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.