Voltages on 5E3 Build

[DC Music]

Hi all,

I've really gained a huge amount of knowledge from browsing this forum... I'm very grateful to everyone who shares their knowledge here. I just recently completed a 5E3 build, and I had a question about voltages & was hoping for some advice. In my build, I used a Classic Tone 40-18078 PT which has options for 710V and 660V & I opted to use 660VCT. Also in the amp is an RCA NOS 5Y3GT rectifier. My voltage readings seem to be a bit low. These measurements are taken with the amp completely live (power & standby switches both on). Would you guys recommend that I rewire using the 710VCT option?

The amp sounds absolutely fantastic to my ears. It's predictably early to break up, but the overdrive is sensational... smooth & thick. Sonically there's not much I'd change.

I'm in Australia so we run on 240V (measured at between 230V and 235V at the wall).

B+ 326V

Filter caps
1 - 326V
2 - 296V
3 - 216V

Filaments 3.25V

V1
Pins 1 & 6 - 102.5V
Pins 3 & 8 - 1.7V

V2
Pin 1 - 124.5V
Pin 3 - 1.17V
Pin 6 - 165V
Pin 7 - 16.9V
Pin 8 - 47.9V

V3
Pin 3 - 313V
Pin 4 - 289V
Pin 8 - 18.2V

V4
Pin 3 - 317V
Pin 4 - 289V
Pin 8 - 18.2V

[mac dillard]

If you like it then leave it along...If you have a new production 5y3 swap it in in place of the NOS..That will give you an increase in voltage anywhere from 20 to 40 volts.

[DC Music]

Hey Mac, perhaps I would have been wise to follow your advice. I decided to rewire with the 710V secondary winding, and now I have nice healthy voltages (now I'm getting around 355V at the 6V6's).

But now I'm trying to troubleshoot an interesting problem which arose today. I'm hoping to get some advice on. I first noticed that when I plug into either of the bright inputs (normal volume down), the volume is already CRANKING; it's as if the Bright volume pot doesn't attenuate below about 8 or 9. Opening the pot up to 10, 11 or 12, you can notice some extra signal being added. Turn the Bright volume pot right down, and the signal stays as if the pot's stuck on 8 or thereabouts. This behaviour wasn't happening last time I tested the amp.

Plugging into either of the Normal inputs, both volumes right down, the amp puts out a conversation-level signal if you play guitar through it. Turning the Normal volume up seems to get normal behaviour. The amp sounds great otherwise.

I'm really stuck trying to troubleshoot this. Tried different tubes in V1 & V2, no change. The only thing I did notice while poking around (with all safety precautions covered) is that some of the signal wires on the preamp side are a bit "microphonic"... in other words if you tap those wires, you can hear some tapping coming out the speaker. I thought this was standard behaviour though.

The really odd thing about this is that the amp wasn't behaving this way before I rewired the rectifier tube for 710V, and I haven't touched any of the preamp-side wiring at all. I checked all my resistors, everything seems to be reading OK.

Does anyone have any thoughts or advice? Could I have a faulty volume pot?

[mac dillard]

Time to get the old chopsticks out. Sounds like a ground problem in the pre amp stage.

[DC Music]

Thanks so much for the reply mac.

I did have that thought too... I ran out of time today to properly investigate it. It might be time to re-do my ground buss & all the pots ground connections.

[mac dillard]

Also look close at the connections to the tube socket pins.

[Bruce / Mission Amps]

Most of the time, if you can't shut the volume off... you have a bad pot.... or, somehow disconnected one of the volume pots from "hard" chassis ground... typically, the normally grounded lug isn't grounded very well.

[DC Music]

Hi guys, thank you so much for the advice. I checked out the problem again today & narrowed it down to a faulty pot. I'd checked and (probably needlessly) re-soldered a couple of ground connections. After thinking about it more closely I couldn't think of another theoretical possible cause for the exact problems I was experiencing other than a bad pot. I replaced the offending pot & voila! Problem fixed.

[tubeswell]

Glad you fixed the problem.

FWIW for future reference, you can pretty much tell if its a pot grounding problem with a quick R-meter check for DC continuity to ground.