5E3 no cathode voltage mystery - PLEASE HELP!

knuckleberryfinn

I've just finished building a 5E3 circuit (it's my third, and around my tenth amp in total, so I'm not a complete beginner) and there is only 0.2 VDC across the 250 ohm 6V6 cathode bias resistor.  There is sound, but it is like a transistor radio - distorted, quiet, weedy.  All the other voltages are very high (458vdc on the plates, 429 on the screens, and proportionally the same down to the preamp).  I'm using a Hammond 291BEX, which is rated for Deluxe Reverb as well as Tweed Deluxe,but thought the 5Y3 would bring them down lower than this.  I was planning to run 6L6s in it, so the high voltages aren't necessarily a problem (I think?!).

However, the cathode voltage is completely mystifying, and definitely the source of the problem.  Here's what I've tried so far...

A different resistor;
Clipping the bypass cap out of the circuit;
Grounding the resistor in lots of different places;
Reflowing all solder joints;
Other valves.

None of these have made any difference.

I am DEFINITELY never building an amp ever again (I don't know why I even started this one - it just gets on my tits) but would very much like to get this working, as I suppose it's going to have to be a keeper.  (Also the cab I made looks really nice).  So, if anyone can help I will be eternally grateful.

Cheers,

Paul




   

Modulus_Amps

Photo's?

No voltage means no current flow, so something is not connected, connected to the wrong place or is the wrong value, if you have voltage at the plate pins and screen pins then the list below will likely solve your issue

Check that the 250ohm resistor actually measures 250Ohm
Check that you actually have the 250Ohm resistor grounded to the first filter cap ground point
Check that the bypass cap on the resistor is the right way around
check that you actually have the bias resistor connected to the right pin on the power valves
Check all the power valve connections are on the correct pins.
Check the 220K grid leak resistors are correct value and connected to the grid pins on one side an ground on the other
One you you have current flow your voltages will drop in the whole amp

The voltage in the amp will come down once you have this sorted.

DJH83004

The HT voltage will be high as clearly the valves are not conducting, however there is too big a differential between the anode volts and screen grid volts, they should be within a few volts of each other, in a non conducting state. So back to basics are you actually measuring on the valve base, e.g. pin 3 anode, pin 4 screen grid, pin 8 cathode. With the valves removed do you still get the same anode and screen grid voltage?. Also with valves out and everything switched off are you getting 250 ohms directly from pin 8 to ground?? 
Suspect this is something you are missing - don't worry it happens to us all ! 
Edit: my suspicion is a problem with the screen voltage, so check that area, around the 2nd stage filter cap and the 5K dropper resistor as well as the above. 

ICBM

A pic would probably help, but as already said the problem is lack of conduction in the power valves.

Could you have connected the cathode resistor to pin 1 instead of pin 8? Or not connected the filaments up correctly? eg to pins 1 and 7 instead of 2 and 7. Just a thought…

jpfamps

I'd also check you have the correct voltage on the power valve screen grids.

No voltage here and the valves will be off.

I would first check the resistance from the valve cathodes to ground. If this is 250 ohm you need to look else where.

ecc83

ICBM said:

A pic would probably help, but as already said the problem is lack of conduction in the power valves.

Could you have connected the cathode resistor to pin 1 instead of pin 8? Or not connected the filaments up correctly? eg to pins 1 and 7 instead of 2 and 7. Just a thought…

Aye, I also wondered if'bugger were lit!

Whenever you build anything electronic it is a good idea BEFORE you put power on the kit to clip a meter to 'chassis' and go round valve or IC pins and check resistances. Often you get a funny reading, especially with stripboard jobs where a track has not been cut or 'blobbed' or 'whiskered' across.

Please don't give up on DIY 'tronics! There are SO few people doing it these days. Many audio forums have a DIY section and most stay empty of new projects for weeks on end.  If you want a bit of one-on-one I have the time unlike many of the chaps here perhaps. Just PM me.

Dave.

normula1

I've just had something similar happen on a preamp I've been tweaking. With all the movement, one of the heater wires had worked loose even though it still looked connected.

knuckleberryfinn

Well I don't know whether I should be proud or ashamed of myself: the screens weren't wired up at all! I knew it had to be builder error, but my eyes just couldn't see properly anymore. In fact, I didn't even need to look at it - I just had this dreadful realisation while I was in the shower.  What a tit.

It sounds bloody lovely now I've added those two wires and cleaned it all up. I'll post a pic in a bit to celebrate.  Thank you all of you, and sorry for being that tit again!

Modulus_Amps

@ecc83 said:

Please don't give up on DIY 'tronics! There are SO few people doing it these days. Many audio forums have a DIY section and most stay empty of new projects for weeks on end.  If you want a bit of one-on-one I have the time unlike many of the chaps here perhaps. Just PM me.

Dave.

Love this Dave and agree that he should not give up, but I think discussions have moved off old forums and onto dedicated facebook groups more an more, that is what I am seeing mostly these days and I don't spend much time on the technical forums like I used to do.

Modulus_Amps

knuckleberryfinn said:

Well I don't know whether I should be proud or ashamed of myself: the screens weren't wired up at all! I knew it had to be builder error, but my eyes just couldn't see properly anymore. In fact, I didn't even need to look at it - I just had this dreadful realisation while I was in the shower.  What a tit.

It sounds bloody lovely now I've added those two wires and cleaned it all up. I'll post a pic in a bit to celebrate.  Thank you all of you, and sorry for being that tit again!

showers are great problem solvers, well done!

knuckleberryfinn

Ha!  Thanks.  In my defence, I was quite ill over the weekend, and decided to distract myself from it by building the amp.

knuckleberryfinn

Here it is...

https://drive.google.com/file/d/1Ka8Hu6vbT0wnz19_9ED5B_-Yt5QssqDm/view?usp=sharing

Modulus_Amps

knuckleberryfinn said:

Here it is...

https://drive.google.com/file/d/1Ka8Hu6vbT0wnz19_9ED5B_-Yt5QssqDm/view?usp=sharing

Cool cab, well done

ecc83

Modulus_Amps said:

@ecc83 said:

Please don't give up on DIY 'tronics! There are SO few people doing it these days. Many audio forums have a DIY section and most stay empty of new projects for weeks on end.  If you want a bit of one-on-one I have the time unlike many of the chaps here perhaps. Just PM me.

Dave.

Love this Dave and agree that he should not give up, but I think discussions have moved off old forums and onto dedicated facebook groups more an more, that is what I am seeing mostly these days and I don't spend much time on the technical forums like I used to do.

Oh! I am sure there are dedicated forums full of  somewhat geeky electronics DIYers! I have no interest in those and they probably find all the help they need. No, I infest just AUDIO and recording forums where the posters are not interested in electronics of itself but as a means to an audio/guitar related aim.

SO many of these guys and gals have NO clue about even basic connectors or current flow, gain matching etc. Fork! Few even own a test meter.

HOW some of these people  learned to read, drive a car, got a job and operate a computer (often WAY better than I!) but cannot follow Ohms Law I do not know!

Dave.

MtB

If the voltages from the rectifier are still high you can drop them a bit through a line of diodes - I think it's approx 10v drop per diode in series.

MartinB

That's a stylish cab; very 50s looking with that archtop peeking out from behind it.  

jpfamps

MtB said:

If the voltages from the rectifier are still high you can drop them a bit through a line of diodes - I think it's approx 10v drop per diode in series.

Err more like 0.7 V per rectifier diode.......

RiftAmps

jpfamps said:

MtB said:

If the voltages from the rectifier are still high you can drop them a bit through a line of diodes - I think it's approx 10v drop per diode in series.

Err more like 0.7 V per rectifier diode.......

Errr no, he's talking about using 10v 5w Zeners

jpfamps

RiftAmps said:

jpfamps said:

MtB said:

If the voltages from the rectifier are still high you can drop them a bit through a line of diodes - I think it's approx 10v drop per diode in series.

Err more like 0.7 V per rectifier diode.......

Errr no, he's talking about using 10v 5w Zeners

Didn't see any mention of zener diodes (which of course can come in a myriad of voltages).

kreggs

I suppose  a NOS 5y3 might drop the voltages too. Or a JJ 5y3. Ive had luck with those.

knuckleberryfinn

Now the power valves are drawing current, the voltages are all down to a normal level (B+ of 372v).  I was thinking about using a 5U4GB and 6L6s but it's plenty loud as it is.  In case I do though, can the Hammond 291BEX supply the necessary current to 2 x cathode biased 6L6GCs?  The transformer is rated at 165mA, though there's lots of talk on ye olde internet that this is a very conservative rating.  In an ill-educated guess working from a 6L6GC data sheet, it's going to be pretty close to the rated limit if the plate voltage is around 410v (which is around the figure I'd expect).