View Full Version : Mission 5e3 gain very low
I have just completed a Mission 5E3 using a Mojo 761 Export PT for 240V operation in the UK. Everything fired up OK and the voltages look about perfect using the Mission Tweedy schematic voltages as a reference. Everything works as expected except the gain is far too low. E.g. using the stock 12AY7 there is only the most mild breakup using the Bright channel Hi input with the Normal volume low. Increasing the Normal volume removes all gain and the overall volume drops. Using a 5751 in V1 was not much better and using a 12AX7 in V1 produces a bit more gain that starts at about 10. I've also used another 12AX7 for V2.
All 4 inputs appear to work as expected, i.e. Bright channel is brighter than Normal channel and both low inputs are a bit attenuated, and the tone control works OK. The lack of gain exists on all 4 inputs. Using a dummy jack to open the jack switches I can get the expected values and trace through to V1. I have re-soldered all connections, traced out the circuit and measured all resistors, checked all grounds for zero impedance.
The circuit is completely standard Mission 5E3 except for:
I originally wired it up with a choke and added 2 470R screen resistors but when the voltages looked quite different to a non choke 5E3 I removed the choke and put the original 5K1 resistor back and left the screen resistors in there. The 2nd dropping resistor is the standard 22K.
I have added a second zener before the PT center tap to drop the B+ from 369V with 1 zener.
The first cap and cathode resistor ground goes to one of the PT bolts.
I used a 300R cathode resistor to reduce the bias a bit.
My guitar uses single coils and the speaker is a know to be working Weber 8 Ohm 12A125 plugged into the 8 Ohm speaker jack.
The voltages all seem OK:
V1 Pin 1 =117V
V1 Pin 3 =1.9V
V1 Pin 4 =6.9VAC
V1 Pin 5 =6.9VAC
V1 Pin 6 =118V
V1 Pin 8 =1.9V
V1 Pin 9 =6.9VAC
V2 Pin 1 =153V
V2 Pin 3 =1.2V
V2 Pin 4 =6.9VAC
V2 Pin 5 =6.9VAC
V2 Pin 6 =192V
V2 Pin 8 =47V
V2 Pin 9 =6.9VAC
V3 Pin 2 =6.9VAC
V3 Pin 3 =350V
V3 Pin 4 =329V
V3 Pin 7 =6.9VAC
V3 Pin 8 =21.6V
V3 Pin 2 =6.9VAC
V3 Pin 3 =351V
V3 Pin 4 =329V
V3 Pin 7 =6.9VAC
V3 Pin 8 =21.6V
1st cap = 362V
2nd cap = 330V
3rd cap = 240V
It seems that there is either a loss of signal somewhere or the amp can't deliver enough current to amplify what signal is there.
I'm running out of ideas to try here. Anyone got any pointers?
I have taken some pictures that may give some more useful clues to this problem:
http://www.cyclecouriers.com/5e31.jpg
http://www.cyclecouriers.com/5e32.jpg
http://www.cyclecouriers.com/5e33.jpg
http://www.cyclecouriers.com/5e34.jpg
http://www.cyclecouriers.com/5e35.jpg
http://www.cyclecouriers.com/5e36.jpg
http://www.cyclecouriers.com/5e37.jpg
http://www.cyclecouriers.com/5e38.jpg
I'm still struggling with this.
What I did today was:
1. Dissconnect all input wires from all 4 jacks and wire in a single Cliff jack with 1M directly to one of the 68K input resistors and also a ground point on the one of the jacks. The symptom of very little gain was exactly the same.
2. I ran a short length of wire with crock clips from the input ground to various ground points.
3. I used the same link from all combinations of grounds to other ground points - no change.
4. I redid the mains ground, first filter cap and cathode resistor ground by bolting a high tensile 6mm bolt through the chassis, fitted with star washers and tightened the hell out of it. I was previously using one of the PT mounting studs and I was tightening the bolt close to the point of shearing - no change.
5. I did a bit more re-soldering of joints I had already re-soldered.
6. I chopsticked about but found no suspect joints.
I finished off by playing my 5F2-A through the 5E3 cab and speaker. That thing is a monster compared to the 5E3 which hardly even begins to purr.
What can it be?
I have traced out the schematic using my board as the source and all resistor values and connections check out fine. I was measuring resistance from adjacent components or links to the pins in the tube with the tube half pulled out to expose the pins.
The only thing I found which was out of spec was one of the volume pots read 835K instead of 1M. I changed it for a 1M pot that measured 1M.
I am now 100% sure that the circuit is correct and if there is a piece of stray solder or wire under the board, I can't detect it by measuring between adjacent components. I have even tried to blow out anything lose under the board with strong blasts of air.
One thing I did notice, is I left off the 4700pF tone cap connection to ground and the gain increased a bit. I always thought that the 5E3 tone stack was pretty efficient so I don't know if this is normal or maybe a pointer to the problem.
Now if I use a 12AX7 in V1 when the volume is on 12 I get more breakup than before but still way less that my 5F2-A. This is driving me crazy and using up valuable time that I should be using playing through the amp.
To resolve any possible wiring issue I removed the board and found there wasn't any disconnected or shorted wires or anything conductive under the board. So I then removed all solder and re-soldered and checked the circuit. This circuit is exactly as it should be.
I put the board back in checked it and the amp is exactly the same. Still no gain.
My plan today is to use a tuner as a signal generator and trace the signal through the amp.
I have been tracing the signal through the circuit. I found the guitar tuner output to be too low so I used an iPod looping a 440Hz sine wave. All 4 inputs give the same reading. All voltages are referenced to ground.
Here's what I get.
The voltage on pin V1 7 (12AY7) is 0.006VAC.
Just after the first coupling (0.1uF) cap and going to the volume pot, I get 0.127VAC on low volume and 0.193VAC on Max volume for that channel. This value decreases with the volume control and I can also see it decrease if I set the volume to 12 and then adjust the other channel volume to full.
I can't read the voltages on 1 or 7 because it jumps about too much. I get just a few mA of DC after each coupling cap.
The voltage going into is V3 (pin5) is 0.955VAC and V4 (pin5) is 4.37VAC on the low volume setting and 10.8VAC on max volume. The other 6V6 is slightly higher with 4.51VAC and 11.3VAC. I can't get the DVM to read the output voltages.
Do these voltages look OK? Is the difference in voltage between V3 and V4 acceptable?
TD_Madden
03-28-2007, 01:26 AM
Suggest you email Bruce at bruce@missionamps.com. He and I had an email exchange a few years ago on one of his kits....turns out I misread the layout AND some resistors....
Suggest you email Bruce at bruce@missionamps.com. He and I had an email exchange a few years ago on one of his kits....turns out I misread the layout AND some resistors....
I have emailed Bruce but not heard anything back. He is normally so quick to reply that I assume he must be on holiday or something. I would certainly like to hear what Bruce thinks is the problem with this one as it's driving me crazy.
pdf64
03-28-2007, 12:57 PM
Hi Gee, 0.006V = 6mV, are you sure that's what is coming out of your ipod, I would expect, and you need, 10x this? Is you meter good above 100Hz? When you've confirmed a decent test signal (aim for 100mVac) then try the same test on your bassman and deluxe ie comparing input voltage to grid voltage to anode voltage. If you follow that through it should be straightforward to narrow the problem down. Good Luck - Peter
Hi Gee, 0.006V = 6mV, are you sure that's what is coming out of your ipod, I would expect, and you need, 10x this? Is you meter good above 100Hz? When you've confirmed a decent test signal (aim for 100mVac) then try the same test on your bassman and deluxe ie comparing input voltage to grid voltage to anode voltage. If you follow that through it should be straightforward to narrow the problem down. Good Luck - Peter
peter,
I was a bit surprised at the low reading. I've just checked my DVM (a Uni-Trend UT60A (http://www.uni-trend.com/UT60A.html)and the AC voltage lowest range is 4V so it looks like the low AC voltages will not be correct. I'm wondering if I should get a new more sensitive meter?
pdf64
03-29-2007, 03:54 PM
Hi Gee, your meter isn't the best but it will serve ok for troubleshooting this (hopefully!). From page 33 of the users manual from that link, the frequency response for Vac ranges is 40 to 400Hz, so maybe lower your test signal to 250Hz. There's free shareware available to utilise your pc/soundcard as a signal generator which might be worth you investigating, try googling for it.
So, to reiterate, get yourself a decent test signal (at least 50 to 100 mV rms at about 250Hz) that you can measure on your meter. Plug that into your bassman and see what you get on the V1 anodes (pins 1 and 7), then try the same on your 5E3. Once you've done that we'll be in a position to move forward on this. Good luck - Peter
Hi Peter,
I just picked up a better meter and will repeat my measurements. I don't have a Bassman but I can repeat the measurents on a 5F2-A.
I made some of the changes that we discussed off line. I moved the 100R filament resistors off the V4 socket that were tied to the cathode, and tied them to ground instead. Also I ran a ground directly to the speaker socket. Unfortunately the amp is just the same - still low on gain. The amp is just as quiet with the 100R filament resistors grounded so I will leave them that way.
One other thing I have noticed is that what little gain there is, appears to be at its maximum when the tone control is maxed. Is this normal behavior for a 5E3?
PS Had an email from Bruce Collins last night. He has been in hospital and will need at least a couple more weeks to recover. Hope he makes a full recovery as very soon as possible.
With a new more accurate meter and the iPod output turned up to a more healthy ~50mV outut level, playing the same 440Hz sine wave here are the voltage readings I get.
Bright Channel, Hi input voltage at 68K resistor junction = 52mVAC
Bright Channel, Lo input voltage at 68K resistor junction = 35mVAC
Normal Channel, Hi input voltage at 68K resistor junction = 52mVAC
Normal Channel, Lo input voltage at 68K resistor junction = 35mVAC
Then using the same signal in the Bright Channel Hi input:
The voltage on pin V1 7 (12AY7) is also 52mVAC.
Just after the first coupling (0.1uF) cap and going to the volume pot, I get 1.27VAC on full volume, tone set to max. This value decreases when the Bright volume control is turned down and I can also see it decrease if I set the volume to 12 and then adjust the other channel volume to full. This voltage also decreases a bit as the tone control is turned down from Max.
So this is an amplification of about 24X. Is this the right sort of amount amplification for a 12AY7?
As an experiment, I changed the 12AY7 for a 12AX7 and the max voltage at the same point was 2.5VAC which equates to an amplification of about 48X. I then changed back to the 12AY7.
I get just a few mA of DC after each coupling cap.
The voltage going into is V3 (pin5) is 27.2VAC and V4 (pin5) is 28.9VAC on max volume. These readings reduce as the volume control is reduced.
So how do these voltages look and how does the gain look?
One thing I did notice is that when I am measuring the input voltage where the two 68K resistors come together, the voltage increases from 52mV to about 62mV when the volume control goes from about 10 to 12. Could this be extra signal being picked by the hum induced by the probe touching the input and then amplified by the circuit?
Tried a different OT today and the problem of low gain is still the same. So I put the original OT back in there. If I play the guitar through the amp I see a maximum of about 10VAC across the 8 Ohm speaker which makes the output about 12.5W. Fully cranked the B+ drops by about 12V. So it seems like the power tubes are working OK.
I also bypassed the 2 voltage dropping Zeners on the PT center tap, to see if they were somehow causing the problem. No change without them.
I changed the V1 bypass cap to 1uF and this did improve the gain. I would say that the gain is about 50% there now. The original 22uF measured OK. I changed the other preamp cathode bypass for a 10uF (all I had) just in case they were part of a faulty batch.
Still trying to track this one down. I have since changed out all of the coupling caps and also tried another set of matched 6V6GT's. The gain is still low.
pdf64
04-03-2007, 06:25 PM
Hi Gee - changing the cathode bypass cap from 22uF to 1uF shouldn't increase the gain. You should be able to measure this using your method above, comparing the 2 bypass caps. Use the control setting that give you the maximum 1st stage voltage output figure ie active channel vol max, other channel vol min, tone max.
The gains you have measured so far for the 1st stage are reasonable (expect about 30 for 12ay7, about 60 for 12ax7).
Your power output seems reasonable - is that 10Vac a pure sounding sine wave or can you hear distortion on it?
I suspect from what you said about the measured input voltage rising as the volume was turned down that there's a ground problem somewhere. Double check all your ground connections (B+ filter cap ground, input socket, volume / tone controls, cathode cap / resistor - mark them off on the schematic as you confirm that they're good).
Hope that helps - Peter
Hi Gee - changing the cathode bypass cap from 22uF to 1uF shouldn't increase the gain.
Yes I agree. I think what it indicates is that either the 22uF cap was faulty (it measures OK though) or the process of replacing it changed something.
The gains you have measured so far for the 1st stage are reasonable (expect about 30 for 12ay7, about 60 for 12ax7).
Your power output seems reasonable - is that 10Vac a pure sounding sine wave or can you hear distortion on it?
I think the 10Vac is undistorted as it souded undistorted. I'm using a DVM and I'm not really sure it the Vac readings will be RMS or average. I wish I had a scope.
I suspect from what you said about the measured input voltage rising as the volume was turned down that there's a ground problem somewhere. Double check all your ground connections (B+ filter cap ground, input socket, volume / tone controls, cathode cap / resistor - mark them off on the schematic as you confirm that they're good).
Hope that helps - Peter
I have done this many times.
Thinking out loud here about the other thread about the volume control not behaving normally.
The low gain problem that I have also exhibits a similar (not conventional) strange volume behavior on both channels. E.g. Using the Bright channel Hi, with the tone on max and Normal volume on min, my Bright volume increases to about 3 then stays at about the same level until about 9.5, and the increases in loudness and gain.
Could this be a clue to my problem?
I forgot to mention that in the above post about the unusual volume behavior, that this is when using a 12AX7 in V1 (and V2).
With a 12AX7 in V1 I would say that the gain is about 75% of what it should be in a a normal 5E3.
Well if you are 100% positive that everything is connected OK and you have eliminated the obvious components, start looking at your layout.
Your grid wires to the power tubes look like they could stand to lose a couple of inches to start with, get these as short as possible. Consider remounting the 1.5K grid stoppers vertically from the socket pin 5 to allow shorter grid wires.
I can't say whether it is actually related to your problems or not, but why daisy chain the pot grounds? Ground the volume pots & the tone pot cap to the back of the pots. Remove any varnish from the pot backs first and stake the wires/components in place whilst the solder cools for a good mechanical connection.
I don't see how a scope would really help you at this stage, if you can audibly hear that its not right ...then it's not right. When you say its only 75% of the gain of a regular 5E3, is there a functioning model that you are comparing to?
You mentioned that your in 240VAC area, perhaps if you tell us where there might be somebody who can help you? There is still the possibility that there is something you are overlooking that someone else might spot.
Nevertheless, shorten those grid wires and get the layout as close to a working 5E3 as possible.
Your grid wires to the power tubes look like they could stand to lose a couple of inches to start with, get these as short as possible. Consider remounting the 1.5K grid stoppers vertically from the socket pin 5 to allow shorter grid wires.
You're right, the grid wires were a bit long and I have already shortened them. The grid resistors are mounted to match Bruce's diagram for the of fitting screen resistors to go with his choke kit. I am not using the choke kit (yet) but have mounted the Screen resistors on the 6V6 sockets. Currently I am shorting the screen resistor leads to simulate the screen resistors not being there. Still the same however.
I can't say whether it is actually related to your problems or not, but why daisy chain the pot grounds? Ground the volume pots & the tone pot cap to the back of the pots. Remove any varnish from the pot backs first and stake the wires/components in place whilst the solder cools for a good mechanical connection.
I have redone the grounding a few times. Currently, the 1st filter cap and cathode resistor & cap are grounded to a large high tensile bolt, fitted with star washers, tightened right up and fitted between the board and the PT. The 2nd, 3rd cap and 220K grid supply resistors are also grounded to the bolt. The other grounds are as per Bruce's layout using the brass plate. I have removed the brass plate, scuffed up the rear of the plate and chassis underneath and tightened up all the I/P jack as tight as I dare. All ground points read no resistance with my 3 different meters. The amp plays well and has no ghost notes or phantom "grounding issue" sort of sounds. I'm not a fan of using the back of the pots really. Each pot ground links back to the Bright Hi I/P jack as per Bruce's diagram.
I don't see how a scope would really help you at this stage, if you can audibly hear that its not right ...then it's not right. When you say its only 75% of the gain of a regular 5E3, is there a functioning model that you are comparing to?
I would like a scope to measure the peak voltage. From the readings I have taken with my DVM it looks like V2b is attenuating the signal. Although it may be the average way that the DVM reads VAC, or as Randy at Mission mentioned, it may be the DVM probe is unsettling the PI readings.
Ideally, it would be great to compare to a set of complete DC and VAC readings for a working 5E3. I know that Bruce has this planned.
The amp with a 12AY7 in V1, behaves like a clean amp with a small amount of breakup and a delicate amount of crunch. Having changed out the V1 cathode cap, now with a 12AX7 in V1 it has a great clean output up until about 6 and then breakup creeps in and on 12 it has a Champ like breakup. I have heard many 5E3 clips and I would be surprised if my expectation of a 5E3 is different to what they really sound like. I also think that the fact that this amps volume behavior is different demonstrates that something is not right. I.e. As I understand it, a 5E3 has a small amount of great sounding clean but this disapears by 2 to 3 when breakup sets in and increases to 12 with little to no increase in overall sound volume.
You mentioned that your in 240VAC area, perhaps if you tell us where there might be somebody who can help you? There is still the possibility that there is something you are overlooking that someone else might spot.
I'm in sunny 240 Volt province of Cardiff in Wales if you know anyone near here.
Thanks. Gary
The Captain
04-06-2007, 12:45 PM
Hi Gee- Ive noticed low gain too with a 240v PT in UK. I was wondering if using the 230v EU primary would make a difference? could well be nonsense idea- I haven't tried as Im much below in tech skill as yourself, but perhaps mains V in UK is closer to 230v than 240v thesedays? As I said I dont fullly know what Im on about, so its a hunch really.
Hi Gee- Ive noticed low gain too with a 240v PT in UK. I was wondering if using the 230v EU primary would make a difference? could well be nonsense idea- I haven't tried as Im much below in tech skill as yourself, but perhaps mains V in UK is closer to 230v than 240v thesedays? As I said I dont fullly know what Im on about, so its a hunch really.
The PT seems to be OK. It doesn't have a 230V tap anyway (only 220 and 240V), but I'm getting the right sort of voltages from it so it is reasonable to assume that the the 240V configuration is the correct one for here in the UK. The filaments are a bit high at 6.9VAC but that's also OK. As far as I know, too high a filament voltage just shortens the life of the tubes slightly.
I have just picked up an Oscilloscope (for £10 off eBay) and done some investigating in my low gain 5E3.
I can see something strange happening when I look at the output from V1. I am using a 440Hz sine input at 100mV peak to peak into the Bright Hi input, tone on 6, Normal Ch on 0.
The input looks fine below:
http://www.computerhomefix.com/a001.jpg
Here is the trace looking at the output from V1, with Volume on 3:
http://www.computerhomefix.com/a002.jpg
Volume on 10:
http://www.computerhomefix.com/a003.jpg
And volume on 12:
http://www.computerhomefix.com/a004.jpg
As you can see from the final picture, the top of the signal is being clipped between volume 10 and 12.
Can anyone explain what is happening here and how to cure it if it is a problem?
Bruce / Mission Amps
04-21-2007, 03:25 AM
I do the same test on my 5E3 and although the picture is not perfectly linear 100%, the signal is not clipped... it looks almost just like your input picture with the peaks being a little more round.
I do the same test on my 5E3 and although the picture is not perfectly linear 100%, the signal is not clipped... it looks almost just like your input picture with the peaks being a little more round.
Hi Bruce,
Are you saying that you don't see the same cutoff that I see on mine?
Is what I am seeing normal or could this be grid cut off in V1?
I don't understand why there is a reduction of peak to peak voltage between volume setting 10 and 12 which is where the clipping starts. With a 12AX7 in V1, that is exactly where the amp starts to really breakup.
Thanks,
Gary
I've been doing some more probing about with the scope and monitoring voltages as the volume is altered.
I can see that there is something that happens when the volume (Bright with Normal on 1, Tone on 12) gets to about 10.
Here's what I measure (with Normal Vol on 2, Tone on 12):
Pre Amp screen supply (3rd cap) = 332Vdc Bright Volume 0
Pre Amp screen supply (3rd cap) = 302Vdc Bright Volume 10
Pre Amp screen supply (3rd cap) = 308Vdc Bright Volume 12
6v6 Screen supply (2nd cap) = 316Vdc Bright Volume 0
6v6 Screen supply (2nd cap) = 270Vdc Bright Volume 10
6v6 Screen supply (2nd cap) = 280Vdc Bright Volume 12
B= (1std cap) = 360Vdc Bright Volume 0
B= (1std cap) = 340Vdc Bright Volume 10
B= (1std cap) = 348Vdc Bright Volume 12
V2 Cathode = 1.1Vdc Bright Volume 0
V2 Cathode = 0.5Vdc Bright Volume 12
V3&4 Cathodes = 20.5Vdc Bright Volume 0
V3&4 Cathodes = 28.1Vdc Bright Volume 10
V3&4 Cathodes = 25.2Vdc Bright Volume 12
Is it normal for the voltages to drop so much as the volume increases?
Also is the amount that the power tube cathode voltage increases by, normal?
Here are some more traces showing the outputs from the PI.
Volume on 2 below:
http://www.computerhomefix.com/a005.jpg
Volume on 4 below:
http://www.computerhomefix.com/a006.jpg
Volume on 10 below: The inverted signal starting to change from the non inverted signal.
http://www.computerhomefix.com/a007.jpg
Volume on 11 below:
http://www.computerhomefix.com/a008.jpg
Volume on 12 below:
http://www.computerhomefix.com/a009.jpg
How do these post PI traces look?
Bruce / Mission Amps
04-21-2007, 07:12 PM
I don't know right off hand but, when I get a few spare minutes...
I'll take the tweed 5E11 Vibrolux chassis I have on my bench off life support and put the 5E3 back up and check it out with the same setup you have.
I don't know right off hand but, when I get a few spare minutes...
I'll take the tweed 5E11 Vibrolux chassis I have on my bench off life support and put the 5E3 back up and check it out with the same setup you have.
I would very much appreciate that Bruce.
Thanks,
Gary
I have been enjoying playing my Deluxe recently with a 12AX7 in V1. The gain is low but what gain there is makes this a great amp.
However, the gain is still lower that it should be and I have some new enthusiasm to track it down. Since my last post I have tried splitting the tone controls as a test and also changed out the 3 power caps - but the gain is still low. I have put back the original tone stack and original power caps.
Today I installed a couple of DPDT center off switches to fine tune the cathode caps for V1 and V2a. I have fitted these before on a few amps and my experience is that there is always a drop in output volume when the center off position is selected, i.e. no cathode cap across the cathode resistor. This is exactly what I find with V2a but I don't get this behavior with V1. On V1 I am using a 0.68uF and a 25uF cap so I would expect to hear a difference. I have checked the circuit which is fine and measured the voltages on the ends of the caps which follow the V1 cathode voltage when that cap is switched in. I believe that the caps are wired correctly yet I don't see the behavior that I would expect.
Can anyone comment on what might be happening here or confirm if this is normal behavior?
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