5e3 build cap size change

[Only1way]

This is a question for the die hard 5e3 Deluxe builders. I have seen the die heart amp builders that go over board to have every component to original specs. Then I see some u-build kits and other amp builders who tweek their amps. Like upping values on components to compensate for today's voltage at what 120v ? It is higher now than it was in the 50.s So the values on the components will be different with higher voltage or what?
What is the best choice between stock or moded? Should the filter caps in the 5e3 stay at 16uf 450v or 20uf 500v? How about 22uF.
Then I have read some threads and talk about changing the cathode resister and bypass cap for the 6v6 pin 8 to different values.
I have read some with 5 watt 270ohm resister with a bypass cap 25uf 50v. I have read about some 400 ohms 10 watt 22uf. What about 300 ohm 5 watt.?
Well witch one is right here? What are your opinion.s

[Chuck H]

Unless your goal is limited to authenticity, whatever sounds best to YOU is "correct". The stock 5E3 is panecea to some players, but certainly not all. Many builders find the need to mod the stock circuit. Bruce at Mission Amps has elevated this to an art form, and many good 5E3 mods can be found by searching this site.

As for voltage... The 5E3 isn't the "Deluxe" with real voltage issues. And bias is bias. If you bias the tubes correctly there is no one "correct" value for rk on the 6V6's. The bypass cap is a matter of tastes, as are the filter cap values. Other common mods include changes to the coupling and bypass cap values in the preamp, a "fix" for the interactive nature of the volume controls and mods to the phase inverter. The 5E3 is a simple and very, very versitile circuit. The mods are usually simple and dramatically effective. It's really amazing that one basic design can morph into a great amp for most players with only minor tweaks.

Search this site and you'll learn a lot of 5E3 'how to'.

Chuck

[black_labb]

on my 5e3 style amp i did without the tube rectifier. instead i used a 47uf filter cap into a sag resistor, which then fed the traditional 16uf cap network. this gave sag (across the resistor into the 16uf network) but because it is sagging from an already filtered stage you dont get as much B+ hum from the lack of filtering. i havent tried it along side a traditional 5e3, but i like the way the sag resistor works. i have a switch in there that allows the sag resistor to be bypassed and also switches to zeners for biasing. this makes the amp quite versatile as it stiffens it up alot (higher voltages overall and nothing to cause squishiness).

to me its the perfect 5e3, but it probably wouldnt suit everyone.

[tubeswell]

Like upping values on components to compensate for today's voltage at what 120v ? It is higher now than it was in the 50.s So the values on the components will be different with higher voltage or what?

The line voltages will only be slightly higher in a 5E3 today if you are using a power transformer that was wound with the same number of primary and secondary turns and had the same amount of iron etc as those Fender 5E3s made in 1955 (i.e.; 12/11 or 9% higher - all other things being equal, which they usually aren't exactly).

But at the sort of voltage levels you'd expect in a 5E3, you don't need to up the cap voltage ratings in the amp to compensate.

And for the most part using higher voltage caps (and resistors) won't detract from the result.

What is the best choice between stock or moded?

A matter of personal preference. The more 'modded' it is, the less it will sound 'stock'. However one mod I like is replacing the first stage 0.1uF coupling caps with .022uF/22nF . This removes quite a bit of flab from the signal path. YMMV

Should the filter caps in the 5e3 stay at 16uf 450v or 20uf 500v? How about 22uF.

22uF is fine. Caps typically tend to be manufactured within a range of actual capacitance values up to 20% either side of the 'rated' value that is acceptable (referred to as 'tolerance'). So a cap with "16uF" stamped on it, might actually measure 20uF. For the purposes of storing short term charge to even out the pulses of rectified current from your rectifier tube (or SS diode), there is not much diff between 20 and 22, nor between 16 and 22 for that matter. A 5Y3GT will work happily with 22uF reservoir cap and will even take 40uF at a stretch - altho' this is harder on the tube (but that is the maximum value reservoir cap I have used with a 5Y3GT. I wouldn't risk putting 47uF in there. Rectifier tubes don't like it if the reservoir cap is too big).

450V vs 500V? - not critical for the voltages we are talking about in a 5E3 power rail. As long as the cap is about 20% above the highest (DC) voltage you are likely to see, you shouldn't have any problems.

Then I have read some threads and talk about changing the cathode resister and bypass cap for the 6v6 pin 8 to different values.
I have read some with 5 watt 270ohm resister with a bypass cap 25uf 50v. I have read about some 400 ohms 10 watt 22uf. What about 300 ohm 5 watt.?
Well witch one is right here? What are your opinion.s

Again the voltage rating of the cathode bypass cap should ideally be about 20% above the maximum rating stamped on the cap. If your cathode voltage is going to be 18VDC, then a 25V cap will work. However, I prefer to use a 63V cap or a 100V cap in this position, because the cap is physically located right above the output tube sockets (where it tends to be hotter).

As for the resistor value, anything from 250R to 330R will give a ballpark bias value assuming the idle plate voltage sits somewhere between 340 and 370VDC.

The power handling rating of the cathode resistor should ideally be at least 2 x what the calculated power dissipation of the resistor is likely to be. So if you get 18V dropped across a 250R resistor, that is .072A (18/250= .072), and your power dissipation will be 18V x .072A = 1.3W. So a 3W resistor will work fine, but a 5W is better, because where the resistor is physically located in the amp is about right above where the 6V6 sockets are, which tends to get hotter.

In terms of the value of the resistor, the way to work it out is by calculating the power dissipated by the 6V6s. One fairly reliable way to do this is:

1) Measure the idle voltage (E) between the cathode and the ground, and divide that by the actual measured resistance (R) of the cathode resistor to get the current (I) flowing through the cathode resistor (E/R = I), which is the current flowing through both tubes. Assuming the tubes are sharing one cathode resistor (as in the stock 5E3 schematic), the current in each tube will be 1/2 what the current in the cathode resistor is. (e.g.; 72mA through the resistor, divided by 2 = 36mA per tube). Take away 2mA from the current flowing through each tube (as a ballpark assumption for what the screen current is in a 6V6 in a 5E3) to get an estimate of the current flowing through the plate of each 6V6 (i.e.; e.g.; 36mA-2mA = 34mA)

2) Measure the idle voltage dropped between the plate and the cathode of each 6V6, which will be the plate idle (DC) voltage minus the cathode idle (DC) voltage (e.g.; 360-18 = 342).

3) multiply the plate to cathode voltage obtained in 2 by the plate current obtained in 1 to get the plate dissipation. (i.e.; e.g.; 342V x .034A = 11.6W) . A 6V6GT typically has a maximum plate dissipation rating of 12W, a 6V6GTA 14W, whilst a JJ6V6S - it is rumoured - can take 16W. For a push pull amp operating in Class AB1, a plate dissipation between 70% and 85% of the maximum plate power handling rating is the 'ideal' operating range. However, because a 5E3 is cathode-biased and uses "fairly low" voltages, you can run the 6V6 closer to maximum. YMMV

[hasserl]

... bias is bias. If you bias the tubes correctly there is no one "correct" value for rk on the 6V6's...

IME 5E3's tend to run the bias very hot, at or over the rated max dissipation value for 6V6's. Attempts to reduce it by using a larger rk negatively effect the tone of the amp, and don't really help much with the bias because as the current is reduced the plate voltage is increased. When you get up to 300 ohms the negative effect on the tone is enough to be audible, going larger just makes it worse. In the end I like to just stick with the standard rk value in these amps, if the bias is too hot then work to bring down the B+. But in all honesty, the bias shift that occurs as the current increases across the power tubes when a signal is applied will keep the tubes from over dissipating, and they will run quite happily like that for a long time.

So, IMO the stock value is the "correct" value for 5E3's. The circuit works and sounds best that way.

[Bruce / Mission Amps]

The reason I started using the 270 ohm 5 watt resistor in all my builds and 5E3 kits (and I believe I was the first to do this) is because after many years of seeing many real tweed amps and measuring every thing possible I could, I found that the average 6V6 biasing resistor resistance, even after many years of operation, was about 265 ohms! It had nothing to do with the actual bias voltage developed on the cathodes or line voltages. Ha ha ha.
Most of the tweed amps were built during a time when the line voltages were supposed to be about 117vac but it was still very common to run into 110v-115v. The raw line voltage on my bench today is... 126vac!!!
That does make a difference with the actual B+ voltages in the amp.

The great sounding amps do run the 6V6 power tubes very hot and they will be idling around 12-14 watts each.
Hasserl's comment about 300 ohms limit has some merit but I don't hear any degradation until I am closer to 360 ohms, and even then it is subtle.
I'm hedging on that one because the feel of the amp starts to change too.

Also, most of the great sounding stock amps I've heard still had filter caps that measured anywhere from stock 16uF, upwards to +26uF. It just doesn't seem to be very critical as long as there is low AC ripple on the OT.
Keep in mind that old worn out filter caps frequently measure out at 150% capacitance then they should be.... but they are still bad.
To be honest, as far as a fresh new build goes, I think they sound better with at least a 20uF-22uF caps at the OT center tap and the screen node.
The preamp B+ node can be anything from 10uF to 22uF with virtually no difference in tone.

I try to get all my customers to build the amp stock first and then tweak a little after they've had a chance to mess with the stock sound.
The #1 thing would be to reduce bass response in any way that is easy for you to do... typically at least 1/2 lower value coupling caps and 1/4 value cathode bypass caps on the preamp stage.

Chuck's comment has the most impact though... what ever sounds good, is good.

[tubeswell]

The reason I started using the 270 ohm 5 watt resistor in all my builds and 5E3 kits (and I believe I was the first to do this) is because after many years of seeing many real tweed amps and measuring every thing possible I could, I found that the average 6V6 biasing resistor resistance, even after many years of operation, was about 265 ohms!

Funny you should mention that. The 5e3 clone thing I built last year has a '270R' resistor that measures 255R.