Recent Posts

21

Tube Amp Building - Tweaks - Repairs / Re: Vox AC15 based on Hoffman AC30 layout

« Last post by Big_Mike on July 25, 2026, 10:56:05 pm »

Thanks Steve, I see Mojotone also has an AC-15 output transformer with a 6k impedance.  Any preference for that OT over the 18W OT with an 8k impedance, or is there really no difference?

22

Tube Amp Building - Tweaks - Repairs / Re: Vox AC15 based on Hoffman AC30 layout

« Last post by sluckey on July 25, 2026, 09:15:28 pm »

The Mojotone 18W power transformer is an excellent choice. It will allow the use of a 5Y3 or EL84 rectifier. The Mojotone 18W output transformer is also an excellent choice. It has 4, 8, and 16Ω secondary taps.

23

Tube Amp Building - Tweaks - Repairs / Re: Vox AC15 based on Hoffman AC30 layout

« Last post by Big_Mike on July 25, 2026, 08:12:59 pm »

Steve, I am looking at buying the iron for this project.  For the power transformer, I was looking at the Mojotone 18W transformer.  Does that look like a good choice?

https://mojotone.com/products/british-style-18w-power-transformer-direct-replacement-for-the-marshall-r-18w?srsltid=AfmBOordnn3HUNxre5svmSYqJLcVqUe-kNmqNgf9RtgwTUTLqt8I4JbN

I decided that I am going to stick with a tube rectifier instead of going solid state.  Let me know what you think.

24

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by wsscott on July 25, 2026, 06:12:40 pm »

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

25

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by shooter on July 25, 2026, 04:19:50 pm »

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.

26

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by sluckey on July 25, 2026, 03:27:27 pm »

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

27

Tube Amp Building - Tweaks - Repairs / Re: Standel 25L15-Do you like it as is or would you like to change something

« Last post by wsscott on July 25, 2026, 02:36:12 pm »

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.

28

Tube Amp Building - Tweaks - Repairs / Re: Help with 1987 build

« Last post by ltkrepp on July 25, 2026, 11:30:54 am »

Thanks all. I got it. I in fact did have the socket wired incorrectly.

29

Tube Amp Building - Tweaks - Repairs / Re: Help with 1987 build

« Last post by sluckey on July 25, 2026, 10:15:37 am »

Measure the voltages on V1 pins 1, 2, 3, 6, 7, and 8. What have you?

30

Tube Amp Building - Tweaks - Repairs / Re: Help with 1987 build

« Last post by ltkrepp on July 25, 2026, 10:08:15 am »

Yes. Traced and put to a meter every socket pin connection to its destination on the board. All went to the correct places onthe board and had continuity.