Mission 5e3 gain very low

[Gee]

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?

[Gee]

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

[Gee]

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?

[Gee]

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.

[Gee]

Still no gain in my 5E3

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.

[Gee]

Hey Bruce where are you?

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]

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....

[Gee]

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]

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

[Gee]

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 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]

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

[Gee]

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.

[Gee]

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?

[Gee]

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.

[Gee]

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.