Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big_Mike on July 14, 2026, 06:09:48 pm

Title: Vox AC15 based on Hoffman AC30 layout
Post by: Big_Mike on July 14, 2026, 06:09:48 pm: Hi guys,

I am gathering materials to build an AC15 from a Hoffman AC30 layout. I get that you simply remove two of the output tubes and increase the cathode resistor, but I would like some help modifying the Hoffman layout as follows:

1. Change from tube rectification to solid state rectification.

2. Add in a Lar-Mar PPIMV.

Could anyone help me with modification of the Hoffman layout? Thanks.
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: passaloutre on July 14, 2026, 09:05:46 pm: Neither of those requires a change to the layout. You can put a rectifier on a couple terminal strips, and the LarMar is built on the pot.
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: Big_Mike on July 14, 2026, 09:31:07 pm: I guess what I am trying to figure out is how to incorporate diodes for the solid state rectifier on the board layout. I think I figured out the wiring points for the Lar/Mar PPIMV. Could anyone help me with a turret board layout diagram for a solid state rectifier?
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: sluckey on July 15, 2026, 09:24:58 am: If I were using Hoffman's board I would just replace the two 100Ω filament resistors with two 1N5408 diodes and make a couple minor wiring changes. Like this...
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: Big_Mike on July 16, 2026, 10:25:22 pm: Quote from: sluckey on July 15, 2026, 09:24:58 am
If I were using Hoffman's board I would just replace the two 100Ω filament resistors with two 1N5408 diodes and make a couple minor wiring changes. Like this...

Thanks Sluckey, much appreciated!
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: Big_Mike on July 23, 2026, 04:28:54 pm: Sluckey, to install a Lar/Mar PPIMV, I assume you remove the two 220K resistors after the phase inverter and install a dual gang 250K pot? Thanks!

Mike
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: sluckey on July 23, 2026, 05:26:43 pm: Yes . Also put a 2.2m between wiper and ground lug of each pot for safety. If you will also have a cut pot then look at page 3 of this pdf to see a neat way to install the mv...

https://sluckeyamps.com/dual_lite/dual_lite.pdf
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: Big_Mike on July 23, 2026, 06:25:04 pm: Quote from: sluckey on July 23, 2026, 05:26:43 pm
Yes . Also put a 2.2m between wiper and ground lug of each pot for safety. If you will also have a cut pot then look at page 3 of this pdf to see a neat way to install the mv...
https://sluckeyamps.com/dual_lite/dual_lite.pdf

Cool, I was wondering how to incorporate a cut pot with a Lar/Mar master and plan to have it with this amp. Thanks for the info!
Title: Re: Vox AC15 based on Hoffman AC30 layout
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 (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.
Title: Re: Vox AC15 based on Hoffman AC30 layout
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.
Title: Re: Vox AC15 based on Hoffman AC30 layout
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?
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: sluckey on July 26, 2026, 05:53:20 am: No preference. I've always just used an 8K OT.
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: Big_Mike on August 12, 2026, 04:06:30 pm: Steve,

One thing I realized after getting a Marshall 18 watt chassis is that the octal tube socket is directly underneath the IEC mains input. I waa going to use an octal tube rectifier, but am thinking now of using an EZ81 9 pin tube rectifier, as that tube socket is away from AC I put.

Question - if I use an EZ81 tube rectifier, I don't need a separate 5 volt secondary on the power transformer, is that correct? I can just use the standard filament wiring, is that right?
Title: Re: Vox AC15 based on Hoffman AC30 layout
Post by: sluckey on August 12, 2026, 04:41:32 pm: Quote from: Big_Mike on August 12, 2026, 04:06:30 pm
Question - if I use an EZ81 tube rectifier, I don't need a separate 5 volt secondary on the power transformer, is that correct? I can just use the standard filament wiring, is that right?
Right and right. You can connect the EZ81 filaments to the same filament winding that is powering all the other tubes in that amp as long as you don't exceed the current rating of the winding. However, many people prefer to have the rectifier filament connected to a separate dedicated winding. The mojo PT you linked in reply #8 has a special dedicated filament winding. It is a 6.3V winding but it has a 5v tap, so you can use either a 5Y3 or an EZ81. See attached pic...