5E3 College Term Paper

[Leftydave]

This is my first post and I thought it was interesting that several of you criticized that college term paper on the 5E3 Deluxe indicating that it took away many of the characteristics that most people liked. I tend to agree, but I’m curious to what the members here thought about the recommendation to place 470 ohm screen grid resistors in the output stage to extend the life of the 6V6s.

Did the 5E3 design go through 6V6s pretty quickly? Does the addition of those resistors change the tone of the amp significantly? I’m attempting my first 5E3 build shortly and was curious. Thanks.

http://online.physics.uiuc.edu/cours...5e3_deluxe.pdf

[Bruce / Mission Amps]

This is my first post and I thought it was interesting that several of you criticized that college term paper on the 5E3 Deluxe indicating that it took away many of the characteristics that most people liked. I tend to agree, but I’m curious to what the members here thought about the recommendation to place 470 ohm screen grid resistors in the output stage to extend the life of the 6V6s.

Did the 5E3 design go through 6V6s pretty quickly? Does the addition of those resistors change the tone of the amp significantly? I’m attempting my first 5E3 build shortly and was curious. Thanks.

http://online.physics.uiuc.edu/cours...5e3_deluxe.pdf

The 470 ohm resistors won't change much of anything but they really aren't needed since the screen current is low in these amps plus, the actual voltage on the screens is quite a few volts less then the plate anyhow.
The 6V6s in the 5E3 will last a fairly long time but they are idling in high current, at around 12-14 watt each, so they do not last as long as they would in a fixed bias amp, which could have it's power tubes idling at 7-10 watts each.
Using screens resistor on the amp won't change the life of the tubes by any real margin.

[bnwitt]

Ah but they don't hurt anything and they're a bit of extra insurance. Also, if you dump the first dropping resistor (5K or 4.7K) in the power supply replacing it with a choke to tighten up bass response, I would definitely put them in there. Fender's current reissue has them.

By the way, I disagree with the entire premise of that paper's "problems with the 5E3" section. So, in my opinion they set out to fix something that wasn't broken. I believe the original design and layout produced awesome tone and very low noise. One man's noise is another man's harmonic ecstacy. The early break up of the amp makes the 5E3 one of the finest sounding amps ever.

[tubeswell]

Ah but they don't hurt anything and they're a bit of extra insurance. Also, if you dump the first dropping resistor (5K or 4.7K) in the power supply replacing it with a choke to tighten up bass response, I would definitely put them in there. Fender's current reissue has them.

FWIW I found that when I put a choke in my 5E3 clone, the voltage of everything went up quite a bit (Plate was about 380 something) and I had to not only put a 4k7 (common) screen-dropping resistor in, but also I had lower the B+ with 2 x 15V zeners to get it sounding nice at around 340-350V B+. (However, now it sounds better than ever)

[bnwitt]

FWIW I found that when I put a choke in my 5E3 clone, the voltage of everything went up quite a bit

Well of course it did. The DC resistance of the choke is around 100 ohms compared to the original 5k resistor. Personally I like the sound of the circuit with higher voltages and having built and sold more than 40 of them with rave reviews fom my customers, it seems many others do as well. You get tighter bass and a little more clean headroom out of the circuit with the choke using the standard PT. That being said, tone is a personal thing and your mileage may vary. If you just want the tighter (non farty) bass, you just need to use a PT with a lower secondary voltage to begin with and your voltages will be closer to the original. Remember, an original today will be running with higher voltages any way as their PT primaries were designed for lower house voltages. Bruce Collins of Mission Amps uses (the last time I checked) adjusted PT's for today's wall voltages to keep things historically accurate. If one wanted a really accurate amp sound from the 50's his kit would be the one to buy for sure.

[cminor9]

This is my first post and I thought it was interesting that several of you criticized that college term paper on the 5E3 Deluxe indicating that it took away many of the characteristics that most people liked. I tend to agree, but I’m curious to what the members here thought about the recommendation to place 470 ohm screen grid resistors in the output stage to extend the life of the 6V6s.

Did the 5E3 design go through 6V6s pretty quickly? Does the addition of those resistors change the tone of the amp significantly? I’m attempting my first 5E3 build shortly and was curious. Thanks.
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I read your term paper and tried several of the mods just for the heck of it. I tried most of them one at a time to see what kind of difference each would make.

I don't use a 12axy for the first tube. I prefer an ecc81 in there, which is closer to the 12ay7.

I added the feedback loop and quite frankly, hated it. That got yanked quickly! I repurposed the switch and hole in the chassis for the next mod.

I put a switch on the cathode bypass cap on the second preamp stage. The mod provides more headroom, but it also takes away some of the oomph. I prefer it stock. I never switch the mod on (take the cap out of the circuit.) If I ever have to drop a 12ax7 in the first position, I'll try the switch. That's why I kept it in.

I upped the cathode resistor on the 6v6s from 270 to 330. I could tell a bit of a difference, though I am not sure if it's lost any of it character warmth and grind. I'd prefer to err on the side of running the tubes a bit colder, so I left that in.

The 470 ohm resistors didn't make a discernible difference. I left em in. From what I read, it can't hurt.

The 0.033 cap in the power supply didn't make any difference for me. I just left it there. I had seen that that in other amps, so I figured it wasn't too odd of a piece to leave alone.

That should cover all of your proposed mods.

I don't anyone should harshly criticise your paper. While most of the mods didn't make much difference to me (or I didn't like them) it was an interesting read. I learned a lot from reading it and from performing the mods to the amp. It's a student term paper, after all, and people should view the paper in light of this fact. You aren't claiming to a superior engineer, you essentially had a hypothesis and set out to dis|prove it. I am glad you put it online. I benefited from it.

Build the 5E3! The most important mod to the amp: lower the values of the 5 filter caps from .1 mfd (you missed that one!). Google this forum to see what those values should be. If you do that, the amp won't really need any other mods. The stock 5e3 is a bit boomy and muddy with guitars with humbuckers. Then again, Leo fender designed this thing with single coils pickups in mind.

[Bruce / Mission Amps]

Ah but they don't hurt anything and they're a bit of extra insurance. ....

Well, this "insurance thing" is something that I'm not sure I agree with 100% and I'll tell you why...
with respect to a 6V6 amp (a 5E3 comes to mind here) at WAY WAY less then 375vdc on the screens, do most builders think the 1w 470 ohm resistor is going to be like a screen protection fuse?

Think about it like this...
The max rating of the screen grid in a 6V6 is about 2 watts but it does have a maximum current rating too.
If it were possible under some high power output condition... for the screens to pull, say 50ma of current, (typical screen current in a 350v to 420v, class AB1 push pull 6V6 amp driven hard and put away dripping wet, is less then 20-30ma) ... then the 1 watt resistor would be under a 1.175 watt load at 50ma.

Ohm's Law:
I^2*R=P .05a x .05a * 470 ohms = 1.175 watts

So, in my opinion, I'd guess a 1 watt resistor would not open up (especially in a fuse like mode) at 50ma of current.. but at 50ma I'd be afraid the 6V6 tube would melt it's frickin' screen wire off and that 1 watt resistor just got a little hot!
What do you think?

[bnwitt]

Well that's not really what I meant by insurance. I was thinking about a voltage drop across the screen resistor to reduce the voltage at the screen due to the higher screen voltage out of the choke.

[tubeswell]

FWIW from my experimentation with my clone, 470R didn't do much to drop the screen voltage (with the choke filter installed) - at best it brought it in line with the plate voltage.

I wanted the screen to be 20V or so under the plate which required something around the 5k ballpark (as a common resistor to both screens). This also helped drag down the plate voltage a bit (as I knew it would). The zeners did the rest

[bnwitt]

2 or 3 volts under plate has been my experience with a 470R. That's all I'm after with it.

[Five_E35]

Fender uses the 470's in their reissue, along with the bombproof rectifier diodes, and a HV fuse. Maybe for when an imported 5Y3 shorts, the diodes take over and cook the imported 6V6GT's at 400 volts, causing screen failure & a screen to cathode short which certain imported 6V6GT's are well known for.

I put 1 watt 470fp's in my recent build, but left out the diodes. I don't know if it applies with the 5E3, but I'd rather smoke a resistor than a power transformer. I left out the diodes because I'd rather blow a fuse or rectifier tube instead of red-plating NOS 6V6GTA's. Most people feel they don't seem to affect the sound and FWIW, Deluxe Reverbs run the screens a few volts higher than the plates, and most people feel that doesn't really matter either with that particular tube.

Regarding your paper, flat frequecy response, and centering the tone control - I don't think any of that really applies to guitar amps and speakers that sign off around 6Khz. I'm not sure what all the inputs on the 5E3 were originally intended for. There were no active pickups, but steel guitars and electric accordians were around and microphones. I don't have an analyzer, but my ears tell me this thing is closer to flat or slightly V-ed with the tone control up around 9, which just happens to correspond to the half-way point electrically of the 1M audio taper pot used in this amp. Seems like using a 1M linear would put the tone control near the middle. Wouldn't a tone control intended to be centered use a linear pot in any case???

[Leftydave]

I want to thank everyone for their replies, I knew I came to the right forum! Not sure which way I'll go on this. I'm sure I'll have a few more questions along the way. Thanks.

BTW, I have a old JBL D130. Has anyone built a 5e3 using a 15" speaker? I guess it would be more like a Pro?

[balcorn80]

I just wanted to add that the hoffman board i built then built my 5e3 amp around already had 470 ohm resistors engineered into the circuit.