Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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 16th August 2019, 02:35 PM   #31
Lingwendil is offline Lingwendil  United States
diyAudio Member
 
Lingwendil's Avatar
 
Join Date: Apr 2008
Location: Oakley California
You're doing a lot of things right here- and the above suggestions are on point. I would use anything from 100-470uF for C1-C4, and I would go for something with a 105C, 10,000hr rating, They'll last a good long while, and at the smaller values are still very affordable from places like digiKey for good name brand commercial parts. C4, C5, and C6 can all be 100nF. Don't bother with audiophile secret-sauce parts here.

Individual cathode bias for each EL84 is a lovely touch, practical and reliable

For the CCS, well, that's up to you. I would go for a plate resistor from 120k to 150k and leave it at that. I happen to find 12AX7 types do well without a CCS so long as the cathode resistor is unbypassed, and the plate load is high. Play with the parts until you get 60~70% of the B+ voltage at the anode. Gain should be 50~ or so at the 12AX7 before feedback. Plenty for EL84 types in this application.


This is going to be one sweet amplifier.
__________________
6SN7 Push-pull Flea Amplifier PCB files are now available!
  Reply With Quote
Old 17th August 2019, 01:30 AM   #32
Kofi Annan is offline Kofi Annan  United States
diyAudio Member
 
Kofi Annan's Avatar
 
Join Date: Jan 2004
Location: US
OK-- lowered the cap values (see attached schematic). I believe increasing the 12AX7A plate resistor would require an increase in B+, which I think would cause some issues (unless I am misunderstanding, which DOES HAPPEN), so I'm inclined to leave it as is.

Also, it looks like I will need to have power transformer that is capable of delivering about 260mA of current at around 275 volts. That seems like a tall order (unless I am misunderstanding which, please refer to the PREVIOUS SENTENCE).

Thoughts on the power transformer? Also, I'm assuming I should go with the Toriody transformers with the CFB taps? Any thoughts on that?

KOFI???
Attached Images
File Type: png EL84PPP v2 - Self Bias Cathodyne v2.PNG (163.9 KB, 243 views)
  Reply With Quote
Old 17th August 2019, 01:24 PM   #33
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Coupla things that I hope will help...

I think R12 and R13 are not in their correct positions. R13 (grid leak) should go from input to (signal) ground before the connection to R12 (grid stopper). See attached schematic.

The way you have it now, R12 and R13 form a voltage divider, albeit one that doesn't knock down the signal voltage by very much.

Also, I noticed that R14, the cathode bias resistor for the 12AT7, is a pretty high value, keeping the plate current of the 12AT7 to less than 2.5mA. Is that enough current sunk to drive the Cmiller of the paralleled EL84s in UL configuration? Would Ia of 3mA or 3.5mA be an improvement, or not necessary?

Quote:
I believe increasing the 12AX7A plate resistor would require an increase in B+, which I think would cause some issues (unless I am misunderstanding, which DOES HAPPEN), so I'm inclined to leave it as is.
The way the 12AX7A is configured now, it will be drawing a healthy (for a 12AX7) 1.2mA of plate current. 12AX7 can work well with less than 1mA of plate current. What would be the harm of increasing the value of R21 (the 12AX7A plate resistor) to 150k? Or you could re-bias that stage with maybe R12 = 220k and R22 = 2.2k. LTspice says that would put 168V on the plate, 1.5V on the cathode, plate current of 0.69mA. You'd get a bit more gain out of the 12AX7 that way, but transconductance would be a bit lower. Would that lowered transconductance cause problems driving a negative feedback loop? It certainly should not be a problem driving the 12AT7 cathodyne (that will be easy to drive, high input impedance, low input capacitance).
--
Attached Images
File Type: png _KOFI_12AX7-12AT7-cathodyne.png (17.0 KB, 220 views)

Last edited by rongon; 17th August 2019 at 01:34 PM.
  Reply With Quote
Old 17th August 2019, 08:40 PM   #34
Kofi Annan is offline Kofi Annan  United States
diyAudio Member
 
Kofi Annan's Avatar
 
Join Date: Jan 2004
Location: US
Quote:
Originally Posted by rongon View Post
Coupla things that I hope will help...

I think R12 and R13 are not in their correct positions. R13 (grid leak) should go from input to (signal) ground before the connection to R12 (grid stopper). See attached schematic.

The way you have it now, R12 and R13 form a voltage divider, albeit one that doesn't knock down the signal voltage by very much.
Correct! Thanks for catching that!

Quote:
Also, I noticed that R14, the cathode bias resistor for the 12AT7, is a pretty high value, keeping the plate current of the 12AT7 to less than 2.5mA. Is that enough current sunk to drive the Cmiller of the paralleled EL84s in UL configuration? Would Ia of 3mA or 3.5mA be an improvement, or not necessary?

The way the 12AX7A is configured now, it will be drawing a healthy (for a 12AX7) 1.2mA of plate current. 12AX7 can work well with less than 1mA of plate current. What would be the harm of increasing the value of R21 (the 12AX7A plate resistor) to 150k? Or you could re-bias that stage with maybe R12 = 220k and R22 = 2.2k. LTspice says that would put 168V on the plate, 1.5V on the cathode, plate current of 0.69mA. You'd get a bit more gain out of the 12AX7 that way, but transconductance would be a bit lower. Would that lowered transconductance cause problems driving a negative feedback loop? It certainly should not be a problem driving the 12AT7 cathodyne (that will be easy to drive, high input impedance, low input capacitance).
--
I upped the plate resistor for the 12AX7A and lowered the bias resistor for the splitter. I also ATTEMPTED to model a cathode feedback transformer, but things look very wonky to me here.

Did I get the CFB arrangement and modeling correct? Also, regarding the power transformer, am I correct that for a stereo supply I would need over 360mA?

Kofi

PS-- Credit where credit is due: I modeled the CFB using the design in this thread.

Quote:
Originally Posted by Kofi Annan View Post
Did I get the CFB arrangement and modeling correct? Also, regarding the power transformer, am I correct that for a stereo supply I would need over 360mA?
Wait-- should be half-duty cycle, so I should only plan for 180mA, right?

Other Kofi
Attached Images
File Type: png EL84PPP v2 - CFB v3.PNG (171.8 KB, 197 views)
  Reply With Quote
Old 18th August 2019, 12:18 AM   #35
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Hmmm... That way of modeling the cathode feedback secondary doesn't look right to me. I have to think about how to do that in LTspice. I don't think I've ever tried.

Does your OPT have an extra secondary winding for cathode feedback? If so, it would be wired like this:
Click the image to open in full size.

or this:
Click the image to open in full size.

I would first design (and model) the amp without negative feedback, and get the best looking circuit you can get before adding NFB. Then once you're satisfied you've done the best you can within the limitations of devices and available voltage and current from your supplies, you can start adding NFB knowing that the underlying circuit will give you the best chance to get optimal results.

Re: power transformer - It's best to have enough current capacity from the transformer to be able to supply the peak current demands the amplifier will draw from it.

Are you figuring each EL84 will draw 45mA plate current with no signal? Then yes, your power transformer has to be rated to deliver at minimum 360mA (45*8), plus the current required from the (class A) driver stages. That's 2.5mA for the two 12AX7 triodes and 6mA for the two 12AT7 triodes, so another 8.5mA on top of the 360mA for the EL84s, or 368.5mA. I'd call it 370mA at minimum. I'd go for 400mA current capacity. That's going to be a pretty hefty chunk of iron.

In VA terms, that would be 250VAC * 0.4A current = 100VA.

Antek makes a toroid with that rating for under $40 -
AS-1T250 - 100VA 250V Transformer - AnTek Products Corp

Edcor makes a 500VCT 400mA EI core monster too (9.75 lbs!), for $75 -
EDCOR - XPWR078

Those are in the USA, so you might need to look closer to home.
--

Last edited by rongon; 18th August 2019 at 12:21 AM.
  Reply With Quote
Old 19th August 2019, 02:56 PM   #36
Lingwendil is offline Lingwendil  United States
diyAudio Member
 
Lingwendil's Avatar
 
Join Date: Apr 2008
Location: Oakley California
I really like Antek power iron, and it's very economical.
__________________
6SN7 Push-pull Flea Amplifier PCB files are now available!
  Reply With Quote
Old 19th August 2019, 06:10 PM   #37
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
The Antek AS-1T250 says that it's rated 100VA (250V 0.4A). It has two 250VAC secondaries (not center tapped). For a stereo amp, I guess you could put a full wave bridge on each of the two 250V secondaries. So, a question: Does that mean each of the two 250V secondaries is capable of delivering 200mA?

If yes, that means the Antek toroid gives you the same voltage and current capability as the Edcor XPWR078 EI cored 500VCT 400mA transformer at less than half the weight. (9.75 lbs for the Edcor vs. 4.1 lbs for the Antek toroid.) And...

The cost for the Antek is something like half that of the Edcor.

Really? If so, looks like Antek all the way.
--
  Reply With Quote
Old 19th August 2019, 06:26 PM   #38
Kofi Annan is offline Kofi Annan  United States
diyAudio Member
 
Kofi Annan's Avatar
 
Join Date: Jan 2004
Location: US
Quote:
Originally Posted by rongon View Post
The Antek AS-1T250 says that it's rated 100VA (250V 0.4A). It has two 250VAC secondaries (not center tapped). For a stereo amp, I guess you could put a full wave bridge on each of the two 250V secondaries. So, a question: Does that mean each of the two 250V secondaries is capable of delivering 200mA?

If yes, that means the Antek toroid gives you the same voltage and current capability as the Edcor XPWR078 EI cored 500VCT 400mA transformer at less than half the weight. (9.75 lbs for the Edcor vs. 4.1 lbs for the Antek toroid.) And...

The cost for the Antek is something like half that of the Edcor.

Really? If so, looks like Antek all the way.
--
That would be awesome! I was looking at chokes last night, however, and it appears that I will need to split the supply out with an individual choke for each channel. The current ratings of chokes are too low unless you go HUGE and I'd rather have the chokes smaller and 'under the hood'.

Make sense?

Kofi
  Reply With Quote
Old 19th August 2019, 07:07 PM   #39
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Yes. Since this looks like it will be a BIG stereo amp chassis, and you'll have individual plate supply windings from that Antek transformer, you might as well go dual mono with the power supply design. It will be more expensive that way, but oh well.

What chokes were you looking at? There are some decent ones available for not much money.

The Hammond 157R looks usable, 2H with 57 ohms DCR at less than $25 each.
If you need cheeeeeap, there's the Triad Magnetics C-24X, 1.5H 50 ohms for less than $10 each.

I don't think you'll need large inductance values since you'll be using a cap input power supply (as opposed to a choke input supply), and 400V rated capacitors will be fine for a 325V B+. There are good 330uF 400V caps available. Perhaps use one of those for the cap after the choke, with a 100uF 400V low ESR/high ripple current rated part for the reservoir capacitor (before the choke).

Or did you want to go more deluxe than that?
--

On further thought...

To arrive at the voltage to expect for the B+, you'd have to factor in DC resistances in series with the power supply. So...

Let's say the filter choke's coil resistance is 50 ohms, and the primary winding resistance of the OPT is 100 ohms. Given that, then...

with 170mA current drawn from each channel, 150R * 0.17A = 25.5V dropped across the winding resistances.

Looking at that Antek transformer's spec sheet, I see that under load, if 400mA is drawn then the secondary voltage will be 241VAC.

Click the image to open in full size.

241 * 1.35 (conservative estimate of rectifier efficiency) = approx 325VDC

That means the B+ to the EL84s might be as low as +300V in the end.

The next step up the Antek 100VA line is the AS-1T275 with nominal 275VAC secondaries. That one shows 264VAC delivered under 350mA load. That would give you a higher raw B+ voltage, something like 355VDC. The 50 ohm DCR of the filter choke would drop about 9V (50R * 0.17A = 8.5V) for approximately +335VDC, then another 17 or so volts dropped across the OPT primary winding resistance, so perhaps +330V delivered to the EL84 plates.

I figure it's easier to drop a few volts than it is to add some. So I'd look at the 275VAC 100VA transformer, Antek AS-1T275, instead of the 250V one. If the B+ voltage comes out too high, you can always put a small value/high wattage wirewound resistor in series after the rectifiers, just before the reservoir cap (power supply input cap). That is sometimes necessary to damp subsonic resonances anyway. 22 ohms would drop almost 4V, for instance, or 47 ohms would drop 8V.
  Reply With Quote
Old 19th August 2019, 08:33 PM   #40
Kofi Annan is offline Kofi Annan  United States
diyAudio Member
 
Kofi Annan's Avatar
 
Join Date: Jan 2004
Location: US
I was just modeling this in PSUD last night and came to a similar conclusion regarding the voltage drop under load. I modeled it at 290VAC and found it came in around ~340VDC. I'll give the 275VAC job a try...

Here's my quick sketchup of the EL84PPP with EI transformers. I'll work up a couple with the toroid power transformer as well.

I believe this is the toroidal output transformer that 45 mentioned. It has cathode feedback taps at 10% and can handle 300mA. Just wondering about DC balance, though-- I think there's a risk of core saturation unless we can ensure balance. Any experience with this?

Thanks so much for all the responses. I could not do this without you guys.

Kofi
Attached Images
File Type: jpg Screen Shot 2019-08-18 at 4.36.26 PM.jpg (264.5 KB, 59 views)

Last edited by Kofi Annan; 19th August 2019 at 08:36 PM.
  Reply With Quote

Reply


Kofi Annan in: "Tube Amp for Multi-Way Speakers"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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Kofi Annan in: "Kofi Makes a 300b DRD... and You Get to Help!" Kofi Annan Tubes / Valves 138 25th February 2019 06:46 PM
Kofi Annan in: "You Can't Spell 'Annan' Without Vancouver!" Kofi Annan The Lounge 8 6th April 2013 10:48 PM
Kofi Annan in: "Speakers for Baby Huey" Kofi Annan Full Range 6 14th November 2011 06:02 PM
Kofi Annan in: "Kofi's Baby Huey" Kofi Annan Tubes / Valves 181 4th June 2011 05:58 PM
Kofi Annan in: "Kofi's Living Room" Kofi Annan Full Range 5 25th May 2008 06:19 PM


New To Site? Need Help?

All times are GMT. The time now is 12:00 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki