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Hoffman Amps Forum image Author Topic: Friends Question about K bias SE outout  (Read 4001 times)

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Offline Ed_Chambley

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Friends Question about K bias SE outout
« on: January 09, 2017, 09:30:18 am »
I had a fellow builder send me a Schematic and asked me to review a build he has done.  It is a simple SE design, but he has no Bypass Cap on the 5881 Output tube and is complaining about hardly no output volume.  He does have a K resistor and since I have never left off the bypass cap I suggested this could be the problem.


He countered that he may have less output, but should have some and if I understand Beam Power tube output his signal should be canceling without some bypass.


So does it cancel when it swings negative or what would this actually cause?  Reading RG Keen, he states a shorted K bypass will actually make the tube act as it is shorted.  I would like to be able to explain this to him as he does not own a scope or an analogue meter and refuses to join our forum.  Very shy.


I did a quick test on a tube tester Champ I have and sine wave turned into sawtooth and I had no real wattage, but I did it very quickly as I do not know what damage could result.

Offline PRR

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Re: Friends Question about K bias SE outout
« Reply #1 on: January 09, 2017, 12:40:06 pm »
Just as in any stage: unbypassed cathode resistor gives about 1/2 gain. You have to pluck twice as hard for the same output.

MAXiumum output is not changed (not enuff to notice). You just have to work harder.

A shorted cathode resistor will make the tube conduct to the max. In a resistor loaded stage maximum output will be small. In a power stage, typically the tube melts before you can call your friend.

Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #2 on: January 09, 2017, 02:42:39 pm »
Thanks I understand it simply by you stating "just as any other stage."


I will let him know.  I guess he needs to trace his signal now.  He mentions he has a faint sound so he probably has a meg where a K should be or something like that.  I still do not know why he does not want to bypass the Power Tube, I mean he does have a volume control.
« Last Edit: January 09, 2017, 03:01:45 pm by Ed_Chambley »

Offline HotBluePlates

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Re: Friends Question about K bias SE outout
« Reply #3 on: January 09, 2017, 07:38:46 pm »
...  I guess he needs to trace his signal now.  He mentions he has a faint sound so he probably has a meg where a K should be or something like that.  ...

Could be any typical new-build issue.

My fave for a little while has been "check the speaker jacks" (if there's more than one).  If an unused, parallel speaker jack has a shorting contact (like the input jacks), it will curiously allow a tiny bit of very weak, distorted audio through the speaker on the other jack.  So that might be a good first-stop.

But wire in the wrong place, hundred-kΩ where kΩ goes, missing ground, part on wrong turret/eyelet, etc.

Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #4 on: January 09, 2017, 08:34:32 pm »
...  I guess he needs to trace his signal now.  He mentions he has a faint sound so he probably has a meg where a K should be or something like that.  ...

Could be any typical new-build issue.

My fave for a little while has been "check the speaker jacks" (if there's more than one).  If an unused, parallel speaker jack has a shorting contact (like the input jacks), it will curiously allow a tiny bit of very weak, distorted audio through the speaker on the other jack.  So that might be a good first-stop.

But wire in the wrong place, hundred-kΩ where kΩ goes, missing ground, part on wrong turret/eyelet, etc.
Thanks, he just left.  I used a scope and his gain stage just past the tone stack had no little signal.  Wiper to grid, not wiper to K.  It was almost cascode without intentions.

Moved the wire and it woke up.  Helped him tweak a little and it actually a nice sounding design.  He does only have one input and it did not like humbuckers well so I made him a switch to change the bias on V1.  Heaters had no CT either.  He is a nice fellow, I don't understand why he will not join the forum here.  He feels like he does not know enough and I laugh at how little I know.

Thanks for the suggestion, I will keep it in mind.

Offline PRR

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Re: Friends Question about K bias SE outout
« Reply #5 on: January 09, 2017, 11:56:22 pm »
> He feels like he does not know enough

Nobody born knowing this stuff.

How IS he going to learn??

Running to Ed's house? Ed doesn't know everything. And what if you go out of town? Or have to wash your hair that night?

Sluckey has talked about a luncheonette(?) where the electronics techs hung out, and talked about cars, girls, and maybe a few electrons. Where the new guy could blurt an ignorant question and the old hands would throw him, maybe a fish, but also a fish-hook so he could do his own fish next time.

Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #6 on: January 10, 2017, 05:00:04 am »
> He feels like he does not know enough

Nobody born knowing this stuff.

How IS he going to learn??

Running to Ed's house? Ed doesn't know everything. And what if you go out of town? Or have to wash your hair that night?

Sluckey has talked about a luncheonette(?) where the electronics techs hung out, and talked about cars, girls, and maybe a few electrons. Where the new guy could blurt an ignorant question and the old hands would throw him, maybe a fish, but also a fish-hook so he could do his own fish next time.
I agree with you and told him the same.  Not much hair to was anymore, but I like the line.

If he would have been hanging around with me BEFORE I learned the little I know about amps, he would have probably feel better about it. He has no idea that radio and old TV repair is more difficult than a guitar amp.  He thinks my learning increased, which is has, but in terms of difficulty radio has guitar amps beat.

Please no one flame the comment above, I mean no disrespect to amp builders.  In building amps, there is much more varied skills needed needed like wood work, tolex or finishes.  I even fold my chassis on most builds.  It is designing amps that I intend to sponge off the knowledgeable folks here.  I am getting close and I thank you both along with many others.  IMO designing is the difficult stuff because just as soon as I depend on say an expected voltage from experience of a previous build, something in the new design changes it more than expected.  Like for instance, how do you know in advance what plate voltages will be.  Data sheet gives approximations which sometimes are not even within 20% due to more load than expected.

This was his first scratch build.  Prior he built 2 kits from Mojo and Mojotone really does a great job with their kits.  I did convince him to start back lurking, but he saw a thread a few years back where someone was humiliated by one of our Moderators.  IMO the guy who got offended read into a lot more than what was really there.  I prefer not to go back and rehash as it is not my style.  It is just what my friend is afraid of.  Yes, immature, but aren't we all in some way.

With some folk, no knowing something seems to me to be considered a character flaw to them.  What a lost opportunity to learn.  You cannot know what you do not know.  Just like I started this thread simply by saying I did not know.  I thought the gain would be similar to a triode stage unbypassed, I just simply had never done it.

BTW, his is now bypassed with a 80uf/50v and the measurable output clean was just under 4 watts unbypassed and just over 8 bypassed.


Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #7 on: January 10, 2017, 05:07:36 am »
I am going to ask this here and if I need to start a thread I will. 

What is a good method of slowing inrush when using SS rectification without using a sag resistor? 

I have noticed with working with these old octal types with SS rectification my plates run up sometimes near 100 volts before settling back to what I have spec'd.   Specifically 6Sl7 and 6sn7 and 6SC7's are the worst and why do not the modern tubes like 12AX7 do the same?  I mean they do some, but not as much.

Is it the circuit, or are the tubes different in this way. Meaning in the same conditions the 12A_7 tubes would behave the same?

Offline sluckey

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Re: Friends Question about K bias SE outout
« Reply #8 on: January 10, 2017, 07:07:53 am »
Quote
What is a good method of slowing inrush when using SS rectification without using a sag resistor?
Use a NTC thermistor in the primary of the PT. The thermistor will have a higher resistance when cold. As it warms up due to current rising, the resistance goes down. Google "thermistor tube amp". Here's one link...

     http://antiqueradios.com/forums/viewtopic.php?t=82798

Quote
I have noticed with working with these old octal types with SS rectification my plates run up sometimes near 100 volts before settling back to what I have spec'd.   Specifically 6Sl7 and 6sn7 and 6SC7's are the worst and why do not the modern tubes like 12AX7 do the same?  I mean they do some, but not as much.
Maybe the 12AX7 simply heats up faster (conducts sooner) than the big ole octal tubes?

There are plenty of successful amps out there with ss rectifiers. Many of the newer amps use a thermistor. Look at this schematic to see how one is used in the Fender Reissue Twin Reverb...

     http://el34world.com/charts/Schematics/files/fender/Fender_65_twin_reverb_%20manual.pdf

What are your specific concerns about inrush current?

A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #9 on: January 10, 2017, 08:05:25 am »
Quote
What is a good method of slowing inrush when using SS rectification without using a sag resistor?
Use a NTC thermistor in the primary of the PT. The thermistor will have a higher resistance when cold. As it warms up due to current rising, the resistance goes down. Google "thermistor tube amp". Here's one link...

     http://antiqueradios.com/forums/viewtopic.php?t=82798

Quote
I have noticed with working with these old octal types with SS rectification my plates run up sometimes near 100 volts before settling back to what I have spec'd.   Specifically 6Sl7 and 6sn7 and 6SC7's are the worst and why do not the modern tubes like 12AX7 do the same?  I mean they do some, but not as much.
Maybe the 12AX7 simply heats up faster (conducts sooner) than the big ole octal tubes?

There are plenty of successful amps out there with ss rectifiers. Many of the newer amps use a thermistor. Look at this schematic to see how one is used in the Fender Reissue Twin Reverb...

     http://el34world.com/charts/Schematics/files/fender/Fender_65_twin_reverb_%20manual.pdf

What are your specific concerns about inrush current?
My concern is only the tube plates rise above the the maximum voltage from the Data Sheets.  I do know we often run 6V6 tubes and EL84's above as well, but since I am not familiar enough with the older octals my concern is killing them.  I mean, these are tubes never used in boxes from many years ago.  Some like the 6SC7 sell for obscene prices.


Of course I am using older pulls when testing the circuit and I have less concern with these, but like the red base RCA's I have I would like to put into use in a few preamps I am making.


Using a Analogue meter one of the 6SC7 for instance will rise to 360 Vdc, but quickly begin to return to about 255vdc when conducting.


I will read what you posted anyway, but are my concerns foundation-less?

Offline sluckey

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Re: Friends Question about K bias SE outout
« Reply #10 on: January 10, 2017, 08:39:52 am »
Quote
but are my concerns foundation-less?
Maybe. The only reason the plate voltage is high is because the tube ain't conducting yet. What's the harm?

I like the idea of the thermistor but I've never used one in an amp. Thermistors are power resistors and heat up like power resistors. This makes them prone to failure. At least, the older types were prone to failure. (Can't say for the newer variety.) They were common on the old "all American 5" tube radios or tvs with series tube filaments. A common problem with the series filament strings was tube filament failure due to high inrush current when the tubes were cold. The thermistor was placed in the series filament circuit to limit the inrush current at turn on. The failure rate of the thermistor was about the same as the tube filament failure rate!

Here's a device that you can use to delay applying B+ for 1 minute after powering on, giving the tubes time to warm up. Will look good next to all those octal tubes too...

http://www.ebay.com/itm/Amperite-115N060-Delay-Relay-Time-Delay-Relay-Glass-Octal-NEW-IN-BOX-/232183484038?hash=item360f36ce86:g:pGMAAOSwgGJYXNlc
« Last Edit: January 10, 2017, 08:42:25 am by sluckey »
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #11 on: January 10, 2017, 10:34:52 am »
Quote
but are my concerns foundation-less?
Maybe. The only reason the plate voltage is high is because the tube ain't conducting yet. What's the harm?

I like the idea of the thermistor but I've never used one in an amp. Thermistors are power resistors and heat up like power resistors. This makes them prone to failure. At least, the older types were prone to failure. (Can't say for the newer variety.) They were common on the old "all American 5" tube radios or tvs with series tube filaments. A common problem with the series filament strings was tube filament failure due to high inrush current when the tubes were cold. The thermistor was placed in the series filament circuit to limit the inrush current at turn on. The failure rate of the thermistor was about the same as the tube filament failure rate!

Here's a device that you can use to delay applying B+ for 1 minute after powering on, giving the tubes time to warm up. Will look good next to all those octal tubes too...

http://www.ebay.com/itm/Amperite-115N060-Delay-Relay-Time-Delay-Relay-Glass-Octal-NEW-IN-BOX-/232183484038?hash=item360f36ce86:g:pGMAAOSwgGJYXNlc
O yes, I know the TV and radio ones.
I like the Octal Delay Realy.


On another note, I ran across a 24 count box of RCA 1625 tubes.  Data sheet refers to 807 tubes, but does no directly say they are the same.  Do you know if they are basically a 6L6?  These have a plate cap and I had previously thought they were a TV tube, but searched the forum and found a schematic K and DL worked out that uses them.

Offline sluckey

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Re: Friends Question about K bias SE outout
« Reply #12 on: January 10, 2017, 10:53:40 am »
I'm not familiar with the 1625 tube.
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline PRR

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Re: Friends Question about K bias SE outout
« Reply #13 on: January 10, 2017, 01:03:09 pm »
1625 is a 807 with 12V heater. It will replace old 6L6, but may be marginal in 6L6GC apps. The plate-cap is a safety issue.

> under 4 watts unbypassed and just over 8 bypassed.

This suggests the driver is unable to slam the power tube properly. Any volts-data on the driver?

Offline Ed_Chambley

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Re: Friends Question about K bias SE outout
« Reply #14 on: January 11, 2017, 11:01:10 am »
1625 is a 807 with 12V heater. It will replace old 6L6, but may be marginal in 6L6GC apps. The plate-cap is a safety issue.

> under 4 watts unbypassed and just over 8 bypassed.

This suggests the driver is unable to slam the power tube properly. Any volts-data on the driver?
No, I did not keep after he left and was happy.  He wanted only clean and that is the way he left with it.  I am sure I will see it again.

Offline John

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Re: Friends Question about K bias SE outout
« Reply #15 on: January 11, 2017, 01:41:38 pm »
Quote
He has no idea that radio and old TV repair is more difficult than a guitar amp.  He thinks my learning increased, which is has, but in terms of difficulty radio has guitar amps beat.


I used to have trouble finding the problem in an amp. Then I started repairing VTVMs for "fun". After chasing gremlins in those old things, amps suddenly became easier.
Tapping into the inner tube.

Offline John

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Re: Friends Question about K bias SE outout
« Reply #16 on: January 11, 2017, 02:32:31 pm »
Vacuum tube volt meter. Hey, I had to google it way back when too!
Tapping into the inner tube.

Offline John

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Re: Friends Question about K bias SE outout
« Reply #17 on: January 11, 2017, 03:52:18 pm »
For sure, I've never learned anything from what I did *right* ;)
Tapping into the inner tube.

 


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