AC30/4 build

[Turbojunkie]

Nice!

I appreciate you documenting this, I've had something like the Hoffman AC30/4 build in mind for a while, so I'll be following this one closely

Noisy EF86's can be frustrating, but they sound so good that it's been worth a few extra precautions to me...try to use a head instead of a combo, try to avoid placing the amp on top of a vibrating speaker cab, use with silicone damping rings, and I've seen that some people even shrink wrap them...

I came across this shock-resistant socket mount while browsing the WeberVST site...anyone tried it?

https://tedweber.com/wp-content/uploads ... -sm9_5.jpg

[sluckey]

These IERC worked great in military equipment for vibration and heat dissipation. They work well on an EF86 too.

https://www.ebay.com/itm/4-IERC-TR6-602 ... 0008.m2219

[Colossal]

Nice!

I appreciate you documenting this, I've had something like the Hoffman AC30/4 build in mind for a while, so I'll be following this one closely.

I really like the Dr. Z Stangray and this is the rough design that I have in mind for this build. I am adding a few bells and whistles, as usual, and maybe even individual biasing for the EL84s. The Dr. Z PT runs 290-0-290 on the secondary, and from what Mark says, it runs pretty high plate and screen voltages. My PT is a Haddon clone and I have modeled its expected behavior. My voltages will run a little hotter because the PT primary is wound for 117V, like the original, and my mains voltage typically 123VAC, however, I am adding additional series resistance to each leg of the secondary (I'm not hot switching though and no standby switch), and then an additional 47R after the rectifier. After this, B+1 should be around 322VDC and 310VDC screens, or so my modeling says. I'm expecting about a 6-8V drop across the OT (also a Haddon clone), so plates and screens should be quite close. I'm planning for 37.5mA per plate and about 12W of dissipation, so the bias resistor is going to be somewhere around 80R per tube, like the original. I have not been able to confirm what bias voltage the AC30/4s ran at, however. The AC30/6 1960 schematic says 10V quiescent and 12.5V at 30W. I am also planning a series RC power supply, where the originals had 22k for the PI and preamp branching from the Screens node.

Speakers will be a pair of 8R Blues wired for 16R in a Vox style 2x12. Just can't go wrong with that.

Noisy EF86's can be frustrating, but they sound so good that it's been worth a few extra precautions to me...try to use a head instead of a combo, try to avoid placing the amp on top of a vibrating speaker cab, use with silicone damping rings, and I've seen that some people even shrink wrap them...

This will definitely be a head build. The EF86 will get a DIY shockmount as well as silicone ring livery. The mil-spec dampers Steve posted are pretty cool though.

[Mark]

Have you seen the shock mounts in Echoette tape echoes, they are the bomb. I wish someone made those again.

https://app.photobucket.com/u/mark1406_ ... 7047ee78c3

[drew]

Have you seen the shock mounts in Echoette tape echoes, they are the bomb. I wish someone made those again.

https://app.photobucket.com/u/mark1406_ ... 7047ee78c3

Your link produces a blank white page, at least on my iPad. Photobucket is garbage for web forums.

[Mark]

Here we go, I downloaded them.

FA2C5FA9-8E1A-4D99-B3B7-A2CC03D2EB5A.jpeg

B44702C3-1CBF-400B-B829-77810208325A.jpeg

CB3BA72A-6ADB-4E28-951C-400CA87B90A7.jpeg

[jazrok]

I've got a Vox AC30 transformer set and have been wanting to build a single channel EF86-based AC30/4 type amp for while. It's a neat amp and has a unique sound. There are some noteworthy modern derivatives such as the Dr. Z Stangray (EF86) and DB4 (5879) (both amps now discontinued) and of course numerous Matchless amps. Here is the basic schematic:

I don't suppose you have a layout for this as well?

[Colossal]

I've got a Vox AC30 transformer set and have been wanting to build a single channel EF86-based AC30/4 type amp for while. It's a neat amp and has a unique sound. There are some noteworthy modern derivatives such as the Dr. Z Stangray (EF86) and DB4 (5879) (both amps now discontinued) and of course numerous Matchless amps. Here is the basic schematic:

I don't suppose you have a layout for this as well?

I don't at the moment, but if I build it on a turretboard, it will be quite close to an 18W Lite IIb layout. Very straightforward. The amp will be intentionally simple; just EF86 preamp, loss-loss Tone control and Volume, phase inverter, and power amp.

[ChopSauce]

I went back to this topic as I envision a significant makeover for my AC4 build, whose sound somewhat deceives me.

Did you made any progress?

EDIT: you already gave all the information I was asking for, about the Carmen Ghia style tone control.

About less theoretical points, now:

- what about cutting a support into soft rubber (layers of bicycle tubes, for example) to insert between the chassis and the socket for the EF86?

Thanks for sharing these many suggestion for improvement, anyway!

[chief mushroom cloud]

https://www.ebay.co.uk/itm/B9A-MCMURDO- ... SwrXhf40V1

[ChopSauce]

Much thicker, so...

Thanks for the link!

[TUBEDUDE]

Sorbothane sheet is available at McMaster Carr in wide ranges of thickness and softness for those of a DYI bent that would like virgin sockets.

[brewdude]

https://www.ebay.co.uk/itm/B9A-MCMURDO- ... SwrXhf40V1

I just purchased a couple of these sockets, although from a different listing. I found some that are new unused.

Thanks for posting.

[jazzbass]

Hi everyone,

I am reading your comments / diagrams / observations on the VOX AC30-4 project with great interest.
It's an amplifier that I would have liked to make but .....

I ask if someone who has finished it can insert photographs and demos specifying what supply voltages they have chosen, what components, if they have made a head or a combo and with which speakers etc.

Regarding the EF86 pentode I have seen some videos of a manufacturer who is no longer there in which he suggested to use a piece of rubber as a vibration damper, fix it to a hole in the chassis larger than those intended for the preamp valves, in which to make a hole to fix the valve base. I think his name was Gabriel ...

I made, many years ago, a Matchless (Muchle$$) from a Ceriatone kit that I really like and it's my main amplifier but the VOX AC30-4 has always fascinated me.

Thank you

Franco