Not sure if voltages are correct ?

[dude]

Hi guys,

Looking for some advice on the voltage of a conversion I'm building. I gutted a Create Vintage Club VC20 and plan on building a 6V6 super lite 18 watt cathode biased, schematic below.

I re-drill the sockets on the side of the chassis as the speaker in the cab was tight. I used tin to cover the old chassis holes. I had a bd for an el84 18 watt lying around so I'll use that. No cap can, no room, using a bridge rectifier and the old PT that came with the Crate, no center tap on HV but has a CT for the a/c heaters. I used a few standoffs and made a power filter supply on one side and the bridge rectifier on the other.

I haven't populated the bd yet as you can see. The a/c voltages at the bridge are 128v each side, the DC B+ is 340v, the a/c filaments are 3.6v each. I think these reading are OK...?

What I think is strange is the B+ at the first filter cap 340v is the same as the screen voltage, both 340V...? I have a 5 watt 2K between the them but why is the voltage the same on the pate as the screen?

The schematic I'm using calls for a 2.7K but I figured with 6V6 I wanted a little more voltage down the rail. I know the schematic is for el84's that I'm using but I'll make the changes and add 470 screen grids and change the PI tail to 22K and 470, maybe I'll do a few other changes too for the 6V6's.

But why do I have the same voltage at the plate and screen? The 2K R reads 2K1 on my meter. I also have a 220K bleeder off the first filter cap. I use a 200 ohm 5 watt sag resister on the standby to B+ (no voltage drop there).

Thanks in advance and if anyone has any other suggested changes to the schematic using 6V6s please share.

al

[sluckey]

What I think is strange is the B+ at the first filter cap 340v is the same as the screen voltage, both 340V...? I have a 5 watt 2K between the them but why is the voltage the same on the pate as the screen?

That's normal. There is no amplifier or any other load on your power supply at this point. No load means zero current flowing thru that 2K resistor and that means there will be no voltage dropped across that resistor. So, all voltages will be the same right now.

[dude]

Thanks, I assume when I finish the board I'll have about 5 volts so so less at the screens than the plates (with a load), correct?

I always thought you wanted a little less voltage at the screens?

The old Crate schematic says 320v B+ (same PT), I get 340v which I like better.

With the 340v B+ (no load), I have a 47uf/450volt filter and the screens a 33uf/"350v". If with a load the voltage doesn't drop at the screens to a little less than 320v is the 350v cap cutting it close? It's very tight but much easier to get a higher voltage filter cap on the screens without the bd in.

So, I take it - with a load on the power supply the voltages will drop to around what the old Crate schematic said which was 320v and the 33uf 350v filter will be OK then....? Or would you raise that filters voltage now. I'd rather not as it was tight getting it in and I'd have to remove the PT to get at it. But much harder with the bd in if it fails....?

I keep buying these old amps and making then sound better as it's much cheaper in cost but they can be a real pain in the A**. But when they turn out nice it's fun to bring one to a gig and every one's like "wow I never heard that amp sound like that", just stokes me. 

PS:where's the spell checker?

EDIT: Pretty stupid question - after I just read the above post about bad caps, 20% of 350 could be less than the 320V I have replace the cap to 450v.

Thanks, al   

[HotBluePlates]

Thanks, I assume when I finish the board I'll have about 5 volts so so less at the screens than the plates (with a load), correct?

The schematic you posted has voltages on it. It shows 25v difference from the plate B+ node to the screen B+ node, with even more dropped across a series resistance on the way to the screens. It should be a good indication of how much lower the screen voltage will be than the plate voltage.

I always thought you wanted a little less voltage at the screens?

It doesn't matter unless the screen voltage is 350v+ and the plate voltage is 35v.

That said, it is common for the screen voltage to be not-very-different-from the plate voltage. Sometimes the screen voltage is a bit higher than the plate voltage (because of no screen resistors and some voltage drop across the OT primary).

[dude]

Hope it sounds good, thanks al