Standel 25L15-Do you like it as is or would you like to change something

[shooter]

^^^^what he said
 I 2nd the motion, the aye's have it, go plug-in n play

[sluckey]

Stuckey's correct.

Who is Stuckey? As much as I've helped you in the past, you should at least get my name right!

[wsscott]

Good advice!

[wsscott]

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.

[sluckey]

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

[shooter]

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.

[wsscott]

Sluckey and Shooter this information is very helpful. Thanks for taking the time to share your wisdom.

[wsscott]

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.

[shooter]

The amp is dead quiet.

so go play outside, when you go look'n for trouble, it has a nasty habit of showing up 

[wsscott]

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?

[Willabe]

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.

[sluckey]

Your not supposed to turn an amp on or off with the volume up. Standard practice.

I've never heard that before.

[Willabe]

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.

[shooter]

amp makes a Pop noise when the amp is turned off

try temp'n in a bleeder R on the B+ line

[wsscott]

Shooter what size?

[wsscott]

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.

[sluckey]

Anti-pop snubber for power switch...

[wsscott]

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?

[sluckey]

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

[wsscott]

Thanks.  It may be awhile before I have enough stuff to order that will exceed the shipping! 

[sluckey]

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.

[wsscott]

I'm able to get a 6 pack of them on Amazon for $5.00 and free shipping delivered by this Friday!

[Willabe]

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?

 

[sluckey]

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.

[Willabe]

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

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.

[passaloutre]

I think the solution here is just to never turn it off.

[wsscott]

I'm sorry to have started a ruckus!

[Willabe]

I'm sorry to have started a ruckus!

No ruckus.

[wsscott]

Sluckey-your anti-pop snubber did the trick!  NO MORE POP!!

Thanks so much.

[shaun]

"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.

[wsscott]

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.

[wsscott]

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?

[wsscott]

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.

[wsscott]

HotBluePlates-When you built this amp, how did you wire the 2.2M and the 1.2M resistors coming off the Input jacks?

[wsscott]

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.

[sluckey]

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.

[wsscott]

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?

[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?

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.

[wsscott]

Sluckey is the attached image what you are suggesting I do?

[shooter]

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!

[wsscott]

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.

[sluckey]

You sure have a knack for taking a very minute detail (that you should already know) and stretching it into an endless discussion!

[shooter]

I signed an NDA

...on a schematic with errors?? 




put the grid R in, plug in n play. assuming no other errors....

[HotBluePlates]

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?? 

The schematic is error-free.
But Telephone Game introduces errors.

[wsscott]

HBP-Thanks.  I guess I read the schematic incorrectly, or there is a nuance in schematics that I’m ignorant about.

[glass54]

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” 
Regards
Mirek

[wsscott]

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.

[passaloutre]

This continues to be the most bizarre discussion I’ve seen on this website

[AlNewman]

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.

[wsscott]

Yes, it appears to have solved the issue.