Mission 5e3 gain very low

[pdf64]

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

[Gee]

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.

[Gee]

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?

[Gee]

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.

[MWJB]

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.

[Gee]

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]

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.

[Gee]

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.

[Gee]

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:

Here is the trace looking at the output from V1, with Volume on 3:

Volume on 10:

And volume on 12:


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]

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.

[Gee]

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

[Gee]

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:

Volume on 4 below:

Volume on 10 below: The inverted signal starting to change from the non inverted signal.

Volume on 11 below:

Volume on 12 below:


How do these post PI traces look?

[Bruce / Mission Amps]

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.

[Gee]

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

[Gee]

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?