Problem witjh Mojotone 5E8-A - Treble pot acting as Volume control

[nadles]

Well I've checked all components and found one resistor of the wrong value. 8.2M instead of 10M. I;ve attached the schematic with the offending component circled in red. I don't know if it is the cause of the problem, but I will swap it out for a 10M. Other components appear to be correct.

[nickb]

Jumping back to the bass path test- the results was as it should be. The volume will reduce at one end at as the pot is grounded at that end.

Based on all the testing I believe that the tone stack is wired correctly. One way I can think of that explains your symptoms is if the tone stack is being driven from a much higher impedance that it should be. Certainly there is some loss of gain somewhere. At this point I would note DC on every tube's pin grid, cathode and plate pin. Also note the DCV of each one the HT+ supplies. The take ACV readings when feeding a 50mV 1Khz sine wave into one input. Annotate the schematic with the results- it already has the DC figures so you can cross check them as you go.

My gut makes me suspect the 12AY7 that is driving the tone stack but it's better to gather data than jump to conclusions.


If you haven't got a signal generator you can you you PC as a sound source and open source software such as Audacity. Adjust the PC volume to get the required signal level. Your ears, wife & neighbors will appreciate it if you use a dummy load while doing this!

BTW the 8.2Mohm vs 10Mohm will make very little difference,

[frus]

nadles, is that a silver mica cap across the two legs of the treble pot? I can't see clearly from your photos, but there should be no cap on treble pot (except the 0.01 from left leg to ground)

EDIT: it's curiuos, there seems to be a 47pF cap on the original Fender layout, but NOT on the schematic. Furthermore, none of the other amps with the same tone stack (5F4, 5E5A, 5E7) exhibit that cap neither on schematic nor layout. Maybe try removing it to see if it makes any difference?

[nadles]

Jumping back to the bass path test- the results was as it should be. The volume will reduce at one end at as the pot is grounded at that end.

Based on all the testing I believe that the tone stack is wired correctly. One way I can think of that explains your symptoms is if the tone stack is being driven from a much higher impedance that it should be. Certainly there is some loss of gain somewhere. At this point I would note DC on every tube's pin grid, cathode and plate pin. Also note the DCV of each one the HT+ supplies. The take ACV readings when feeding a 50mV 1Khz sine wave into one input. Annotate the schematic with the results- it already has the DC figures so you can cross check them as you go.

My gut makes me suspect the 12AY7 that is driving the tone stack but it's better to gather data than jump to conclusions.


If you haven't got a signal generator you can you you PC as a sound source and open source software such as Audacity. Adjust the PC volume to get the required signal level. Your ears, wife & neighbors will appreciate it if you use a dummy load while doing this!

BTW the 8.2Mohm vs 10Mohm will make very little difference,

Thanks Nick. I didn't think that the 8.2M would be the problem, but I was living in hope .

I'll take the readings and try the sine wave.

[Steve A.]

Back of the board


Front of the board

Here are pictures from Mojotone if anyone wants to try comparing the two. (With Mojotone sparing no expense in components you'd think that they could have supplied the Fender wire in the colors that Fender used in their old amps.)





So how come these pix come out as thumbnails? Do you need a link to be able to use the IMG tag?

Steve Ahola

P.S. I don't think that replacing the 10M resistor with 8.2M is causing the problems although you might try disconnecting one end to remove it from the circuit altogether just for the hell of it.

[Steve A.]

It has been suggested that the amp may be working as it is designed to work:

The Fender "E Series" tone controls are very different from most other Fender amps. They offer such a wide range of control, many guitar players have trouble adjusting the amp between songs. It takes a lot of getting used to.

I think that you might be right on here.

1. The treble pot acts as a volume control (affects both channels). With the treble pot on 0 the volume is very low and can barely be heard. start turning the treble pot and the treble increases as does the volume.

The grid of V4A is connected to the treble wiper. With treble set to 0 the signal going to the grid is shunted to ground through a .01/400uF cap which I believe would cut the volume considerably along with making the sound darker. As you turn the treble pot up more of the signal from V3B passes through the 250pF cap which would add brightness to the sound.

2. The amp is not as loud as expected. With the volume and treble on 12 (full) the amp is loud, but not as loud as i would expect from a 40W amp. The sweet spot for the amp is Vol 7, Treble 7 and bass 3. At these settings the 5E8-A is a little louder than my Deluxe Reverb clone with the vol on 3.5. I was expecting the volume to be closer to my JTM45 clone.

Your low power twin uses a cathodyne phase inverter rather than the long tailed pair in the DR and JTM45. The schematic shows 400vdc going to the plates which is kinda low for 6L6GC's. FWIW the Fender Field Guide lists the 5E7 Bandmaster at 26 watts and it has 410vdc going to the plates (the circuit sure looks a lot like the 5E8A.) Also the bias voltage for the 5E7 is -40vdc while that of the 5E8A is -42v. Your selection of speakers can also make a big difference in the output volume.

3. Filter caps discharge themselves as soon as the power is switched off. With the amp on, the filter caps have 455 V. Within approx 10 seconds of turning the power off the Filter caps read 8-11 VDC. (I've tested this both with the meter on the cap as it discharges, and without the meter - results are the same.).

If the caps are discharging too fast it tells me that the charge is being bled off to ground. With the amp off and the caps completely discharged check the DC resistance to ground at different points in the B+ supply.

Steve

P.S. Perhaps you can upload some sound clips demonstrating the amp as you adjust the controls (call out what you are doing so we don't have to guess.) loudthud could tell you if the controls are working normally for the 5E8A.

[nickb]

Before you go crazy on this, I'd like to make sure of one thing. It has been said a couple of times before in different ways but the tone stack on this is not the best. The treble adds highs and the bass adds lows. So, if you set them both to zero, you pretty much get nothing. Turn both the controls to max - is it as loud as it should be? If not then go and do the DC/AC tests.

[loudthud]

There is a link to Duncan's Tone stack calculator on my profile page in the Visitor Messages section. If you are on a Mac, go to the website and bug Duncan about the Mac version. He has been promising one for a while.

Posted below is the results of the E-Series tone stack. Note that with the Treble at 0% and Bass at 50% the response falls at 6dB pre octave from 100Hz to about 3kHz. That's bound to sound pretty muffled.

[Steve A.]

There is a link to Duncan's Tone stack calculator on my profile page in the messages section. Posted below is the results of the E-Series tone stack. Note that with the Treble at 0% and Bass at 50% the response falls at 6dB pre octave from 100Hz to about 3kHz. That's bound to sound pretty muffled.

I never realized that what the "E-Series" was in Duncan's TSC. Thanks for the heads up! I think it is fun to just play with the controls from 0-0 to 10-10 and everything in between which is pretty amazing! At 10-10 the midrange is very scooped just like a BF tone stack but it is centered around ~650Hz rather than ~200Hz. With treble set 7 watch how the graph changes as you raise the bass from 0 to 10. (Be sure to change R3 from 100k to 220k for the 5E8A.)

Here is a link to Duncan's site:

TSC

Steve Ahola

[Diablo]

I checked out your observations using my 5E8A clone. Both the treble and bass controls effect the volume by quite a bit. The tone stack is quite interactive. Typically I leave the bass control at 6 (middle of pot), and dial in the treble as needed (usually around 3-4), presence control at 6. I use the normal channel, not the bright channel. Mine is also modded to use EL34 tubes as I think the amp sounds better with them than 6L6s or 5881s.

[nadles]

I checked out your observations using my 5E8A clone. Both the treble and bass controls effect the volume by quite a bit. The tone stack is quite interactive. Typically I leave the bass control at 6 (middle of pot), and dial in the treble as needed (usually around 3-4), presence control at 6. I use the normal channel, not the bright channel. Mine is also modded to use EL34 tubes as I think the amp sounds better with them than 6L6s or 5881s.

Thanks Diablo. How loud is your 5E8A?

I've read reviews of the reissue which say that it will compete with a drummer at 3-4 and a 6 you need ear plugs. Mine is not that loud.

with volume at 3-4 you can talk over it and with all controls on 6, its as loud as my Deluxe Reverb with volume on 3.

I've been comparing it with other amps and I don't believe the 5E8A is as loud as it should be. With all controls on 12 it would be as loud as a cranked Deluxe Reverb.

I'll try and record some sound clips at different settings o give an indication of how it sounds.

I also need to make a dummy load to try the sine wave and AC measurement. That sine wave is most unpleasant

BTW what settings should be used when playing the sine wave and tracing the AC?

[loudthud]

On the volume-power issue it could just be bias. Looking at the voltage readings in the first post the B+ is on the high side, bias is more negative than seems normal. The bias on the phase inverter looks reasonable. Did you do anything to set the bias? Is there a bias pot? Do you have a bias probe or can you install 1 ohm resistors between the power tube cathodes and ground so bias current can be measured?

Edit and Comment: I wish people wouldn't just blindly follow Fender schematics and layouts from the 50's. These old amps need safety updates (especially international builders), ways to measure / adjust bias and proper heater circuits.

Grounding is another issue. Trouble usually happends when somebody trys to do a star ground with spade lugs on a transformer bolt. In that case you would be better off following what Leo did.

[Steve A.]

On the volume-power issue it could just be bias. Looking at the voltage readings in the first post the B+ is on the high side, bias is more negative than seems normal.

I agree that the amp need a bias adjustment pot. I would try copying the circuit that Fender used in their reissue. (They recommend an idle bias current of 33.3mA per tube.) The Fender reissue also uses 12AX7's for the initial preamp tubes instead of the 12AY7 used in the original. It also uses 2 initial preamp tubes- one stage for each input jack which is really cool because you can bias each tube stage differently and get away from the Hi/Lo input jack trick that Fender used on so many amps. The Reissue also uses a 47pF cap from the wiper of the treble pot to the 250pF cap. It also uses a special audio taper for the two volume pots so the numbers on the reissue might not line up with the numbers on the Mojotone kit. It seems like we have some apples and oranges here...

Here is a link to the Fender reissue schematic- and a big cheer to Fender for posting these on their website! I used to have to borrow them from the amps at the local music store and make copies. (For awhile you could find schematics on the European Fender sites but not the US one.) People are going to reverse engineer designs if they really want so why not just make them readily available?

http://support.fender.com/schematics..._schematic.pdf

Steve Ahola

[Diablo]

Thanks Diablo. How loud is your 5E8A?

I've read reviews of the reissue which say that it will compete with a drummer at 3-4 and a 6 you need ear plugs. Mine is not that loud.

with volume at 3-4 you can talk over it and with all controls on 6, its as loud as my Deluxe Reverb with volume on 3.

I've been comparing it with other amps and I don't believe the 5E8A is as loud as it should be. With all controls on 12 it would be as loud as a cranked Deluxe Reverb.

I'll try and record some sound clips at different settings o give an indication of how it sounds.

I also need to make a dummy load to try the sine wave and AC measurement. That sine wave is most unpleasant

BTW what settings should be used when playing the sine wave and tracing the AC?

My amp is plenty loud, much louder than my Deluxe Reverb clone. Mine also has the 12AY7 tubes like the original from the 1950s. I agree with Loudthud that I think your bias setting is too cold on your power tubes. My clone has adjustable bias. If yours doesn't, you can install one. Take a look at the Weber schematic for the bias circuit below.

https://taweber.powweb.com/store/5e8a_schem.jpg

[nickb]

BTW what settings should be used when playing the sine wave and tracing the AC?

I have some ROUGH numbers. These might be off to some degree. Maybe someone who actually has one of these in front of them and give you their measurements.

Set the presence to minimum (high resistance), Treble and bass to midway and volumes to min. Apply 50mV at 1Khz rms to input 1. Monitor the output across your 4 ohm dummy load and slowly increase the volume. When you get to 10V rms on the output I would expect the volume to be at 20% (i.e ground side to wiper 200K ohms)

With it set up like that, take Vac readings through the signal path. Please take the greatest care!

You should be able to get to about 16Vrms before clipping starts but it's going to be very hard to know your at clipping without a scope.