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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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Old 8th August 2019, 08:36 PM   #11
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by aardvarkash10 View Post
this is a thread screaming for Pete Millett's Engineer Amp. Link to Pete's page, or search for the many threads here.

Fast build time, scalable up to around 200W if George and other extremeists are to be believed, proven board and circuit, and Pete's usual documentation and happy free consulting if your questions are halfway intellegent or better.
Thanks for the idea, aardvarkash10. Unfortunately, I'm concerned that the fixed bias will be an issue and that the aesthetics of having a visible / hot / touch-able / ouch-able heat sink won't work for my friend. He will likely never calibrate the bias when it drifts out of whack and the optics would be a no-no. Note that this will be going into a restaurant he owns and I'm trying to limit his legal liability.

Sorry!

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Originally Posted by Parafeed813 View Post
It looks like I'm not the only one with a 12yo at home, lol
Wow! There's two of us? Let's get another and start a support group!

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@Eli
How about another high gm pentode like the EF184 as replacement for the 12BY7?
I don't need the gain as my version is too sensitive as it is with trioded EF86. I'm not keen on heavy gNFB but might give it a try.
Quote:
Originally Posted by Eli Duttman View Post
The EF184/6EJ7 looks interesting as a voltage amp. The 6922 cascode has (IMO) the big advantage of using an in production type.
Readily-available tubes are a indeed requirement for this project, I'm afraid.

Eli-- I am working up a version of the schematic for the Citation amp you recommended using the 6922 front end. I'll post my version shortly to see if what I drew up is what you were talking about (spoiler alert: it probably won't be as I tend to misunderstand things-- just Google my posts and you'll see what I mean...).

Also, how about the PP-1C from Allen Wright? Would that be a decent candidate?

Eli,

Whipped up this version of the Citation V using the 6922 approach you drew up. Is this what you were thinking?

Not sure if this is exactly correct so feel free to critique.

Kofi
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File Type: png Citation V with 6922.PNG (170.2 KB, 472 views)
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Old 9th August 2019, 03:24 AM   #12
Eli Duttman is offline Eli Duttman  United States
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What is shown as R7 & R8 (2X 500 Kohm parts) is, in fact, a 1 MOhm trim potentiometer. The "set & forget" pot. is adjusted to yield 3 V. of 6922 self bias.

PSRR in cascodes is very poor. At a minimum, well filtered B+ is needed and regulation is (IMO) indicated.

The GNFB loop was omitted.

Quote:
I'm concerned that the fixed bias will be an issue and that the aesthetics of having a visible / hot / touch-able / ouch-able heat sink won't work for my friend. He will likely never calibrate the bias when it drifts out of whack and the optics would be a no-no. Note that this will be going into a restaurant he owns and I'm trying to limit his legal liability.
The H/K Cit. 5 is very much a "fixed" bias amp. So, let's use its "idiot resistant" Mullard style small signal circuitry, but "clone" a self biased O/P tube setup. The Heath W5M provides that template.

Edcor's CXPP100-6.6K O/P trafo will provide the magnetic headroom needed to ensure that the GNFB induced deep bass error correction signal does not saturate the core.

When legal liability rears its FUGLY head, a substantial tube cage is a must. Keep those mischief making little fingers out of places they definitely don't belong.
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File Type: gif Heath W5M Schematic.gif (84.9 KB, 462 views)
File Type: jpg Heath W5M Voltage Chart.JPG (56.3 KB, 457 views)
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Old 9th August 2019, 05:27 AM   #13
rhone is offline rhone  Australia
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Quote:
Originally Posted by Kofi Annan View Post
Also, how about the PP-1C from Allen Wright? Would that be a decent candidate?
I have built the PP-1C and then turned it into the PP-2C, its big brother. Clean and clear amps, Allens sonic signature, but not super powerful as they are biased into Class A. Also contain silicon bits and pieces which some are allergic to.

Whatever you build, please document it in a big thread including all the dumb things you did - your previous threads were priceless and very informative. Unless, heaven forbid, you don't do dumb things anymore?
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Old 9th August 2019, 02:15 PM   #14
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by Eli Duttman View Post
What is shown as R7 & R8 (2X 500 Kohm parts) is, in fact, a 1 MOhm trim potentiometer. The "set & forget" pot. is adjusted to yield 3 V. of 6922 self bias.
Right-- I represented this as two 500K resistors as I wasn't sure how to implement a trim pot in LTSpice. Too dumb / lazy to figure that out.

Quote:
PSRR in cascodes is very poor. At a minimum, well filtered B+ is needed and regulation is (IMO) indicated.
Here comes the ignorance:

If I wanted to regulate the cascode supply with some transistors, would that do the trick? Maybe with the 10M45S? Or if I just added a decent-sized choke to the supply would that do?


Quote:
The GNFB loop was omitted.
I am definitely going to need a parent to help with the GNFB implementation...


Quote:
The H/K Cit. 5 is very much a "fixed" bias amp. So, let's use its "idiot resistant" Mullard style small signal circuitry, but "clone" a self biased O/P tube setup. The Heath W5M provides that template.
Hmmm... so, I updated the schematic in LTSpice. Does the attached look correct? It appears this would run the 6L6GCs at around 98mA. I assumed a 330R cathode resistor would be OK here, but maybe lower would be better?

Quote:
Edcor's CXPP100-6.6K O/P trafo will provide the magnetic headroom needed to ensure that the GNFB induced deep bass error correction signal does not saturate the core.
Eight weeks of lead time. Sheesh. Better get the order in now.

Quote:
When legal liability rears its FUGLY head, a substantial tube cage is a must. Keep those mischief making little fingers out of places they definitely don't belong.
Speaking of which, anyone know how I can ensure that my friend / his restaurant bear the legal liability of someone gets hurt? Maybe I need a lawyer to draft something?

Quote:
Originally Posted by rhone View Post
I have built the PP-1C and then turned it into the PP-2C, its big brother. Clean and clear amps, Allens sonic signature, but not super powerful as they are biased into Class A. Also contain silicon bits and pieces which some are allergic to.
I really would like to build this one, but I'd be concerned that 8W wouldn't do it for a multi-way speaker and a bunch of people in a restaurant. Maybe I could adapt it?

Quote:
Whatever you build, please document it in a big thread including all the dumb things you did - your previous threads were priceless and very informative.
Will do, as always!

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Unless, heaven forbid, you don't do dumb things anymore?
Ha ha ha ha ha ha ha haaa haww HAWW HAAWWAAAWHAAAAGGHHH *COUGH* *COUGH* *COUGH* uhhh.. uhh. uhhhhhh......

No. Never happens anymore. Never. Sometimes. Mostly.

Kofi

6L6GC Amp Self Bias.PNG
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Old 9th August 2019, 09:34 PM   #15
kevinkr is offline kevinkr  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Make sure the thing is in a properly grounded, fully enclosed metal box, properly fused, that there are no accessible points with HV on them, and the tubes are not accessible without removing a cover first. Make sure that the end user understands that it must be properly ventilated and that regular maintenance is required. (For which you must be paid.. )

Note I am not a lawyer, but on this front news might not be so good - without an LLC I doubt you can be legally excused from personal liability in the event that misuse or a malfunction causes property damage or personal injury. The end user may indemnify you against such loss but that will not be binding on the insurance company insuring his restaurant. The risk is pretty small but I do recommend you talk to someone qualified to advise you in this regard since you raised the issue.

In some jurisdictions there may be further complications around requirements for UL/CSA/cUL certification for anything used in a public venue. (LA comes immediately to mind.)
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Old 9th August 2019, 10:58 PM   #16
Kofi Annan is offline Kofi Annan  United States
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Thanks, Kevin.

I am definitely concerned about this issue. I am confident that I can make the amplifier safe for proper use and handling, but I can't guarantee someone won't drop it while it's plugged in, spill a mai tai in it and slurp it off a tube with their tongue, attempt to hold up their end of a double dare by sticking their fingers in a socket, etc.

I already have an LLC (for a management consulting business), but I am thinking product liability insurance would be a good thing to have. I've reached out to a few lawyer friends but no one seems too keen on responding as of yet.

Also, tube cages-- I don't think Atlanta has a Tube Cage Dept location here. Where does one find these?

Kofi
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Old 10th August 2019, 01:20 AM   #17
kevinkr is offline kevinkr  United States
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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Hammond makes nice chassis and matching cages. I recommend aluminum as the steel chassis are hard to work with. (Kills punches, etc.) You can get them from Antique Electronic Supply or through their distributor business.

Long ago I thought about making a little FM stereo solid state/tube hybrid receiver for commercial use, but the business tanked long before I ever got there.

Yes, LLC should help. You probably need a lawyer familiar with product liability law I would suspect. Maybe you can make him an amp in return for legal advice!
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Old 10th August 2019, 07:57 PM   #18
45 is offline 45  Italy
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I would go for a parallel push-pull with EL84's. Pentode with 10% cathode feedback for 30W power easy. Possibly more.

A reasonably priced transformer for this would be the Toroidy 4K:
TTG-CFB4000PP - Tube output CFB transformer [4kOhm] Cathode Feedback Push-pull - Shop Toroidy.pl

The front-end only needs to be capable of swinging more volts (i.e. 10% of the output voltage). But it's not a big deal really as the input impedance of the output stage with CFB is more benign.

N.B.
When reading the specs, where it says 10% Ra for cathode fbk windings in reality is 10% of total primary turns. So it wrongly specified but the transformer works quite well.

Last edited by 45; 10th August 2019 at 07:59 PM.
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Old 10th August 2019, 10:22 PM   #19
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by kevinkr View Post
Hammond makes nice chassis and matching cages. I recommend aluminum as the steel chassis are hard to work with. (Kills punches, etc.) You can get them from Antique Electronic Supply or through their distributor business.

Long ago I thought about making a little FM stereo solid state/tube hybrid receiver for commercial use, but the business tanked long before I ever got there.

Yes, LLC should help. You probably need a lawyer familiar with product liability law I would suspect. Maybe you can make him an amp in return for legal advice!
Thanks again, Kevin. I just found a lawyer that will help me with this issue. I'll keep you posted.

Quote:
Originally Posted by 45 View Post
I would go for a parallel push-pull with EL84's. Pentode with 10% cathode feedback for 30W power easy. Possibly more.

A reasonably priced transformer for this would be the Toroidy 4K:
TTG-CFB4000PP - Tube output CFB transformer [4kOhm] Cathode Feedback Push-pull - Shop Toroidy.pl

The front-end only needs to be capable of swinging more volts (i.e. 10% of the output voltage). But it's not a big deal really as the input impedance of the output stage with CFB is more benign.

N.B.
When reading the specs, where it says 10% Ra for cathode fbk windings in reality is 10% of total primary turns. So it wrongly specified but the transformer works quite well.
Thanks for the reply, 45!

I will look into that once I have the EL34 fully modeled. I believe that there are some complications related to the balance of the EL84s in the parallel pair, correct? I would love to make an amp that had all of the benefits of an EL84 push pull only MORE POWER!

Any designs you would recommend?

Also-- @Eli: I have been noodling around in LT Spice with the Citation V design and input scheme you posted and wanted to see about using the the EL34 (I happen to have a quad of those already).

Would you (and everyone) mind taking a look at the design I attached and tell me how horribly awful I am at this? Also, would I need to have a regulated supply if I CLCRC the snot out it (see PSU below the audio circuit)?

Kofi
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File Type: png EL34 Amp Self Bias with PSU.PNG (201.9 KB, 292 views)
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Old 11th August 2019, 04:13 AM   #20
Eli Duttman is offline Eli Duttman  United States
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Because of current hogging, forget PPP EL84/6BQ5s in a self biased amp.

Different O/P "iron" for EL34s than 6L6 family tubes. Edcor's CXPP60-4K should be OK, but make certain to roll infrasonic noise off at the amp's I/P. A single RC high pass pole whose F3 is in the 16 to 18 Hz. range gets the job done.

There's little to be gained by reinventing the wheel. Use Mullard's original 5-20 as the starting point, for a self biased PP EL34 O/P stage.

BTW, Mullard was trying to sell tubes that they manufactured, when the 5-20 design was published. The topology is fine, but the high RP/low gm 12AX7/ECC83 is a poor choice for the LTP.
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