Building a 5F6-A from the ashes of an old Italian PA amp

[pietro.castelli]

I then proceeded to test the effect of my little contraption.
First of all, the pilot light turned on when I flicked the switch:

Then I measured the heaters voltage at the power supply:

and at the pilot light:

You might argue that 6.67 volts was still okay, but the mains voltage was particularly low when I took the measurement.
In my previous tests, I've seen the power supply produce more than 7 volts at the heaters.

 

[2L man]

I then proceeded to test the effect of my little contraption.
First of all, the pilot light turned on when I flicked the switch:

Then I measured the heaters voltage at the power supply:

and at the pilot light:

You might argue that 6.67 volts was still okay, but the mains voltage was particularly low when I took the measurement.
In my previous tests, I've seen the power supply produce more than 7 volts at the heaters.

Something very strange if filament AC voltage is more than 7V one day and 6V on another!!!

Connect somekind load to it.

Transformer output voltages should come proportional to input voltage and 6V vs 7V is about 17% difference which should never happen for Mains voltage.

 

[pietro.castelli]

Something very strange if filament AC voltage is more than 7V one day and 6V on another!!!

Connect somekind load to it.

Transformer output voltages should come proportional to input voltage and 6V vs 7V is about 17% difference which should never happen for Mains voltage.

Hi,
6V is the reading after the dropping diodes.
The highest voltage I’ve read on the heaters at the power supply is 7.06V. The lowest was 6.67V. That’s about a 5% difference which is the coherent with the fluctuation I get sometimes here on the mains voltage

 

[pietro.castelli]

Today I finished the heaters circuit, wiring all the tubes sockets:

Meanwhile I’ve also started working on the stand by switch, where the B+ from the rectifier is going to be wired.
I’ve added a capacitor and resistor in parallel which should remove any pop:

The capacitor is 0.047 uF 630v and the resistor is 100k 2W

 

[pietro.castelli]

Yesterday, I wired the two bridge rectifiers for the B+ and Bias supply.
I must admit it kicked my butt a bit more than expected. It's a lot of wiring to tuck in a relatively small area. Also, working with eyelets is far less enjoyable compared to turrets. When you have a single eyelet hosting multiple components leads plus wiring, it is a tight squeeze.

This is the wiring bundle so far near the power section (still waiting for lacing tape), with the grounding strip I've mounted to the chassis.

I've also started to assemble the filter capacitors board:

And finally, an overview of the chassis so far:

 

[Marcomel79]

Looking very nice Pietro. Someting my father always told me: people never ask how long it took, but they always ask who made it

 

[pietro.castelli]

Looking very nice Pietro. Someting my father always told me: people never ask how long it took, but they always ask who made it

That's a very good lesson to learn!
And I also inherited my passion for tinkering with things and building stuff from my father!

 

[pietro.castelli]

Today I did some more work on the tube sockets,
adding the screen resistors, wiring the OT primary, and so on:

Wiring is still a bit of a rat’s nest. Still waiting for the lacing tape to get things tidied up. I’ve also added some taps to measure the bias of each tube.
I know it’s not the most accurate way of measuring, but it helps getting in the ballpark:

Finally I’ve populated the main circuit board with most of the components:

I must say I’m pretty happy with the way the
build is coming along so far.

 

[StingRay85]

Amazing work so far.

Be aware that heater voltage under load might still drop, and your board to drop voltage might become redundant

 

[pietro.castelli]

Amazing work so far.

Be aware that heater voltage under load might still drop, and your board to drop voltage might become redundant

Thanks!
Yes, I’m aware but it isn’t a big deal.
Worst case scenario I’ll remove the diodes and
keep the virtual center tap only.

 

[pietro.castelli]

After two much needed weeks of holidays, the amp build continues:

First I changed the routing of the output transformer primary to tubes’ plates connections as well as some mistakes I’ve made wiring the tube sockets. Thanks @Marcomel79 for spotting them!

Since I’ve finally received the lacing tape I’ve ordered, I also started tiding up the wiring:

Meanwhile I’ve also wired the filter capacitors board, and made some progress with the tedious job of attaching all the hook-up wires to the main circuit board.

 

[Marcomel79]

Looking like a work of art!

 

[StingRay85]

This looks like Hiwatt wiring quality, amazing

 

[2L man]

Your building skill is very good and look neat!

Chassis electrical role is Safety Earth! If both ends soldering strips "buses" have connect to Chassis there will flow some HV current thru Chassis and it open possibility for hum and noise enter to audio.

Safety Earth wire can have EM noise because it always has resistance and impedance and often run long length with Live and Neutral. Electric regulations has already about 50 years forbid to flow current in SE.

HV0VDC / Signal Reference should have only one connection point with Chassis and then no secondary current leak to Chassis because it does not have other return point. If does not matter is the connection close to power supply or close to input jacks but signal input is more common place.

 

[Marcomel79]

Your building skill is very goon and look neat!

Chassis electrical role is Safety Earth! If both ends soldering strips "buses" have connect to Chassis there will flow some HV current thru Chassis and it open possibility for hum and noise enter to audio.

Safety Earth wire can have EM noise because it always has resistance and impedance and often run long length with Live and Neutral. Electric regulations has already about 50 years forbid to flow current in SE.

HV0VDC / Signal Reference should have only one connection point with Chassis and then no secondary current leak to Chassis because it does not have other return point. If does not matter is the connection close to power supply or close to input jacks but signal input is more common place.

If you look carefully the first tag is not connected to the other ones...

 

[2L man]

If you look carefully the first tag is not connected to the other ones...

It look like it is!

 

[Marcomel79]

It look like it is!

View attachment 155830

The other one

 

[pietro.castelli]

Looking like a work of art!

This looks like Hiwatt wiring quality, amazing

Thanks a lot guys!
I'm rather OCD, so I'm trying to do a neat build, but I'm still a rookie (this is my second amp), so I make a lot of mistakes

If you look carefully the first tag is not connected to the other ones...

It look like it is!

View attachment 155830

If I got @2L man's comment correctly, he suggests separating Safety Earth from grounding.
I indeed disconnected the last tag of the stripboard to avoid ground loops, but the first tag carries the earth connection, and the following one carries ground connections. I should have probably separated the first tag instead.

Just to simplify things, here's a rough sketch of how things are connected at the moment:

 

[Marcomel79]

Thanks a lot guys!
I'm rather OCD, so I'm trying to do a neat build, but I'm still a rookie (this is my second amp), so I make a lot of mistakes






If I got @2L man's comment correctly, he suggests separating Safety Earth from grounding.
I indeed disconnected the last tag of the stripboard to avoid ground loops, but the first tag carries the earth connection, and the following one carries ground connections. I should have probably separated the first tag instead.

Just to simplify things, here's a rough sketch of how things are connected at the moment:

Thats exactly what i thought you were gonna do. It does make more sense to connect the earth ground separate and sll the other grounds together!

 

[pietro.castelli]

Thats exactly what i thought you were gonna do. It does make more sense to connect the earth ground separate and sll the other grounds together!

Of course, it makes a lot more sense. I don't know why I've connected Earth that way, but at least is is going to be an easy fix. As I've mentioned, I still have a lot to learn
Thanks to you and @2L man for pointing me in the right direction!

Cheers!