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What value should an EL34 Cathode bypass capacitor be in a CCS?

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Posted on May 6, 2016 at 18:58:15
DAK
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 I am using an LM317T to set the bias voltage in a push pull el34 amp and i wonder if the bypass cap should stay the same value as if it were bypassing a resistor or should it be some other value since we are dealing with a SS device? regards, Dak

 

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RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 6, 2016 at 21:15:42
shane.
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 This is one situation IME where it sims worse but sounds so much better !!

Maybe use a different CCS, but either way to answer your question would be to forget the capacitor altogether if you are in Class A; coupled cathodes to common CC sink, et voila.


Shane

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 7, 2016 at 09:13:11
DAK
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 The amp is PP class AB , i believe. How does one know if it is class AB or class A push pull?

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 7, 2016 at 12:23:55
JKT
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 Setting aside the level of bias current, if the cathode has a capacitor bypass then there is an AC path between the output tubes plate and its cathode. With a PP pair, in a sense then, they are in parallel. One tube can be pushed into cutoff while the other conducts (AB1). With a high impedance CCS "tail" and no capacitor the only available AC path for the signal is plate to plate and cathode to cathode. In a sense they are in series. Like the old style Christmas tree lights where if one burns out the whole string goes down. Sense it is impossible for one tube to conduct with the other cutoff under these conditions you have imposed Class A operation. The higher in impedance the CCS the smaller any imbalance AC currents traipsing though the PS picking up PS colorations. With a bypass cap there will be no suppression of the AC imbalance currents.

FWIW my mono-blocks output stage is PPP differential with four triode KT120s running off a single cathode CCS with no bypass cap. The purity of forced Class A is something one should hear before considering the trade-offs (less power etc.) compared to Class AB1.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 7, 2016 at 12:54:55
DAK
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 So, you are saying that there is no harm in running the amp without a bypass cap? The main drawback will be a loss of power vs class AB1. What would be the output power for PPEL34 if i were to forego the bypass cap?
In my amp i converted from cathode resistor bypassed with 200uf capacitor. I am also wondering if i can use a smaller perhaps higher quality cap that would sound better? thanx for the help, Dak

 

Wow, PPP KT-120's, posted on May 7, 2016 at 14:36:33
nerdorama
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 JKT,
What sort of operating point do you use on them? I'm trying to plan an amp project and KT-120's, or 150's are on my possible list. What is the driver stage to push all these in triode? My present amp is a PP differential output stage but not nearly as powerful as yours. I like the sound I have but want a bit more umph.
Sorry if this is a thread diversion, but I just had to ask.
Thanks,
John
De gustibus non est disputandum

 

Allen Wright, forced differential Class A, posted on May 7, 2016 at 18:20:44
Tre'
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 "The addition of a current source to the power tube's cathode converts a conventional PP amplifier to an all-differential, all-Class A amplifier. In fact, the current source actually prevents the circuit from entering into Class AB."

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 7, 2016 at 19:42:46
JKT
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 If you use a cap you should consider an Ultrapath connection. The cap goes from the B+ side of the transformer primary directly to the cathode. That bypasses the PS filter cap. The usual value is on the order of 80 to 100uF. This can be determined by experiment. If bass is rolled off make it bigger.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Wow, PPP KT-120's, posted on May 7, 2016 at 19:58:59
JKT
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 600V B+ with cathode bias (~65V bias) and 70 to 80mA plate current. At present total current set to 290mA. I came up with a zero tempco CCS for the output stage "tail" CCS using a Fairchild KA431 (low tempco TL431) to control a big ass power MOSFET. I can set the bias with the amps cold and 3 hours later it's within +-0.25mA.

They're driven by a Lundahl LL1660 IT 2.5/1 with each pair of KT120s driven by its own secondary. A 6BX7GT 300V p/k @19mA/side drives the IT. It's direct connected to the 6SN7 input stage. The input and driver stages have their own CCS fed shunt regulated supplies.

Output is ~37W.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: Allen Wright, forced differential Class A, posted on May 7, 2016 at 20:00:11
JKT
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 What I said.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

Yes, I was just reinforcing what you said. (nt), posted on May 7, 2016 at 21:00:44
Tre'
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 8, 2016 at 02:38:59
DAK
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 Tomorrow, i will try the amp with out the bypass cap and see how it compares with the bypass cap as it is now configured. It should be an interesting A/B comparison. cheers, Dak

 

RE: Sorry, that did not work for me, posted on May 8, 2016 at 16:06:54
DAK
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 I removed the bypass caps and the amp went into motorboating when a signal was present. I have been incrementally making slight changes to this amp and when i switched from resistor setting bias to CCS or actually CVS with the LM317. i also removed the resistor on the input grid of the input 6sn7 tube. So i used a straight piece of signal wire from the input rca jack instead of a 1K resistor before the grid. Before i removed the bypass caps as i was testing the new CVS on the amp there were a few putt putts when the amp was first powered up. I am guessing the power tube section is unstable before they warmed up sufficiently. Just several putt putts before disappearing and not reappearing again. But now with the cathode bypass capacitors disconnected the amp is oscillating continuously.
Any suggestions as to what i should do to alleviate this problem? Besides putting everything back to how it was. I am thinking if i should reduce the amount of feedback from the 16ohm tap to the input tube. Should i go further down the rabbit hole or get out when i can? cheers, Dak

 

RE: Sorry, that did not work for me, posted on May 8, 2016 at 16:27:53
Tre'
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 The only way to learn is to jump into the hole.

I hope you still have a resistor from the grid to ground.

How much feedback are you using?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Sorry, that did not work for me, posted on May 8, 2016 at 17:03:10
DAK
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 The fb resistor is a 1w Takman the colors are either brown/brown/black/red/grey or grey/red/black/brown/brown
the color is faded some so the brown could be gold or grey could be silver. I have not worked on this amp for awhile so i dont remember. For PP el34 a fb resistor of at least 5K would be the usual, so, i am guessing around 7K. I wanted to play with the LM317 so i did this mod to the amp. Truthfully, the amp sounded better with the ccs and i would not want to go back to the resistor biasing. So, having bypass caps is not a deal breaker for me.

 

RE: Sorry, that did not work for me, posted on May 8, 2016 at 17:23:40
Tre'
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 Well, as has been said, without the cap you will force Class A operation.

That will limit the maximum output power but should be cleaner (less distortion) up to the Class A limit.

You might try disconnecting the FB all together. If that stops it from oscillating you could use a pot wired as a rheostat and slowly adjust it to see how much FB you can use before it starts causing problems.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Sorry, that did not work for me, posted on May 8, 2016 at 18:03:52
DAK
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 Yeah, i guess , l will play with the fb level and see how that affects the amp's stability. cheers, Dak

 

RE: Sorry, that did not work for me, posted on May 8, 2016 at 20:42:35
Triode_Kingdom
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 Have you wired the LM317 as a voltage regulator, or as a current source? I'm asking because your first post on this said you were using it to set bias voltage. Maybe you could sketch a schematic and post it?





 

RE: The lm317 is a constant voltage source, posted on May 8, 2016 at 21:12:45
DAK
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 The voltage is set with a 19.6 ohm resistor. Input is at the cathode. out is connected to the 19.6 ohm resistor which goes to ground. Common is set to ground. regards, Dak

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 09:32:48
Triode_Kingdom
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 I don't understand your nomenclature. The LM317 has three terminals: Vin, Vout and ADJ. In any event, wiring it as a voltage regulator won't work without a parallel cap. You need a current source to force Class A operation.



I'll just add that the LM317 is not a good choice for this job. It's a noisy device, and many people who have used it for this purpose report poor quality sonics.


 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 11:03:17
DAK
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 OK, nomenclature can be confusing.
If you look at the actual TO220 device from the front, the side with the label. The first leg goes to ground, the center leg has the voltage setting resistor to ground and the right leg is at the cathode.

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 11:56:41
Al Noakes
 Most of the signal goes through the cap . If these devices are bypassed with a cap , the quality of the cap is more important than the quality of the CCS . I would do it by ear with some 22uF 63V film caps and add in parallel for best sound .

Al

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 12:17:22
Tre'
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 I agree but what's the point of a CCS if it's bypassed with a cap?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 12:22:21
Triode_Kingdom
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 OK, that sounds like you have it wired as a current source. The problem is, 1.25/19.6 is only 64 mA. You need more than that to center the EL34s in Class A. Does it motorboat even at low levels (without the cap), or just above a certain threshold? How much voltage is on the cathodes at idle? What mode is this amp - pentode, UL or triode? Are there grid stoppers on the output tubes?

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 12:48:38
JKT
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 With motorboating I would also look for the possibility of low frequency coupling within the B+ supplies to the different stages in the amp. Relying on the caps in PS RC filters to shunt noise and signals to ground thus isolating different stages becomes more problematic the lower the frequency of the offending signal.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 13:03:46
JKT
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 My phono pre, line stage, and mono-blocks all consist of LTPs with CCS "tails". Over the years that I've been perfecting these most of the sonic improvements have come from "improved" CCSs. To quote The Donald: They make for a "huuuge" difference. There are better alternatives out there for "tail" current sources (current sinks in this case) than the LM317. One place to start reading would be Walt Jung's Current Source 101 articles.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 13:07:08
JKT
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 I should add Morgan Jones 4th edition to the list.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 13:15:03
DAK
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 Hi, JKt, yes, in its cathode bias w/resistor, the amp was running 70ma at the cathodes. I thought that was too high so i dialed it back a little. The amp is running UL mode but there are no grid stoppers on the suppressor grids. There are 1K resistors on the input grids. As soon as the amp received a signal, it started motorboating.

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 14:50:47
Triode_Kingdom
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 The LM317 shouldn't cause that. It's likely there's another issue in the mix. See JKT's post below. If you have a scope, you can look at the stages ahead of the outputs to see if unwanted signal is leaking back through the B+ lines.

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 14:53:28
Tre'
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 He'll need to disconnect the GNFB first.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: The lm317 is a constant voltage source, posted on May 9, 2016 at 15:36:22
DAK
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 Yeah, i did not think it was the lm317 but removing the bypass cap exposed some other situation which has now come to light. I will check the the voltages of the front end tubes and see if something jumps out there. cheers, dak

 

RE: The lm317 is a constant voltage source, posted on May 10, 2016 at 01:26:44
Al Noakes
 It's useful for maintaining balance in a pushpull amp , easy to modify a cathode bias network this way .

I would prefer fixed bias and a smart centre reading meter

Al

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 10, 2016 at 06:38:25
PakProtector
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 "The purity of forced Class A is something one should hear before considering the trade-offs (less power etc.) compared to Class AB1."

You can force this Class A operation with loading and operating point as well. Quite useful in any case...:)
cheers,
Douglas

That, and the both phases conduction during low power AB biasing is *NOT* Class A, it is merely low power AB. At maximum power, if both are not conducting, it is *NOT* a Class A amp.


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: The lm317 is a constant voltage source, posted on May 10, 2016 at 08:19:20
Tre'
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 "It's useful for maintaining balance in a pushpull amp...."

DC balance? I don't see how with only one shared CCS.

Without a bypass cap the impedance of a CCS is high giving a large error feedback signal that maintains dynamic AC balance. At any given moment that error signal is negative FB for one tube and positive FB for the other, forcing AC balance.

With a bypass cap there's no corrective signal.

In terms of DC, the total current will match the set current but why would that current necessarily be evenly divided between the two output tubes?

I don't see the mechanism to accomplish that.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: What value should an EL34 Cathode bypass capacitor be in a CCS?, posted on May 10, 2016 at 10:21:34
JKT
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 "You can force this Class A operation with loading and operating point as well."

True. However, as long as there exists an AC signal path from an output tube's plate to its cathode any AC imbalance in the PP pair will generate imbalance currents which in the usual case flow through the PS. A stiff cathode current source will act to enforce AC balance. That's where a lot of the "purity" stems from.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

RE: The lm317 is a constant voltage source, posted on May 10, 2016 at 10:30:15
JKT
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 "In terms of DC, the total current will match the set current but why would that current necessarily be evenly divided between the two output tubes?"

You can use two CCS, one for each half and AC couple the cathodes with a capacitor. The trade-off is a capacitor in the signal path. Something I've wanted to try just to see how it would affect the sound. Matching tubes for my PPP mono-blocks can be a bitch.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

 

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