5E3 performance

[dfoxdavies]

Thanks for taking a look. I'll check that volume pot tomorrow.

[angelodp]

That black cap appears to be grounded on the pot surface?? Clip that tail off so it's not grounding out. The 470pf cap, is it touching the pot? Also the Tone pot wiper looks like the wire might be extending to the outer lug....maybe not.

[dfoxdavies]

I can see that the capacitor leads appear close to the pot bodies, but they are not touching. I have clipped/moved them so they are definitely not a problem. The connections to the pot lugs do appear that there may be a solder bridge to the pot body, but this slightly different view shows the gap.

20240728_111528.jpg

I think if there had been contact, particularly with the pot lugs, there would be zero output from the bright channel, right? Which is not the case.

I'm trying to understand the mechanism by which channel interaction occurs. The unused input jack sockets are grounded out (by the tip shunt). I would guess this would be to prevent hum (?). The two channels then share a cathode bias resistor, which I don't think carries any signal. After V1, the output from the two anodes are linked by the two 100K resistors. So here is a direct path for signal to pass to the other volume control, but no amplification. In fact, as it passes through 200K of resistance, the signal would be attenuated. as seems to happen in my amplifier.

Is this some mystical feature of the 5E3 design, or is there an electronic explanation for the phenomena?

[angelodp]

Ok so that cap is not grounded out.

On the 5E3 tone/vol

https://el34world.com/Forum/index.php?topic=10386.0

That link has a very good explanation.

[sluckey]

I'm trying to understand the mechanism by which channel interaction occurs.

The interaction is due solely to the backwards way the volume pots are wired. The signal input is connected to the VOL wiper and the signal output is taken from the end of the VOL pot. Then the end lugs of both VOL pots are connected together to go to the next stage. You have to look at and understand the schematic to clearly see the interaction. There are a couple more tweed era two channel amps that are wired exactly like the 5E3 and they also have this interaction.

Most amps have the volume pot wired in a more conventional way, ie, signal input is on the end lug and the output is taken from the wiper.

[cdemike]

I'm trying to understand the mechanism by which channel interaction occurs. The unused input jack sockets are grounded out (by the tip shunt). I would guess this would be to prevent hum (?). The two channels then share a cathode bias resistor, which I don't think carries any signal. After V1, the output from the two anodes are linked by the two 100K resistors. So here is a direct path for signal to pass to the other volume control, but no amplification. In fact, as it passes through 200K of resistance, the signal would be attenuated. as seems to happen in my amplifier.

Is this some mystical feature of the 5E3 design, or is there an electronic explanation for the phenomena?

In addition to what sluckey said, I'd just want to add that the grounding of the unused channel's grid should make no difference since the interaction happens after the gain stage in question; grounding the stage's grid shouldn't really matter in terms of interactivity between the channels' volume controls beyond the unused channel not unnecessarily adding noise. I think about it this way: if you separate out the channels from each other, treating the unused channel triode and associated plate load resistor as a resistive path to ground (roughly Ra + the plate load resistor's value), you can easily see how the unused channel's volume control works as an adjunctive tone control. Try drawing out the signal path from each channel this way and you'll see it.

[maxkracht]

I think if there had been contact, particularly with the pot lugs, there would be zero output from the bright channel, right? Which is not the case.

Most likely a wire from one of the volume pots touching ground would cut the output, but touching doesn't mean good contact. Slightly touching can mean resistance, same with a bad solder joint. Can also be intermittent, so play loud enough and it vibrates more open or more closed.

[dfoxdavies]

Over the weekend, I have been able to spend more time with the amp.

The substitution of the 5Y3 for the 5U4 has resulted in more grit and overdrive. Power output (volume) seems to be fairly unchanged. It's pretty loud for 15W.

I have been researching on YouTube for videos demonstrating the interaction of the 3 controls. I have been educating myself on what to expect from these interactions. I think I may not have been as rigorous in my assessment as I should have been. The amp cleans up significantly with guitar volume pot changes, this may have colored my results. I've read some opinions that because high volumes are achieved early in the pot rotation the guitar volume may be a more usable method of controlling overall volume. Running the amp more wide open has allowed me to appreciate more of the tonal interaction between the volume pots. I think it's going to take me a while to 'unlock' some of the tones within this amp.

Thanks to everyone who has taken time to read and think about my issues, especially to those who have offered suggestions. I have taken something from every reply, increasing my knowledge with each one.

I would like to include here a couple of external pics of the amp. Having bared my soul with the less than stellar interior shots, I am more proud of how it looks outside.

Front.jpg

Back.jpg

[martin manning]

Very pretty on the outside, I agree. It's never too late to tidy up on the inside ;^)

[LOUDthud]

Those pics of the inside of the amp are some of the best I've ever seen, sharp, in focus and well lit. What camera or phone was used ?

[angelodp]

Quite nice cab, enjoy.

A

[dfoxdavies]

Yeah, that was my phone. A Samsung Galaxy A51

[maxkracht]

Cabinet looks excellent! No shame in an imperfect looking interior, we all start there or much much worse.

I still think the volume dropping at higher settings indicates a problem. If all of the wiring is correct to the schematic, I would guess it is some sort of high frequency oscillation. Your amp can be working very hard to amplify frequencies you cannot hear and at some point on the volume dial it loads things down enough to cut the output. Shortening the wires going to your tubes might cure this, changing preamp tubes could help, and there are a number of ways to cut the offending frequencies if the first two options don't work. If you're happy with the amp, and the power tubes aren't red plating, it might be ok to ignore it or leave it for another time.

[dfoxdavies]

The current situation, whether affected by the rectifier change or some other wiring adjustment, is that the bright channel does no longer exhibit the volume drop between 11 and 12 on the dial. The normal channel still drops volume above 11. Previously, I think they both had problems above 10.

I will take another look at this as an on going issue.