Heathkit A9-xx chassis

[Tom Bavis]

A bit more info on gas tubes: http://www.tubecad.com/june2000/page3.html

 

[knockbill]

Thanks Tom,,,
Very good read,,, I'll need to bookmark that, and use it for reference...

Regards,
John

 

[knockbill]

Got it...
Holds at 210-213VDC with no screen load, 330VDC B+ (seems low for a 6L6 amp, but its what the schem calls for)...
Now just the anxious wait for the mailman!!

Thanks for the help, so far,,, There will be more questions!!!

Regards,
John

 

[gadget73]

the beauty of a gas regulator is that it should hold stable output voltage no matter what the load is, provided the circuit is set up correctly. You should be able to wire that to the screens and not see it move at all.

 

[knockbill]

That's what I'm counting on,,, it moves a few volts with no load, other than a meter, and a rough draft PS running it,,,
Can't wait for the parts to get here!!!

Regards,
John

 

[knockbill]

I got one channel built on a board... Put a 100 ohmR in series with the screen, and get .16V across it at idle with a 6L6GC,,, and .24v across it with 7581 tube...
The regulators read 231V on the screens, which seems way too high... The 0C3s test good,, (30+ where 10 is good)...
Using a 10K series resistor, with only one channel for load... I tried 6.2K, and 5.6K Resistors, but the screens stay about 230V...
The amp sounds good, has a hum, which is probably due to all the test leads, going to two separate boards...

How big a problem is the screen regulation, as the drop across the 100 Ohm resistor is within the spec Kevin posted? I'll take V reading at all the pins, and see if anything else seems out of spec...

Regards,
John

 

[knockbill]

Tube readings seem reasonable,,, the regulated screen V does't hold rock steady, fluctuates between 211-216V or so, at idle,,, goes up and fluctuates with CDP input...

Using 4.7K series R, with one channel,,, I'll have to modify the PS, with both running...

Is this as close as these tubes operate? I replaced the unmatched pair, with a pair of NOS RCAs.... Just curious, as the amp sounds good, so far,,, and B+ is pretty stable...

Regards,
John

 

[kward]

At idle the voltage should be quite stable. Have the gas tubes been used in a different circuit before? That might cause the behavior you are seeing.

 

[knockbill]

I tried them in the RCA amp, and also in the different evolutions of the amp/PS I'm building now... they were about the same, (fluctuating Voltage)... never saw them dead steady at 210V, in either amp... I have a few pair 0C3s,,, none of them were exactly stable, all test good...
Right now, B+ is 334VDC, screen V drop thru the 100R is .14-.19VDC fluctuating,,, and regulator V is 213-223 fluctuating, at idle...
I tried a different 6L6, it's a metal tube, and tests good in the tester, but draws too much current, .25-.30, in this amp, so I took it out...

Series resistor is 3.04K ohms, to get the readings "in the ballpark", there is only one channel built...

Regards,
John

 

[knockbill]

Pretty sure the fluctuation problem is the PT,,, not big enough for this project, I'll look for another one...

Regards,
John

 

[kward]

John, so how did this turn out? Did you get it working?

 

[knockbill]

Hey Kevin...
Yep, somewhat,,, I have 4 PS connected to it, at the moment!!! Two doublers I built with stuff I had on hand,,,One to power the regulators, and one chassis,,, and another one to power the other chassis...B+ is too low tho, about 290V, calls for 330V, so I'm not running it much, as the current is a little high for the tubes...
There's DC heater supply on one side,,, with an old train transformer lighting the tubes on teh other! Believe it or not, its quiet,,, no hum or noise at all,,, can only hear teh CDP purring, between tracks...

Sounds very good, so far,, I expect it to be outstanding, when its all sorted out with a decent PT,,, hoping to find something useable at a Hamfest this weekend...

Thanks for asking,,, I'll post progress, as it happens...
Regards,
John

 

[knockbill]

Making a little more progress,,, got a PT that can power the B+, but its 6V coil can't handle the load... Using a filament trans with it,,, I have a couple bigger PTs, so may try to use one of them, to save some space on the chassis....
Really sounds good so far...

Regards,
John

 

[knockbill]

Here's the final layout on the A9 chassis,,, the brown paint threw me off so I made it gray... Using a SS PS, and loosing the 5U4 to gain space, this should work...Re cut the holes to spread the output tubes a little more... the big PT will go in the back left corner, and OPTs on the right side... Going to dress the line wires tight to the chassis corners, and see if it stays quiet... If not, the PT will go in teh front right, and output tx on either side in back...
Some fun, eh!!!!!!

Regards,
John

 

[kward]

Looking good. So you got the gas regulator tube supply figured out? As I recall you were looking for a beefier power transformer?

 

[knockbill]

Hey Kevin,
yep I did, ended up with 4.7K resistor to stabilize them...I found a half dozen PTs at a hamfest a couple weeks ago, started a different thread on em, and got them well sorted out... The one I'm going to use is big, but doesn't have a CT or a 5V tap, so SS full bridge seems the right way to go... there are two identical 6.3V coils on it, so it will have a separate one for each channel...
Just need a few bigger WW resistors for the latest PS version, and we'll see how it works out...
Thanks for your help, so far...

Regards,
John

 

[knockbill]

Getting closer...

Regards,
John

 

[knockbill]

I linked a couple other threads here, to try to keep this build together... Had to ask some specific questions, as it progressed...
Mouser order came today so I got a little more done on this amp...
Put the dropping resistors in the heatsink,,, Here's the Heat sink thread...
http://www.audiokarma.org/forums/showthread.php?t=608250

Still chasing the distortion problem, Here's the thread on it...
http://www.audiokarma.org/forums/showthread.php?t=606618

More info PT sizing, etc,,,
http://www.audiokarma.org/forums/showthread.php?t=601089

Regards,
John

 

[knockbill]

Getting closer...

Got it running with the 6L6GCs, and "test" OPTs, thru CDP no preamp, and the rebuilt Advents... Sounds good, music sounds natural... needs better/bigger OPTs, and then maybe adjust the FB to match them...

Regards,
John,

 

[knockbill]

Try it with what you ordered. May be fine.

If the gas tube pulses or does not light, add in parallel to the 6.8K resistor anything you've got in the neighborhood of 20K to 30K ohms (3 watt or larger). That will bring the total resistance down a bit and should address it, if it happens at all.

The regulators pulsed slightly on deep bass at high volume... I just dropped the resistor from 4.7k to 4k,,, its much better now... May go a little lower, but hardly any pulsing now...

Regards,
John