Help with 60Hz hum in Foreplay I in custom chassis

[Paul Birkeland]

I would suggest you build just a cathode follower buffer preamp to use with the KT-550.   You could parallel the halves of the 12AU7 to do this.  If you'd like, I could draw something up, it wouldn't be that complicated to adjust your build.  This would reduce your system gain by another 24dB.

[atalcott]

That sounds fantastic Paul. I’d love to see a schematic and try it out. Thank you!

[Paul Birkeland]

There's not a ton of HV current available from that little PT, hence the super low operating current, but this will work much better with your system.

[atalcott]

Thanks Paul. Everything else is perfectly clear but I haven’t seen than overlapping circle symbol before and am confused there. My best guess is that they just represent the existing transistor/LED network, and the resistors on the board need to be adjusted to the values listed? Am I in the ballpark? I take it the “3mA” is the “super low operating current” you refer to.
If you weren’t tied to the original PT how would you adjust the power supply to bump that up, and what advantages would that provide?

[Deke609]

overlapping circle symbol

Constant current source. In this case, the C4S: Camille Cascode Constant Current Source.

cheers, Derek

[Paul Birkeland]

Thanks Paul. Everything else is perfectly clear but I haven’t seen than overlapping circle symbol before and am confused there. My best guess is that they just represent the existing transistor/LED network, and the resistors on the board need to be adjusted to the values listed? Am I in the ballpark? I take it the “3mA” is the “super low operating current” you refer to.
If you weren’t tied to the original PT how would you adjust the power supply to bump that up, and what advantages would that provide?

That is the CF white board that would come with the anticipation.  The VA boards are not used. 

If I didn't have to use that PT, I would just run more current.  More voltage doesn't really matter IMO.

[atalcott]

That is the CF white board that would come with the anticipation.  The VA boards are not used. 

If I didn't have to use that PT, I would just run more current.  More voltage doesn't really matter IMO.

Got it. Still not totally clear on the changes around the CF board though. Currently R2 on the boards is connected to the B+ on one side via the "C" lead, and to the gate of the MJE340 and the anode of the first LED on the other side. In your schematic there appear to be just two connections to the CF board now, with the new grid leak resistor R1k connected to the MJE340 source along with the cathode (after the new bias resistor Rbias). But that would take R2 out of the circuit, and your schematic also indicates a change in R2 from 100k to 150k, so obviously that doesn't work. If R2 were connected to R1k via lead "C" then I wouldn't think R2 would need to be 2w, since it's just ultimately connected to the grid... Schematic of the CF board attached so you don't need to look it up.

I'm thinking about building this as a separate unit since 1) seems like a shame to use both tubes when only one is needed, 2) I don't really need source selection and could make it that much more compact, and 3) I have my eye on an SE KT120 build down the road that actually needs a high-gain preamp like the Foreplay.

[Paul Birkeland]

I didn't draw the connection from the C4S to B+, but that does need to be made.  R2 connects to B+, just as it does in the original Foreplay Anticipation upgrade.

The new grid leak resistor (rlk, which is 1,000,000 ohms, not 1K) and the 1K rbias connect to the collector of the MJE340, which is the easiest pad to identify on the board.

The B+ connection is easily identified because it will only have the free end of R2 connected to it.

The ground connection is the other connection that feeds the smaller R1 and feeds the 2N2222 transistor.

[atalcott]

Great, that all makes sense, thanks. If I build this as a separate unit, would any of the PCBs for the newer CCS upgrade kits work? If so, can I buy those separately?

[Paul Birkeland]

You could use the SEX 3.0 C4S upgrade.  The LEDs need to be turned around and you'd need a pair of PN2222 and a pair of MJE340 transistors.

[atalcott]

Over the past month I've been ordering parts for the buffer preamp and trying to figure out how to lay it out. Today I managed to finally get the first pass built to a point where I could power it up slowly and take some voltage readings. Pics are attached for reference. So far it's just a chassis plate with front and back pieces to hold the input/outputs and volume . Not sure where it's going to end up, looks-wise.

B+ is at 115v with 120v supply. Grids are at 2.18v and cathodes are at 5.8v. Both sets of LEDs on the board fire up.

Right now I have two problems: There's no sound in one (red) channel and the PT is running very hot. The sound out of the channel that's working is pretty good, so that's encouraging at least.

I haven't been able to find any obvious bad connections in the red channel, the input isn't shorted to ground... I'll just have to keep experimenting with that one.

The overheating PT is what I could use suggestions on. The amp is pulling 240mA, and if I'm reading the AES sheet right the max rating for this PT-442 is 315mA so that should be enough headroom. What other readings could I take that might point me in the right direction?

As far as the layout goes, I wanted to try something with the tube horizontal for a lower profile, inputs and outputs on the back, volume knob on the front. Beyond that, I did my best. I was limited to 6" depth because of the steel I could find was a 6" x 18" sheet of 22ga. Seemed to make sense to have the tube in the center with the socked facing inwards for best access to the terminals, and then things just sort of filled in from there. The ground scheme is: Inputs/volume pot/chassis --> CF boards/heater center tap/outputs --> PS Caps --> DC supply negative. It's pretty compact I guess, but too cramped around the terminal strip (that holds all the connections to the B+, two ground lugs, and CF boards and cathodes/grids) and the volume pot. I appreciate any feedback if anything looks way weird or is a no-no. First try at a layout so I expect there'll be a lot to learn from mistakes here.

[Paul Birkeland]

You have a backwards diode in your rectifier bridge.

[atalcott]

Ah, thanks Paul. Bonehead move. I flipped the diode from the B- to AC so that's fixed. Evidence attached.

After the switch the PT is still running very hot. Could I have damaged it with the backwards diode?

[Paul Birkeland]

That's somewhat possible.  I would pull the tubes, power up the transformer, then measure the AC voltage of the two windings.  A damaged transformer is very likely to show problematic voltages on the unloaded windings. 

[atalcott]

With the tube out, the AC voltages are 63v and 10.5, which is obviously not right. Looks like I fried it, so I'll get another one coming from AES.