Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: imgumby001 on October 11, 2020, 12:11:13 pm
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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!!
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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).
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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 :icon_biggrin:
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.
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Sorry guys,
I’m having trouble posting photos.
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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.
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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
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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.
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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
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How are you measuring this "-43mA"?
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thanks!
I see the 1 ohmer's are built in already
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Pin 5 on the 6l6 to ground.
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use pin 8 to ground, the measured volts will equal tube current, ohm say so :rolleyes:
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Yes sir! Immediately!
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Pin 8 on the 6L6’s reads near 0.
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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?
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Missed the AC
what do you have on F1 F2, Vac to ground where i circled
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14vac.
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Sorry, read that wrong. 14vac was the measurement between the two. F1 measures less than 4vac, and F2 is 10vac.
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Schematic does not agree with Layout as far as heaters go. I wouldn't put any tubes in until heater circuit is resolved.
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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?
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No. Not unless the joint is a fail -- and seems unlikely.
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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?
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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.
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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?
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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.
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I get 7v on both readings.
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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.
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I get 7v on both readings.
You are fine, don't change anything.
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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...
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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.
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Ok, well that’s exactly what I have. Looks like my math needs some polishing!
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That's a weird way to do heaters...
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... 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 -43Vdc at pin 5?
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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?
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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.
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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
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But before all bets are in...... I'm betting on the pentode/triode switch wiring!
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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.
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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.
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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.
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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!!!
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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.
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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.
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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
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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.
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-45.9vdc to -29vdc.
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-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.
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Dont we need to know the plate voltage also? And the measurement from pin 8 to ground?
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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. ;)
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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.
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I have a 22k resistor I’m going to try when I get home. I’ll pull r55 and toss it in there. Hopefully that will get us in range better.
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Oh ok
My experience with that transformer is it will be about 395v to 400v b+. Ofcourse depending on consistency in manufacturing
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I just went and measured my kld jtm25 (I suspect same transformer)
B+ is 399v with the tubes installed. Been meaning to change the bias resistor in it but my 6l6'S are marked 6l6gb being 19watt tubes. If infact the markings are correct.
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Ok, awesome! I’ll compare those readings to mine when I get home.
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I have a 22k resistor I’m going to try when I get home. I’ll pull r55 and toss it in there. Hopefully that will get us in range better.
Don't do that. According to Reply #13, the stock bias already has the 6L6's near cut-off. Replacing R55 with a 22K resistor will only make it worse.
With the tubes installed and the amp warmed up, note the voltage on pin 3 of the power tubes. Take the number "21" and divide it by the voltage on pin 3. Adjust the 20K Bias pot until the voltage on pin 8 equals the quotient. Now check the voltage on pin 3 again and repeat the process. Note the resulting bias voltage at the junction of R51, R55, and C54 then post that voltage here along with the voltage on pin 3.
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The number 21 being 70% dissipation of a 30watt tube?
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The number 21 being 70% dissipation of a 30watt tube?
Yes
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Yes sir! I’m on it right after dinner!!!
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I have absolutely no voltage on pin 8 of the power tubes. I’m trying to troubleshoot it now...
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Please excuse my ignorance. Clearly you must be talking about mV. I’m going to bust out my analog meter and get those readings. My apologies.
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My results are ambiguous to me. I don’t know how to interpret this. With the 20k bias pot all the way down pin 3 is at 396vdc. Divided by 21 I get 18.857. However, when I set my analog to mA I barely get any needle movement. Please tell me what I’m not understanding. ( Realizing, of course, that may be so much information no one has time to explain it all to me! )
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21 divided by 396 equals 0.053. Adjust the bias pot until you have 0.053V on pin 8.
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My humble apologies. My meter can barely detect that. I believe I’m set there currently, or, as close to it as I can read on the needle.
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Ok, I’m there. The junction is at -38.4vdc, and pin 8 is currently set to 0.053v. Sorry for my confusion.
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Out of curiosity what is your b+ voltage? And what is you pin 3 voltage now?
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B+ is at 380vdc, and pin 3 is 398.2vdc Jammie.
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Hmmm I would think the 398 would be b+ AND 380 would be pin3
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I’m currently playing it and just took a reading 383.7 on B+ & 397.4 on pin 3. On a side note. There’s an incredible volume difference between the normal and lead channels. Is that normal for yours?
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Mine is a jtm45 so it has a normal and a bright channel roughly the same volume.
The master volume marshall models have a high gain and low gain channel.
When the master models are setup correctly they can get a huge clean blooming tone. That's how alot of the greats ran them with a pedal in front
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Clearly mine is far from right.
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:icon_biggrin: part of the tone search.
What speakers are you using?
Still red plating and humming?
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I’m in it boss. I was hoping to make a neat amp on the cheap by buying this kit. Clearly I underestimated the complexity and concept of kit, and cheap.
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I’m running a 6.5” eminence in a custom cab, and a 10” celestian also in a custom (ahem...) cabinet. And yes, if I crank it she’ll red plate. Post rewire of the pentode/triode switch there’s an intense hum of I switch to pentode. Like I said before, there’s also no volume on the normal channel. Not no, but low, I guess. It sucks cause the clean is very clean at mid vol and gain. But if I pull my fingers off the fretboard she buzzes, which I’m assuming is grounding.
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That sucks! Personally I wired mine up originally without the penrose tried switch and without the send and return loop. Just to save trouble shooting. If needed.
But mine is a point to point. And it doesn't like to be grounded anywhere except through the pots with the shielded cable. Then its quiet.
There bbn is a video on YouTube of a kld jcm25. And that one seems to sound pretty good.
You'll get it. Just got to keep trouble shooting
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Also I'm 99.9% certain that you have 2 wires backwards. Plate voltage should be higher then pin 3
Mine is showing b+ of 399v and pin 3 is 388v.
Did you measure b+ at the standby switch? Or where it says b+ 2 on the board
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On the board bub. I’ve been looking at the schematic trying to figure out how to rewire without the switch. It was a cool idea, but I’d rather have an amp that’s loud when cranked, but sounds great, than an amp that’s sounds like crap, but is bedroom playable!
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Ok
Take a measurement at the standby switch
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I will bro, in the am. Thanks for all your help. Means a lot to me. From everyone. Means a lot.
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So, today B+ is reading 404.8vdc and pin 3 is real near to that. I really want to do away with the pentode/triode switch, but I’m afraid I’ll blow the amp up. It looks straightforward on the schematic, but I’m unsure.
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One major mess up figured out. The arcing in my pentode triode switch was a poorly executed solder joint that id covered up with heat shrink. The solder had contacted the switch chassis tab and was grounding out! I fixed all the joints and re heat shrunk everything. It’s not as pretty as when first done, but it’s sounding much better. At low volume and gain the cleans are down right clean!! Nice glassy tones, with good chime and some warmth to boot. However, the pentode setting has considerable buzz in its own right, add any amount of gain and all bets are off. The sound goes in the toilet- gritty, muddled, and just downright awful. I’m assuming the buzz is unfound ground issues. My real question surrounds the gain. Should I be looking more at the preamp section for that type of gain issue or at the power side? I’ve switched out the tubes in both sides with little appreciable difference, so I’m tentatively ruling that out for now. I’m not sure how to proceed with my troubleshooting at this point.
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Still red plating?
To bypass the pentode/triode switch take the yellow and green wires and hook one to where p1 is marked on the board and the other to where p2 is marked on the board. Then both p12 and p2a hook directly to where b+b is marked on the board ( r30&r31 junction)
Also double check all of the shielded wire connections. The line all of the way through the circle is the conductor/inner most wire and the line on the outside circle is the shield/bare wire.
Also be sure that when soldering the shielded wire that loose strands didn't short/make contact with the conductor. Or that when you soldered the shield and or conductor didn't melt through the insulation of the inner and cause a short
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Copy that!!! I may redo all the shielded sections with better quality wire. I’d suspected perhaps that was a weak link for some time now. Thx bub!!!!!
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If its still red plating or if the pentode tried switch makes a notable difference in hum/buzz. I would just bypass it until everything else is worked out. Then come back and try and get it working properly.
Not going to be alot of volume difference between pentode triode. SPL at 1 watt 1 meter . If the speaker is 97db at 1 watt 1 meter then 12 watts or so is still going to be 97db. To me it is more of a tonal difference then volume difference.
The shielded wire in jtm25 was actually pretty good quality.
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To bypass the pentode/triode switch take the yellow and green wires and hook one to where p1 is marked on the board and the other to where p2 is marked on the board. Then both p12 and p2a hook directly to where b+b is marked on the board ( r30&r31 junction)
Remove all of the wires from the Pentode/Triode switch. Remove the wires connected to P1, P2, P1A, P2A, and B+B on the board.
The yellow wire from the OT goes to P2 on the board and the green wire from the OT goes to P1. P2A on the board connects to P1A and then P1A connects to the junction of R30(10K), R31(2.2K), and C16(47uF) aka B+B on the board.
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Yeah I just figured he already had the wires from p1a and p2a routed to the switch so just take both from the switch and connect to b+b . And the green and yellow wires may need to be swapped around. If oscillating is happening (mine did have to be swapped). Could unsolder the wires at the switch and just hardwire the b+b and p1a and p2a together and leave them full length.
But your directions are much clearer and easier to understand. For sure!
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You guys are awesome!! I have to repair our dryer today, so I’ll get on this after. You two are the best!!!!!
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Well, that’s done, and it’s a definite improvement. That being said, it couldn’t sound worse. I’m still red plating at 3/4 gain/volume, with the other all the way up. It’s not loud. There’s no way you could play with a drummer. Like, no way. There’s still the extreme difference between the inputs in volume as well. The only thing I can think of now is to reflow every single joint in the whole amp and hope I inadvertently hit that as the issue. I’m not holding my breath. One thing, I finally received my tube bias meter, and There’s a pretty far distance between the power tubes. Any combo of the four I have here at the house is a pretty far span. 7ma is the diff between the matches set, and the other two are 10+ depending which socket or pair I’m comparing. So, ya, booo. But, thanks for everyone’s help, assistance, and patience with me. I’m learning by the day, and hopefully I’ll be able to be a contributor at some point.
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received my tube bias meter
your schematic shows there are 1 ohm resistors on the cathodes of the power tubes.
measuring VDC across that R IS the tubes current, no meter and extra things to cause more problems required.
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Yes sir. Copy that.
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Ha! Nobody yell at me, but, and I don’t know why it didn’t occur to me before, I had an dumb-dumb epiphany and remembered I have a bunch of vintage preamp tubes in my upper shop storage. I pulled all new preamp tubes and filled her with vintage. An old jj in v1, a Holland mans mullard from an ao-43 I have in v2, and an unknown origin Sylvania that you can barely read save a touch of green and a severely faded 12ax7. And holy shnikes Batman, she finally sounds good!! Make no mistake, I’m still red Plating at high gain/volume combination, on the lead channel only, but it sounds clear, low to no buzz or hum, and right before she reds up sounds super SMOOTH!! I’m wondering if I can even get 6L6’s to run right in the current configuration. It’s clearly not the tube or the socket, so it must be the general bias limitations of the bias pot and associated resistors/caps in the bias circuit. Maybe a pair of EL’s would do better?!?!
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It’s clearly not the tube or the socket, so it must be the general bias limitations of the bias pot and associated resistors/caps in the bias circuit. Maybe a pair of EL’s would do better?!?!
I thought the red-plating followed the socket.
If you are going to reflow any joints, start with pin 5 on the socket that red-plates.
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Any combo of the four I have here at the house is a pretty far span. 7ma is the diff between the matches set, and the other two are 10+ depending which socket or pair I’m comparing.
Does the same tube have different idle currents depending on which socket it is in?
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Does the same tube have different idle currents depending on which socket it is in?
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Yes, it sure does!! Also, I went ahead and flowed ALL points on the pcb, both the power/preamp board and the input/pot board. I’m pretty sure it helped with the overall functionality and noise issue. I must have a bias or voltage problem. That’s being said, I’ve gone over and over the component order according to the schematic. They’re all where they’re supposed to be and values have been verified and re verified. Either the schematic is wrong somehow or all my power tubes are garbage!!!
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Yes, it sure does!!
With the Master Volume at minimum and the Bias pot untouched, please post the idle current for each 6L6 when installed in the V4 socket and when installed in the V5 socket.
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V4- nos Soviet #1-393.1vdc
nos Soviet #2-388.2vdc
JJ - 391.4vdc
Unknown 6L6-384.9vdc
V5- nos Soviet #1-389.2vdc
nos Soviet #2-385.4vdc
JJ- 395.1vdc
Unknown 6L6-390.7vdc
The jj and the “unknown” tube are old used tubes I had from buying tube lots. The NOS Soviets I purchased for this build.
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One thing, I finally received my tube bias meter...
What is the make and model of this tube bias meter?
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With the Master Volume at minimum and the Bias pot untouched, please post the idle current for each 6L6 when installed in the V4 socket and when installed in the V5 socket.
there are 1 ohm resistors on the cathodes of the power tubes.measuring VDC across that R IS the tubes current,
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Amptata T5 Vacuum Tube Tester EL34 KT88 5881 6L6 6550 Amplifier Bias Current
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Shooter,
I didn’t use that little meter to measure the current readings above, I used my fluke and got actual current readings. I do as I’m told boss!!
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Well I'm pretty sure el34's wont fix the problem.
What is the pin 8 to ground voltage? Maybe measure all of the pin voltages and post here. With the readings of just the nos tubes. Then swap sockets with them and post those readings
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Shooter,
I didn’t use that little meter to measure the current readings above, I used my fluke and got actual current readings. I do as I’m told boss!!
The readings you posted above are voltage readings. Not what the boss requested.
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Amptata T5 Vacuum Tube Tester EL34 KT88 5881 6L6 6550 Amplifier Bias Current
If it works, I would like to know the current readings for each tube when they are in the V4 socket and then in the V5 socket.
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Pin 8 to ground is .6mA on v4 & .5mA on v5.
First set-
Tube 1 - V4
1. .6mA
2. 10.6vac
3. 387vdc
4. 365.1vdc
5. -37.1vdc
6. 0
7. 3.7vac
8. .65mA
Tube2 V5
1. .5mA
2. 10.5vac
3. 376.3vdc
4. 358vdc
5. -36.5vdc
6. 0
7. 3.5vac
8. .5mA
Letting the tubes cool then swapping. Readings to follow.
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Is that with the bias meter?
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No, these are pin voltages. I thought that’s what you asked for. I’ll cool her and slap the bias meter in...
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That's fine. Continue listening to the pro's. They will be more help
I Was trying to figure why the pin 1&8 voltages were .65 and .5 instead of 65.00mv and 50.00 mv
I suspect v4 is the red plating tube?
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V4-34mA
V5-48mA
Those are take with the little meter.
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Those probably are the correct readings, but my little analog meter is hard for me to read. I have no formal electronics training(as if everyone couldn’t tell), so I may read/say a bunch of nonsensical stuff. But, I’m trying as hard as I can to do as I’m told, and listen.
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What happens when you swap the tubes
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After swap-
V4-48mA
V5-33mA
Values are approximated due to the way the meter is numbered. I think it’s pretty close though.
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So the red plating follows the tube? What does the one that red plates read?
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It’s the one running the higher voltage, and yes, the plating follows the tube. But, the other tube is clearly getting high current. You can clearly see the blue excited electrons when the lights are dim in both tubes. Don’t get me wrong, I’m ordering a different set on payday, but I’m weary to to run them only to burn one up potentially. That being said it may be the only way to make a definitive determination. The jj red plates also though. Idk, aside from this red plating issue the amp is finally a playable thing. I’ll be super sad if I can’t get it under control.
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If the VDC and current are correct;
I get ~~~~~13.2W on the lower current tube
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Yeah not even running 50%.
I would get rid of that pentode/triode switch.
Tone and functionally before dufangly thingy's
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Oh no, that switch is already gone! It was the number one thing that has so far saved this build!
If the VDC and current are
correct;
I get ~~~~~13.2W on the lower current tube
So, I should have double that?
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I’m not sure how my question ended up inside the quote...
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Pin 8 to ground is .6mA on v4 & .5mA on v5.
First set-
Tube 1 - V4
1. .6mA
2. 10.6vac
3. 387vdc
4. 365.1vdc
5. -37.1vdc
6. 0
7. 3.7vac
8. .65mA
Tube2 V5
1. .5mA
2. 10.5vac
3. 376.3vdc
4. 358vdc
5. -36.5vdc
6. 0
7. 3.5vac
8. .5mA
Letting the tubes cool then swapping. Readings to follow.
With these numbers I can see why it would be red plating.
But with your bias probe the current seems fine. Actually a little low and shouldn't be red plating.
If you go to robrobinettes site there is a bias calculator
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Also ... what are the markings on the tubes? As far as 6l6gc, or what not
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so we can "assume" for now 1 tube is bad, the one with higher current and red-plating, mark it and leave it out.
with BOTH tubes out;
measure VDC at PIN 5 V4, tube side to ground. should be steady. repeat for V5
IF so;
keep measuring and "bang" the chassis a couple times and make sure it's steady
IF so;
measure at the same place and adjust the bias pot 1st max ccw, record VDC. then max cw, record VDC
post those values
leave the pot at the MAX negative VDC
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Also ... what are the markings on the tubes? As far as 6l6gc, or what not
The NOS tubes are unmarked- but were purchased as 6L6GC’s.
so we can "assume" for now 1 tube is bad, the one with higher current and red-plating, mark it and leave it out.
with BOTH tubes out;
measure VDC at PIN 5 V4, tube side to ground. should be steady. repeat for V5
IF so;
keep measuring and "bang" the chassis a couple times and make sure it's steady
IF so;
measure at the same place and adjust the bias pot 1st max ccw, record VDC. then max cw, record VDC
post those values
leave the pot at the MAX negative VDC
I get -44.2vdc to -28.1vdc on both sockets. Leaving at -44.2 as per my instructions.
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the 3 colors are the "other" paths for problems
NO POWER, wait ~15min
insert a tube into a socket
now;
ohm each path end to end
leave your meter at each end, then wiggle poke prod the physical circuit, so pin 3 to OT "top" you'd verify zero-ish ohms, then wiggle tube, meter dance?, wiggle wire at pin 3, meter wiggle?
ASSUME there is voltage, so wooden stick, face not to close :icon_biggrin:
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the 3 colors are the "other" paths for problems
NO POWER, wait ~15min
insert a tube into a socket
now;
ohm each path end to end
leave your meter at each end, then wiggle poke prod the physical circuit, so pin 3 to OT "top" you'd verify zero-ish ohms, then wiggle tube, meter dance?, wiggle wire at pin 3, meter wiggle?
ASSUME there is voltage, so wooden stick, face not to close :icon_biggrin:
I’m not sure I’m doing this right. There are no readings on most of these points, save continuity on the grounds. I removed the switch, so that point is moved, but I’m not getting any movement from my meter, or real readings of any kind. What am I missing?
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pin 4 to the junction of C14/R31 should be about 1k
pin 3 to OT should be 0, but you might have to sharpin your probe tip to "pierce" the OT wire real close to where it comes out of OT
pin 1 to 8 should be 0
pin 1 OR 8 should be 1ohm to ground
do the pin tube-side, then install a tube, bottom side to...........other end, then wiggle tube
you're looking for mis-wire, bad solder, broke trace if it's PCB, anything that could cause a current imbalance besides a bad tube
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Yes sir!! Copy-copy!!
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I was enlisted, AND new my Father :icon_biggrin:
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:icon_biggrin:
The Russian 6p3s-e always has some blue glow inside the plates. As well as some sovtek 6l6wbg and the relabeled sovtek and russian military tubes. And is normal for THOSE tubes. They also are 25 watt tubes not 30 watt. So red plating is bad but blue inside the plates is normal.
If they are the regular 6p3s and not the 6p3s-e they are rated for max plate voltage around 400 volts or so but still 23-25 watt tubes
Edit:
Robrobinettes bias calculator is showing the 6p3s as a 20watt tube and the 6p3s-e as a 20.5 watt tube.
So with about 400 volts on the plates 35mv or so across the the 100ohm cathode resistors would be plenty hot!
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agree, I always consider 6L6 a 20W tube
the 35mV is across the 1 ohm, not 100ohm though
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Do you mean because a 6l6gc is a 30 watts max tube and at 70% is 21 watts?
If his nos russian tubes are the 6p3s or 6p3s-e variety then they are 20 watts max and 14 watts at 70%
So my thinking is if he bought them nos advertised as Soviet tubes they are probably 20 watt max.
Not matched very close. What supplier was that?
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I did 1 pSE 6L6(EH) build, typically I run 90-100% but they sounded best at about 70% :dontknow:
I bought same tubes for a repair, they sounded best and behaved well at 55-60% (according to old notes)
"generalities" are a good start, but at the end of day, the amps are basically an individually "tuned" thing
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Hmmmm......
From what I have seen on the internet. There are at least 2 different 6p3s-e tubes and a 6p3s standard that is closer to a 6l6g than a 6l6gc. But most resellers are claiming 6l6gc equivalent even though some are not.
And all of them have different specs. The later ones being relabeled by numerous manufacturers. And supposedly upgraded?
Depending on the particular ones gumby has. They could be running way over max.
The bias calculator is showing either variety of 6p3s as a max of 20 watts. More of a super 6v6
Interesting!
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I was enlisted, AND new my Father :icon_biggrin:
No offense bubba, just force of habit.
:icon_biggrin:
The Russian 6p3s-e always has some blue glow inside the plates. As well as some sovtek 6l6wbg and the relabeled sovtek and russian military tubes. And is normal for THOSE tubes. They also are 25 watt tubes not 30 watt. So red plating is bad but blue inside the plates is normal.
If they are the regular 6p3s and not the 6p3s-e they are rated for max plate voltage around 400 volts or so but still 23-25 watt tubes
Edit:
Robrobinettes bias calculator is showing the 6p3s as a 20watt tube and the 6p3s-e as a 20.5 watt tube.
So with about 400 volts on the plates 35mv or so across the the 100ohm cathode resistors would be plenty hot!
You’re absolutely right!! I’m sorry I didn’t look at this closer/sooner. Here’s the add;
For sale are NOS new matched duet of 6L6GC vacuum tubes $13.99/pair. A representative picture is shown.
They are made by the Famous Russian Reflector factory, 6n3c's designed for up to 50 watts/pair of 6L6s.
Very nice warm sound.
_____
former feedback about these
"just put tubes in my 1973 pro reverb ive had since 1976 ,very pleased good sound"
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Just FYI- the only tube actually labeld 6L6GC is an actual old labeld Sovtek, and its red plating as well. The other unbranded tube is labeled 6L6. The NOS tubs from the add aren’t labeled at all. Clearly I didn’t know what I was looking at when I bought them, thinking they were actual 6L6’s.
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Luckily, and at the very least, I was smart enough to order some NOS Sylvania’s last night! I did also order another to match the Russian tube that seems to be working ok.
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I would put both nos tubes in. Then the one that is reading high with the bias meter adjust bias to 35-37 if possible. Taking note of the plate voltage also.
Then see if it still red plates. I'm betting they are the 20 watt max tubes. So 70% would be 14watts.
Could be to late
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I’m thinking it’s too late for the one, but I have another on the way, form a different seller. I think it’s obvious to everyone they weren’t an actual matched set, but what can I expect at that price. Just one more lesson learned on this deal. We’ll consider it a small price to pay...
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Hopefully you ordered from a different supplier! Some mismatch isn't bad. But that is alot.
They apparently didn't burn the tubes in before matching.
The red plating tube is probably still good. The Soviet tubes are pretty robust but severely misrepresented .
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The sylvania's will probably need r55 raised to get full bias range
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lets make sure there's music and no glows before raising anything
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lets make sure there's music and no glows before raising anything
Copy that!!!
The sylvania's will probably need r55 raised to get full bias range
So, what value do you think will do it? Or, would it make sense, albeit a harder part to source, would changing the bias pot to a unit with a broader range be a better overall fix/upgrade? I’d still like to be able to run some EL’s if I get my hands on some.
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I wouldn't change anything yet. Just from what I have seen I would speculate that will be the case though.
The power transformer is only rated at 3 amps on the 6.3 volt taps. I would have to double check but I think el34's will draw about 1.8 amps per tube. Compared to 6l6 varieties that draw 1 amp.
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If it were 1.5a of draw that would do it, right? I swear I saw mullard el34’s draw 1.5a heater current.
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Correct el34's draw 1.5 amps. 12ax7's draw 300ma per tube. 3+.9=3.9 Amps .
I put some el34's in the jtm25 last night. And it uses 4 12ax7's so 4.2 amps total heater current. Then measured the voltage across the heater on the el34's and 12ax7's and still had 6.3v on the el34'S and 6.5v on the 12ax7's
The power transformer didn't seem to be running any warmer than usual after 20 minutes so maybe the el34's would be ok.
Maybe later I will pull the tubes and measure then it will be more clear on the extra load of the el34's.
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Thanks bubba!!! I didn’t pull the trigger on any 34’s yet, so I’ll wait for your final numbers to come in on that before I make the ultimate call. Im still considering trying to source a bias pot with a broader range and seeing if that doesn’t create a sweet spot for easy bias adjustments for a greater number of tubes, I.e. the 6l6 variety, both american/British and Russian variants, and EL’s. Doesn’t seem like a bad idea at this point.
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So here’s my latest findings. Received a new NOS 6P3S. It’s matches the cooler running one that has NOT been red plating in the amp. Warmed it up good in v4 position after removing the red plater from that socket. Took it inside and hooked it to the 30w 12” EH that I’ve got in a sealed cab. Leaned into her slowly and she sounded pretty good. Keep in mind I’ve got the bias voltage turned as low as the pot will adjust. Cranked the gain and volume and the amp came to life for the first time! Sounded sweet!! Great bottomy tone, that was rich, if not a bit weak in the treble side. Still, thick and rich. Just when I was starting to feel safe, red plate on the v4 tube! So, not wanting to just chalk it up to bad tubes I swapped positions between the power tubes, and what did I find? The red plate stayed with the socket! Now it never did that before. That specific tube had not fully red plated when the other one was in, so this is the first time I’ve seen it go hot. At this point v4 is my major issue. Short story long, it seems like I’m moving toward either;
1. Getting an actual matched set of 6l6gc’s
Or
2. Changing a resistor to restrict the voltage going to the power tubes.
Does this sound about right to everyone?
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swap tubes, does it follow tube?
the red Plating stays at the socket
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Maybe tighten the tube socket pin contacts. Also wouldn't hurt to pull the board and have a peak to be sure that when wiring something didn't get grounded or shorted.
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I ordered a set of the "matched" 6p3s tubes out of curiosity. Hopefully be here tomorrow.
Damn curiosity!!
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You're up against the wall with the bias adjustment, yet you are still red plating. Time to actually do something.
-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.
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Maybe tighten the tube socket pin contacts. Also wouldn't hurt to pull the board and have a peak to be sure that when wiring something didn't get grounded or shorted.
I actually wired it out of the chassis. Not that it would be a bad idea to look at it.
You're up against the wall with the bias adjustment, yet you are still red plating. Time to actually do something.
-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.
It would be ridiculous to argue with that!! Looks like tomorrow I’m gonna be swapping resistors.
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V4-34mA
V5-48mA
Those are take with the little meter.
Ok I'm trying to learn something here so dont get me wrong .
If the bias meter used here is reading correctly. Shouldn't these numbers be within range for 6l6 tubes?
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(Assuming in a known good amp)
probably, B+ is a missing element
I like to monitor while playing hard since PP tends to go somewhat current crazy when pushed, so what "appeared" ok at idle might be to much at full roar
for this thread, i'm still not convinced the test equipment, or methods, isn't a contributing "issue", besides the issue with V4
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(Assuming in a known good amp)
probably, B+ is a missing element
I like to monitor while playing hard since PP tends to go somewhat current crazy when pushed, so what "appeared" ok at idle might be to much at full roar
for this thread, i'm still not convinced the test equipment, or methods, isn't a contributing "issue", besides the issue with V4
Look, I couldn’t agree more! There’s no arguing I’m a complete rookie. I appreciate the “schooling” I'm receiving from you guys. There’s no possible way I could have figured any of this out without y’all. That being said, I’m trying real hard not to be an idiot, and I genuinely want to be good at this and fully understand how to do this on my own. Hell, I’d like to one day be good enough to help other rookies!! Tonight I’ll take a photo of all my test equipment so you can see what I’m using. As far as the implementation of the tools, user error is definitely a reality.
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Something has gone afoul. Maybe retention and clean the tube sockets?
IF the 387v on pin 3 (the plates) is correct?
Here are 70% numbers for bias
6p3s= 36.2ma
5881 23watt=41.6ma
5881 26watt=47ma
6l6gc 30watt=54.3ma
Just trying to learn something. So a true 6l6gc shouldn't red plate? Or a 5881wxt+ 26watt tube. According to his measurements. But a 20 watt tube very well could be?
I'm not doubting Sluckey he for sure knows what he is talking about. I'm trying to understand how increasing the negative voltage is going to stop the red plating? The plate voltage and ma across the 1ohm resistor is showing they should be within range. Or maybe on the cold side?
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The more I think on it, it seems to me my initial mistake was not identifying the correct tubes in the first place. I’m going to order a set of GC’s AND a set of EL’s and see where we land. I think Slucky’s real point was it’s time to do SOMETHING different as we’re not really getting anywhere at this point with the current setup. Makes sense in my head anyway!
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So a true 6l6gc shouldn't red plate? Or a 5881wxt+ 26watt tube. According to his measurements. But a 20 watt tube very well could be?
I'm not doubting Sluckey he for sure knows what he is talking about. I'm trying to understand how increasing the negative voltage is going to stop the red plating? The plate voltage and ma across the 1ohm resistor is showing they should be within range. Or maybe on the cold side?
Let's say an amplifier is running at 400V and it has an OT with a 4K primary. With the proper screen voltage, a 30W 6L6GC will work fine in this amplifier. It will operate above the maximum dissipation for part of the positive signal swing, but not that much above and there is plenty of time when it isn't above maximum dissipation.
Installing a 20W tube and leaving the idle at 21W will cause that tube to run above its maximum dissipation for the entire positive signal swing and to exceed the maximum by up to 20W. If the tubes are borderline red-plating, decreasing the idle current can give them a little more time under maximum dissipation and just might hold off red-plating. Or not. Idling clear down to Class B is not a viable solution and it might not prevent red-plating, anyways.
The real problem is that the 20W tube needs a higher OT primary impedance and/or a lower HT voltage so that the time spent above maximum dissipation and the magnitude of the excess is reasonable.
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Let's say an amplifier is running at 400V and it has an OT with a 4K primary. With the proper screen voltage, a 30W 6L6GC will work fine in this amplifier. It will operate above the maximum dissipation for part of the positive signal swing, but not that much above and there is plenty of time when it isn't above maximum dissipation.
Installing a 20W tube and leaving the idle at 21W will cause that tube to run above its maximum dissipation for the entire positive signal swing and to exceed the maximum by up to 20W. If the tubes are borderline red-plating, decreasing the idle current can give them a little more time under maximum dissipation and just might hold off red-plating. Or not. Idling clear down to Class B is not a viable solution and it might not prevent red-plating, anyways.
The real problem is that the 20W tube needs a higher OT primary impedance and/or a lower HT voltage so that the time spent above maximum dissipation and the magnitude of the excess is reasonable.
Thanks 2deaf! Some 6L6GC’s are on the way, along with some EL34’s just in case the 6L6’s don’t sound to my liking. The pn3p tubes were an inadvertent mistake.
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So if the voltages were as advertised on the power transformer 360v b+ that may have ended up with 350v or so on the plates.
Then the 20watt 6p3s may have worked fine?
Well I ordered a pair of the 6p3s to try. But I have been using the 8k primary classic tone 18watt marshall transformer
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I don’t have the schematic in front of me, I can’t remember what the it’s primary resistance is. The 6n’s will still serve me at some point, I didn’t hate the sound coming out of them. I’d really like to finish this project, I’ve got four more right behind it!! ;)
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Supplied transformer is 3.4k
My gut instinct thinks your going to have to swap bias resistors to get above 50% with the el34's or 6l6gc.
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That just seems crazy to me that they would mess the schematic up that bad. The only real test is to do it and see. Got nothing better to do tomorrow. I’ll swap them in the am and post my voltage results.
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Dont know if I would get crazy just yet the may be fine.
I was just blabbering my thoughts.
I have 2 sets of nos 6v6's one rca and the other jan sylvania I think? And 2 sets of 6l6gc, a set of el34's and a set of 6l6gb
The 6l6gb's biased fine stock as did the jan 6v6's. All the others I had to change the bias resistor.
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You blabber away bubba, it helps me get my schemes in order!!! I’m going to try it just to see. I won’t have the other tubes until the end of next week. Besides, it’s no big deal to put back whatever I change!!
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Right on
Just remember to be careful with those pcb traces. All master model Marshall's that I know of had PCB's out of the factory but they were a little more forgiving
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V4-34mA
V5-48mA
Those are take with the little meter.
Ok I'm trying to learn something here so dont get me wrong .
If the bias meter used here is reading correctly. Shouldn't these numbers be within range for 6l6 tubes?
No. -34v is not in the range you were given by Sluckey earlier.
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No. -34v is not in the range you were given by Sluckey earlier.
That's pretty funny and everything, but I don't really think that it's cool.
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I understand that. I was saying 34ma or 34mv across the 1ohm resistors.
2deaf cleared the matter up with the graph and explanation.
Normally a 20watt max tube such as the 6p3s would be completely fine at 400v and 34ma dissipation 400v and 48ma bias and a 20watt max is over max.
As stated 2deaf cleared up why the 20watt max tube could still be red plating while still operating within it's safe dissipation range.
I respect sluckey and know he knows what he is talking about.
I wanted to know why it would matter. Give a man a fish and you feed him for a day but teach a man to fish and feed him a lifetime..... I wanted to learn how to fish so to speak.
Now after 2deaf give the example I'm getting some bait. :icon_biggrin:
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This morning a paralleled in a resistor with R51 to see what difference I could get on the red plating front. It made a definite difference, I just chose too low of a value for a significant parallel adjustment. I also pulled all the low grade shielded wire off the inputs and pots and replaced it with some nicer coax. I triple checked all the values of the components on that board, and they all checked out good. The presence pot still does nothing. The really sounds terrible at high gain/volume. The red plating is present still, but dramatically reduced. I’m hopeful, but really not happy with the job I’ve done so far. At least I’ve learned a lot.
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Still v4 red plating? Even if swapping tubes?
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I didn’t swap today, but, as of last test, yes. The order from nesstube won’t be here till probably Wednesday, so, I’m working with the 6n3p’s still. Side note, I’m very lucky. I wasn’t aware, as in I didn’t know, that 6L6’s could be a problem if they’re actual 6L6’s. My Sylvania brand tube is an actual 6L6. I’m lucky in that I didn’t melt it down.
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Have you tried swapping the green and yellow out put transformer wires?
Maybe unhook the presence control wire. Or retention the tube sockets. They may not be making good contact.
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Ok ..... my 6p3s tubes arrived . The one tube did the blue electron glow also.
So I did the sluckey mod by swapping the resistors. Now mine had a 22k in it instead of a 27k.
Now I get a range of -47.8 to 59.3
Haven't tried the tubes again that was with no tubes in the sockets.
I believe sluckey was shooting for -40 to -55 range.
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I didn’t swap today, but, as of last test, yes. The order from nesstube won’t be here till probably Wednesday, so, I’m working with the 6n3p’s still. Side note, I’m very lucky. I wasn’t aware, as in I didn’t know, that 6L6’s could be a problem if they’re actual 6L6’s. My Sylvania brand tube is an actual 6L6. I’m lucky in that I didn’t melt it down.
I dont know anything about ness tube but on there website they show nos sovtek 6l6gb's and state they cant be used in a 6l6gc amp. But the picture is clearly of 6p3s tubes!
So I looked at their new production tubes and they have ness 6l6gc's again the picture is clearly a 6p3s.
I have been running the 6p3s with bias resistors in normal positions with the following voltages and no red plating.
Pin 1= 35.2mv
Pin 3= 384v
Pin 4= 355v
Pin 5= -33.5
No red plating at all. But have the blue glow. Now I studied that on the net and. It seems that it may very well be fine. Dont know? But that's the word on the net
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How does it sound?
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I like them.
Really brings out that steely wooly sound. And let's the brassy ringing strat bell tone come through. Kinda different not really bright . Hard to explain. Jtm45 is based on the bassman and known to have flubby bass but these tubes tighten that up.
Definitely got that hendrix marshall thing going on. Humbuckers and some knob tuning and its into ac/DC territory.
So they let the guitar sound like the guitar being played. Where the other tubes I tried always colored the tone or exaggerated bass or treble. The only other tube I tried in it and liked better were the old original sovtek 6l6gb(not the ones mess is selling)
I played the heck out of it. And no problems so far
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Just to be clear you DID swap both bias resistors, or no? And, what are the values of those resistors in you amp?
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Originally I left them like in the schematic . Where your are 27k and 15k. Biased to 13-14 watts. Had blue glow but no red plating . Then swapped the two resistors around opposite from the schematic and could only get 7.5mv bias. But very little blue glow.
I ended up swapping back to the schematic placement. And left it there and been playing no problem. No red plating. I really like the sweet pick attack on these
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Copy that. I’ve now swapped mine and get relatively little voltage to work with. Obviously there’s no red plating. It’s late here, and everyone is in bed, so I can’t crank it, but, clean seems very friggin clean at low volume. Dirty is awful. Even at low volume, when I crank the gain to full there’s a terrible noise generated by the speaker, almost a grinding. Is this oscillation? I’ve just swapped all the preamp tubes back to the brand new shuagang’s(sp?). I’m about to do some low volume work and see how that sounds...
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What I’m noticing is at full gain, at any volume save all the way down with the master, it’s terribly crackly and buzzy, just awful. But if you roll it back just a bit it clears up, and your left with good tone. It’s and alpha pot, but, bad pot maybe? The Chinese preamp tubes sound fine. I guess the real test will be in the morning when I can come out and blast the volume and see if it will red plate. I’m guessing by your (jammied) findings it’ll be a no.
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To bias them plug in your bias meter and also take a pin 3 voltage reading .
From your previous voltages it looks like the higher voltage on pin 3 was 387v start at about 32ma.
Be sure to check each tube on pin 3 . If you start getting over 36ma to 37 ma . The chance of red plating increase. So for purposes of this test neither tube with your bias meter should be set higher than 36ma. 14watts is close to 70% of the 6p3s
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I can’t even approach that with the current config. I’m maxing out at about 29ma. Pretty dramatic shift. Here in a bit I’m going to go play it and keep the mA meter attached so I can monitor the max voltages reached. It’s just a curiosity, but I’d like to see where it goes. Maybe it will be playable, I guess we’ll see.
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What is the pin 3 voltage on each tube (with tubes installed)? Also are both tubes at 29ma?
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Sorry, I was testing to recall from last night. Standing here fully cranked it’s only producing 20mA to the hot tube(red plater), and right at 10mA to the cool runner!! Pin 3 voltage for v4 is 399.4vdc, and v5 is 399vdc straight up. I’m think what I’m seeing is definitely under powering these, and if I were to want to run them I’d need to swap the 27k now in r51 position for say a 22k-ish. Does that sound right?
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R51 should be a 15k and r55 should be a 27k according to the schematic?
That's what they need to be.
But with the voltages the power transformer puts out you dont want anything over about 36ma to 37ma on either tube
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If you cant get enough ma on the bias probe with r55 being 27k up it to 33k. And leave r51 as 15k for now
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20mA to the hot tube(red plater), and right at 10mA to the cool
I don't think one side should ALWAYS be higher, randomly yes, but consistently over many tubes tells me there's something not right there.
If the amp has NFB remove and retest current
IF no change, leave NFB off and swap OT primary leads and re-test
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Guess he could put the tube that is in v4 into v5 and vice versa just to be certain. Whether it is the tube or the socket.
I think the out put transformer primaries need swapped. Just my personal thoughts.
But with the voltages the power transformer is producing. 36ma is way plenty current on the 6p3s.
Even changing to 6l6gc or el34 . The bias voltage isn't going to allow for biasing
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20mA to the hot tube(red plater), and right at 10mA to the cool
I don't think one side should ALWAYS be higher, randomly yes, but consistently over many tubes tells me there's something not right there.
If the amp has NFB remove and retest current
IF no change, leave NFB off and swap OT primary leads and re-test
Sorry, NFB?
Guess he could put the tube that is in v4 into v5 and vice versa just to be certain. Whether it is the tube or the socket.
I think the out put transformer primaries need swapped. Just my personal thoughts.
But with the voltages the power transformer is producing. 36ma is way plenty current on the 6p3s.
Even changing to 6l6gc or el34 . The bias voltage isn't going to allow for biasing
Ok, gonna swap the ot leads and retest. Here’s hoping for an awesome outcome!!!
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Swapping the ot leads leaves it sounding lifeless and probably the worst it’s sounded so far. I’m going to go out on a limb and say they were probably landed on the right points to begin with. But, I still have r55 and r51 swapped, so, should I swap them back and retest BEFORE I swap the ot leads back? The mA range seems to have remained the same for both power tubes.
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Did the presence control work? I would go ahead and swap the resistors
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Easter eggs keep getting added to the hunt, not good for Tshooting.
1st decide if 2X the current is "normal"
IF not
you need to make 1 change and verify, and note what you did (tube A still 2X I)
pull A install tube B (no change)....all tubes 2 X when in V4 position
Move on
swap OT primaries - done
there has to be a logic to the method otherwise it's just monkeys and darts
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Agreed
And it will never stop red plating if running a 20watt max tube at 19 watts idle.
The biggest problem I see right now is improper bias.
Change one thing at a time. Swap the tube from v4 to v5 using the pair he has in it right now
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V4-34mA
V5-48mA
Those are take with the little meter.
Then 2 posts later he swapped the tubes between sockets and the v4 and v5 number were the opposite
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improper bias.
the bias goes to both tubes from the same "source", but one side is reacting different than "normal"
so bias is probably a symptom of the problem
so set the bias max neg and leave it for now (guitar playing is down the road)
next verify every tube in both sockets - logically, with labels and notes and currents
then we've eliminated "bad tubes" (kinda for now)
that leaves "why V4"
output were swapped
so
swap inputs
un-do everything V4 and re-do
.........
10mA at idle isn't the end of the world, actually pretty normal i'm thinking
once that's all checked n verified
then set bias for 12-14W and evaluate with guitar and monitor current
if current is good and guitar sound sucks, Then we move left :icon_biggrin:
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After swap-
V4-48mA
V5-33mA
Values are approximated due to the way the meter is numbered. I think it’s pretty close though.
This post is after the two tubes between sockets. 48ma at the plate voltage is like running a true 6l6gc at 72ma at the same voltage.
It clearly followed the tube in this test.
But yeah eliminate as many variables at a time as possible.
Shooter knows and we are both getting at the same thing only in different ways
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Did the presence control work? I would go ahead and swap the resistors
Yes, it seemed like it did respond after the ot primary swap.Easter eggs keep getting added to the hunt, not good for Tshooting.
1st decide if 2X the current is "normal"
IF not
you need to make 1 change and verify, and note what you did (tube A still 2X I)
pull A install tube B (no change)....all tubes 2 X when in V4 position
Move on
swap OT primaries - done
there has to be a logic to the method otherwise it's just monkeys and darts
I’m going to go down shooters list above and try to figure all these aspects- after I return the resistors to the correct placements according to the schematic. Yes?
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Good news!
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Resistors are swapped to schematic. Voltages read as they were. Socket evaluation yielded no issues. Tested all three pn3p tubes in v4&5- original cool tube and replacement tube w/broken key read near identical- hot tube reads approx 8mA higher in both sockets. Did NOT return ot wiring - warmed appropriately and played- max vol/max gain- on lowest bias setting NO red plating, even on hot running tube- volumes seem louder/punchier, despite being very, very over gainy- will swap to vintage pre amp tubes from Chinese high gain tubes after recording all voltages- removing mA meter from power tubes as it seems to be effecting overall tone. Ya...
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So, I think it finally happened...(sniff, sniff). It finally...
Sounds RAD!!!! Who would have guessed a properly wired, and biased tube amplifier would operate properly??? Looks like the original issue was the improperly wired pentode/triode switch, then the incorrect rewire, and then improperly biasing. My list of thank you’s is very long, but if you helped me on this project I can’t thank you enough. Fortunately for me there’s all of you, UNfortunatley for all of you I have no less than five more amp projects, literally lined up on my bench! So, short story long, I’d like to say, once again, thank you all from the bottom of my guitar loving heart!!!
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Good news buddy!
Wish there was a good supplier for matched 6p3s tubes. I have seen any supplier for them yet that actually burns the tubes in and then matches them.
Almost better off buying the cheap bushel basket fulls from Russia then sorting yourself
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I’m going to get a set of the keyway replacements HotPlate recommended in your 6n3p thread. At that point I’ll have a nearly perfectly matched set!
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Well, it’s red plating again. I’ve tried various tubes- 6L6GC’s, EL34’s, and a bunch of the 6P3S’s and they all red plate at full vol w/full gain. Both sockets are doing it w/v4 tubes usually showing red first. One pair of the 6P3S ran for about 20min before any red showing, which is weird, right?
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What's pin 3 voltage and the bias probe readings? In ready to play with volume , gain, and tone controls turned to zero?
To get this amp biased you need to take the pin 3 reading along with the bias probe reading. And then figure dissipation or use one of the online bias calculators.