? You better stick with the layout that you used to build the amp. Otherwise you'll likely end up in a dark rabbit hole and I ain't gonna pull you out.

I can rotate the pots on my drawing if you want. But I'll wait until you are through messing around. LMK.

BTW, I wouldn't bother with installing bias test points. What you have is fine. Besides, you're gotta have the chassis opened to turn the bias pot.

[dwinstonwood]

Thanks sluckey; I'm done messing with it. 
Yeah, I guess it would be nice to have the pots rotated 180 degrees on the layout reflect the way they are mounted. At least it's a lot easier for me to read things that way. Though, you've already gone to more than enough trouble!

Hopefully, by this time tomorrow night I will have it test mounted in the chassis with its own speaker. The 10K pot won't get here until mid-week. I should have time to engrave the front and back control plates Tuesday.

Don't laugh at the name. I haven't really decided what to call it yet.

[sluckey]

I kinda liked the name "Musing".   

[dwinstonwood]

Hey Steve, this is a bit off topic, but in another recent thread you referenced your AB763 Bandmaster build. I noticed that both the Bandmaster and the AA864 used the same 125P7D PT (at least on the schematics).
Does your amp have the original 125P7D, or did you actually find a current production PT that provides 430VDC on the plates?
Thanks.

[sluckey]

My bandmaster has the original PT.

[sluckey]

Some updated drawing. I'm closing the book!   

[dwinstonwood]

Super! Thank you sluckey for keeping my gutterball project in the lane.

Your layouts and schematics are excellent. Appreciated.

I hope some people build this amp. It's not that expensive if using the Hoffman Stout chassis. And, it has a few (not too complicated, even for me) options for gain: setting a hi or lo input with different grid stoppers, keeping or leaving out the voltage divider and/or the gain pot, and using different V2 tubes - 12AX7, 5751, 12AT7, etc.

I ended up preferring the amp with lower gain, even though I started out wanting it higher! I have a 47K input grid stopper, a 5751 V2, and both the voltage divider and the gain pot. But, changing one resistor, removing two others, and swapping one tube can make it into a different amp (especially with humbuckers).

Cheers