5E3 odd voltages between V1 & V2 and other newbee stuff

[Wallington]

Hi,

First off, I'm a long time lurker and would best be described as a blithering idiot when it comes to electronics - they should probably have something akin to a driving test before they let people like me buy a soldering iron, so please be gentle with me!

I've been modding a 5E3 over the past few month and getting pretty happy with the tone now, but looking to make some more tweaks

As it stands, I've updated the first pair of coupling caps to 0.022 and the PI pair to 0.047 and I'm also now running the GZ34 / 6L6 combination (17w dissipation) and have upgraded the cathode bypass cap and resistor to 50v and 10w accordingly

The PT has a 3A out and I'm reliably informed that the OT is good for the job

I also have a 4k7 5W feeding from the power filtering caps

With the GZ34, 6L6GT and the 4k7, I'm seeing plate voltages of 347v and screen voltages of 325v respectively

V1A plate voltage (12AY7) is 133v
V1B plate is 131v
V2A plate is 174v (12AX7)
V2B plate if 214

So here's my question (s)

Question 1 - if I understand correctly, I can't increase the plate voltages, but swapping the 4k7 for a 5k1 would lower the screen voltage? If I'm after more dynamics from the power section, would this be a good thing to do?
Question 2 - the only 5k1s I can find are 2W - is that man enough with the 6L6s?

From what I can see, my V1 voltages are low and my V2 are high? (the Weber 5E3 diagram suggests 163/161 and 164/200)
Plate resistors are tested as 100k in all cases but V2B which is 56k

Question 3 - How can 1 tube be high and the other low if they are being fed from the same 22k dropping resistor?
Question 4 - How do I fix this? ..I'm assuming dropping the value of the 22k dropping resistor will increase v1 voltages but push V2 further over? ..do I need to change the plate resistors and if so, what values for the plate and cathode resistors for V1?

Lots there I know, but any help would be hugely appreciated! ..it's so close to being a fantastic amp - just needs a little more dynamics

[frus]

1) 5E3 is especially noted for its compression, which is kind of opposite from 'dynamics'. If you put a bigger dropper resistor, the compression would increase, not decrease. I would put a smaller one. of course, you would need to increase the following 22k resistor IF you want to keep the preamp voltages the same

2) No, I'd put 5W just to be safe. OF course, if you put smaller resistance in (for instance 2k7), you may get away with it, but you are bound to find more standard values (like 2k2, 2k7 or 3k3) in 5W

3) Weber also suggests 12AX7 for V1, so. Your voltages with 12AY7 look spot on to me. V2 voltages are also in the ballpark (10% variance is normally expected, depending on the tube)

4) Don't, it's fine

[Wallington]

Thanks @Frus - really appreciate the help!

Can I ask another newbee question - I'm assuming that if I'm going to tweak the values of the dropping resistors, I need to keep the total resistance of the two the same, if I don't want to mess with the preamp voltages?

..so if I have a total resistance of 4k7 + 22k = 26k7 to begin with and I go to 3k9 for the first resistor (quick scan suggested 3k9 was the next lowest down @ 3-5W), I need to aim for as close to 22k8 for the second resistor?

Stock values seem to mean that I'd stick with the current 22k and that would mean an 800R drop in overall resistance and a corresponding increase in preamp voltages?

[Chuck H]

Actually, the voltage at each node depends largely on how much current each node is drawing. In other words, it's not a mater of net resistance. A smaller value resistance to the screen node will increase preamp volts such that you would need to increase resistance over your 26.7k assessment. But I wouldn't recommend it. That large preamp rail resistance is also responsible for a considerable amount of compression. As noted, the 5e3 isn't the go to amp for big, dynamic tones. Many builders do the rectifier and power tube swap to a gz34 and 6l6's to get a little more power and dynamics. Beyond that there ARE things that can be done, but they aren't as simple as component value changes. It would be more extensive involving tearing down and redesigning the whole power supply rail with different cap values and maybe a zener involved.

The simple, but less effective route is as frus said. Change the screen dropping resistor to a lower value. Like 1.5k/5W or 2.2k/5W (I would add 470R individual screen resistors too with this). This WILL increase preamp voltages quite a bit. But that's only a problem if you don't like the sound. The tubes will be fine. In fact if you want to reduce sag, improve linearity and increase dynamics a bit more you could increase the preamp voltages still further by reducing the value of the 22k preamp rail dropping resistor. The tubes are ok with that. The next step in addition to the above mentioned alterations would be to increase the value of the first filter cap to 47uf (the rectifier tube will be fine). Don't expect to turn your 5e3 into a plexi Marshall though. It's still going to be less dynamic than that.

[Wallington]

Hi Chuck H,

Thanks for the input and the info - I need to do some electronics 101 (never covered this at school other than a passing nod to Ohm's law) so the steer is much apprecaited

Looking at the amp, it looks like I have 470Rs between pins 4-6 of the output tube sockets, so I'm assuming these are the screen resistors you've mentioned?

That being the case, I'll order up some dropping resistors and do some experimenting with that before making any changes to the filter caps - it doesn't need much tweakery to get it just so for my ears, so hopefully this will just bring it into the sweet spot

[Wallington]

Hi All,

Just thought it would be right and proper to report back following some experimentation following you're kind input

I've now dropped the screen dropping resistor value to 3k3 5W and I'm really happy with the sound of the amp - really nice balance between dynamics and 'swirl' now!

I've noted that the screen voltages on the 6L6s are now in the 340ies volt range, so now within a few volts of the plate voltage - they started out with a 40v delta, so I'm assuming a correlation between the difference and power stage compression?

I expected the pre-amp voltages to go up and they have, but only by a couple of volts. Oddly, V2B has dropped to 171v from 214v and I don't understand that? ..I can't even see where I could have taken an incorrect measurement to get that number in error?

I'm sorely tempted to leave it as is and enjoy it now, but I might also try lowering the preamp dropping resistor to push the preamp plate voltages up into the 160s again, or leave as is and put a Vox-influenced V1 - 220k plate / 1.5k cathode resistor combo in and try running a 12Xa7 in there?

..decisions decisions!

[Chuck H]

I have to warn you, if you leave it as it is for too long you will get used to the tone and dynamics and your playing and sensibilities will acclimate to it. Then any changes you make will seem discordant with what you expect the amp to do In other words, as a player you will start to exploit the amps particular musical properties. Then if you change the amp it won't behave as you've grown to expect an amp to be. This has been my experience anyhow. Ever buy a new, nice or expensive amp and find yourself wondering why it's not as good in many ways as your old crappy one? This is why. As a designer I've found it most productive to take a project very close to the tonal image in my head before pausing for more extended evaluation.