5E3 - Why does it distort above 360 plate volts?

[Iplayloud]

Hi I just acquired a beater 5E3 clone. After some fixing up I notice how the plate voltage is low compared to a SF Princeton Reverb. Giving it less headroom, like further in the opposite direction from a Twin.
It has 330v and from what I read anything above 360v would make the amp distort at any volume. Is this simply because of the lack of NFB? If yes, then NFB was probably space age electronics when it was discovered.. even though distortion is interesting for us on guitar. NOTE: correct 12AY7 in V1.

BTW I know I can't raise the B+ without changing the PT and I'm not really looking to do this but just curious about the low voltage. Also it does distort on the bass channel if I chugg the big strings at low volume.

[eschertron]

I'm interested in how this discussion plays out (no pun intended!), as my understanding is that higher B+ equals greater headroom for the output tubes. It may have something to do with how the load line intersects the knee of the power tube's curve at different B+ voltages?

[g1]

If I'm reading your post correctly, someone somewhere has stated that a 5E3 will distort more as you raise the B+ beyond 360V.
If we are going to discuss this, we should at least know the source of the statement. Can you link us to where you saw this?

[Iplayloud]

Ok I can't find where I read it, I read dozens of different forums and sources. Thinking back it doesn't really make sense as raising the plate voltage would rather increase headroom and lower distortion apparently.

Sorry for the possible misinformation in the title, I can't change it now that replies have been posted.

It's my first older/tweed style amp and I just have a bunch of questions. Thanks in advance for responses.

1. Why do kits have a low plate voltage, simply to be faithful to the original models (browner tone) ?
2. Is the 5E3 a good candidate to jumper channels like a Marshall plexi?
3. Would it be a good idea to un-share the cathode of V1 and use different values like a Plexi Lead amp? (giving each a different character, when bridged one volume controls the low end while the other controls the top end)
4. Why is V1 not a 12AX7, good idea to put one in there?
5. Why no mixing resistors where the 2 inputs meet, like a pair of 270k on a '59 Bassman, when a volume is maxed the coupling cap is direct in the grid of V2a.

[mozz]

You can buy any voltage transformer you want for a deluxe. Do you know what trannys are in it? Have you actually measured the 330v? What rectifier tube are you using and have you tried any others?

I think you have it backwards, lower voltage will cause more distortion, 360v is a good starting point for a deluxe clone.

[Chuck H]

The info came from here. Post #2

http://music-electronics-forum.com/s...ad.php?t=11590

I'm guessing that with the higher B+ no effort was made to adjust the bias resistor. I still find it hard to believe that with the stock bias resistor and 370Vp the amp will distort on all settings. Though it's possible the tubes would be over dissipating at idle.

EDIT: As far as I can tell, most people report plate voltages between 330V and 340V for a stock 5e3.

[soundmasterg]

I have a completely stock 1956 Fender 5E3 and the B+ voltage right off the rectifier is 378V. That's with today's higher wall voltages...it was probably measured with around 122-125VAC from the wall but I didn't note it down when I measured it last. It doesn't distort at any/every volume either. It distorts when I turn it up just like any good tube amplifier should.

1. Why do kits have a low plate voltage, simply to be faithful to the original models (browner tone) ?
2. Is the 5E3 a good candidate to jumper channels like a Marshall plexi?
3. Would it be a good idea to un-share the cathode of V1 and use different values like a Plexi Lead amp? (giving each a different character, when bridged one volume controls the low end while the other controls the top end)
4. Why is V1 not a 12AX7, good idea to put one in there?
5. Why no mixing resistors where the 2 inputs meet, like a pair of 270k on a '59 Bassman, when a volume is maxed the coupling cap is direct in the grid of V2a.

1. Every kit is made based on whatever original amp they are cloning. Given that tube amps can be highly variable it isn't surprising that there is a range of voltages that they could have and be perfectly stock.
2. You can jumper the channels if you like. Try it and see. When using the amp, you get more volume and better tone if you are using one channel and you have the unused channel volume half way up.
3. Try it and see. It is a bit of soldering and experimenting but that is the best way to learn. I like the stock 5E3 circuit myself but there are tons of variations that people have done over the years.
4. V1 is a 12AY7 because that is how it was designed. If you put a 12AX7 in there, it will function, and will have a bit more volume and distortion at the same volume settings. The 12AY7 sounds better.
5. That is the way it was designed, and that contributes to the behavior I mentioned in my answer for #2. The channels are very interactive and a lot of people like that feature of the 5E3.

Greg

[pdf64]

I have a completely stock 1956 Fender 5E3 and the B+ voltage right off the rectifier is 378V. That's with today's higher wall voltages...it was probably measured with around 122-125VAC from the wall but I didn't note it down when I measured it last...

I think it would be super useful for someone with a real 'as stock' original 5E3, variac and meter to supply the 5E3 via the variac, and tweak it so as to put 6.3Vac and 5Vac on to the heater circuits, and then measure the primary Vac, secondary HT Vac, HT Vdc (ie at reservoir cap + & OT CT node), Vdc after the 5k dropper at the screen grid node, and power tube cathode Vdc. Actual values of the cathode resistor and 5k HT dropper would help to complete the picture.

[Steve A.]

When I built my Mission Amps 5E3 clone Bruce helped me design 2 mode switches to clean up the sound. One was a split plate load on one of initial gain stages and the other changed the values in the RC network between the two 12AX7 stages. If I can't find the drawings I will reverse engineer the mods. (Back then I wanted the option to get really nice clean sounds like on a Deluxe Reverb but of course without the BF tone stack curve.) The purpose of the split plate load was to simulate the gain of a 12AY7 while using a 12AX7 for the initial preamp tube but it sounds really sweet with a 12AY7, too.

Steve A.

[Enzo]

Even then they won't all be the same. If your 250 ohm resistor measures 247 ohms, that doesn't mean they all do. Nothing is precision here. Individual tubes will draw differing amounts of current, which will shift B+ voltages, not to mention the B+ dropping resistors themselves will be of differing values, along with all the other resistors.

To quote from the layout drawing:

"Voltages read to ground with electronic voltmeter, values shown + or - 20%."

[soundmasterg]

I think it would be super useful for someone with a real 'as stock' original 5E3, variac and meter to supply the 5E3 via the variac, and tweak it so as to put 6.3Vac and 5Vac on to the heater circuits, and then measure the primary Vac, secondary HT Vac, HT Vdc (ie at reservoir cap + & OT CT node), Vdc after the 5k dropper at the screen grid node, and power tube cathode Vdc. Actual values of the cathode resistor and 5k HT dropper would help to complete the picture.

It probably would be really useful, but who has the time? I could check that and post it, but I wouldn't be able to get to it for 6 months, in which time I would likely forget that I was going to be doing that, and the OP would likely have moved on to some other project. Big car projects/repairs suck. (Thats what I've been working on for a year and a half in every spare moment.)

Greg

[Iplayloud]

Thanks for the replies and to Chuck H who found the post. My PT is a Hammond 272DX (115 60 103).
My 5y3 rectifier is an old tube. I would probably get a higher B+ with a newer Sovtek 5y3 which I read are actually a 5v4 with a multiplier of 1.25 instead of 1.1 for a real 5y3.

I'll probably get a Sovtek rectifier to get a bit more headroom, and will need to read on biasing this amp. I've biased plenty of negative grid biased amps but not any with cathode biased.