Problems? Maybe? Measuring bias

Discussion in 'Amp Tech Center' started by zachlovescoffee, May 4, 2021.

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  1. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Hello good folks of TDPRI who have so lovingly embraced all of my insane questions I use to drop all over FenderForum.

    I just picked up a 69' Princeton Reverb and I'm slowly tidying up out of. spec resistors and capacitors, replacing the rectifier tube (5U4GB), which has basically melted the PT and other minimally invasive procedures to improve safety and soundness (screen and grid resistors [done]). Basically, if it's on the Rob Robinette suggested mods diagram, I am likely going to do it eventually. Haven't decided on Negative Feedback Loop. Won't be doing Stokes or anything that changes power.

    Most of the bypass caps have drifted considerably (original Mallory white and blue small). The same is true for the coupling caps (big red dudes) and the bias filter cap. Right now there are two NOS RCA 6V6GT tubes that have likely been assailed by that 5U4GB rectifier for a while along with a mix bag of what appear to be NOS preamp 12AX tubes in V1-V4.

    For measuring bias, I used the Rob Robinette method to measure my bias. I'm concerned that I'm either doing it incorrectly or something is WAY off with this amp (or my meter).

    NOS RCA 5Y3GT tube results

    V5 plate (pin 3) voltage = 424VDC but the measurements kept jumping around 418-416
    V5 pin 3 to cathode = -420 VDC
    V6 plate (pin 3) voltage = 422 VDC but the measurement kept jumping around from 417-415
    V6 plat 3 to cathode = -419 VDC

    OT Voltage Center tap was 428 VDC

    V5 voltage drop = 8 VDC
    v6 voltage drop = 9 VDC

    V5 OT resistance = 151.9
    V6 OT resistance = 151.8

    V5 measurement: 8 / 151.9 = .052 or 52 milliamps
    V6 measurement: 9 / 151.8 = .059 or 59 milliamps

    SMOKING HOT? The amp is also quiet as hell. To get any form of reasonable output I have to dime the volume control.


    NOS Raytheon 5V4GT tube results

    V5 plate (pin 3) voltage = 444VDC
    V5 pin 3 to cathode = -440 VDC
    V6 plate (pin 3) voltage = 444 VDC
    V6 plat 3 to cathode = -440 VDC

    OT Voltage Center tap was 428 VDC

    V5 voltage drop = 16 VDC
    v6 voltage drop = 16 VDC

    V5 OT resistance = 151.9
    V6 OT resistance = 151.8

    V5 measurement: 16 / 151.9 = .105 or 105 milliamps
    V6 measurement: 16 / 151.8 = .105 or 105 milliamps

    SMOKING HOT? The amp is quiet but not as quiet as the 5Y3GT. It sounds better overall.

    It's been about 10 years since I've had my probs and meters inside a chassis so I am coming back up to speed and I hope I am heading in the right direction. I'm looking forward to your assistance in gut checking me. I have a GZ34 heading toward me that I am also going to try in there.
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    A more accurate method for measuring the voltage drop is to meter from the CT to 6V6 tube pin3. Essentially the same terminals where the resistance of the primaries are metered. This can be done hands free with the use of clips. Remember these terminals have over 400 volts on them. The voltage drop will measure the difference of the two high voltages. The difference will only be a few volts.

    The use of 1 Ohm 1% resistors from 6V6 pin8 to ground can also be used. In this case mV DC will be measured across the 1 Ohm resistor. The mV = mA due to the 1 Ohm resistance. This method measure Plate and Screen current together.
     
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  3. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Okie dokie. I have some Dale 1R 1% I'll solder on tonight and measure and report back in mV. Do you see anything discernibly wrong or totally crazy with what I've posted? Or is it all garbage? :)
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So far it looks like the 6V6 are running too hot.

    What is the negative bias voltage? It can be measured from the center terminal of the intensity pot to ground.
     
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  5. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    The jumping around may be caused by the trem, which is still on even with the knobs turned down. The bias wiggle is probably what you're seeing. Hit the pedal, turn it right off.
     
  6. Wally

    Wally Telefied Ad Free Member

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    Fwiw, the 5U4 is the stock tube. The GZ34 will push that B+ on up the scale. A 5R4 drops more voltage than does the 5U4 if you are looking to get into vintage voltage levels. Has the amp been recapped...fresh electrolytics???? the can and the bias cap for sure need to be replaced.

    As for that PT, it is the weak link there. As muchxs was wont to remind us, that is the same PT as used in the
    Champ and other single-ended Fenders and is challenged to deal with the current draw of this Prin Rev...especially if the amp is to be used by someone other than the “student” for which its original purposes was intended. What do you mean by ‘basically melted the PT’? It seems to be working, right? With low output, one might worry about the OT. I suggest all new electrolytics before proceeding. The last fellow to whom I personally gave this advice did not heed it....and a cap failure took out that weak PT. The Hammond 290AEX is a good replacement, in case you might need to know.
     
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  7. ArcticWhite

    ArcticWhite Tele-Afflicted

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    Are you saying you're running 52 year old filter caps on an amp that's worth over a thousand dollars?
     
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  8. Wally

    Wally Telefied Ad Free Member

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    I am suggesting to shut this amp down. Unless I have lost it all, here is what I see I; your 5Y3 test.

    V5—8/151.9=.0526 ma..call it .053ma
    V6–422x.059=.059ma
    V5— 424x.053=22.47 watts/14= 160% of max plate dissipation
    V6– 422x.059=24.89/14=178% of max plate dissipation

    that is not hot...that is rough the roof. Surely the tubes are redplating???.

    your situation as described with the 5V4 is beyond that. I do not understand how there is 428vdc to the OT center tap and there are 440vdc on the plates. You do not have a voltage DROP there, so I do not know what to think.
    I also do not see any reason to be measuring from the plate to the cathode, which is at ground, in this fixed biased amp.
    I prefer to read Aiken on these matters.https://www.aikenamps.com/the-last-word-on-biasing
    Rob’s site is confusing to me because there is not a clear delineation between fixed and cathode biased circuits. I am also in the small camp that uses the transformer shunt method to come to much quicker and just as accurate numbers. I do not advocate that anyone use this transformer shunt method, though.
     
  9. zachlovescoffee

    zachlovescoffee TDPRI Member

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    just soldered in a 1R 1% to pin 8 and the other end to ground. I took a reading in mV using my probes at both sides of the resistor. For both tubes I measured 0 mV.

    this seems suspect.
     
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  10. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Yeah I hear you. It doesn’t make any sense to me what do ever. All of the bypass caps are out of spec, bias cap too (resistors are fine).

    Is it possible every single cap is so far out of spec (bypass, filtering, coupling) that it’s driving these values?

    btw I will be re-caping it this week
     
  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Was the amp *on*?

    As Wally wrote this may be too hot for testing.

    I thought you had replaced the bias cap. I think that cap should be replaced before you test further.
     
    Last edited: May 4, 2021
  12. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Will do! It must clearly be way out of spec. My new caps will be here Thursday so the first one I’ll replace is that and the cap can then I’ll report back here with new voltages.
     
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  13. Wally

    Wally Telefied Ad Free Member

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    I cut out the main caps to assure a dead amp. All work follows that. All new electrolytics. Replace bad resistors. Every nut, bolt, and screws is tightened. All grounds checked. All connections checked. Good general service on everything. Three way AC cable with best and safest wiring. Cleaning and retensioning all sockets. Make all ready in a ‘dead’ circuit before applying power with a limiter. See what you have then. Hope for the best.
     
  14. zachlovescoffee

    zachlovescoffee TDPRI Member

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    I think I figured out my error with the 1R measurement.

    cathode is tied directly to ground. Was I supposed to remove that ground connection and replace it with the 1R to ground? I think yes.
     
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  15. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Ignore this post
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes the 1 Ohm resistor replaces the ground wire.
     
  17. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Okay I'll make that change and report back in later today after I swap in the new caps. I had to replace a handful of 100K resistors last night that were out of spec. Believe it or not, 95% of the resistors are still within 5% of their rated readings. Given how crappy the caps are, I am utterly surprised.

    I ran across this gem () today. Take a look at minute 2:45+. This fellow indicates that he's had to remove the cathode ground and put some resistance in place to bring the tubes into a safe dissipation area. I don't think I've seem this method before. Is this pretty common and do you think I'll have something like this in my future?

    Also, if I have to do that, how does that affect the 1R 1% situation? I'd like to pipe those resistors out to an external bias test point so I don't have to open the amp all the time. Thanks LLC!
     
  18. schmee

    schmee Doctor of Teleocity Ad Free Member

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    I have no problem with 425v or so but that bias needs to be down probably in the 17-19ma range. You need to figure that out and not run the amp when it's above 40ma. It's probably redplating.
    Before replacing a bunch of stuff get that solved.
    Have you replaced the filter cap can yet? Tried new 6V6's?
    I would not do mods, those amps are superb as is and stand on their own because they are a bit different than the Fender larger amps. Turn off the vibrato.

    I think you said it has old RCA power tubes (?) I have had bad luck with those. Get yourself a new set of tubes for testing if nothing else (JJ) One set of RCA NOS took out the output tranny in my Deluxe Reverb first gig.
    I had two sets of old RCA that when installed would be up in the 70-90ma range before adjusting! They can be amp killers.
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Well... If you wanted to keep the 1 Ohm resistors in place they would be placed in series with the *new* cathode resistors.

    Ime this type of mod is not common. I doubt this mod is in your future.;)

    Evidently Uncle Doug rethought the cathode resistor bias method and has reverted this amp with an addition of a bias pot. He wrote this in the comment section of the video you posted:
    "I have since converted the PR to an adjustable bias method, like the DR uses, and made a video demonstrating how it was done and how well it worked. This eliminated the cathode resistors and restored the amp to grounded-cathode (fixed bias) status, where 70% of max PD is appropriate... :)"

    Comment: Imo 70% of max PD is appropriate but lower percentages may be desired and are appropriate as well.
     
  20. Wally

    Wally Telefied Ad Free Member

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    I would have changed the resistance in the bias circuit to achieve a safe current draw. It seems to me that what he has done there is to introduced a mixed Biasing ...fixed and cathode. Not my cup of tea. and...it is rare to have an exact match, ime. I pulled out a UOS pair of RCAs to test the ‘72 Prin Rev I recently restored. Voila! They were a dead-on match...27.6ma on [email protected] tube. Yes, they were running a bit hot, but this was just for bringing the amp online. The NOS set I put in the amp were Biasing a bit cooler. Fwiw, the trem was strong even at that high plate dissipati9n point.

    That said, I await insight from others who are above my pay grade. I am lucky that I have a good number of NOS RCA 6V6s that I have matched. With the method that Uncle used there, one thing is that the next set of tubes will require redoing that cathode Biasing situation...changing the resistances, etc.
    and...Uncle needs a guitar that is in tune, doesn’t he?
     
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