ADA Depot - A Forum To Support Users of ADA Amplification Gear
ADA Preamps => Original MP-1 => Topic started by: timkimball on June 18, 2015, 06:30:12 AM
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I suddenly have low gain in both the tube configurations. SS is fine.
I replaced D16 with an 1N4004 as in the 'gain loss' thread. I started a new thread because this seems to be a different issue, or at least I'm not sure if that fix was related to the tube section only.
The low volume that comes out is still distorted, and the OD1 LED still functions as it always has.
Any ideas?
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Hey Tim, I'm sure MJMP and others will chime in to help. So dumb question, so you had the gain loss and then replaced D16 and it didn't fix it ?
If OD1 LED is flashing there should good signal in "that" tube stage, though OD 2 ? (so could be a tube ? (in OD 2)) worth a try. Do you have decent signal at FX loop send (plug direct into one MT200 channel) or Headphone out (do half insert (or full) and plug into MT200 (or use insert cable for stereo (RTS > TS/TS))), but just to trouble shoot.
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Most of the time it's a bad tube but you can also check the voltage,should be around 190Vdc.You can measure this at either R1 or R4 on the tubeboard.
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Hey Richard,
Yep, I replaced R16 and no change. I noticed the OD1 LED because my main setting always has it clipping a bit. I'll mess around and see if OD2 will light. A friend is loaning me some tubes this week so I'll have some to switch out.
Thanks for the help guys. I'll post back once I have some news, I just haven't had a chance to get into it yet.
Tim
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Do you have an OD1 and OD2 led (old mp-1's) or Input and OD1 led (newer MP-1's)?
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Hey Tim, no worries, let us know how you go, I'm sure between us all we'll get to the bottom of it :thumb-up: and get you up and rocking again.
Cheers R
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My MP-1 is a newer one then, the LEDs are input/OD1/Master/EQ ( I was typing at work when I mentioned trying to get OD2 to light)
I can max out every parameter in Tube Clean and Tube Dist will only get the OD1 LED to light.
I tried different tubes with no change. I have 3 that I switched out in different combination with the old ones. They were not new but my friend said that they were working when he pulled them from his amp.
The voltages on the tube board: R1 = 190VDC and R4 = 184VDC, these are from the low side of the resistor/ lead towards front of the MP-1
I checked the traces to D16 and am fairly sure I didn't break one.
Thanks for the help
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Hey Tim. MJMP will know better but 184v at R4 is probably not good ? But he will update us :thumb-up:
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Well normally one side of the R1,R4 should measure 190V,the other side around 20V less so about 170V depending on the tube.This can vary a bit and only applies to the orginal tubeboard (no mods).
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I switched around the 5 tubes I have, this time keeping better notes, and get consistent voltage (within +/- 0.2V) with all the tubes so I'm ruling out tube issues.
6.8V drop on R1 (V2) and 13.0V on R4 (V1) vs the 20V you stated...sounds like both sides are having problems. Much less current running through these than there should be.
I'm getting 197 V out of the rectifier vs 190V
One thing I noticed is that the ground wire coming out of the board at one point shorted to pin 9 on V2. The circuit makes no connection to this lead (heater tap) but I can't see where 4 or 5 are going either. I don't know what to think of it.
I haven't done any mods to this other than the input jack. I've owned this since about 1995. It looks like there was some re-soldering done on the battery and a couple random spots on the main board (just from shiny flux spots), as far as I know the tube board is original.
I'm not sure what to do next, but I have the parts for the Noise Mod. I'm thinking I will do that, maybe I'll by chance replace a bad part with this mod or just jar something loose...I figure I have nothing to lose by doing it.
The ISP Theta pedal is looking pretty good right now though.
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The ironic thing is that now when my MP-1 is out of commission... my MT200 power amp isn't having any issues with the protect mode! Starts up right away, all the time.
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Mmm, hard to see why the MT200 protect mode would affected by the MP1, generally the protect mode issue is caused by a Neon light and sensor. (posted about elsewhere http://adadepot.com/index.php?topic=690.0).
Did you get to trying/checking FX send and/or headphone signal ? Although as you have SS output fine, it would seem MJMP has you looking in the right places.
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Well 13V is still ok,just tested some of the mp-1's i have and they varied between 13V and 18V.Now that 6.8V seems to low for me so you could check that section out.Power it off and check the Ra (100k) and Rk(1.1k) resistors.
That's not pin 9 but pin 1 on V2 were the shield wire is connected.The heater traces going to pin 4 and 5 run on top of the board (it's a dual sided pcb).These are hard to see if all components are fitted.
Also measure the resistance between the OD2 shielded wire(between the shield and signal wire).Should be very high.
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I am back in action!!! :whoohoo!: R2 on the tube board was open, replaced that and seems that all is well. :thumb-up: I have the same voltage at both R1 and R4 (~184v).
I'm just going to hot glue between the tube socket and that shield wire and get everything back together. Thanks for the help Rnolan and MJMP. Thanks for the info on the tube voltages, without the help (and the schematics and info on this site) I'd be out shopping for a new preamp.
I was a little bit looking for an excuse to buy something new, already had my buddy lined up to have it sent to his house (under my wife's radar). But now that I heard my MP1 again, I know I don't need anything else.
Rnolan, yah the amp and preamp are completely independent, just funny that the amp was giving me crap on and off but now it consistently started up when it was of no use to me. It'll act up again now that my preamp is working, haha. I've read about the lamp and fix, it just hasn't completely crapped out on me so I haven't done anything about it. I had checked the FX loop to make sure I was getting the same results with it in or out and had the same results, hadn't mentioned it
Again, Thanks!.
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Hey Tim, great news :thumb-up: :whoohoo!: and at our end very happy to help :wave: Check that post re MT200 (if you haven't already), you can use a torch apparently if it's acting up (bandaid but apparently works).
Ok so now it's time to sort out your tubes, patches, patching (leads) and FXs LoL. Lots of posts here with good advice and experience.
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Ah nice,you fixed it,must say i only had this happen once and a long time ago that one of the anode resistors failed.
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Hey MJMP, I know your generally a man of few words (antithesis of me LoL and my long raves and diatribes). So R2 on the tube board failed (and this is very rare), it's an Anode resistor (= resistor in the Anode part of the tube circuit (to ensure the correct voltage is applied to the Anode in the tube ??)), so (if your up for it, I'm interested to learn more) could you please explain what that meant for the circuit ?
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It's rare in a MP-1 but it can happen,but it happends more in tube heads,usually with low wattage resistors they use now and I see it a lot in older marshall's and fenders where they use carbon composite and these tend to go up in value after years of use.Last i worked on a 76 JMP and one shifted from 100k to almost 500k. :o
So if the Ra (anode resistor) goes up in value the current will drop and the gain will increase.But this will give less linearity and a higher output impedance,so for ECC83/12ax7 tubes the standard is 100k and 220k,higher then this and current will be to low and the sound becommes fizzy and will not be good anymore.You can see this when you start with drawning load lines (see pic).It's just ohms law to draw it,take the Va voltage and devide it with Ra.This will give you the max current.As you can see the 47k line crosses the lines where they are very straight (so lineair,BTW this is from the ECC83 datasheet Ia vs Va).
100k is lower but as you can see the lines it crosses isn't so lineair anymore and it's even worse for 220k.And from there you can choose the bias point so you can calculate Rk etc.... waaay to much to explain.Works for preamp and power tubes.But it can also be done with just plain formulas and calculations.Both will work but using the datasheet way (it's more geometry) isn't so easy.But i'm sure there's a lot of info about it on the net.
For instance here's an example on how to calculate a design.
http://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
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http://www.ax84.com/static/p1x/p1-ex-theory.pdf
Here is a very good pdf on tube design.
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Hey MJMP, thanks my friend, very edifying :thumb-up: I have a little reading to do LoL
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No problem :thumb-up: