layout vs schematic Express

[IaXy]

Hi all,

This is my first post here. I have howver been lurking and reading up. I am looking at building an Express. Looks quite interesting.

I got the schematic UR12 modified to be as the Francesca, but I don't see a layout.

I found https://tubeamparchive.com/download/file.php?id=820 this one that Ritchie modded, but the layout does not match the schematic.

So which one would be the correct one to use?

Thanks,

Bob

[UR12]

Can you be a little more specific. What doesn't match?

[IaXy]

I didn't go through the whole layout. I stopped after the fist few differences.

The first few filter caps and the bleeder resisters are before the standby switch on the layout and the schematic they are after the standby switch.

Granted, this probably won't make a diff in tone, but it was enough to flag, to ask a question.


Bob

[gearhead]

That particular one is actually a change from the exact Francesca build. UR12 can explain it better, but it is safer to do it the way indicated on the modded Version A1b (standby before any filter caps).

[IaXy]

hmmm.... Why would you put a pair of caps and bleeder resistors before the standby switch? I never keep my bleed resistors in circuit anyway. I allways put them on the power switch.

I first thought it was a mistake. You got my mind running on this one. I cannot figure out why you would want this... Can someone explain?

Bob

[gearhead]

The caps and bleed resistor location -before- the standby switch is exactly how Francesca was built. You'd probably have to ask Ken Fisher why he did it that way. Moving them to the other side was UR12s safety-oriented contribution, I believe (UR12 chime in)/

Being new on the scene myself, is there any reason why B+1 (OT trannie) would benefit from getting supplied power, with stabilized-capacitors, immediately after the standby switch is toggled? That's the only reason I can think of Ken's design.

[UR12]

The caps and bleed resistor location -before- the standby switch is exactly how Francesca was built. You'd probably have to ask Ken Fisher why he did it that way. Moving them to the other side was UR12s safety-oriented contribution, I believe (UR12 chime in)/

Being new on the scene myself, is there any reason why B+1 (OT trannie) would benefit from getting supplied power, with stabilized-capacitors, immediately after the standby switch is toggled? That's the only reason I can think of Ken's design.

Actually I never moved them. They were drawn that way on the original Kelly 90 schematic and I haven't changed that part. My first Junkyard Express is wired up like Franchesca and I have seen no ill affects as far as performance from using Ken's way.

My take on this, and anyone who has a differing opinion I welcome a comment, is that if you do it Ken's way you still leave b+ on the two paralleled 40u caps and the bleader resistor with the standby switch open. IF you were running a tube recto then this may in some obscure way lengthen the life of your recto as there would'nt be a current surge charging the caps when you flip the standby switch. Most tube rectos list the max input cap at 30 - 60uf and the charging current surge that larger caps pull could exceed the current rating of the recto. The 2 40uf caps in parallel is equivalent to 1 80uf which exceeds most of the ratings on the tube rectos.

Having said that, Ken is not using a tube recto in Franchesca but uses standard 1N4007 SS diodes. In this configuration the diodes are rated at 1 amp and would have no problem with the initial current surge of a 80uf cap so because of this I don't see any reason to keep the B+ on these two caps and remove it from all of the others. Most manufacturers put the standby switch after the rectifier(s) and before the filters which removes ALL B+ from the amp's components and IMHO would seem a bit safer to me.

It's interesting to note that with the 80uf cap as the first cap after the rectifiers and using a 200k bleeder resistance the caps will discharge 99% of their voltage in about 73 seconds. They can charge to 99% in just somewhere around 400 micro seconds depending on the internal resistance of the cap and power supply.

So I guess to fix the schematic, since my goal is to have it match Franchesca, then I will change the Standby switch configuration to match Franchesca and the layout. I will post another schematic of the power supply section as it is now for anyone wanting to use it as an alternate way instead of the way Franchesca is wired.

Thanks Bob for the heads up. This is a work in progress to get a true schematic of Franchesca so that other people don't run into the same problems we have already work out.

[UR12]

I have corrected the Franchesca schematic to reflect the standby switch location change.

[IaXy]

I was just wondering why... I thought maybe it was like that to keep B+ on the CT of the OT so when the standby switch was switched on it would stop from making a popping sound. I don't know, anyway, if Ken had it like that, which I was looking at Francesca's pics, and as far as I can see it is, then it must be for a reason.

Has anyone built one like the schematic or are you building it like the layout? just wondering.

Bob

[gearhead]

Actually I never moved them. They were drawn that way on the original Kelly 90 schematic and I haven't changed that part. My first Junkyard Express is wired up like Franchesca and I have seen no ill affects as far as performance from using Ken's way.

Grack, you're right. The A1b schemo you were using had them wrong. I musta gotten it confused with the faulty bias point location.

[UR12]

I was just wondering why... I thought maybe it was like that to keep B+ on the CT of the OT so when the standby switch was switched on it would stop from making a popping sound. I don't know, anyway, if Ken had it like that, which I was looking at Francesca's pics, and as far as I can see it is, then it must be for a reason.

Has anyone built one like the schematic or are you building it like the layout? just wondering.

Bob

There is no B+ on the CT using Ken's method until the Standby switch is flipped. The only thing that has B+ on them are the first 2 caps and the bleeder resistors. This would defeat the purpose of having a standby switch by leaving b+ on the plates of the power tubes. A wire comes from the caps to one side of the Stby sw and the other side of the sw goes to the 25w 1k resistor and the CT on the OT.

I built mine using the pictures of Franchesca as the schematic wasn't even close when I started my build. I think the layout came later and was done by Nik at cerriatone. I've never really tried Nic's layout. He had a few values changed like using 22uf and 47uf caps instead of 20uf and 40uf (Which probaly won't hurt a thing). I guess he was using stock values. available in Malasia. Richie also made some changes to the layout to make it more like Franchesca but I haven't done anything with that layout.

[IaXy]

yeah your right. I was looking at the schematic.

So, the two bleeder resistors and the first two 40uf caps are before the standby switch, then after is the 1k resistor and the CT.

If this is the way it is supposed to be then this is the way I will do it.

Thanks,

Bob

[Ron]

I was just wondering why... I thought maybe it was like that to keep B+ on the CT of the OT so when the standby switch was switched on it would stop from making a popping sound. Bob

That's basically the answer. Those caps charge up in a couple of AC cycles when the amp is first turned on and not when the standby switch is initially thrown, so you get much less of a pop.

Less safe? Maybe a little, but the bleeders will discharge them in about 1/RC seconds (~1 sec) when the power is switched off. No one should be touching parts when it's plugged in anyway, unless you're me.

[Mark]

Dear Dana

While we are looking at the written Francesca circuit. The input isn't correct. The diagram shows the earth of the shielded cable being attached to the plate of the first stage. Looking at the pictures, the earth of the shielded cable is attached to the earth of the input jack.

I think most people would work off the photo's and a couple of circuits diagrams. However, this isn't always the case.

Sorry I didn't mention it sooner.