5E3 help/advice needed

[soundguruman]

I do not see the heater to ground connection.
What I do see looks proper.
F1 & F2 with a grounded center tap.

I see two green wires going to the pilot light.
Then, after that, do they go straight to the next tube filament?
Kind of hard to see in the photos...I'm sure I could be mistaken.
But it kinda looks like one is grounded to the chassis lug.
Look again, there could be a boo boo.

[stephenhartley]

Thanks for your help - I'll have a go as soon as I've some time off work, and let you know how I get on.

[stephenhartley]

Just a couple of questions

'make sure the 6.3v filament string is referenced to a DC voltage such as the power tubes cathode biasing resistor.'

How exactly do I do this?

'...along with the screen node hi-v cap'

Which is the screen node hi-v cap?

The jack sockets are switchcraft type repros - got them off the internet can't remember where. I studied a few diagrams in weber's book to check & recheck the input jack wiring but may have got it wrong - will check again.

[Gingertube]

There is a pointer to a classic HiFi preamp problem in the text above. The problem goes away at a particular volume control setting, that is at a particular value of impedance to 0V looking away from that grid.

That is a pointer to ultrasonic parasitic oscillation in that stage (which you heasr as massibe hum/buzz).

Swap tubes and see if it changes. Tidy up lead dress.

AND THEN if still there:

You said that you have the wire from the 220K summing resistors to the following grid is shielded, good. What you want is for that wire not to feed the grid directly but feed a grid stop resistor of say 10K. The resistor body should be right up against the tube socket pin, then connect the wire which used to go to the grid pin to the otherside of that 10K. The shield of that wire need only be connected to 0V at one end, that tube stage cathode resistor 0V end is the best place.

Cheers,
Ian

[stephenhartley]

A bit of progress but not there yet

Started with Bruce's earlier suggestions of working through pre-amp to try and isolate level of buzz.

Jumper lead to earth and pin 2 of v1 - buzz a little quieter and more trebly

2nd jumper lead to earth and pin 7 of v2 as above.

Jumper from earth to pin 2 of v2 - buzz disappears altogether.

Tried removing one of input sockets from each channel (the one with 1m resistor) and buzz got progressively louder & louder - had to switch off completely.

Jumper lead from pin 8 (cathode of v2) - buzz disappears for about a second then returns - I presume this is because of cathode cap discharging and recharging? Is this of any significance?

Next step is to try Loudthud's & Bruce's earthing suggestions.

Thanks once again for all the help on here - what a great site

[Steve Conner]

There is a pointer to a classic HiFi preamp problem in the text above. The problem goes away at a particular volume control setting, that is at a particular value of impedance to 0V looking away from that grid.

That is a pointer to ultrasonic parasitic oscillation in that stage (which you heasr as massibe hum/buzz).

No, I think the explanation is that the same hum is getting into two places, both before and after the volume control. Each stage reverses the phase, so at one particular volume control setting the hum will cancel.

Are you testing with a guitar plugged into the input jack? Have you used shorting jacks that ground the tip contact when no plug is inserted? All unused inputs should be shorted to ground, or you can get all sorts of buzzing and oscillation.

[R.G.]

Hum cancellation by introducing offsetting hum is known in academic/theoretical circles as "The Method of Offsetting Errors".

[stephenhartley]

All working nicely now - thanks for all the help.

The main problem was that I hadn't earthed the heaters - my first build had a centre tap on the PT 6.3v supply, but I've used a mojotone on this one & it doesn't have a centre tap.

There was a bit of oscillation on the normal channel & I've minimised this by screening and moving about a few wires.

The sound is awesome and LOUD. Initially, with a 270r cathode resistor, I was reading 44mA per tube. It's down to 36mA by upping the cathode resitor to 330R.

The plate voltage is 390v - quite high for a 5E3? - cathode voltage 24v

This is giving 26 watts

Seems very high for a 5E3 but I'm not complaining as it sounds great.

It has a GE 5y3GT rectifier, two Russian tung sol branded 12ax7s and a pair of Russian military 6V6s

I've swapped out the 15w celestion blue for a reconed 60s Goodman's Audom 60 (now rated at 50w)

[Bruce / Mission Amps]

From my earlier reply

"The main thing to do in a simple amp like this is:
make sure the 6.3v filament string is referenced to a DC voltage such as the power tubes cathode biasing resistor."

I didn't read every comment from you but did you ever make mention of a non center tapped filament supply?

I feel like you owe me some brain cells.....

[loudthud]

Questions that never got ask or answered:

1) Voltage checks including AC volts on the heater pins.

2) Resistance checks on the heater pins.

3) Is the hum 60 or 120 Hz?

4) Does the hum fade out or stop immediately when you click the standby switch off?


I consider this boilerplate for the next hum thread.

[soundguruman]

All working nicely now - thanks for all the help.

The main problem was that I hadn't earthed the heaters - my first build had a centre tap on the PT 6.3v supply, but I've used a mojotone on this one & it doesn't have a centre tap.

There was a bit of oscillation on the normal channel & I've minimised this by screening and moving about a few wires.

The sound is awesome and LOUD. Initially, with a 270r cathode resistor, I was reading 44mA per tube. It's down to 36mA by upping the cathode resitor to 330R.

The plate voltage is 390v - quite high for a 5E3? - cathode voltage 24v

This is giving 26 watts

Seems very high for a 5E3 but I'm not complaining as it sounds great.

It has a GE 5y3GT rectifier, two Russian tung sol branded 12ax7s and a pair of Russian military 6V6s

I've swapped out the 15w celestion blue for a reconed 60s Goodman's Audom 60 (now rated at 50w)

Oscillation is cured by running the grid wire under the plate resistor.
Or, twist grid and plate wire together, a bit.
Well, as long as the plate voltage is higher, the watts are going to be more.

[Steve Conner]

Glad to hear you got it working... Which Russian 6V6s are you using, these ones?
6p6s, Tube 6p6s; Röhre 6p6s ID3547, Beam Power Tube

At 390V and 26W output you might have better luck with the 6P3S instead.
6p3s, Tube 6p3s; Röhre 6p3s ID3565, Beam Power Tube

[stephenhartley]

The Russian 6V6s have a white label on them with lots of numbers and 'Electronka 6 P 6 S'

Is the 6p3s a 6L6?

Sorry I wasn't clearer about the heater wiring - the original filament wiring on the Fender circuits shows earths at each tube - I changed it following advice on my last build without realising that there was a difference in the PTs.

I'm relatively new to electronics but am learning fast thanks to all the help on here.

Just need to fit the faceplate then it's finished - will try running the grid wire under the plate resistor/twisting wires.

Have managed to make a decent faceplate using a sheet of perspex over a faceplate printed on gloss photo paper - have used a graphics programme (Corel).

Will put up pics.

[Bruce / Mission Amps]

The Russian 6V6s have a white label on them with lots of numbers and 'Electronka 6 P 6 S'

Is the 6p3s a 6L6?

Sorry I wasn't clearer about the heater wiring - the original filament wiring on the Fender circuits shows earths at each tube - I changed it following advice on my last build without realising that there was a difference in the PTs.

I'm relatively new to electronics but am learning fast thanks to all the help on here.

Just need to fit the faceplate then it's finished - will try running the grid wire under the plate resistor/twisting wires.

Have managed to make a decent faceplate using a sheet of perspex over a faceplate printed on gloss photo paper - have used a graphics programme (Corel).

Will put up pics.

Well, I was, jokingly yanking your chain... I do find that many threads like this would have be answered very quickly if the novice builder had revealed more key information.... not your fault.
The remedy is so simple, I didn't think to ask either.

[stephenhartley]

Thanks - I hadn't quite understood what you meant by 'referencing the filament to the cathode resistor' - didn't realise it meant grounding it.

Just looking at the circuit and I presume that the advice on running the the grid wire under the plate resistor from soundguruman relates to the preamp tubes, as I don't see resistors on the power tube plates.

I reckon I might be pushing my luck running the 6V6s at 13w each, so I might up the cathode resistor to take it down a bit more.