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Fine Tuning Princeton Reverb Build

Discussion in 'Shock Brother's DIY Amps' started by jchabalk, Oct 12, 2020.

  1. jchabalk

    jchabalk Tele-Meister

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    (I posted last week about starting to track down a new low-wattage build to plan out for the coming winter months. I’ve been reevaluating things and i think my time would be better spent going deep(er) on an amp i’ve built already. I have 8 amps kicking around here that i’ve built over the last 10 years or so. I started with kits, then moved on to sourcing parts for standard circuits and substituting parts for known changes - much of it though not fully understanding how and why things work the way they do.)

    I figured i’d start by digging deeper into the Princeton Reverb (AA1164) i built a few years ago using the Hoffman layout. It works great, sounds great, everything measures to spec. I do get a bit of what sounds like phase inverter distortion so i’ve been watching a bunch of videos and reading about how the phase inverter works in different amps (something that’s escaped me). I’ve found this video to be super helpful.

    As i’ve started wrapping my head around how to start looking in to this I’ve found that my power tube bias is a bit different between the 2 power tubes (a “matched set” of Tung-Sols from TubeDepot). Measuring the voltage across 1 ohm resistor to ground on the cathode of each power tube. i get:

    V5: ~18mv / milliamps
    V6: ~22mv / milliamps

    I set the bias at this point so i had an average of 20 milliamps. From what i’ve been learning about cathodyne phase inversion the bias should be equal between the 2 tubes (assuming the tubes are in fact matched and the amp is built following the correct schematic)

    Is this level of variance acceptable? and maybe more importantly is this something that could lead to distortion?

    I’m going to spend some time on this later this evening and was trying to get my game plan together for measuring and learning some more. I don’t have a current set of other voltages at the ready but am going to get them later today

    Here’s the tube set:
    V1: 12AX7 EH (Electro Harmonix)
    V2: 12AT7W (Tung-Sol)
    V3: 12AX7 (Tung-Sol)
    V4: 12AX7 (Tung-Sol)
    V5: 6v6 (Matched - Tung-Sol)
    V6: 6v6 (Matched - Tung-Sol)
    V7: 5AR4 (Sovtek)


    IMG_3472.jpeg

    IMG_3461.jpeg

    IMG_3462.jpeg
     
    Last edited: Oct 12, 2020
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  2. BigDaddy23

    BigDaddy23 Tele-Holic

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    Nice build, mate! Try a 470K grid stopper on pin 7 of the phase inverter - IME, this fixes blocking distortion most of the time. You may also have some crosstalk happening between wires....that can manifest as a zappy sound too. Pay close attention to the lead dress around V4 and keep grid/plate wires as separate as possible. Chopstick and play.
     
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  3. dan40

    dan40 Tele-Afflicted

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    Nice looking build! I would feel just fine with a 4 ma mismatch and many tube dealers would probably consider that a fairly close match. A perfect match is fine if you are shooting for zero distortion and every last watt of power that you can squeeze out of the amp but that is not always desirable in a guitar amp.
     
  4. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The OT primary CT is never right on center, so one tube in a matched set is going to idle hotter than the other. Measure the DC resistance on each side to see what I mean.
     
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  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Okay, so these aren't new tubes. What's the voltage for each? What were they at when you installed them? If you've had any red plating in one, it will drift hotter and hotter. Have you checked?
     
  6. jchabalk

    jchabalk Tele-Meister

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    Thanks a lot for the suggestions. I got a bit sidetracked tonight - i’ve never had this happen but it was so utterly confusing i figured i’d share.

    I was reseating my tubes earlier before getting into this troubleshooting and one of the tube shields wasn’t coming off - like it was stuck. I’ve had the retaining nubs on the side of not really fit too well before so i was massaging it a little and it came free. great. when it came off a small bit of glass fell off in my hand too.

    It turns out it was the tip of the tube and it had somehow gotten wedged in the spring that’s in the retaining clip and when it broke off and let all the air back into the tube - although i didn’t know that immediately.

    When i fired the amp up it started sounding like the reverb tank was crashing, then it got quiet, and then it seemed simply dead (no sound at all). So this amp that’s been working perfectly for a couple of years and that sounds great is now dead. Fast forward - i pulled the chassis and was checking voltages and all seemed fine, but still totally dead.

    I finally notice the discolored tip of the tube (V4 if i remember correctly) see that the tube doesn’t have it’s nice pointed tip, yank it and replace it with another of the same 12ax7 i have and it’s all back to normal working perfectly.

    V4 is the tremolo and phase inverter of course, just got it all sorted out.

    So i’m going to give it another shot tomorrow

    :rolleyes:

    IMG_2932.jpeg
     
    Last edited: Oct 13, 2020
  7. BigDaddy23

    BigDaddy23 Tele-Holic

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    No good! Ironically, I did a similar thing the other day in my Super Reverb....12AT7 though and that little pointy pit had a bit of a hook to it which hung up when removing the shield. PITA!
     
  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I had the same thing happen with a new Tung Sol 12AX7. Gotta be careful with those tube shields.
     
  9. jchabalk

    jchabalk Tele-Meister

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    This makes me feel a lot better, i was having one of those evenings of troubleshooting where the problems kept compounding ;)
     
  10. jchabalk

    jchabalk Tele-Meister

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    I let the amp warm up for a while this afternoon and measured voltages across the amp. I run my amps through a variac. My wall voltage is generally 120VAC on the nose. This tends to result in a heater voltage higher than 6.3VAC, so i've adjusted the variac to 117VAC which gives me a heater voltage right at 6.3VAC - so all of these measurements are made with a mains voltage of 117VAC

    Things i'm trying to confirm:
    • Nothing is operating massively out of spec
    • I have noise, it's pretty light and more prevalent when the tremolo is running, i'll try to post a recording later (i don't want to chase it if it's not bad so i'm mostly looking for validation that it's not worth going after, will also check and adjust wiring around v4 as part of this
    • Interested in a good way to shield the amp while it's in the cabinet. I have copper tape i could line the roof of the cabinet with but i want to make sure it's secured properly

    Power transformer, secondary side
    Rectifier (yellow, GZ34 pin 2 to 8) 5.13VAC
    Heater (green, 7025 pins 4/5 to 9) 6.3VAC
    1/2 high volrage (red, GZ34 pin 4 to gnd) 321VAC
    1/2 high voltage (red, GZ34 pin 6 to gnd) 322VAC

    6V6 Bias power supply ALL TUBES (VDC)
    input to 100K resistor 408VDC
    input to diode 451VDC
    grid (pin 5) 6V6GT(blue) V6 -36VDC
    grid (pin 5) 6V6GT(brown) V5 -36VDC

    DC Power supply ALL TUBES (VDC)
    Node A 408VDC
    Node B 395VDC
    Node C 319VDC
    Node D 242VDC

    Output transformer resistance (ohms):
    155ohm Center tap to 6V6GT(blue) plate, pin 3, while still warm V6
    136ohm Center tap to 6V6GT(brown) plate, pin 3, while still warm V5

    6V6 plate dissipation 6V6GT(brown) V5,V6
    Output xformer CT to Plate drop (VDC) 0, 0
    Plate voltage (pin 3, VDC) 397VDC, 397VDC
    Grid voltage (pin 5, VDC) -36VDC, -36VDC
    Cathode voltage (pin 8, VDC) .02VDC, .02VDC
    Plate current (amps) .024VDC, .022VDC

    9 Pin tubes plate dissipation 12AX7(1) 12AX7(2) 12AT7 7025(12AX7)
    T1 Plate (pin 1, VDC) 151VDC, 383VDC, 152VDC, 236VDC
    T1 Grid (pin 2, VDC) 0, 0, 0, 0
    T1 Cathode (pin 3, VDC) 1.2VDC, 7.2VDC, 1.15VDC, 2.35VDC
    T2 Plate (pin 6, VDC) 156VDC, 383VDC, 153VDC, 181VDC
    T2 Grid (pin 7, VDC) 0, 0, 0, 0
    T2 Cathode (pin 8, VDC) 1.11VDC, 7.2VDC, 1.2VDC, 52.8VDC

     
  11. BigDaddy23

    BigDaddy23 Tele-Holic

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    Your voltages look good mate.

    Copper tape is great for shielding the roof of the cab - it will make contact with the chassis lip which forms the 'cage.' It shouldn't move or sag if the adhesive is good.

    Re the noise and the tremolo......any noise will sound worse when the trem is engaged. Our ear picks the oscillation change which draws our attention. If it's a trem thump in a bias wiggle circuit like this, you can use a diode across the outer lugs of the intensity pot to tame that. Here's a pic of my 6G2 Princeton which shows the diode on the pot. Note the stripe orientation.
    7EA577E4-973A-4209-A748-BE6CDD13D8D1.jpeg
     
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  12. jchabalk

    jchabalk Tele-Meister

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    excellent, thank you! the noise in my tremolo is i think directly related to the bias wiggle. To the ear it sounds like you're turning the volume up and down - which is effectively the result of what the trem circuit does right?

    So the little bit of hum in the amp output gets louder and softer. Overall this is a pretty quiet build and it's not really noticeable while i'm playing. I'll see if lead dress changes anything and this may be where i leave good enough alone
     
  13. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    Put a 12" speaker in it!

    The Princeton truly comes alive with a 12" speaker.

    Nice build!
     
  14. BigDaddy23

    BigDaddy23 Tele-Holic

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    That's right - it changes the amplitude of the signal wave (not the frequency). If you plug in the footswitch or a shorting plug into the Tremolo RCA socket, the tremolo will cease to oscilate. That will help you assess the noise floor. That said, I totally agrre that there is no need to chase down problems if they aren't a problem!
     
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  15. jchabalk

    jchabalk Tele-Meister

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    I have a few other amps here that i've built, i've got a 1x12", 2x10", and a couple of 2x12" cabinets. I'm planning on spending a few evenings playing through them all with this amp, i've got a 12" on my mind for this amp too.

    Currently running a 10" Jupiter LC in it - which i really like. I'm trying to figure out if a 12 is going to add something notable to it. I know that playing my 5e3 with a 12" weber that the bass and fullness really coming through above and beyond either of the 10" speakers i've got.
     
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  16. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    I had Princeton and put a 12" speaker in it. Made a world of difference. The amp sounded good with a 10" but the 12" opened it up completely. The sound was less spiky, fuller, a bit more compressed. Fuller.

    I had also built an Allen Accomplice kit Deluxe Reverb, and put two 10" speakers in it. Never cared for the sound. Then I tried hooking it up to my Fender Deluxe Reverb that had a Weber 12" AlNiCo speaker in it. I haven't played 10's ever since. I even replaced my Super Reverb with a Pro Reverb because the Pro has 2 12's. I'm just not a fan of that snappy attack from 10's. It's a bit too aggressive, all the time. Have fun with it!
     
  17. jchabalk

    jchabalk Tele-Meister

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    I experimented with a piece of grounded aluminum foil over the chassis last night and it works pretty well at cutting down the hum. There's a little more background noise when i have the amp on my work table than when it's in the cabinet on the floor too and it cuts it down.

    This morning i picked up a length of bronze window screen at the hardware store with the idea that i'd cut it to size and secure it to the inside top of the cabinet like the old amps used to have. It too appears to have the same effect on cutting noise as the foil i used last night did. I was testing the screen with my ohm meter and it was kind of all over the place making me wonder if there's a coating on this stuff - it's definitely woven too rather than brazed/soldered. I made an attempt at soldering it and it is *not* easy to solder.

    Wondering if anyone else has used this stuff? I was going to staple or flush screw it to the top of the cabinet and the top flange of the amp chassis would press against it tightly once i snug up the mounting screws.

    IMG_2947.jpeg
     
  18. BigDaddy23

    BigDaddy23 Tele-Holic

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    Could you lightly sand the surface of the mesh that makes contact with the chassis? That should ensure a good connection for grounding.
     
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  19. jchabalk

    jchabalk Tele-Meister

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    For sure, was going to file the contact surface on the chassis too. It's weird stuff. When i got it i thought it would be easy to solder but it sure isn't.

    I then made the rookie mistake (because i looked it up online *afterwards*) of using a blowtorch - which i thought was a brilliant idea of course (and i never get to use my blowtorch). I put flux on it figuring that would let me lay down a nice bead of solder all around the perimeter? In reality it just burns holes in the stuff and the solder drops onto the ground. a lower temp flame probably would work better and i might try it again on a piece of scrap.
     
  20. BigDaddy23

    BigDaddy23 Tele-Holic

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    You shouldn't need to solder to it at all - it just needs metal to metal contact with the chassis which comes when you cinch up the chassis strap screws. I wouldn't bother scratching up the chassis if it is bare metal (ie unpainted) which yours appear to be.
     
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