Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on March 11, 2026, 02:08:43 pm
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I've always been a great fan of Chet Atkins, and loved the sound of his Gretsch's and his Standel 25L15 studio amp. When I was a teenager, they were a rarity, but considered as the Holy Grail where I lived. So I've recently been thinking of building a replica. I've found what I think to be is a pretty accurate schematic, and the circuit looks pretty simple. The amp just has 1 channel, 25 watts, 15" JBL P130 speaker, and Volume, Bass and Treble pots. I don't play gigs, but I have a couple friends that do, and so I would be building this for the fun of doing it, but also making it available to buddies to use.
So I'm hoping I can get some input from those who own or have used this amp, and can give me some thoughts on things they like, or would like to change or have added to the amp. It's designed for 807 power tubes, and I've never seen those in an amp. I've read that 6L6's could be used instead. But would that affect the sound that I really like.
If it was going to be used by someone who has a studio for an amp that players could use when recording, would it be an attractive addition to the studio, and is there something more that the amp needs or should have that would make it even more attractive for a studio.
Thanks for your comments.
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would that affect the sound that I really like.
just guessing, but..
pretty sure 90% of that sound you like, came from the mans fingers n soul...
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HotBluePlates is the man to talk to.
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To build one, you need the right information; the result will be a truly, truly clean amplifier, t'hat was the purpose of that amp
As far as I'm concerned, building one and then modifying the design wouldn't be worth the effort.
Franco
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Power transformers and output transformers? Can you get some similiar ones as the original?
I believe those two components has a LOT to do to a spesific sound of an amp.
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Yes, I'm pretty sure I can get the transformers with the proper specs. Agreed on their impact on sound.
I've talked to a couple of buddies who gig, and they said one thing they would like is a switch to adjust the wattage of the power output because they play in different venues where a 25 watt amp might be too much. So a switch that could change the wattage to 5W, 15W, and 25W for example.
I think this is something different than an attenuator. I think maybe Marshalls have something like this. I know I've seen it on some amps, but I don't know what's going on. I'll do some research.
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To maintain the original tone I don't think a VVR will be appropriate expecially with the 25L15
To me the right way will be an attenuatore, but a very good one
Franco
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can you post the schematic, i've looked at about 1/2 Doug's n their all sand.
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Shooter-sorry but I had to sign a NDA and can’t share it.
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:laugh:
it is tube though?
self biased or fixed?
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Yes, its all tubes, cathode/self-biased, running 807 output tubes as the original.
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I've always been a great fan of Chet Atkins, and loved the sound of his Gretsch's and his Standel 25L15 ... It's designed for 807 power tubes, and I've never seen those in an amp. I've read that 6L6's could be used instead. But would that affect the sound that I really like. ...
If you use the original 807s, you have the hassle of top caps but you can buy NOS 807s (https://www.amplifiedparts.com/products/vacuum-tube-807-beam-power-amplifier) all day long for half the price of some Chinese 6L6s (https://www.amplifiedparts.com/products/6l6wgc-str-blackplate-tube-amp-doctor-matched-vacuum-tube).
That said, you can ditch the top caps and use 6L6G/GB/GC/5881 as you please without any significant effect on the amp's sound.
I've talked to a couple of buddies who gig, and they said one thing they would like is a switch to adjust the wattage of the power output because they play in different venues where a 25 watt amp might be too much. So a switch that could change the wattage to 5W, 15W, and 25W for example.
Chet Atkins, not Eddie Van Halen, right? Who cares if "25w" is too loud, because you simply turn down the Volume control.
You will also find the amp's Tone Controls use negative feedback, and the amp is extremely-clean sounding unless you turn Treble & Bass full-up. That being the case, there is zero benefit to "reducing power output."
Further, the design of the entire amp is geared to "Clean, Clean, Clean." Each stage of the preamp and power amp handles a larger signal than the stage before, so that no stage is over-driven.
So I'm hoping I can get some input from those who own or have used this amp, and can give me some thoughts on things they like, or would like to change or have added to the amp.
HotBluePlates is the man to talk to.
Standel 25L15 Inspired Amp (https://el34world.com/Forum/index.php?topic=14263.msg134513#msg134513)
It's been a minute...
The most important change was that I added Bias Adjust and Bias Balance lifted essentially verbatim from the Williamson Amplifier (https://www.r-type.org/articles/art-107b.htm) (R14 through R22). Adding some meter pin jacks, I can install any two 807s and have them exactly biased up rapidly.
If you use a JBL D130 (and not a paper-dome version), you may find the upper treble a little steely. I added a Vox-like Cut control at the phase inverter output to reduce extreme high-treble as desired.
There is a T-filter that imposes a fixed midrange-scoop. What if you want more/less midrange? I added a control that reduces the amount of scoop by adding a 1MΩ pot between Ground and the resistor-to-ground in the T-filter.
What if you don't like the center-frequency of the mid-scoop? That was a little more complicated: I added a dual-pot and a pair of build-out resistors that allowed the resistance of the pair of resistors forming the "Top of the T" to be changed; this changes the center-frequency of the scoop.
... I've found what I think to be is a pretty accurate schematic, and the circuit looks pretty simple. The amp just has 1 channel ...
There are really 2 channels, 2 Volume controls. But one is hidden on the underside of the preamp chassis, and they share the T-filter and Tone control section.
I didn't need 2 channels, so I ditched the input 12AX7 and used a 7-pin 6AV6 (or 6AT6 if desired). Once again, you can buy NOS all day long for cheap, cheap cheap (https://www.amplifiedparts.com/products/vacuum-tube-6av6-ebc91-dual-diode-triode).
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If you want to go on the cheap there are G-807 and if you want to go cheaper than that, 1625
Franco
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Main design points we know;
1. Real class A 25w power amp with 807 tubes (6L6GB with different pinout/bottle).
2. There's 2 chokes in the power supply.
3. Each stage can take more input power then it's given, so it doesn't distort.
4. 15" JBL D130
5. T filter mid range scoop.
6. NFB around the tone controls.
Questions about design still not clear;
1. Do we know how many and what type/# of preamp and PI tubes?
2. And which type of PI?
3. NF around the power amp?
4. Grid bias for power tubes?
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HotBluePlates-If you use 6L6GC's instead of the 807's, what changes to its wiring would be needed? Obviously you would no longer have the anode cap to deal with, and it would free up some space, so might make the build more efficient.
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1. Do we know how many and what type/# of preamp and PI tubes?
12AX7 (1 stage per channel) ---> 12AT7 ---> 12AU7
2. And which type of PI?
Split Load / Concertina / Cathodyne. (pick your favorite name)
3. NF around the power amp?
No.
4. Grid bias for power tubes?
It's a cathode-biased amp.
HotBluePlates-If you use 6L6GC's instead of the 807's, what changes to its wiring would be needed?
Octal socket instead of 5-pin socket (https://www.amplifiedparts.com/products/socket-5-pin-ceramic-plate-bottom-mount).
No other changes necessary. You're not trying to get higher power output from the 6L6GCs, even if the tubes themselves can dissipate more heat.
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HotBluePlates-would you still add the Bias Adjust and Bias Balance pots if you use the 6L6GC's instead of the 807's?
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would you still add the Bias Adjust and Bias Balance pots
It's a cathode-biased amp.
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Maybe it’s just a single pot to balance the tubes so that the bias point for both tubes is the same.
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would you still add the Bias Adjust and Bias Balance pots
It's a cathode-biased amp.
The most important change was that I added Bias Adjust and Bias Balance lifted essentially verbatim from the Williamson Amplifier (https://www.r-type.org/articles/art-107b.htm) (R14 through R22). Adding some meter pin jacks, I can install any two 807s and have them exactly biased up rapidly.
So, I think HBP made his adjustable grid bias and balance to match up nos 807 tubes he bought?
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BHP-If you wanted to use an OT with 3 taps and selectable for 4, 8 and 16 ohms, what would you recommend on how to wire the circuit? Would this affect the NFB circuit?
Also is the T-filter that you refer to a Prescence control, or something else?
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HotBluePlates-would you still add the Bias Adjust and Bias Balance pots if you use the 6L6GC's instead of the 807's?
Objection: Asked and Answered, because
The most important change was that I added Bias Adjust and Bias Balance lifted essentially verbatim from the Williamson Amplifier (https://www.r-type.org/articles/art-107b.htm) (R14 through R22). Adding some meter pin jacks, I can install any two 807s and have them exactly biased up rapidly.
An amp does not need to be fixed-biased to have bias-adjustment, and an amp does not need to be fixed-biased to have bias-balance. Click the links guys, and look at the schematic.
... Would this affect the NFB circuit?
3. NF around the power amp?
No.
No Negative Feedback around the power section.
Guys... you're killing me with the repeated questions about stuff that was already answered. Maybe that "pretty accurate schematic" ain't so accurate?
When I built my example of the 25L15, I received information on the circuit from someone who was provided access to an original by a collector, and documented every detail of that particular amp (there is a possibility that other individual examples were tweaked for their original buyer). Now the 25L15 is not "magic" and you can cop the sound close-enough using a blackface Fender amp set for a clean sound. But the only negative feedback in the amp is inside the preamp to make the Treble/Bass controls function. Apparently Bob Crooks didn't feel power section negative feedback was necessary to damp the speaker.
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I've noticed on a lot of older amps, ie. from the late 50's to mid-60's, like the Standel, and the Valco/Gretsch/Supro, and probably some others, that the chassis' were made with their ends being made with blocks of wood instead of the aluminum/steel of the chassis being bent up to form the end pieces like the rest of the chassis.
Any thoughts on why they did it that way. Was it to provide for a solid mounting surface that wasn't of a conductive material, and maybe helped avoid ground loops?
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I think it’s just easier to manufacture. I’ve made chassis this way, with old safe deposit boxes, cut to length and plugged at each end with blocks of wood. Inspired by my Supro Thunderbolt.
Also, why is everyone always so cagey about this schematic? These amps are 60 years old.
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... Also, why is everyone always so cagey about this schematic? ...
In my case because:
... I received information on the circuit from someone who was provided access to an original by a collector, and documented every detail of that particular amp ...
Right or wrong, the gentleman who provided me the information on this amp asked that I not make it public. He was concerned he might get sued by the modern Standel company (as unlikely as that is/was).
I gave him my word, so that's that.
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I've thought of adding a Presence pot to the circuit. Any thoughts on that? Any suggested circuit layout to use?
Thanks.
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If there’s no negative feedback, then you can’t use a traditional presence control.
It sounds to me like this amp (assuming you can get your hands on an accurate schematic) doesn’t require any changes. I’d try it as is before trying to shoehorn mods in there.
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found this in the "public" domain
might not be the mystery amp but is is a pic n you can color on it:)
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No that’s got a bridge rectifier and it’s running EL84’s not the 807’s and the 5U4.
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The schematic for the preamp is in the wild, seems to be accurate. The power supply is built on hollow state recto 5u4g. Build that and you're at 90% of the original. Use Hammond hi-fi OT with NOS 6L6GB tubes to circumvent the 807 and plate cap cautions.
--Pete
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May be here you can find some info
https://gretsch-talk.com/threads/standel-25l15.105/page-2 (https://gretsch-talk.com/threads/standel-25l15.105/page-2)
https://www.guitartubeamprepair.com/standel-25l15-schematic-pdf (https://www.guitartubeamprepair.com/standel-25l15-schematic-pdf)
Franco
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I've thought of adding a Presence pot to the circuit. Any thoughts on that? Any suggested circuit layout to use?
You would have to add negative feedback around the power section to implement a Presence control (that simply reduces negative feedback at high frequencies).
When used with a JBL D130 with an aluminum dome, the amp/speaker combo has plenty of top-end presence. I did the exact opposite of what you're considering, by adding a way to reduce treble at the phase inverter to offset the D130's brightness.
found this in the "public" domain
might not be the mystery amp but is is a pic n you can color on it:)
wsscott has an accurate schematic already. I forgot, but he obtained it from the same source as I did.
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What type of non-electrolytic, 600V caps would you use in a build of this Standel, ie. polypropylene, polyester, 715P or 716P, Orange Drop, Mallory, etc.?
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What type of non-electrolytic, 600V caps would you use in a build of this Standel, ie. polypropylene, polyester, 715P or 716P, Orange Drop, Mallory, etc.?
You mean for coupling caps?
I used no-name polyester coupling caps. They do the job well enough, are not overly-large, and the amp sounds good!
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Thanks.
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Since I'm considering using 6L6's instead of 807's for the output tubes, I want to resolve my concerns over the issue of arcing between the Plate and Grid of the 6L6's, which I understand was one of the reasons that the 807's were used in this amp.
Is that really a concern anymore? I haven't seen any numbers for the Plate voltages on this amp, so I don't know if it runs that much hotter than other 6L6's do in an amp like this.
The specs for the Output Transformer show 60W, 6600 ohm, CT, 200ma.
Any thoughts?
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There is no issue of arcing between output tube elements in the 25L15.
I don't know why Bob Crooks chose to build his amps with 807s instead of 6L6s. There is nothing about the amp's behavior that makes use of the 807's higher plate voltage rating.
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OK, let me ask a dumb question-certainly not my first.
In Reply #13 above, its commented that this is a Class A amp.
Would someone please tell me what makes this a Class A amp and not a Class AB? Is it just the wattage?
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If you look at "sound" as a sinuscurve (one frequency), class A amplifies 360 degrees of that, while AB cuts it in half (180 degrees). This means that the output tubes rests half of the cycle (class AB). In class A there is "full power" all the way..
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Nope.
I will habe to correct myself.
That was single ended vs push pull..
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Class a a/b c d are all "operating conditions"
the amp is just that, it amplifies, IF the drive signal "pushes" the amp into "clipping" you've entered the "next class"
the operating conditions are typically set with "bias" or G1-Cathode VDC
you won't pass any EE classes with said explanation thu:)
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So why would this amp, that has 2 6L6’s, is cathode biased, has a cathodyne phase inverter, B+ of 400VDC or higher, and 25 watts output be considered Class A? If the design of the circuit doesn’t create the distinction, is it just the way it “plays” or “sounds”, and if so, how can one identify the Class without ever hearing it?
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read this...
https://www.aikenamps.com/index.php/the-last-word-on-class-a
then read this...
https://www.aikenamps.com/index.php/is-the-vox-ac-30-really-class-a
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I read both of those articles, and understood about 10% of the technical analysis--but they were helpful.
Rightly or wrongly, what I get from the technical explanation is that the bias setting is the main factor that affects whether the amp is Class A or AB. If that's correct, then by just looking at the schematic you can't tell which class it is because either Class can have the physical design attributes of the other.
If this conclusion is wrong, or much too simplistic, I would appreciate your comment.
Thanks for your time and guidance on this question.
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Correct. You can't tell just by looking at the schematic. You need to look at the signal on the plate.
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the amp is Class A or AB.
"The Amp" is simply an amplifier, it has no Class.
now you have to determine "how it operates" as an amplifier ….Class
so yes, bias is a major contributor, so is the input, or drive signal
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I've started building this amp, and have a question about placement of the fuse holder in relation to distance from the AC mains power line.
This amp has a Power Supply chassis mounted on the floor of the cabinet, and the Pre-Amp chassis mounted to the top of the cabinet. The PS chassis is 8" deep front to back, is 2.5" tall, with the PT mounted at the left-rear side of the chassis. The rectifier tube is mounted at the front edge of the chassis in line with the PT.
The most accessible location for the fuse holder is on the front left side of the chassis about midway between the PT and the Rectifier tube.
So where is the best location to bring in the main AC power line to connect it into the fuse, etc. Or does it really matter? I could put it on the rear left side behind the PT but on the 2.5" rear face of the chassis, not on the top, or I could bring it to the 2.5" face of the left side of the chassis, closer to the fuse holder.
I think I would prefer the AC power line on the rear, but I'm seeking input from you guys who have done this many more times than me! Thanks.
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Here's a photo with the cover of the fuse holder placed on the left in front of the PT, where I would prefer it.
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Most of us normally put the fuse holder right next to where the power enters the chassis. That allows the "un-fused" hot lead to be the shortest possible. For a production amp, that would be imperative.
For a personal, one off amp, you can put it anywhere, but definitely consider keeping that AC away from your signal path as best you can.
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Thanks. If I put the fuse holder at the rear of the chassis it's awkward to get to, so that's why I wanted it up front.
I have thought of bringing the AC in at that point on the left side of the chassis where I would put the fuse holder, so I'll have to see if it will work with the layout in the cabinet.
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I'm working on the bias circuit in this cathode biased amp, and the schematic gives the option of making the cathode bias resistor adjustable using a 100 ohm pot plus a resistor vs. just a single value resistor.
This option calls for a 100 ohm pot plus a 68 ohm 2 Watt resistor. I failed to order the 68 ohm 2 watt resistor, I just have 1/2 watts on hand. But I do have a 100 ohm 2 watt resistor. Would substituting this resistor create issues in adjusting the bias--or otherwise?
Thanks.
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Kinda hard to answer without access to a schematic.
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But I do have a 100 ohm 2 watt resistor.
more R = less I (I= E/R)
so you are "safer"
start WITHOUT the adjustability, get the basics dialed in, then tinker...so install the 100, figure out what the tubes are dissipating, figure out if they go into cut-off, figure out how it sounds......
then tinker
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Thanks Shooter-I figured there might be a math analysis involved that only some people would know. Certainly not me!
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:laugh:
it's not "math", it's the language of electronics :icon_biggrin:
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Does anyone know how the new Standel amps back light their front panels? I think Magnatone also has some models that are backlighted. They look really cool, and I'd like to try it some time.
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HBP did it on his Standel
The light is placed around the panel
Go here
https://el34world.com/Forum/index.php?topic=13326.msg129564#msg129564 (https://el34world.com/Forum/index.php?topic=13326.msg129564#msg129564)
And read the post from Doug
Franco
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Thanks!
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I've almost completed building the Power Amp section, and wondered if there is a way to test it. This section is where the AC comes in and contains the PT, OT, 2 chokes, and the rectifier and 2 6L6's. The on/off switch is in the preamp, not this section. Since I haven't started construction of the Preamp, I don't have a way to inject a signal, or anything else, but I want to make sure this section of the amp is wired properly.
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How can you continue to ask for help on this project when you won't share the schematic? I understand honoring someone's request to keep the secret, but if everyone honored that secret then you wouldn't have the schematic. Think about it!
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wondered if there is a way to test it.
sure, no problem, just provide the following;
total expected;
filament current/voltage
preamp current/voltage
PA current/voltage
then you can calculate a dummyload for testing
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Thanks Shooter.
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Assume the attached Gretsch schematic of the 6165's power amp has the AC power line and fuse attached at the PT's primary windings. Other than that, no controls and nothing from the Preamp is connected.
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To test if it works you can use the preamp of one other amp or a signal generator or arrange a partition resistor from the output of a small transistor radio
Franco
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I have a question about filament wiring.
I've seen some builders run the 6.3VAC filament wires from the Power Transformer to a terminal strip, and then run 1 pair of wires from there to the Pilot Light, and another set to the pre-amp tubes. Is there any benefit to doing it that way vs. just running the wires directly from the PT to the Pilot Light and then from there to the pre-amp tubes?
Also, does it matter which pre-amp tube's filaments get wired first in the string? Assuming you have 3 common preamp tubes, should you start with V1 which is farthest away from the PT or start at V3 and finish at V1?
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the key to good filament wiring is keeping it "uniform" and away from sensitive AC wires or any AC source where noise coupling can happen. (inductive coupling)
so it doesn't matter to your questions as long as nothing can "couple" into or out of the filament chain n induce 60hz into the amps signal path.
believe this is Steve's example of "clean filament wiring"
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Thanks Shooter. Sluckey does nice work!
I have normally used 20AWG wire for wiring up the filaments. Is that a good choice?
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20AWG is fine, but you could easily use 22AWG as its ampacity is about 7 amps or more, depending on insulation.
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Thanks for the tip. Good to know if you're out of 20.
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From Merlin.
Use the example on the far right, red/white wires.
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Thanks.
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First connect the Power Tubes, then PI and down till V1
https://valvewizard.co.uk/heater.html (https://valvewizard.co.uk/heater.html)
Franco
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I've seen some builders run the 6.3VAC filament wires from the Power Transformer to a terminal strip, and then run 1 pair of wires from there to the Pilot Light, and another set to the pre-amp tubes. Is there any benefit to doing it that way vs. just running the wires directly from the PT to the Pilot Light and then from there to the pre-amp tubes?
It doesn't matter, except one may be easier given your physical layout.
... does it matter which pre-amp tube's filaments get wired first in the string? Assuming you have 3 common preamp tubes, should you start with V1 which is farthest away from the PT or start at V3 and finish at V1?
It doesn't matter, except that you will find your own preference for performing mechanical assembly.
For my own 25L15-style amp, I decided routing the heater wiring over the tube socket would give the shortest path, smallest "hum loop," stayed at roughly right-angles to all important components/wiring, and was feasible with the ~22ga teflon-insulated wire I was using. I planned ahead to orient the sockets so that the heater-pins were horizontally aligned for this.
(https://i.imgur.com/6M2c9Ur.jpg)
It's been too many years, and I don't remember which end I started with. But today I would probably start on the left-hand side of the image, work my way across each socket, then finish with the run from the last socket to the XLR jack that I used for the power connection to the preamp chassis.
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I finished the build of the amp a few days ago and its in testing mode. But no shorts or major problems. It worked. The only issue I had was that the schematic I was following was a little confusing to what I'm used to and I wired the pots in "reverse", so I had to rewire them so Full CCW is "Off". It's really clean and bright. I haven't checked the output wattage yet. I've got it set for a bias of 65ma.
One thing I'm trying to figure out is the way the 2nd Input/Volume for Channel 2 works. Input 1 and Volume 1 work normally like any other amp, and turning Volume 2 pot has no effect on its output volume.
However, if I'm using Input 2 which I would describe as having a softer/smoother tone, maybe more "jazz-like", it only produces an output signal if I turn the Vol. 1 pot past its "Off" postion. It seems like it's sort of like a Master Volume for Channel 2. I don't know if I've wired it wrong or this is the way it's designed. The more I turn up the Volume 1 pot, the louder the signal, but increasing the Volume 2 pot also increases the signal too, but not as much as Volume 1 does. So there's some kind of interaction.
One other thing in using Channel 2, is that I get oscillation and high screech if both Volume 1 and Volume 2 pots are turned up really high.
This seems to be the only "issue" for me to figure out. Its dead quiet.
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We really can’t help you without a schematic. The pots are wired in reverse because the tone stack is a negative feedback loop
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I understand. I don't believe the amp I'm working with has a negative feedback circuit, but I could very well be wrong.
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The Standel schematics I’ve seen have a negative feedback loop tone control circuit. Of course we don’t know what circuit you’re building.
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HotBluePlates-I finished the amp and have been testing it and have a question about some voltages and the bias, if you have the time.
I checked voltages yesterday, as well as the bias, and it seems like the voltages are fine for the Power transformer's secondary (356/712 VAC), and the OT (374 VAC), but the bias is too cool based on Rob Robinette’s bias calculator. I have Plate to Cathode voltage of 349 volts, and with the 6L6GC’s that I’m using I should have a bias of around 80–85ma, but the highest I can get it using the adjustable bias pot mod that you designed, I can’t get it past 75ma.
The Power Supply's cap nodes are reading: 387, 373, 296, and 378.
The Pre-amp's cap nodes are reading: 318, 211, 274 and 287.
Also, V1’s plate voltage seems low. I’m using a 7025 tube vs. the 12AX7, and the plates are running 93.1 and 90.4 VDC. They’re getting their power from Node 3 of the cap and it is outputting 296VDC going into those 270K resistors.
Any thoughts. I've looked for circuit voltages but can't find any that are relevant for this circuit.
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plates are running 93.1 and 90.4 VDC.
the signal coming in is 150mV, it'll swing maybe 2VacPP, those #'s are fine.
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Shooter--How did you calculate the 150mv incoming signal value? Just so I can learn something.
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no calc, just measure the average guitars output, you'll probably find it's ~~~ 150mV
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One thing I'm trying to figure out is the way the 2nd Input/Volume for Channel 2 works. Input 1 and Volume 1 work normally like any other amp, and turning Volume 2 pot has no effect on its output volume.
However, if I'm using Input 2 ... it only produces an output signal if I turn the Vol. 1 pot past its "Off" postion. It seems like it's sort of like a Master Volume for Channel 2. ...
When I built my iteration of the 25L15 I omitted the 2nd channel. Somewhere I have a paper copy of the schematic, but it is buried somewhere in a box. Below is the version of the preamp schematic that has been floating around for a long time (and which is probably right for the 2nd channel).
Notice that:
1. The 2nd channel Volume is wired "backwards" like a tweed Deluxe: its wiper is the Input and the non-grounded outer-lug is the Output.
2. The 2nd channel Volume Output is connected to the Input of channel 1's Volume control. There is an isolation-resistor in between, mainly to keep Channel 2's Volume from affecting the response of the T-filter tone circuit (whose response would change if you turn Channel 2's Volume control high, due to bleed-back through Channel 2's coupling cap to the plate load resistor for Channel 2's input triode).
So what you're describing sounds "normal" for this circuit. You gotta turn the Channel 1 Volume high to keep it from reducing the output of Channel 2.
Recognize that this "2nd channel" was not accessible via a control-panel jack: it was hidden on the underside of the preamp chassis (ditto for the Volume control for the 2nd channel).
- The legend was that the 25L15 was only intended for use by "a Star."
- Supposedly "no-2nd-channel" was a desired feature: now no other guitarist could "sit in" and "plug into the Star's amp."
- But there was a "hide-a-jack" 2nd channel, if "the Star" wanted to let someone else play along through their amp.
- But the sound from the 2nd channel was supposed to either be quieter, or be able to be completely shut-off if "the Star" turned off the main Volume control.
(https://www.diyaudio.com/community/attachments/1953-standel-25l15-preamp-schematic-b-jpg.394263/)
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... the bias is too cool based on Rob Robinette’s bias calculator. I have Plate to Cathode voltage of 349 volts, and with the 6L6GC’s that I’m using I should have a bias of around 80–85ma, but the highest I can get it using the adjustable bias pot mod that you designed, I can’t get it past 75ma. ...
349v x 0.075A = about 26 watts ---> plenty hot enough for 807s or 6L6GBs
I wouldn't bother trying to run it hotter. It won't really get you any more "goodness" in this amp.
Or you can shrink the value of a fixed-resistor in the output tubes' cathode circuit to increase the plate current.
The Pre-amp's cap nodes are reading: 318, 211, 274 and 287.
Also, V1’s plate voltage seems low. I’m using a 7025 tube vs. the 12AX7, and the plates are running 93.1 and 90.4 VDC. They’re getting their power from Node 3 of the cap and it is outputting 296VDC going into those 270K resistors.
Close enough for Rock n' Roll.
(or even Chet Atkins tunes...)
(296v - 90v) / 270kΩ = 0.76mA ---> plenty enough current per triode
As Shooter says, a guitar's pickup probably delivers only ~100-150mV and if the stage gain is ~60 then the output signal is only 6-9v. There's plenty of headroom for an Input Stage.
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HBP--Thanks for the input. Looks like I've got the wiring done correctly and it does sound really clean--even through the cheap Fender Champion 20's speaker! I'm trying to decide what speaker to get for this amp now that it looks like it deserves something nice. I'm probably going to go with a 12" vs. 15", but which make, I don't know.
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I wanted to know what the Output wattage of the amp is at the speaker, so I connected an 8 ohm dummy load and injected a 150 mVrms at 1Khz signal. Using my scope I adjusted the sine wave until it started to flatten and then backed off for a clean wave form. At that point it is reading 11.3 Vrms. So if you square that number and divide by 8 ohms, you get right at 16 watts.
That's pretty far off from the 25 watts output its supposed to produce.
Yet your math shows 26 watts.
So what am I doing wrong?
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349v x 0.075A = about 26 watts ---> plenty hot enough for 807s or 6L6GBs
Tube's dissipation, in watts
So if you square that number and divide by 8 ohms, you get right at 16 watts.
Audio signal, in watts
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Ahhh, so then what is a "25 watt amp" being measured for, ie. tube dissipation or audio signal?
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so then what is a "25 watt amp" being measured for,
:laugh:
the buyer, salesmen just do what salesmen do
tube dissipation is for the tech/builder, so we know that $100 tube set won't let all the magic smoke out before the guitarist can get all giddy.
I build a 23W (measured at speaker), 5 guitarist said "NO WAY that's a 23w amp, it's gotta be closer to 40W" so I converted 23wrms to PP and they were happy :icon_biggrin:
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OK, now that's the answer, ie. multiply by 1.414 to get PP, and its what I wanted to see!
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Turn your 150mv signal up until the output *just* starts to visibly distort on a scope. Then measure your output voltage.
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I"ll give that a try.
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... 16 watts.
That's pretty far off from the 25 watts output it's supposed to produce. ...
Differences of Power can be expressed in Decibels: 10 x log (16w / 25w) = -1.94 dB
Differences of Loudness can be expressed in Decibels: +10dB is commonly perceived as "doubly-loud."
Speaker Sensitivity can be expressed in Decibels: dB SPL at 1 meter, for 1 watt of applied power
You will find it is pretty easy to find a speaker whose Sensitivity Rating is 2dB higher than some other speaker you might use. Which then means you can easily make up the loudness-difference of "16w instead of 25w" with a more-sensitive speaker.
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I was thinking that I must have built the amp incorrectly in some way, or a component isn’t up to spec, if the amp’s wattage is 40% lower than what it’s supposed to be.
Or maybe my method of measuring the output is wrong.
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maybe my method of measuring the output is wrong.
more like an "understanding" of the many ways in which Watt's gets used to equalize apples n oranges
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I was thinking that I must have built the amp incorrectly in some way, or a component isn’t up to spec, if the amp’s wattage is 40% lower than what it’s supposed to be.
Or maybe my method of measuring the output is wrong.
Again, you measured the output power with an input signal of 150mv. That's a completely valid thing to do; the answer wasn't wrong, it just wasn't what you actually wanted to measure. The amp was outputting 16W under those conditions.
But generally, when someone says "this is a 25W amp" they are either: (A) referring to the maximum clean output power, or (B) more interested in marketing than physics, as shooter alluded to.
I'm thinking your amp can put out more than 16W of clean output, but to measure that you need to turn it up until *just* before visible clipping. Even this is an inexact science, as harmonic distortion occurs well before the clipping becomes visible on a scope
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Sorry, I probably wasn't clear. The 150mRms input signal was with the Volume pot all the way down, and it raised up to 16VRms by my turning up the Volume pot, until at that point it showed beginning of distortion on the scope.
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oops.
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Calculate your power output using the peak voltage (15.9V, right?), then convert that to RMS...
Why bother? He already measured 11.3VRMS across an 8Ω load. That equates to 15.96WRMS. No conversion needed.
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Stuckey's correct.
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I was thinking that I must have built the amp incorrectly in some way, or a component isn’t up to spec, if the amp’s wattage is 40% lower than what it’s supposed to be.
Maybe they don't really make "25 watts." Maybe that was just a guess. Or maybe the output section makes 25w with a higher supply voltage (which then allows same-tube-current to create a larger voltage drop across the OT primary, and same-current x more-voltage-drop = more-power).
I never bothered measuring mine. It sounded good when I finished assembling it, and that was proof-enough for me.
If the amp seems to be working right otherwise then I recommend you stop "looking for problems," and just enjoy the amp.
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^^^^what he said
I 2nd the motion, the aye's have it, go plug-in n play
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Stuckey's correct.
Who is Stuckey? As much as I've helped you in the past, you should at least get my name right!
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Good advice!
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I was following a member's topic on his Princeton build which is having low output, and Sluckey's analysis that the Phase Inverter mod he did was the cause of the problem. As I understand it the grid voltage was too low in relation to the cathode voltage and created too big of a difference between the two, and limiting current. I apologize if I totally missed the explanation.
Anyway, that made me think about the voltage I'm showing on the cathodyne PI in the amp I built. Attached is a schematic of a Standel 25L15 that I found that is similar but not the same as my schematic, but the PI part of the circuit is 99% identical. So with that in mind, my question follows:
The PI tube is a 12AU7 and it goes to two 6L6GC output tubes. It is V3B in the circuit.
The V-3B's Voltages are as follows:
Plate: 243.6VDC
Grid: 61VDC
Cathode: 81.6VDC
I read Merlin's article on cathodyne phase inverters and, if I understood correctly, he notes that the more "equally loaded" or in balance the voltage of the grid and cathode are, then the more headroom and output power the amp will have.
So it seems to me that the difference in voltages that I have of 20VDC seems much too out of balance.
I don't know how to determine what those voltages should be, or if I've gone down a rabbit hole. Any input is appreciated.
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The grid circuit of an AC coupled cathodyne is very high impedance so you cannot accurately measure the grid voltage because your meter loads down that high impedance circuit. The 61v you measured is meaningless. To know what the actual grid voltage is connect your meter to the bottom of the grid leak resistor. Then you will see that the grid voltage is very close to the cathode voltage.
As I told you earlier, the actual voltage on the grid is not important. It's the voltage between the grid and cathode that is important. But you got to understand how to measure the grid voltage properly. Of course, you could always put one probe directly to the cathode and the other probe directly to the grid to get the real bias voltage.
There are three high impedance boot strap biased circuits that we commonly see in guitar amps. One is an AC coupled cathodyne. Another is an AC coupled cathode follower. And another is the LTP PI. You cannot accurately read the grid bias voltage of any of these three circuits simply by measuring the grid pin to chassis ground voltage. The reading will always be unbelievably low, just as you saw in your amp. If there was really a 20V difference between grid and cathode the tube would be hard cutoff and your amp would produce no sound.
Due to the inaccurate grid voltage readings of the above three boot strap biased circuits, I no longer list the grid voltage on my schematics. Instead, L list the voltage at the bottom side of the grid leak resistor. For example, look at these schematics...
https://sluckeyamps.com/dual_lite/dual_lite.pdf
https://sluckeyamps.com/phoenix/phoenix.pdf
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what he said^^^^
you can get a reasonable sense of "imbalance" by measuring the AC signal coming out, they should be ~~~~~~~ equal n 180 out of phase
the "max clean" amplitude can be used to determine if there's enough "clean drive" signal for the PA tubes to push them into compression.
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Sluckey and Shooter this information is very helpful. Thanks for taking the time to share your wisdom.
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Like HotBluePlates, I decided to use XLR plugs and sockets along with Belden type cable for the wiring needed to connect the separate PS chassis section to the Preamp chassis section. There are 2 sets of cables. One carries the signals from the output tubes, and the other carries Mains AC, rectified DC from the main filter caps, and 6.3 VAC for filaments. I built the cables myself rather than off the shelf units. They work fine.
So these cables connect to XLR sockets mounted on the 2 chassis. My question is whether the housing for these sockets need to be grounded in any way or is their connection to the chassis sufficient? The amp is dead quiet. If their mounting loosened from the chassis could that cause any noise, hiss, hum, crackle etc.?
Thanks.
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The amp is dead quiet.
so go play outside, when you go look'n for trouble, it has a nasty habit of showing up :icon_biggrin:
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I’ve noticed that this amp makes a Pop noise when the amp is turned off, unless the Volume pot is turned off before flipping the switch. The loudness of the pop seems to be dictated by the Volume pot’s setting. This amp doesn’t have a Standby switch. It doesn’t matter if the guitar is plugged in or not.
Any suggestions on how to remedy this?
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Any suggestions on how to remedy this?
Yes, turn the volume all the way down before you turn off the amp.
Your supposed to do that with any/all amps. Your not supposed to turn an amp on or off with the volume up. Standard practice.
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Your not supposed to turn an amp on or off with the volume up. Standard practice.
I've never heard that before.
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Your not supposed to turn an amp on or off with the volume up. Standard practice.
I've never heard that before.
It's to keep the speaker from thumping.
Just common sense.
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amp makes a Pop noise when the amp is turned off
try temp'n in a bleeder R on the B+ line
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Shooter what size?
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Shooter-I installed a 470K/3W resistor between the first node of each cap can, (ie. there is a 4 node can for the Power Supply and also one for the separate Preamp), and ground, but no difference. The pop is still there. But at least I have bleed resistors installed for the cap cans.
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Anti-pop snubber for power switch...
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Sluckey-I have a Vishay 400V .01uf film polyester cap that I got from Amplified Parts, but I don't know if that's a Class X. Would that work?
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Class X (or class Y) is important. Use this .047µF and a 470Ω resistor...
https://www.amplifiedparts.com/products/capacitor-kemet-r41t-series-polypropylene-safety-class-y2
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Thanks. It may be awhile before I have enough stuff to order that will exceed the shipping!
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Sluckey-I have a Vishay 400V .01uf film polyester cap that I got from Amplified Parts, but I don't know if that's a Class X. Would that work?
You could certainly use that cap as a temporary test but for reliability you should replace with a class X or Y ASAP.
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I'm able to get a 6 pack of them on Amazon for $5.00 and free shipping delivered by this Friday!
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So you are going to buy parts and do the work to install them, to 'fix' something that's normal, popping/thumping, knowing you could just turn the volume pot down 1st and that will stop it from popping/thumping when you turn the amp off?
:think1:
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I have 15 amps and I never touch the volume control when I switch the amps off. None of them pop when I turn the power switch off. Obviously some amps must pop or there would not be an anti-pop switch snubber circuit floating around the net. Even Robinette has this circuit on his site. Maybe I'm just lucky or have better building skills than some others.
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I have 15 amps and I never touch the volume control when I switch the amps off. None of them pop when I turn the power switch off. Obviously some amps must pop or there would not be an anti-pop switch snubber circuit floating around the net. Even Robinette has this circuit on his site. Maybe I'm just lucky or have better building skills than some others.
Long time ago but, I seem to remember, all the amps I played through/used in high school and in the clubs, mostly BF Fenders, always popped when you turned them off with the volume up.
If a guy turned off his amp or the PA and the speakers popped/thumped they'd tell the guy turn down the volume 1st and that wont happen. And if you went to a stereo store they'd tell you and do the same. Same for any PA systems and studio power amps at the music store.
My computer Altec Lansing SS stereo pops if I turn it on/off with the volume up. All though my Bose TV stereo system does not pop with the volume up when turned on/off. Bose probably put in a soft start up/shut down circuit to keep that from happening.
I always used a standby switch. So, turn on pop/thump was different, see below. And an amp without a standby but with a tube rectifier shouldn't thump at turn on, rectifier tubes heater needs to warm up before passing dcv. Tube amp with SS rectifier, still needs tube heaters to warm up. But full SS amps probably will because of instant power supply voltages at turn on and no tube heaters.
Merlin shows an anti-pop switch snubber circuit for amps that have the reservoir B+ filter cap before the standby switch. To keep the B+ dcv in rush down when you flip the stand by on and stops the speakers from thumping.
Go down to; Implementing a Standby Switch (The Least Bad Way) in the Cons list; "3. Can cause an unpleasant thump in the speaker when switching."
https://www.valvewizard.co.uk/standby.html (https://www.valvewizard.co.uk/standby.html)
And those Carling standby switches Fender used, after years of the contacts arcing at turn on because of the dcv on them, they would get burned and the contacts would 'bounce' when turned on causing the speakers to pop/thump on old amps.
I seem to remember these things being talked about here over the years and in different tube amp books.
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I think the solution here is just to never turn it off.
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I'm sorry to have started a ruckus!
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I'm sorry to have started a ruckus!
No ruckus. :icon_biggrin:
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Sluckey-your anti-pop snubber did the trick! NO MORE POP!!
Thanks so much.
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"If I put the fuse holder at the rear of the chassis it's awkward to get to, so that's why I wanted it up front"
Welcome to the world of logistics, by which I mean difficult choices. In amp building, real estate is everything, and the logistics of how and where to place everything can take many hours of planning and building - and rebuilding. Sometimes, I've had to rebuild an amp half a dozen different ways before I understood what I need to do to get it sounding how I wanted. The more I build, the better I get at understanding why things have to be a certain way. I've received great feedback over the years from forums, and EL34 World is as good as it gets, but it has always come down to hours spent at the workbench. It can be very frustrating sometimes, but also deeply rewarding when you learn something new because it will apply to everything you do going forward.
About the only tip I can offer, looking at your amp pic, is that you have a small chassis. Bigger is always better, especially when learning a new circuit or design. Close proximity of components often leads to electro-magnetic field issues and cross talk in the lead dressing. A small chassis means that it's easier to paint oneself into a corner. But you may already know all this. Hope the build goes well.
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Thanks-Yes, I completed the build without any real problems, but I did spend a lot of time working out the location. I'm really pleased with the sound. Nice and clean. One thing about this build is that the power supply is in one chassis and will sit on the floor of the cabinet, and the preamp is in another chassis and is at the top of the cabinet. So that separation works really well. The preamp is a much bigger chassis and I've got plenty of room to spare, ie. nothing is crowded.
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This amp has 2 shorting Switchcraft jacks. One is for the Main Volume, and the other is more like an Accessory Jack and it is configured so the signal from that jack will never be as loud as the signal from the Main Input jack.
Sometimes, for no apparent reason that I can find, I get crackling noise and if I fiddle with the plug that's in the jack it comes on and off and sometimes doesn't pass any signal at all.
I've done the tap test and no reaction.
So I'm wondering if I've either got a bad jack or if my wiring is incorrect--which I doubt.
If the amp is on and nothing is plugged into either of the input jacks, I can hear low level noise from the speaker.
But when I plug the guitar cable into the Main Input jack it goes dead quiet with no background noise, and it passes a clean signal.
So my question is whether there should be any low level noise coming through the speaker when nothing is plugged into the jack? I thought the shorting type jack sends everything to ground and it should be dead quiet. Seems like I shouldn't be hearing anything when nothing is plugged in.
Thoughts?
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I was reviewing the schematic and it shows a 2.2M resistor coming off the tip of the input jack and going to ground, and a lead also going from the tip to the grid of V1. There is no grid stopper resistor on the grid. Yet a layout I was provided shows that 2.2M connected between the coupling cap and the grid on one end of the resistor and the other end going to GND.
So maybe that's the issue. I have that resistor connected between the Tip and the GND on the Input jack, and perhaps it should be connected between the Grid and ground.
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HotBluePlates-When you built this amp, how did you wire the 2.2M and the 1.2M resistors coming off the Input jacks?
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I've drawn the part of the circuit with the 2.2M resistor in question, and attached the image.
So I'm trying to confirm where that resistor should be wired. I currently have it wired between the tip on the jack and the shield on the jack, with the switch and the shield lugs jumped together. Then the tip is wired to the cap, which in turn is wired to the grid on the 12AX7. A layout by someone else shows the resistor connecting to the grid pin on one end, and the other end of the resistor to a ground point.
The jack is a Switchcraft switched mono jack, I just didn't have a symbol for it in my DIYlayout program.
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The circuit in your drawing WILL NOT WORK! There are several simple fixes. Here are two...
1. Move the resistor to the right side of the cap so that it is connected directly to the tube grid.
2. Replace the cap with a 10K to 33K resistor. Mount the resistor close to the tube socket. This is my preferred fix.
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Thanks Sluckey.
So with option 1, the resistor connects the cap to the grid and there is no connection from the grid to GND? So it's not intended to be a grid leak resistor?
And so the tip of the jack would just be wired directly with the cap, then the resistor, then the grid?
I wonder why the schematic was drawn as is shown, but the layout is as you suggest except the layout shows one end of the resistor connected to the grid along with the cap, and then the other end of the resistor is connected to a GND.
Would the way that I have wired it based on the schematic be the cause of the intermittent unusual noises, etc. that I describe? Is that an impedance issue that's going on and the guitar's input cable is affecting it in some way?
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So with option 1, the resistor connects the cap to the grid and there is no connection from the grid to GND? So it's not intended to be a grid leak resistor?
THE TUBE MUST HAVE A GRID LEAK RESISTOR CONNECTED TO GROUND!
Would the way that I have wired it based on the schematic be the cause of the intermittent unusual noises, etc. that I describe? Is that an impedance issue that's going on and the guitar's input cable is affecting it in some way?
No and no. The actual amp cannot be wired like your simple drawing.
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Sluckey is the attached image what you are suggesting I do?
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we of the tech world live by a schematic, you have a mystery one, so it's REAL hard to help, you can cut n paste just the section you have questions on, nobodies gonna steal your work!
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Like HotBluePlates, I signed an NDA, so I'm legally bound not to publish it. I would otherwise be happy to share. But I appreciate everyone's help under the circumstances.
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You sure have a knack for taking a very minute detail (that you should already know) and stretching it into an endless discussion!
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I signed an NDA
...on a schematic with errors?? :icon_biggrin:
put the grid R in, plug in n play. assuming no other errors....
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HotBluePlates-When you built this amp, how did you wire the 2.2M ... resistors coming off the Input jacks?
Sluckey is the attached image what you are suggesting I do?
My amp has only 1 input jack, because I chose not to have the "Hide-a-Jack channel."
I wired my Input jack like this most recent schematic fragment: Tip --> 0.01µF --> Input Tube Grid, with 2.2MΩ to Ground.
I signed an NDA
...on a schematic with errors?? :icon_biggrin:
The schematic is error-free.
But Telephone Game introduces errors.
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HBP-Thanks. I guess I read the schematic incorrectly, or there is a nuance in schematics that I’m ignorant about.
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Possibly!! 😆😆
Maybe time to review Leo’s BF series, Dumble’s ODS (modified Leo amps) and Sluckeys amps.
You’ll definitely get a few good ideas and discover some “best practice” :laugh:
Regards
Mirek
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So I guess my description of the "noise" in my Reply 131 above was caused by the wiring error of not having the grid leak resistor installed in the correct location.
The first image of the schematic that I drew and posted is exactly how it is drawn. And it shows the 2.2M resistor connected to the tip of the jack and not between the cap and the grid. So if that's drawn correctly, I really don't know how I would have known to have installed the resistor at another location. I thought I was supposed to install components as they show their connections in the schematic, but I guess that's not always the case. But, we don't need to get into a discussion on that point.
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This continues to be the most bizarre discussion I’ve seen on this website
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So I guess my description of the "noise" in my Reply 131 above was caused by the wiring error of not having the grid leak resistor installed in the correct location.
Since you've changed it to the correct position, is the noise gone now? Capacitors block DC voltage, and the grid of your input tube needs a DC reference to ground to bias correctly.
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Yes, it appears to have solved the issue.
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HBP-Thanks. I guess I read the schematic incorrectly, or there is a nuance in schematics that I’m ignorant about.
Back in this post (https://el34world.com/Forum/index.php?topic=33394.msg372748#msg372748) you included a schematic from some source.
If you inspect it the connections are:
- Input Jack Tip connection
- Coupling cap
- Resistor to Ground
- Tube Grid
That pattern is consistent for both channels, and it reflects how we guided you to arrange these parts in the most recent discussion.
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HBP-The point I was trying to make is that the schematic that I got, and presumably the one you also got, with the NDA is drawn "incorrectly". Take a look at your's and you'll see what I mean-unless you got something different.
Since the amp seems to be working great now, I think this topic can be closed. Thanks for everyone's input.
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Since the amp seems to be working great now, I think this topic can be closed
the schematic that I got, and presumably the one you also got, with the NDA is drawn "incorrectly".
since you're now working from a "different" schematic, does that nullify the NDA?? :icon_biggrin: