First Build Problems- KLD kit Build

[imgumby001]

Hello folks,

Just completed my first build. It’s a kit from the Chinese company KLD. It’s supposed to be a JCM 800 circuit, or their idea of what one would look like of it had a pcb instead of an eyelet/turret board. First off, it sounds terrible, especially with any kind of gain added. That being said, it’s relatively quiet with the gain and volume down. No buzz or hiss to speak of. Secondly, even with my bias on the 6L6’s set to -43mA at full gain and moderate volumes one of the power tubes red plates!! The lead channel has all the crunch gain, and there’s none on the normal channel, and my presence knob does absolutely nothing. I’ve added an extra ground from the pcb’s ground point for the output Jack’s to chassis ground, and was thinking of adding another from the pots/input jacks as well. I’ve checked all caps, diodes, and resistors for correct values, polarity, and the like, and stabbed every component with a stick I can find. I’m at a total loss. Please help me if you can. I will happily measure any voltage or value you think would be pertinent. Thanks all, have a wonderful day!!

[acheld]

Lots of very talented folks on this forum, but will need some basic information about what exact model and what you have done.

Best if you could post a schematic of your build and some photos.

The general mantra around here is that if your build was wired correctly, it would work correctly -- the vast majority of non-working builds seem to fall into this category.    So check your wiring against the schematic.  And again.   Sometimes it takes a few times to get it right (you can ask how I know this).

[shooter]

even with my bias on the 6L6’s set to -43mA at full gain and moderate volumes one of the power tubes red plates!!   

negative current shouldn't red-plate 


swap tubes, does it follow tube?
add 1 ohm resistors from cathode to ground so you can get an accurate current measurement, -43vdc isn't very good for calculating.

[imgumby001]

Sorry guys,
I’m having trouble posting photos.

[imgumby001]

Attempting the schematic...

https://i.imgur.com/XmYqIZp.jpg

Here’s a link to my Imgur post. I can’t seem to get the pics or schematic to load. They’re too big apparently.

[imgumby001]

Ok, so that was a fail. I’m so sorry, I’m just not very good at the posting pictures thing. Here’s some links...

https://i.imgur.com/sCayo0n.jpg

https://i.imgur.com/T7XleNT.jpg

https://i.imgur.com/UUlDRYH.jpg

https://i.imgur.com/AFB5qLH.jpg

[imgumby001]

So, the red Plating stays at the socket, but the other socket is on the verge. You can see the blue glow of electrons colliding in the tube.

[shooter]

you have to scale your pic's to <1M to post.

Restrictions: 4 per post, maximum total size 2048KB, maximum individual size 1024KB

blue isn't a good indicator, red though is

[PRR]

How are you measuring this "-43mA"?

[shooter]

thanks!


I see the 1 ohmer's are built in already

[imgumby001]

Pin 5 on the 6l6 to ground.

[shooter]

use pin 8 to ground, the measured volts will equal tube current, ohm say so 

[imgumby001]

Yes sir! Immediately!

[imgumby001]

Pin 8 on the 6L6’s reads near 0.

[imgumby001]

Here’s something I just found, and it seems wrong. I’m measuring the AC2/6.3v solder point where the 6.3v secondary mounts to the board and I’m getting less than 4v ac. On the AC1/6.3v secondary solder point I get over 10v. That’s not right, is it?

[shooter]

Missed the AC
what do you have on F1 F2, Vac to ground where i circled

[imgumby001]

14vac.

[imgumby001]

Sorry, read that wrong. 14vac was the measurement between the two. F1 measures less than 4vac, and F2 is 10vac.

[2deaf]

Schematic does not agree with Layout as far as heaters go.  I wouldn't put any tubes in until heater circuit is resolved.

[imgumby001]

When I pulled the secondaries off the board for diagnosis they all measured correct. That being said, the original 6.3v leads were too short from the factory to reach their solder points on the pcb. I had to lengthen them to reach. Is it possible the extra length is causing resistance?

[acheld]

No.  Not unless the joint is a fail -- and seems unlikely.

[imgumby001]

So I pulled the 6.3v secondaries again, just to be triple sure if my readings, and sure enough! When they’re pulled from the board they read 7.5v ea. But connect them to the board and v1 reads 3.8 and v2 reads 11! I pulled both power and preamp tubes and remeasured with no change found. So, why is my voltage at those points changing so far off what the schematic lists they should be? Is it the power trans? If it is, why are my unconnected primaries reading near what they should?

[AmberB]

According to the schematic, the heater winding looks like a center tap heater winding.  Was there 3 wires coming out of the power transformer for the heaters? 
The imbalance of the heater voltage tells me that the heaters are not properly referenced to ground.

edit...If you're getting 7.5 volts on each leg of the heater when unloaded, that seems reasonable.  Getting 11 volts to ground on 1 side tells me that you may be wiring the heaters wrong from the power transformer.  Between the 3,8 volts and the 11 volts, it looks like you may have it wired for 12.6 volts on the heaters.

[imgumby001]

Yes. There sure are, there’s ac1, ac2, and ac/ov. There are specific labeled eyelets on the pcb to land them. Is there a way to fortify the ground path for them?

[2deaf]

So I pulled the 6.3v secondaries again, just to be triple sure if my readings, and sure enough! When they’re pulled from the board they read 7.5v ea. But connect them to the board and v1 reads 3.8 and v2 reads 11! I pulled both power and preamp tubes and remeasured with no change found. So, why is my voltage at those points changing so far off what the schematic lists they should be? Is it the power trans? If it is, why are my unconnected primaries reading near what they should?

Those readings are expected relative to ground, although 7.5V is a little high.  But maybe they aren't loaded.

Measure between AC1/6.3V and AC/0V.  Then measure between AC2/6.3V and AC/0V. 

[imgumby001]

I get 7v on both readings.

[2deaf]

According to the schematic, the heater winding looks like a center tap heater winding.  Was there 3 wires coming out of the power transformer for the heaters? 
The imbalance of the heater voltage tells me that the heaters are not properly referenced to ground.

edit...If you're getting 7.5 volts on each leg of the heater when unloaded, that seems reasonable.  Getting 11 volts to ground on 1 side tells me that you may be wiring the heaters wrong from the power transformer.  Between the 3,8 volts and the 11 volts, it looks like you may have it wired for 12.6 volts on the heaters.

The schematic is incorrect but he has it wired correctly.  I'll draw it out and show you why his voltage readings are expected.

[2deaf]

I get 7v on both readings.

You are fine, don't change anything.

[imgumby001]

I still don’t understand why I’m red plating at full gain/volume, and why the gain stage sounds so terrible. I also can’t understand why my presence pot does nothing, and why my pentode/triode switch, while working according to the voltage readings I’m taking, has no effect on the volume level, or sound overall. Geez, I sure do sound like a rookie...

[2deaf]

Half of the heater winding is connected to the power tubes.  The other half is connected to the preamp tubes and it has an artificial center tap connected to ground.  The preamp half will read 6.3V across it, but only 3.15V from either side to ground due to the artificial center tap.  The voltage from one side of the power tube half to ground will be the 6.3V across that half plus the 3.15V from there to ground.

If there is 7V across each half, the readings will be 3.5V and 10.5V.

 

[imgumby001]

Ok, well that’s exactly what I have. Looks like my math needs some polishing!

[AmberB]

That's a weird way to do heaters...

[2deaf]

... even with my bias on the 6L6’s set to -43mA at full gain and moderate volumes one of the power tubes red plates ...

I don't understand this statement.  Does one of the power tubes red plate when you play with enough volume at the speaker?  But it doesn't red plate at idle when you're not playing and the volumes are down?  Is there any chance that "-43mA" is really -43V_dc at pin 5?     

[imgumby001]

Yes, I could totally have misread/misunderstood that reading. If it is -43vdc, than what is that telling me? I have the bias turned all the way down on the bias pot. Do I need to add resistance to that circuit?

[sluckey]

Carefully measure the voltage on pins 3, 4, 5, 8 so we can get a full picture of what's going on. The voltage on pins 3 and 4 will be big positive voltages. The voltage on pin 5 will be a moderate negative voltage, usually approx. -50V. And the voltage on pin 8 will be a small positive voltage, usually approx. 50mV.

[jammied]

I built the jtm25 from KlD .


I will share my experience. The other guys on here are way more educated but here I go.


The heaters do for sure wire different then vintage/original.


Kld doesn't included hookup wire. I used some cloth covered fancy wire and had problems with arcing between wires. So I replaced most of the cloth wire with regular PVC coated wire.


My jtm25 is pretty particular to where you run a ground wire off the board to chassis. The hookup schematic doesn't show any other grounds from the board. Just grounds through the shielded wire from pots.


I had some arcing at a 6l6 socket that give me problems. So cleaned up my hookup on the socket.


The power supply in4007 diodes went bad. (Maybe I overheated them while soldering?) So I had a fred diode bridge rectifier I swapped for the regular 4 diode setup. Now when I grounded the bridge rectifier to ground by the power transformer the amp had a bad hum. So I ran the ground to the board and problem solved no more hum.


The bias supply doesn't have alot of range. My nos matched rca grey glass 6v6's wont bias beyond 5watts per tube without a resistor change. The jjel34's I tried would only bias to about 50% also. Jan Phillip's 6v6's bias fine and also the sovtek 6l6gb bias fine.


For sure double check all of the solder joints and tube sockets.




Sorry I'm not much help! I'm curious as to what you find

[jammied]

But before all bets are in...... I'm betting on the pentode/triode switch wiring!

[2deaf]

Yes, I could totally have misread/misunderstood that reading. If it is -43vdc, than what is that telling me? I have the bias turned all the way down on the bias pot. Do I need to add resistance to that circuit?

I would like to know what the voltage range is at the junction of R51 (15K), R55 (27K), and C54 (22uF) with the power tubes removed.  Then I would like to know what the voltage range is at pin 5 of each power tube with the power tubes still removed.  This will tell me if you have the appropriate bias voltage and possibly what sort of meter you have.

[imgumby001]

Thanks everyone. Been at works all day. I’ll get those voltages when I get home, around 6pm mountian. As I was waiting for replies last night I did a great deal of house keeping inside the chassis. After shortening a bunch of wires I broke the connection from one of the standby leads to the pentode /triode switch. Soldered everything back together, and when I fired it up to test I threw the standby and The pentode/triode switch started arcing and popping! I’m not sure what’s happening as I didn’t change anything and everything is well insulated. Not that I couldn’t have made a soldering error, it just didn’t appear any different than the first time I soldered it. That switch may be fried.

[jammied]

I may have bet correctly!! Well fairly closely.


I dont know nuthin but I would get rid of that switch and hook the output transformer directly to pin 3 off the 6l6's.

[imgumby001]

You guys are awesome!! If I were to remove the switch what would I do with all the extra wiring? Should it be obvious by looking at the schematic? I’m just getting home and getting cleaned up for the work day. After dinner I’ll fly in and take those readings. Thanks all!!!

[jammied]

Dont take my word for it.


I would get the voltages first as requested.


If I remember correctly that output transformer has a yellow wire and a green wire. One goes to the pin 3 of one 6l6 and the other to pin 3 of the other 6l6. These two wires are at one end of the switch and also run to the power tubes pin 3


Then the the 3 remaining wires of the switch would tie together.


Might need professional input on that.

[imgumby001]

Thx bub! I may just get a better quality switch, but I’m undecided at the moment.

So, at the junction I read -45.7vdc. V4-pin 5, and V5 pin 5 both read the same -44.9vdc. I’m using both the Fluke 323, and T5-600 at my bench presently. The 323 has my alligator negative lead for single hand probing, and the T5 is what I use to measure across stuff. I also occasionally use my GB Instruments analog meter as the conditions require.

[jammied]

You should measure all of the 6l6 pins. Pin 3, pin 4, pin 5, pin 8


Need to set the meter to mili volts for pin 8. Pin 3 needs 500v setting

[2deaf]

So, at the junction I read -45.7vdc. V4-pin 5, and V5 pin 5 both read the same -44.9vdc. I’m using both the Fluke 323, and T5-600 at my bench presently. The 323 has my alligator negative lead for single hand probing, and the T5 is what I use to measure across stuff. I also occasionally use my GB Instruments analog meter as the conditions require.

OK.  So now I know your meter has a 10M input impedance.  What I want to know now is what the voltage is at the same junction with the 20K Bias Pot at one extreme and then at the other extreme, again with the power tubes removed.

[imgumby001]

-45.9vdc to -29vdc.

[sluckey]

-45.9vdc to -29vdc.

That bias voltage range is more suitable for EL34s. 6L6s would be happier if that voltage range was shifted upward to about -55v to -40v. You can do this by decreasing R55 and/or increasing R51. Simply swapping R55 and R51 just might get it.

[jammied]

Dont we need to know the plate voltage also? And the measurement from pin 8 to ground?

[imgumby001]

I can get all those readings tonight. Also, maybe I SHOULD try some el’s. I know I have one. I’ll have to check my “back stock” to see if I have another. ;)

[2deaf]

The documentation shows 260V for the HT winding putting an upper limit of 366V for B+.  The hottest 6L6 in my drawer required a bias of -32.4V to get 57mA of plate current with 366V on the plate and 366V on the proximal end of a 1K screen resistor.  A brand new, middle of the road, GT branded 6L6 required a bias of -31.1V.

-45.9 is going to give you a very low voltage on Pin 8.