Go Back   Home > Forums > Amplifiers > Tubes / Valves

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

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 15th December 2012, 08:57 PM   #1
Gluca is offline Gluca  Italy
diyAudio Member
 
Gluca's Avatar
 
Join Date: Nov 2004
Location: Back to Italy
Default RIAA triode or pentode

well I know ... there are zillions of discussions and schematics out there and I am considering many of them, but would somebody of you share bad/good experience with pentode RIAAs?

gianluca
  Reply With Quote
Old 15th December 2012, 09:23 PM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Use a high transconductance low noise type like the D3A(7721) or E810F(7788) and it should work quite well based on my very, very limited experience. Input capacitance will be quite low with just a little care.

Another option is a cascode built around the ubquitous ECC88, Russian 6S3P or 6S4P, or the 5842/417A - any of these will also give you low miller capacitance as the lower tube in the cascode will have relatively low effective mu.

Gains of 46 - 50dB or more are achievable in the first stage with care.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 15th December 2012, 09:27 PM   #3
Gluca is offline Gluca  Italy
diyAudio Member
 
Gluca's Avatar
 
Join Date: Nov 2004
Location: Back to Italy
Kevin, do you mean triode strapped D3As or E810FFs or just plain pentodes?

G
  Reply With Quote
Old 15th December 2012, 09:34 PM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Hi Gluca,
No, I actually mean pentode connected D3A, and 7788.. Or alternately triode cascodes which could incidentally include a D3A in triode connection.

I made the mistake of using the D3A in triode in the design prior to my current muscovite design (analog source) and the total miller capacitance and strays at more than 300pF is a killer.. This is not the best sounding phono stage I have designed by a long shot, how it interacts with the cartridge is a big problem eliminating most MM from consideration..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 15th December 2012, 11:03 PM   #5
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
I guess the reason you're asking is noise. With triodes, to minimise noise you need to operate at the lowest possible anode current you are happy with. High gm helps too, but anode current is the main thing to think about.
With pentodes there is additional noise introduced due to screen current partition. To minimise that you need to minimise screen current relative to anode current. In other words, you need to run with low anode current, very low screen current, and high gm is a bonus if you can get it.

Having said that, pentodes are probably not as bad as some would have you believe. For example, the equivalent input noise of an EF86 over the audio band is only about 2uV, whereas a reasonable triode might be 1.5uV. (OK, a really well chosen triode might achieve 0.5uV, but be prepared to pay for Telefunkens or something!)

You could of course use a triode at the input, but pentodes for later stages where the noise is no longer important, but then you may have to think about linearity instead.

Last edited by Merlinb; 15th December 2012 at 11:06 PM.
  Reply With Quote
Old 16th December 2012, 12:11 AM   #6
diyAudio Member
 
Chris Hornbeck's Avatar
 
Join Date: May 2011
Location: Little Rock
There's a must-read article in the first (Volume 0) issue of Linear Audio. Frank Blohbaum (mit der Umlaut) has an eye opener about pentodes and noise.

All good fortune,
Chris
  Reply With Quote
Old 16th December 2012, 01:15 AM   #7
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
TANSTAAFL will be with us forever. Name your poison: pentode partition noise or triode Miller capacitance. Cascodes are interesting because CMiller is low, along with avoiding partition noise. The "gotcha" with a cascode is abysmal PSRR. IMO, hybrid cascodes of either permutation deserve consideration. Allen Wright successfully stacked a high gm triode on top of SS, but a high voltage MOSFET stacked over a high gm triode bears looking into too.

A high μ, high gm, low RP, low CMiller triode to keep in mind when scoping a RIAA preamp out is the 6GK5. A low noise pentode to consider is the Octal 6AC7. Don't forget the silicone rubber damping rings and DC heater supply.
__________________
Eli D.
  Reply With Quote
Old 16th December 2012, 02:18 AM   #8
diyAudio Member
 
Chris Hornbeck's Avatar
 
Join Date: May 2011
Location: Little Rock
Frank Blöhbaum

Chris
  Reply With Quote
Old 16th December 2012, 08:46 AM   #9
diyAudio Member
 
Join Date: May 2007
Note Frank Bs circuit is made for high NFB applications. His PFL200 phonostage has an open loop bandwith of 13kHz and ca 35dB NFB. Unfortunately he has removed his schematics from Intactaudio/forum.
  Reply With Quote
Old 16th December 2012, 09:47 AM   #10
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by Chris Hornbeck View Post
There's a must-read article in the first (Volume 0) issue of Linear Audio. Frank Blohbaum (mit der Umlaut) has an eye opener about pentodes and noise.
Yes, I saw his presentation at ETF and read the article. What's interesting (to me) is that he hasn't really overcome the partition noise issues in pentodes. His circuit, at essence, is a cascode with a bipolar on top, triode on bottom with the screen used as a plate. The plate in his pentode has hardly any effect on the circuit- I built one on the bench, disconnected the plate, and got almost identical results.

This isn't that surprising since using the screen of a pentode as a plate in low noise circuits is a long-known trick.
__________________
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it." — Upton Sinclair
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pentode to Triode Switch Sherman Tubes / Valves 40 6th February 2013 12:06 AM
Pentode to Triode Operation Monjul Tubes / Valves 13 21st September 2010 11:27 PM
Triode versus Pentode Ricky Tubes / Valves 98 20th March 2008 08:43 AM
Pentode to Triode trick. Diy martin Tubes / Valves 4 7th November 2005 11:00 AM
triode=>pentode Lico Tubes / Valves 3 7th April 2004 07:08 PM


New To Site? Need Help?

All times are GMT. The time now is 06:04 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2