Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Suggestion for a tube buffer
Suggestion for a tube buffer
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th January 2018, 10:55 PM   #1
indaco is offline indaco
diyAudio Member
 
Join Date: Jun 2016
Location: Pizza country (Italy)
Default Suggestion for a tube buffer

Hi everyone,

I'm trying to design a tube buffer based on a pretty linear tube operating in the medium voltage range. In this case, the voltage I can obtain is about 120 V (from a voltage doubler). I took a look on two possible tubes like ECC88 or EF95, but if there are other possible candidates they are welcome. I read a very nice tube is 6SN7 but has minimal distortion only for fair high anodic voltages.
The project wich I refer to is here:

Audiocostruzioni - Il Portale dell'alta fedelt? amatoriale

I'd like to bring some change to preamp stage, relating to voltage and maybe the tube too. Thanks.
  Reply With Quote
Old 8th January 2018, 03:47 AM   #2
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
To a fair approximation: tube is tube.

At a given signal level, THD will rise about as fast as supply voltage falls.

120V has not been commonly used for line amps.

However there are "tube line amps" run at 12V (no specs).

Have you considered a Voltage Quadrupler?
  Reply With Quote
Old 8th January 2018, 04:20 AM   #3
Lingwendil is offline Lingwendil  United States
Now with high-fructose ASSAULT-gluten!
diyAudio Member
 
Lingwendil's Avatar
 
Join Date: Apr 2008
Location: Oakley California
Suggestion for a tube buffer
A 6DJ8 would work reasonably well at that voltage, but a cathode follower is reasonably high in PSRR, so a quadrupler wouldn't be a bad idea, and then you can run a 6SN7 well.

Also, you can run a CCS instead of the load resistor to give back some of that voltage the triode is standing on, giving the tube more breathing room.


Here's a overengineered and lovely design from our very own Sy that I have built (the non-servo version will work great in the majority of applications, and you can even build it without the input transformer if you please) and will do pretty well-

Heretical Preamplifier

I'm fixing to build another with affordable (but high quality) edcor input transformers, and a 6N16B russian submini tube for my input selector build.
__________________
Check out My 6SN7 Push-pull Flea Amplifier project! PCB COMING SOON!

Last edited by Lingwendil; 8th January 2018 at 04:29 AM.
  Reply With Quote
Old 8th January 2018, 07:08 AM   #4
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
For a voltage quadrupler, the 1/2 wave parallel topology provided works reasonably well. Large valued caps. in the multiplier are essential and only 1/8 of the AC RMS current being rectified is available as B+ DC. TANSTAAFL.

I suggest you scan the archives for my posts regarding "hash" filters. Following the multiplier, a "hash" filter and then a LC reservoir section will result in "clean" DC. Schottky diodes in the multiplier eliminate switching noise, but the big caps. necessary still result in noise that should be removed.
Attached Images
File Type: jpg Half Wave Parallel Multiplier.JPG (77.4 KB, 469 views)
__________________
Eli D.
  Reply With Quote
Old 8th January 2018, 09:13 AM   #5
indaco is offline indaco
diyAudio Member
 
Join Date: Jun 2016
Location: Pizza country (Italy)
Yes, I was thinking to quadruplicate the voltage (keeping in mind that each step I will lose mA), anyway if the plate current is fair low to keep all stable I will consider it. I'll take a look for some topology in PSU, like those you said.
At the moment the trasformer I would use has three secondary outputs: 5,6 - 28,5 and 43 VAC. So it can be useful for the heaters too (for 6N... etc tube types)

A last question, somewhere I read another way to raise the voltage...at the secondary output of the first transformer another transformer, set reversed so 12V->220 , was connected to get the desired voltage. Is this a good idea? It seems you may risk to burn something...

I read it here: Tube buffer
  Reply With Quote
Old 8th January 2018, 12:05 PM   #6
kodabmx is offline kodabmx  Canada
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
Another idea would be to use 6P43P tubes connected in triode. They will work nicely at 40V. Scroll down to the bottom.


Tube Tester Files - EL82, EL84, EL86 Soviet Clones
  Reply With Quote
Old 8th January 2018, 03:21 PM   #7
indaco is offline indaco
diyAudio Member
 
Join Date: Jun 2016
Location: Pizza country (Italy)
Yes, it would't be a bad idea, maybe for the future, but for now I'd prefer to go to a known route. By the way, I made some calculation for the cathode follower with the ECC88, I hope it's right:

V(cathode) = 220V - 150V = 70V
R(c) = V(plate)/i(c) = 220/22 -> 10 kohm (2 watt)
R(b) = V(bias)/Ia = 4/0.007 -> 560 ohm (1 watt)

The last ones both derived from the ECC88 diagram, by choosing the parameter Ug=-4V and Ia=22 mA, with Ua=220V
RI(internal resistance)=5k
RL(load) =10K
Av = (RL*mu /(RL + ra)) / (1 + [RL*mu / (RL + ra)]) = 0.96 (from datasheet)
Rin = ((RI + (mu+2)*RL) / (RI + (2*RL))) * Rg = 1 Mohm (choosing Rg=68K)
C(in) =0.33u
C(out) =4.7u
Attached Images
File Type: gif cathode_follower.gif (1.6 KB, 399 views)
  Reply With Quote
Old 8th January 2018, 04:54 PM   #8
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
GoatGuy's Avatar
 
Join Date: Dec 2012
Location: SF Bay Area
Quote:
Originally Posted by indaco View Post
Yes, it would't be a bad idea,
Seems OK. Dividing the power supply into ⅓'s is good. 70 volt VK (kathode in german) relative to ground. 220 V VA. Plenty of "buffer" headroom.

In point of fact, unless you have unusual front-end requirements, it almost makes no difference. Choose VK to be anything from 20 V to 150 V (for your 220 V power supply). There are rarely input signals requiring more than ±7 VP₂P (peak to peak). So "70" is as good as any other number mid-range.

The big "gotcha" tho' is that you'll have to go for a "raised grid". Not sure any of your diagrams have that. Essentially, you set up a high-Ω voltage divider to be the "raised" voltage you're trying to achieve. Say "70 volts": Shooting for a CLEAN voltage divider (you don't really want to amplify power supply hum) is key.

Using the notation RHI and RLO, I'd use a

RHI = 220 kΩ
RLO = 68 kΩ … in series.

this gives a Thevenin equivalent resistance of REQ = 51 kΩ and a divided voltage of +68 volts.

Connect grid of buffer to that via another 330 kΩ resistor. Place (on the voltage divider) a nice small 2.2 μF capacitor (250 V) for "filtering". 68 V leg to ground.

Then you'll get your 70 volt VK. As you intend.
It'll be running at 6.8 ma at 10 kΩ RK. ½ watt. Use a 1 watter if you like.

GoatGuy
__________________
John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink
  Reply With Quote
Old 8th January 2018, 05:10 PM   #9
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
GoatGuy's Avatar
 
Join Date: Dec 2012
Location: SF Bay Area
PS the "SYclotron" heretical preamplifier is a good idea too. Only problem is to get really high quality transformers. And they're “pretty spendy” for the good ones. I myself like cap-coupled. Caps work, they're cheap, the "sonic impact" is far, far, far oversold.

PPS: the RHI and RLO resistors were chosen not "out of the blue" but with something in mind: coming up with a 68 volt "grid supply" that'd have an equivalent resistance below 100 kΩ. I actually have something just as arbitrarily chosen (in values) working right at the moment. Been in operation for near 15 years. "It works".
__________________
John Curl's Golden Rule…: 100 kHz bandwidth, 3 μs risetime, 100 W mean output, 100 V/μs slew rate, 2 Ω dynamic load, 20 amp min current source/sink
  Reply With Quote
Old 8th January 2018, 07:02 PM   #10
indaco is offline indaco
diyAudio Member
 
Join Date: Jun 2016
Location: Pizza country (Italy)
Quote:
Originally Posted by GoatGuy View Post
Seems OK. <snip>

If I understood well is this a fixed biased CF you say? Effectively, among other things it offers a lower input impedance too
  Reply With Quote

Reply


Suggestion for a tube bufferHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
FS: Yaqin CD-1 Tube Buffer w/Mullard 6DJ8 Tube. Adds tube warmth to any system. matevana Swap Meet 2 15th June 2013 12:35 PM
Tube buffer suggestion for LM4702 amplifier? Dxvideo Tubes / Valves 5 22nd November 2012 10:15 AM
Suggestion for this tube preamp fastvideo Tubes / Valves 7 14th July 2008 09:02 PM
subminiature tube suggestion dsavitsk Tubes / Valves 13 22nd December 2005 06:09 PM
kind suggestion for an high current buffer. beppe61 Solid State 11 15th October 2005 06:36 PM


New To Site? Need Help?

All times are GMT. The time now is 06:20 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki