Fender 5C3 build with 6L6's

[jjman]

I worked on a friend’s Ampeg Mercury with 6v6s. All 3 newer Sovtek 6SL7's were humming in v1. I tried floating the CT via moving it to the 6v6 cathodes to no avail. Heater balance pot also didn't help. I hooked up a 6.x volt DC walwart to it and it still hummed. Moved some grounds around with no improvement.

I picked up a few NOS (maybe even used somewhat) 6SL7's and the hum disappeared. I've read that this tube is temperamental with hum.

[M Fowler]

Is pin #6 on the V1 grounded and the ground of the volume control also is ground with this #6 pin and the cathode to input jack.

[David Root]

In my experience all old octals with relatively high gain, 6SL7, 6SC7, 6SQ7 etc. will hum any chance they get.

Use 6V3 DC it's so easy, nooooo hum!!

[M Fowler]

Just saying I built this Fender Bassman 5b6 and there is no hum but I have the low level signals, medium and high voltage separated. 6SL7 and 6SC7 tubes.

Also with the Gibson EH150 but has 6SQ7 that needs the detector plates pin 4 & 5 grounded to eliminate hum.

[Cliff Schecht]

So I tried a few more heater setups today. I first tried running the first preamp only off of rectified DC, seemed to add a lot of hash and crud to the sound. Also was getting some hash that I think was thrown in from the reservoir cap charging. No good. So I dropped in an LDO, one that happened to do 3A. I built up a board and mounted the SMD LDO to said board, threw on a heatsink and fired the amp up. Still too much noise. I then tried running all of the heaters off of regulated DC. The DC after the rectifier dropped to 6.3V when I ran everything off of the LDO so I was only getting about 5.7V at the output. Once again, no good. I finally ripped everything from the LDO forward out, dropped in some 3A rectifiers and an 8200uF cap. Flipped on the amp and was quite surprised that the noise, well at least any of the 60/120Hz crap, was gone. There is still a good amount of noise just from the CC's everywhere in the amp and sloppy wiring but it's manageable now, no more than a real tweed Deluxe for sure. I just need to add a series resistor now to drop the heaters from 7V down to 6.3V. I'm glad to be close to calling this amp done finally!

[Cliff Schecht]

Damnit! So I finally had a DC heater configuration I was pretty happy with, but I was hearing a goofy "swirling" type sound in the high end that seems to sweep as the distortion waned out. Does anybody know what is causing this? I haven't tried other tube sets yet but this will probably be my first fix after I figure out the noise issues.

I wasn't sure if the DC heater setup was causing this swirling though so I ripped them out (powering all of the heaters off of DC) and went back to an AC setup with DC elevated 100 Ohm balance resistors. I turned on the amp and immediately noticed an increase in hum, so it's once again looking like DC heaters are necessary..

I'm getting close to going the easy route and throwing in a buck/boost SMPS that can handle the current. A relatively simple solution would be to take an already existing TI EVM and modify it for my needs. I've already found a part that can handle the startup current and the constant heater current (5A peak, 3A continuous) without any fuss and the buck/boost topology guarantees a clean 6.3V. The nice thing is this SMPS has a built in switch (simple), runs at upwards of 2 MHz (small components, EMI is easy to handle) and I can get free samples or maybe even a free EVM through my connections..

[Cliff Schecht]

The excess noise was definitely heater related. I threw the DC heater circuit back in (running the entire amp) and referenced both ends to the power tube cathode through 100 Ohm resistors. I flipped on the amp and measured almost exactly 6.3V DC. Sweet. Rolled up the guitars volume and noticed immediately how much of a difference the elevated DC heaters made in the noise floor.

I'm still hearing a sort of sizzling in the high end but I'm not sure this is an issue as much as it is just inherent in the amp design. It may just be me hearing things too..

[rocketeer]

I would definitely run the heaters off of a battery to make sure your hum problem is heater related. I have an original 5C3 (serial number 0350) and built a clone of it 30 years ago. I had moved transformer locations to make the unit more weight balanced and had huge hum problems due to PT location in reference to the first preamp tube. I noticed in your pictures that you built it with a non original chassis layout and your first preamp tube is right next to the PT. There's a good reason that on good amp designs that the first preamp tube is as far away from the PT as possible. Running your heaters off a battery will at least let you eliminate one possible source of hum.

[Cliff Schecht]

DC heaters killed the hum problems in the amp for sure. I'm satisfied with that. My problem now is that I'm still hearing a nasty fizzy sound in the high end which turns out to be a goofy crossover distortion artifact. I'm attaching a scope shot so others can maybe chime in. I tried reducing the bias (they were at 82%, now at 66%) and tacking in some large grid stoppers (10k) with no improvement in the crossover buzz. Anybody else seen this before (especially the dip on the down-going excursions)?

[Cliff Schecht]

I also notice that one tube is conducting more than the other.. Perhaps I have a leaky cap that is causing some DC offset? How could I check for this without removing the cap?

[Cliff Schecht]

Measured at full distortion that the grid shifts up to 18V and the cathode up to 32V. I'm going to try a Zener clamping circuit tomorrow and see if this fixes the problem.

[Colossal]

Measured at full distortion that the grid shifts up to 18V and the cathode up to 32V. I'm going to try a Zener clamping circuit tomorrow and see if this fixes the problem.

Cliff,

You might know this, but the issue of the nasty, trebly, distortion products is a popular topic that resurfaces from time to time with the 18W crowd. It seems that most are successful in building "normal" 18W clones without any issue, but some "lucky" ones are cursed with the ugly crossover distortion for what seems to be no obvious reason (all things being equal: perhaps their individual choices of components, iron, and tubes however are not equal).

The solution as investigated and documented by Paul Ruby is a zener clamping circuit and became known as the Paul Ruby Fuzz Fix or something like that. I've enclosed a PDF write up of it and there are some cool scope traces that show it perfectly. I have come to understand that crossover distortion can occur when the grids go positive from severe bias excursion. I suppose a driver tube rigged as a cathode follower for each side of the PI would prevent the grids from positive bias, but of course that would mean an extra tube. I have also wondered if a combination of fixed and cathode bias could be used, at least to keep the grid voltage from going above cutoff. I was thinking a lead could be taken from the push side cathode and tied to a left pull side grid leak and vice versa so one side is always helping the other to recover faster than it otherwise would under cathode bias. I don't know if this would work though...just speculating aloud.

Please feel free to call bullsh!t on me though and set me straight as needed My grasp of power amp design is still in its formative stages. I read Kuehnel late into the night but it takes a while for it all to stick into a cohesive picture.

[Cliff Schecht]

I consider myself well versed in SE amp design but I'm no push pull expert. I understand the concepts fine and even what causes the distortions (wanted and unwanted). I just haven't built as many push pull amps and am still learning the curiosities that happen with them.

I'll read over that PDF in a bit, I understand the issue and what the Zener clamps do but I love staring at others scope shots .

[Cliff Schecht]

Just peeked at the schematic and realized that the top 6V6 of the output pair has a G1 resistance of 1M which is then fed into the grid of the bottom of the PI. This is definitely over the recommended impedance of 500k in the datasheet, unless I'm missing something about how this circuit affects input impedance..

So how would I install the diode pair with this circuit? Would I install one pair across the 250k resistor on the bottom and just put in the diodes to ground on the top side?

[Cliff Schecht]

Diode clamps worked like a charm. The amp sounds great now! It rips when cranked and cleans up decently to boot. Should sound most excellent with some slide.