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Heathkit A9-xx chassis

knockbill

knockbill

Lunatic Member
I have a Heathkit A9 chassis,,, a little research finds it to be an old integrated mono 6L6 PP design... Also have a few 6L6s and similar family tubes, and was thinking of trying a stereo SE amp using it... Restoring it would not be easy, as I don't have the original transformers...

I don't want to cut it up, if its valuable, but, think there were a lot of these made over the years... I have some PTs, as well as a couple OPTs that may work...

Think its a feasible project? Better use of this chassis? Any ideas are welcome...

Regards,
John
 
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All you have is the chassis and nothing else ? Think of it as a blank slate, I would say 80 percent of the value of a tube amp is in the transformers, with them
Gone you have a metal box, not a heathkit a9...
 
maxhifi said:
All you have is the chassis and nothing else ? Think of it as a blank slate, I would say 80 percent of the value of a tube amp is in the transformers, with them
Gone you have a metal box, not a heathkit a9...
Click to expand...

Its a A9xx chassis, that someone gave me... its stripped, cleaned and repainted,,, It came with new caps to rebuild it, and the pots, switches, sockets etc, but I guess they gave up on the rebuild, and sold the transformers...

Figured I could it to use it for a project, but the way the holes are laid out, it may not work anyway...

Just thinkin out loud...

Regards,
John
 
If the chassis has been nicely restored why not sell it on, with the caps, pots and switches, for another person to use in a monobloc restoration? I had one that I bought while on a holiday abroad and had to leave the 115V PT, chassis and old caps behind. The OT is good though, and I've used all the Heathkit bits elsewhere.
 
I'm getting the feeling that its worthless as an A9xx, without the transformers...
I used the new 715 Orange Drop caps, VC pot, and the original hum pot, it came with, to restore a pair of Heathkit pre amps...
I have the chassis left, and am thinking of building a 6L6 SE amp on it...
Actually, working on the PS as we speak!!

Regards,
John
 
Here's a pic of a preliminary layout on the A9 chassis... I've been working on a couple of PS ideas, but don't think I have a big enough transformer for the B+...
I'll start a thread, when I come up with a viable plan...

Regards,
John
 

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I'd try and move the power transformer away from the outputs as much as practical. Maybe up in the front corner where the yellow transformer is. That should let you route the power wiring away from the signal wiring. Looks like a square hole for an IEC type power connector is back there anyway. I'd also put the rectifier close to the power transformer for the same reason. Generally you want your small signal tubes (6sn7?) physically close to the output tubes just so you don't end up with long leads between stuff.
 
knockbill said:
Here's a pic of a preliminary layout on the A9 chassis... I've been working on a couple of PS ideas, but don't think I have a big enough transformer for the B+...
I'll start a thread, when I come up with a viable plan...

Regards,
John
Click to expand...

Your audio transformer on there looks a bit toasty...:smoke:
 
Yep, I like short wires,,, basically just laid it out to see how many holes I could use up, and see how it looked without cutting any new ones!!! Ideally, it should be as you said..
Its not looking like this chassis is going to work for this project,,, It's in pretty good shape, other than the square power hole, so I'll save it for a trade or ??? .... Want to build a A-9xx !!!!
It would be easier to start with an uncut chassis, and I have a couple boxes I can use...
I still need a B+ transformer, I think, but, I found a schematic that looks good...

Regards,
John
 
Here's another box I had,,, seems to lay out better, and I can put the ugly yellow trans under the hood... Need to find something for the hole in teh middle , tho...

Regards,
John
 

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gadget73 said:
can cap. Or a voltage regulator tube for regulated screens or something. Those just look cool.
Click to expand...

I have teh cans for teh A-9... but never liked teh look of em on top... funny you should mention gas regulators!! Have some 0C2s, and 0D3s,,, been looking at schems to use em,
Can't figure out how to put em in the amp circuit tho... any ideas?

Regards,
John
 
Dandy said:
Or place the power tubes one behind the other, and the smaller tubes further apart.
Click to expand...

Yep, possibly,, ideally, I'd like the driver components to reach the output tubes, and be equal distance apart,,, seems they have a lot shorter leads than they did in the old days,,,
I think looks cooler to see the tubes "head on", also...

Here's a modified schem I found in another thread,,, any ideas on adding the gas regulator? good middle hole filler, hope they aren't noisy!!!
Appreciate your input...

PS,, does the VC look right?

Regards,
John
 

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Last edited:
Re the VC, I wish I knew how to calculate the correct value. Something to do with the relationship between impedance and sensitivity. 220k is the value for the SCA-35 and also for the last 6BQ5 build I constructed from an auction bought PCB. Both are PP amps though. I use a 50k pot for a passive to control two monoblocs.

I hope gadget73 can advise on this and the regulator. So, I'm just an interested bystander at this point. :)
 
I think the VC is OK,,, I just replaced the 470K with a 500K pot,,, The gas regulators are too cool to stay in the boxes of tubes, nice purple glow... I'd like to see one front and center on this guy!!!

Gonna start building one channel, and see how it works...

Regards,
John
 
gas regulators have different output voltages, but I believe you can run them in series to add to the voltage you need. basically it ends up used like this

03017.png



You'd sub your B+ supply for the battery, but the idea is basically the same. The regulator tube just pins the voltage to some amount above ground, depending on the particular tube you're running. Its really just a precisely made neon lamp, if you've ever played with those. A modern equivalent is the zener diode. Its used in pretty much the same way, though they don't look nearly a cool.
 
Looking at the schem for the RCA PA/ theater amp... it uses 2 OC3s in series for 210V on the screens of the output tubes (6146A)... seems the primary of the plate transformer is in series with pins 3 & 7 of the gas tubes... That's just to open the B+ supply, if the gas tube is removed, correct?

So, I have to build the amp with the schematic I have, and measure the screen V, then use a regulator tube that will eliminate the screen circuit, and provide the right V...

How is the plate resistor calculated? The RCA amp uses a 10K 65W resistor!!
The amp I'm building uses 6L6 SE,,, Its got two resistors, and a cap taking the screen to ground, so, if I can use one of the regulator tubes I have, it looks like I can eliminate the cap/resistors, which would be good, cause I don't have em!!!

Any more thoughts are welcome...

Regards,
John
 
without looking at a datasheet, that sounds right. I know they do have a pair of pins internally connected so that the supply opens if the tube is removed. Don't know which they are off the top of my head though.

Resistor is calculated to drop the voltage from whatever the input is to the regulated voltage level. Current flow through the resistor is whatever ohm's law comes up with for the voltage drop, plus the current pulled through the stage. You often end up needing a fairly high wattage resistor if its dropping an appreciable amount, or dealing with a lot of current. Screens draw very little, so most of what you'd have to get rid of is the drop from the main B+ supply down to the screen voltage.

You still need the resistors in the chain. Output of the regulator tube is relatively clean, but it needs about an 0.1uf across it to filter out any hash generated internally.
 
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