Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on June 11, 2021, 06:06:21 pm
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Guys,, I have a new PR build that has the nastiest distortion I have heard in a long time. I have tried everything I know of to find the source of the problem and I'm just pulling my hair out here.
I've tried different new tubes in every slot but the rectifier. I've tried reducing the capacitor on V1 cathode. I've got Merlins 470K grid stopper on the PI. Tried a different reverb pan.
I'm including a video of the sound. The distortion goes away when I disconnect the reverb pan. Cables are good. It has Sozo caps metal film carbon film and metal oxide resistors.
This is 4 on VOL 5 on treble and bass https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_sf_aa1164.pdf
https://youtu.be/qRx4MihaDFI (https://youtu.be/qRx4MihaDFI)
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Might need photos. How certain are you it's not a wiring issue? I had a similar(ish) issue when I built mine. I missed the parallel connections on that 12AT7 socket.
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The AA1164 chassis is playing through an Eminence Texas Heat in a Mojo 1x12 Lite cabinet. It does this with all speakers I have and it's own Emi 1058 in it's cabinet.
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Well i would like to post some photos but my Canon SL1 photos are evidently too large.. V2 wiring is correct, I''ve wired many Fender reveb drivers. Give me some time to reduce the size of the photos or host thm somewhere and link to them. PITA Or figure out how to take smaller image size with this camera.
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Don't yell at me, like that guy on another forum.
> photos are evidently too large.
The Canon surely came with FotoChop type software, which can crop re-size cut colors and more.
There's also this How-To thread: https://el34world.com/Forum/index.php?topic=464.0
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pic
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Sorry I'm just learning this camera. The yellow coupling cap on the far right is not .02uf - I had reduced it to .003uf to reduce bass frequencies in my troubleshooting.
I know transformers are the last thing you would suspect but I'm thinking it might be this reverb trans. The reverb driver is a JJ 81 and the recovery is a JJ 83 both brand new, and they have both been changed out. The power tubes are JJ6V6S. The PI is a JJ. The board wire is 22ga teflex. The sockets are better quality ceramic. probably chinese. I usually use Beltons but I like these ceramics from AES too. The filter cap can is a JJ 40/20/20/20. The rectifier is a Tube Amp Dr select GZ34. V1 is probably a brand new EH 12AX7. The pots are Alpha. Jacks are Switchcraft.
PT is 290AX OT is 1760E Hammonds. Reverb trans is a generic Fender product. Said its ror the reissue Pro and I looked at it's scematic and that pcb amp's reverb driver circuit and tank is the same. It's made by Schumacher pn E-037099.
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I'll post more photos tomorrow.. right now I am frazzled.
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I know transformers are the last thing you would suspect but I'm thinking it might be this reverb trans. .
Disconnect the Reverb from the PI input point. Does it go away?
silverfox.
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I know transformers are the last thing you would suspect but I'm thinking it might be this reverb trans. .
Disconnect the Reverb from the PI input point. Does it go away?
silverfox.
Hi, I will check that out ..
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The distortion goes away when I disconnect the reverb pan.
Big clue. Chase this one.
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Well I know it's not cables or pan because it does it with other cables and pan.. will dive in deeper tomorrow
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Disconnect the Reverb from the PI input point. Does it go away?
silverfox.
Ok I lifted the wire from the wiper of the reverb pot, esentially that takes the reverb wet signal off of V3B grid pin 7, I turned the amp on and it was loud, but I forgot to hook up the reverb tank, so I shut it back off hooked up the reverb tank cables and turned the amp back on and then there was no sound, nothing. I reconnected the brown wire to the reverb pot wiper terminal and then the amp sounds as before, maybe a little better, but still has the raspy distortion when hitting the bass strings hard. Or when hitting a chord hard. I think I noticed that the distortion seems to be worse when you first start playing through the amp and as it warms up it diminishes. I went through all the components and traced the wiring according to the schematic and layour and I don'r find anything amiss.
Here are the diagrams https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_sf_aa1164.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_sf_aa1164.pdf)
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Seems to be something in the reverb recovery stage. Seeing as how you’ve swapped out that tube already, try retensioning the socket pin clamps, and chopstick the resistors around V3a (including the 470k in series with the pot wiper)
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Thank you, I have chopsticked this thing so many times.. I touched up one solder join which was the 220k on the secondary of the reverb tranny.
Checked the pins on the new sockets and they are all very tight. I will post the tube voltages later today, maybe someone can spot something I'm missing.
I checked and adjusted bias and its running at 9.5w at idle, since these are JJ 6V6 that is fine for me.
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scope?
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scope?
I don't have one but this is one of those times that makes you more in inclined to purchase one.
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Yep, once you get into signal path audio stuff they take the guess work out
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Vs Filaments 3.2/3.2 Trem off (it works great) vol on 0 reverb turned down
V1-1 155.2 6 152.3
2 .002 7 .000
3 1.291 8 1.336
V2-1 396.7 6 396.7
2 .007 7 .007
3 8.05 8 8.06
v3-1 156.7 6 155.8
2 .000 7 .002
3 1.258 8 1.318
V4-1 183 6 187.2
2 .001 7 14.64
3 1.276 8 55.4
V5-3 408.7 V6-3 410
4 401.8 4 402
5 -34.36 5 -34.42
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Can you recheck those wonky cathode voltages?
ie V1 3&8
V2 3&8
V3 3.
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Sorry about that.. I corrected them above. Still have the problem with reverb connected the chords have a nasty distortion overlay. Unplug the cables and its gone.
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V4 6 187.2
7 14.64
8 1.318
Pin 8 should be about 50V on the cathodyne stage
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V4 6 187.2
7 14.64
8 1.318
Pin 8 should be about 50V on the cathodyne stage
Before the edit, it was :icon_biggrin:
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Pin 8 should be about 50V on the cathodyne stage
corrected
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What 0V common layout have you used? A PR is so far from any type of theoretical model that I guess you just have to go with something that’s proven good.
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It's the basic 1964 Fender Princeton Reverb circuit AA1164 with some updates that were common on the larger amps such as the Deluxe Reverb. 1.5K grid stoppers and 470 screen resistors on the 6V6s, adjustable fixed bias, GZ34 instead of 5U4G w/safety diodes, and a RAW pot installed in input #2. 1 ohm resistors. The link is in 1st post. I've built a few and all have worked very well like you would expect them to.
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That documentation is unreadably blurry, ought to be deleted, this is far better https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_aa1164.pdf
The Fender layout doesn’t really show the 0V common arrangement, we just know that eg the brass plate behind the pots is used, all 0V common to chassis connections are soldered locally.
Whereas modern clones tend to use solder eyelet lugs at the PT bolts.
So right off there’s a difference.
My point being that as well as hum, poor 0V common arrangements can cause parasitic oscillation that can sound similar to the distortion in your video.
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Understood. No brass plate and all preamp grounds on the pot side of the eyelet board are terminated near the input jacks, exception being the reverb transformer and the jacks and that 220k resistor on the reverb fs jack on the tube side of the board, like in the Fender layout.
I just turned the amp on reverb tank connected and reverb turned off using the fs jack. Played through the amp and it is really distorted with the vol on 4. Then I disconnected the reverb tank return cable and it was still very distorted. Then, while still playing, I disconnected the reverb send cable and the distortion went away. I know this is telling me something but what?
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With all reverb cables connected as normal, pull the 12AT7 driver tube. Does distortion disappear? If so, get a bunch of 12AT7s and roll them.
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The distortion is reduced considerably when the cables are hooked up and the driver tube is pulled. There is still some there though. I tried a JAN 12AT7WC ECG branded tube and it sounds better than the JJs but still distorted. I have tried at least 3 different JJ 12AT7 tubes with no change.
The driver plate is supplied by the same ps node that supplies the screen grids on the 6V6s. Is the distortion getting coupled into the 6V6s that way?
I am going to terminate the ground on the reverb transformer closer to where the wire enters the chassis.
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Well.. I'm holding my breath.. but PDF64s comment above about grounding got me thinking. I noticed I deviated from the layout by terminating the reverb trans black gnd wire at the termination point under the reverb return jack which is also where V3 pin 2 the 220K resistor also terminats. I moved the ground point to the inside bolt of the reverb transformer which is very close to where the reverb trans wires enter the chassis. Fired up the amp and that seems to have fixed the problem. But I'm holding my breath and will wait a bit and fire it up again after I eat some breakfast.. fingers crossed.
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Nope distortion still there. Taking reverb send cable off gets rid of it. Things I know. It's not tubes. It's not cables or tank. It is possible it may be due to vibration.
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Nope distortion still there. Taking reverb send cable off gets rid of it. Things I know. It's not tubes. It's not cables or tank. It is possible it may be due to vibration.
So it's on the reverb transformer side? (is that what you mean by 'send cable'?)
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Yes the cable the sends signal to the tank.
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Yes the cable the sends signal to the tank.
If you rig up an 8 ohm speaker to the send cable’s RCA Jack on the amp chassis, you may be able to hear if the distortion is coming from the reverb transformer.
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Hooked reverb signal from the driver out to a speaker and attenuated the main speaker so I could hear the reverb signal and it is pretty clean without distortion, so can rule out the reverb trans.
Turned the attenuation down on the main speaker and there is slight distortion on it while hooked up like this.
Going to get out my listening amp later today and see if I can isolate exactly where the distortion is being generated.
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Hooked reverb signal from the driver out to a speaker and attenuated the main speaker so I could hear the reverb signal and it is pretty clean without distortion, so can rule out the reverb trans.
Turned the attenuation down on the main speaker and there is slight distortion on it while hooked up…
As the speaker hooked up to the RT produces a pretty clean signal, but the main speaker is distorting, it probably isn’t the RT, but it could be the power supply node that the RT primary is hooked up to. It could be that filter cap node isn’t grounded, meaning it’s not decoupled, which would cause signal ‘feedback’ through the power rail when a signal load is applied through the RT primary. Theoretically, this would cause a fizzy sounding distortion. (Assuming it’s the screen node on a cap can, I wonder if that node is faulty? You could try jumpering that filter cap node with another filter cap (that is properly grounded)).
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I have entertained thought that it might be that filter cap node.. it's a JJ cap can.. guess it could be a bad ground situation inside the can.. the ground for the can is solid to the chassis. Thanks for the ideas I will give that a go too. The screen Vs look good but that could be different under load.
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do both send and return tubes share the same ground point?
some Fender re-issue schematics, the return is with the pre amp and the send with the PI PA section
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They're both grounded at the preamp ground point
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Well, the send circuit is a power amp, fed from the screen grid’s HT node.
So you may want to amend the 0V common point it uses to reflect that.
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Hope I'm not duplicating a previous suggestion. Have the coax cables been checked yet? Can you put the Reverb pan in a different amp to test it? Check the socket pins fit?
silverfox.
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Yes. I have used different cables and pans with same symptoms. Sockets are brand new and tight little buggers. Takes significant effort to pull a small bottle out.. I surmise the contacts are quite good.
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Guys I think I'm going to call this amp good now. Not sure what exactly has remedied that nasty distortion but it isn't doing it now, for the last couple of days actually. I think a combination of things helped
as in moving the ground for the reverb transformer and lowering the bias to a cooler setting. It also seems to have improved after I ran the reverb driver out to a separate speaker and put the main OT output on an attenuator. Why I don't know. I also don't know if pulling tubes in and out and changing them made any impact. This has just been the weirdest problem I have encountered in a quite a while. I am happy I didn't have to resort to an amplifer priest to come and exorcise it. Thanks to all for the ideas and help. Much appreciated.