5E3 build questions

[Randall]

First off, Hello, I'm new here. My name is Randall from Portland, ME. I have begun my first amp build project and have a few questions.

There is a coupling cap between the sides of the 12AX7 PI that is not labeled on the original schematic. Anyone know what it is?

I am interested in the star grounding method instead of the chassis as ground path method. What is the preferred way to isolate the input jacks and pots without drilling out the holes to accommodate isolation washers? And while I'm at it, does this really make a lot of difference?

What wattage powers would you put in for the 5K and 22K in the power supply, and the three 100Ks and 56K feeding the plates of the preamp tubes?

And finally for now, I am going to ditch the ground switch for a standby. What value cap should I use?

Thank you in advance. I am building a combo for myself and a chassis for a friend at the same time. I guess I should ask for advice on what speaker everyone likes, too. For now I am looking at the Weber alnico.

[LeftyStrat]

There is a coupling cap between the sides of the 12AX7 PI that is not labeled on the original schematic. Anyone know what it is?

Mostly likely 0.1uf 400, same as the rest.

I am interested in the star grounding method instead of the chassis as ground path method. What is the preferred way to isolate the input jacks and pots without drilling out the holes to accommodate isolation washers? And while I'm at it, does this really make a lot of difference?

Switchcraft makes plastic 1/4" phone jacks that shouldn't require drilling out the holes. I'm not sure it will make that much difference in this circuit, though I defer to others on this.

What wattage powers would you put in for the 5K and 22K in the power supply, and the three 100Ks and 56K feeding the plates of the preamp tubes?

At least 2w for the 5k and 22k. 1/2w for the plate resistors.

And finally for now, I am going to ditch the ground switch for a standby. What value cap should I use?

Not sure what cap you are talking about. The standby switch just breaks the B+ going to the tubes. Though with a tube rectifier it's not really needed.

[xtian]

Hi and welcome!

Look at the 5e5 schematic: there's a 0.02/600v cap there, in the PI.

Star grounding can help, and there is one version where the main ground to chassis is very close to the inputs. So you can leave your inputs grounded to chassis, or use Cliff jacks and ground elsewhere.

Power string dropping resistors can be 2 watt, others, including 100K plate resistors can be 1/2 watt. You can get better specs off Ceriatone, Weber, and other similar schematics.

Modern builds should not use the "death cap" that was part of the old grounding circuit. Again, refer to modern schematics.

Go get 'em!

[ToneMerc]

And finally for now, I am going to ditch the ground switch for a standby. What value cap should I use?.

Typically you will find values .01 to .05 across the standby switch. You can a small or large 600v film capacitor. I like to use a .01 1kV disc capacitor as the lead spacing works well the switch terminal spacing.

Welcome aboard

TM

[Firestorm]

First off, Hello, I'm new here. My name is Randall from Portland, ME. I have begun my first amp build project and have a few questions.

Welcome.

There is a coupling cap between the sides of the 12AX7 PI that is not labeled on the original schematic. Anyone know what it is?

My schematic shows .02.

I am interested in the star grounding method instead of the chassis as ground path method. What is the preferred way to isolate the input jacks and pots without drilling out the holes to accommodate isolation washers? And while I'm at it, does this really make a lot of difference?

No need to isolate the pots. The best variation on star grounding (IMO) uses one point for all the power amp related grounds and another for the preamp related ones. The preamp grounds usually do well grounded at the input jack, so maybe no need to isolate those either.

What wattage powers would you put in for the 5K and 22K in the power supply, and the three 100Ks and 56K feeding the plates of the preamp tubes?

I'm sure Fender used 1W for the 5K, but I'd be inclined to go 3W, even 5W. The 22K should propably be 1W; 2W even better. For the plate resistors, 1W will be a little quieter than 1/2W.

And finally for now, I am going to ditch the ground switch for a standby. What value cap should I use?

Good idea to ditch the ground switch. Of course, you'll be using a three-prong cord. Not sure what cap you're thinking of for use in a standby circuit.

[Randall]

Thanks for the quick replies. The standby switch cap I'm talking about is what I see on most other Fender schematics going from the hot side of the switch to ground. I had thought this was to absorb the hit of switching the HV. I realize a tube rectifier takes a while to come up, but it's also nice to switch off the HV during breaks and leave the heaters on.

I am not inclined to re-invent this circuit by modding it too much. But the death cap is a no brainer in my book. If folks here think leaving the inputs chassis grounded is fine, I would actually rather do that. That being the case, what is consensus on the brass plate?

[xtian]

Hi and welcome!

Look at the 5e5 schematic: there's a 0.02/600v cap there, in the PI.

Star grounding can help, and there is one version where the main ground to chassis is very close to the inputs. So you can leave your inputs grounded to chassis, or use Cliff jacks and ground elsewhere.

Power string dropping resistors can be 2 watt, others, including 100K plate resistors can be 1/2 watt. You can get better specs off Ceriatone, Weber, and other similar schematics.

Modern builds should not use the "death cap" that was part of the old grounding circuit. Again, refer to modern schematics.

Go get 'em!

[Structo]

Thanks for the quick replies. The standby switch cap I'm talking about is what I see on most other Fender schematics going from the hot side of the switch to ground. I had thought this was to absorb the hit of switching the HV. I realize a tube rectifier takes a while to come up, but it's also nice to switch off the HV during breaks and leave the heaters on.

I am not inclined to re-invent this circuit by modding it too much. But the death cap is a no brainer in my book. If folks here think leaving the inputs chassis grounded is fine, I would actually rather do that. That being the case, what is consensus on the brass plate?

Yeah, that is the infamous "Death Cap".

If and when that cap ever fails, you will have 120vac on the chassis.

You don't need it with a three prong cord with ground.

[Phil_S]

And finally for now, I am going to ditch the ground switch for a standby.

Hello and welcome! If you are building a 5E3, you can skip the standby switch with no ill effects. Since I read this http://www.valvewizard.co.uk/standby.html I haven't used a standby and don't miss it. It is one less thing to do.

In case you miss the important stuff buried in Merlin's article, he says, "Contrary to popular belief, the standby switch is not there to prolong valve life-span. The theory is that if the HT is applied while the cathodes are cold they will be 'stripped' by ions crashing into the unprotected cathode. However, this simply does not happen. It is an urban myth borrowed from transmitter and cathode-ray tube technology NOT ordinary 'receiving' valves. "

[M Fowler]

I don't plan for a standby switch in my custom builds but people think they just have to have one.

I recommend everyone shut their amps off when they exit the stage heading to get a front row seat at the bar.

Mark

[Randall]

Something I am not getting is what I have read concerning input grounding. Some say go with the brass plate, it's fine. Some say lift the inputs and ground with a wire directly back to a common point. And Mojotone skips the brass plate and instead runs an 18g copper wire across the back off all controls and then to the chassis, which I don't understand because the pots are already grounded to the chassis by the shaft mount. What am I missing? Is this because their chassis is painted black?

[jjman]

A cap from a standby switch, to ground, is a filter cap. Leaving it out would be bad.

[Firestorm]

The standby switch cap I'm talking about is what I see on most other Fender schematics going from the hot side of the switch to ground.

That's the main filter; in BF amps, Fender kept it charged while the amp was on Standby by putting it on the hot side of the Standby switch. In the 5E3, that would be the first 16uF cap. If you want a Standby switch in your build, the hot side would go to the output of the rectifier and the 16uF cap; the cold side would go to the 5K resistor and the OT center tap.

Something I am not getting is what I have read concerning input grounding. Some say go with the brass plate, it's fine. Some say lift the inputs and ground with a wire directly back to a common point. And Mojotone skips the brass plate and instead runs an 18g copper wire across the back off all controls and then to the chassis, which I don't understand because the pots are already grounded to the chassis by the shaft mount. What am I missing? Is this because their chassis is painted black?

Fender used the brass plate because it was easier to solder to than the chassis itself. The grounding isn't quite as random as it appears because there are small insulators between the brass plate and the chassis at each pot and typically one solid solder connection at the power amp side of the brass plate. But what I do is skip the plate and run an 18g copper wire behind the pots (not soldered to them) grounded at the input jack. I make all my preamp grounds to that wire. Then I use a second ground point for all the power amp-related connections over at the other end of the amp. Seems to work.

[JazzGuitarGimp]

Hi Randall, and wecome!

It looks like you're already getting great advice, but I see no one has addressed your question on speaker choice yet. To which I offer my $0.02:
I have the Tone Tubby Alnico Hempcone in my 5E3 and love it.

Cheers!
Lou

[ToneMerc]

The standby switch cap I'm talking about is what I see on most other Fender schematics going from the hot side of the switch to ground.

That's the main filter; in BF amps, Fender kept it charged while the amp was on Standby by putting it on the hot side of the Standby switch. In the 5E3, that would be the first 16uF cap. If you want a Standby switch in your build, the hot side would go to the output of the rectifier and the 16uF cap; the cold side would go to the 5K resistor and the OT center tap.

Something I am not getting is what I have read concerning input grounding. Some say go with the brass plate, it's fine. Some say lift the inputs and ground with a wire directly back to a common point. And Mojotone skips the brass plate and instead runs an 18g copper wire across the back off all controls and then to the chassis, which I don't understand because the pots are already grounded to the chassis by the shaft mount. What am I missing? Is this because their chassis is painted black?

Fender used the brass plate because it was easier to solder to than the chassis itself. The grounding isn't quite as random as it appears because there are small insulators between the brass plate and the chassis at each pot and typically one solid solder connection at the power amp side of the brass plate. But what I do is skip the plate and run an 18g copper wire behind the pots (not soldered to them) grounded at the input jack. I make all my preamp grounds to that wire. Then I use a second ground point for all the power amp-related connections over at the other end of the amp. Seems to work.

Firestorm, Fender used the brass plate because it made assembling the controls in the angled chassis less labor intensive. The pots,switches and components were stuffed and soldered to the brass plate as a sub assembly before moving to the next stage of building. Also, that " insulator" wasn't an insulator, but a fiber paper keeper to prevent the pot shaft from falling back through the brass plate hole prior to the knobs being put on when the assembly was mounted in the chassis. Think about it, not much chance of the plate being actually insulated when the pot shaft threaded bushing also contacts the brass plate and the chassis as well.

Sometime back I purchased about a dozen fully assembled late 70's control panels from someone that worked at Fender in the 70 and 80's. They were exactly as they came off the production line, all the fiber "insulators" were on the outside of the control plate, with none of the pots having nuts on them.I could take the plate assemblies turn them upside down and shake them, the pots would not fall back through because of the keeper.

TM