5E2 Build Questions (rather long): bias, noise, etc.

[greg]

Hello, all

I have build a 5E2 circuit out of an old Motorola record player. The OT is a beefed up Champ version from Allen amps.

I have a couple of issues with the amp that I could use some direction on.


First, my screen voltage is higher than my plate on the 6V6. Is this expected with this circuit? Should it be addressed?

Second: I'm having difficulty calculating what my cathode bypass resistor should be. At present, I have it set per the schematic, but my voltages are considerably higher, so I know I need to do something here, but I'm not yet in the know on how to calculate it.

Here are my voltages on the 6V6:
Plate: +345
Screen: +351
Cathode: +20
Measured 42.4 MA between the plate and the screen.

Third: I changed the plate resistor on the 12AX7 from the specified 100K to 220K (what I had available). Would this bias the tube incorrectly? I know it biases it differently - but is it now "wrong"? I have heard of dropping or raising plate resistort to change the gain, but I don't know that that meant that I'd also need to change the cathode resistor to set the proper balance for this goal of getting less gain or, in my case, more gain).

Fourth: I used only one input jack, and I chose a 68K grid load resistor - but I just realized that this setup looks to parallel the resistors when the one of the inputs is used. My question, therefore, is which value should I use? 68K? 56K? 33K?

Fifth: I found the amp to be rather dark initially. I upped the negative feedback resistor to 33K (again, what I had on hand). This means less feedback (less of a blanket on the sound), correct? I put it on a switch to take it out altogether, and it certainly did open up the sound (I like both), but this leads me to the question - with the negative feedback connected, the amp is dark - I turn the treble to max and the bridge pu of my strat is still pretty rounded out. I put in a switchable bright cap across the volume control (47pf and 500pf) and now it is brighter, but it is pretty hissy. I did use (modern) carbon comps througout, but I think it is likely something else. Chopsticking the leads didn't change this at all. For grins, I switched out the bright cap (thinking that their proximity to other components might be a problem), then turned off the negative feedback and cranked up the treble - Yep - pretty hissy in that configuration too.

Sorry for the long post. Any direction would be much appreciated.

Oh yeah - one more question. Why does this design use the B+ from the other side of the choke, leaving the B+ at the top of the first filter cap unused? Is it paralleling the first two filter caps to make a higher value?

Thanks in advance,
Greg

[MWJB]

Add a screen grid resistor from the B+/Screen supply (they are tied together on this amp) node to pin 4 of the 6V6. 1.5K at 2-3W might do it?

If you have 20v at the cathode, divide that by 470ohms (cathode resistor value) and you get 42mA, less a little for screen supply...as you say. This is fine, not a problem.

Your voltages are not high, they may be a little higher than the schem but in reality all 50's & 60's Fenders are.

220K plate resistor will darken down the tone slightly, no it is not wrong as long as it is to your taste. If the amp sounds a little dark/grindy, go back to 100K.

If your layout is good you don't actually need another resistor apart from the 1meg on the input jack.

Lots of things could be causing your hiss - tube, layout, etc. Switching out the NFB pushes up gain & probably exacerbates any noise issues. Try the 100K plate resistor to brighten the amp.

You don't say what preamp voltage you have at 12AX7 pin 1, reducing the value of the 22K power supply dropping resistor will bring up the preamp voltage for a brighter sound too.

The 5E# amps are when Fender typically started switching from resistors to chokes in the power supply, the 5E6/7/8 etc., use the choke and a filter cap either side as a brute force filter to reduce hum, it also drops B+ voltage a little. A downside is that the screen supply & B+ supply come from the same node so screen voltages will normally exceed plate voltages, unless you fit a screen grid resistor to each tube socket. Though I don't necessarily think that this is why Fender changed the practice...they probably just decided that a resistor was cheaper when considering 1000s of amps, plus they seemed to be looking for more cleans so probably didn't want the B+ drop from a choke filter.

[greg]

Thank you for the information and suggestions. I'll see how the "proper" plate resistor works out and I'll also add the screen grid resistor.

Is there any harm or lack of performance to be expected with the current configuration of the screen being positive in relation to the plate? Seems like a strange thing for Fender to have let slide, since we're only talking about one resistor here.

Greg

[J Martin]

Just add another resistor and filter cap after the plate node to feed the screen. Slightly higher screen voltage was common in a lot of older amps, but some modern tubes can't take it for long depending on how hot the tubes run.

[MWJB]

It's common in more modren amps like SF Champs & Princetons too. Nobody much notices until they get the voltmeter out.

If you simply run a screen grid resistor from the downstream side of the choke you don't need another filter cap.

[MWJB]

Clarification: "from downstream side of the choke to 6V6 pin 4"

[sportster4eva]

Hello, all

Oh yeah - one more question. Why does this design use the B+ from the other side of the choke, leaving the B+ at the top of the first filter cap unused? Is it paralleling the first two filter caps to make a higher value?

Thanks in advance,
Greg

The earlier Fenders did it that way to reduce hum. Since a single ended design is more prone to hum, the choke smooths out the ripple that's left after the first cap. On later amps, they switched to using paralleled caps right to the OPT, and a resistor to the next cap, where the screen voltage is tapped off, like the 5f2a.
I think using the choke and a screen dropping resistor is a better idea personally.

[greg]

Thanks to everyone for the replies.

Greg

[greg]

Just wanted to give everyone an update - I added the screen resistor to bring the voltage down below the plate. I ended up having to use a 4.5K and now I have it 8 volts lower than the plate.

Being that I have it "south" of the choke, I did not add the extra filter cap. The amp is working and sounding good - thanks for your suggestions.

I am wondering if having the screen lower than the plate is to protect the tube or if it performs an electronic function to allow the tube to operate properly. If the latter is the case, I still don't see why this wasn't added at the factory. Through this process, I have also noticed that another of my 6V6 builds has the same thing (higher screen voltage than plate voltage) and I'm going to address that as well, but I'd be interested in knowing really what I am doing here, and what the target really is (i.e., how much lower should the screen be than the plate and why).

Thanks again,
Greg

[MWJB]

With the sort of voltages that you typically find in a 5E/F2, or Champ the screen voltage is not likely to cause premature failure.

Having the screen lower that the plate makes the tube function properly. It wasn't added at the factory because resistors cost money and the lack of them wasn't going to cause warranty claims (few of the 5E# amps had them - Twins, Pros, Supers, Bandmasters, Bassmans all lived without them and are still regarded as very desirable amps).

Note later Princetons & Champs went back to a resistor between main B+ & screen, like the 5E3.

Exactly how much lower the screen should be is a matter for you to experiment with/personal preference. Big PP fenders with 470ohm screen resistors & chokes between the OT CT & screen supply may run just a couple of volts below the plate, sometimes the same as the plate. 5F1/5F2A/5E3 run tens of volts below the plate.

The target, as always, is a tone that you like.

[greg]

Thanks (again) MWJB

You statement "The target, as always, is a tone that you like" is so true, but being that my current understanding of the actual function of each particular component is obviously limited (function/safety/tone shaping and the combination of any or all of these 3), I'd be in dire straights without the guidance from the forum. Thanks for the clarification that this particular component is one where function, safety and tone are all factors that come into play. I suppose I'll find this to be the case with the vast majority of components in a guitar amp.


Greg