Champ Build Trans Hum and high B+

[shaun]

Just built a Champ 5F1 using a P-TF22772 from tubesandmore, which they recommend.
It's putting out 343vac per side when loaded with all three tubes - 5Y3, 6V6, 12AX7. (Schematic calls for 325 per.)
I'm getting 396v of B+ at node A. Rather high.
384v on the plate of the 6V6. Also high.
12AX7 pin 1=212v, and pin 6=205v.

The trans has a slight hum when I put my ear up to it - probably normal. But I'm also getting a loud hum though the speaker of the amp. (Using a Hammond P-T1760C OT, 5K into 8ohm spkr.) I've checked grounding issues 100 different ways - nothing there, although my limiting lamp has a glow that is a bit too bright for my liking, suggesting a ground problem. I've considered the orientation of the OT to the PT - seems fine compared to other builds I've done/seen.

So I'm wondering if there could be a problem with the PT. The hum I hear from the PT is the same frequency as that coming through the speaker - probably 60 cycle hum. But even when they hum, I wasn't sure PTs went bad. But what do I know?

I guess I'll order a new PT and see what happens with a replacement, but of course I've cut the leads on the other one and won't get my $$ back if there's a problem with it. And I'd like to understand what happens when/if transformers go bad.

Thanks all, and Happy New Year.

[sluckey]

I doubt the PT is bad.

[shaun]

I think you're right. I tend to build Frankensteins, so the variables get ridficulous, especially for someone like me. However, I just realized I'd floated the 6.3v using 100k res instead of 100ohms. Doh!

[PRR]

How's the *current* in the 6V6? Measure its cathode resistor. Right value? Right voltage?

If severely mis-biased the amp can suck twice normal current, which makes double ripple, and also little ripple rejection. (Even before Frankenstein happens.)

[shaun]

Thanks, PRR. I'll take a look at that in the morning.

[shaun]

Good call, PRR. The standard 470ohm cathode resistor had me at 18.9 watts dissipation. Rob Robinette's calculator advises me that a 750-800 cath res. should bring that down to around 11 watts, which is what I want for this particular build.

Thanks for your help!

[shaun]

So, following PRR's advice, I replaced the 6V6 470 ohm cathode resistor with a 776 ohm (actual reading). This tamed the plate dissipation somewhat. I'm now down to 13.81 watts, which is about 115% I think. But my B+ also shifted. Node A went way up to 430v. I replaced the 5Y3GT (TAD brand) with a JJ 5Y3S, and Node A came down to 407 - I was surprised, but I guess the GT in this case means extra get-up-and-go.

It's kind of cool to see how the balance of power shifts around inside the circuit as various items are changed out. It helps me understand that a well designed schematic allows the energy in the system to sort of sit comfortably within it a stable fashion.

Anyway, here's where I'm at with what should be a super simple build:

Rec plates: 345vac
Node A: 407
6V6 plate: 397
Cath Res: 776 ohm
V-drop: -27

I have worked very closely with the 5F1 Fender schematic - not using my usual hodge-podge of old stuff. I bought new PT and OT. New tubes, too.

This all suggests to me, in my newbyness, that the PT is putting out too much voltage - 20v per side. And because it hums a little more than usual, I wonder about it. I have a new one coming in the mail, so I'll find out in a week, but as PTs don't usually cause such problems, well....I should have used the variac to check it thoroughly before installing. But I trusted its newness.

[DummyLoad]

current production 5Y3GT are not really a 5Y3GT - they have a significantly lower plate resistance than USA NOS 5Y3GT types. source and install a USA made NOS tube and the B+ will drop another 20-30V.

magnetic components' classictone line winds a PT with lower volt taps to address the issue.

http://www.classictone.net/40-18109.pdf

--pete

[shaun]

Thanks, Dummyload. Very good info.