Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Meltunes on March 08, 2020, 09:39:34 pm
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Hi All -
This is my third amp build and I'm having a bit of a challenge on the gridiron getting this amp the last 5 yards into the end zone for a score... I'm looking for some guidance and assistance in tracking down the source of the distortion that I'm getting when the volume is turned up anywhere from 7-10. This amp is a DIY build from Mojotone's PR AA1164 head kit. I have a sound sample where I'm hitting the strings pretty hard (ok, really hard), which is when the distorted sound can be heard.
I've connected the amp to different guitars and also to different speaker cabs, and all of those scenarios don't eliminate the noise. It also doesn't matter if the reverb tank is connected or not, or when the tank is connected, whether the reverb is turned up or not, the noise is still there.
I have voltage readings for all the tube pins, pics of the chassis, and I have a sound sample file that I can share (though m4a files don't appear to be an allowed file type on the forum), it just depends what is going to be helpful to you in helping me diagnose and remedy this issue.
Thank you in advance for any guidance and assistance you can provide!
Best Regards,
Meltunes
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Here's a link to the sound sample...
https://soundcloud.com/user-176418093/pr-aa1164-volume-at-10 (https://soundcloud.com/user-176418093/pr-aa1164-volume-at-10)
Meltunes
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tracking down the source of the distortion
does it happen with NOTHING plugged in?
what is your bias set at?
what happens when you pull V1?
here's a schematic to help
https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf
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So the issue is the blips of distortion that kinda sit on top of the signal?
Does turning the treble control down (ie rather than the vol control) get rid of it?
Have you got access to an oscilloscope?
it may end up being helpful to see some detailed, high resolution photos of the build, so look intohow to do that. eg upload to a hosting site such as imgbb and provide links.
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I appreciate the comments so far... I'm going to try to address the points as fast as I can, but I work a pretty demanding job during day so my replies will be in the evenings...
@Shooter: what is my bias set at?
I'm getting the following voltage readings on my output tubes:
6V6 V5 - Pin3 404V, P4 396V, P5 -30V
6V6 V6 - Pin3 405V, P4 397V, P5 -30V
I calculated my plate dissipation as thru the 6V6 anode~GZ34 cathode resistance and voltage drop method:
6V6 V5 - 165ohms resistance, -2.89V voltage drop, 409V plate voltage, 17.52mA plate current (calc), 7.2W plate dissipation
6V6 V5 - 148ohms resistance, -2.29V voltage drop, 410V plate voltage, 15.48mA plate current (calc), 6.3W plate dissipation
Here's pics of the chassis.
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> distortion that I'm getting when the volume is turned up anywhere from 7-10. ... I'm hitting the strings pretty hard (ok, really hard), which is when the distorted sound can be heard.
Am I missing something? ANY amp will distort. If you turn it full-up. And hit it really hard.
IMHO that's an important musical accent. Dah-da-DAH! Dah-da-DAH! The naked steel string does not have the harmonic expressiveness of a violin's gut string when bowed violently. We get that "shout" tone by distorting the amp.
I may be missing a point. The dogs are asleep so I'm not going to play distorted samples; anyway between my speakers and my ears I can't be the judge.
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Hi Meltunes
Referring to Shooters reply regarding el34's schematic (Hoffman Princeton Reverb), I would love to see the 1R resistors on pin 8 of V5 and V6. Then you could get real good idea of Anode Currents.
Recently completed an AB763 (yes different PI) BUT the amp is really clean to about 7 on Vol (B+T at mid position and 1kHz, 40mVrms input) then progressively the sinewave is trashed at max Volume. This is all "steady state" measurement/indication on the CRO. As PRR indicated, giving the guitar strings a little thrashing will invoke the distortion you have, so I would say its "normal"
Also just have a look at the link below:
http://fenderguru.com/amps/princeton-reverb/
Summary
"The Princeton Reverb is the smallest blackface/silverface Fender amp with both tremolo and reverb. With a 10 inch speaker run by a 12-15W dual 6V6 amp, it delivers a true “American” Fender tone with punchy, responsive lows together with chimey highs. When it is cranked it tends to sound a bit “browner” than the bigger two-channel amps, meaning more breakup in the lower frequencies and mid-focused tone. Much of this is due to an unefficient phase inverter circuit design."
Trust this helps :icon_biggrin:
Kind regards
Mirek
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> distortion that I'm getting when the volume is turned up anywhere from 7-10. ... I'm hitting the strings pretty hard (ok, really hard), which is when the distorted sound can be heard.
Am I missing something? ANY amp will distort. If you turn it full-up. And hit it really hard.
+1
Normal behavior IMO.
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Hi Guys -
I appreciate your comments and feedback, this really helps me understand my amp's behavior better... perhaps I was expecting too much???
@glass54 - I have some 1 ohm 1 watt metal oxide resistors that I can install this weekend... would those have an adequate wattage rating?
@PRR and Sluckey - I really appreciate your thoughts on my amp's behavior at full volume. I just wasn't expecting to hear that cutting in/out rat-a-tat-tat as the chord tones tail off...
This weekend I'll add the 1 ohm resistors to the output tube anodes and provide an update on the anode current readings. I'll also try strumming less hard and see how much that cleans up the tones...
Question: Are there any simple resistor/cap substitutions or mods to minimize the distortion at full volume? I've got the chassis on the bench and now would be the time to try it...
Thank you for all your help so far!
Meltunes
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Are the power tube grid stoppers actually in circuit? Photos are somewhat indistinct but it looks like the wires are connected directly to terminal #5 on each socket.
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He has grid stoppers mounted on the sockets between pin 1 and pin 5 as found in many AB763 amps. However, they do nothing. He needs to move the yellow wires from pin 5 to pin 1 for any benefit.
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...I'll also try strumming less hard and see how much that cleans up the tones...
Question: Are there any simple resistor/cap substitutions or mods to minimize the distortion at full volume? I've got the chassis on the bench and now would be the time to try it...
Is your goal to have less distortion with the volume up high? It should be trivial to reduce system gain, such that the vol control has to be turned up to 10 to get the same gain (and hence distortion) as it currently gets at 6.5. The downside being that a guitar with weaker pickups might not be able to drive the amp to full power with vol at 10.
Or is your goal that you want the distortion to sound nicer, get rid of those blips?
Whichever, get those grid stoppers wired in circuit.
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get those grid stoppers wired in circuit.
+1
not sure if you might be describing blocking distortion from the PI, one thing if the R's don't get you happy, try knocking the .1uF to .01uF, might help choke back some unwanted freq's :dontknow:
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re: @glass54 - I have some 1 ohm 1 watt metal oxide resistors that I can install this weekend... would those have an adequate wattage rating?
Assuming 25mA current per tube, P=I²R, then (0.025)² x1 = 0.0006W.
I think you'll be right with your metal oxide Rs :laugh:
Regards
Mirek
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Hi Everyone -
Thank you for your feedback!
@pdf64, sluckey - I will move the yellow wires from pin 5 to pin 1. Hopefully this will help... yes, I would like to get rid of the blips, cutting in/out that is heard in my sound clip as the chord tones tail off...
@shooter - I'll check to see if I have a 0.01uf cap on hand I'll have to order some if I don't.
Question: My PI plate voltage is high at 233V compared to Fender's schematic at 200V. Could this be contibuting to the issue?
My other voltages seem in-line with voltages I've seen on schematics for this circuit. I will attach an Excel with a table of my voltages so you can see the readings.
I'll have a chance to move the output tube grid stoppers this weekend...
Thank you for your comments so far I really appreciate your help!
Meltunes
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Question: My PI plate voltage is high at 233V compared to Fender's schematic at 200V. Could this be contibuting to the issue?
Fender schematic note 1 says voltages are +20%
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Hi All -
I really appreciate your feedback so far!
@sluckey - you're right, Fender's schematic does say +/-20%... my eyes must have focused on the second note regarding resistor tolerance of +/-10%, my bad. Thank you for pointing out the voltage tolerance. Okay I won't concern myself with that voltage reading then.
You guys correctly pointed out that I needed to move grid control wires to pin 1 from pin 5 so that my grid stoppers are actually doing their job, somehow I missed that step. Examining my screen resistors wiring now, I can see that I also need to move the other pairs of yellow wires to pin 4 from pin 6. Sheesh, not sure how I missed those changes...
I'm hopeful that those two corrections will make a difference in the cutting in/out rat-a-tat-tat sounds that can be heard in my sound clip posted earlier in this thread as the chord tones tail off...
@shooter - I've got some 0.01uf caps on order, so swapping out the 0.1ufs is something I can try after correcting my wiring mistakes on the grid stoppers and the screen resistors.
Thank you for all your help so far! I really appreciate being able to tap into all your knowledge and expertise!
Meltunes
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FYI the original PR didn't have grid stoppers and screen resistors. Both are considered improvements.
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The heater voltages look a little low (5.8V)?
What’s your actual wall voltage, and what wall voltage was the PT designed for?
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The heater voltages look a little low (5.8V)?
Every filament pin shows 5.8V. I wonder what his reference is? I bet if he puts one meter probe on pin 7 of a 6V6 and puts the other meter probe on pin 2 of the same 6v6 he will have some different numbers.
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I think that the 5.8V must be Vac across the heater lines (rather than an ac or dc voltage referenced to circuit common) because the rectifier heater, at 4.6V, is down from its nominal by the exact same 0.92 ratio.
Which points to the PT primary being fed a slightly less than intended nominal voltage, eg a 240V primary used with a 220V supply.
Which though not ideal shouldn’t be a problem per se; but knowing that would help to put everything else in context :think1:
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Hi Everyone -
Thanks for your replies and feedback, I really appreciate it as I/we zero in on the issue with my amp!
@pdf64, sluckey - I'll recheck my heater voltages. Off hand, I don't recall whether those readings were taken across the tube pins or from common ground. Its been several months since I took them and there's been *alot* of voltage checking and oscilloscope trace viewings since then...
@glass54 - Thank you for providing the calc to estimate the power handling needed by the 1 ohm resistors.
@shooter - Amplified Parts estimates that I'll receive my 0.01uf caps early next week. I was hopeful that I'd have them by Saturday (tomorrow), so testing with those (if needed) will have to wait until the next weekend.
Question: RE: Installing the 1 ohm resistors... Right now I have both output tubes Pin 8 grounds tied together and going to the star ground point. Do I install the 1 ohm resistors on Pin 8 and solder the other leg to the ground wire(s) that runs to the star ground? Or do I install the 1 ohm resistors on Pin 8 and the other leg to the chassis right by the tube, and leave the ground wire(s) as is? Any guidance on this will be much appreciated!
I'm looking forward to doing some work on my amp this weekend!
Thank you for all your help and guidance!
Meltunes
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do I install the 1 ohm
EACH tube cathode gets 1 resistor, the "other ends" of the resistors get twisted, soldered and connected to ground.
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Have you got access to an oscilloscope?it may end up being helpful to see some detailed,
Oscilloscope can be used for troubleshooting of the distortion. By measuring from different points on the signal path you might get the root cause localized. This kind of distortion could be visible as a high peak on the scope.
/Leevi
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Hi Everyone -
Your feedback and comments are really appreciated!
@shooter - thanks for the clarification on the 1 ohm resistors installation, I plan on installing them as you suggest.
@leevi - yes, i have a scope. I'd really appreciate some guidance on the primary circuit check points to check after I get the corrections made to my wiring of the grid stoppers and screen resistors. Correcting my wiring mistakes might solve my issue... (fingers crossed).
This weekend I was able to make the wiring corrections to the grid stoppers, but I haven't had enough time to make the corrections to the screen resistor wiring, and also add the 1 ohm resistors to the tube grounds.
I won't be able to get back to finishing these wiring changes until next weekend... so i will have an updated audio sound sample file after the output tube mods changes. Again, fingers crossed that the grid stoppers and screen resistors are the mods needed to clean up the distortion that can be heard in the audio sound sample that I posted at the beginning of this thread.
Thank you for all your help, assistance and guidance!
Meltunes
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Update: Yesterday I was able to complete the grid stoppers wiring change (move wire to Pin 1 from Pin 5), and screen resistors wiring change (move wire from to Pin 6 from Pin 4) on both tubes, as well as add 1 ohm resistors in series with the cathode ground. So I thought today I'd just check my bias run a sound check, hoping the wiring changes would resolve the cutting in/out distortion, and be off to using the amp.
Well, not so fast... my amp is now blowing the fuse after about 15 secs on power up. Whaaa??? I didn't expect this.
So I put in a new fuse so I could check continuity thru the IEC connectors, thru the fuse connections to the power switch, both the hot and the neutral sides check out fine. I checked continuity of the heaters from the pilot lamp thru each of the tubes working back from the rectifier to V1, all those check out okay. I checked my grounds for the output tubes, and the preamp sections to the star and they all check out okay.
Really, the only changes I made since the last successful power up were to make the grid stopper and screen resistor wiring changes, and adding the 1 ohm resistors to the cathode grounds.
I'm stumped, and could really use your expertise in troubleshooting why my fuse is blowing.
Thanks!
Meltunes
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Really, the only changes I made since the last successful power up were to make the grid stopper and screen resistor wiring changes, and adding the 1 ohm resistors to the cathode grounds.
That is probably where the problem is.
Pull the rectifier tube. Does the fuse still blow?
Measure the negative voltage on pin 5 of both output tubes. What have you?
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+1
follow Sluckey's suggestions
fwiw;
So I thought today I'd just check my bias
when unsure ALWAYS do that without power tubes and make sure you have it at grid pins of power tubes socket, If it looks good and can adjust (if adjustable) then set and now maybe sound check.
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@sluckey - thank you for the quick reply... The fuse does not blow when I have the recifier tube pulled. The grid negative voltage on both V5 and V6 measures -30.8V.
@shooter - thank you for the reminder...
One other thing I want to mention is to make the wiring changes, I had to pull the power switch, fuse holder, and standby from the panel and move them around so I could get to the output tube pins to solder the wires and resistors in place. Not sure if that is important or not...
Can you walk me thru what I should check next?
Thank you for all your help and assistance, I really appreciate it!
Meltunes
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I had to pull the power switch, fuse holder, and standby from the panel and move them around
There's your problem, now trace out the wires against the layout and schematic, you'll probably find something touching, we can't touch, it's the new order :icon_biggrin:
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Hi All -
I appreciate all the help to get my amp sounding great at all volume levels!
@shooter - I spent some time tracing out the wires against the layout and schematic, and peering into the chassis with magnifying glasses and flashlights... the good news is there was no metal-to-metal contact that shouldn't be there. So that checked out.
Turns out I had a bad output tube. Confirmed with a new set that arrived yesterday. No more blown fuses! I biased the amp to an average plate dissipation of 8.15W using the OT center tap-plate method.
I tried reading the voltage across the 1 ohm resistors I installed at the cathodes, but I don't seem to be getting a reading... perhaps my DMM isn't sensitive enough? I have it on the 200mV setting.
With that done, now I'm back on track for another sound check, just need an empty house for a little while. And that looks like it won't happen for a couple days, so I'll just have to wait until I get the chance to see how the amp sounds now at volume with the grid stoppers in circuit.
Thank you for all your help, suggestions, and guidance so far! I really appreciate it!
Meltunes
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Hi All -
I appreciate all the help to get my amp sounding great at all volume levels!
@shooter - I spent some time tracing out the wires against the layout and schematic, and peering into the chassis with magnifying glasses and flashlights... the good news is there was no metal-to-metal contact that shouldn't be there. So that checked out.
Turns out I had a bad output tube. Confirmed with a new set that arrived yesterday. No more blown fuses! I biased the amp to an average plate dissipation of 8.15W using the OT center tap-plate method.
I tried reading the voltage across the 1 ohm resistors I installed at the cathodes, but I don't seem to be getting a reading... perhaps my DMM isn't sensitive enough? I have it on the 200mV setting.
With that done, now I'm back on track for another sound check, just need an empty house for a little while. And that looks like it won't happen for a couple days, so I'll just have to wait until I get the chance to see how the amp sounds now at volume with the grid stoppers in circuit.
Thank you for all your help, suggestions, and guidance so far! I really appreciate it!
Meltunes
Are your power tube cathodes ONLY ground by the 1 ohm resistor meaning you have no other cathode ground? Easy to check your meter, go get that dead AA battery in your junk drawer in the kitchen.
Seriously, clip it in a cap while they are draining and your meter should begin dropping when it is in range.
Also, if you raise the voltage on your PI it will INCREASE headroom. The problem with a cathodyne PI usually because If you balance it and add a grid stopper it helps blocking distortion. Changing the first bypass cap to 4.7uf will remove bass frequencies which sound horrible generally distorting. Increasing the first filter cap to 40uf will tighten up the bottom end as well.
None of the bypass caps in any Princeton AA1164 I have tweaked have anywhere near the stock bypass because to compensate the tone stack Bass is way over the top and will pass so much you will think you cabinet is rattling.
I also increase the voltage on V1A to 180VDC. Some say the Bias Vary Tremolo doesn't work as well with hotter bias, but Princetons sound much better when they are running to the hot side. I have had to fiddle with every tremolo circuit in every amp I have built to get what I want. Princeton is no different. It has a great Tremolo running 6V6GT at 70% dissipation.
READ: http://www.valvewizard.co.uk/cathodyne.html
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Update on sound check - the amp sounds fantastic now with the grid stoppers in circuit! With the Volume on 10 there is a nice natural breakup, none of the cutting in/out is present anymore... thank you everyone for your eagle eyes that saw I missed wiring the grids correctly with the grid stoppers installed... thank you!
@Ed - thank you for your detailed response. It certainly seems that there is a degree of individuality to every amp, and to get each amp sound great takes a separate path to get there... I'm learning alot thru this process, but am very much in the mode of trying to learn as much as possible so this gets easier/faster to the goal with each build.
Now for where my amp stands... it sounds great when at full volume, but I am getting some hum thru the reverb circuit/tank when I turn the Reverb to 3+ with the other pots down all the way. Specifically, the hum is inaudible until about 3 on the dial, then gets a little louder until 4-5 when it levels off the rest of the way (Reverb on 10).
I thought it could be the result of a ground loop. Other threads on this topic in this forum suggested to isolate the Tank Input RCA jack from the chassis. I did that and there was no change to the level of the hum.
I could use help troubleshooting this...
Attached is a pic showing how I have the RCA jacks wired (similar to Hoffman's PR AA1164 pics). The test I did today was to remove the Reverb Input RCA jack from the chassis to see if that eliminated the hum. It did not.
The chassis is on my bench and the Reverb tank is out on my bench about 2 ft. from the chassis.
Here's the sound clip of the hum...
https://soundcloud.com/user-176418093/reverb-tank-hum-reverb-on-6
Here's pic of the Reverb RCA Jacks...
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Maybe check the following thread
/Leevi
https://el34world.com/Forum/index.php?topic=25242.msg273346#msg273346
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Hi All -
@Leevi - I read thru the thread you provided as reference (thank you)... what resolved your clipping issue, was it the voltage divider you installed? Or was it resolved by installing a grid stopper on your driver tube? Your issue (if I understand your thread correctly) was clipping in the reverb circuit. Is the diagnosis for clipping and hum the same with regards to a reverb tank circuit? I'm asking the question because I don't know the answer.
In my amp with the chassis on my bench and with the reverb tank connected (also out on the bench), I am getting a noticeable hum with the Reverb on 3+.
How do I troubleshoot this besides isolating the Tank Input RCA jack from the chassis? (which I've tested and didn't help).
Here's the link to a short sound clip of the hum:
https://soundcloud.com/user-176418093/reverb-tank-hum-reverb-on-6
I really appreciate the guidance and assistance on this issue!
Best Regards,
Meltunes
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Because of the big amount of information of this topic I have not followed all the details, sorry.
I understood you have at least two issues: crackling tone (distortion) when playing loud and the reverb tank hum.
The link I shared is related to the crackling which was caused by the reverb driver. The sample you have shared sounds similar issues as mine.
The grid stopper/voltage divider helped in that.
What comes to hum I propose that you take the tank away from the cabinet and verify that hum is not caused by the transformers.
/Leevi
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Thank you to all who are lending their hard-won expertise to help me diagnose and resolve the issues with my amp build.
@Leevi - I've got the tank on the bench about 2-3' away from the chassis. Moving the tank in different positions doesn't change the hum.
But, yesterday I noticed that the hum is there when I turn the volume up (all other pots on 1). So the hum is present with both the volume pot and the reverb pot, when turned individually or both up to 10.
I swapped out the V1 tube resulting in no change.
I made a couple lead dress changes so I could press the plate wires to the chassis and elevate the grid and cathodes above the plate wires. Now they don't criss-cross each other. But this didn't result in any improvement in the hum.
Question... What should be my next steps in diagnosing the hum I'm getting when I turn the volume pot up? The hum gets louder as the volume is turned to 10.
Thank you for your help and assistance! I really appreciate it!
Meltunes
Here's pics of V1-V2 and V3-V4 as they are now.
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If you turn the vol up to (say) 5, and then twiddle with the reverb level and/or the treble and/or bass controls, does the hum change/lessen/go away altogether?
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Hi All -
@tubeswell - I set the volume at 5, and then individually turned up each of the Treble, Bass, and Reverb pots... Treble and Bass changed the pitch of the hum slightly but didn't make the hum necessarily any louder. However, the Reverb pot as it was turned up changed the pitch of the hum, and also made the hum louder.
While I had the amp powered up, connected to my guitar speaker and humming, I took one of my DMM probes attached one end to the chassis with an alligator clip, then moved the probe tip to each ground point starting with the input jacks thru the amp's pot lug grounds ending at the star ground point. I was attempting to see if I perhaps had a bad ground point somewhere in the preamp, but the hum didn't change (thought it might if there was a bad ground... not sure if that logic is sound or not, please correct me on this if I'm thinking about this wrong.)
I did notice a slight (very slight) buzzing added to the hum when I touched the input jack(s) ground lug. Don't know if that is meaningful or not, but just in case I thought I should mention it.
What does the changing hum pitch (slightly) with the Treble and Bass controls, and changing hum pitch (slightly) and level (moderately)
with the Reverb control indicate? What should I look at/try next?
Thank you for your assistance! I really appreciate it!
Meltunes
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did you ever get a meter that could measure across the 1 ohm?
just don't want a 2nd bad PA tube :icon_biggrin:
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> Reverb pot as it was turned up changed the pitch of the hum
Pitch? Or timbre?
Like a different string? Or same string different pluck technique?
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Update:
@PRR - Re-listening to what is happening to the hum's character as I turn up the Reverb pot, it does not change pitch, but only gets louder.
I mistakenly thought it changed pitch but it does not.
So both the Volume pot and the Reverb pot increase the loudness of the hum.
What should be my next troubleshooting steps to isolate where the hum is originating from?
I really appreciate all the help! I know this amp is going to have great tone... its there, just need to isolate/eliminate the hum...
Meltunes
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What's up with the 470k going to the phase inverter tube? That's not original.
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What's up with the 470k going to the phase inverter tube? That's not original.
That's a Rob Robinette "improvement" mod. So is the grid stoppers and screen resistors on the power tubes.
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Update:
When I pull V3 out there is no hum at all, both Volume and Reverb controls all the way up and the amp is dead silent. When I pull V4 (after re-installing V3), there is virtually no hum too (its barely perceptible).
When I pull V1, there is hum on the Reverb control, and when I pull V2, both Volume and Reverb have hum.
I could use some suggestions on what to do next to isolate the source of the hum... suggestions???
I appreciate all the guidance you guys are providing! ... I'm learning more at every step...
Best Regards,
Meltunes
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Question: Since my amp is no longer distorting (that's been fixed), but now I'm asking for help troubleshooting a hum issue, should I change the subject line to indicate a hum issue? The current subject line is no longer relevant... I just don't know what the forum protocol is about something like this (changing the subject line)...
Please advise!
Thanks
Meltunes
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When I pull V3 out there is no hum at all, both Volume and Reverb controls all the way up and the amp is dead silent.
V3 is the reverb recovery triode and PI driver triode. So with that tube out, there is no signal path to the PI. So the hum is coming from before the PI
When I pull V4 (after re-installing V3), there is virtually no hum too (its barely perceptible).
V4 is the LFO triode and PI triode. But its not the problem, because the hum goes when you remove V3 (but leave V4)
When I pull V1, there is hum on the Reverb control...
V1 is the input triode and TS recovery triode. With this tube out, you still have hum on reverb level because the reverb level is related to V3 (which is still in). However, with the reverb level cut and V1 still in, you get hum from vol control, because V3 contains the PI driver triode which is in both the dry and wet signal paths. So try dealing with V3 first
...and when I pull V2, both Volume and Reverb have hum.
V2 is the reverb driver. It's not directly in the electrical signal path (because it's kinda like a signal terminus - it drives the reverb pan input transducer). So pulling it out won't affect any direct signal path hum.
Re: V3, do you have substitute 12AX7 tube you could put in (just to firstly check that the hum is not a 'tube problem')?
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+1
maybe try this?
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Hi All -
@tubeswell - thank you for the explanation of what each section does in layman's terms, and your suggestion on where to focus my attention.
@shooter - thank you for identifying what to try next!
I'll have time tomorrow (Saturday) to make the wiring lead dress change and I'll update on what I find...
Thank you! ... ! really appreciate the help!
Meltunes
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Update:
@shooter - I made the wiring change, and also while at it, I changed the lead dress for V4 Pin 2 and Pin 3 as well. These changes can be seen in the photos attached.
Unfortunately, the hum is the same.
Here's some additional troubleshooting I did today while I had some time to poke around in the chassis...
(1) Reflowed solder in the eyelet where the 47 ohm resistor, 2.7K ohm resistor off the extension speaker jack, the 25uf/50v electrolytic, and the 1.5K ohm resistor off V3 Pin 8 are all joined, due to pressing on that node with a chopstick made the hum slightly louder. After reflowing solder there, pressing the connection didn't change the hum any longer.
(2) I pulled the input jacks off the chassis to isolate them just to see if that made any change to the hum, which it did not.
(3) I checked the ESR of all the electrolytics in the amp, and the 25uf/50v cathode bypass caps all measured 0.47-0.51. The bias supply 100uf/100v measured 0.11.
(4) The ESR of the cap can measured, 40uf/500v (lug stamped Y) = 2.6, 20uf/500v (lug stamped X) = 4.6, 20uf/500v (lug stamped U) = 3.8, 20uf/500v (lug stamped O) = 5.1.
(5) Swapped in new 12AX7s in both V3 and V4... did not help.
(6) Leaving the standby on, Volume up, and Reverb up, while powering up the amp, there was a few seconds with no hum initially, but the hum returned when the rectifier began to heat up.
I hope one or more of these observations can help you guys with your hard-won experience point me in a direction of what to try next...
I appreciate your time and attention to helping me get my amp working...
Meltunes
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Reverb hum seems to have become a very common issue in the last 20 years. Certainly seen a lot of it on this forum. And there has never been a definitive solution. It most always seems to be associated with the reverb recovery/mixer tube. I suspect the new 12AX7s are not quite up to the task. Try using a 7025 like Fender used in the AB763 amps.
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Before we talk about re-doing the grounding, have you got a really hot soldering iron (something like 100W+) with a big flat tip? and some decent solder flux/paste?
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1) bond the reverb return jack and reverb footswitch jack ground lugs together.
2) tie the 220K grid leak for the recovery amp to the return/footswitch ground lugs
3) move the reverb driver amp 1Meg grid leak to the tube socket and ground it to the return/footswitch lugs - e.g., relocate the 1Meg from the board.
4) the reverb transformer secondary MUST connect DIRECTLY to the tank - you have the black wire going through the chassis to get to the tank. e.g., BOTH reverb transformer secondary leads (green & black) should be connected to the SAME jack.
FWIW, i don't like the fact that the fender amps use the screen supply node for the tank driver amp B+ - that circuit could be much more hum/buzz-free if there was a dedicated RC filter network for the B+ of that stage.
see attached.
--pete
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Today's Update:
@tubeswell - I have an 80W at 900F degrees with 1/4" flat tip, and a 70W variable temp maxes at ~900F degrees with 1/8" flat tip that I've used to build my amps.
@dload - I didn't have time today to solder in a 1M ohm resistor from the footswitch RCA jack to V2 Pin7 as in your illustration (Thank you for your illustration!), but I did move the reverb transformer black lead to the reverb input RCA jack, and also joined the reverb output RCA jack and the footswitch RCA jack ground lugs together.
With just those wiring changes the hum is still there... just thought I'd check at this stage before continuing with pulling the 1M ohm resistor from the board and installing it directly on Pin 7 also as in your illustration. You'll see in the photo of the wiring changes made so far that the 1M ohm resistor is soldered to the reverb footswitch jack, but is not in circuit at the tube (yet).
I got an idea to use an audio frequency analyzer from another poster's thread on this site, and my amps hum frequency was analyzed with a peak frequency of 157Hz.
I double checked the frequency (mainly because I didn't believe it) with a separate audio tone generator app, set the audio generator tone at 157Hz, and confirmed that it sounds like my amp's hum.
I am hoping this information helps narrow down what is going on... I will continue the process of moving the 1M ohm resistor to V2 Pin7 from the board.
I am grateful for all your help!
Meltunes
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What does the ground on your cap can look like?
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Hey Pete, I'll give it a shot. I'm working on a new build and could squeeze in a dedicated RC filter for a 12AT7 reverb driver. What values would you suggest?
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1K 1W - 22uF 500V
--pete
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Hi All -
@dload - I will have some time tomorrow (Tues) to complete the changes you suggest for the 1M ohm resistor.
@tubeswell - attached are pics of my preamp grounding, the star ground, and the mains ground.
Thank you for all your help! I'm grateful for everyone's contributions to this project I still have a ways to go.
Meltunes
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additional preamp grounding photos...
Meltunes
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looks like your pre ground (green wire along pots) goes to the mains star ? if so it shouldn't
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What Shooter said. Try moving the pre-amp ground to the input jack. You may also want to try installing a separate 500V 20uF filter cap for the pre-amp, and ground this at the input jack (and separate and move the pre-amp ground returns to this new ground return point)
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Grounding.
http://www.valvewizard.co.uk/Grounding.html
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Hi All -
a couple questions for clarification just to make sure I'm following your suggestions correctly...
Q1. Should I still move the location of the reverb driver 1M ohm grid leak resistor to the grid pin of V2 Pin7, from the board as was suggested by dload?
Q2. Should I relocate ONLY my preamp daisy chain ground to one of the input jacks from the star ground?
Q3. Can I solder the preamp daisy chain to the buss wire attached to the back of the pots or does it need to be soldered directly to one of the input jacks?
Q4. Should I relocate any other grounds from the star ground to the input jack?... heaters, bias circuit, output tubes etc.?
@willabe - thank you for the valvewizard document... I read it, but not entirely sure I understood everything. It will have to sink into my noggin a bit..
Thank you! I appreciate everyone's help and expertise!
Meltunes
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The trouble with cap cans that have a common ground lugs for all the caps, is that it makes it quite difficult to get a quiet noise floor in this sort of amp where you have a noise-sensitive reverb recovery stage that is supplied by the pre-amp filter cap node. There's a whole bunch of reasons for this (which would make for a long-winded debate here), but the goal of good design is to get control of as many variables as you can.
Simply put, this means choosing not to use the cap can for the pre-amp filtering. Use a separate cap, where you have better control over how the pre-amp ground returns get grouped. (You can still keep the cap-can and use it for supplying the (other) 'high-current' parts of the circuit e.g. reservoir cap and smoothing/screen node cap, heater winding return, output tube stage returns etc)
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@tubeswell - thank you for the additional information... I appreciate the time and patience (from everyone) to explain things and help me understand how the circuits interact with each other etc.
So to recap, this is what I think my series of next steps are in order of escalation of effort:
(1) First, separate the preamp grounds from the star ground, and relocate this group of preamp grounds to the input jack.
(2) If that doesn't resolve the hum issue, install a new separate 20uf/500V cap for preamp circuits filtering.
(3) Disconnect the existing cap can's preamp filtering cap which at that point would be out of circuit.
Thoughts? Do I have this right?
Thank you all for your help and assistance!
Meltunes
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You really need to 1, 2 and 3 together. 1 won’t work without the other two.
The idea is to separate the grounds. You can’t do that effectively without separating the filter caps. The filter caps are the mechanism by which circuit impedance is fixed between the HT rail and the ground return (HT and the ground 'look one and the same' to the signal). If the ground return has any 'micro-wobble' in it, the filter cap will 'reverse shunt' this into the HT line. Forward current ‘flows’ from the +ve node of each cap, through the respective circuit components that get their power supply from that cap, and then through to the ground return. This is happening in all the different supply node stages, and in the output stage, its happening to a greater degree than the input stage because there's more current being returned to ground. To minimise ground hum, you want the circuit components ground returns going back to the same ground return as the filter cap from which those components get their power supply. The preamp node is the most sensitive to noise, because it’s currents are the lowest in the amp. So the logical thing to do is bundle them apart , so that Interference from higher current circuits is minimised.
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@tubeswell - Thank you for the explanation! It is starting to make more sense what is happening and why. Can you suggest where I should position/install the new filter cap in the chassis?
Below is a photo of my amp build...
I will be most grateful for guidance on this...
Best Regards,
Meltunes
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circles are radial Ecap, rectangles axial Ecap "ideas"
web up radials, I've found smallish that will shoe-horn in many places.
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@shooter - thank you for the suggestions... I really appreciate it!
I've got a couple filter caps on order and will update when I've received them and have the new one installed... hopefully will have the parts by Saturday so I can work on it this weekend.
I'm looking forward to this mod! ... can't wait to hear how the amp sounds sans hum...
Thank you!
Meltunes
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What Shooter said. The best place(s) are near the preamp components that the preamp filter cap will be supplying (for the shortest possible wire runs)
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@tubeswell and @shooter - Thank you for the suggestions and guidance...
...while I'm waiting for my filter caps order to be delivered (USPS says delivery will be Monday... bummer),
question about the ground tie point to the chassis...
Would it be better to drill a hole in the bottom of the chassis next to input jacks for a post so I can use ring terminals on the preamp ground return, the negative of the filter cap, the ground lug of the input jacks, and bolt them to the chassis?
Or just solder the preamp grounds and negative of the filter cap directly to the input jacks ground?
Does it matter which way they get tied together (on the input jack or bolted to chassis)?
Also, should the input jacks be isolated from the chassis? If yes, I'll have to get some fiber washers or something like that....
Thank you for all your help! I really appreciate it... this is new territory for me...
Meltunes
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You want to minimise the overall number of ground return points to the chassis. I vote for using the input jack sleeve terminal. Doing it that way, I’d solder a short sturdy single strand bit of copper wire (say 3 or 4 inches long) as a signal ground buss to the Input jack sleeve. Then, using individual lengths of smaller gauge single strand hookup wire, run the separate preamp ground returns to this buss (including the preamp filter cap ground, the cathode grounds, and volume, treble, bass and reverb level grounds- but not the tremolo pots). Keep all these hookup wires clumped together at the input jack end of the buss right next to the jack.
The reason for doing it this way, is to keep the other (Floating) end of the ground buss wire free, in case you want to experiment with Merlin’s galactic ground return method later on. YMMV
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@tubeswell - oh man, so glad you elaborated on how to do this... ok, ground returns straight to the one of the input jack sleeves...
A couple questions on the Tremolo circuit ground return... also really glad you specifically called this out (I would've not known to do this...)
Does the Tremolo Speed pot ground go to the star ground?
If I'm reading the schematic correctly, the Tremolo Intensity pot Lug 1 makes its way to ground by being connected to the bias circuit ground. I've installed a bias potentiometer following Hoffman's diagram, so my Tremolo Intensity Lug 1 attaches to the bias pot, then makes its way to ground thru the bias filter cap and resistor.
Can you confirm that I'm understanding the grounds for the Tremolo correctly?, i.e. Speed pot to star ground, Intensity pot leave as is.
...Still waiting for filter caps to arrive...
Thank you very much for all your help (and the forum!)!
Meltunes
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The trem speed pot can go to the star ground (because the trem circuit gets it’s power supply from the smoothing (screen) cap supply node (and the trem functions via the output tubes). The intensity pot can stay where it is, as long as the bias supply is also grounded with the other ‘high current’ returns. (The bias supply works through the output tubes).
The reservoir and smoothing cap nodes (The main Pii filter) i.e. the cap can ground, should also ground at the same place. The heater ground reference (or heater elevation circuit ground) should also ground here, along with the High Tension winding CT, and the 6V6 cathodes (or the bias measuring resistors that are in series with the 6V6 cathodes - if you are using these).
Then, if you need/want to go to Merlin’s galactic ground later on, it’s a simple matter of moving this cluster to the floating end of the ground buss wire that I suggested earlier. And this is what I was talking about earlier when I said that ideally the amp only wants one signal ground return to the chassis if minimising ground hum is the goal. But the reason that you’d do it this way (I.e. a galactic ground instead of a star ground point), is you get better control of the sequence in which the high-current vs low-current returns are ‘stacked’ together- with the low current returns being immediately at the chassis (at the input jack), without the high-current returns being in between.
But see how you go with the split ground first. Split grounds work well in lots of amps. (The only real reason for keeping a galactic ground option open in this case, is the noise-sensitive reverb recovery stage, and the star ground is potentially problematic in this case).
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@tubeswell - Thank you again for the detailed explanation of the why's and what's, I really appreciate being able to learn from folks like you who contribute your valuable insights and expertise gleaned from hard won experience to those of us starting on our journey...
I will update when the filter caps have arrived in the mail, and I've made the alterations to my grounding scheme!
Thank you, I am very grateful!
Meltunes
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@tubeswell - I was drawing up a sketch for myself prior to making the mods and I have an additional question regarding the cathode grounds... does the PI cathode ground tie to the preamp ground, or does the PI cathode ground tie with the output tube grounds at the star ground point?
Thanks!
Meltunes
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does the PI cathode ground tie
The PI is considered part of the PA, so it gets grounded with the PA
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I wouldn’t ground the PI with the output tubes (just to be different 😉) While the function of the PI is to drive the output tubes, the PI itself is a low current Stage. But if you want to experiment with it, put the PI ground on a bit of wire that you can move between the preamp or output grounds
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put the PI ground on a bit of wire that you can move between the preamp or output grounds
+1
I tend NOT to build things with PI's :icon_biggrin:
the PI swings a BIG signal, so that's what I try keeping away from small signals, they can get all tangled up in tangled wires :icon_biggrin:
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@shooter and tubeswell - Thank you both for the clarification of where the PI cathode ground should go... I'll try routing the PI cathode ground with the preamp grounds at the input jack first and see how that works out...
I *should* receive my filter caps order in the evening today according to USPS tracking, so I'll be able to start on these grounding mods in the next couple of days... will update when I've "git 'er done"...
Thank you for all your helpful explanations and insights, I really appreciate it!
Meltunes
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quick update: ... I didn't get quite finished rewiring the grounding as recommended by @tubeswell and @shooter, but almost have it done... have a busy work week ahead so likely will be next weekend when I can finish up and report on how the new grounding scheme works out.
... looking forward to providing an update, fingers crossed the amp is sans hum!
Meltunes
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UPDATE... spoiler alert - SUCCESS!
I finished the rewiring of my preamp grounds, fired up the amp, and the hum is COMPLETELY GONE!!!
THANK YOU @tubeswell and @shooter! Your suggestions and guidance (and patience answering my questions) has resulted in guiding me to successfully completing a build of a great sounding Princeton Reverb! Man, does this amp sound amazing!...
I really could not have gotten here without your help, so I really appreciate it !!!
I fabricated a steel sheet metal shield for the top of the chassis to enclose the whole chassis in a metal box in the cab, and this amp is super quiet now...
Below are a couple pics of how I ended up wiring up the preamp and the separate new preamp electrolytic cap (again, thank you @tubeswell for the suggestion and @shooter for ideas on where to place the e-cap in the chassis ! )
@Willabe, thank you for the link to Merlin's site... I bought his book and am currently reading the chapter on grounding. I'll be reading the whole book it is a great resource!
Best Regards,
Meltunes