5e3 output transformer hum

[gerry113377]

I have a 5e3 that when the volume gets turned up the output transformer starts to hum and crackle and there is no speaker output at all. If the volume is turned down all the way, all the voltages are normal. If I turn  the volume all the way up, the b+1 goes from 380v to 420v and both the b+2 and b+3 drop in voltage. Any ideas?

[shooter]

Any ideas?

start with the speaker, jack, wires, grounds
if nothing
pull all the pre amp tubes (leave PI n Power)
symptoms change?
IF not
it's in the PA section

[gerry113377]

When I pulled the preamp tubes the sounds went aways, just silence. When I was checking everything, I found the 1m resistor on the input jack was reading 0 ohm. I removed the wires going to that input jack channel. The amp should still work on the other channel right?

[shooter]

not sure your skillset so;
I would put ANY audio into the PI ******make sure it's BEFORE the coupling cap********
so you have to cobble a couple wires and gator clips, if possible use a battery device for audio
one wire to AMP chassis, the other to the coupling cap


make sound?


EDIT: leave tubes out

[sluckey]

I found the 1m resistor on the input jack was reading 0 ohm. I removed the wires going to that input jack channel. The amp should still work on the other channel right?

That resistor should read zero ohms when nothing is plugged in. Put the wires back The tube needs them to be connected to the jack.

Has this amp ever worked?

[gerry113377]

Yes, the amp worked. It's a mojotone kit. I tried to mod it to a robrob deluxe. I triple checked everything. I swapped the blue and brown power tube plate wires also. I have since removed the NFB and 10% power mods Still have the MVolume and lead channel mods installed. The first filter cap is 30uf and the power tube grid stopper resistors are 5.6k not 1.5k. If I feed a 800hz signal through the input, I can here the 800hz tone in the output transformer but not out the speaker. The bright channel volume is where the crackling sound is coming from. When I turn it up it starts to crackle in the output transformer. The normal channel doesn't do it. I will try what you said, the bypass, tomorrow, I'm at work now. Can I use A 1/8w 1m resistor for the input jack? That's all I have, the old one is damaged. I am a novice at tube amps but have been soldering pedals for a few years.

[shooter]

post a schematic of what you have


your "symptoms" sorta indicate the OT secondary circuit is NOT connected to the speaker correct.
sticking the 800hz into the PI should narrow that quick.
BUT.....understand the test, otherwise we'll just dig another rabbit hole

[gerry113377]

It's wired exactly like this. The only difference is I don't have the aux jack installed, so I only have one speaker jack instead of two. Black is going to S & Sw and green is going to T. The orange and yellow wires are taped off separated from each other (4 & 16 ohm) going nowhere. The red primary center tap is wired to pin 8 on the 5y3 then jumped to the b+1 instead of being wired directly to b+1. I don't have a standby switch installed.

[Willabe]

Can I use A 1/8w 1m resistor for the input jack?

M=meg, m= milli, 1M resistor.

[gerry113377]

So do I connect the audio signal to a pin on V2?

[shooter]

NO!!


There has to be a coupling cap between the audio and pin 2.
so follow pin 2 BACK on the schematic, that will lead to a coupling cap (guessing .02uF but I don't have your schematic)
inject the audio BEFORE that cap. 
leave preamp tubes OUT

[gerry113377]

Here is the schematic. V2 pin 2 is the grid for the gain stage and pin 7 is the grid for the PI.

[shooter]

since the PI is shared;


unsolder the MIDDLE TOP wire on your MV
solder one side of a cap (anything from .01uF to .1uf) to that wire (AFTER you have ohmed the wire to make sure it goes to pin7)
connect one wire from an audio source to the unsoldered end of the cap, connect the other wire ground wire to the chassis


turn on audio
power up amp
if you don't hear any sound within 30-45 seconds power DOWN


MAKE SURE YOUR SPEAKER is connected

[gerry113377]

Thanks! I just de-soldered the output jack and and touched the output transformer secondary direct to the speaker cable and I have sound through the speaker. The weird thing is I checked resistance on the jack and it was good, and I know it was wired correctly. So, I am ordering new jacks today. I'll let you all know what happens.

[sluckey]

since the PI is shared;


unsolder the MIDDLE wire on your MV
solder one side of a cap (anything from .01uF to .1uf) to that wire (AFTER you have ohmed the wire to make sure it goes to pin7)
connect one wire from an audio source to the unsoldered end of the cap, connect the other wire ground wire to the chassis


turn on audio
power up amp
if you don't hear any sound within 30-45 seconds power DOWN


MAKE SURE YOUR SPEAKER is connected

That won't work! Leaves the PI grid open (as far as dc is concerned) and you lose bias voltage on the cathodyne.

[shooter]

Thanks, missed that.  EDITED my post


at least He finally found the problem I suggested at the beginning

[gerry113377]

You sure did! But at least now I know how to bypass the preamp section, valuable troubleshooting skills! Thanks!

[shooter]

 called half splitting.  with large systems, you want to narrow the focus pretty fast, otherwise you'll be inside 10 different cabs hunt'n n peck'n till next Tuesday.


they key though as Sluckey pointed out, DOUBLE check the "logic" before you run off and post 

[gerry113377]

OK, I've been doing a lot of reading and buying tools. I'm still new at this but slowly catching on. Some skills develop in the real world with experience, not from studying. This is my second amp build (rebuild). I'm getting there. Thinking about doing a Bassman or 18 Watt next.

[shooter]

Either one is a good choice, BUT study!  It will help cement what you "learned by doing"


Archives of favorite topics (el34world.com)
read through the "highlighted" threads here.  the one's specifically on building/fixing


How to design valve guitar amplifiers (valvewizard.co.uk)
White Papers (aikenamps.com)
read through these pages, there are more like it if you get bored 
they will help understand "why" I screwed up with injecting a signal and much more.
the more you know and understand, the cleaner your builds, less errors, better tonal tweaks, etal.
enjoy the addiction 

[gerry113377]

After installing the new jacks, all is well. It appears even the input jacks were garbage so the 1M input/grid leak resistors weren't connected also. I think that was where the crackling sound was coming from. So I replaced all the jacks, 4 inputs and the 1 output. Now the voltage doesn't fluctuate any more. I have a constant 389v on the B+1 and 325v on the B+2. The amp sounds great. The master volume is wonderful! Before it was just way too loud! Is there such a thing as being too loud? Now I just need to hook back up the 10% power and NFB mods and I'll be finished for now until I make a cab. Thanks for everything!