DeathTrap400 : : A Pretty Damn Good 300B Amp - Page 3 - 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 15th October 2012, 02:58 AM   #21
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Nice find on the 6N30P data. I found the 6N6P data through TDSL.

~Tom
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 15th October 2012, 04:10 AM   #22
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Tom,
I have some Power and Output trannies sitting on the shelf for a couple of 300B monoblocks. One of those "I'll get round to" projects that I never have - so this looks good. Also have a pair of JJ 300Bs and some ECC99.

A few questions:
1) Vcascode - I take it that this is set at around say 30V less than +400V rail to limit the voltage across the LT3092 to less than its max 40V rating. Is that correct?
2) I haven't seen that MOSFET + CCS Chip style CCS before. Have you tried anything simpler for the CCS load (A Buddha CCS or something similar). I have no experience with CCS as anode loads but have used them a lot as diff amp tails and have never managed to get any better performance than a cascode bipolar transistor arrangement. Past a point device/lead capacitance becomes the limiting factor (at the high frequency end).

Thanks,
Ian
  Reply With Quote
Old 15th October 2012, 05:07 PM   #23
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
I've attached the "supply" portion of the schematic. This amp is fed from a +400 V regulated supply (I use my 21st Century Maida for this). The -225 V is generated by a string of zeners with a MOS follower.

Quote:
Originally Posted by gingertube View Post
Tom,
I have some Power and Output trannies sitting on the shelf for a couple of 300B monoblocks. One of those "I'll get round to" projects that I never have - so this looks good. Also have a pair of JJ 300Bs and some ECC99.
Sounds like a good start for a project...

Quote:
Originally Posted by gingertube View Post
1) Vcascode - I take it that this is set at around say 30V less than +400V rail to limit the voltage across the LT3092 to less than its max 40V rating. Is that correct?
That's exactly right. I use Vcascode = 15 V less than B+, but you could use 30 V as well. You'd have less headroom for the input valve, but that's mostly of academic value. The circuit has more than 100 V of headroom. An extra 15 V isn't going to make much difference.

Quote:
Originally Posted by gingertube View Post
2) I haven't seen that MOSFET + CCS Chip style CCS before. Have you tried anything simpler for the CCS load
It's a standard IC-based CCS with a MOS cascode. There's nothing mythical about it. It just works... And it's actually pretty simple. R1, R2 set the current. The LT3092 provides a current that is very stable with temperature. In addition, it's a quite low noise current source. C2 further rolls off the noise, and C1 provides HF compensation to improve the transient response. Q2 is there to keep the voltage across the IC to a reasonable level - it also dissipates all the power in that circuit. Heatsink needed!

I looked at a handful of CCSes before I settled on the LT3092. But most discrete CCSes are very temperature dependent and I don't like the idea of bias points shifting around as the amp heats up. I designed a couple of temperature compensated current sources, but they were all of higher complexity than the IC + a few passives and still required a cascode to survive the voltage. So I went "simple is good" and let Linear Technology worry about the temperature compensation...

I did try the IXYS 10M45 CCS. That'll work too, but I like the sound of the LT+cascode much better. I speculate that the improvement in sound is because the output impedance of the cascoded CCS is much higher (orders of magnitude) than that of the 10M45.

I'm not familiar with the two circuits you mention. If you have a schematic, I'd be interested. I'm curious...

BTW, I intend to get some PCBs made professionally for this project. I'll be selling the excess boards from my website. I'm hoping to get the project done before Christmas, but would prefer to not make too many promises regarding availability date. Just letting you know it's in the pipe...

~Tom
Attached Images
File Type: png 6N6P_ECC99_12BH7A-300Bdriver_Supply.png (28.9 KB, 1051 views)
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

Last edited by tomchr; 15th October 2012 at 05:11 PM.
  Reply With Quote
Old 15th October 2012, 06:16 PM   #24
diyAudio Member
 
Join Date: Oct 2012
Since this use a transformer input, is it possible to shift the cathode bias of first stage to -225V? Then DC-couple can be realized between 1st stage output to 2nd stage input? This can remove the coupling capacitor C4A.
  Reply With Quote
Old 15th October 2012, 07:26 PM   #25
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
With Vak = 180 V, there isn't enough head room on a 225 V supply to run the input stage. Of course, one could run the CCS from the B+ and put the cathode of the input tube at -225 V. This would dissipate rather large amounts of power in the cascode MOSFET. That's probably manageable if the MOS can sit on an external heat sink.
However, with such an arrangement, the bias of the output tube is now dependent on the characteristics of two tubes. And the Vak varies a lot more than Vgk both as the tube warms up and from tube to tube, so controlling the bias on the output tube will be a challenge.

There is (at least) one solution to this. I tried using a 70 H inductor/choke load on the input tube. If one end of the choke is grounded and the other connected to the anode of the input tube with the rest of the amp DC coupled from there, it actually provides nice control of the output tube bias. One can use cathode bias on the 300B pretty easily. I really like the simplicity of that circuit. A couple of "light bulbs" some resistors, and an inductor. BAM! You have an amp. I built one, actually. It was inspired by Jack Elliano's 300B DRD but rigged as I describe above.
I ended up going back to the circuit I posted yesterday for a couple of reasons: The THD on the input tube is heavily dependent on the bias point of the tube (DUH!) But to control the bias point when the anode is grounded through an inductor, a variable supply is needed. This isn't hard to do, but it adds complexity and with the tube I used at the time (d3A, I think) there's enough tube-to-tube variation that the supply would have to be customized to each tube. In addition, the lowest THD I could get with that circuit was on the order of 0.5 % and it was rather frequency dependent. I'd have to check my notes if you want more data. I auditioned a few different biasing options for the 300B and liked fixed bias the best. So my conclusion was that with a DC coupled amp, I would trade off amp complexity in exchange for supply complexity and get worse performance that I get with my current circuit.

I'm not a fan of capacitors in the signal path either. But I find that a quality polypropylene cap is transparent if used properly. By using a cathode or source follower, one avoids the issues with blocking distortion as well.
In the 300B DRD circuit, you exchange the cap for an inductor in the signal path instead. Tradeoffs, tradeoffs.

Sorry for the long-winded reply, but I've spent the past two or three years fiddling with this 300B amp, so I have lots of stories to tell...

~Tom
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 21st October 2012, 05:14 AM   #26
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Folks,

I've tested d3A, ECC99, 6N6P, and 12BH7A as driver tubes in my 300B driver design. Attached are my end-to-end measurements (amplitude, H2, H3, impulse response).

RED = 12BH7A-300B
BLUE = ECC99-300B
YELLOW = 6N6P-300B
GREEN = LME49811-based sand amp for reference.

Measurements of the THD of the amp with resistive load reveal the following:
12BH7A: 0.39 % THD @ 1 W; 4.8 W @ 1 %; 9.7 W @ 2 %; 10.2 W @ 3 % THD.
6N6P: 0.16 % THD @ 1 W; 9.7 W @ 1 %; 11.4 W @ 2 %; 11.8 W @ 3 % THD.
ECC99: 0.18 % THD @ 1 W; 8.9 W @ 1 %; 9.6 W @ 2 %; 9.8 W @ 3 % THD.

The input tubes were biased as follows: 12BH7A: Vgk = -11.2 V; Ia = 12 mA. ECC99: Vgk = -6.0 V; Ia = 12 mA. 6N6P: Vgk = -6.2 V; Ia = 15 mA.
The cathode followers were running at 4.4 mA. For the cathode followers, Vak was roughly: 12BH7A: 230 V; ECC99: 125 V; 6N6P: 170 V. These voltages were chosen for optimal THD.

I'm surprised that the 12BH7A distorts so much. I wouldn't have thought that from looking at the plate curves. I'm guessing it has to do with the source follower. 12BH7A is not a very good candidate for a source follower due to its low gm. And I suspect it isn't entirely happy at the 4.5 mA I'm running through that source follower. I did try upping the current, but that made negligible improvement.
I'm thinking that the 12BH7A is probably better suited for 300B amps with the anode of the input tube (12BH7A) connected directly to the 300B grid (via a cap if necessary) rather than through a cathode follower. Maybe a source follower could improve the THD, but I have decided to not use a source follower as I prefer the sound of a good cathode follower.

Listening tests:
To my ears, on my system, the 12BH7A sounds a lot like the d3A. It's a nice tube. No question about it. It reveals a lot of detail, but to my ears, it's a bit fuzzy in the highs. The lisp on hard consonants is more pronounced. And at higher SPLs I found it kinda straining to listen to.
The ECC99 is nice as well. I don't find it as revealing as the 12BH7A/d3A and perhaps a bit harsh, but for a current production $15 tube, it's pretty damn good! This tube was my favorite until I was led to the 6N6P.
In my opinion, the 6N6P is the best compromise between harshness and precision. My amp likes that tube.

The end result is the same as last time I posted. My board will support 12BH7A, ECC99, and 6N6P by configuring a couple of wire jumpers. One can wire in a pair of d3As in place of an ECC99 if desired.

~Tom
Attached Images
File Type: png AmpComparison_Amplitude.png (83.3 KB, 944 views)
File Type: png AmpComparison_Impulse.png (53.1 KB, 911 views)
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 21st October 2012, 11:34 PM   #27
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
First stab at component placement. I see a few obvious optimizations already, but it looks like a stereo driver will fit on a 4x5 inch board.

~Tom
Attached Images
File Type: png 300B_AMP_R2p0_Layout.png (90.0 KB, 869 views)
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 25th November 2012, 01:38 AM   #28
grufti is offline grufti  United States
wombat
diyAudio Member
 
grufti's Avatar
 
Join Date: Jun 2004
Location: Hillside
Any news? I'm interested.

Last edited by grufti; 25th November 2012 at 01:39 AM. Reason: Explained myself a little better
  Reply With Quote
Old 25th November 2012, 03:10 AM   #29
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Yes. I have news. The past few weeks have involved power supply work and mucking with various rectification schemes. Attached is where I ended up. I tried Schottky diodes, regular diodes, X caps, etc. and wasn't able to measure any significant difference between one type or the other so I went "standard" with the supply.

Tomorrow I hope to get some work done on the amp board. I've been mulling over how to make it work with the chassis layout I have in mind. I think I might make the amp board a long, skinny one. Like 3x5 or 3x6 inch with holes for the 300B sockets.

Don't change that channel! We'll be right back...

~Tom
Attached Images
File Type: png 300B_PSU_R2p0.png (126.6 KB, 770 views)
Attached Files
File Type: pdf 300B_PSU_2P0.pdf (26.6 KB, 114 views)
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
  Reply With Quote
Old 27th November 2012, 05:01 PM   #30
diyAudio Member
 
Join Date: Jun 2008
I have a 845 SET that uses a 6AC7 pentode and a 300B as drivers. It's wanting in many ways and so I've been following this thread with great interest. Or perhaps I should say trying to follow...

It's the cathode follower that I'm having trouble following. I retread MJ and Boskie on the subject last night and am still in the dark. There is a big gap in my understanding. Perhaps someone can shed some light.

It's my understanding that the cathode follower will have just under unity gain and will need to swing considerable voltage to drive the big triode. Given the near unity gain, the grid swing can be no less than that across the cathode resistor, no?? Yet the ECC99 Grid voltage lines max out at about -20 volts. So, how is it possible for this ECC99 to swing more than say 10 volts pp?

School me.

Last edited by Captn Dave; 27th November 2012 at 05:07 PM.
  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
6AV5GAs Re-Visited -OR- Pretty Darn Good SE Amp hareynolds Tubelab 27 11th May 2014 02:19 AM
PGP (Pretty Good Poweramp) syn08 Solid State 263 4th April 2013 06:28 PM


New To Site? Need Help?

All times are GMT. The time now is 06:50 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