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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

JMPrinceton Reverb

Discussion in 'Shock Brother's DIY Amps' started by Snfoilhat, Oct 18, 2017.

  1. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA
    Junco_Partner_Reverb_Amp_DEC2017.jpg
    If it makes it through startup and doesn't sound awful, I'll have to post an updated schematic. Made a ton of substitutions based on what I had on hand. Cheers
     
    telemnemonics and Darkness like this.

  2. symbiotic

    symbiotic TDPRI Member

    On the back panel toggle switch, I think I see a connection that was not soldered. Check it out?
     
    Snfoilhat likes this.

  3. telemnemonics

    telemnemonics Poster Extraordinaire

    Age:
    58
    Mar 2, 2010
    Maine
    I'm curious to hear this amp!
    The PR chassis/ cab is perfect and the Marshall sound(s) is what I want to hear, so this will be interesting.
    For me an 18w TMB works, with el84 or 6v6.
    Mostly I play my 6v6 18w, but use an AD pedal for reverby chores.
    It will be interesting to hear how the reverb sounds with some distortion, hopefully you have some tweakability in there.
     
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  4. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA
    OK!
    The amp runs. The cold clipper works, the overdrive voice at really high volume levels is sweet, but just as y'all predicted the signal coming into the wet path is too high (I believe), because I'm getting fizzy distortion at dwell settings above 4 (microphony is another possibility I guess). I was getting strange measurements on the 12AT7 and swapped in a known-good 12AX7 (because I didn't have a known-good 12AT7, only a UOS stereo-pull), but the reverb is pretty weak. A decent sound, but not a Fender reverb sound, as one might anticipate. When I post all the voltages, maybe someone can help me troubleshoot the 12AT7--cathode current seemed way too high and I was afraid the tube was bad, but maybe that's not the case.

    Master volume is useable but the taper isn't great. I think the overall amp topology is good and only some small component changes are called for, maybe tweak the interstage attenuations, maybe make the fat switch less subtle.

    I only have one combo cab right now, so this will be familiar to some from my harmonic trem 'princeton' amp. On the lookout for a new combo cab to house this so I can ultimately run them together.
     

    Attached Files:

    telemnemonics likes this.

  5. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    An as-built schematic with complete set of voltages would be good at this point
     

  6. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA
    Junco_Partner_Reverb_Amp_schematic_5DEC2017.png
    Text in red represents values as built.

    VDC
    B+0 (before sag resistor) 347
    B+1 334
    B+2 321
    B+3 286
    B+4 265

    V6+V5 plates 327
    screens 319
    cathodes 20.5
    Vpk 307
    Ik 76 mA
    Ip 72 mA
    PD 11W/tube, 92%

    V4a plate 176
    cathode 22

    V4b plate 165
    cathode 22
    (forgot to measure bias resistor-tail resistor intersection)

    V3a plate 143
    cathode 1.9

    V3b plate 139
    cathode 1.9

    V2a+b plate 315 (12AX7)
    cathode 3.5
    Vpk 311
    Ip 3.5 mA
    PD 25%

    V1a plate 141
    cathode 1.8

    V1b plate (bias cool) 173
    cathode (bias cool) 2.8
    plate (bias cold) 222
    cathode (bias cold) 3.7

    With my 12AT7 in V2 while the amp was still on the light bulb limiter, i was measuring 20V across the bias resistor (Ik=20 mA) and that was more than I expected, so I pulled the tube. Looking back on it, that's only 10 mA per triode and well within what this tube can handle.

    Edit for correction: I subbed it in just now (with the amp all assembled to quickly test it), and the reverb disappeared. Maybe biased into cut-off? Maybe a bad tube. Need to look into it further. Getting pretty happy with the sound of the 12AX7 (see post below).

    My note on the schematic that the AA1164 supplied 450V to the reverb driver is a typo. Should read 400V.
     
    Last edited: Dec 6, 2017

  7. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA
    One quick update, since I finally put the soldering iron and the Internet down for 10 minutes and actually _played_ the darned thing: there is plenty of reverb. Maybe it's the "it's OK for me" that some people who try oddball reverb drivers end up with, but I think it's pretty drippy. My 6G15ish amp is at a friend's place so I can't make a direct comparison today--I guess that'd be the gold standard for spring reverb.

    There is definitely fizzy distortion though, and so definitely an issue.

    Edit for negligence

    Speaking of issues: I was on autopilot early in the build, and wired the preamp tube sockets for a center-tapped 6.3VAC supply, just as I have always done, totally forgetting that this PT requires running the 12A_7 filaments in series within each bottle. Jeez I have been running each filament at 2X the required voltage!
     
    Last edited: Dec 6, 2017

  8. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA
    Update, happy days are here again

    Fixed that heater wiring mistake. Began a fresh start-up procedure to take new voltages unloaded then loaded then off the light bulb limiter, which I'll post below. Swapped back to the 12AT7 in the reverb driver position (same tube--different numbers!)


    VDC
    Sag resistor 351
    B+1 339
    B+2 324
    B+3 295
    B+4 276

    V6+V5 plates 331
    screens 322
    cathodes 21
    Vpk 307
    Ik 74 mA
    Ip 70 mA
    PD 11 W/tube, 91%

    V4a plate 197
    cathode 18.5

    V4b plate 196
    cathode 18.5
    bias resistor 16.9, bias voltage 18.5-16.9=1.6

    V3a plate 172
    cathode 1.7

    V3b plate 163
    cathode 1.7

    V2a+b plates 319 (12AT7, bias resistor 1K. I think I could go lower following the silverface and Rivera-era amps)
    cathodes 5.0
    Vpk 314
    Ip 2.5 mA X2
    PD 0.8W X2

    V1a plate 183
    cathode 1.4

    V1b plate (bias cool) 207
    cathode (bias cool) 2.1
    plate (bias cold) 244
    cathode (bias cold) 2.9
     

  9. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    So you tried subbing pre-amp tubes (for the 'fizzy distortion')?
     

  10. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    Also, if it were me, I would put a 20uF-ish reservoir cap at the rectifier (before the sag resistor)
     

  11. Phrygian77

    Phrygian77 Tele-Holic

    655
    Apr 30, 2016
    Crawfordville, FL
    Why not order a JAN 12AT7 or two? Thankfully those are still some of the cheapest NOS tubes available, and at the price, I wouldn't use anything else for a reverb driver or a blackface PI.
     

  12. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA
    Thanks for the solid advice. I have 1-2 hours of real playing time on the amp now, and can hopefully give a better impression of what's going on. First, it's pretty good!

    Current tubes are
    V1 12AX7 new JJ
    V2 12AT7 old Webcor-branded from the donor amp
    V3 12AX7 new JJ
    V4 12AX7 new JJ (I took my phase inverter from the brown Vibrolux, not the PRII, so I think this is the correct tube, but am open to other considerations)
    V5 12AB5 old Webcor-branded from the donor amp
    V6 12AB5 old Webcor-branded from the donor amp

    The 12AT7 is driving the tank better than the 12AX7 I tried earlier. I measured more current, more plate dissipation, and my ears tell me it's wetter.

    I'm still running the dwell at 4, but at 4 I'm getting more dwell, if that makes sense. I'm still running dwell at 4 because with either tube type, beyond 4 the fizz comes into the sound. 4 and lower I only hear a momentary burst of it with strong pick attack. So working hypothesis is something about the signal coming into the reverb driver. Could be as simple as too much signal, or too distorted, just like Phrygian77 suggested earlier.

    The fizz is something extra, that I can hear behind the clean (at low pre-gain settings) or behind the overdrive sound (at high pre-gain settings, low master vol). I haven't listened closely to it dimed to see how much noise gets into the output when the power tubes are really going, because the amp is loud! Easily small venue rock show loud.

    The natural hairy/gritty clean and OD sounds (when the dwell is set low and the fizz disappears) are what I have been trying to accomplish for a good long time. Smoother. There is a ton of complicated high end that kind of wobbles (maybe what people usually call sparkle?) but not icepick. Really liking how the two treble controls work together, and I like this speaker. Thanks!
     

  13. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    314
    Apr 8, 2016
    Oakland, CA


    This recording doesn't do the amp justice, unfortunately, but I think all build threads should end with some kind of sounds to share. It captures some of the high, bell-like ringing I really like, but not the all the grit or the psychological sense of loudness, which I really enjoy in the sounds I'm getting.

    Here is an edge-of-breakup setting. I think you can hear enough clarity that some people will interpret this as a clean guitar track, but I was strumming lightly save for a few notes. Power chords would have a moderate OD sound (though I neglected to capture any here). The most touch-sensitive circuit I have built yet.

    Fat on Clipper Bias cold Volume 7 Tone 3 Dwell 4 Reverb 4 Cut 3 Master Volume 2

    One room microphone (iphone:rolleyes:) off-axis and picking up ambient noise (like the pick) because it clipped so much when placed closer to the speaker. Now I want to learn more about recording.
     

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