Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Sep Sat 21, 2019 4:37 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 18 posts ] 
Author Message
 Post subject: Removing output tube cathode cap to reduce gain
PostPosted: Jul Wed 24, 2019 10:18 pm 
Member

Joined: Nov Wed 27, 2013 5:59 am
Posts: 668
Location: Metzger Oregon
First a little background, I have 3 or 4 amps taken from consoles for my vintage hi-fi tinkering. I prefer to “restore”, as much as possible, the stock circuit making changes only where needed to make the amps run stand-alone.

The problem I’ve run into is that many of these amps were never intended to be connected to anything but the preamp/tuner they came with, and they have more gain than a typical stand-alone amp. When connecting a preamp, there is very little usable volume range – and I prefer to have less gain in the amp anyway to reduce any noise picked up from the cabling. I started out “tube rolling”; trying to replace the 12AX7 with something having lower gain. This helped some, but I prefer not to shove tubes into a circuit that they are not designed for. Then it occurred to me, when working on old radios and phonos, often a cap can be added to the cathode of the output tube (if there wasn’t one) to increase gain, the opposite should work in my case, remove the cathode cap to reduced gain, plus I get a little local feedback as an added bonus. My question is, would there be any harm in doing this? I can’t think of any possible issues, but wanted to avoid damaging things if there is something I might not have thought of.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Thu 25, 2019 12:45 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26422
Location: Annapolis, MD
Whenever you have an unbypassed cathode resistor, you have what some call "local feedback". This reduces the stage gain and also reduces distortion.
You have to keep in mind the difference between the DC and AC behavior. If you simply add (or change) a cathode resistor, you get the local feedback, but you also change the bias.

To the specific question: If you don't change the DC circuit, you cannot hurt anything by changing or eliminating the bypass cap.

_________________
-Mark
"Measure voltage, but THINK current." --anon.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Fri 26, 2019 12:44 am 
Member

Joined: Nov Wed 27, 2013 5:59 am
Posts: 668
Location: Metzger Oregon
Thanks Mark, that's what I thought, but it help to have some confirmation.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Fri 26, 2019 3:08 am 
Member
User avatar

Joined: Jun Wed 08, 2011 2:33 am
Posts: 9143
Location: Ohio 45177
I would think you might sometimes get away with putting a 12AU7 in a 12AX7 spot for less gain? I think the guitar amp people get away with that. Well it is not as if anything will "blow up" if you try it. At least in preamp positions, I would not do it in phase splitters.

_________________
Reddy Kilowatt says; You smell smoke? Sorry about that!


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Fri 26, 2019 4:23 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5296
Location: Rochester NY USA
12AY7 is very close to 12AX7 in characteristics, gain of 40 vs 100. Would likely work better than most other dual triodes.

Best way to reduce gain is to add negative feedback - reduces distortion and flattens frequency response. But if amp already has feedback, adding more may make it unstable. Better to attenuate input signal with a voltage divider.

BTW, removing cathode cap won't change gain on a push-pull amp stage. Removing it on a single-ended output stage will also increase bass cut-off frequency, since it raises output impedance. (though if an amp has negative feedback, gain will not change much).

_________________
My web page: https://bit.ly/2rxq4qx


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Fri 26, 2019 8:38 pm 
Member

Joined: Nov Wed 27, 2013 5:59 am
Posts: 668
Location: Metzger Oregon
Using the 12AU7 is exactly what I did as well as the 12AT7, but I was concerned that I was now using a tube that was incorrectly biased, and thought maybe a different approach would be better.

Tom: I hadn't though about push-pull, but every amp in question is in fact push-pull output so likely my idea wouldn't work anyway. I am pretty sure there is already some feedback being applied, but I will look into that further first, as I don't remember for sure. I'll see if I can locate some 12AY7s too, that sounds like a good sub. Would there likely be an issue with the phase inverter? In each case, the tube is used with 1/2 supplying gain, and 1/2 inverting phase for the PP output tubes. One amp in question is a Magnavox that uses 6EU7s, and from what I can tell, there isn't anything else that would sub without a pinout change. That amp does use feedback, and I don't have it up and running yet, so it might work okay as-is.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Sat 27, 2019 9:34 pm 
Member
User avatar

Joined: Jul Mon 08, 2013 2:48 pm
Posts: 360
While there's no risk removing the power output stage cathode biasing resistor decoupling cap, the effect on the the overall gain in a PP type amp will be insignificant, if not detectable at all (though an increase in distorsion and poorer overload margin may result). Using lower amplification factor tubes could help but some circuit parts values will need to be adapted to bias the tubes properly,etc... Increasing feedback (reducing the value of the global NFB resistor) will proportionally reduce the gain but will very likely lead to instabilitry issues. Too much gain means too many amplifying stages, thus the easiest way would be by-passing the 1st stage and feeding your signal directly into the 2nd stage. If you post a schematic of your amp(s) here we could guide you more effectively...


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Mon 29, 2019 4:52 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 4624
Location: Vancouver Island, Canada
Tom Bavis wrote:
Better to attenuate input signal with a voltage divider.

This is a good way, as you can adjust gain by simply changing resistor values. The voltage divider goes in after the preamp stage and bleeds off the excess.
I also agree with bypassing a preamp stage if there is more than one.

12AY7 is a good tube if you can find them. They are used in some Fender amps, and I think some reverb circuits. So the prices for those are high, and since the gain factor is 40, my book says 44, there will be less noticeable difference than with the 12AU7 you already have, which has the lowest mu factor of 17.

Keep your eyes peeled for another version of the 12AU7, the 5963, a computer tube. Shh, don't tell the guitar guys.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Jul Mon 29, 2019 5:54 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 20207
Location: Warner Robins, GA
My suggestion is to use a 100K stereo volume control on the input.

That way you can set the gain to where the output just does start to distort as the preamp also just does start to distort.

The way you do that is turn up the preamp until right before the point of distortion as seen on an oscope with the prwamp being fed a 400Hz signal at the maximum input voltage the preamp is specified to handle.

You then adjust the 100K pot until the amp is right before the point of distortion with the amp fed into a dummy load of the impedance the amp is designed to drive.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Fri 02, 2019 3:58 pm 
Member

Joined: Nov Wed 27, 2013 5:59 am
Posts: 668
Location: Metzger Oregon
Thanks for all the ideas. Strangely enough, I happen to have 5963 tubes in one of the amps currently. From what I read, though, they were designed to be rugged and for digital use, so linearity may not be as good. That said, they sound good enough in there. I will experiment with some 12AY7s at some point, and use a voltage divider on the input after that.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Sat 03, 2019 3:15 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 4624
Location: Vancouver Island, Canada
Guitar players don't worry about linearity, it's all about tone and good ole Southern mojo. :lol:

For guitar amps, I used to like the 5963 just fine in a multi stage preamp, using one side, gain of about 20 mu, followed by one side of a 12AX7, gain of 100 mu.

Two duo-triode tubes can be wired as two paralleled circuits, with switches to change plate voltage from the low gain tube to the high gain.
Isolation transformer, 35Z5/50L6, low watt speaker or a line level out.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Sat 03, 2019 5:34 pm 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 11634
Location: Powell River BC Canada
In amplifiers with multiple feedback loops the cathode bypass
of the output stage affects the low frequency response and changing that can
alter performance (example make it oscillate with some loads) .

In the circuit below the 6L6 cathode bypass rolloff if is 80 Hz.


Attachment:
W N E  124 A shown as 12 watt 7 and a half ohm output  VE7ZSO.jpg
W N E 124 A shown as 12 watt 7 and a half ohm output VE7ZSO.jpg [ 407.59 KiB | Viewed 520 times ]


Attachment:
we  124 FEEDBACK.jpg
we 124 FEEDBACK.jpg [ 347.23 KiB | Viewed 520 times ]


.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Sun 04, 2019 1:06 am 
Member
User avatar

Joined: Nov Sat 26, 2011 4:09 am
Posts: 9573
Location: Texas. USA
Tubologic wrote:
While there's no risk removing the power output stage cathode biasing resistor decoupling cap, the effect on the the overall gain in a PP type amp will be insignificant, ......
You should point out that this does not apply if its a Class AB amp, which most are. Its valid for only the 'A" portion of the waveform but will distort badly as soon as it tries to enter 'B'.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Sun 04, 2019 6:15 pm 
Member
User avatar

Joined: Jul Mon 08, 2013 2:48 pm
Posts: 360
Flipperhome wrote:
Tubologic wrote:
While there's no risk removing the power output stage cathode biasing resistor decoupling cap, the effect on the the overall gain in a PP type amp will be insignificant, ......
You should point out that this does not apply if its a Class AB amp, which most are. Its valid for only the 'A" portion of the waveform but will distort badly as soon as it tries to enter 'B'.


This is technically correct but even in class AB PP amps the decrease in overall mid-band gain will be very small, and that's what the OP is trying to achieve.

Also, please read (and quote) my complete message where I wrote:


...the effect on the overall gain in a PP type amp will be insignificant, if not detectable at all (though an increase in distorsion and poorer overload margin may result).

To make it clearer: removing the cathode bypass capacitor in a class AB PP amp will allways result in degraded performance.

In a class A amplifier the use of the bypass capacitor generally reduces the intermodulation distorsion, though it may either decrease or increase harmonic distorsion.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Sun 04, 2019 7:51 pm 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 11634
Location: Powell River BC Canada
The concept of amplifier gain vs apparent level or what is heard as louder or softer
needs scrutiny.

Taken at a single frequency the amplification is the ratio of the power levels at the
input and the output at an agreed upon distortion level.

To calculate this, the power provided by the signal source must be known, as well
as the power provided to the load the amplifier is driving.

The power provided by the signal source is often not an issue since it usually
a voltage adjustable factor.

The output power provided to the load is a factor, since that is dependent on (in this
conversation) how big and good the tubes are. So, any class of amplifier needs
output wattage capability. And as anyone with an ancient single ended class A triode stage, or a
Philco 20 with push pull 71-A tubes, finds out, As the tubes get weak, so does the sound.

Back full circle to the OT. Remove the cathode bypass to reduce gain, as long
as you can turn up the 'volume control' to make the sound loud again.

Am I right Max ? :wink:
Attachment:
Max Headroom.JPG
Max Headroom.JPG [ 48.02 KiB | Viewed 468 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Mon 05, 2019 3:33 am 
Member
User avatar

Joined: Nov Sat 26, 2011 4:09 am
Posts: 9573
Location: Texas. USA
Tubologic wrote:
Flipperhome wrote:
Tubologic wrote:
While there's no risk removing the power output stage cathode biasing resistor decoupling cap, the effect on the the overall gain in a PP type amp will be insignificant, ......
You should point out that this does not apply if its a Class AB amp, which most are. Its valid for only the 'A" portion of the waveform but will distort badly as soon as it tries to enter 'B'.


This is technically correct but even in class AB PP amps the decrease in overall mid-band gain will be very small, and that's what the OP is trying to achieve.

Also, please read (and quote) my complete message where I wrote:


...the effect on the overall gain in a PP type amp will be insignificant, if not detectable at all (though an increase in distorsion and poorer overload margin may result).

To make it clearer: removing the cathode bypass capacitor in a class AB PP amp will allways result in degraded performance.

In a class A amplifier the use of the bypass capacitor generally reduces the intermodulation distorsion, though it may either decrease or increase harmonic distorsion.
I really don't know what you're trying to get at. In a Class A PP amp removing the bypass cap, and there likely isn't one, won't meaningfully affect gain because the cathodes are operating PP, just like the anodes are, so the result is nearly DC. That's why triode Class A PP amps usually don't have a cathode bypass cap.

In a Class AB amp there, again, won't be any meaningful change in gain during the 'A' portion of the waveform for the same reason. However, when it attempts to enter 'B' that no longer applies and, if there's no bypass cap, the amp WILL distort, not just "may result (in)." To talk about overload "margin" is superfluous because it would ALREADY be 'overloaded' long before max power is achieved. In a PP amp the bypass cap isn't there to 'increase gain'. It's there to stabilize the cathode voltage so it doesn't rise appreciably when the amp goes into 'B'. This, btw, is why a Class AB amp can't sustain max power. If you attempt continuous max power the cap charges up, throwing off bias (among other things).

So, to summarize the Class AB case, he'd end up with essentially the same gain but less power with lousy 'overload' characteristics.

Removing a cathode bypass cap to 'reduce gain' only applies in the single ended Class A case.


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Mon 05, 2019 6:51 pm 
Member
User avatar

Joined: Jul Mon 08, 2013 2:48 pm
Posts: 360
Flipperhome wrote:


In a Class AB amp there, again, won't be any meaningful change in gain during the 'A' portion of the waveform for the same reason. However, when it attempts to enter 'B' that no longer applies and, if there's no bypass cap, the amp WILL distort, not just "may result (in)." To talk about overload "margin" is superfluous because it would ALREADY be 'overloaded' long before max power is achieved. In a PP amp the bypass cap isn't there to 'increase gain'. It's there to stabilize the cathode voltage so it doesn't rise appreciably when the amp goes into 'B'. This, btw, is why a Class AB amp can't sustain max power. If you attempt continuous max power the cap charges up, throwing off bias (among other things).

So, to summarize the Class AB case, he'd end up with essentially the same gain but less power with lousy 'overload' characteristics.

Removing a cathode bypass cap to 'reduce gain' only applies in the single ended Class A case.



Exactly what I meant and said in my posts above.

Bis Repetita ...


Top
 Profile  
 
 Post subject: Re: Removing output tube cathode cap to reduce gain
PostPosted: Aug Tue 06, 2019 4:58 am 
Member
User avatar

Joined: Nov Sat 26, 2011 4:09 am
Posts: 9573
Location: Texas. USA
Tubologic wrote:
Flipperhome wrote:


In a Class AB amp there, again, won't be any meaningful change in gain during the 'A' portion of the waveform for the same reason. However, when it attempts to enter 'B' that no longer applies and, if there's no bypass cap, the amp WILL distort, not just "may result (in)." To talk about overload "margin" is superfluous because it would ALREADY be 'overloaded' long before max power is achieved. In a PP amp the bypass cap isn't there to 'increase gain'. It's there to stabilize the cathode voltage so it doesn't rise appreciably when the amp goes into 'B'. This, btw, is why a Class AB amp can't sustain max power. If you attempt continuous max power the cap charges up, throwing off bias (among other things).

So, to summarize the Class AB case, he'd end up with essentially the same gain but less power with lousy 'overload' characteristics.

Removing a cathode bypass cap to 'reduce gain' only applies in the single ended Class A case.
My apologies' for repeating what was already said.


Exactly what I meant and said in my posts above.

Bis Repetita ...


Top
 Profile  
 
Post New Topic Post Reply  [ 18 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB