Is Anyone Willing?

[The Ballzz]

Howdy Doody,
I'm hoping someone here might be willing to help me look at some details and decide which way to meld three different power supply scenarios for my 5E3 build. Here are the three schematics. On the "modern" Fender, there are layouts, etc on the same link:

Modern Fender: http://support.fender.com/schematics/gu ... ematic.pdf

Mojo Schematic: http://www.mojotone.com/Amp%20Kit%20Sch ... 9&gc=clear

Mojo Layout: http://www.mojotone.com/Amp%20Kit%20Sch ... 9&gc=clear

Original Fender Schematic: http://vintagefenderamprepair.com/wp-co ... ematic.pdf

I have some specific questions on which way to go here. I'll start at the wall power coming in:

A) I kinda like the idea of switching both the hot and neutral, as on the Modern Fender 57. Opinions?
B) It seems that the .1-275v (C-14) cap "might" be a good thing, but just not sure what it's actually doing. Is it helping to filter the incoming AC and if so is it a good addition to the older style? It's funny how it's labelled C-14 on the schematic and chassis wiring diagrams but labelled as C-15 on the service diagram!
C) Is that thermistor, labelled RT-1, of any real importance and should it be included? The claim is that it is for surge protection.

Now that I've micro-disected the primary, I'll delve into the secondaries:

1) I'm using a ClassicTone 40-18078 PT with center tapped 6.3v. Is there any benefit, one way or the other, to either use the center tap or cap off the center tap and create a "faux" center tap with two 100R resistors to ground? I don't plan to use the original scheme of grounding one side of the heater string, unless there is some good reason/benefit?
2) Are the diodes, D-1 & D-2, of the Modern schematic, going to pins 4 & 6 of the rectifier, a good addition to the old design? They shouldn't have any effect on the sound , but it seems they might help "smooth" the AC a bit, before rectification?
3) If I decide to go with a standby, the modern method of putting it on the center tap of the PT seems to be a better way than to have it on a hot rail?
4) That last point brings up a couple other things. First, I pondered long and hard to figure out in what manner R-26 & R-27 were electrically "Elevated" until a friend pointed out that it was likely referring to being literally "physically" elevated to minimize heat issues! Also, when looking at the modern wiring diagrams, I notice that ALL ground terminations (other than the main AC supply ground) seem to be only terminated at the front panel brass plate. Is this a suitable arrangement?
5) Whether or not I actually use the 470R screen resistors will depend on a complete voltage survey upon first fire up. I'm planning to start with the higher 355v-0v-355v PT taps and experiment with a few different rectifiers before trying the 330v-0v-330v taps if the voltages are too out of line.

I know this is long list of questions, but I want to do this right the first time instead of going back to trouble shoot and repair.

Thanks,
Gene

[Stevem]

A) Yes break both sides of the line.

B). The cap blocks line noise to a degree and it's cheap enough to pop in.

C) yes, a slow power up on SS items like the diodes extends there life by a large factor, and that's where a lot siimiconductors fail, at power up!

[martin manning]

1) If you have a CT, use it. Some believe that 2x 100R will more accurately center the heater voltage, but IMO, meh.
2) The Si diodes are backing up the vacuum rectifier. Belt and suspenders.
3) Nope. The same issues re hot switching are still present.
4) I would ditch the brass plate. Put star washers between the pots and jacks and the chassis, and route the preamp grounds to a ground lug near the input jacks.
5) A small screen resistor (100R, say) will help prevent oscillations, and larger ones will be about screen voltage at idle and compression due to screen voltage sag when playing.

[sluckey]

B) It seems that the .1-275v (C-14) cap "might" be a good thing, but just not sure what it's actually doing.

It's a line noise filter. It should be a special Class X rated cap.

1) I'm using a ClassicTone 40-18078 PT with center tapped 6.3v. Is there any benefit, one way or the other, to either use the center tap or cap off the center tap and create a "faux" center tap with two 100R resistors to ground?

Whenever there is a real center tap I use it mainly for simplicity and as a personal choice. However, there is a valid argument to use 100Ω 1/2W resistors instead. Consider this... No speaker is connected for whatever reason and you are wailing away on the guitar with the amp dimed. It's very likely that the no load OT secondary will generate a high voltage signal that is reflected back through the primary to the tube plates, pin 3. It's also common for this high voltage signal to arc from pin 3 to pin 2 (heater) on the socket. This arc often causes a very low resistance path to be burned between pin 2 and 3. Now you have B+ on the filament. If you have 1/2 watt faux center tap resistors, they will quickly burn out without causing any damage to the PT. BUT, if you have a real center tap it will consist of big size copper wire that will not burn out. Now that B+ will be shorted to ground and the high current may very well burn out the OT primary and also the PT HT secondary (both very small wire. Hopefully, the line fuse will protect you. Maybe not. This scenario is the reason you will find HV diodes from plate to ground on many high powered amps.

2) Are the diodes, D-1 & D-2, of the Modern schematic, going to pins 4 & 6 of the rectifier, a good addition to the old design?

I think so, although I have never put any in my stuff. They will provide some protection for your filter caps in the event the tube rectifier should develop a short between a plate and cathode.

3) If I decide to go with a standby, the modern method of putting it on the center tap of the PT seems to be a better way than to have it on a hot rail?

At casual glance this seems like a good idea. But I promise you, there will be a big voltage across that switch when the contacts are open. Putting the STBY switch in the CT works fine in a cathode biased amp. But, if you have a fixed bias amp that uses a negative bias voltage that is dependant on that same CT (ie, common Fender design with a bias tap on the PT HT winding) and you open that CT with a switch, the filter caps in the bias circuit will be highly stressed and may even fail.

I notice that ALL ground terminations (other than the main AC supply ground) seem to be only terminated at the front panel brass plate. Is this a suitable arrangement?

There are many successful ground schemes. I like to separate my preamp and power amp grounds.

[The Ballzz]

WOW!
I love this place, and you folks as well! Great, clear and concise answers!

So, my take and understanding is:
A, B & C) Yep, go ahead & generally duplicate the primary of the Modern schematic.
2) Diodes = GOOD/OK.
1 & 3) You folks make a good argument for using the heater center tap, as well as adding the suggestion from other threads about fusing that center tap in place of the standby switch.
4) While I've seen many variations, it would seem the best bet for grounding would be to send the HT center tap directly to C-13, to be shared by C-11, C12, C-10, R-19, R-20 & R-23. Then use one wire from that buss/ground rail to a lug on the chassis to be shared with the fused heater center tap.
Then join all other grounds to one point on the chassis near the input jacks.
This does, however leave, (in my mind) the age old conundrum of whether to consider the ground of the PI/driver tube as part of the power section or pre-amp section for grounding purposes? Or, should the PI half of the tube ground with the power and the driver half ground with the pre-amp, or.....?
5) Figuring that playing with the screen resistors on a couple of substitution boxes connected between pins 1 & 4 of the power tubes (using pin 1 as a junction point) should get me where I like it!

I love having a good plan! Measure twice & cut once!

You Folks ROCK! Thanx Again,
Gene

[Colossal]

I'm planning to start with the higher 355v-0v-355v PT taps and experiment with a few different rectifiers before trying the 330v-0v-330v taps if the voltages are too out of line.

FWIW Gene, I finished a modded 5E3 not too long ago using a 350-0-350 PT. The rectifier is an NOS RCA 5Y3GT. Plate voltages are actually quite low so starting with 355-0-355 is reasonable. B+ using modern 5Y3s will probably be significantly higher, so something to take under consideration.

[The Ballzz]

I'm planning to start with the higher 355v-0v-355v PT taps and experiment with a few different rectifiers before trying the 330v-0v-330v taps if the voltages are too out of line.

FWIW Gene, I finished a modded 5E3 not too long ago using a 350-0-350 PT. The rectifier is an NOS RCA 5Y3GT. Plate voltages are actually quite low so starting with 355-0-355 is reasonable. B+ using modern 5Y3s will probably be significantly higher, so something to take under consideration.

DOH!
Yep, besides CP Sovtek and JJ 5Y3's , I have a couple UOS 5Y3's and a fairly full selection of other OS octal rectifiers. Naturally, I won't try any of the higher current draw models, but will likely experiment a bit. Gazing into my crystal ball, I'm guessing that the Sovtek would likely be "OKAY" with the 330v taps, but way outa the ballpark with the 355v taps! We shall see what we shall see and I'll keep ya all posted. I'll likely start a new "pictorial" build thread as I get going.

I'm pretty stoked for this build, in case no-one has noticed!

Thanx 4 The Thoughts,
Gene

[The Ballzz]

Here's my planned revision (in red) of the Modern Fender Schematic:


Watcha All Think?
Gene

[The Ballzz]

So, given this is my first Fender style, combo layout build, I'm looking for opinions on the heater wiring. I see a lot of builds laid out like these. I've done several Marshall head style builds and had good luck with heaters tucked into the chassis corner. like the first pic. I've since been schooled that even with my Marshall style layout, the wires should actually hit the pins from over the top, even when tucked. What do you guys think? Tuck them or stand 'em up? FWIW, I believe at least one of those 5E3's is a Carl's Custom Amps.
Thanks,
Gene

[Stevem]

Yup, that's the way to fly in regards to the heater wiring.
If you want to go a bit over board you can shield the wiring to V1 and V2.

[The Ballzz]

Yup, that's the way to fly in regards to the heater wiring.
If you want to go a bit over board you can shield the wiring to V1 and V2.

Thanks, we'll see if there is a need for the shielded wire! I finally got all the parts and am halfway through the build. I will start a new "build thread" with pics, etc. Still taking/organizing pics as I go!

I did somewhat average many of the tips here and even bucked a few, for example: I did go for turret board construction and used the brass plate that turned out to be quite convenient for pot/jack layout and wiring. I did not use the brass plate as the main ground point as per original units.

Thanks To All 4 The Help!
Gene

[xtian]

Cool, looking forward. I have a 5e3 in the works also. Just picked up an old Triad R-11A power transformer (350-0-350). I think I'm building a head version, and I'll very certainly be breaking a lot more traditions than you! I'm starting with Hoffman's layout (hey, Doug!).

[The Ballzz]

Yeah, I started with the Mojo layout, but moved a couple things to facilitate running the HT center tap directly to the negative of the main reservoir/filter cap and sending that to the star on it's own lead.

I then tied the cathode, and other main caps, along with the grid leak grounds through one wire to that same star along with filament CT and speaker ground. All other grounds are tied right next to the input jacks.

I also bagged the stand by and fused the 6.3v center tap in place of the switch.

If it's noisy at fire up, I may consider "elevating" the heaters by tying that center tap to "hot" side of the cathode bias resistor/cap! I've never had to do this before, just leaving option open!

We Shall See What We Shall See,
Gene

[martin manning]

I also bagged the stand by and fused the 6.3v center tap in place of the switch.

There is essentially no current flowing flowing through the filament CT except in the case of a specific type of tube short. That fuse might be better used in the HT center tap where it will protect against any type of excess HT current draw except a rectifier short, which you've mitigated by backing up the vacuum rectifier with Si diodes.

[The Ballzz]

I also bagged the stand by and fused the 6.3v center tap in place of the switch.

There is essentially no current flowing flowing through the filament CT except in the case of a specific type of tube short. That fuse might be better used in the HT center tap where it will protect against any type of excess HT current draw except a rectifier short, which you've mitigated by backing up the vacuum rectifier with Si diodes.

I could only find those particular diodes at a fairly outrageous price, so decided to wait on them, maybe I'll add them later.

6.3v CT is already cut/soldered to the fuse holder, although I left enough length to go to the lug later if I want to change it. This is only the first of likely several of these amps, so I'll make some refinements as I go.

It may be interesting to note that in 40+ years of cranking tube amps to (and possibly past) their supposed limits, I've only had one failure severe enough to take out a fuse and never a transformer! That amp was a black face Super Reverb that had been modified by Don, "aka Dawk" Stillwell to use 6550's along with oversized iron and the reverb loop modified to be added gain stage(s). That amp was pretty much a fiasco, all around!

I'm not too worried! We'll see how it looks upon initial fire up. If a voltage survey shows them to be at or near limits, I may look into added protections. Otherwise, I might simply just "run naked with scissors" for a while!
Gene