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


It is currently Oct Thu 24, 2019 6:19 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 14 posts ] 
Author Message
 Post subject: Stereo tube amp on a budget part 2
PostPosted: Nov Tue 14, 2006 5:51 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4759
Location: Freehold NJ 07728
Tonight I worked on my mini stereo amp. I rewired my filament string from Bills revised schematic and I noticed I made an error wiring the power supply. I corrected my error but left the amplifier per my schematic to see how it sounds. Wow, it sounds pretty good, its amazing how loud 1 watt really is. My ears are ringing as I was listening to all types of different music. I tried a different audio transformer and it had better bottom end but not by much.

I will now play with the tube amp circuit to see what I can squeeze out of 1 watt. Overall I am happy with the results. I will remove the pilot light in the middle and put a volume control in its place. Here is a photo with the basement lights off but tubes on.

Bill, thanks for your input and I appreciate the input from other forum members.

Regards,
Sal Brisindi

Image

_________________
Need capacitors, resistors and other parts--> http://www.tuberadios.com/capacitors


Top
 Profile  
 
 Post subject:
PostPosted: Nov Tue 14, 2006 6:44 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12549
Location: Mpls, Minnesota
Good move to remove the neon pilot lamp, they create a lot of noise and don't belong in an amp.

Looks good, keep us informed of your progress.

Dave


Top
 Profile  
 
 Post subject:
PostPosted: Nov Tue 14, 2006 4:57 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 14800
Location: Fernandina Beach, FL
easyrider8 wrote:
Good move to remove the neon pilot lamp, they create a lot of noise and don't belong in an amp....

Dave


Unless you feed them with B+. Then they become an indicator of when your B+ plus is up and they don't cause hum.

_________________
Don


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 4:36 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4759
Location: Freehold NJ 07728
My Lil' Sal tube amplifier is finished. Like I said before, you will be amazed how loud 1 watt really is, and I don't even think I hooked them up to efficient speakers.

It can use more bottom end, but over all I am happy with it. I tried a real audio transformer on it and it did sound a little better. I pretty much left the amplifier circuit from the original schematic I posted on my website but rewired the filament string and added the 10 ohm resistor per Bill. I probably should fuse it and probably will.

I added 2 220uf capacitors that got rid of the hum totally!

This was a fun little amp to build and is real small, 6"x 4"x 1.5". Here is how the inside looks.

Image

Enough playing, on to the 6L6 tube amp!

Regards,
Sal Brisindi

_________________
Need capacitors, resistors and other parts--> http://www.tuberadios.com/capacitors


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 4:48 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 27947
Location: Detroit, MI USA
That 220 uf cap which appears to be connected right off the cathode of the 50DC4 is going to kill that tube.....the tube manual usually says what the max value of the input filter cap can be. I know it's nowhere near that big, probably 100 uf or even less. You can safely use a value that large after the filter resistor though.

It looks very nice, and it's good to hear you are pleased with the results. One watt into really efficient speakers will drive you out of the room, but you need more power than that plus an output transformer with more iron to get stronger bass response. You might consider using a tapped loudness control circuit rather than a simple volume control, it does boost the low end at lower listening levels and may give you just the improvement you are looking for.

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 5:22 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12549
Location: Mpls, Minnesota
40uf is the max that tube is rated for. Best to change it before you destroy the tube.

Dave


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 5:27 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4759
Location: Freehold NJ 07728
Mr. Detrola wrote:
That 220 uf cap which appears to be connected right off the cathode of the 50DC4 is going to kill that tube.....the tube manual usually says what the max value of the input filter cap can be. I know it's nowhere near that big, probably 100 uf or even less. You can safely use a value that large after the filter resistor though.


I probably can move the positive leg of the 220uf capacitor to the other side of that 100 ohm but that is where the other 220uf is connected. I really needed the large capacitors to kill the hum, which is all gone.....

Thanks,
Sal Brisindi

_________________
Need capacitors, resistors and other parts--> http://www.tuberadios.com/capacitors


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 5:52 am 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 27947
Location: Detroit, MI USA
If you can find the room, then try replacing the 100 ohm resistor with a filter choke having around that DC resistance, probably one salvaged from an old low power amplifier. That will allow you to use smaller electrolytics.

But it should be fine with only about 100 uf or less for each of the two filter caps, that's usually the highest value that was ever used in commercially built circuits. I wouldn't move the first 220 uf over and parallel it with the other 220 and 100, that will give you a total of around 540 uf on that side, waaaay too much. It has to work and not hum with only around 100 uf or something else is wrong.

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 7:16 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12873
Location: San Jose, CA USA
Another option would be to add another 100 ohm resistor between the cathode and the first cap. That should save the tube and may improve filtering slightly. B+ will drop a little, but that's probably no big deal.

_________________
Tom K6VL


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 1:19 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4759
Location: Freehold NJ 07728
Tom,
I may add 100 ohm resistor between the cathode of the 50DC4 and the filter capacitor.

I do have small chokes on hand but there is no room under the chassis to install it.

I know what the specifications state but why would a larger capacitor kill the tube? Its not like the dc switches on immediately, the voltage come on gradually (within 2 seconds) so its not like the tube will see a short across until the cap charges up. Does anyone have an answer?

Thanks,
Sal Brisindi

_________________
Need capacitors, resistors and other parts--> http://www.tuberadios.com/capacitors


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 1:58 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 564
Location: Wisconsin
When it's off for a while, the cap is empty. As it comes up, the cap looks like a very low resistance while the rectifier is warming up. So, th e current draw through that tube will exceed its maximum ratings. It's only when the capacitor is fully charged that the current is within design specs.

The smaller the cap, the less current drawn as the capacitor is charged, plus a smaller cap will charge faster.

Tom


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 3:24 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 5326
Location: Rochester NY USA
As the cap is increased, the diode conduction period decreases, and peak repetitive current increases. The diode's cathode can only support so much peak current - if it gets too high, arc-over can occur.

I would drop the input cap to 40 or less, and add a stage of R-C filtering for the output tube screen and first stage plate supply - they are much more critical for hum than the plate supply to the output transformer, since hum there gets amplified.


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 3:26 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13891
Playing Devils advocate, I got into the same discussion about 40mf being the max these little rectifiers were rated to handle... May be what's in the tube manuals, but not what the mfgr's practiced... 50-60mf was common tied directly to the cathode and I even found a AA5 Philco that used a 70mfd... Possibly 100mf is too much, especially on a hot restart when the cathode is already conducting...

_________________
Tom


Top
 Profile  
 
 Post subject:
PostPosted: Nov Thu 16, 2006 6:21 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
The peak current is also limited by the transformer's DC resistance. You might get away with higher capacitance in some circuits, but why take the chance?


Top
 Profile  
 
Post New Topic Post Reply  [ 14 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 4 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB