Fairchild 627 from scratch

[Slenderchap]

I can turn them back on in the on-line store ... but there will be a lead time as we have run out of some stuff.

 

[Ricardus]

Agreed, i was hoping to get a pair too @Slenderchap

And since I'm generating so much business for him, my two kits get a discount!!

 

[DaveP]

I had to make a decision about the position of the knobs. It’s possible to get an idea of the measurements from several of the available photos. We know the front panel is 19” long and 7” high so careful interpolation from the skirts of the knobs can give an answer to about the nearest 1/8”.

This gives the centres of the top row of knobs 2” from the top and the lower row about 1.75” from the bottom. The central knobs are 4.5” apart and the outer knobs are 3.375 (3-3/8) from the central knobs. The central tube is approximately in the centre or maybe a little below it.

I decided to make my measurements equal at 1.875 (1-7/8) from the top and bottom edges to give more space for the lettering below. The centres of the outer knobs are 4” from the sides making 3-1/4” between the knobs; this leaves more room for the cog/pot assemblies.



I have also finished the fixing frame and all the countersunk screws. The distances chosen for these screws fits the dimensions of the RS tag strips, so that I have convenient places to mount them on the back as required.



The next step is to make the rest of the chassis panels.

Best
DaveP

 

[Winetree]

HAVE YOU SEEN THIS?

http://www.audiomaintenance.com/acatalog/aml-17-033_extended_info.html

 

[Bo Deadly]

Audio Maintenance Limited is the real deal. They give you full schematics and detailed vector graphic drawings in PDF so that you can zoom in on anything perfectly to scale. And the price is very reasonable.

 

[DaveP]

Thanks for the link Winetree and Bo Deadly.

The object of these "from scratch" projects has always been to make them point to point and to avoid pcb's. I try to make them as they used to be made and with materials that anyone can purchase. The idea is to encourage people to try for themselves, as I understand it, that is what GroupDIY was all about. I know that many people like kits because someone else has hopefully done all the difficult stuff for them, nothing wrong with that. My photo is 22 years old so this is also a record of soon to be forgotten arts and crafts.

best
DaveP

 

[Ricardus]

Thanks for the link Winetree and Bo Deadly.

The object of these "from scratch" projects has always been to make them point to point and to avoid pcb's. I try to make them as they used to be made and with materials that anyone can purchase. The idea is to encourage people to try for themselves, as I understand it, that is what GroupDIY was all about. I know that many people like kits because someone else has hopefully done all the difficult stuff for them, nothing wrong with that. My photo is 22 years old so this is also a record of soon to be forgotten arts and crafts.

best
DaveP

And your work is about as good as it gets.

But that is something I wouldn't attempt, so I'm happy to build that kit and hope he restocks soon.

 

[HerbertR]

Thanks for the link Winetree and Bo Deadly.

The object of these "from scratch" projects has always been to make them point to point and to avoid pcb's. I try to make them as they used to be made and with materials that anyone can purchase. The idea is to encourage people to try for themselves, as I understand it, that is what GroupDIY was all about. I know that many people like kits because someone else has hopefully done all the difficult stuff for them, nothing wrong with that. My photo is 22 years old so this is also a record of soon to be forgotten arts and crafts.

best
DaveP

Hi Dave, while "studiing" the schematic of the 627 - which Inductor L1 on the primary winding of the ungaped T3 15k/600 will you use? The original was a 300Hy 5mA UTC S23 as far I could find in the database. Any replacements out there?...
And are the HF/LF Boost "Tripple"-Potentiometers Log or Lin ones? Audiomaintainance has declared them as example A 47K/22K/47K..... ones in their building instruction. That should be Log Pots or am I wrong?
Greez Herbert

 

[DaveP]

Hi Herbert,
I will try one Hammond 156C which has 150H at 8mA, but if that loses some low end I will use two of them in series.
I imagine the triple pots were originally wire-wound so they will be linear. Log pots have about 3 or 4 linear sections so it is more difficult to get them to match up in a circuit like this.

I had to do another job for a friend, but now I am back on the case and the chassis is being put together, more pics soon.

best
DaveP

 

[HerbertR]

Hi Herbert,
I will try one Hammond 156C which has 150H at 8mA, but if that loses some low end I will use two of them in series.
I imagine the triple pots were originally wire-wound so they will be linear. Log pots have about 3 or 4 linear sections so it is more difficult to get them to match up in a circuit like this.

I had to do another job for a friend, but now I am back on the case and the chassis is being put together, more pics soon.

best
DaveP

Dave, great!!! I think I will also do this build PTP. Super affordable idea with the Hammond choke in series....
Linear Pots for boost... Okay, hmmm...... "self" combining 100k and 47k "Gangs" on Quadpot Shafts together...?.. and the same with 4K7 and 10k's. ... Or your Idea...
Fantastic project...

 

[DaveP]

After a lot of thought trying to visualise everything in 3D I settled on this design with vertical tubes and a removable base. The vertical panel behind the tubes is also removable to get access to the 6E5 magic-eye tube. The back panel has input and output XLR sockets and the central hole for the power supply cable.



The space between the tubes is to enable access to the 6E5 with the tubes in place and to provide a place to hang an inverted transformer if required. With a prototype design there are always unforeseen issues so you need spare space and plenty of spare tags to make changes if required.



The brass cogs needed drilling out from 6mm to ¼” and so did the shaft connections. The central section of the shaft connectors is 7mm diameter, so I cut one end off and drilled half of the cog to 7mm. I put some super-glue around the hole and pressed it into place with a vice. I will cut the ¼” brass rod to make two spindle extensions. I dismantled a couple of old burnt-out pots to use the mountings on the front panel.





The next stage should see the chassis finished and a panel for the triple pots and the 6E5 tube base.

Best
DaveP

 

[DaveP]

This shows the chassis partially assembled to fit the internal panel for the triple pots and the tube base for the 6E5.



I have to sort out the right angle for the 6E5 so I have not yet fitted its tube base. The tube passes through the hole on the front panel to locate on the tube base.



The cogs have to be fitted with the outer pots at maximum and the inner pot at minimum. This is because they revolve anti-clockwise, I have to remember to wire them in reverse too.

Now this is done I can finish all the rest of the panels

Best

DaveP

 

[dustbro]

This shows the chassis partially assembled to fit the internal panel for the triple pots and the tube base for the 6E5.



I have to sort out the right angle for the 6E5 so I have not yet fitted its tube base. The tube passes through the hole on the front panel to locate on the tube base.



The cogs have to be fitted with the outer pots at maximum and the inner pot at minimum. This is because they revolve anti-clockwise, I have to remember to wire them in reverse too.

Now this is done I can finish all the rest of the panels

Best

DaveP

As always, I enjoy watching your builds.
thanks DaveP

 

[DaveP]

We go back a long way dustbro, thank you
best
DaveP

 

[DaveP]

The chassis is now finished.

There are two removable panels for access to the upper components and a large removable base for access to the main circuit.

The internal panel for the pots/cogs slides in through slots in the top, it is fixed by screws either side accessible from above or below.

The whole lot weighs in at 2.6kg or 5 ¾ lbs of aluminium.

I have changed the input and output controls to conform to modern standards as you can see in the modified schematic.

In the original circuit, the input transformer is loaded by the 600 ohm triple pad and the 15k resistor on the secondary. I don’t want to change that arrangement, so I have subbed in a 10k pot shunted by a 680 ohm resistor to maintain the loading. At the output I have used a 1k pot shunted by a 1.5k resistor to maintain the loading ratio of the original as far as possible.

The next stage is to find a home for all the main components before wiring-up begins.

Best

DaveP

 

[HerbertR]

Dave, do know what is R30 500K trimmer for? Is that for controlling the 6E5 intensity driven by the 6SQ7 through changing the grid voltage or am I wrong?
Do you also have any suggested Testpoints or Voltages? Except of PSU Voltages for sure....
Thanks for your great project ...

 

[DaveP]

Herbert, R30 is to set the intensity of the 6E5 via the 6SQ7 as you say. I will know better how to set it up when its finished, but as far as I can see, it will be like a pre-set at the back, not accesible on the front panel. For example: If you want the output level to be +6dB, then you would adjust the 6E5 to maximum while the output is at +6dB. Hope that helps.

best
DaveP

 

[DaveP]

I have found a home for the large components keeping the input away from the output stage and mounting the chokes in a right angle plane to the IPT &OPT. I decided to use two chokes to be on the safe side to get 300 H in series. I am not sure which way to wire them yet, any ideas? They are facing each other so the external fields may interact. The central pot is R33 which sets the level of the indicator tube.



I am waiting for the second delivery from RS (the first got here OK) the switch for the low cut got lost because they only put on half my address. It really annoys me that they split the deliveries, which effectively doubles the carbon footprint. Have you noticed that we can’t contact delivery companies to correct addresses because their system won’t allow it, I guess for security reasons, that’s progress for you!



The heaters are now wired up (except 6E5) and the busbar is now in place with the chassis connection only at the input socket. The tag strips allow me to start to add the resistors and capacitors next.



Best

DaveP

 

[HerbertR]

I have found a home for the large components keeping the input away from the output stage and mounting the chokes in a right angle plane to the IPT &OPT. I decided to use two chokes to be on the safe side to get 300 H in series. I am not sure which way to wire them yet, any ideas? They are facing each other so the external fields may interact. The central pot is R33 which sets the level of the indicator tube.



I am waiting for the second delivery from RS (the first got here OK) the switch for the low cut got lost because they only put on half my address. It really annoys me that they split the deliveries, which effectively doubles the carbon footprint. Have you noticed that we can’t contact delivery companies to correct addresses because their system won’t allow it, I guess for security reasons, that’s progress for you!



The heaters are now wired up (except 6E5) and the busbar is now in place with the chassis connection only at the input socket. The tag strips allow me to start to add the resistors and capacitors next.



Best

DaveP

Looks excellent. I also bought 2*150H Hammond Chokes for serial use per channel. They were cheap, about 14 Euro pP. I also think facing the windings of these chokes could cause small interferences. I will try them mounted in series or in 45° angle. At least I made good experiences mounting 8 unshielded In/Out/Interstageaudio edcors for a 670 build in 45° angle position in a 4U 19 inch case sidewall.

 

[DaveP]

The main amplifier components are fitted and when tested and gave the following Voltages:-

V1 6J5 Va=105V Vk=2.8V

V2 6SG7 Va=229V Vg2=208V Vk=35V (with 25k pot)*

V3 6E5 Va=12.8V Vg=0V Target current =2.7mA

V4a 6SN7 Va=93V Vk=3.4V

V4b 6SN7 Va=202V Vk=7V

V5a 6SN7 Va=118V Vk=4.1V

V5b 6SN7 Va=236V Vk1=18.2V Vk2=12.4V

V6 6SQ7 Va=184 Vk=1.6V

The total current drain is 27mA at HT/B+ 236V

*The cathode voltage is elevated by an HT voltage divider. It can be varied from 35V down to 0.4V by the 25k pot.

I have checked the power dissipation of every resistor and they are all less than 0.4W. The two series chokes have a DCR of 6.92K and the current is just under 5mA which checks with the spec of the UTC S-23.

The 6E5 has a green glow and I can now see the right angle to fit the tube base.


As you can see from the pics, there are still all the feedback components to fit and the front panel to sort out.

The cables for the connections to the front panel are a bit of a mess at the moment, but the important thing is that the tubes all work as they should. Now I can go on to the next stage....
Best

DaveP