Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on July 29, 2024, 12:52:29 pm
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Hi there!Just a few pics of my new build. I have a volume issue (the amp lacks volume and doesn't sound as punchy as it should) I need to fix. I have to play it on 6 in order to sound as loud as it should on 2...
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Nice build, very tidy. Looks like a chassis kit form Songwut in Thailand.
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Looks like a chassis kit form Songwut in Thailand.
Spot on! ;)
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He really makes nice chassis and plates.. Very authentic.
So does your low volume occur in both channels? Do you have an idea what the problem is?
Bias set correctly ?
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Yes, it's on both channels. My B+ is too high at 495V but that's not the reason why there is a lack of volume...
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Nice! I'm sure someone here can help you find the problem.
Built to the standard Fender schematic, or some mods?
Have you checked and recorded voltages at pins?
Have you checked all the resistor values? It's pretty easy and common to make a color band mistake.
Have you considered renumbering the vol knobs? :icon_biggrin: Sorry.
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Have you considered renumbering the vol knobs? :icon_biggrin: Sorry.
:laugh: I'm just getting into the fixing so yes, I need to do the whole voltage chart of the amp... The sound is good though, so I'm not really used to an amp that works but not perfectly. Usually they just don't work. :w2:
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Well HVolts being that high would indicate that the tubes aren't conducting much. Try checking bias again and setting it around 65% max P diss at idle.
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This is the layout that I used for this build, by Rob Robinette:
Here's my voltage chart. Can you spot anything?
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Great way to double check a new build with problems;
https://el34world.com/Forum/index.php?topic=17701.msg178630#msg178630
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+1 with Willabe. Quick look - V3 - missing voltage on pin 8. V5 - volts are wacky. V6 - pin 2?
The Vibroverb schematic I have is hard to read - but it does have the Fender intended values on it.
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see pic
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Have you calculated the clean output power of the amp? I'm willing to bet what you're experiencing is the difference between pot tapers, namely the 30% vs 10% log pots. If you can dial in the full power and go up from there to see a distorted waveform at max volume, then I'd look at the pot taper.
Alternatively you can just map the resistance as you turn the pot up.
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Hi everyone and thanks for your input!
I do have an 8V reading at pin 8 of V3, my mistake sorry! The readings on V5 are still the same, no mistake on my part there and V6 pin 2 does read 65V.
@SEL49, thanks a lot for spotting this! I added a small wire to the ground bus wire, it's now grounded.
I haven't sold the volume issue and @stratomaster: I'm sorry but I don't know how to measure the clean output power of the amp. The amp starts delivering a fine sound/volume at 4 on the knob (I should already be deaf by then!) and its sweet spot around 7. Above that it starts sounding messy and the distortion is not pleasing to the ear.
Thanks again!
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It looks like you haven't installed 1 ohm 1 watt 1% resistors on the cathodes of the power tubes. Or maybe I just can't see them. Having those makes it easy to set a safe bias level.
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It looks like you haven't installed 1 ohm 1 watt 1% resistors on the cathodes of the power tubes. Or maybe I just can't see them. Having those makes it easy to set a safe bias level.
Indeed I haven't, but I have the Hoffman bias checker so it's not a problem.
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OK. So I have checked all the component values and my wiring, including a continuity check. I didn't spot any mistake. I don't understand why I don't get any voltages on V5 pins 3 and 8. I put another tube there and got the same readings. I don't know if that's why the amp sounds like a small amp volume-wise though since V5 deals with the tremolo.
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Have a trem pedal? Same result with trem on or off?
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OK. So I have checked all the component values and my wiring, including a continuity check. I didn't spot any mistake. I don't understand why I don't get any voltages on V5 pins 3 and 8. I put another tube there and got the same readings. I don't know if that's why the amp sounds like a small amp volume-wise though since V5 deals with the tremolo.
If there's a problem with the tremolo that is causing the LDR to go low resistance then you'll be bleeding a ton of signal to ground. This would affect the Vib channel more than the Normal though. However the mix resistors act as voltage dividers here, and the normal channel has one less gain stage--so the Normal channel isn't completely unaffected.
What happens when you disconnect the wire going to the wiper of the Intensity pot?
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Hi!I'll be away from home for a couple of weeks, so I'll get back to it when I'm back. Thank you!
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What happens when you disconnect the wire going to the wiper of the Intensity pot?
I'm back.
Nothing changes. The volume is the same whether the wiper is connected or not and I still have 0V readings on V5 pins 3 and 8.
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Have a trem pedal? Same result with trem on or off?
Yes, same results
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Do you have voltage on the right lug of the intensity pot?
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Do you have voltage on the right lug of the intensity pot?
Here is what I have, I don't get it...
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What happens when you disconnect the wire going to the wiper of the Intensity pot?
I'm back.
Nothing changes. The volume is the same whether the wiper is connected or not and I still have 0V readings on V5 pins 3 and 8.
Ok so that eliminates the entire tremolo circuit as a potential cause for low volume.
Do you have a signal generator, or a way to inject a sine wave into the input of the amp?
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Unfortunately no, I don't have any way to generate a signal generator.
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Unfortunately no, I don't have any way to generate a signal generator.
Phone app to headphone jack to 1/4" plug?
Alligator clips to the tip and sleeve of a 1/8" stereo cable plugged into the headphone jack of a laptop?
An old Casio keyboard where you plug into the amp and tape a key down?
Think outside the box. We need a steady sustained signal--preferably sine wave, but sawtooth will work in the case of the keyboard.
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OK, I'll try. And so once I have a sine wave, what do I measure in the amp?
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OK, I'll try. And so once I have a sine wave, what do I measure in the amp?
AC voltage (RMS if your meter has it) at the grid and plate of all of triodes except those in V5.
I'm looking for relative magnitudes here, so eq at 5 with reverb and Vibrato turned off and volume between 3 and 6 should be fine for this purpose. Just don't change volume or EQ between measurements. Pick a frequency between 400hz and 1kHz. The exact one doesn't matter for this purpose.
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Hi!Here's a little update on this issue. A friend of mine and I looked up into the amp and we've narrowed the issue down to the tremolo section. Disconnecting the tremolo from the circuit doesn't increase the volume at all, it's even the opposite. We're still not getting any voltage readings on pins 3 and 8 of V5. We changed the capacitor that goes there but to no avail. I'm currently in the process of removing the whole components board to check if I haven't made any mistake with the hidden underboard wiring. It takes a lot of time for a very hard to fix problem.
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Does the amp have the expected level of gain when the volume is set to 10, full CW?
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I suspect it does.
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What's with the suspecting and not measuring? I gave you an easy way to measure the gain of each stage without the need for a scope. This is a simple test that would have taken a few minutes to perform and instead you're swapping caps and taking the amp apart. And reexamining areas of the circuit that we ruled out earlier.
To be clear, you do have a problem with your Tremolo circuit. But by disconnecting the wiper to the Intensity pot you removed the effect of that circuit on the signal while maintaining the 50k load from the pot. This decoupled the two circuits, so if you still had the volume problem it's not from the tremolo.
I think what pdf64 is getting at, and your answer seems to confirm, is pot taper. If you're at full gain at max volume then there is no other explanation than the pot taper isn't matching what your reference amp has installed.
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OK stratomaster. I have a 1/4" stereo cable and a 1/8"s stereo cable. Can you walk me through with what you would like me to do with them? Thank you!
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Generate a sine wave. The quickest way I can think of is to download a signal generator app for you phone, tablet, or laptop/PC.
Connect the 1/8" to the headphone or speaker out of your device. Use an adapter or alligator clips or anything you can think of to get the signal into the amp. Tip is signal and sleeve is ground. You can ignore the ring.
Connect plug tip to jack tip and plug sleeve to jack sleeve on input 1. You can even use a plug that is connected to nothing to defeat the shunting of the input jack.
I don't care how you do it, but get the sine wave into the amp.
Once there set the frequency to 400Hz. And adjust the volume if your device until you get 150 to 200mV AC. You can measure this with your meter.
Set all EQ controls to 5, volume to say 3, and reverb/trem off.
Now go to every triode grid and measure vAC, except V5. Write them down.
V2a: xx V
V2b: yy V
Etc
Then without changing anything on the amp or the device feeding the signal measure the vAC on the plates of each triode. Write these down too.
If your gain stages are set up properly you'll see a 50-60x difference between the vAC at the plate vs the grid of any given triode.
This will establish whether or not your amp is properly amplifying.
If this sounds like a lot, then just pull out the volume pot and measure the resistance between the wiper and the two outer lugs with the pot set to 50% rotation. I'm willing to bet you have a 10% taper pot and are comparing it to an amp with a J taper. If this is the case, then the only problem with your amp is the tremolo. We can tackle that next.
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Just a suggestion: build your self an audio probe. See R.G. Keen’s Geoflex. You can hook it to a”Walkman” and use it to inject a signal. Start at the PI work back to the input. Also see listening amp over on EL 34 world. Last time this happened to me I had a whisker on the shielded cable shield grounding at one of the pots.
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stratomaster, where am I supposed to measure 150 to 200mV AC? Thanks!
I suppose I should measure the AC voltage at the tip of the input jack. If so, I measure 60mV AC. I can't go above that, my mobile phone is set to its max output already. Would that be enough for the measurements?
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Can't hurt to try.
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OK, so here's what I have in AC with the 400hz signal going through:
V1 pin2: 9.4mV pin 6: 0mV pin 7: 5mV V2 pin 2: 60mV pin 6: 0mV pin 7: 151mV V3 pin 2: 350mV pin 6: 0mV pin 7: 2.6V V4 pin 2: 64mV pin 6: 0mV pin 7: 50mV V5: not measured V6 pin 2: 0mV pin 6: 0V pin 7: 0.1mV. Thank you!
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Good first attempt but you'll have to redo the measurement.
You're missing the plate (pin 1) measurement for every "unit 2" triode, and your pin 6 measurements shouldn't be 0. I think you're out of range on your meter if you manually set it to mV. Also V3 pins 2 and 7 are wired together so they should read identical. This tells me you have either an unreliable input signal connection or problems placing the probes securely enough to get a good reading.
I'll take a step back and explain what's happening as I think that's what's missing.
Signal is entering each triode at the grid and exiting at the plate. The signal is amplified in the process. A healthy triode in this amp will produce a voltage at the plate that is about 50-60x higher than the voltage present at the grid. The reverb driver will be the oddball, but measure it anyway. So if you're hearing signal through the amp then there's no way to not have signal present at the plates.
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OK, I have done it again but the only pin 1 I can read is on V1, all the rest go down to 0V...I should probably add that clipping the alligator clip on V6 pin 2 shuts down the signal...
Hope this helps...
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I don't think you're doing what I asked. The signal only goes into the input of the amp. You don't inject the signal into every grid independently.
In this exercise you're giving a steady input signal and measuring how the voltage changes through the signal path.
You should only be touching probes to the tube pins. I'm unsure why you would be clipping anything to V6, and your description of shunting the signal implies something is wrong with what you're doing.
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I'm clipping an alligator clip instead of touching the pin with the probe so that my reading is more accurate. The signal is just going into the input 1 of the normal channel.
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What is your meter set to?
The only mV range measurement should be V 2a grid. The rest should be volts to tens of volts (except in the reverb circuit).
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I change the setting to get the most accurate reading possible. Most of the time it's in the 200mV range but sometimes I use the 2V or the 200V range. For instance for V1 pin 1 I used the 200V range.
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If your grid voltage is 0.06v (mV range) and the expected amplification factor is 55, then your plate voltage will be 3.3v. This is out of range for your mV setting and could be why you're not getting plate measurements.
Also I just noticed you are only injecting at Normal channel. You need to inject into the Vibrato channel to measure the voltage gain in that portion of the amp.
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Will do it again tomorrow with the signal injected in the Vibrato channel. The lowest mV range I have on my MM is 200mV so yes, it may not be accurate enough to measure 0.06mV... Crap!
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Will do it again tomorrow with the signal injected in the Vibrato channel. The lowest mV range I have on my MM is 200mV so yes, it may not be accurate enough to measure 0.06mV... Crap!
Reread. 0.06V this is the same as 60mV. You don't need to set your meter lower. You need to set it higher. Look at your numbers for V1a. It looks like you're getting a voltage gain of 40 through the stage. This looks like a reasonable measurement and was likely done correctly. You should see numbers like though through the rest of the stages.
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Hi!Here's a new one. I injected the signal in the normal channel to get proper readings for V1 and then injected the signal in the Vibrato channel for all the other tube sockets measurements. I hope I'm doing things right this time and I hope it helps! Thank you!
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These look better. The stand out is V4B. This is the final gain stage after the reverb circuit and before the PI. You're showing 300mV out and 18mV in. This is a gain factor of 16.7. Based on the other measurements you should be expecting about 40. Even 40 seems low to me, but it's been a while since I've done this myself on an AB763 style amp.
Try a different tube in V4. If that doesn't work, then look at the components around V4b to make sure there are no build errors. Double check resistor values on the plate and cathode. Ensure the cathode cap isn't backwards or has come disconnected. A lot is happening this area of the circuit and just downstream (vibrato circuit ties in, normal channel mixes in, etc) so it can be easy to make an error.
Edit: I found the Fender Vibroverb Custom schematic online and it has this exact information on it by stage, AC in and AC out. It's the measurements given in an oval flag at each triode symbol. Divide the out by the in for gain factor. It's not a 1 for 1 between this schematic and your amp (the Cesar Diaz mods affect gain), but maybe the Super Reverb Reissue, which has similar voltages on the schematic, is closer.
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The K bypass caps are not in backwards.
It appears in your pictures that your feedback from the speaker jack isn't connected to the 820ohm resistor. What is the 820 ohm resistor connected to?
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I have checked (again) everything that is connected to V4. All the resistors are correct and within values. The caps are placed how they should be. The wiring too.
@mresistor the 820ohm resistor goes to the speaker jack.
I have also tried another tube in V4 and it didn't change anything.
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Oh ok sorry I zoomed in on the full view of your chassis and now I see the wire from sp jack to under the board and then to 820R.
Curious have you disconnected the feedback to make sure the OT is in phase?
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Curious have you disconnected the feedback to make sure the OT is in phase?
No I haven't tried that. So I simply disconnect the yellow wire that connects the 820ohm resistor and the speaker jack?
Yes.
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Will you please measure the volume pot taper?
You can either measure the resistance to ground of the wiper with the amp powered down or reinject a sine wave and measure the AC volts into the pot vs out of the wiper. Set the pot to about 50% rotation for either measurement.
If you measure around 100k using the resistance method then you have a 10% taper pot, and the response will be different from the J taper that Fender used in these originally. With a 10% pot "7" sounds more like what "4" should sound like.
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Will you please measure the volume pot taper?
You can either measure the resistance to ground of the wiper with the amp powered down or reinject a sine wave and measure the AC volts into the pot vs out of the wiper. Set the pot to about 50% rotation for either measurement.
If you measure around 100k using the resistance method then you have a 10% taper pot, and the response will be different from the J taper that Fender used in these originally. With a 10% pot "7" sounds more like what "4" should sound like.
I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.
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I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.
This is your problem. Wrong taper pot (as mentioned very early in this thread).
Put in a J taper and you'll get the behavior you're used to.
Now we get to work on the tremolo.
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I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.
This is your problem. Wrong taper pot (as mentioned very early in this thread).
Put in a J taper and you'll get the behavior you're used to.
Now we get to work on the tremolo.
OK. So, to understand what I'm doing, with the right pot I was supposed to get 500K at 5 and not 78k? Am I right? I've always ordred CTS 1Meg pots and never had that issue before. How strange! Maybe they sent me the wrong ones this time!I don't have CTS pots to replace them but I have Alpha ones.
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There are a few versions of audio taper pots. The most common are 10%. This is what you have and what most amps use.
The next is J taper or 30% audio. This is what Fender used on the volume and treble pots. You'll see "2-35" on the old schematics near the pots. These pots reach about 30% of their max resistance at 50% rotation.
If you ordered J taper but got 10% then they sent you the wrong thing. If you ordered 10% then you ordered the wrong thing.
The pots you have will work just fine, just know that they will behave differently from what you expect of a vintage Fender. All of the sounds are still there, just at different pot settings. If you want the vintage Fender behavior, put in the right pots.
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This is what I ordered:https://www.tubesandmore.com/products/potentiometer-cts-audio-solid-shaft
They never mention J taper or 10/30% anywhere. So was the volume pot supposed to read 500K at 5 so I know if the Alpha ones are the right ones?
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30%. So ~300k. The Alpha you have sounds like a linear pot of it's 500k @50% rotation.
This is what you're after: https://www.tubesandmore.com/products/potentiometer-fender-1m-j-taper-solid-shaft-panel-mount
They have a CTS branded option as well, but that one seems to be a low torque pot and liable to be unsuitable in loud combo amps.
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They have a CTS branded option as well, but that one seems to be a low torque pot and liable to be unsuitable in loud combo amps.
This;
https://www.tubesandmore.com/products/potentiometer-cts-j-taper-solid-shaft-panel-mount
If so, how are you getting "low torque pot"?
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Curious have you disconnected the feedback to make sure the OT is in phase?
No I haven't tried that. So I simply disconnect the yellow wire that connects the 820ohm resistor and the speaker jack?
Yes.
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I've just measured the resistance. With the volume on 5 I get 78k (red probe on the middle lug of the pot, black ground probe on the left lug). I rotated it from 0 to 7, I get 0k on 0 and 198K on 7.
50% rotation on a 1-10 knob is 4.5.
To measure this properly, the right lug needs to be disconnected from the circuit ie treble pot wiper.
Whatever, it points to the pot having a 10% taper, where 50% rotation would be about 100k on a 1M pot.
Whereas these Fenders normally have a 25-35% taper, eg 250k to 350k at 50% rotation on a 1M pot.
Perhaps just turn it up a bit more to achieve the desired level?
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They have a CTS branded option as well, but that one seems to be a low torque pot and liable to be unsuitable in loud combo amps.
This;
https://www.tubesandmore.com/products/potentiometer-cts-j-taper-solid-shaft-panel-mount
If so, how are you getting "low torque pot"?
User review. I said "appears to be" for this reason. I know I have CTS 1M J tapers in my shop somewhere. No clue if they are the ones sold on that page or purchased elsewhere or if they were the Fender branded CTS. The ones I have seem ok for amp use, but I've used low torque pots accidentally before and had the amp self adjust on stage.
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Thanks everyone! I'll change the pots tomorrow and I'll also disconnect the feedback to see what happens. Then hopefully we can tackle the tremolo issue!
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Hi.
This amp makes me want to cry. :sad2: I changed the volume pot for the Alpha 1MAudio one that I have but unfortunately on 5 it only gives 100k, not much better than the 78K I had with the CTS. It's not a J taper either. It reaches 300K only on 8. There really is a problem with the volume anyway. On 6 it starts giving out a playable volume but only with the strat on 10. If I lower my strat's volume to 6 then it's like playing an acoustic guitar...
I disconnected the feedback wire and to my surprise the amp worked as badly as it does when it's connected... Nothing changed!
I need to order good J taper pots. In the meantime if you have other ideas, please shoot! I've built dozens of amps, it's the first time that I'm having such a hard time debugging one...
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... I changed the volume pot for the Alpha 1MAudio one that I have but unfortunately on 5 it only gives 100k, not much better than the 78K I had with the CTS. It's not a J taper either. It reaches 300K only on 8. ...
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To measure this properly, the right lug needs to be disconnected from the circuit ie treble pot wiper.
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Alpha Taiwan standard audio taper is 15% IIRC.
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Alpha Taiwan standard audio taper is 15% IIRC.
I'll try and find the proper CTS pots here in Europe so they arrive more quickly.
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Hi sleepless I read through this thread again and notice we still don't know what the power dissipation is at idle.
The Hoffman bias checker does the same as 1 ohm 1% 1w resistors do connected between the cathode and ground. Having these installed makes it a ton easier than breaking the connection of the tube to socket and then inserting a bias checking device. I have a Hoffman bias check tool that I use for clients amps but mine have the resistors installed on the Ks.
Please tell is the mv reading with your tool for each power tube and the Plate voltage at that time. Both Vol knobs to 0.
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Hi sleepless I read through this thread again and notice we still don't know what the power dissipation is at idle.
The Hoffman bias checker does the same as 1 ohm 1% 1w resistors do connected between the cathode and ground. Having these installed makes it a ton easier than breaking the connection of the tube to socket and then inserting a bias checking device. I have a Hoffman bias check tool that I use for clients amps but mine have the resistors installed on the Ks.
Please tell is the mv reading with your tool for each power tube and the Plate voltage at that time.
Hi mresistor. I installed the 1ohm 1 watt resistors on the 6L6GC sockets two days ago just in case. What measures would you like me to take precisely? Thank you!
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Please tell is the mv reading with your tool for each power tube and the Plate voltage at that time. Both Vol knobs to 0.
V5: 26mA V6: 22.7mA Plate voltage (pin 3 of the power tube socket) is 438V for both V5 and V6.
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You are running the amp ice cold. 438 x .027 = 11.86W at idle. 6L6 is a 30W tube and 70% of 30 = 21Watts.
I would bias the amp much hotter.
crank the bias up.
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Also remember that you are measuring K current and that it includes screen current. So going 70% at idle is really less than 70% subtracting out the screen current.
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Hi!I've just set the bias at 45mA. It's much better in terms of volume. Much better.
There still is a problem with the amp though: I still don't have any voltage reading on pins 3 and 8 of V5. They read 0V where the schematic shows I should get 2.3V.Thanks for your help!
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I still don't have any voltage reading on pins 3 and 8 of V5. They read 0V where the schematic shows I should get 2.3V.Thanks for your help!
Perhaps the trem needs turning on?
ie plug in footswitch and try it both ways
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Hi!I've just set the bias at 45mA. It's much better in terms of volume. Much better.
There still is a problem with the amp though: I still don't have any voltage reading on pins 3 and 8 of V5. They read 0V where the schematic shows I should get 2.3V.Thanks for your help!
Take a look again at your wiring and voltages. 2.3v is just for pin 3 and 12v on pin 8. Your measured plate voltages are indicative of essentially no current flow as well. They should be 280v and 400v respectively.
With both cathodes reading zero, and with the tubes not sharing plate or cathode wiring, whatever is wrong has to be affecting both triodes. My money is on a bad tube. Check if it's gone atmospheric (white cloudy residue). And try swapping in another tube even if it isn't visibly bad.
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I've just set the bias at 45mA. It's much better in terms of volume.
How did you set the bias at 45mA?
You can't just set it there without checking the power tubes plate dcv and doing the math for what ever dissipation % you want.
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I've just set the bias at 45mA. It's much better in terms of volume.
How did you set the bias at 45mA?
You can't just set it there without checking the power tubes plate dcv and doing the math for what ever dissipation % you want.
His plates were at 450v per post 8 with bias cold. He's ok at 45mA even if the plates didn't drop with hotter bias.
Granted I don't know how he settled on 45mA. I just know he won't hurt anything there.
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Your teaching him the wrong thing.
You ALLWAYS do the work of doing the math. It's soooo easy these days, you can use an on line bias calculator. Doesn't get any easier than that.
https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm (https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
Tube Bias Calc (https://robrobinette.com/Tube_Bias_Calculator.htm)
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Hi everyone!I calculated my way to 45mA. 435V plate voltage x 0.045mA = 19.5W. I'm a bit below 70% for a 30W (21w) tube so I'm good I think.
I now have 1.9V on pin 3 of V5 with the tremolo on using the footswitch (pdf64 you were right!)
Now I have a loud hum on the Vibrato channel as soon as I turn the volume up. Normal channel sounds quiet. A tube maybe?
I've swapped V5 for another tube and I get the same hiss/hum. It's not the tube!
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I calculated my way to 45mA. 435V plate voltage x 0.045mA = 19.5W. I'm a bit below 70% for a 30W (21w) tube so I'm good I think.
That's good that you didn't just guess.
You should have then calculated them for 60% dissipation next just to see where it came out.
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So, just to recap.
Setting the bias to 70% solved your volume issues?
Now the normal channel works as expected, but you have a hum on the vibrato channel only, which is only affected by the volume control?
I'd look at ground connections in V2 and the tone stack, as well as the inputs, check to make sure your input jacks are switching properly.
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Maybe still missing another ground? See attached - It's part of V5, the CF for trem driver and the ground for V6, the LTPI - if it's missing, the ground path for the LTPI is taking a path through the NFB 820R via the OT secondary. Also, can't see pots, is one side of the Intensity pot grounded?
--Pete
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So, just to recap.
Setting the bias to 70% solved your volume issues?
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Working back from the voltages and referencing them to anode characteristics, it doesn't seem that Fender ever intended to idle 6L6GC at such a high dissipation.
Bear in mind that 60% of a design maximum anode dissipation rating (eg as per 6L6GC's 30W) is equivalent to 70% of about what would be its design centre rating (26W).
Certainly more recent Fenders with that info available seem to idle somewhat cooler, eg below 15W here https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb.pdf
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Hi!
Yes, it seems that setting the bias at 45mA solved the volume issue. I think I'm nearly done with this amp. The only remaining problem is the hum/frying noise that I have. I said before that the normal channel worked properly but actually it does have a little hum as well, though a lot quieter than the vibrato channel.
@pdf64 I can lower the bias down to 40mA, I would be slightly under 60% max dissipation.
Here's a pot-side picture. @AlNewman: the ground connections are there. The little bridge between the ground buss and the 47ohm resistor is underneath the board.
I also shot a short video so you can hear the noise I'm getting.Thanks again for your help!
https://youtu.be/w9g6NB-I6Qs?si=J-4O2IcSyJ1vHDWC (https://youtu.be/w9g6NB-I6Qs?si=J-4O2IcSyJ1vHDWC)
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Idle the 6L6GC as hot as you like, I'm just noting that it shouldn't be necessary to do so in order to get the amp working right.
eg the Super Reverb RI is specified at 433V on the screen grids with 33mA cathode current.
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Idle the 6L6GC as hot as you like, I'm just noting that it shouldn't be necessary to do so in order to get the amp working right.
eg the Super Reverb RI is specified at 433V on the screen grids with 33mA cathode current.
I get you. Need to keep looking then!
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That sounds like a bad carbon comp plate resistor.
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That sounds like a bad carbon comp plate resistor.
Should I test or replace each one of these?
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You can try pulling the preamp tubes, 1 at a time, starting with the 1st tube. Pull the tube and listen if the hiss went away. If hissy/crackling went away, it's thse plate resistors. If it doesn't go away, then put that tube back in, pull next tube, and so on.
If you think you've found the tube stage that's hissing, change those plate R's. It may be a few.
Could even be a dirty pot, but those usually only act up as you turn the pot. But sometimes their so dirty they still hiss when not turned. It can also be a coupling cap leaking dcv to a pot. When you turn a pot that has dcv on it it does that, as long as there's current flowing it make a hissing/crackling noise. :dontknow: And that could be why the amps not acting right volume wise, dcv on pots at the grid changes that tubes bias.
It could also be a hissy/noisy tube. You have to poke around until you find it.
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Ok, I'm not getting any noise when I pull V6 out.
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Pulling V6 cuts all signal from reaching the power amp. It tells you very little diagnostically except the noise is in the preamp--which we already knew from it responding to the volume knob.
Since it's common to both channels I'd be inclined to cover the chassis with something conductive (a baking sheet is what I use) to rule out ambient signals as the source.
If you have no luck with that then you'll need to strategically ground grids to pinpoint where the noise is coming from.
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I would use 1W metal film resistors on plates. The only reason I see to use CC resistors there is for aesthetics. MF are much quieter. There are some cool Dales that would look great in there.
CMF60100K00FKEK
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Maybe still missing another ground? See attached - It's part of V5, the CF for trem driver and the ground for V6, the LTPI - if it's missing, the ground path for the LTPI is taking a path through the NFB 820R via the OT secondary. Also, can't see pots, is one side of the Intensity pot grounded?
--Pete
Yes the grounding is good in those places.
I will order metal film resistors for the plates but honestly I've always used carbon comp there and never had such a noise issue with them. I tested all the plate resistors and they're all giving me the right value. Some are a bit above 100K, others a bit below 100k but they're within a 10% margin, so unless a resistor can give its proper value and still be faulty, they're not the culprits here.
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The old cracked carbon R's were the most problematic. Something about the cracked carbon chunks expanding and then the R absorbs moisture? Then they sputter. Carbon comp. R's can do the same.
So the R can measure to spec but still be noisy. Sometimes you can get a brand new carbon comp, CC, R and it will be bad. Or it's been sitting around in inventory for years and has gone bad.
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I've had noisy plate resistors of all types except metal film. The vishays referenced above have served me well. However, that doesn't mean your noise is from the plates. I think you need to do some more troubleshooting and investigation before you swap them out. I've been able to leave original plate resistors in well used 65 year old amps because the source of noise was something else.
Cell phones, wifi, digitally controlled soldering stations, fluorescent and LED overhead lighting, etc are all electrically noisy. You can't judge how noisy the amp is until it's shield is complete.
You may find, as Fender did later in the decade your build reproduces, you need shielded wires to the grids for the first 2 stages to get good quiet performance.
Additionally, I find the noise floor on most (easily 3 of 5) new production 12AX7s to be atrocious. You'll likely have to roll your tubes to find the quietest combination. Put your noisiest one in V5.
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The only reason I see to use CC resistors there is for aesthetics.
There is a reason to use carbon R's. They will distort when they have a high dcv on them. But only with a high dcv on them. So for the most past you would only use them for plate and CF K R's.
Ken Fisher used to tell people in Vintage Guitar mag. if you get a hissy/crackling carbon plate R, make sure to put in a carbon R, or the amp will sound different.
There's a short tech paper, only a few paragraphs, that talks about it. Might be on R.G. Keens web site?
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I also shot a short video so you can hear the noise I'm getting.Thanks again for your help!
https://youtu.be/w9g6NB-I6Qs?si=J-4O2IcSyJ1vHDWC (https://youtu.be/w9g6NB-I6Qs?si=J-4O2IcSyJ1vHDWC)
Stratomaster, have you listened to this?
It's very bad.
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And yes I love those Dale R's too. :icon_biggrin:
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Yes. I agree it's bad. I've heard worse from ambient sources though. Even bad reverb cables can put similar junk in the signal. Putting a baking sheet over the amp takes minutes. There's no harm in ruling out benchtop noise before taking action.
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But I've never heard anything like that from air born interference. Not that kind of crackling.
From bad plate R's, dirty pots, bad solder joints, yes. But yes, put a metal shield plate over the top of the chassis and rule out air born interference or not. :dontknow:
Easy to do and well worth it.
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The only reason I see to use CC resistors there is for aesthetics.
There is a reason to use carbon R's. They will distort when they have a high dcv on them. But only with a high dcv on them. So for the most past you would only use them for plate and CF K R's.
Ken Fisher used to tell people in Vintage Guitar mag. if you get a hissy/crackling carbon plate R, make sure to put in a carbon R, or the amp will sound different.
There's a short tech paper, only a few paragraphs, that talks about it. Might be on R.G. Keens web site?
http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm
There's a case to be made for CCs for anode load resistors in common cathode gain stages, see link.
With cathode load resistor in cathode follower stages, I don't see the same case can be made.
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http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm (http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm)
There's a case to be made for CCs for anode load resistors in common cathode gain stages, see link.
With cathode load resistor in cathode follower stages, I don't see the same case can be made.
Very interesting article!I placed a baking sheet on top of the chassis and nothing changed. I still get the crackling.
Are these the resistors you recommend?https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film (https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film)
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...
Are these the resistors you recommend?https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film (https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film)
Those are for power applications, eg screen grid current limiting in output valves, HT droppers.
Look for a product line that's intended for low noise but can accommodate at least 350V.
Personally I use 1/2W CC for anode loads.
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Are these the resistors you recommend?https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film (https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film)
Actually these are the ones I was talking about, they are precision 1% rated at 500v. This site is Mouser Europe.
https://eu.mouser.com/ProductDetail/Vishay-Dale/CMF60100K00FKEK?qs=LPGv226TKJKhQHe%252BlJF0yQ%3D%3D
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I think either resistor will work fine in that application. Dealing with the big suppliers you'll often find resistors on sale, and can buy 50 for the same price as what some of the smaller suppliers charge for 5.
Before going on a shopping spree though, it would be best to pinpoint the actual problem, because as already mentioned it could be any number of things. Solder connection, leaky capacitor, bad tube, etc. If you rig up a lead with a .1uf/600v capacitor, and systematically ground the signal to chassis, it won't take long to find exactly what the issue is.
Since it seems to be before the volume pots, I'd start at the input side and work towards the volume. When the noises disappear, eureka.
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Hi!I just shot a short video of what I'm getting with the "probe" AlNewman suggested. I already changed the 100K resistor, 22nF cap and 500pF cap on the right side of the board. It didn't change the crackling. I think a video is better:
https://youtube.com/shorts/K7Gf4JYetg8?si=tiSf9fNDhDbnVp-a (https://youtube.com/shorts/K7Gf4JYetg8?si=tiSf9fNDhDbnVp-a)
What would you do? Thank you!
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I guess I did a poor job of explaining the principle of what you're trying to do.
When you dump signal to ground anywhere the noise is present, the noise will disappear. But if you start at the input and work your way through the signal chain, you can find the exact component where the noise disappears.
So, ground out the guitar input. Noise disappears? That means the noise must be introduced somewhere in your input jack.
Noise still there? That means the noisy component/connection must be after that point. Move to the next component in the signal chain, which would be after the grid stopper on the 1st gain stage...etc, etc...
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I guess I did a poor job of explaining the principle of what you're trying to do.
When you dump signal to ground anywhere the noise is present, the noise will disappear. But if you start at the input and work your way through the signal chain, you can find the exact component where the noise disappears.
So, ground out the guitar input. Noise disappears? That means the noise must be introduced somewhere in your input jack.
Noise still there? That means the noisy component/connection must be after that point. Move to the next component in the signal chain, which would be after the grid stopper on the 1st gain stage...etc, etc...
The points where I placed my "probe" in the video are the only ones which suppress the noise. Everywhere else it doesn't change anything. So I actually don't understand what I did wrong in my video. :dontknow:
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Here's a schematic of your amp.
Can you put a red dot on all the points you tested in your video which made the noise go away?
https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_ab763_schem.pdf
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There you go.
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I think he wanted you to keep tracing back to the input jack.
You showed it responds to the volume pot so that means it's further towards the input than you showed by probing. I came to this conclusion because the volume pot at minimum equals grounding the grid of V2b. So probing the plate connection isn't the latest area in the signal flow that killed the noise.
With this probe you'll probably want to ensure it's discharged before you probe grids. But I think you need to at least check the plate connection of V2a.
The 3 areas you probed near V2b plates are basically identical from an AC perspective, so no smoking gun yet.
The two places near PI input were killing both channels and killing the output of just the Vibrato channel. Didn't tell us much diagnostically other than the Vibrato channel is the biggest contributor to noise.
Keep probing further towards the input.
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Hi folks!I think I've found it! I touched the grid of V2 with my probe and the crackling ceased. I reflowed the solder joint. I've been playing the amp on 6 for half an hour and it is dead quiet. I'm going to put back the components I had replaced one by one and order the proper pots and a few tubes anyway but I think I've finally solved the issue: a bad solder joint and a low bias. Jeez that was long!I can't thank you all enough! I wouldn't have made it without you. A million thanks!
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Good job, you stuck with it and you found it. :icon_biggrin:
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Sweet