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Old 2nd April 2008, 01:48 AM   #11
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None of the voltages you measured look out of place to me. Maybe one of our cleverer readers can spot something.

Your EL84 output tubes are biased differently from the schematic shown for the KN734. While the KN734 is predominately "fixed bias", yours is purely cathode biased (also known as self bias, or auto bias). See the "-20V SOURCE" on the lower right corner of the KN734 schematic? This negative supply is fed onto the grids of the output tubes. This keeps them lower than the cathodes and biases the tubes.

Your amp apparently does not have such a fixed bias supply. Instead, the cathodes are kept at a positive voltage (above the grids) by means of a resistor between cathode and ground. It is almost certainly the 100 ohm resistor you spotted attached to one of the caps. Most likely, that 100 ohm resistor is connected to all four EL84 cathodes. It is common practice to "bypass" such a cathode resistor with a capacitor. The bypass capacitor allows AC signals through the tube to be shorted to ground. The gain of the tube will be lower without the bypass cap. It may also affect frequency response of the circuit (can't remember). Look here for an example of four EL84 output tubes sharing a common cathode resistor (R13) which is bypassed with a cap (C80):
http://www.geocities.com/vintageaudio/st35a.jpg

I'm concerned that you say the grid (pin 2) of one of the EL84 isn't connected to anything. I'm thinking pins 1 and 2 are internally connected on the tube. Perhaps pin 1 is wired up in a manner similar to pin 2 on the other three tubes?

Now I really wish I had a schematic for your exact model of amp. I'm trying to guess how it's wired up based on the voltages you are seeing. Out of the four 12AX7 tubes you've got, I'm guessing that one half of one tube is being used as an audio frequency amplifier (call it an AF amp, gain stage, or preamp) and the other half of the same tube is getting used as a cathodyne phase splitter... or is it a paraphase splitter? You'll need this arrangement for each side (left and right), so that takes care of two of the tubes. If they did it this way on your amp, it would be something similar to what you see on V3 and V8 of the KN734.

The other two 12AX7 are probably used for the phono preamp or for providing additional gain needed to overcome insertion losses from the bass and treble controls.

I'll keep thinking on this one. Don't give up hope yet. I'm sure the solution will be fairly simple, once it is found. Do you have access to a digital camera? A couple of closeup photos of the wiring underneath the chassis, as well as a photo of the layout from the top might be helpful.
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Old 2nd April 2008, 02:04 AM   #12
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I'm not sure if this helps, or just further confuses the issue. Regardless, here's another schematic for you to stare at and think about. It's another push/pull EL84 design. An ECC83 tube is the same as a 12AX7.

http://www.schematics.ca/modules/PDd...cid=68&lid=487
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Old 2nd April 2008, 02:58 AM   #13
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I'm staring at your voltages, and assuming the 12AX7 you've numbered #1 and #2 must be the AF amp and phase splitter. I say this because they're the ones with the high(er) voltages on them.

Can you check if pins 1 and 3 (plate and cathode) of these 12AX7 are connected to the EL84 output tubes? There will almost certainly be a capacitor in series between them.

If so, I'm trying to figure out why you are reading 20 volts on the grid of these tubes. If the circuit is anything like V3/V8 in the KN734, or V4/V7 in the A230, I'd think that grid should be a lot closer to zero volts (or slightly negative).
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Old 2nd April 2008, 01:34 PM   #14
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I need to learn to read the schematics a little more carefully, or at least not look at them too late at night. The PI (phase inverter) shown in both the KN734 schematic and the A230 schematic is a cathodyne. The little square block next to the indicated voltage on the grid refers to a note which (unfortunately) you can't see on the KN734 drawing. Basically, it says the -0.7 volts is measured with respect to the cathode, not ground.

I'm reasonably sure you've got tube #1 for one channel (left or right) and tube #2 for the other. The first triode in that tube (pins 1, 2, 3) is the cathodyne PI. The second triode (pins 6, 7, 8) are the AF amp. The question now is why is the grid (pin 2) 40 volts lower than the cathode (pin 3)? It should be less than one volt lower, not 40.

See if you can use the KN734 schematic as a template, and try to sketch out the wiring around tube #1 on your amp. If you can make out the color codes, read the resistor values of your amp and mark up the schematic. Hopefully, there should be some similarity between your tube #1 and V3 on the KN734.
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Old 2nd April 2008, 02:24 PM   #15
kirk57 is offline kirk57  United States
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Ty-

Once again I thank you for spending time helping me with this!

I will take your points in order:

>Most likely, that 100 ohm resistor is connected to all four EL84 cathodes

yes, it looks like that goes to pin 3(cathode) on all of the EL84.
Those are the yellow wires in the pics. Is it possible the bypass capacitor is the 50uf 50v cap that's in the can with the three larger ones?

> I'm concerned that you say the grid (pin 2) of one of the EL84 >isn't connected to anything. I'm thinking pins 1 and 2 are >internally connected on the tube. Perhaps pin 1 is wired up in a > manner similar to pin 2 on the other three tubes?

Yes, that looks to be the case. Hopefully you can confirm from the pics. You must have a lot of experience with these circuits
(or a REALLY good guesser!)

Thanks for the HK230 schematic. Is this one closer to the KN928?

> Can you check if pins 1 and 3 (plate and cathode) of these >12AX7 are connected to the EL84 output tubes? There will >almost certainly be a capacitor in series between them.

Yes they are connected to the grid of the EL84 (pin2 on three of them, pin 1 on the other one) via a .047 ufd 600w caps (the large brown ones in the pics)

One thing I did find was there is a 47K resistor connected to
the 16ohm speaker tap that is broken. I have never used the 16 ohm tap so I don't know if this matters, still should be replaced.
BTW what power rating would these resistors be?
I have a lot of 1/4 watt resistors but those would be too small I'd think....

I hope the jpg is OK, file size is a constraint.

Kirk
Attached Images
File Type: jpg el84s 1-4 with 6ca4 upper left small.jpg (60.4 KB, 499 views)
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Old 2nd April 2008, 02:40 PM   #16
kirk57 is offline kirk57  United States
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OK I found a better way with the jpegs....
this is the top view
Attached Images
File Type: jpg top view small.jpg (75.3 KB, 498 views)
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Old 2nd April 2008, 02:45 PM   #17
kirk57 is offline kirk57  United States
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A larger view of the EL84s and rectifier
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File Type: jpg el84s 1-4 with 6ca4 upper left small.jpg (91.8 KB, 488 views)
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Old 2nd April 2008, 02:58 PM   #18
kirk57 is offline kirk57  United States
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>See if you can use the KN734 schematic as a template, and try >to sketch out the wiring around tube #1 on your amp. If you can >make out the color codes, read the resistor values of your amp >and mark up the schematic. Hopefully, there should be some >similarity between your tube #1 and V3 on the KN734.

OK I'll give this a shot. Here is a pic of that tube.
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File Type: jpg 12ax7 1 small.jpg (99.5 KB, 466 views)
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Old 2nd April 2008, 03:02 PM   #19
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> Once again I thank you for spending time helping me with this!

No problem. I've had a peculiar fascination with EL84, so I've collected a bunch of information on them.

>yes, it looks like that goes to pin 3(cathode) on all of the EL84.
>Those are the yellow wires in the pics. Is it possible the bypass
>capacitor is the 50uf 50v cap that's in the can with the three larger
>ones?

The 50 volt cap is definitely the cathode bypass. I thought last time you said it was 10uF, which sounded too small. 50uF is a reasonable size. This cap shouldn't ever see more than about 10~15 volts, so the 50v rating is just fine. Just watch out that this cap doesn't ever go leaking too much. If it shorts to ground, the output tubes won't have enough bias on them and they'll conduct too much. You might see the plates on the EL84 start to glow cherry red if this happens.

>Yes, that looks to be the case. Hopefully you can confirm from the
>pics. You must have a lot of experience with these circuits
>(or a REALLY good guesser!)

Mostly a good guesser. I wish I had more experience with these designs, but I was born after the golden age of tubes. It's that perverse fascination I mentioned above that drives me to learn about them. You can figure out an awful lot of stuff with the help of guys from the forums and Google. It doesn't hurt that a lot of these amps seem to share a very similar design.

> Thanks for the HK230 schematic. Is this one closer to the KN928?

It's hard to say. I still don't know exactly how the KN928 is wired. I know the output stage is cathode biased (like the A230) and not fixed biased (like the KN734). I expect all three of these amps have similar AF/PI designs, utilizing a single 12AX7. I know the A230 uses 12AU7 tubes in the phono and tone control sections, while your amp has no 12AU7. So, no, it still isn't an exact match for yours.

>One thing I did find was there is a 47K resistor connected to
>the 16ohm speaker tap that is broken. I have never used the 16
>ohm tap so I don't know if this matters, still should be replaced.
>BTW what power rating would these resistors be?
>I have a lot of 1/4 watt resistors but those would be too small I'd
>think....

Could a big problem there. I'm thinking the 47K resistor you are looking at is connected more or less like R55 on the KN734 circuit. You'll also find it in the A230 design at R22. That's a negative feedback (NFB) loop. It takes a sample of the signal from the output of the amp, and feeds it back into the input out of phase. It reduces distortion, lowers the overall gain of the amp, and dramatically improves the output impedance (and damping factor) of the amp. Triode designs can get away with little or no NFB, but in pentode amps it is almost a necessity.

Worst case would be if that resistor is a dead short, but I'd expect the amp would become an oscillator, howling and squealing horribly. If it went open circuit, you should have a lot more gain (louder) but lousy bass response.

>I hope the jpg is OK, file size is a constraint.

It's tough to make anything out from the tiny picture. Try hosting the original photo at a site like Photobucket.com. Make an account, click on the links to upload your pictures, check the box under your uploaded photo, then press the button at the bottom labelled "Generate HTML and IMG code". Then copy the link in the box titled "IMG clickable thumbnails for message boards - recommended" and paste it into your reply.
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Old 2nd April 2008, 04:36 PM   #20
kirk57 is offline kirk57  United States
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Ty-

OK I've uploaded the pics to Photobucket:

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.


A couple of things: I was right the first time, the bypass cap is 10uf 50v, not 50uf.

If I replace the 47k resistor, what power rating should I get?
Is quality an issue here? (i.e. metal film vs carbon vs unobtainium
$50 resistors from the back of the Absolute Sound?)

Looking at the KN-734, I see what looks like 2 12ax7s, but that's actually one isn't it? The diagram shows each one as a triode,
but this tube is a dual triode as I understand. Am I reading this right?

I'm gonna owe you a case of beer (assuming you like beer that is)

Thanks

Kirk
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