DC in signal path between stages

[dennyg]

I'm trying to diagnose source of ugly (thin, fizzy) tone within preamp of a high gain design, and have a diagnostic bit that may lead to the cause.
With no signal (input grounded), i'm getting a 50mv DC reading relative to ground after the coupling cap on the grid lead between stages 2&3.   Replaced the cap; no change.  Disconnecting the lead from the cap, i get no DC on the grid of stage 3.
Any ideas?

[Tony Bones]

It's normal. There is always some grid current and this is the voltage drop across your grid leak resistor. Assuming 1M ohm grid leak resistor, it implies 50nA of grid current. I've seen worse.

[dennyg]

Thanks Tony - yes using a 1M pot between 2&3. So if i hear you correctly, the grid current is induced by the noice signal from stage 2, because when i disconnect the coupling cap the grid voltage drops to zero. I measured same on a mid-gain build and have zero dc on 3rd stage grid. So still scratching my bald head.

[Tony Bones]

The 50mV is caused by current through the 1M resistor; V=IR. I think we can be sure about that.

I suggested that the current was coming from the grid, which is not uncommon. But I should have read your opening post more carefully.

If disconnecting the cap causes the voltage to drop to zero then the current must have been coming through the cap. It's called leakage. 50nA is not enough to worry about in itself, but if it's a failing cap - if it gets worse  - then it might become enough to worry about. Paper in oil caps are prone to leakage. Some of them are really bad, but the ones I've looked are stable and don't change much over time. (Unless the oil leaks out, which is the case with a lot of Vitamin-Qs, but you'll know because the outside of the cap will be oily.)

I don't know what kind of cap it is. If it's cheap you could just replace it. I don't know if it'll fix your fizzy distortion though... Sorry if I'm not much help.

[Ed_Chambley]

Do you have a Schematic?. High gain means a lot of different things to people.

[dennyg]

Ed - attached is schematic for the pre-amp; built as-is using term strips and gothic ring layout.  It's powered by a standard SEL PA with KT88 that I've used in couple other builds, and using a separate 6.3V heater supply tranny with voltage doubler circuit, regulated at 12V DC.  All voltages are spot on with their load-lines etc, but I've never had any DC voltages appear at idle in my other builds along the signal path following a coupling cap hence suspicion.  I'm attempting to use a true star grounding scheme for the first time - star points at reservoir and smoothing caps for each triode, connected to chassis near input jack.  The B+ and heater yield flat DC on a scope and is very quiet at idle. It's fizzy at low volume, hence I believe the issue is isolated to pre-amp.  I've attached gut shot too.  Note the 3rd triode in upper right is disconnected - that's for a clean second channel.  ANY help much appreciated. 

[dennyg]

And gut shot

[Tone Junkie]

My question would be have you done high gain before or owned a high gain amp. The higher the gain the more hiss you have when you turn the volume down. if that's something you already understand my apologies. you never know until you know some ones background. Higher gain amps are very subjective to lead dress issues. Having chased some to the point of taking it apart and totally rebuilding it I know your pain. There is not always an easy answer. But these guys here are great they have gotten me thru numerous problem builds.
Bill

[dennyg]

I had a 5150 that hissed like a snake but I guess with 6 gain stages that's what u get. This Uber is my 3rd high gain build and my fave - when it's working properly but it's temperamental with this fizz issue. I actually chose it knowing it would be a challenge but wanted to learn good layout design with terminal strips and higher the gain the less room for error.

[Ed_Chambley]

Have you checked you pots for DC?  I cannot see much in the photo.  Are you getting any DC across the 33K cathode resistor on V2B?


Might just for checking replace the 1 meg pot with a resistor.  I am not familiar with this preamp at all, but 33k seems small feeding a tone stack with another 33K.  Most designs have a larger Cathode resistor than slope.


Probably will not fix the issue, but it is the only thing I see.

[dennyg]

Thanks for the reply Ed - well, yes the 33K Rk does have DC because it's a CF stage.  I've tried replacing the VR pot feeding stage 3 with 1M R and same.
I did connect the clean channel up last night which goes direct to PA via channel switch, and it sounds just dandy.  Fed it into stage 3 and same ol' fizz, so I'm gonna focus on stage 3 which does have that curious cap bootstrap feeding positive back from the CF into the split plate load. My gut is it's a ground issue - maybe my star ground lead is bad, who knows. 

[shooter]

Most designs have a larger Cathode resistor than slope.

Like you, I'm not familiar with this CF, but I would think there isn't much "clean" to be had but since it is the dirty channel ?

I have an amp with a similar problem, my DC Rises with signal, "backing up" 2 gain stages from the PA tube, but at idle it's fine and as long as it's played "clean" there's no problem.  I suspect mine is a parasitic.  all caps n tubes have been changed, even B+ taps and caps

[MakerDP]

Just curious, are you using EL84's for your output tubes?

[Ed_Chambley]

Most designs have a larger Cathode resistor than slope.

Like you, I'm not familiar with this CF, but I would think there isn't much "clean" to be had but since it is the dirty channel ?

I have an amp with a similar problem, my DC Rises with signal, "backing up" 2 gain stages from the PA tube, but at idle it's fine and as long as it's played "clean" there's no problem.  I suspect mine is a parasitic.  all caps n tubes have been changed, even B+ taps and caps

I am not sure what the grid voltage on the CF is either.  Seems like it would be too low, but I am not slick enough to calculate it from the schematic.  Usually you find a cap at the output on most CF designs too.  This is another thing that is different.


The difference in output impedance and voltage gain is small comparing a 33K and 100K.

[MakerDP]

It makes sense that the design is sending more current to ground than to the tonestack because the bootstrapping dramatically increases the total gain produced by the previous stage and the CF. That may be by design.

The low value of the cathode resistor also increases the amount of soft clipping in the CF itself. The bootstrapping and this together is brilliant for a high gain design.

I still wonder if he's using EL84s... that could be the issue with the fizz right there.

[drgonzonm]

I did a web search for cathode follower, and noticed the web CF had a cap, on the first half of the blue line shown in reply 11.  Without the cap, you have DC voltage to the CF grid originating from the B rail feeding the other half of the tube. 

I am not familiar with all CF designs, but yours differs from a common CF design. 

[dennyg]

Glad to see some interest in the topic - a few responses to your comments and questions:
* This pre-amp has been built successfully as is by several folks over at AX84; I've had mine working and it kills for high-gain tone; I just can't help myself and keep messing with it try to find the source of this recurring instability. Bottom line is the issue isn't in the design, it's with the novice skills of the builder (me!)
* DC-driven CF's are fairly common particularly on Marshall's; my first build is similar to JCM800 pre-amp with an SEL PA, and is now my primary gigging head at a whopping 18watts.  BTW, the Uber build uses an SEL KT88 PA. Not an EL84.
* I think shooter commented on grid current backing up - in my limited experience, it's normal to have some fairly significant grid current in high-gainers when cranked due to 20V+ signals hitting stage 3 grid.  But at idle i don't think there should be any, unless maybe it's biased hot?  Stage 3 on this one uses a 1.5K Rk yielding a .9V bias which is on the hot side, so maybe that could create a bit of grid current?

The DC on grid 3 could be a red herring too.  My main issue is a crappy, anemic, fizzy tone - with all voltages to spec. 
Ironically, I don't intend to keep this amp after I get it working - I want to tear down the pre-amp and try Hoffman's plexi circuit with an SEL PA - but not until I figure out this whacky instability issue!

[Ed_Chambley]

Ok, I removed my drawing since the slope is not grounded.  Duh!


Still puzzled tho.  Cathode Followers are something I have been trying to understand more of.  That is why I jumped directly to it.  The bootstrap which normally is connected at intersection of a plate load resistor halved, but here not exactly half but close.  What is not common is a bootstrapped stage that is also bypassed. 


Bootstrapping for more gain: Because the cathode follower acts as an impedance buffer we can use it to 'bootstrap' the previous stage. This is done by splitting the previous stage's anode resistor into two equal parts and connecting a capacitor from the cathode follower's cathode, to the junction of the two resistors [see right]. The cathode follower then effectively multiplies or bootstraps the value of the lower anode resistor R2, making it look like a constant-current source. This will increase the gain of the previous stage so it becomes almost equal to the mu of the valve! An added bonus is that if you don't use a cathode-bypass capacitor, the gain is still very hgh (about 85-90 with an ECC83). Leaving the cathode unbypassed reduces blocking distortion and reduces 'fizzy' sounding overdrive, giving better crunch. Usually you sacrifice a lot of gain though, but not in this case! So I guess you can have you cake and eat it too...


I copied the above from the Valve Wizard.  May give you an idea.  It helped me understand what is happening around your CF and previous stage.  I attached the schematic clip which shows the values he has when discussing this.

[dennyg]

+1
Merlin's pre-amp book has been my bible since becoming obsessed with this crazy tube amp hobby coming up on 3 years. 
What I could really use from you amp veterans is some help diagnosing this sad fizzy tone.  After my first turret board build, I was drawn to 'old school' term strips - i just like their flexibility and seeing all the wires.  I've learned the gothic ring layout style.  I stare at this beast almost every night thinking what the heck could it be?  Voltages are good; the PS has no ripple; DC heater supply is flat line; the scope offers no clues.  I'm happy to share more gut shots, see if anything stands out as obvious. 
Any ideas would be so much appreciated. 

[shooter]

I've had mine working

without the fizzy?
if so, what was the "next" thing you did?

from my experience, fizzy has turned out to be a bad socket (arcing), a bad tube, and bad solder.

I did connect the clean channel up last night

when you did this, you bypassed 3rd gain, CF, and TS?
If so, inject it at CF, work?, If not at TS, work?, OR just do a shotgun re-wire

[dennyg]

Hey Shooter -
1) the most recent change I made which precipitated the insidious fizz was to go from a hybrid star/bus to a pure star (3 star points with dedicated wires to chassis)
1) I took your suggestion and wired the clean tube output directly to the CF, and no fizz.
2) Chopsticking the 3rd stage found excessive 'wiggle' at pin 1 - thinking that pin may not be making good connection but it's a ceramic socket that i can't dig into to squeeze the pin so i'm gonna replace that socket over the weekend and will report back. 

your suggestions are so spot on to what is helpful from the pros - anything leading with "in my experience..." is hugely helpful for us novices trying to find our way

[shooter]

ceramic socket

they've been problematic for me, switched o the Beldons

I think you're on to the fun stuff!

[dennyg]

Shooter - I replaced the ceramic with a Beldon and that solved the problem!!!  I've been struggling with this for months - but now I've got a new troubleshooting tip in my arsenal.  Thanks so much for the help!
Also figured out why stage 3 has DC on the grid.  Referenced Merlin's book on grid current which starts around .9V Vgk for a 12AX7 - and this stage is biased hot at .945V - so it's normal.