6021 Cascode - 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
Thread Tools Search this Thread
Old 22nd September 2009, 10:20 PM   #1
diyAudio Member
Join Date: Jan 2004
Location: Indiana
Default 6021 Cascode

I'm building a sub miniature guitar amp built with 6021 tubes. I based the amp on the "Powerman" amp and added a cascode boost section a la the "Firefly" amp.

I wasn't able to really find anything on using 6021's in a cascode setup and I know just enough to be dangerous! Could someone point me in a direction of some information on how to design this stage properly?

I have been just taking some stabs in the dark on component values and its funtional but the sound is very rolled off and muddy. B+ is about 115 volts.

Thoughts comments suggestions?

  Reply With Quote
Old 23rd September 2009, 12:29 AM   #2
diyAudio Member
Miles Prower's Avatar
Join Date: May 2005
Location: USA
Blog Entries: 7
No mystery about cascodes. It's simply a two stage amp with a grounded cathode stage driving a grounded grid second stage. Like every other multi-stage amp, start from the end and work backwards.

Here's how I designed a cascode. (Attached) Start with the second stage. In this case, assume a Vpp= 275Vdc, and play around with loadlines until an Rp= 68K gave the best compromise between THD estimate and output swing. (In this case, THD= 5.4%, which would be barely acceptable. 6BQ7s are hard to design for, having no audio pretenses whatsoever, and really require a higher Vpk to linear up. However, in this case, that THD estimate applies to just the one stage, not both. As with the solid state version, cascoding improves overall linearity. Also, this particular cascode was half of an LTP which nulls out even order harmonics, and if the THD is mostly h2, then you lose most of it anyway.) Next, use the plate characteristic to estimate u and g(m). From that, calculate r(p). You need to know that in order to figure out what your r(k) is likely to be. r(k) then becomes the plate load for the grounded cathode stage.

Next, run a loadline for the input stage. This is always a good idea to see if the first stage has enough swing to swing the output to its theoretical maximum. Here, we're OK in that regard, and good to go. No need to move the input loadline by adding external parallel resistance.

Finally, figure out what your point-to-DC ground voltages are. Then you're done. Build it to see if it really works as designed. In this case, it did, and measured results were within 5% of design nominal values.

Originally Posted by Skitch72 View Post
I have been just taking some stabs in the dark on component values and its funtional but the sound is very rolled off and muddy. B+ is about 115 volts.

Thoughts comments suggestions?
The A Number One bugaboo with cascodes is their eeeeeeeeenormous output impedance. Combined with the capacitances of a load, this will not play well at higher frequencies. After completing this cascode design, connecting the o'scope probe directly to the output gave results that looked positively hideous. A 1000Hz square wave had very tilted tops, the measured f(h)= ~20KHz. It looked like the slowest thing I'd ever built, either solid state or hollow state. This was, of course, an illusion caused by the capacitances of the scope probe, the connecting cable, and the Ci of the scope's vertical deflection amp.

Once connected to a cathode follower, the true picture emerged. The measured f(h)= 117KHz (-3.0db) and square waves had nice, flat tops with excellent rise and fall times, when the measurements were taken at the output of the cathode follower.
Attached Images
File Type: png 6BQ7A-Loadline.png (120.0 KB, 109 views)
There are no foxes in atheistholes
  Reply With Quote
Old 23rd September 2009, 12:58 AM   #3
diyAudio Member
Join Date: Jan 2004
Location: Indiana
Perfect! Just what I needed....I am pretty new to load lines but I think 95% of that makes sense...off to the drawing board.

Thanks Again,
  Reply With Quote


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
6021 Amp project..looking for some input Skitch72 Tubes / Valves 1 12th May 2009 04:27 PM
Cascode Buffers acoustixman Solid State 43 26th August 2008 03:04 PM
Borbely Cascode Onvinyl Solid State 25 10th September 2007 03:48 PM
Cascode PSUs mcai7et2 Solid State 3 19th February 2004 10:44 AM
6021 opinions Glass_painter Tubes / Valves 1 28th May 2003 09:51 PM

New To Site? Need Help?

All times are GMT. The time now is 06:03 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2