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5E3 Build w/ B+ Gone Awry

Discussion in 'Shock Brother's DIY Amps' started by Huddy, Nov 1, 2020.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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%...
     
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  2. Huddy

    Huddy Tele-Holic

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    The reason I had the standby was because I'm just using it it as a head and not making a full clone other from the circuit. Wanted to be able to demo/switch between speakers quickly. Apparently that it still doesn't do much in regards of OT Safety if removing the load shortly and not having to turn it off completely. Meh... I'll just leave it "On" rather than have a hole in the chassis.

    I just followed along with the Uncle Doug video - cathode voltage divided by resistor value divided by 2 tubes to get the milliamps x plate voltage.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    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.
     
    Last edited: Nov 2, 2020
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  4. Huddy

    Huddy Tele-Holic

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    Doing backwards math at the voltage that I have currently I'd need somewhere around 450 Ω to get to "normal" operating plate dissipation. While that's nearly double, is it not considered a huge change in resistance compared to a 10 kΩ trimmer pot?

    I originally had some Tung Sol 6V6 GTs in it and noticed they were getting super warm and put in a 6V6S set - still warm but the math was better. Chassis was still warm to the touch after being on for a 10-15 mins.

    Yes without tubes and in play mode B+ was anywhere from 530-520 depending on the cycle of my dishwasher ;-)

    And I ordered a few 5Y3 pulls on eBay that were $10/ea w/ free shipping. That didn't seem awful to me provided they functioning and don't blow something in the amp - PT?. Would that diode mod from RobRob's site be a worthwhile safeguard?
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    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.
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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.
     
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  7. Huddy

    Huddy Tele-Holic

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    LOLOLOL this is totally evil but definitely something I would do ;-)
     
  8. tubegeek

    tubegeek Friend of Leo's

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    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.
     
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  9. Huddy

    Huddy Tele-Holic

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    Yes yes. I was referring to the trimmer in a fixed biased amp like a blackface fender or something. When adjusting the bias in that type of amp the plate voltage can change 5-10 volts pretty easily in my little bit of experience. I've got a healthy stock of various 5w resistors so I'll get to clipping this evening. I've got some old/used/pull 5Y3s on the way and I know that I've got one around the house somewhere that's not a new production just got to find it. (crosses fingers and knocks on wood)

    The higher B+ is also throwing off my preamp tubes with their voltages in the 200 VDC range which I'm sure is less than desirable. Is there a difference between adjusting the 22 kΩ resistor between B+1 and B+2 vs adjusting the individual plate load resistors?
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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...
     
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  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    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.
     
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  12. tubegeek

    tubegeek Friend of Leo's

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    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.
     
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  13. Huddy

    Huddy Tele-Holic

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    Okay very good to know. Raising the Cathode resistor and cooling off the bias would raise the overall B+ which would raise the voltages further down the chain.

    I think the wise thing to do is at this point is... Wait until my 5Y3s come in and see if I get something producing a more acceptable B+ and regroup from there.
     
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  14. Huddy

    Huddy Tele-Holic

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    Fantastic. Thanks a ton ;-)
     
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  15. tubegeek

    tubegeek Friend of Leo's

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    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.
     
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  16. Huddy

    Huddy Tele-Holic

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    Something less permanent could be a Vaciac I assume? What you're explaining is like a DIY version of the AmpRx Brown Box that's like $350?
     
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  17. tubegeek

    tubegeek Friend of Leo's

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    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.
     
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  18. tubegeek

    tubegeek Friend of Leo's

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    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.
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    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.
     
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  20. Huddy

    Huddy Tele-Holic

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    So I pulled out the variac a made it so the plates were seeing 360-365 vdc. w/ the 6V6S set I'm at 11w of plate dissipation using the same method I used before - just being quick with it. A lot less volume but it sounds SOOOOOOO much better. Before it was just loud and clean - now I've got breakup. Maybe that's because of the preamp tubes seeing voltages in the mid 160s. The AC coming in as 101 VAC. filaments are 5.5-5.6 vac. I don't know how big a factor that is. Really crossing my fingers on these 5Y3s I've got on the way.
     
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