a little OT, but I need a pointer from guys that know what they are doing

[Tom Mobley]

So, out of the clear blue, it turns out a car buddy of mine I've known since 1969 has been sneaking out to Karaoke events and really enjoying himself.  I had no idea.

 

anyway, now I discover this guy, who is as talented a metalsmith as you'll ever see, is about half-done building himself a cigarbox guitar. (!)  outtasite.  anyway, he asked me if I know anything about low-power amps.  (!!!!)  Why yes, I do know a couple things about them.  So now, I need to quickly build some kind of small tube amp.  And I mean quickly.  I have in hand some iron I bought from Heyboer back when I was into recreating an HF-81 type deal.  There's a power transformer that Heyboer souped up a little for me and 2 replica output trannies.   I have a couple NOS 6CA4 rectifier tubes for that deal and a handful of EL84 Russians I got from Craig many years ago.  Plus a box of misc 12AX7.

 

I'm thinking a circuit heavily modeled after/ripped off from the HF-81.   I could use a Heresy woofer or the Industrial k24 version or some aftermarket equivilant. There was a thread recently about building a Super Heresy that mentioned several candidates.  I'm having trouble getting enthused about cannibalizing my beloved '77 Heresys I bought new back in 78.  I could do something temporary though.

 

What saith the group?  Am I barking at the wrong moon or within reasonable striking distance?

 

I've got a detailed blueprint of the HF-81 around here somewhere.....

[Tom Mobley]

hah,  found one.

 

anybody see anything wrong with this?

 

 

[Marvel]

Tom,

Ask one of the mods to move this to the tube amp section. There are a few builders on there who could help and probably get a quicker response.

 

Bruce

[jacksonbart]

Looks confusing, but otherwise O.K.

[dtel]

I asked over in the tube section for them to look here to help out. I didn't want to just move it on Tom, hope it helps.

[Tom Mobley]

dtel, thanks for being considerate.  PLease go ahead and move it if that would be good.  It's been so long since I've been regularly I didn't even realize there was a tube amp section.

[dtel]

Ok no problem, probably should have done that at first, good luck.

[tube fanatic]

Attached is the schematic for the HF-81.  Not sure about the amp depicted in the schematic you posted above.  The former is not going to be an easy amp to clone (even if it's only one channel) by any means.  Layout and lead dress will be critical.  Also, why does your friend need 14 or 15 watts for karaoke if he's driving a Heresy?  What is the output voltage of his pickup or pedal?  Does the amp need to have sufficient gain for a mike as well?  What are the power xfmr specs and  what is the impedance of the opt primary?  

 

Maynard

 

 

hfe_eico_hf-81_schematic (1).pdf

[mike stehr]

2 hours ago, tube fanatic said:

Not sure about the amp depicted in the schematic you posted above.

I'm thinking it's the Eico HF-14. Which was a mono power amplifier using the same power amplifier circuit as the HF-81 integrated amplifier.

A Champ clone of some sort would be a option...SE 6V6...nice and simple.

I built a homebrew geetar amp of a Champ clone using a 6AQ5, scrounged from console stereo parts.

[Tom Mobley]

yes, that's the schematic for an HF-14.  I have a power trans and 2 output trans for an HF-81 deal I was working on back around the turn of the century.  I also have NOS EZ-81, a bunch of misc 12AX-7 and some Russian 7189 I got from Craig back then.  I was thinking to finally find a use for these parts in a 12-14 or so watt amp driving a K-22 or K-24 or something smaller if I can find a good candidate.  I guess those guys playing cigar box guitars and canjos use pretty small stuff.  I'm used to KHorns so this stuff seems pretty small to me.

 

This amp really looks pretty straightforward to me.  I have a factory built HF-81 to look at if things get weird.

 

They use "inexpensive" piezo pickups.  $10 - 15 level stuff.  I think most of them plug into a DI or direct box to get the level up some.

 

anybody know if there's a 8" driver that's similar in quality to K-22 or 24?

 

this is great, the cigar box guitar manifesto:

 

http://www.cigarboxguitar.com/the-cigar-box-guitar-manifesto/

 

why not?  I'm going to have to admit (Real Soon Now) that I'm thinking about doing this myself.  There's not likely enough years left in me to be a classically trained bassist like my son, but maybe I could have some fun.

 

 

[audiowize]

That amp will be short on gain for guitar use, and the input impedance might be a bit low.  I would put another 12AX7 stage ahead of the first one, with a 1M pot in front of it. You'll need another 10K resistor and 22uF capacitor to decouple this 12AX7 from the next one.

 

Make the 22K feedback resistor into a 50K pot in series with a 10K resistor to give you variable distortion/gain/damping.

 

The schematic above is going to have a hell of a time producing much (if any) class B power.  This might be desirable tonally, or it might not.  Be prepared to make a voltage tripler/quadrupler off the 6.3V winding to go to fixed bias if necessary.  OTOH, if this sound is something you like, put an L-pad between the amp and speaker so that you can get all that yummy crossover distortion at any volume level.

 

There still isn't a lot of room for tone controls, but you can kind of graft that in along with the additional input stage that I mentioned above.

[Marvel]

Guitar amps usually have a rising output response to compensate for the high freq. rolloff off most passive pickups, too. It can make for a pretty mellow sound with a regular hifi amp.

 

Bruce