DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 22 Oct 2018, 12:54

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 23 posts ]  Go to page 1, 2, 3  Next
Author Message
PostPosted: 27 Jun 2009, 14:24 
Offline
Project Author
User avatar

Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
Hi all,
Having many headphone amps I dug through my archive and found my original bic 6088/mosfet hybrid . What I found was that the mids and highs sound great with my Grado 125's, so I tried some econo Sennheisser HD202's closed back and discovered that the bass filled in quite nicely. The sound is great for AC/DC to Enya (dont laugh).

So I decided to make another with the least amount of cost and price, using metal film resistors and most Radio Shack parts. I did not choose to case this amp up as some folks spend $15 on components and then $60 on a hammond and knobs. This is for the beginner who is looking for a great Saturday project and can listen to the amp the same day.

The amp is focused around the 6088 submini tube http://townsendtubeworks.com/6088.htm (not affiliated with the site). This tube was used for early hearing devices and they are plentiful and inexpensive. I suggest you purchase them in lots of 5 or more because they are cheaper and the filaments are very fragile. If you make a mistake with the heater voltage they become a flash bulb :worried:

The Pentode (datasheet http://tubedata.itchurch.org/sheets/138/6/6088.pdf) is directly heated and in this build the screen is attached to the plate, or triode strapped.

The original design used a 10k ohm plate resistor, I changed it to a 20K pot in a variable resistor config to vary the current and properly set the bias of the mosfet for maximum voltage swing.

The Mosfet is in a basic class A follower configuration with 75mA of Q current (13v regulated smps). I reduced the source resistor to 2W for space. Dont forget to heatsink your Mosfets and Voltage regulator!

The heaters are regulated with a LM7805 and a 68Ohm resistor to drop the voltage to ~1.2v, If you cant source a 68 ohm resistor a 100 and 220 in parallel will work. I have plenty of tubes so I have pushed the heaters up to 1.5, it will shorten tube life, but that can be a matter of months or years.

You can use RCA inputs with a 3.5mm jack out, the choices are infinite. One thing is certain you must use a coupling capacitor anywhere between 220uF to 1000uF. Headphones hate DC Trust me :up:

Just a word of caution: As with any headphone amp, never test with your best headphones, with headphones on your ears. Make sure to label your input and output because the initial inrush will fry your sound card if you get them backwards. One way to avoid such hazards is to use a male 3.5mm on the input and a female 3.5mm on the output.

I added a on/off led indicator for looks and just to make sure I dont forget to turn it off.

The build is very simple, using a radio shack 417 hole 276-150 makes the build easy because of the the rails in the middle, The project can be made with as little as 4 jumpers as most of the components terminate at either rail.

Have fun with this one!


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.



Top
 Profile  
 
PostPosted: 27 Jun 2009, 18:43 
Offline
Editor
User avatar

Joined: 28 May 2008, 21:53
Posts: 4565
Location: Winnipeg, CANADA
Hi Rogers - Thanks for sharing. Looks like a fun little project. At 1.25V and 20mA for the heaters, I don't imagine the tubes get very warm.

The tube look interesting for use as an all battery powered preamp. A rechargeable NiMH D cell is typically 1.2V so that would be good for the heaters. Then a couple (or more) 9V batteries could be used for the B+.

Did you happen to snap a photo of the underside of the board?
Cheers, Gio

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


Top
 Profile  
 
PostPosted: 27 Jun 2009, 19:36 
Offline
Project Author
User avatar

Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
Hi Gio,
Here is a pic of the bottom, disregard the red and green wires as they are my in and out using an external volume control, the wire is coated with that super insulator that can be a pain to solder. If I was to case this up I would have wired it from the top.

The resistor can be left out if using the correct 68 ohm dropping resistor. The wires from the inputs to tube pins 4 are elevated over the rails and do not short. all in all very easy to build


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.



Top
 Profile  
 
PostPosted: 28 Jun 2009, 07:40 
Offline
User avatar

Joined: 31 Dec 2008, 15:34
Posts: 441
Hi Rogers

It looks great, I like the easy setup of yours = respect. ( i'm learning a lot from you guys in here ). :up:

I wish i had some 6088 pentode tubes, so i could try it out :P , but it seems that i only got the EL84 pentodes, i guess it's not usable in this construction. :down:

Would it be useful to serialcouple the two 6088 filaments :lightbuilb: , so it would be supplied with 2,5 V and maybe the regulator, has an easier job to regulate 2,5 V instead of 1,25 V ? ( just a thought :geek: ). Also the 68 Ohm dropping resistor must be changed to another value, if a different current must be draught ( again, just a thought ).

:?: I was wondering about something ....Is the job for the tube only to reverse the phase ...or what ?

Regards from Kramme :mrgreen:


Top
 Profile  
 
PostPosted: 28 Jun 2009, 09:43 
Offline
Project Author
User avatar

Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
KrammeAcoustics wrote:
Would it be useful to serialcouple the two 6088 filaments :lightbuilb: , so it would be supplied with 2,5 V and maybe the regulator, has an easier job to regulate 2,5 V instead of 1,25 V ?

I was wondering about something ....Is the job for the tube only to reverse the phase ...or what ?

Hi John,

I tried to run the tubes in series in the original build, but since they are directly heated cathode only one tube will work. Both tubes need a ground connection.

The tube and potentiometer are used as 1) voltage amplification and 2)As a voltage divider to set the bias on the gate of the mosfet.

We can use this design because the source supply is a low voltage, the circuit is very similar to my other amp just a different tube and no cathode resistor.

This amp is best suited for people who are listening to music videos and movies on their computer. As Gio stated the filaments can be run with standard rechargeable D or C cell batteries and the B+ with a 12v rechargeable. I will have to get one of my ups batteries charged up and see how long it will last, the whole amp draws 400mA of current.

This is a great candidate for a portable amplifier running smaller Li batteries, the problem is the power supply and charging circuit will be much more sophisticated and surface mounted for space. That might be my next direction :up:

Rogers


Top
 Profile  
 
PostPosted: 28 Jun 2009, 11:56 
Offline
Editor
User avatar

Joined: 28 May 2008, 21:53
Posts: 4565
Location: Winnipeg, CANADA
Great, my real curiosity of the bottom was the LED as I seemed to have missed the note in the text first time around. ;)

Ironically, Mark sent an email noting that he purchased a K272 - stereo tube preamplifier - headphone driver kit.
Image
See the attached manual - no schematic is included in the manual.
Attachment:
K272-JAN6418-sub-mini-pentode-headphone-amp.pdf

What I found interesting in the manual and something you may want to try are tube dampers. Check it out, it looks like plain rubber grommets will work quite nicely.
Attachment:
JAN6418-sub-miniature-pentode-tube-dampers.jpg

How warm do these tubes get and do you see the grommets melting at all?
Cheers


This post has a file attachment. Please login or register to access it. Only Registered Members may view attached files.


_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


Top
 Profile  
 
PostPosted: 28 Jun 2009, 22:26 
Offline
Project Author
User avatar

Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
GIO,
I am using 13v B+ and the tubes are room temp at most, the 5v regulator is the hottest and the mosfet and source resistor next. I have to try the grommets, the amp has no problem unless you tap or bump it when it is operating. There will be a very high pitch ring as the filaments are vibrating. That kit is nice to see, there has not been much out there in sub-mini other than DIY. Thanks for the link.

The LED is just an indicator, a little ground effects and reminder to shut the unit off before going to bed :D

Rogers


Top
 Profile  
 
PostPosted: 29 Jun 2009, 18:44 
Offline
Project Author
User avatar

Joined: 06 Jun 2008, 18:23
Posts: 4950
Location: Australia
I am new to these tubes and have found there are quite a few brands out there. Is any one aware of which is refuted as the best tubes. I'm talking distortion levels etc.

_________________
Projects: "Lagoon" - tube preamp with cathode follower | "retro-Oatley 6J6" - 6J6 push-pull headphone amp with OPTs | "retro-Hiraga" - Jean Hiraga Le Monster
Website: retro-thermionic


Top
 Profile  
 
PostPosted: 29 Jun 2009, 19:13 
Offline
Project Author
User avatar

Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
The 6088 is not on the top of the audiophile list of tubes but for listening to ipod and standard cpu audio cards it does the job. Here are a couple of links to different submini tube sites. The one I really would like to know is what Pete Millett used in his ttvj portable hybrid. This amp received many great reviews. There are some nudies but I can not see the tube numbers.

http://amps.zugster.net/projects/subminiature
http://www.ttvj.com/index.php?main_page ... ts_id=1062
http://www.tonepublications.com/MAGPDF/TA_012.pdf

Rogers


Top
 Profile  
 
PostPosted: 29 Jun 2009, 21:30 
Offline
Editor
User avatar

Joined: 28 May 2008, 21:53
Posts: 4565
Location: Winnipeg, CANADA
The tone audio notes that heater draw is about 15mA so that narrows it down a bit. Perhaps a 6814? The article makes a note about the ringing when moving the amp around.

Peter is fairly approachable so it may be worth asking.

Cheers

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 23 posts ]  Go to page 1, 2, 3  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 12 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy