5E3 Build w/ B+ Gone Awry

Good troubleshooting here; great advice. Did you get rid of the standby switch? You should: Google 'Valve Wizard standby.'

I’m reading on a small screen, so without reviewing, are you using Rob's 'cathode resistor voltage drop' method? Simple and precise. Measure VDC plate to cathode for the first Calculate button, then VDC across the cathode resistor for the second. Done.

And the folks above are just right. A 5E3 is working fine at plate dissipation up to at least 115%...

 

Yes it does. Small changes in resistance (EDIT: such as 250ohm to 300ohm) does not alter the plate voltage or bias much.

For a 5E3 you are starting rather high with 380 VAC. The B+ will be high which will be problematic for bias even when increasing the bias resistor. Perhaps you can get away with it using JJ 6V6s as they can handle more than 14W.

EDIT: The Hammond 290BBX may provide 355VAC when loaded, so the 380VAC may have been a misleading number.

As D'tar said in post #14 the rectifier circuit is not complete.
The 362V number is not accurate in this instance because the circuit is not complete without the CT in play. When switched off of standby the rectifier circuit is complete, which then includes the CT.
The actual B+ will be the much higher 420V as the OP stated with tubes in place.

Rectifier tube? Get a vintage new or used 5Y3.

 

Fixed bias vs cathode bias is apples to oranges. I should have phrased the resistance comment differently. Nearly doubling the resistance would have been a more accurate description.

The TS wattage ratings have been found lacking when compared to other 6V6. That is not to say they sound bad, they just red plate easier.
The JJ seems to be able to be abused both with voltage and current.

I always use that mod. Easier on the tube. Probably need it more with new production tubes. lol.

 

Good. Leave Standby "On" or (easy and foolproof) just jumper around the switch so the hole isn't empty. You can use it to test your buddies' ears -- "now switch it! Hear the difference?"...

As for the Uncle Doug method, I *think* Rob's is the same but he helps make sure you measure *plate-to-cathode* and he does the math and other fiddly bits for you.

 

A trimmer pot for the 6V6 cathode would not survive unless it was a high wattage pot, those are not a common item.

Put something like double the biggest value you think you could need in, and then clip other values in parallel. 2W resistors. Like start with 2K, add 1K in parallel (that'd be 667 ohm equivalent) just to get a feel. 2K 1K 1K in parallel gives 400 ohms.

Then you can put a permanent single R once you get it how you like it. Or just solder in the testers.

 

What's your exact bias now? What's your B+ (tubes in and operating), plate-to-cathode value, and your voltage drop across the cathode resistor? For extra credit, you can measure the resistor's actual ohms (amp off) and fine-tune Rob's calculator.

I ask because your goal of getting bias in the 'normal' range by upping the cathode resistance is an iterative process and a slippery slope; 450 may not end up doing what you want. And by normal do you mean 110-115% (real-world normal) or ≤ 100% (what many people assume, incorrectly, is the goal)?

Why am I dwelling on this? Well. If you're concerned about your preamp voltages, upping the cathode resistor will (if I understand right) *increase* B+ in the preamp...

 

Yes there is a difference. I'd leave the plate resistors alone. OMMV.

As tubegeek said about the bias resistor, installing higher resistance dropping resistors and then bypassing them to achieve smaller resistances may be a quicker way to find a resistance that will work for this amp.

 

Gotcha re:trimmer.

All your q's are more or less related to the same idea: ohm's law.

A current through a resistance results in a voltage drop. The bigger the R or the bigger the I, the bigger the V.

V=I×R

When you draw more current through your 6V6 you are increasing the voltage drop through any resistance that comes before it. In this specific case, you aren't dealing with an actual resistor, the R is the internal resistance of the 5Y3, and it's not strictly a constant value. But we can say the internal resistance is something around 1K because a ROUGHLY 50mA current results in a ROUGHLY 50V drop. (I'm just getting us in the order of magnitude. It's closer to 1K than 100 or 10K.)

If you jack up the current 10mA, you'll have something close to 10V less B+ coming from the first filter cap.

That will set the starting point for the next dropper resistor, and some voltage will drop through it, based on the screen current and the preamp tubes currents.

The next dropper, the 22K will drop something like 40V - only the two preamp tubes are pulling current through it, maybe 2 mA, 2mA * 22K ~=40V.

I'd personally worry ONLY about the first filter cap voltage at first. Get the plate dissipation somewhere between 90 and 110% of 14W, if you can, and see where you are.

If the preamp plate supply is way high then, sure, I'd mess with the 22K, bump it up to 33K or 47K if that voltage is crazy high once the 6V6 is squared away.

Changing the plate loads is a) more work and b) increasing them will give you just a smidge less distortion and who wants that? While increasing the dropper will filter the power supply to the preamp a smidge quieter, there's nothing bad about that.

 

Look, B+ too high in a Champ is a very common problem, and the 5Y3 is probably your best case scenario. (It's a relatively inefficient rectifier.)

Pretty much anything you do to lower B+ will involve heat: more current through the 6V6, more voltage drop through larger resistors, you get the idea.

I'd say you might want to look into a vintage voltage rig, to lower your wall voltage maybe 10%.

If you search my username and the word "keen" here, or if you browse around @robrob 's site, you'll see what I'm talking about. You take a surplus power transformer, run the 120, 125 wall voltage into it, and tap off something more like 110 to feed the amp. A pretty good solution.

 

Yes exactly. It's going to do exactly what a Variac would do but with only two settings, typically. A Variac is continuously adjustable and some can actually give you a little HIGHER than wall voltage if you crank them all the way.

 

You can make the transformer in an external box with an AC socket and plug ANY amp into it - so it doesn't need to be permanent, FYI.

 

It is widely known as a buck-boost transformer or bucking transformer. I have heard them referred here on TDPRI by many other names such as buckminster or vintage voltage adapter. You can assemble one for maybe twice the cost of a 12V 3-4 amp transformer.