new project 12AQ5 S.E.

[ALBATROS1234]

i whipped this up today because i wanted to see what my new tubes sounded like compared to my 1st amp design/build. i built the basic circuit on breadboard then subbed resistors and caps until it started sounding decent. i dont have a tone control yet because i am having a weird problem. ultimately i want to add a tone stack and possibly tremolo and/or reverb . also i may add a second power tube for parallel s.e. i just wanted to tweak basic preamp and tone before adding on a ton of complexity. anyway it sounds decent at low and moderate to medium loud volumes, it is breadboarded so there is some hum just like my 1st build (which went away when i soldered it to a circuit board.)on this amp when i start to turn it up loud to check headroom and performance i get a choppy helicopterish oscillation, maybe 2 clicks per second approx. not as much on single notes as on chords.theres also a touch of screech when loud. perhaps i need a 3rd filter cap?

other than that it actually sounds pretty good, not necessarily better than my 1st build just different. the first was the 7f7/6f6 se which has a raunchier overdrive(in a good way) which sounds like low tech garage nasty when i crank the drive like a cheezyish mesa boogieish/ marshallish todays project with the 12aq5 it sounds smoother more vintage supro like perhaps due to the 12aq5 which as you guys know is closer to a 6v6 than anything. anyway i have heard of parasitic oscillation in tube amps, is this whats happening?  why is it only at loud volume?

[DummyLoad]

what's Vrk of 12aq5? anode r values for 12ax7?

[shooter]

read my mind
also what's the volts ACrms that's driving the PA tube

[ALBATROS1234]

the c (preamp)node is putting out 284vdc which i split in 2 voltage on plate 1 is 108vdc that one has the 1.5kohm kr and the 2nd plate has 124vdc due to the larger 2.2kohm as per the bottom lefthand corner of my schematic.the plate of the power tube is getting 266vdc after it passes through the ot and screen is getting 286vdc

im not sure what you mean dummyload? i know its an oddball tubeset . the preamp is a 12volt heater version of a 6SL7(12SL7) an the power tube is a 12 volt version of a 6AQ5 which i got because acoording to the tube manual they are essentially a 7pin 6v6 but noone wants them because they dont say 6v6 and they have lower max voltage . i got 5 tested nos for 10 bucks and thought they would make for a bit of fun.i like their tone its just the chop chop chop noise when i hit a chord when turned up.

[ALBATROS1234]

one thing i found odd is it seems that there are 2 inputs for the grid on a 12AQ5 . 1 and 7 seem to both be connected to the input grid. i assumed they wer interchangable so i went to pin 7.

[DummyLoad]

please review marked-up schematic attached for clarification.


--pete

[sluckey]

one thing i found odd is it seems that there are 2 inputs for the grid on a 12AQ5 . 1 and 7 seem to both be connected to the input grid. i assumed they wer interchangable so i went to pin 7.

That's pretty common. If you use your ohm meter you'll find that pins 1 and 7 are indeed connected together inside the glass. I just ran into that with a 6AQ5.

[ALBATROS1234]

thank you sir. the plate resistors are 220k ohm 3watt. voltage drop across the cathode resistor is 19.4 vdc.

[shooter]

so you're at 6.4W and should be able to drive the tube with 20ish Vac

If you dime te Volume, and bring up gain from 0 to ......... where does it start getting ugly?

If you dime gain and do the same with volume, 0 to ..... what happens?

EDIT forgot, maybe add a 33k R to grid of V1a

[sluckey]

I know this is just a drawing issue, but look closely at the power supply node C.

[PRR]

> look closely at the power supply node C.

Hey! No hum!

Seriously: getting goofy when turned up is often a clue that output is sneaking into input and making oscillation. Sometimes chopsticking while goofy makes it worse (maybe better)- change that part of the layout.

[ALBATROS1234]

so basically the first volume control is gain(drive/pre) and ther second is a master vol. i put in a grid stopper first a 25k then a 53k. both helped, the larger a tad more. i also switched the 2.2k cathode resistor to 1.5k so now both triode cathodes have the same resistor but different caps.

with gain/pre cranked the sqealing/oscillations start around 1/4 turn up. if i back the gain up a 1/4 turn it helps a bit but still cant get the master up past half. the oscillations seem(?) to have quickened in pace, although i am not 100% sure.

with master cranked i can turn up the gain/pre about 3/4 before it squeals and oscillates .

there is alot of background hum, though that could be due to being breadboarded and long lengths of wire as well as alligator clip jumpers everywhere. that said it seems louder than the hum from my 1st project.

i think im going to take out the 3 bypass caps 1 at a time and see what happens with one or the other gone. it just seems that without bypass caps the overdrive greatly diminishes and i would like to have an amp that gets that sweet single ended grind. not looking for metallica tones but want 70s angus young /jimmy page classic rock crunchy goodness.

[ALBATROS1234]

this is ridiculous it will not no matter how i try to post the phone snap of my power supply circuit board even though it was taken under identical conditions of the above posted pics. this has happened before at this forum. thats why on the other thread some of my pics are sideways. the forum software kept saying my right side up pics were "too large" dont get it will keep tryin!!!!ugh!!!

[ALBATROS1234]

[ALBATROS1234]

there we go i have to upload it to photobucket and paste a link here. alot of trouble but there you go. as you can see i wondered about this 2 cap config but i have seen it done this way in other guitar amp schematics. should i add another cap?

so black wire is node "A" red is "B" and white is "C"

and by the way i did draw the schematic wrong the "C" node is not attached to ground it just jumps off the end point and goes to the V1 plates as shown

[sluckey]

this is ridiculous it will not no matter how i try to post the phone snap of my power supply circuit board even though it was taken under identical conditions of the above posted pics. this has happened before at this forum. thats why on the other thread some of my pics are sideways. the forum software kept saying my right side up pics were "too large" dont get it will keep tryin!!!!ugh!!!

You need to understand what too large means. It doesn't mean you're trying to put an 8x10 picture in a 5x7 frame. It means the file size (byte count) to too large. There are several ways to reduce the file size on your phone. Probably the simplest is to change the camera resolution. Or you can crop the pic size (trim with scissors to fit the frame). Or, do as I do. Put the pic on a computer and use a graphics program to change the resolution and/or just crop it.

Doesn't matter how you do it, but you must get the file size (byte count) under 1024KB. This restriction is stated directly under the Attach File input box. Goes like this...

     "Restrictions: 4 per post, maximum total size 2048KB, maximum individual size 1024KB

[ALBATROS1234]

i understand that its not the physical dimensions sluckey, its gotta be that the snaps of the schematics are mainly a white background with a touch of black drawing and the pic of the power supply is in color,therefore eating up more bits of memory to post at that place. on my ww1 aircraft modellers forum i posted many pictures which were color snapshots it must be a limitation of this forum software or maybe even an option to be selected by its administrator. i just meant that it didnt seem like it was a very large file compared to the hd files posted at the other forum in which i am a member. never had problems posting a simple phone pic before which is being called too large.no biggie just took some extra steps to make it work. i made it large in size so details could be seen. this is the way we do it at the other forum so you cal see all the super detailed cockpits etc.

back to the amp, i took out both preamp bypass caps and it helped considerably although i lost gain and it became quite muddy. so i added a couple bright caps on the volume pots. still getting some screech at higher volumes. this the break up is quite early it doesnt get totally clean even with the gain down and volume at reasonable levels.

with the power tube bypass cap removed as well it gets a large volume cut and starts sounding anemic. perhaps i can try a smaller sized cap in that role.

[sluckey]

maybe even an option to be selected by its administrator.

That's right. That limit is stated in plain view when you go to attach a file.

You can't look at a pic and tell what file size it really is. You will have to keep the file size below 1024KB to post it on Hoffman's forum. I know there ore plenty other forums that allow larger file sizes or may not have any file size restrictions. But this ain't one of them.

You should be able to look at your pic info on your phone to see what the file size is. If it's too large you will have to use one of the options I mentioned earlier to reduce the file size. Sometimes I can't get the file size small enough. In those cases I upload the too large file to my website and just post a link in the forum.

[ALBATROS1234]

what about a nfb loop. technically i can use this one and my first one , i really have no need to crank it very load in my bedroom but i want to get it right . as some may recall after working perfectly for like a month i finished rerouting wires and mounting everything on a chassis an now my 1st amp is feeding back/squealing if i turn the gain and treble up too high. the new one also has the oscillation problem though. perhaps i should try a nfb loop, perhaps not all the way from the ot to the 1st stage and maybe use a pot so it can be adjusted from nothing to 500k of resist? just thinking and wanted opinions.

[shooter]

but i want to get it right

Once I decided to "create" rather than "copy" you need a good set of tools, and technics.
even if you cut n paste amp sections like I do, stitching them together and getting them right almost mandates a scope.  Also a good "test bed" BB.  I start at the wrong end, I create a solid PS and 1 tube PA.  for testing I "drive it" with a known good source, usually some SS device that I can get a couple volts RMS to give the PA something to amplify, once that works cleanly, I build my pre/drive whatever.  That way when I have problems I know it's on the left side of the schematic
Then it's scoping the signal, balancing it for my PA.  Once that's cooking with gas I usually drag in a real musician and "tune" the preamp to taste.

all the standard best practices, wire lengths, wire lay, component placement, etc.
Hang in there, once the light-bulbs start glowing, the frustrations become just a minor step

[ALBATROS1234]

i may fully copy an amp schematic one day but, i like the experimentation and cracking problems. i do have a scope its a a heath 4552 25mghz dual trace. i have a top of the line dmm and an old heathkit ig-102 that was given to me recently. i was told they werent sure if it worked. so i take it apart because the selector switch wouldnt do its job, it was a mechanical problem and although i can get a sine wave on the scope i can get it to play a tone through my amp which i found odd, i'll figure it out. same guy that gave me the ig-102 gave me an old simpson 260 series 3 analog meter, it doesnt work yet even after cleaning the leaking battery crust and filing contacts etc. so i guess i have the basics to start playing, just new to tubes and my layout needs improvement as well as a deeper understanding of the inner workings of the tubes.

[ALBATROS1234]

I figured out most of the problem. It was a bright cap, I took it out and problem solved. It was too large in size. I thought i grabbed a .0047 pf but it was .0047nf. I put the smaller size in at it's  much better except if I turn gain to 100% I get a slight alarm buzzer type noise. Guess I gotta trace that down next

[DummyLoad]

>> I thought i grabbed a .0047 pf but it was .0047nf.


.0047pf is of no use in the audio spectrum.


--pete

[ALBATROS1234]

I thought that was the value of a fender tweed style bright cap. I know there's a range from .047pf to 500pf or that's what I thought I read . Must have been a bad cap then. Or else I messed up testing the value.i someyimes repurpose some components which I take from  old electronic stuff. Mostly stuff from the 90s is usually something people give me and I desolder components and group them in zipper bags with like types resistors, electrolytics , other caps etc. Many are asian and have gibberish written on them so I test capacitance value with my multimeter.  It was too large and was causing a problem or busted but with a different much lower value cap i had lost the helicopter noise and now just has a minor tweak to get it finished

By gibberish written on caps I mean no value that I recognise. Probably a part number. It is very hard to trace down value by searching the web. For instance I just picked a cap up. It says "sa2B474M RUalt0u"  this cap is .4 uf which is what it tests

[shooter]

500pf

the one I remember most is 120pF, I find the bright "feature" about the same as the standby "feature"

[PRR]

> .0047pf is of no use in the audio spectrum.

Yes. A foot of cable is 30pFd. Less than that is about no-effect in any normal audio circuit. 0.0047pFd is like 0.002 inches of cable added to a circuit-- I can't cut a wire that short. 0.0047pFd is about 1,800MEG at the TOP of the audio band, so no-effect.

[DummyLoad]

I know there's a range from .047pf to 500pf  

a .047pF cap is very, very, very, very small value - used in microwave and UHF circuits. not used in audio ever. period. typically we use 100pF and up.  a .047pF is 2000x smaller than 100pF cap...

http://www.sengpielaudio.com/calculator-RC.htm

use 100pF and up for treble peaking caps. 100pF/120pF/220pf/270pF/390pF/470pF/500pF are typical values. popular variants of marshalls use 1nF (also written as 1,000pF or 0.001uF). some other designs use more, some less. 

--pete

[ALBATROS1234]

Seems I was mixing up nano and pick. I looked in the robinette site and it was taking about a 10pf in fender amps that should be removed. He suggested a 470. Anyway I checked and the one I replaced that was to big was 470 nano and I put In a 470pf. It definitely effects the tone. For whatever reason the larger one was causing oscillation problems