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opinions of the heathkit w-A9C iron

I fixed the filament voltage problem I talked about. Are you saying that if the filament is not lit, it would explain the excessively high voltage on pin one and 6(the plates) of the 12AU7?

TIA
mark
 
I think you found the problem. No, I tied 4 and 9 together. I also think I may have cooked my PS tranny or it was bad when I got it I am getting about 26vac on each leg of my filament taps. I am too tired to do more tonight. Tomorrow I will isolate the filaments and see what voltage I get.

Also, r32 on the right channel is 0v, left is .67v.

Thanks for the help!!!!!!!!!!!!!!!!!!!!!
Mark
 
I see what you are getting at. Sometimes the obvious eludes me(most times actually). I'll try to fix the filament tomorrow.

I usually use the 6u8 variants and never have to worry about the 12v filament taps. This is why.......
 
Yeah the tube isn't conducting, or conducting very little. Check the size of the cathode resistor. Also check the filament voltage across either pin 4 or 5 to 9. Make sure both pins 4 and 5 are wired together-- for a 6.3 v supply.
 
One good thing, I really haven't ever seen a bad transformer put out more voltage,it's almost always less with a heat buildup
 
I isolated the two different filament supplies. Both are okay, about 6.6vac output. I also rewired the filaments on the 12au7. Connecting pins4/5 as one leg and the other to 9. Now I get music through it.

Even so, the 12au7 plates are still way too high. I'll re-measure and post, but something like a90 on pin one and 260 on pin 6. :thumbsdn:

Anymore insight as to the cause. Oh the cathode bias resistor has about 0 on the left channel and 1.2 on the right channel.
 
Jcricket said:
Even so, the 12au7 plates are still way too high. I'll re-measure and post, but something like a90 on pin one and 260 on pin 6.
Click to expand...

Do you mean 190V on pin 1?

I ran a load line for the fist stage of this amp as per the voltage readings and resistor sizes posted on the schematic. I was thinking it might be a misprint, but no, the schematic is smack dab on the money for the first stage.

So there are only a few things that can be wrong:
1. rail voltage not as expected
2. plate resistor not as expected
3. cathode resistor not as expected
4. filament voltage not as expected
5. DC on the grid where none is expected
6. bad tube

I think so far we've ruled out 1 and 4. Or have we ruled out #1? What is the exact rail voltage (voltage at the top side of the 100K resistor with respect to ground) while the amp is idling? Otherwise, that leaves 2, 3, 5, and 6 as possible suspects.

You might want to temporarily ground the grid (pin #2) of the first 12AU7 triode with an alligator clip straight to the chassis so you can guarantee it's at ground level while you measure the other items. Specifically measuring rail voltage, voltage drop across the 100K plate resistor, and voltage drop across the 820 ohm cathode resistor will give you what you need. There is probably something amiss in one of those three. Could be a bad tube. Ruling out all other possibilities would leave a bad tube as the only other option.
 
I'll do some measurement and get back to you. One thing I did note. When the filament taps were disconnected, they read right at the 6.6vac across each set of taps. Once I re-connected them to the filament circuit, it read about 27vac to ground. hmm...............

more to come
 
more info and measurements

pin 1 plate = 185.2v
pin 6 plate = 261v
pin 3 cathode resistor = 1.16
pin 8 cathode resistor = 66

on the rectifier output - 399v
b+ to the OPT's = 385v
front end drivers, right after the junction of c1 and r45(after the 3k resistor in the PS) = 328v

I forgot to re-measure the filament supply. I did try two other sets of tubes on the 12au7 - no difference.

Puzzled I be
me

Edit: Also I did ground the input, however, I did place a cap in series to block any dc that might come in.
 
I am going to go double check my filament circuits and be sure I didn't screw up something.

could it be that I am using 6l6gc tubes instead of 6l6 tubes. I doubt it, but what do you think?
 
I just checked filament voltage. Across the heaters I am getting about 6.54v, slightly high, but not awful. I think I also figures out the hum, I forgot to install the dual 100 ohm hum balance on the filament supply. The filaments ae not referenced to ground in any way. Could this be it?
 
I referenced them too ground through a pair of 150 ohm resistors. It is dead silent now. Even so, the plate voltages on the 12au7 are still way too high. Now the filament voltage to ground is 3.12 per lag. Perfect.
 
uh, yeah, it is. My brain is dead(or should be). where is the foot in mouth, head up you Bu** emoticons............

I'll try the 12au7's and see. I cannot imagine why I did not catch that. STUPID, STUPID, STUPID.


Just out of curiosity, what would happen if I ran a 12ax7 in here?

thanks,
mark
 
:whip:

Ha. no worries. I've done that a few times meeself.

Well the amp wouldn't blow up, as you found out, since it's running both stages below the max dissipation for a 12AX7. But gain is going to be way more than a 12AU7 which is going to throw off the sensitivity of the amp as well as the feedback factor will be off.

With a 12AX7 for the second stage phase splitter, its load line is really steep, and the bias point is clear up at 3 mA or so, so it's running hot for a 12AX7, and there's only about 40V swing allowed on one side before it runs into grid current, so it will probably offer quite a bit of distortion as compared to a 12AU7.

Probably best to stick with the 12AU7 that it was designed for.
 
I put in a 12au7(a real 12au7), and what do you know., It works wonderfully.

Thanks again to all who helped. In the next few days I will take a few more voltage measurements and post them. I'll also put it on my O-scope an see if I can get some pics of how it tests. Here are the voltages on the 6l6gc. The plate of the 6l6gc is seeing 362v, the cathode is at 21.3v, and the grid is at 309v.
 
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