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


It is currently Sep Thu 29, 2022 2:34 pm


All times are UTC





Post New Topic Post Reply  [ 58 posts ]  Go to page Previous  1, 2, 3  Next
Author Message
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Thu 22, 2022 12:04 am 
Member
User avatar

Joined: Nov Sat 13, 2021 1:15 am
Posts: 47
Location: Cleveland, Ohio
JGTubeamps wrote:
Perhaps you could use this one.


I appreciate you taking the time to draw that up for me, John. I'd really like to work my way though my own design though, and have something I can say I (with a lot of help!) did myself.

I noticed you have a 1.5k grid stopper and 470 screen stopper resistor on the 35L6 - I was thinking of adding those myself (maybe it'll stop some kind of oscillation? Worth a shot), so I think I may borrow that part of your design and go put those in now.

_________________
Nothing for it, really.


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Thu 22, 2022 2:19 am 
Member
User avatar

Joined: Nov Sat 13, 2021 1:15 am
Posts: 47
Location: Cleveland, Ohio
Update:

I've implemented the changes as outlined in the new schematic below. The popping noise appears to be gone (!!!), but the music does start to turn to static at maybe 3/4 volume. Can this be fixed? Is this just as much as a 35L6 will give?

Screen voltage to the 35L6 is still about 145 vdc. I'd feel more comfortable if this was lower. Would it be worth it to install a Zener diode and take care of both the overvoltage and the voltage sag all at once?

Attachment:
IMG_20220921_220808055[1].jpg
IMG_20220921_220808055[1].jpg [ 602.42 KiB | Viewed 390 times ]

_________________
Nothing for it, really.


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Thu 22, 2022 8:12 am 
Member

Joined: Mar Thu 21, 2019 2:53 pm
Posts: 415
Location: Niosia / Cyprus
Blorpcandy wrote:
Update:

I've implemented the changes as outlined in the new schematic below. The popping noise appears to be gone (!!!), but the music does start to turn to static at maybe 3/4 volume. Can this be fixed? Is this just as much as a 35L6 will give?

Screen voltage to the 35L6 is still about 145 vdc. I'd feel more comfortable if this was lower. Would it be worth it to install a Zener diode and take care of both the overvoltage and the voltage sag all at once?

Attachment:
IMG_20220921_220808055[1].jpg


What is the meaning of " does start to turn to static " ? Something like clipping?
Could you take voltage measurements of all the pins of the tubes ?

_________________
_________
Website: https://johnamplifiers.webs.com/apps/photos/


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Thu 22, 2022 10:16 pm 
Member
User avatar

Joined: Nov Sat 13, 2021 1:15 am
Posts: 47
Location: Cleveland, Ohio
I think it may be clipping, yeah. I haven't bothered to bust the scope out to have a look at it yet, though.

Here's all the voltages I measured; I'd also like to note that on the last schematic I forgot to pen in the 0.1 uf cap between pins 4 and 8 on the 35L6.


Attachment:
Soloist Amplifier Tube Voltages.png
Soloist Amplifier Tube Voltages.png [ 13.58 KiB | Viewed 364 times ]

_________________
Nothing for it, really.


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Fri 23, 2022 4:50 pm 
Member

Joined: Mar Thu 21, 2019 2:53 pm
Posts: 415
Location: Niosia / Cyprus
Voltages look ok except G2 which is higher than the recommended voltage according to the datasheet.
Do you have an audio signal generator and o-scope? A visual inspection of the output signal and the signal at the triode plate will reveal the performance of the amplifier.

_________________
_________
Website: https://johnamplifiers.webs.com/apps/photos/


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Fri 23, 2022 10:41 pm 
Member
User avatar

Joined: Oct Sat 03, 2020 9:41 pm
Posts: 333
Location: Greenville, SC, USA 29615
You definitely need to reduce the voltage on the screen grid. Either by using a higher resistance in series with the screen, or by using a resistive divider.

Using your figures, if we take the source B+ voltage as 157.4V and the screen voltage as 144.8V, that's a drop of 12.6V across the existing 500 ohm resistor, which means the screen is drawing 25mA. But the tube spec calls for only 3mA when there's no signal. The specified max screen dissipation is 1 watt. 25mA at 145V is 3.6 Watts - you are in danger of burning out the screen grid.

_________________
Faith is ... a belief in things not seen. (Heb 1:1)


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Fri 23, 2022 11:28 pm 
Member
User avatar

Joined: Nov Sat 13, 2021 1:15 am
Posts: 47
Location: Cleveland, Ohio
Jay, would we not also consider the 1.33k resistor in the calculation of the power to the screen grid? That 157.4 volt reading was directly off the cathode of the 35Z5 and not after the filter resistor; sorry if I didn't make that clear.

If this is the case, with the equivalent of a 1.53k resistor before the screen, it looks to me like it should be drawing 8.23 mA for a power dissipation of 1.19 watts - still high, but not crazy.
Either way, I'm gonna go get that voltage down and report back.

_________________
Nothing for it, really.


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 12:52 am 
Member
User avatar

Joined: Nov Sat 13, 2021 1:15 am
Posts: 47
Location: Cleveland, Ohio
Tacked in a resistive divider, addendum below. Voltage is at a much more comfortable level, as marked.

Sounds to me now like there's a high-pitched squeal at volume, with the screen voltage dropping at the same time. If there was no speaker plugged in I'd think it was the output transformer arcing.

With that voltage drop, I'd assume maybe it's trying to draw more current than it has to work with; it looks like the divider will supply 4.99 mA though, which I'd think would be enough since at maximum signal it needs 5.5-7 mA, and I wasn't anywhere near max. Is this one of those problems another capacitor would fix?

I don't own a function generator, but I do have a small oscilloscope I can poke around with if need be.

Attachment:
IMG_20220923_203541459[1].jpg
IMG_20220923_203541459[1].jpg [ 544.8 KiB | Viewed 318 times ]

_________________
Nothing for it, really.


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 1:26 am 
Member
User avatar

Joined: Jul Fri 18, 2008 9:02 am
Posts: 2662
Location: Alton IL 62002 Near ST. Louis
Add a .1uf across the 40K resistor on the screen grid .

_________________
Mark
(Conti the brain damaged robot )
PG-18-23909 General Radiotelephone Operator License


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 1:56 am 
Member
User avatar

Joined: Oct Sat 03, 2020 9:41 pm
Posts: 333
Location: Greenville, SC, USA 29615
Have you looked at the 35L6 data sheets? It seems that the ideal operating point is with grid 1 (control grid) close to zero volts relative to the cathode. Since your circuit takes the control grid to ground, you could probably operate with the cathode grounded also. The more negative the grid is relative to the cathode, which is the same as saying the more positive the cathode is with respect to the grid, the less static current through the tube. Your cathode resistor is giving you a bias of -9.3V, which is pretty close to cutting off the tube. More current through the tube (smaller or zero cathode resistor) will give you higher gain and lower distortion.


Attachments:
35L6GT.pdf [379.13 KiB]
Downloaded 5 times

_________________
Faith is ... a belief in things not seen. (Heb 1:1)
Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 2:56 am 
Member
User avatar

Joined: Sep Mon 16, 2013 1:42 am
Posts: 5864
Location: Tucson, Arizona U.S.A.
Location: Tucson, Arizona U.S.A.
RetireeJay wrote:
Have you looked at the 35L6 data sheets? It seems that the ideal operating point is with grid 1 (control grid) close to zero volts relative to the cathode. Since your circuit takes the control grid to ground, you could probably operate with the cathode grounded also. The more negative the grid is relative to the cathode, which is the same as saying the more positive the cathode is with respect to the grid, the less static current through the tube. Your cathode resistor is giving you a bias of -9.3V, which is pretty close to cutting off the tube. More current through the tube (smaller or zero cathode resistor) will give you higher gain and lower distortion.

Where did you get that from? The first graph in the posted data sheet says that with 110 volts on the screen and 0 volts on the grid the plate current is 100mA! That's way beyond reason. Also, the typical operation says -7.5 volts on the grid or 180 ohms cathode resistance although at different operating conditions.

Don't operate any amplifier with an output transformer without a load. That risks burning out the output tube or transistor, the output transformer, the tube socket, or whatever else might be associated.

The high negative grid voltage when the output is distorted looks like you are driving the grid of the 35L6 positive causing it to draw grid current. In other words, you are pushing it beyond it's capability. Be aware that this is a rather low power amplifier. The power specified in the data sheet is at a rather high distortion level (10%) and doesn't include the losses in the output transformer. Realistically you might get 1W out of it. If you have an efficient speaker that would be fine but if the speaker has low efficiency, the result may be disappointing.

_________________
Jim Mueller

Who's that swimming in the punch bowl?
It's Walter Wart, the freaky frog!


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 3:40 am 
Member

Joined: Mar Thu 21, 2019 2:53 pm
Posts: 415
Location: Niosia / Cyprus
Add the capacitor from G2 down to ground as battradio suggested but I would go for a 10uF or 22uF instead of 0.1uF.
Also use your o-scope, attach it to the speaker terminals and watch for any abnormal waveform. Output signal should be a replica of the input signal. I suspect that the amplifier is oscillating or perhaps you push it too hard as Jim suggested.

_________________
_________
Website: https://johnamplifiers.webs.com/apps/photos/


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 3:20 pm 
Member
User avatar

Joined: Oct Sat 03, 2020 9:41 pm
Posts: 333
Location: Greenville, SC, USA 29615
Jim Mueller wrote:
Where did you get that from? The first graph in the posted data sheet says that with 110 volts on the screen and 0 volts on the grid the plate current is 100mA! That's way beyond reason. Also, the typical operation says -7.5 volts on the grid or 180 ohms cathode resistance although at different operating conditions.


Sorry, Jim. I was looking at the first graph of "Average Plate Characteristics" and it looked to me like operating dead center on the graph would be desirable, with the highest gain and the best linearity. But as you say that would cause the plate dissipation to exceed the limit. I don't know why they published that graph when most of the curves are actually outside the Safe Operating Area.

Also, to compound my embarrassment, I didn't look up (and still don't know) the exact definition of the "Ec1" labels on the curves. I assumed that was the grid voltage, but now I'm not so sure. Is there a reference somewhere to find the definitions of Ec1, Ec2, Ef, Eb, Ic1, Ic2?

_________________
Faith is ... a belief in things not seen. (Heb 1:1)


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 3:47 pm 
Member
User avatar

Joined: Oct Sat 03, 2020 9:41 pm
Posts: 333
Location: Greenville, SC, USA 29615
Just a question: what's the rating of the output transformer being used? If the transformer is too small, the magnetic circuit will "saturate" and cause distortion. If the turns ratio of input to output is incorrect, it will limit the undistorted power to less than the circuit is capable of.

_________________
Faith is ... a belief in things not seen. (Heb 1:1)


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 4:18 pm 
Member

Joined: Mar Thu 21, 2019 2:53 pm
Posts: 415
Location: Niosia / Cyprus
RetireeJay wrote:
Is there a reference somewhere to find the definitions of Ec1, Ec2, Ef, Eb, Ic1, Ic2?


Ec1=Vg1, Ec2=Vg2, Ef=Vh, Eb=Va, Ic1=Ig1, Ic2=Ig2

_________________
_________
Website: https://johnamplifiers.webs.com/apps/photos/


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sat 24, 2022 9:11 pm 
Member
User avatar

Joined: Jul Fri 18, 2008 9:02 am
Posts: 2662
Location: Alton IL 62002 Near ST. Louis
https://edcorusa.com/collections/tube-a ... ransformer

The transformer he is using would work well with a 6550 , so it is not saturated or over driven . See my first two posts on this thread .

_________________
Mark
(Conti the brain damaged robot )
PG-18-23909 General Radiotelephone Operator License


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sun 25, 2022 12:20 pm 
Member

Joined: Jan Wed 15, 2020 11:29 pm
Posts: 2237
Location: AUSTRALIA
Location: Queensland.
JGTubeamps wrote:
I have noticed in the above posted schematics that none includes an isolating transformer. According to the modern safety standards this is considered dangerous especially in my country where the mains voltage is 240V. In this case the unit should be enclosed in a plastic case and no metallic parts should be exposed . However handling the IP cable and jack may present a risk.


+1

It might be hard for constructors of equipment, given the plethora of hot chassis sets from history, to understand the folly of them and get to grips with modern safety standards related to line operated apparatus.It is also not very practical for home constructors to make plastic cased double insulated apparatus with no metal parts projecting through the case wall. And plastics are poor at mitigating fire risk. A metal chassis is the better option.

The Engineers who built "power transformer-less" sets of yesteryear were fully aware of the dangers of them.

In some countries like AU & NZ they were more or less outlawed by regulations, all radios & TV's had power transformers. But in other countries (like the USA & UK) the business machinery (and the bean counters) wanting light weight sets without power transformers, effectively trumped the Engineering team's wishes and instructed them to design these dangerous devices. Although, in the UK they might have been forgiven to some extent in that many domestic dwellings even in the 1960's + 70's still had DC supply outlets and the transformer-less sets were, like the band, AC/DC capable.

So my advice before doing anything else; get a power transformer and go to 6.3V heater tubes (you could even move to a better output tube like a 6L6), or use the isolation transformer method that you have moved to, but don't forget to connect the chassis to the Earth of your dwelling's power connector with a 3 wire cord.

Connect your metal chassis to the line power system Earth. This way you will have no dangerous issues interfacing to the input of your amplifier.

AES in the USA have numerous Hammond power transformers in stock. I would also recommend that you use a chassis (panel) mount fused (+/- switched) IEC line connector, so that you are not left with a line cord dangling from your amplifier.

Now you could ignore completely all the above advice, as there is no obligation to follow it, it is just a forum here after all, not a police state full of electrical safety inspectors.

However, it would not be a nice situation if anybody had to say "I told you so" after you, or somebody you love, or a stranger one day, for that matter, has a nasty accident with your creation, maybe many years after you are gone. Apparatus has a habit of falling into the hands of others, if you wait long enough.

Recently I built a line powered supply for a vintage National Radio, built into a painted Hammond steel chassis. I made some references to the safety requirements. You might want to read it, to see what I was getting at. If you want to take the time & effort to make a build to this sort of safety standard, you will not be responsible for your own or anybody else's electrocution. It is often forgotten that most of the task is one of Mechanical Engineering and a lesser challenge of Electronics Engineering, to make safe line powered equipment.

http://www.worldphaco.com/uploads/POWER ... _1-10A.pdf

A really good example of not using a power transformer and/or not earthing the body of "equipment" is the "Suicide Shower Head" These are hot exposed element wires placed in the stream of running water, inside a shower head directly powered by line power, popular in South America:

https://slightnorth.com/south-american-suicide-shower/

Generally, in most countries the water heating elements in a water tank systems have an earthed jacket around the element wires and an earthed tank. This is the way the shower heads could have been, if they had spent more time & money on them. What is deemed safe in one country is not necessarily deemed safe in another.


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sun 25, 2022 7:28 pm 
Member
User avatar

Joined: Jul Fri 18, 2008 9:02 am
Posts: 2662
Location: Alton IL 62002 Near ST. Louis
https://frank.pocnet.net/sheets/127/6/6L6WGB.pdf


https://frank.pocnet.net/sheets/129/e/EL34.pdf

A 6l6GB/5881 or 6CA7/EL34 will both work well with your output transformers as well as a 6550 or 6B4G if you want to use a triode .

My favorite pentode is a 5881 ,

What speakers are you using , keep in mind the power out and listening distance will determine the the speaker that can be used with satisfactory results .

I use KEF Corelli ( https://hifi-wiki.com/index.php/KEF_Corelli ) as my computer speakers with a 8 watt tube amp , the Corelli's are 96DB efficient ,
with 1 or 2 watts not many speakers will sound well in a large area .

_________________
Mark
(Conti the brain damaged robot )
PG-18-23909 General Radiotelephone Operator License


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Sun 25, 2022 7:36 pm 
Member
User avatar

Joined: Jul Fri 18, 2008 9:02 am
Posts: 2662
Location: Alton IL 62002 Near ST. Louis
http://www.diytube.com/getsetgo/getsetgo_manual.pdf

Look at the 6SL7 diver section of this amp .

_________________
Mark
(Conti the brain damaged robot )
PG-18-23909 General Radiotelephone Operator License


Top
 Profile  
 
 Post subject: Re: Simple Three Tube Stereo Amplifier May Need Another Tube
PostPosted: Sep Mon 26, 2022 1:06 am 
Member
User avatar

Joined: Nov Sat 13, 2021 1:15 am
Posts: 47
Location: Cleveland, Ohio
Jeez, you leave for one day and everything happens!

Lots of great suggestions since I left - allow me to first thank everyone who has tried to help me out so far. I can at least say the job can't ever be a total loss since I've learned so much.

I began this project for a good friend of mine; he's always wanted a tube amplifier but never had the money to purchase one of any reputable name and doesn't have the equipment to put one together himself.
Now, we've spent the last four years in college together, and as the end begins to draw near a small part of me fears that we will go our separate ways and I figured a small, handmade amplifier would be a nice gift to say thank you for all of the good times.

Of course, loans are expensive and so is wood so for the electronics I wanted to try and use what I had laying around. Those nice Edcor transformers came from auction recently, and the only tubes I had in my case of (mostly television) tubes that liked the transformer impedance were those 35L6's. This initially seemed to work out great, since with a rectifier tube in the string I could eliminate a power transformer and save quite a good chunk of change. I realized the 1-watt limitation of the 35L6, but I was (and am still) having troubling qualifying exactly what 1 watt of audio sounded like.

The rest of the story is written out in this thread. The only parts I had to purchase were the discrete components and some tube sockets. Of course, a lot of work needed to be done to bring my initial design into something sensible - something it actually isn't, yet. The writing may be on the wall that the design was flawed from the start, and while that's not really ideal, it is okay - that's just how progress is.
So, for now, I'm going to work this design until the end of its rope, and if I do need to restart from scratch, you bet I'll be back here to toss around ideas and come up with a bill of materials that works.


Anyways, if you're not here for my life story and just want to figure the amp thing out, that starts here.
______________________________________________________________________________________________________________________________________________________________________

Here's a link to what the scope is showing me - I used YouTube and played a 500 Hz tone to get a nice sine wave. I'm not all that sure what I'm looking at or for, but I do observe that the distortion only shows up on the first half of the sine wave, which I have a feeling is important. Interestingly, I tried 2 KHz too, and found that at both frequencies it took a lot more from the volume knob to get it to distort - with regular music, it only took maybe an 8th of the knob, with the sine waves taking maybe a quarter. I did add a 22 uf cap across the second divider resistor.

_________________
Nothing for it, really.


Top
 Profile  
 
Post New Topic Post Reply  [ 58 posts ]  Go to page Previous  1, 2, 3  Next

All times are UTC


Who is online

Users browsing this forum: No registered users and 5 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB