nasty distortion in low-power tweed twin (5D8)

[alange5]

I'm building an amp based on the 5D8 tweed twin (cathode biased 6L6 pair) and I'm having some trouble with very harsh distortion.  I built the amp single input, single channel with octals in V1 and V2.  Here's the original 5D8 schematic:





And here's how I currently have it wired:





Not pictured: a 5U4 in place of the two 5Y3's.


I'm experiencing some harsh, fizzy distortion starting at about 3 on the volume knob.  The harder I dig into the strings, the harder it distorts.  Playing gently yields a clean tone.  Anything past noon on the volume sounds like a fuzz pedal.  I'm thinking it's originating in the preamp, as changing the 6SL7's to 6SN7's dramatically reduces the volume, but the distortion onset and characteristics remain the same. Same goes for PI tube substitutions.


Originally, I didn't have the 270k to ground on V2's grid.  Adding it reduced the distortion a bit.
My voltages in the preamp seemed low (a little less than 100v on V1's plate and less than 1V on V1's cathode).  I dropped the B+ resistors from 10K to 4.7K, which upped the voltages a bit and slightly delayed the onset of distortion to about 5 on the volume.


The most dramatic change has been experimenting with plate resistor values.  Lowering V1's plate resistor to about 50k dramatically increased the clean headroom - same goes for the 100k on V2's plates.  But the distortion is still there, especially with heavy pick attack.


Other info: 6L6's biased at about 80% dissipation with a 220 ohm cathode resistor.  Output transformer primary is about 9K with an 8 ohm load.


I'm having a hard time wrapping my head around what's happening in the amp, what type of distortion I'm experiencing, and how to go about fixing it.  Any insight would be very much appreciated.


-Andrew

[shooter]

did you try disconnecting the NFB?
did you try swapping primary wires on the OT?

find a data sheet for the pre tube that shows "typical" resistance coupled amplifier values.  comes in handy for setting up a pre tube for normal operation, biasing a 6SN7 is NOT the same as 12AX7

[alange5]

did you try disconnecting the NFB?
did you try swapping primary wires on the OT?

find a data sheet for the pre tube that shows "typical" resistance coupled amplifier values.  comes in handy for setting up a pre tube for normal operation, biasing a 6SN7 is NOT the same as 12AX7

Indeed, I've tried both.  From what I've seen, at typical 12AX7 operating voltages in a Fender-type circuit, the 6SL7 usually gets about 250k on the plates and about 1.5k-2.5k on a bypassed cathode, which is why I'm confused - my problem seems to be helped my lowering the plate resistor, not raising it.

[tubeswell]

You many not have enough/any decoupling going on in the Pre-amp power supply rail (for the High Tension Supply).


With the amp switched off (and the filter cap discharged), using your R-meter, check that you have DC continuity between the ground lead of each filter cap and the amp's chassis. And then check that you have DC continuity between each filter cap's positive pole lead and the relevant dropping resistor lead in the power supply rail.


With the amp on, and your DC-meter in hand, take VDC readings at each filter cap +ve pole decoupling node on the HT supply rail, and at each plate and cathode (and the tail for the LTP).


And report back.

[sluckey]

Change that 470Ω cathode resistor on V1 to a 1500Ω. Any better?

Edit... That 6SL7 would be happier with a 4.7K cathode resistor and a 220K or 270K plate resistor.

[alange5]

With the amp on, and your DC-meter in hand, take VDC readings at each filter cap +ve pole decoupling node on the HT supply rail, and at each plate and cathode (and the tail for the LTP).

Confirmed continuity on both the + and - sides of the B+ rail.

Change that 470Ω cathode resistor on V1 to a 1500Ω. Any better?

Yes.  Bumped it up to about 2K and the result is similar to what I experienced when I lowered the plate resistor - more clean headroom and higher cathode voltage.


Voltages: (note: V1 cathode resistor currently 2K, and B+ dropping resistors still 4.7K)


B+ nodes:
A: 385v
B: 336v
C: 310v
D: 292v


V1
plate: 180v
cathode: 1.8v


V2a
p: 170v
c: 1.7v


V2b
p: 325v
c: 170v


V3a
p: 215v
c: 1.2v


V3b
p: 240v
c: .8v

[alange5]

Edit... That 6SL7 would be happier with a 4.7K cathode resistor and a 220K or 270K plate resistor.

That change made a pretty big difference.  I'm still getting distortion at high volume and heavy pick attack, but it's much better than it was.  Should I use similar cathode/plate values on the V2 6SL7?

[sluckey]

Should I use similar cathode/plate values on the V2 6SL7?

I would give it a try. So, why did you change over to 6SL7s? Just wanted all octals?

[alange5]

Should I use similar cathode/plate values on the V2 6SL7?

I would give it a try. So, why did you change over to 6SL7s? Just wanted all octals?

I was given a chassis with octal sockets, and I have a decent amount of 6SL7's and 6SN7's on-hand.  That, and I've never worked with them, so I figured it would be a good learning experience.

[tubeswell]

V3a
p: 215v
c: 1.2v

V3b
p: 240v
c: .8v

These have the same plate supply voltage and load resistance, yet V3b seems to be running colder than the other.


This leads me to suspect that the bias for V3b is somehow different, or that you have different Rp and Rk values in V3b to V3a, or the triodes are way off being 'matched'.


How did you wire the grid supply connection for V3b?



Other than that, recheck the Rp and Rk values for V3b, and if they're okay, try a tube swap.

[alange5]

These have the same plate supply voltage and load resistance, yet V3b seems to be running colder than the other.

This leads me to suspect that the bias for V3b is somehow different, or that you have different Rp and Rk values in V3b to V3a, or the triodes are way off being 'matched'.

How did you wire the grid supply connection for V3b?

V3 is a 12AX7 - Grid connection is wired per the Fender layout:



 I'll double check the resistor values when I get back to my workspace on Monday.  It's possible the triodes aren't matched - it's an old unlabeled 12AX7, though in the various stages of troubleshooting I've gone through, I've tried a few different PI tubes, and none of them had much affect on the distortion.

[sluckey]

Put the B+ dropping resistors back to the stock 10K values. Does that help?

[alange5]

Put the B+ dropping resistors back to the stock 10K values. Does that help?

Not much, if at all.  Adjusting plate and cathode values on V2 doesn't seem to have much effect either.  It's still a crunch-fest after about 50% on the volume.

[DummyLoad]

see attached. if it still has nasty distortion after both mods, then you have a wiring error or the pickup(s) of the guitar are too hot. are you using active pickups?


--pete

[alange5]

see attached. if it still has nasty distortion after both mods, then you have a wiring error or the pickup(s) of the guitar are too hot. are you using active pickups?


--pete

I'll give it a shot.  I'm using passive single coils to test the amp.


Something else I noticed - the amount of distortion is largely dependent on the tone controls.  High amounts of treble and/or bass brings on distortion early even at lower volume levels.  I'm curious if something is going awry with the 6SL7 cathode follower feeding the 12AX7 PI.  I've never seen an amp that uses that arrangement, so I have no reference for values. 


If clipping the bypass caps doesn't work, I'm planning on building a listening amp and tracing the whole thing.  I'll report back.  Thanks for all the help.

[alange5]

see attached. if it still has nasty distortion after both mods, then you have a wiring error or the pickup(s) of the guitar are too hot

I removed both caps, and there's a definite decrease in distortion.  With both caps removed, at lower volume I can really dig into the strings without it distorting, which was not the case with the caps in place.  But raise the volume up around halfway, and the distortion comes back.


I also breadboarded a couple noval sockets and subbed in 12AX7's in V1 and V2, but the results are more or less the same.


I've also tried various tube swaps - In the preamp I've tried 6SN7, 12AY7, 12AT7.  For power tubes I've tried 6L6GC, 5881, and 6V6.  In the rectifier I've used 5AR4, 5Y3, and 5U4.  Different tubes yield different results in terms of overall volume and character, but the distortion characteristics are always the same.  I've also tried different output tube bias points - as low as 60% and as high as 95%


At this point I'm looking at the output transformer.  It's a HUGE old RCA.  I believe it came from a theater amplifier, or some sort of PA.  If it's not the transformer, I'm stumped.

[shooter]

6L6's biased at about 80% dissipation with a 220 ohm cathode resistor

do you have any way to look at your audio signal - scope?

if you're still cathode biased, measure the DC volts at the power tubes cathode, as an example call it 10vdc
any "drive" signal that is less than 10 will get amplified "clean".  the same is trueish for the preamp section, so you're left with figuring out where it gets ugly.  If you have ugly coming into the PA <10 it's still ugly, If it's >10 and ugly, it's just gonna get uglier

[tubeswell]

... At this point I'm looking at the output transformer.  It's a HUGE old RCA.  I believe it came from a theater amplifier, or some sort of PA...

When you build an amp using any old OT, you're likely to get any old result. Might need to locate a tweed twin OT.

[alange5]

do you have any way to look at your audio signal - scope?

Unfortunately I do not.

if you're still cathode biased, measure the DC volts at the power tubes cathode, as an example call it 10vdc
any "drive" signal that is less than 10 will get amplified "clean".  the same is trueish for the preamp section, so you're left with figuring out where it gets ugly.  If you have ugly coming into the PA <10 it's still ugly, If it's >10 and ugly, it's just gonna get uglier

If I remember correctly, I have about 20-25v at the 6L6 cathode.  Could you elaborate on what you mean by "drive signal less than 10"?

[shooter]

I have about 20-25v

You bias voltage is a good indicator for how much "drive" signal (VAC) you need to bend/distort the PA tubes.
so your grid sets 20-25v below 0, so a 12Vac rms sinwave on the grid will not clip/distort the power tubes.

BUT If you overdrive each preamp tube, you want to knock it back to a "normal" level before it moves to the next preamp tube.  That "normal" level can be "ballparked" by knowing the cathode VDC of your preamp tube. so V1a's cathode VDC is 1.2vdc, to "keep it clean" keep the drive into V1a around 600mVac.  The signal at the plate of V1a might be something like 4.5VAC, soo you want to knock it back with a voltage divider to something close to V1b's cathode VDC, unless you want preamp distortion, then leave it at 4.5vac, overdrive v1b and listen, like it, keep it

[alange5]

Thanks for the explanation.


After some more testing, I can confirm the problem is the OT.


I temporarily wired in an OT from a p-p 6V6 amp. It’s now loud and clean without a hint of distortion. Looks like I’ll be ordering a new OT.


Thanks again for all the help. Although it ended up being a bit of a goose chase, I enjoyed playing around with different values in the preamp.