5E3 E Caps

[Mike_J]

The Hoffman and original Fender 5E3 schematics had 16mF, 16mF and 16mF for the B+, screen and preamp E caps respectively. I have 30mF, 20mF and 20mF in there now. When I built the amp many years ago I am pretty sure I started with the 16's but had a hum. Fixed it by going with the caps mentioned and a power choke. Very quiet after that but much less breakup than what a 5E3 should have. Note: selected the 30mF cap for the B+ because I am pretty sure you do not want more than 32mF if using a 5Y3 rectifier tube.


I presume there has been success using the 16mF caps or they would be on all the schematics. Has anyone experimented with this to see what the impact is on the 5E3 tone or maybe feel by using higher valued caps.

[SILVERGUN]

By using a choke instead of the power resistor in the power supply you have taken away some of the "break up" magic. This would be more relevant than your cap values.
What are your voltages?

[dude]

I agree, get rid of the choke. Don’t think the 16uf’s had anything to do with the hum. Could have been your filament wiring, did you have a center tap for filaments? If not you must use two 100 ohm R’s to ground off heater wires for the 6.3v to ground.

[Mike_J]

By using a choke instead of the power resistor in the power supply you have taken away some of the "break up" magic. This would be more relevant than your cap values.
What are your voltages?

Haven't put it back together yet. Have repopulated the board and removed most of the shielded wire. Had to wait for a few parts that came today. Glad you don't think the cap values are significant as I know the amp was quiet with these caps in it. May replace them individually after I get the amp up and running. Depends on whether I like the tone as is.

[Mike_J]

Could have been your filament wiring, did you have a center tap for filaments? If not you must use two 100 ohm R’s to ground off heater wires for the 6.3v to ground.

Had a center tap which I used. Plan on using two 100 ohm Rs tied to the positive end of the cathode resistor and cap ala the Mission Tweedy Deluxe documents. Will tape off the center tap. Has anyone done this? What were your results?

[shooter]

Has anyone done this? What were your results?

I've elevated ground, wasn't impressed quit doing it.

A PP amp has "bilt in" noise cancelation, unlike SE. built correctly there shouldn't be any hum, so you don't need to waste the extra wire - fwiw 

[PRR]

> A PP amp has "bilt in" noise cancelation, unlike SE. built correctly there shouldn't be any hum

There's dozens of ways hum gets in.

Fixing some does not fix "all".

If I run my well-pump to a ditch I get dirty water. If I run it to a pressurized pipe, dirt can't get in against pressure. But the pipe just carries any dirt in the well.

Push-pull *tends* to null B+ ripple. But if your single-ended stages (most of an amp) have B+ ripple or heater hum, the "clean" push-pull stages just amplify it as part of their job.

[Mike_J]

> A PP amp has "bilt in" noise cancelation, unlike SE. built correctly there shouldn't be any hum
Push-pull *tends* to null B+ ripple. But if your single-ended stages (most of an amp) have B+ ripple or heater hum, the "clean" push-pull stages just amplify it as part of their job.

Am I missing something? The 5E3 is a SE amp if I am not mistaken. The question was is the positive side of the cathode cap a good place to tap into DC to let the heater AC ride on and secondly is it even worth the trouble? The amp was pretty quiet using the center tap of the heater wires so it is probably not necessary but I saw it used on the Mission Amp, 5E3A in the schematic library, which was the reason for the question.


Thanks
Mike

[brewdude]

Pretty sure that a 5E3 is a cathode biased push pull amp.
I have had good results connecting the heater center tap (or artificial center tap) to the PA cathodes in cathode biased push pull amps—SE amps too.

[SILVERGUN]

> A PP amp has "bilt in" noise cancelation, unlike SE. built correctly there shouldn't be any hum
Push-pull *tends* to null B+ ripple. But if your single-ended stages (most of an amp) have B+ ripple or heater hum, the "clean" push-pull stages just amplify it as part of their job.

Am I missing something? The 5E3 is a SE amp if I am not mistaken. The question was is the positive side of the cathode cap a good place to tap into DC to let the heater AC ride on and secondly is it even worth the trouble? The amp was pretty quiet using the center tap of the heater wires so it is probably not necessary but I saw it used on the Mission Amp, 5E3A in the schematic library, which was the reason for the question.


Thanks
Mike

Just a terminology hiccup.
SE = Single Ended - usually one output tube amplifying the whole signal, all the time
http://www.valvewizard.co.uk/se.html

PP = Push Pull - usually 2 or 4 output tubes where the signal is split into opposing phases and amplified
http://www.valvewizard.co.uk/pp.html

SE- think Champs, some old Princetons
PP- think just about every other amp Fender produced, including the 5E3

Yes, the positive side of the cathode cap is an acceptable place to tap DC to elevate the heaters.
In your case it is probably not worth the extra 10 minutes because the amp was quiet with the center tap grounded. There is no special mojo to the Mission Amps design. They tried it, the amp was quiet, so they left it in....or, they had some noise, tried it and left it in.
You can charge more if you use extra resistors. 

[Mike_J]

Thanks for the lessons and the information. Was exactly what I was hoping to learn.

[PRR]

> 5E3 is a SE amp if I am not mistaken

Google disagrees but that's not what I was saying.

Push-pull cancels some buzz, but there are many sources of hum/buzz. One is heater-cathode leakage. Even "small" h-k leakage may cause large hum/buzz in low-level preamp tubes.

Centering heater power around AC ground (grounded CT) helps and is our first step in all but the crudest guitar amplifiers.

An additional measure is to center around AC ground with a small DC addition. More than a few volts, less than a hundred volts. The power output stage usually has such a DC voltage and is convenient.

Not all tubes have heater-cathode leakage. If the ones in it now do not, then there may be "no benefit" in a DC heater bias. However a large fraction of tubes do. In a factory, selecting low heater-cathode leakage tubes in low-level stages for low hum/buzz operation is a drag. Many makers apply the DC bias so-that they may use the heater-cathode leaky tubes without complaint. Many DIY folks, when convenient, apply the DC bias knowing that even if "these" tubes are OK, some day one will fail and another tube installed, which may have heater-cathode leakage. Naturally this will happen at a wedding or prayer-fest where you have to sit "silent" between tunes. The Preacher or the Bride-Father glaring at your buzzy amp.

[Ed_Chambley]

By using a choke instead of the power resistor in the power supply you have taken away some of the "break up" magic. This would be more relevant than your cap values.
What are your voltages?

What brand has the best magic?


Don't give me no skinny stinking leads.

[SILVERGUN]

By using a choke instead of the power resistor in the power supply you have taken away some of the "break up" magic. This would be more relevant than your cap values.
What are your voltages?

What brand has the best magic?


Don't give me no skinny stinking leads.

I was looking around but don't see a "Don't like" button with a big thumbs-down.

I'm gonna have to bring this up at the next meeting.

[pdf64]

...When I built the amp many years ago I am pretty sure I started with the 16's but had a hum. Fixed it by going with the caps mentioned and a power choke. Very quiet after that but much less breakup than what a 5E3 should have...

Firstly, I'm wary of relying on memory alone after a day or 2, never mind many years 
Always try to keep a notebook and record sufficient info that another person could recreate what's been done; 'future me' being effectively another person 

With regard to the original hum with the spec 16uF HT caps, bear in mind that one of them may have been bad or had a bad solder joint (5E3s typically being a 1st amp build).

With regard to the break up, have a looky at any pentode / beam tetrode power tube info and it can be seen that when Vg1-k =0V (the A1 / AB1 clipping point) max plate current (and hence power output) is determined by Vg2; as that increases, so does plate current.
So if the 4k7 g2 HT dropper is replaced by a low resistance choke, Vg2 will be much higher, hence the max plate current and power output will increase.