Mark IIB to IIC+ Conversion
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I have a 33k , and a 10k in parallel ? with the two 47uf's. As you can see, there once was two resistors next to the 33k. It came from factory that way. After that reversed biased diode, that 470 is attached to one side of the 10k. Ov
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Thanks for sharing! Looks like you have a single 33k the same place I do. Interesting how cold yours is biased.
To check plate current, I measured the resistance of each side of the OT primary when the amp was off and charged down, and then measured the voltage drop when it was on. Then Ohm’s law gives the plate current. This is my favorite way to check it because you don’t have to break the circuit and there’s no screen current involved
To check plate current, I measured the resistance of each side of the OT primary when the amp was off and charged down, and then measured the voltage drop when it was on. Then Ohm’s law gives the plate current. This is my favorite way to check it because you don’t have to break the circuit and there’s no screen current involved
Ty kindly for the lesson. Please explain why you said my bias was cold. Did you base it on visual or my comments on my negative voltages on power tube grids? I was always so confused with bias due to the negative ( - ) before the number. Under and over biased. Taking myself back to my old school days, in math, the number line. Zero being the center. Anything to the left is a negative number, and to the right positive. With that said, for an example, say -80 is lower than -40 even though the 'number' is higher. With all of my personal experience trying to sort this under bias and over bias thing in my little brain, if tubes are ' under' biased, does that mean that the negative ' number ' is too low? Going towards right on the number line? Which makes tubes run too hot or even redplate? And the opposite with over biased. Too cold, as you stated. The negative 'number' is too high, further left on the number line? Please simplify this dilemma for me.
Different 6L6's . Some would be hotter than others by touch. And EL34's would always feel cooler by touch (only on outer tubes) .Side note: years ago, my grid voltages on power tubes would always hover around -38vdc on outer and - 50vdc respectively. I have know clue as to why except maybe AC voltages changed on my block. Thank you for your time Jay! Blessings. JimThe word bias refers to a DC offset between the cathode and grid that controls the plate dissipation at idle. The greater the offset, or distance between the points on the number line, the lower the idle dissipation will be. In Class AB, tubes are typically biased such that the plates dissipate 60-70% of their max rating at idle. Anything above this range I’d consider hot, and anything below I’d consider cold.Ty kindly for the lesson. Please explain why you said my bias was cold. Did you base it on visual or my comments on my negative voltages on power tube grids? I was always so confused with bias due to the negative ( - ) before the number. Under and over biased. Taking myself back to my old school days, in math, the number line. Zero being the center. Anything to the left is a negative number, and to the right positive. With that said, for an example, say -80 is lower than -40 even though the 'number' is higher. With all of my personal experience trying to sort this under bias and over bias thing in my little brain, if tubes are ' under' biased, does that mean that the negative ' number ' is too low? Going towards right on the number line? Which makes tubes run too hot or even redplate? And the opposite with over biased. Too cold, as you stated. The negative 'number' is too high, further left on the number line? Please simplify this dilemma for me.
woodbutcher65
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Here's how to understand bias. The bias voltage is applied to the grid in the tube, between the plate which is at a high positive voltage, and the cathode, which is grounded. Since the flow of electrons is from negative to positive, as the electron itself has a negative charge, this means that in the tube, the plate will pull electrons from ground through the cathode and the space between the cathode to the plate.
We need a means to control this flow of electrons. Because with no grid voltage applied, or no grid in the tube, it just flows from cathode to plate in an uncontrolled flood that will likely soon destroy the weakest part of the tube, the cathode.
So we apply a voltage, a BIAS voltage, to the grid placed between the cathode and the plate. The voltage is negative, so it is source of electrons, and it acts as an obstacle to the electrons being pulled off the cathode. The more negative the bias voltage, the more it blocks the current flow from cathode to plate. Because electrons, being negative, will repel other electrons. Remember, the plate is positive, it's attracting electrons. So there is a current flow, and that current flow is between the grid and the plate, but the electric field around the grid is what is controlling the flow from the cathode.
We need a means to control this flow of electrons. Because with no grid voltage applied, or no grid in the tube, it just flows from cathode to plate in an uncontrolled flood that will likely soon destroy the weakest part of the tube, the cathode.
So we apply a voltage, a BIAS voltage, to the grid placed between the cathode and the plate. The voltage is negative, so it is source of electrons, and it acts as an obstacle to the electrons being pulled off the cathode. The more negative the bias voltage, the more it blocks the current flow from cathode to plate. Because electrons, being negative, will repel other electrons. Remember, the plate is positive, it's attracting electrons. So there is a current flow, and that current flow is between the grid and the plate, but the electric field around the grid is what is controlling the flow from the cathode.
Ty for your replies WB and Jay. I got my head wrapped around that.
. Those lil " explosions " from cathode to plate also releases random negatively charged electrons flying everywhere. Which I believe is why the positively charged screen helps to attract them. I have a DRG C+. Say that my outer tubes have a -48vdc. And hypothetically I would want to run them a little hotter, there's a 220k grid resistor on them currently. Being it's negative again, if I wanted to lower that negative voltage to say , just for example, to -40vdc for experimental purposes, would I increase the value of that resistor to lower that negative dc voltage? Man, after just watching a YT video, I'm even more confused. Is the plate voltage a fixed voltage? Why I ask is in this video, this tech adjusted the bias via a bias pot to fine to the % of plate dissipation and whenever he turned the bias pot, the plate voltage changed. The plate voltages in my C+ currently is 500v. Some years ago it was around 480v. What made it jump? Blessings, Jim. I have a question about my 470 ohm screen grids later, if everyone doesn't mind. 
Jay, you populating the preamp card? Can't wait.
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woodbutcher65
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Plate voltage does change as a consequence of the loading. You see, no power supply is perfectly "stiff", they sag a little when they are loaded. Higher quality, higher current supplies can handle heavier loads without their voltage dropping appreciably.
In truth, the power supplies in our ampifiers are relatively weak and they definitely sag under load. Just measure the plate voltage as different numbers of power tubes are plugged into the circuit and you will see that more tubes plugged in results in lower plate voltage,
and plate voltage will also drop when you take the amp out of standby.
We don't really concern ourselves with the bias voltage, because it's the bias CURRENT that does the work. It's pure Ohm's Law in play here. To raise the current, the supply voltage has to be increased for a given load. So we adjust the bias supply to deliver whatever voltage is necessary in order for the proper amount of current to be delivered.
If your amp's plate voltage is drifting higher, then the dropping resistors values need to be looked at. It's very likely that if they're carbon film or carbon comp resistors in the voltage divider ladder, that the values are now higher than they were when the resistors were new. They should be replaced with new resistors that are in spec.
Lower value series dropping resistors values will mean less voltage at the supply nodes.
Plate voltage of 500 volts means you're risking destruction of any weak power tubes. You want that voltage problem taken care of.
If you should encounter an original Mesa RP11A preamp board, please take photos of it from several angles and post them here. And if you can post photos of the BOTTOM of the preamp board, by all means please do so. Just in case you encounter one.
In truth, the power supplies in our ampifiers are relatively weak and they definitely sag under load. Just measure the plate voltage as different numbers of power tubes are plugged into the circuit and you will see that more tubes plugged in results in lower plate voltage,
and plate voltage will also drop when you take the amp out of standby.
We don't really concern ourselves with the bias voltage, because it's the bias CURRENT that does the work. It's pure Ohm's Law in play here. To raise the current, the supply voltage has to be increased for a given load. So we adjust the bias supply to deliver whatever voltage is necessary in order for the proper amount of current to be delivered.
If your amp's plate voltage is drifting higher, then the dropping resistors values need to be looked at. It's very likely that if they're carbon film or carbon comp resistors in the voltage divider ladder, that the values are now higher than they were when the resistors were new. They should be replaced with new resistors that are in spec.
Lower value series dropping resistors values will mean less voltage at the supply nodes.
Plate voltage of 500 volts means you're risking destruction of any weak power tubes. You want that voltage problem taken care of.
If you should encounter an original Mesa RP11A preamp board, please take photos of it from several angles and post them here. And if you can post photos of the BOTTOM of the preamp board, by all means please do so. Just in case you encounter one.
I haven't made much physical progress since the last update, but still chipping away. I'm getting close on the preamp board design. One of the things I've learned through this process is that Boogie used plated through holes all the way back to at least the Mark IIB. This surprised me because the older boards were DIY style, I wonder what their process was. I am not setup to do plated through holes at home, so I'm trying to figure out how to design such that I don't shoot myself in the foot with underside traces if mods are needed. This is particularly tricky with the physical space limitations associated with my IIB.
Another thing slowing me down is that I've been having a lot of fun playing through the power section. I'm using my Triaxis as a preamp, plugging into the preamp / power amp junction so the EQ and beyond is active. The taper on the graphic pots I used is very close to my Boogies, I don't notice a difference. I was planning to sell my Triaxis someday because it always sounded too compressed to me, this is not the case with this power section. I wonder if the 2:90 is the culprit. I haven't measured the power output, but this power section is friggin loud. No problem keeping up with a drummer using a clean sound through a 2x12
Another thing slowing me down is that I've been having a lot of fun playing through the power section. I'm using my Triaxis as a preamp, plugging into the preamp / power amp junction so the EQ and beyond is active. The taper on the graphic pots I used is very close to my Boogies, I don't notice a difference. I was planning to sell my Triaxis someday because it always sounded too compressed to me, this is not the case with this power section. I wonder if the 2:90 is the culprit. I haven't measured the power output, but this power section is friggin loud. No problem keeping up with a drummer using a clean sound through a 2x12
Ty for your reply! Yes, I will. Mine has a RP11A board. I should have noted that my PT is a 105. Not sure if that matters or not regarding the high plate voltages. Last time I was inside her, few weeks ago, those two 1k 2 watt carbon comp resistors on power board measured darn near perfect. Are those the droppIng resistors? You'll notice the LDR supply burnt. M. Bendinelli rebuilt it a while back. Blessings, Jim. If you would like closer picts, I'll get to it.
Holy crap, yeah it did!
And yes, those resistors drop voltage in the power supply, although not very much since their value is relatively low and there isn't much current. Their primary purpose is to decouple the different power supply nodes and increase filtering. It was common practice to oversize carbon composition resistors if you wanted a stable resistance value, rule of thumb was to use double the power rating you needed. I used five watt metal oxide in my build so that they could handle the voltage
woodbutcher65
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What I need, and these photos DO help, is to try to identify the traces underneath where the larger components block the view. Basically that's underneath all the larger capacitors.
I'll spend a long evening with these photos and try to figure out the trace patterns as well as I can.
If I had that amp in my hands I'd be all over that PC board until I had every trace well mapped.
I'll spend a long evening with these photos and try to figure out the trace patterns as well as I can.
If I had that amp in my hands I'd be all over that PC board until I had every trace well mapped.
Ty for the replies! Yes, Mike told me that those 680 2w resistors can get pretty hot! lol. I had amp on stand by for a few hours while leaving room. Luckily I came back when I did! I believe they have upgraded the 680 to a wire wound 3 watt. Let me tell ya WB, I spent many hours tracing out the preamp board after that lil accident with the canned air (sigh). Remember? We talked about it via pm's. I do know where some, if not all of the under side traces lead to. If you have any questions , let me know. Trouble is that I can't draw on my gadget to show where one lead or component connects to. Which would be sooo much easier. I can do my best and try to explain via typing . Blessings, Jim. Side note: which version of the C+ does everyone think is best? Slo Cones? Or " The final version "? Me, I find SC's a lot more accurate. A couple components differ from what's in my C+ .
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Jay, is that an image of A IIB preamp card? And about the 2:90's, I kinda liked mine until I repaired and recapped it.
That’s an image of the card I’m designing for the IIC+ circuit. It’s based off of RP11s
Ty. Man, I would loooove to rebuild my preamp!! I need all new preamp sockets too. Jay, a tech on YT claims that tantalum caps love to fail in Mesa's. My 1984 C+ has some on the preamp board. How do I know if they fail? I lifted a leg on a few and they measured within specs. Thoughts? About that canned air accident, after I completely recapped it, I reflowed the input jack. While amp was fully powered on, not thinking, I sprayed some canned air and I hit the edge of preamp board near tone stack. Needless to say there was a mini " explosion". All of the Spragues OD's turned brown. They eventually turned back to their orange color. That happened 4 years ago. I reached out to other ppl over this. Some said that I need to replace components due to this, not trusting the readings. Some ppl say that if it works, your probably ok. I checked every component that I could. They all tested in spec. Some say although the Orange drops and caps measure in specs, the ESR may be wrong. I mean, the amp works but is it working at its full potential? That's the question that drives myself crazy. Thoughts on that too my friend? Blessings, Jim.
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I’d be wary of what you hear from amp techs on YouTube, I haven’t been very impressed by any I’ve seen. There may be some folks that repair electronics for a living lurking around, they would have better insight than me. I’ve never had one fail but from what I’ve read, tantalums are sensitive to over voltage and tend to fail short, so I’d say as long as you’re developing a cathode voltage they wouldn’t be shorted. I’m not familiar with the canned air you’re referring to, I’ve never used it on an amp so not sure I’m following. But for the orange drops, I’d say as long as they’re blocking DC and the amp sounds good, it’s not worth messing with or stressing over.
If you ever replace your tube sockets, try to remember to snap some pictures of the front and back of the preamp card when you have it out!
Man, I'm sitting here admiring your work!
Impeccable! I need to ask, why did you put another 1M R to ground? I've looked at many EQ cards. I can't see that extra Meg anywhere. That or I'm blind as heck! lol. I'm like a sponge wanting to learn. Aaand, which transistor is backwards? 
. Blessings. Jim.Thanks for the reply. Ok, ill leave the tantalum be.Yes, I checked all of my voltages. There is a few volts on every cathode in preamp circuit. And every Orange drop is blocking DC. And that canned air, That's that stuff ppl buy to blow off dust and whatever from usually computers. Let me tell ya brother, DO NOT use it in a live circuit!
. It took a week for my eyebrows to grow back. lol. And yes , I will snap some picts of preamp card. Jim.Man, I'm sitting here admiring your work!
Thank you!
Not sure what you mean by extra 1M, I used a 1M instead of the 470k shown for R33 in the schematic below that I got from the sloclone site. The one between the caps on the right is R34 and the one in the top right corner is R33 as shown. If you're referring to R34 as extra, it was present in my Mark IIB EQ board which I based mine off. I think R33 was 1M on my IIB and my Mark IV.
The backward transistor is the darlington which is Q1 in the schematic below. It's the transistor on the far right in the picture I believe.
