Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: x44d80 on March 12, 2015, 10:57:09 pm

Title: Advise needed on a possible scratch build Hoffman AC30
Post by: x44d80 on March 12, 2015, 10:57:09 pm

I have a drawer full of leftover parts and would like to try a Hoffman AC30. The power trans I have is a 540/680v 150ma combo and the output is a smallish 40w 4kohm.  I want to put these on a homemade unpunched 5e8 chassis I have. The only reason I choose the AC30 is I have 8 el84 tubes I pirated from organ pulls with nothing to put them in.   I'm not smart enough to figure the math and if the transformers are up to this task. Before I order a board from Doug I'd like some thoughts on if the transformers might work or if I should think of a different circuit.

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: tubenit on March 13, 2015, 05:25:41 am

Here is a tube data cheat sheet that you can use as a starting place to figure this stuff out.

A pair of EL84 in push/pull may use 90 ma.   So, four would use 180 ma.   And I would want some margin of ma current.  So, I would not attempt this with less then 200 ma and would prefer 250 ma if available.

So, the answer is "no" from my perspective.  Your transformer would not have enough current available for the EL84's quad.

However, you could build an AC15 just fine with that trannie using the 270-270 taps.

with respect, Tubenit

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: x44d80 on March 13, 2015, 07:44:10 am

Thank you for the tube data sheet, it is very helpful. I will look into another circuit.

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: tubenit on March 13, 2015, 08:14:37 am

Considerations for EL84's could include:

HoSo56
18w Stout
AC15
Plexi 18w

With respect, Tubenit

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: MakerDP on March 13, 2015, 05:39:09 pm

If you like the idea of an AC30, I would also recommend the AC15 or a 15-watt version of a Matchless DC30. I haven't looked at Doug's AC30 board but I'm sure it would work with an AC15/DC15 as-is, with a few value changes here and there of course.

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: kagliostro on March 13, 2015, 05:52:11 pm

I agree, Doug's AC30 build with only a pair of power tubes is a good 15W

---

Give a look here, may be you find Sluckey's Dual Lite very interesting

http://home.comcast.net/~seluckey/amps/dual_lite/dual_lite.htm (http://home.comcast.net/~seluckey/amps/dual_lite/dual_lite.htm)

K

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: x44d80 on March 13, 2015, 07:51:56 pm

I thank you all for the suggestions.  I will look at all of them. I want to thank all of you on the forum and Doug for his website.  With your help I was able to build 4 amps with absolutely no previous experience.  Thanks a million.

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: drew on March 15, 2015, 08:15:58 pm

If you're still wanting to build an AC30, there's a thread over on ampgarage.com where a guy made one with an inexpensive Antek torroidal power transformer - link: http://ampgarage.com/forum/viewtopic.php?t=27318 (http://ampgarage.com/forum/viewtopic.php?t=27318)

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: x44d80 on March 26, 2015, 06:46:35 pm

Update. I found a tranny with 250ma, it's the size of a small car so I bent up a new larger chassis to hold this monster.  I have a question before I punch  the holes for the power tubes.  What is a good distance between the EL84 tubes for heat dissipation? I really searched this forum before I posted but didn't find a satisfactory answer. Thank You!

Title: Re: Advise needed on a possible scratch build Hoffman AC30
Post by: kagliostro on March 27, 2015, 04:34:02 am

On Vox AC30 power tubes holes are (center to center 40mm) so don't go under this measure


K