no sound in cab but sound in output transformer

[Jaakers]

what does this mean?
I build my hi octane ax84 myself.
I get no sound out of the cab but there is noise in the output transformer the more volume I open.
If I touch the input guitar cable you hear extra noise in the output transformer.

Can someone give me additional insights?

How to debug this?

[sluckey]

Check the wiring of the speaker jack. Or the speaker cable. Or the cab wiring.

[Jaakers]

Thank you.
I was thinking this way as well and ordered a new output transformer.
It's working now so the output transformer was broken.

Now I still have static hum which increases if I open gain1 and 2.
Anyone an idea what might cause this?

[Jaakers]

I've been reading quite a lot since last post and one way to reduce the humm might to use the center tap of the power transformer of the filament heaters.
if in the attached picture I take the grn/yel center tab wire and connect it to pin 3 of the output stage en disconnect A from pin4 then I installed the center tab and I MIGHT have reduced hum?

[Jaakers]

this seem to have blown the power transformer or at least the 6,3 volt part. The light aint burning anymore...

[shooter]

so the schematic you posted doesn't show a CT for the FIL windings, so where did you get the FIL windings have a CT?  a meter is handy to verify you smoked it

[Jaakers]

thanks for the reply.
in attachment you can see what i did.
and the other is a picture of the hammond 369ex. I connect the two green to the filament and the CT(grn/yel) I connected as the other picture.
This seems to be wrong...

[sluckey]

this seem to have blown the power transformer or at least the 6,3 volt part. The light aint burning anymore...

Are the tube filaments lit?

[Jaakers]

not anymore.
the power transformer get hot and that is it.

[shooter]

Not sure what went wrong, but again, the pic you posted, 300sch.jpg don't match what Hammond shows for a 369ex

https://www.hammfg.com/files/parts/pdf/369EX.pdf

I'd un-due what you did, pull all the tubes and measure the tranny secondary's, then  if you have 6.3vac, and B+ vdc, add 1 pre tube and see what you see

[Jaakers]

this is indeed the 369 you posted.  I attached the grn/yel thing like in the schematic.
i understand this must have been working?

this morning i checked again and touched the wires a little bit and maybe the two filaments were in contact which means a short circuit. anyway the light bulk worked again... more to follow in the weekend.

[Jaakers]

I did something wrong with the wiring and the heater filaments got shorted.
No harm done to the power transformer.

However coming back to the topic. I took the center tap of the 6.3 wiring and brought it to ground.
This did not solve the issue at all.

Am getting desperate at figuring out.

How can I determine 60 to 120 hz oscillations? Been searching to find answers but not easy to interpret them...

[shooter]

How can I determine 60 to 120 hz oscillations

both my meters have a freq function, scope is a great way.  listen to each til ya puke, then in the future they can be discerned by ear, mostly.

[Jaakers]

ok again a step forward.
the fluke 123 scope sais 100hz in the signal path. I live in belgium...
I changed all the 47uF capacitors but they were all ok and the new ones also.
is this possible and if yes?
what am i missing?

[Jaakers]

pulling the first pre amp tube kills the hum.
so...

suggestions? :-D

[shooter]

swap in another V1 just to eliminate the tube, after that, socket pins, cable dress, grounds, solder connections, proximity to noisy house stuff.........

[Jaakers]

i changed the ct from the main PT to the central bus and the humm is gone. Previous it was connected to another point on the chassis. Dont think all the hum is gone but at least the next point has taken over.

Now there is a lot of hiss which is untraceble by the fluke?

Still not satisfactory but there is progress...
Hiss is easy to solve, not? :-D
and how?

There is also one wire if you poke it you hear him in the speaker. this needs to be solved as well... its the first coupling cap wire from the 12ax tube...

Thanks for the feedback given so far...

[mresistor]

120hz vs 60hz

[Jaakers]

is the blue capacitor an extra white noise blocking mechanism?

[sluckey]

is the blue capacitor an extra white noise blocking mechanism?

You could think of it as such. It's actually a high frequency LNFB circuit that reduces gain at high frequencies, thus reducing white noise hiss.

[Jaakers]

thanks,
I was reading this topic and in the attached schematic I found it and by your comments given.
https://el34world.com/Forum/index.php?topic=17957.0

I will also remove the carbon resistors which are still in the machine. 5 in total.

I also bought 2 0.022uF caps this noon but already forgot what for.

thanks anyway!

[ALBATROS1234]

It sounded like an open circuit between the ot and speaker.  I have a couple of Silvertone 6v6 amps I pulled from reel to reel and they have leads wire to the output jack where if the wires are not hooked up to internal speakers or the speaker cable is not plugged in output transformer will make crackling sputtering buzzy noises. Like it's arcing or something. Once I plug in the cable it works fine. I bet is wired wrong or there's an open circuit

[Jaakers]

it seems i finally managed to get most of the hum and noise out of my amp.
it sounds much better full gain then clean.

tone controls have moderate effect on the tone...

i play the amp through a peavey 2 15" bass cabinet. maybe this is the cause?

another thing i observed. with a joyo ultimate drive effect the noise is back and the effect sounds completely not good.
is this solely related to the pedal or is my amp partly responsible for this?

[Jaakers]

these two 47pF caps did solve the hiss but also the nice sound. So this is not the solution.

is there something else i can try?

[Willabe]

You can try going smaller. It might work better.

Try 20pF and/or 10pF.

[Jaakers]

Is it possible there is always hiss in this amp?
http://www.ax84.com/static/hioctane/AX84_Hi-Octane_101004.pdf

Can I get it silent full gain? Spending way too much time on this matter.
It's time to finish this machine.

[sluckey]

Gain and noise (hiss) are directly proportional. Higher gain means higher noise. No way around that. There are simple band aid fixes such as the 47pF trick, or shielding, and there are more elaborate circuits to improve signal to noise ratio, such as log amps. But I think that hi octane amp is just gonna have a certain amount of hiss that you will have to accept.

[Jaakers]

thanks for the replies.

I consider this project finished then.
Now up to woodbuilding.

[ALBATROS1234]

how loud is the hum or hiss. ALL tube amps will have some hum and/or hiss. is it unacceptably loud? i have scratch built around 10 amps as well as modifying as many and the amount of hum/hiss each one has is different. i have only had 1 which the noise is pretty loud, the best solution i found which helped sustantially was a humdinger pot btw the filament leads. it had no center tap so i tried an artificial center tap which helped but not enough. the pot made it better but not perfect. but unless the amp is cranked it cant be heard while im playing so its what i live with. can you hear the hum and hiss when you are playing?

[Jaakers]

So once again a new post.

I kinda figured out this amp is not what I want.

How can I convert this machine to less gain, more headroom with most of the components still in there?

Here are a few examples of preamps:
http://www.ax84.com/corepreamps.html

I bought a book which introduces me:
https://www.ampbooks.com/mobile/books/basic-theory/

Anyone suggestions or leads?

[sluckey]

How can I convert this machine to less gain, more headroom with most of the components still in there?

The easiest way is to simply eliminate the first gain stage.  Refer to the schematic you linked in reply #26 above. Disconnect the wire on V4 pin 2 and tape the end of the wire. Disconnect the wire on V4 pin 7 and reconnect to V4 pin 2. Ground V4 pin 7. Play guitar and see how you like it.

EDIT... corrected tube numbering. V4 is the correct tube designation...   sluckey

[shooter]

on V1

for clarity, the schematic shows it as V4?   

[Jaakers]

can I still do something useful with my first preamp stage except creating gain?

 

[sluckey]

tremolo

[Jaakers]

ok I'll have something to research again.

I can do this:
http://www.valvewizard.co.uk/trem1.html

and then I can recuperate my gain 1 gain control to convert it to tremolo control...

is there a catch?

[sluckey]

is there a catch?

Yeah. Think small. Do the mod. Test it. Hey, you may not even like it, but get that working before you throw more mud at it.

[Jaakers]

bon, this seems easier then in reality.

my queeste for an example how to do this with my amp didn't result in a schematic.
the problem seems to be my single ended output transformer?

where can i input this tremolo signal in the amp?

edit:
ok 5 minutes later I found an example:
http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/M2/Images/KalamazooM2Schem.jpeg

edit2:
is the attachment more or less correct? remove R11, I think?

[sluckey]

is the attachment more or less correct? remove R11, I think?

No. Don't connect into the cathode follower. Connect to the cathode of V3b. I would start by removing C10 and R15. May need to experiment a bit with the cathode resistor.

If this doesn't work out well, try connecting to V4a cathode.

[Jaakers]

it's time to do the math you are indirectly saying? :-D

[sluckey]

I'm not indirectly saying anything.

[PRR]

it's time to do the math you are indirectly saying? :-D

I don't see how you read that.

Most cathodes are "near ground". But the cathode follower you have pointed-at has cathode FAR above ground. Tying that to the trem oscillator cathode will just muss things up, no sound, maybe small smoke.

You have a couple or three other "groundy" cathodes to play with. A clip-lead is your friend.

[Jaakers]

I was actually refering to this sentence. "May need to experiment a bit with the cathode resistor."

Cant't this be solved by doing the math correct?
I'll figure it out this weekend by reading some theoretics and buying all the necessary parts at noon today.

[sluckey]

Cant't this be solved by doing the math correct?

No. Grab a handful of resistors and a gator clip lead. Solve problem experimentally. Probably take 5 minutes or less.