807 pp build project - whose in? - 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 1st December 2009, 09:43 AM   #1
diyAudio Member
 
aardvarkash10's Avatar
 
Join Date: Mar 2007
Location: Auckland, NZ
Default 807 pp build project - whose in?

OK I've plagued a few of you over this a few times in the past, but now I'm ready to build.

Here's the givens: The opts are ex a jukebox running 7868 power tubes. Specs on the jukebox schematic indicate the opts are 6.6k primary and around 25W. There is no u/l tap, and the secondaries are multi-tapped. We can assume they are not audiophile quality.

The ptx is from the same donor. It has a bunch of interesting windings - two heater windings, a 5 volt and a centre tapped HT that appears to be around 360-0-360.

Rectification to be decided. Filtering to be decided. Regulation to be decided.

The power tubes will be 1625s - 807s in different trousers. Pentode mode. The STC spec sheet gives a set of specs for AB1 that need 360V at the anode, 270V G2, -22.5V bias, 45 pp drive for 26.5W output. Cool! Matches my iron reasonably.

Now the help. I want minimum tube count - I just like the simplicity. So I'm looking to dissimilars like the ECF802 to provide gain and splitter - the pentode section for gain, the triode for cathodyne splitter.

The ECF802 is capable enough - the Ik ratings for both sections are ok - 15ma for the pentode, 10ma for the triode, so drive shouldn't a too much of a problem should it?

Limitation is Vkf at 100 volts, but with only 45Vpp drive required I should be able to stay under that.

What the hell - I've got a handful of them and I don't intend to spend on anything other than passives in this project. If you have suggestions, bear that in mind.

Feedback will be required obviously to deal with the combination of pentodes and under-educated opts. I'd like to keep it short and simple so a schaded approach seems the answer. But can it be used in the gain/splitter configuration I'm proposing? And if so, what do I need to figure in the math, cos to tell the truth, both Schade and Broskie have me a bit fuddled in the detail.

Or do I just get basic with some global NFB to the ECF802 pentode?

Comments invited.
Attached Images
File Type: jpg jal-200b.jpg (213.3 KB, 421 views)
File Type: jpg jal-200c.jpg (211.7 KB, 401 views)
__________________
"Folks, you can't prove truthiness with information. You prove truthiness with more truthiness. In a process known as truthinessiness." - Stephen Colbert, The Colbert Report
  Reply With Quote
Old 1st December 2009, 09:47 AM   #2
Colt45 is offline Colt45  Serbia
diyAudio Member
 
Join Date: Feb 2002
Location: Canada
Send a message via ICQ to Colt45
real men don't need feedback.
  Reply With Quote
Old 1st December 2009, 10:57 AM   #3
diyAudio Member
 
Join Date: May 2007
Go for a real triode/pentode: 6F12P with both sections Gm close to 20mA/V and triode mu of 100! The pentode has 70 mu when triode-strapped. They should be run at 10-20mA. Cheap on Ebay. The pentode is great for driving Schaded circuits.
  Reply With Quote
Old 1st December 2009, 11:00 AM   #4
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
You mentioned that there are multiple taps on the OPT, which simplifies tweaking of NFB (without changing the circuit).

I'd leave VAS cathode unbypassed for starters and apply feedback from the secondary (to VAS cathode resistor) only if needed. I wouldn't mess with plate to grid feedback unless absolutely necessary - you first need to figure out whether you're getting enough volume anyway.

Other than that it sounds pretty straightforward. Pentode section of ECF802 appears to be pretty linear at high current but gets rather ugly past Vg1 = -2V and/or at low current so you'll have to bias it close to the Pd curve and pick the operating point as close to Vg1 = 0V as you can afford to (IR LED bias to remain close to 1.2Vish ?). Keep Vg2 at or above 150V to maintain more headroom (you could go for red LED in case of Vg2 = 200V).

Good luck with your build and remember to keep us updated
  Reply With Quote
Old 1st December 2009, 11:26 PM   #5
diyAudio Member
 
pauldune's Avatar
 
Join Date: Oct 2009
Location: Netherlands
You could use a simple ecc81 or a ccs loaded ecc88. Just like I did. both schedules are here:
Optimizing a 807 PP amplifiers input stage and phase splitter...

Gain of both is about 30x. so with a 0.5 vrms input you should reach your 45 vpp.
Both have reasonably low Ri, so that should be good.

Or is only a dissimilar an option?

/joke mode on/ If you build the CCS version, you can experience the burn-in effect yourself, dont worry, in a few days the sound gets a lot better... /joke mode off/
  Reply With Quote
Old 1st December 2009, 11:49 PM   #6
diyAudio Member
 
pauldune's Avatar
 
Join Date: Oct 2009
Location: Netherlands
Quote:
Originally Posted by pauldune View Post
You could use a simple ecc81 or a ccs loaded ecc88.
Sorry, saw too late that you already have the ECF's...
Good luck...
  Reply With Quote
Old 2nd December 2009, 12:00 AM   #7
diyAudio Member
 
aardvarkash10's Avatar
 
Join Date: Mar 2007
Location: Auckland, NZ
hey! Yeah - all my stuff takes significant burn in. It takes a loooooooong time to accept that the sound it makes is as good as it will ever be :-(

Hehehehehehehehehehehe

Forgot to put that in the info above - I aim at a 2V pp input so I need a voltage gain of around 50 (or a bit more) at the pentode before some feedback...
__________________
"Folks, you can't prove truthiness with information. You prove truthiness with more truthiness. In a process known as truthinessiness." - Stephen Colbert, The Colbert Report
  Reply With Quote
Old 2nd December 2009, 12:27 AM   #8
diyAudio Member
 
pauldune's Avatar
 
Join Date: Oct 2009
Location: Netherlands
Quote:
Originally Posted by aardvarkash10 View Post
hey! Yeah - all my stuff takes significant burn in. It takes a loooooooong time to accept that the sound it makes is as good as it will ever be :-(

Hehehehehehehehehehehe

Forgot to put that in the info above - I aim at a 2V pp input so I need a voltage gain of around 50 (or a bit more) at the pentode before some feedback...
Mu of the pentode section, triode strapped is about 40.... is that too low?
Has some nice characteristics in triode....
http://www.tubes.mynetcologne.de/roe..._as_triode.pdf
  Reply With Quote
Old 2nd December 2009, 12:30 AM   #9
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 7
I already did an 807PP project, though my design was a good deal different. I did, however, make some discoveries that are apropos.

807s like to make high order harmonics. Though the spec sheet calls for THD= 1.8%, a lot of that is h5 and higher. Running open loop, it sounds quite nasty on some program material (Ozzy became all but unlistenable) though Andre Rieu wasn't so bad. 807s definitely need the assistance of local NFB. In my design, this was parallel feedback taken to the grids of the cathode follower grid drivers to "skip over" the 807's reverse transfer capacitance to avoid making a high pass filter with the feedback resistors. As per O. Schade's recommendation, the local NFB was trimmed so that 10% of the plate Vp was fed back. (~ 7.0db(v) of local NFB)

Since the 807 is an RF type, if you don't feed it RF it'll make its own. That means plate stoppers, and screen stoppers. For that, I made coils (N= 10; #18AWG; ID= 7/16th inch; space wound in parallel with 100R / 2W C-comp resistors, mounted right at the plate cap.

For screen stoppers: 1K5 / 0.5W C-comp resistors were needed to remove the last of the snivets that showed at plate current cutoff.

Once that was done, an additional 6.0db(v) of gNFB was needed to clean up the rest of the pentode nasties, and improve woofer damping. Get the open loop design right, and you don't need gargantuan amounts of gNFB. Too much, and you start to get that solid statey sound. Yuck!
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 2nd December 2009, 01:12 AM   #10
diyAudio Member
 
aardvarkash10's Avatar
 
Join Date: Mar 2007
Location: Auckland, NZ
yep and thanks Miles - I'd kinda taken it as read that the anode and screen stoppers that both you and shoog (and other 807/1625 exponents) recommend would be de rigeur.

I have Vixen schematics liberally strewn around my hard drive, so I will study it again and get back if I have specific questions.

Thanks for you ever present and patient guidance!
__________________
"Folks, you can't prove truthiness with information. You prove truthiness with more truthiness. In a process known as truthinessiness." - Stephen Colbert, The Colbert Report
  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
pre amplifier build project FOXYE Analog Line Level 3 2nd October 2009 04:04 AM
New Project Build Thread ryoung Multi-Way 5 19th April 2008 02:15 AM
Project - build your own driver! reinart Multi-Way 0 16th May 2005 07:46 AM
speaker build project rhowrhow Everything Else 0 20th February 2005 05:11 AM


New To Site? Need Help?

All times are GMT. The time now is 01:10 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