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Old 6th January 2019, 01:57 AM   #21
Lingwendil is offline Lingwendil  United States
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Need 3-5W tube amp
Quote:
Originally Posted by rpi View Post
The project from Lingwendil also looks attractive. I like the small form factor of the
toroid transformers. Is it possible to use toroid output transformers with that Spud amp?


Thanks!
Toroids are fantastic for the power supply, or for push pull, but not for SE unless you use specific versions constructed for the use. For SE traditional types still present the best options.

Unless, you could to do a push pull triode connected version of the EL86, toroids will work well there. It isn't traditional, however, so you may want to do some testing there before you commit. You'll get an easy 5~8 watts there depending on your selection of voltages and operating points.
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Old 6th January 2019, 01:58 AM   #22
rpi is offline rpi  United States
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Sure Doug, as soon as I'll have anything to show I'll post it here.
Here is the sketch for that crazy idea with stepper motor which lifts the board with tubes:

Click the image to open in full size.

I mean the stepper motor with lead screw similar to this:
Amazon.com: Iverntech NEMA 17 Stepper Motor with Integrated 210mm T8 Lead Screw for RepRap Prusa i3 3D Printers Z Axis or CNC Machine: Industrial & Scientific
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Old 6th January 2019, 02:58 AM   #23
Eli Duttman is online now Eli Duttman  United States
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Originally Posted by rpi View Post
Thank you Eli Duttman. I'm all ears right now. Is there any real implementation of the amp
which you described? I'm not afraid to make the one from scratch. I just don't have any
experience in building tube amps though I completed many other audio projects.
The 12AX7 section into "12" W. O/P tube is the sort of thing found in low cost console amps. With the right parts, that sort of setup can be both "boombox" or HIFI, at your whim. Plug the right (high efficiency) good speakers in and your listening room is filled with 1st class sound.

Quite a bit can be "borrowed" from the Magnavox schematic I'm providing. "Maggotbox" used ultra-linear (UL) mode O/P tubes. We will use full pentode mode, for both technical and cosmetic reasons. Our power supply will be SS rectified, for reasons of cost and space. BTW, 6BQ5 = EL84.
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File Type: jpg Magnavox SE 6BQ5 Console Amp.jpg (177.9 KB, 444 views)
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Old 6th January 2019, 05:02 AM   #24
rpi is offline rpi  United States
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I have tried to summarize my understanding of two suggested amplifiers.
The listed pros and cons of those amps are just for my particular project.

pcan - Spud Amplifier with 6E5P Tube
Pros: very simple - only one tube (6E5P) per channel, only 4 resistors and 1 capacitor per channel, only one voltage for output stage (200V). The tube has nice glowing.
Cons: low wattage (about 1W). Large output transformers.
Price:
The tube 6E5P is about $5 on eBay
Output transformers ?
Power supply transformer ?
Total: ?

Eli Duttman - Magnavox type amp
Pros: only 2 tubes per channel. Small number of passive components - 7 resistors, 2 capacirtors per channel. Good wattage ~5W ?
Cons: requires two different voltages for two amplification stages (180V, 200V). That may need more complicated power supply/transformer.
Price:
The first amp stage (12AX7) is about $10 per tube on eBay
The output tube (6BQ5/EL84) is also about $10 on eBay
Output transformer (GXSE15-5K ?) is $43
Total: 10*2 + 10*2 + 43*2 = $106 + ?(power supply can be probably borrowed from Spud Amp if it's possible to use only 200V)

Right now I'm biased toward the Spud amp because of its simplicity.

I just need to clarify the cost of transformers.
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Old 6th January 2019, 06:00 AM   #25
H713 is offline H713  United States
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One thing you might consider with your stepper drive is that I've found some stepper drivers to be extremely noisy devices. Tube amplifiers are a high impedance circuit, so this could potentially cause some noise problems. I wouldn't let this discourage me from implementing it (it would make the whole project significantly cooler), but be aware that it could potentially be problematic.

My vote is for the Magnavox type amp- 1 watt is very small. Those would have to be crazy efficient speakers to get a reasonable SPL out of it.

No need to worry about the two voltages- one extra resistor and capacitor in the power supply and you're good to go.

If you're worried about the cost of the 6BQ5 tubes, the 6AQ5 is a slightly smaller (7-pin base) tube that is available for a few dollars each. They are electrically equivalent to a 6V6, and a SE 6BQ5 design could easily be modified to use 6AQ5s. If you can find some used output transformers for a 6V6 or 6BQ5 from an old console amp, you might be able to save some money over the Edcors. These seem to pop up on eBay all the time (or maybe it just seems that way to me since I'm never looking for them).

You only need one 12AX7, by the way, since they have two triodes in one tube- save 10 bucks.

This Antek Toroid would do for a power transformer. I've had good luck with Antek toroids in the past. They seem to be fairly conservatively rated and well shielded- good for a tube amp.
Also, I second the suggestion for a 0A2 or 0D3- they glow pretty colors.
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Old 6th January 2019, 06:49 AM   #26
Eli Duttman is online now Eli Duttman  United States
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I too looked at the AS-05T240. Unfortunately that item's rectifier winding is good for only 50 or so mA. of B+ and each of the "12" W. O/P tubes needs roughly 40 mA.

$29.73 is the combined cost total for a Triad N-68X and a Triad VPS24-1000 and that's less than the cost of an AnTek model which will supply enough B+ current.

A total of 4 tubes: 1 12AX7, 1 0A2, and 2 "12" W. O/P tubes are needed for a "knockoff" of the Magnavox amp. Remember what I said about Russian 6Π15Π-EV stock. Less than $21 buys 4 tubes, which means you get a working set and a set of spares. Matching is a non-issue in this SE/GNFB setup. Sockets correctly wired for the 6Π15Π accept EL84/6BQ5s, with zero changes. Should a good deal present itself down the road, it's a plug & play situation.
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Old 6th January 2019, 07:47 AM   #27
H713 is offline H713  United States
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[QUOTE=Eli Duttman;5655367]I too looked at the AS-05T240. Unfortunately that item's rectifier winding is good for only 50 or so mA. of B+ and each of the "12" W. O/P tubes needs roughly 40 mA.

$29.73 is the combined cost total for a Triad N-68X and a Triad VPS24-1000 and that's less than the cost of an AnTek model which will supply enough B+ current.


My mistake- I thought that transformer had a pair of high voltage windings like some of their other transformers. Would have been perfect. Anyhow, if a toroid works better on your chassis there is always the AS-1T250, but I have to say I like the price of that Triad N68X. I'd figure with a doubler configuration you'd probably have about 100 mA of available B+ current- more than enough.
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Old 6th January 2019, 08:04 AM   #28
pcan is offline pcan  Italy
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Need 3-5W tube amp
Before committing to the 6E5P design, I strongly suggest you to test if 1W is enough for your speakers. You may use one of those 1$ - 1W class-D chip amp boards from ebay; they distort the sound in a nasty way if overdriven, while the tube amp will not, but you will get the idea. The schematic proposed by Eli Duttman is a classic, proven circuit with more power and better power handling.
The schematic and a picture of the bottom of the 6e5p amplifier are attached to this post. You don't need matched tubes. Eli Duttman idea about the gas discharge regulator tube is excellent. I believe that on this amplifier you may use two 85A2 regulators in series; the recomended current trough this type of regulator is 6mA. The cathode current trough the 6e5p is about 30mA. Anode current and plate impedence are low, this means that the output transformer is not critical and you may use cheap toroidal transformers. The big transformers you see on my build were meant for KT88 tubes, but they turned out to be of subpar quality on the intended application. On this circuit they sound perfect. You may use much smaller transformers, such as the 3500 ohm version of the GXSE15 (I believe that the regular 5K version would also work, altough with lower output). A word of caution if you want to use cheap power toroidal transformers as output transformers: because this circuit is single ended, you need to oversize them a lot to avoid saturation. Try 100VA. Some are good for this application, other aren't, you may want to try out.
I build my smaller prototypes on clear plastic containers, they are easy to drill and placement of components is simplified. The L shaped aluminium bar on top is essential to avoid deformation due to the weight of the transformers.
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File Type: jpg 6e5p schematic.JPG (121.1 KB, 424 views)
File Type: jpg 6e5p bottom.JPG (379.9 KB, 382 views)
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Old 6th January 2019, 08:37 AM   #29
Guerilla is online now Guerilla  Denmark
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Quote:
Originally Posted by Eli Duttman View Post
We will use full pentode mode, for both technical and cosmetic reasons. Our power supply will be SS rectified, for reasons of cost and space. BTW, 6BQ5 = EL84.
Im very sorry to kind of hi-jack. I have often heard builders say they dislike pentode sound. Could you describe its sound and behaviour compared to UL and SE?

Also I have seen someone mention that a pentode poweramp is a transconductance amplifier. Is this true?Your proposal maybe something I would like to build for tweeter duty in an actively speaker. Thanks. And @ Rpi Best of luck with project

Last edited by Guerilla; 6th January 2019 at 08:43 AM.
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Old 6th January 2019, 03:39 PM   #30
Fenris is offline Fenris  United States
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I built a nice little guitar amp using a pair of 6gv8 tubes in push-pull. It makes about 10 watts in pentode with lots of good guitar distortion, but can be changed to a lower powered, low distortion triode mode by changing one component. The output uses an $11 Antek toroid transformer, a $10 switchmode power supply, and a 12v wall wart. Each channel is built on a 4" x 4" PCB and uses about $25 of components. 6F5P tubes can be substituted for 6GV8 to save a few dollars.
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