5e3 Voltage Question

[morton]

Hi all, second build. Hoffman 5e3 in chassis I punched my self. Modified the layout to add a MV removing the 1M resistor for V2. Amp sounds good, but with MV at full volume and normal/bright volumes over 9 there is a rhythmic thumping sound  (oscillations), thought might be interference from the long run from V2 and the board to the MV pot. Also voltages look fairly normal (from what I can tell based on the data page info) except for V2:

Pin 1:145
Pin 3: 1.2
Pin 6:175
Pin 8:1.5
Any help would be appreciated.

Thanks,
Gary

[Willabe]

Looks real good! 

But why do you have 2 red wires coming of the rectifier tube going to the filter cap can?

On the long grid wire runs try using shielded cable, shield grounded at 1 end only. 

[morton]

Thanks, I was thinking of using the shielded today (day late and a dollar short, typical for me). The 2 red wires from the rectifier are the B+ for the OT and the board rail. I was thinking that if I ran the OT lead to the rectifier I could twist it and the board rail together to eliminate some hum. I can always change it if it is a problem. How doe the V2 cathode voltages look?

Thanks,
Gary

[Willabe]

It's easy enough to change out a few wires for shielded cable.

The 2 red wires from the rectifier are the B+ for the OT and the board rail. I was thinking that if I ran the OT lead to the rectifier I could twist it and the board rail together to eliminate some hum. I can always change it if it is a problem.

I don't understand what your trying to describe, but it my be a problem.

[sluckey]

V2 pin 8 should be about +40V. You didn't ground one lug of the MV did you???

[morton]

The Hoffman layout showed the B+ supply for the OT as well as the board B+ coming from pin 8 on the rectifier so that was how I wired it. For a shorter run I could have landed the OT feed on the board, but I figured I would wire it as shown and then twist the board B+ and OT wires together. Is this a potential problem?

Sluckey-yep, I did. The mod was one that Kagliostro had posted back in April, looked like it grounded to the pot ground rail. Bummer, I'll unsolder the jumper and see what I get on 8.

[morton]

Pin 8 43v DC. Odd oscillation is completely gone. Thanks a bunch.

[Willabe]

The Hoffman layout showed the B+ supply for the OT as well as the board B+ coming from pin 8 on the rectifier so that was how I wired it. For a shorter run I could have landed the OT feed on the board, but I figured I would wire it as shown and then twist the board B+ and OT wires together. Is this a potential problem?

Ok, you mean the OT's center tap (CT)?

Yes the B+ dcv comes from pin 8 of the rectifier tube. But it goes to the 1st B+ filter cap, in this case 1 section in the filter cap can. Their both the same electrically so running the CT to pin 8 and twisting the 2 together wont reduce any hum because their both the same B+ dcv not B+dcv and it's ground return wire in a twisted pair.

I don't think it will cause a problem but it's not nessary.

[morton]

Thanks, I figured that I would try it, as i could always shorten it and feed the OT direct from the board if it was a problem.

[Willabe]

It's good to keep wires short(er) when/where you can, especially with gride wires and wires that carry a lot of current, like the OT CT.

Glad Sluckey got you the fix!      

Your build looks real nice! 

[morton]

Odd situation now. Didn't have time to check it much last night, but with the the ground from the MV removed and the pot fully attenuated to zero there is still sound from the normal or bright volume. Normal/bright volumes dimes it's probably like setting them at 1-2 (0-10 scale). Before the ground was removed from the MV pot it attenuated the normal/bright volumes completely.

[sluckey]

That's just the way that mv works. If you just gotta have zero sound when the MV is on zero, make this slight change...

[morton]

First to admit I struggle with schematics to actual physical layout, I can follow it but I have a hard time visualizing where it goes in the board. Looks like I add back the 1m resistor, connect one leg of the .022 cap on the board to pin 3 of the MV and then add a cap (.022?) from pin 3 to 2 on the MV pot? Connection from pin 3 to the board stays the same. Original layout from Kaligiostro I was using attached. Thanks in advance.

[sluckey]

...

[morton]

Thanks, really appreciate it. I'll work on it tonight.

[morton]

Mod worked great Sluckey, thanks. Had a 500pf bright cap on the MV initially, but it made the distortion sound off. I also used rg174 for the 2 wires to the MV and changed out the wire from V2 grid 1 to the tone pot as in my layout it ended up being a pretty long run (thanks for the advice Willabe). Amp is quiet now and sounds great.

[Willabe]

Good deal!