6X8 Pentode/Triode Musings - diyAudio
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 9th March 2009, 09:05 PM   #1
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Default 6X8 Pentode/Triode Musings

Miles Prower mentioned the 6X8 pentode/triode in another thread.

I wonder if something could be made of this "oddball" type in a phono preamp. I see 2 ways around the common cathode connection: red LED bias and grid leak bias. I'm partial to grid leak biased 2nd gain blocks, as they yield better bass extension. Grid leak bias in the 1st gain block requires cap. coupling between the 47 KOhm cart. loading resistor and the pentode's g1. Given a 20 MOhm grid leak value, a transparent PTFE cap. will be affordable. Perhaps 1 of the time constants can be obtained by parallelling the 20 MOhm part with capacitance.

The $64 question is whether the noise factor of the pentode is adequate.

Comments please.
__________________
Eli D.
  Reply With Quote
Old 9th March 2009, 09:38 PM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
I use these for voltage regulator error amps. You can make an unbalanced differential with the triode section as the noninverting input that receives the reference voltage. The pentode half becomes the inverting input that provides the gain. Connect its control grid to the voltage set/sense divider.

I was using 6KE8s for this, but these guys are a good deal more expensive, and 6X8s are quite common, nobody wants them, and I can get 'em dirt cheap.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 12th March 2009, 12:09 AM   #3
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Bump.

Anybody care to comment on the phono preamp concept?
__________________
Eli D.
  Reply With Quote
Old 12th March 2009, 01:19 AM   #4
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by Eli Duttman
Anybody care to comment on the phono preamp concept?
Since this type was intended for use as a cathode coupled mixer/oscillator, it looks like the pentode section would be quiet enough for a phono preamp. It was designed for use in a typically low level part of the receiver. It also looks to have some very low internal capacitances as well. The triode section looks to have good linearity as well. You can always get around that common cathode by applying fixed bias, or adding a bit of positive grid bias to offset the cathode bias.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 12th March 2009, 09:44 AM   #5
diyAudio Member
 
Geek's Avatar
 
Join Date: Sep 2004
Quote:
Originally posted by Miles Prower
...and 6X8s are quite common, nobody wants them, and I can get 'em dirt cheap.
Speaking of which, I have a big Ziploc full of the dang things. You want more?

Cheers!
  Reply With Quote
Old 12th March 2009, 10:09 AM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
I'm partial to grid leak biased 2nd gain blocks
Eli, what does this do for headroom and overload performance? The second hole is where the signal is the highest...
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 12th March 2009, 04:58 PM   #7
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
6X8s are quite common, nobody wants them....Speaking of which, I have a big Ziploc full of the dang things. You want more?
So did I. They were in the lot that I just sold though. I have too many other triode - pentodes that aren't joined at the cathode and have better specs.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 12th March 2009, 05:10 PM   #8
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: UK
Noise and microphonic effect are quite typical troubles with non-specialized tubes in this application.
If you don't need to buy anything except such cheap tubes I think the best response is trying all the possible solutions you mentioned.

Other possible cheap candidates you might consider are the 20EZ7 (basically a low noise and low cost 12AX7 with 20V filament supply and different pin-out) and the 5755.

IMO, the 5755 as input tube of a MM phono stage can really stand out.
120V anode voltage and 0.5 mA could be a good working point (the bias is -0.5V).

In my experience the best phono amps all had a passive split RIAA, with the 75 us network after the second stage. With some expedients (basically a design choice), such network can have a negligible insertion loss.

45
  Reply With Quote
Old 12th March 2009, 06:35 PM   #9
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by Geek
Speaking of which, I have a big Ziploc full of the dang things. You want more?

Cheers!
Yep. I can use 'em.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 12th March 2009, 06:59 PM   #10
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by 45
Noise and microphonic effect are quite typical troubles with non-specialized tubes in this application.
Not necessarily. You never know until you try. The spec sheet for 6BQ7s make no mention of any sort of audio useage at all. Yet, when used to make a cascoded LTP with active tail load, these really rawk. Click the image to open in full size. More gain than the usual solutions (12AX7, 6SL7) none of that pentode nastiness problem mentioned with pentode LTPs. These, too, are cheap since they have no audiophool pedigree. I also discovered that the Sylvania and Motorola versions with the parallel connected heaters aren't microphonic at all. (Different story with the ones with series connected heaters.)

Besides, the fat cathodes score extra glowey bottle coolness points.

6BQ6GTBs also are "non-specialized" with no mention than anything other than TV horizontal deflection duty. Sounds as good as PP 6V6s, but with better than twice the output power. Given the need for pulling bigamps through the deflection coils, these require less impedance transformation, so that the OPTs are easier to design, and work better. Also, cheap with no audiophool pedigree. (But they've been going up lately. I guess the good folks at the VT store figured something was up when I bought a whole bunch at $1.30 a pop.)
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  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
C3g driving 2a3: Triode or pentode? Klimon Tubes / Valves 7 15th May 2009 10:31 AM
PP-Triode , PP-UL , PP-Pentode-strapped-as-Triode percy Tubes / Valves 23 22nd January 2007 12:36 AM
SE Pentode or PP Triode ? percy Tubes / Valves 37 8th October 2004 10:21 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 04:56 AM.


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