My DIY Tube Amps

For the serious and not so serious in audio: DoItYourself and Tubes (SET, PP, Vintage).

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My DIY Tube Amps

Postby 6a3fan » Sat Dec 14, 2013 12:17 pm

I have been building tube amps for almost 20 years. Now I have the following amps: 300B SET, 2A3 SET, 2A3 PP, RH84, 6EA7 SET, 6L6 SE UL, EL84 PP, ST70 rebuilt. My preamps: 6EU7 RCA phono with 6CG7 line, and 12AX7 phono with 12B4 line. I've also recently experimented with SE mosfet amps. I built an ZCA amp, and a preamp with Salas NJFET phono and a SE moosefet line amp.

My daily driver is my 2A3 SET. I made all the iron in this amp: the power transformer, the choke and the two output trannies. The circuit is a copy of the JE Labs simple 2A3 with 6SL7 voltage amp direct coupled to a 6SL7 cathode follower. The output stage is straight RCA tube manual: 300 volts B+, cathode biased to 60 volts, with plate current of 60 mA. I used a mix of resistors: Allen Bradley types, carbon film and metal film. The cathode caps are Black Gates, while the coupling capacitor are Russian PIO. The 2A3s are Sovteks, the 6SL7s are Russian. For directly heated tubes, I prefer tube rectification for their slow turn on. The rectifier tube is a vintage Sylvania ST 5V4. Filter capactors are Cerafine and Sprague.

Photos show the amp, the under the hood and the scope traces at 10 kHz which shows minimal ringing. Remember that the amp does not have global feedback.

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My other 2A3 amp is PP following the JE Labs 2A3 PP circuit with notable exceptions. The long tail pair is loaded by a choke I asked Edrel to wind. The bobbin is two chambered, with both coils wound such that the center tap of both coils comes from the outer layers. I think that this arrangement is similar to the Brooks 2A3 amp. Also, noting the caution in the RCA tube manual that PP 2A3s should be balanced, I took a page from the Williamson amp to balance the output tubes. Essentially, the Williamson takes a positive voltage from the cathode circuit and injects this to the grid using a potentiometer to attain balance. Finally, while I copied the voltage doubler arrangement of the JE Labs circuit, I used an indirectly heated tube rectifier as a series element for the soft start feature.

This amp uses Chinese 2A3s, and NOS 6SN7s: RCA for the voltage amp, and Sylvania for the long tail pair. The rectifier is a NOS Mullard GZ37.

Following are photos of the amp, below the hood and the scope traces of both channels at 10kHz. Remember that the amp does not have global feedback.

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For many of my amps, I use aluminum channels to make my chassis. I put the channels side by side and put on wooden sides. The chassis are stiff and strong. However, the wear and tear on the chassis are easily seen in the photos, as these amps have been rebuilt at least a couple of times.
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Re: My DIY Tube Amps

Postby 6a3fan » Sat Dec 14, 2013 12:20 pm

I probably made a mistake posting the photos from photobucket. How is this done again please?
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Re: My DIY Tube Amps

Postby tony » Sat Dec 14, 2013 1:10 pm

i also use photobucket, point to the image and select "get links" then click on the image link, finally, paste to your post....
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Re: My DIY Tube Amps

Postby Gino » Sat Dec 14, 2013 2:39 pm

Type [img ] your photo url here [ /img]

Photobucket has Share links for forums. Some forums display that correctly. Then when you click on your posted photo, it brings you to the photobucket website. What I do is i delete code ahead of [img ] the url her [/ img].

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Re: My DIY Tube Amps

Postby AbadSantos 7 » Sat Dec 14, 2013 3:40 pm

Congrats 6a3fan!!! :clap: :clap: :clap:
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Re: My DIY Tube Amps

Postby tony » Sat Dec 14, 2013 4:17 pm

he has been in touch with me thru pm's, he is an expert pala.... ;)
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Re: My DIY Tube Amps

Postby 6a3fan » Mon Dec 16, 2013 4:58 pm

Master Tony calling me an expert is flattering. But I am just a persistent student.

The following is my most compact amp. I use it in my bedroom driving the impressive 4" Sony TV speaker in a vented box. Source is a regular CD player. The tube is a 6EA7 dissimilar triode. One section is similar to a section of a 6SL7, while the power triode has a plate dissipation of 10 watts. The high gain section is capacitor coupled to the power triode. No global feedback is used. Both sections are cathode biased.

Again the chassis is made from my favorite 4" aluminum C section with wooden side ends. I hand wound all of the transformers: the power tranny, the two filter chokes (one per channel) and the output transformers. Even with its limited power, this is one of my favorite amps with a very sweet sound.

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The square wave scope trace of course shows some rounding of the leading edge but there is no obvious ringing.
Last edited by 6a3fan on Mon Dec 16, 2013 8:00 pm, edited 1 time in total.
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Re: My DIY Tube Amps

Postby 6a3fan » Mon Dec 16, 2013 5:12 pm

I am still not successful posting photos. I tried just pasting on the link and removing the code, but it's not working. Do I need to change settings in my photobucket account?
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Re: My DIY Tube Amps

Postby tony » Mon Dec 16, 2013 5:21 pm

photobucket user too...
there is a feature with photobucket that you can use, just point to the picture you want to post,
click on the small wheel at the top right corner of the picture, a dropdown menu will appear,
select the get links, a new window will appear, point and click to the image codes, and paste,
that is what i do with my pics...
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Re: My DIY Tube Amps

Postby Gino » Mon Dec 16, 2013 6:08 pm

Try [ img]http://s121.photobucket.com/user/kuton_photos/media/6ea7SEamp.jpg[/ img]

I deliberately put spaces on the codes so they will appear. Leave no spaces.
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Re: My DIY Tube Amps

Postby 6a3fan » Mon Dec 16, 2013 7:57 pm

Finally did it. Thank you Tony and Gino for helping.
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Re: My DIY Tube Amps

Postby tony » Tue Dec 17, 2013 10:21 am

my pleasure, any plans for the 6AQ5's ....... ;)
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Re: My DIY Tube Amps

Postby 6a3fan » Tue Dec 17, 2013 10:32 am

Yes, two plans in fact:

1. Stereo SE amp with 6AU6 driver in triode mode for a full 7 pin setup. I have not been successful using Schade feedback so this will likely be UL with feedback from OTP secondary to input tube cathode.
2. A line amp similar to the 6V6 line amp in DIY Audio.
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Re: My DIY Tube Amps

Postby tony » Tue Dec 17, 2013 10:57 am

the 6AQ5 is like a small 6V6, i happen to like.... 8)
silicon ray in hongkong made a line amp using the 6au6 and 6v6 in a
cascaded common cathode with global negative feedback...
he used both pentode tubes in triode mode, and so i used real tiodes,
the 13fm7 instead and so far listening tests are positive....
i will do a line amp using the 12en6 and 6bc4 combo in that topology...
it seems that the inspiration for silicon ray was the Musical Fidelity x10D buffer which had a lot of following...
if you are interested to hear this preamp, contact Buknoy, he is in marikina and should be near you...
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Re: My DIY Tube Amps

Postby buknoy » Tue Dec 17, 2013 11:16 am

tony wrote:the 6AQ5 is like a small 6V6, i happen to like.... 8)
silicon ray in hongkong made a line amp using the 6au6 and 6v6 in a
cascaded common cathode with global negative feedback...
he used both pentode tubes in triode mode, and so i used real tiodes,
the 13fm7 instead and so far listening tests are positive....
i will do a line amp using the 12en6 and 6bc4 combo in that topology...
it seems that the inspiration for silicon ray was the Musical Fidelity x10D buffer which had a lot of following...
if you are interested to hear this preamp, contact Buknoy, he is in marikina and should be near you...


yes sir if by any chance you are free and you want to hear master tony's creation feel free to shoot me a pm and you are
very much welcome, I'm from sto. niƱo marikina near bayan :)
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Re: My DIY Tube Amps

Postby Jaagut » Fri Dec 20, 2013 10:44 am

thank you sir 6a3fan,

i just have learned from your sample amps about the 3 cuts of C aluminum frames for a nice casing. :clap:

looking forward for more shares of your tube amps and tips too. :)

regards.
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Re: My RH84 Amp

Postby 6a3fan » Fri Dec 20, 2013 8:57 pm

The RH84 is different from my other Single Ended amps as it has a pentode output stage, with plate to plate or Schade feedback. It uses EL84 output tubes. The input tube is a 12AT7 of which I have good examples. I have another non-triode SE amp, a copy of the Fi Primer 6L6 UL amp with cathode feedback from the secondary of the output transformer.

This amp has James output transformers, but the power transformer and choke were wound by me, using DEECO cores and formers. The power transformer tends to get a bit hot, most likely because of iron losses as I used good sized wire.

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The scope traces of a 10kHz square wave are a bit more ragged than some of my other amps. Perhaps this is caused by the design, as the James transformers have a good reputation.I am also intrigued by the upper trace that seems to show crossover distortion on the trailing edge, although of course this cannot be because this amp is single ended and is operated Class A. Perhaps the masters can tell me what's going on.

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Re: My DIY Tube Amps

Postby tony » Fri Dec 20, 2013 9:07 pm

The power transformer tends to get a bit hot, most likely because of iron losses as I used good sized wire.


the trick here is to compute for a 260 volt primary, that will keep your power traffos from running very hot...
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Re: My DIY Tube Amps

Postby 6a3fan » Fri Dec 20, 2013 9:24 pm

That's a nice trick to know and I will certainly try that with my next power tranny. I used to compute only at 240 VAC.

I read somewhere that a DC component in the AC power line may also cause transformer heating. Is this at all possible?
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Re: My DIY Tube Amps

Postby tony » Sat Dec 21, 2013 7:35 am

6a3fan wrote:That's a nice trick to know and I will certainly try that with my next power tranny. I used to compute only at 240 VAC.

I read somewhere that a DC component in the AC power line may also cause transformer heating. Is this at all possible?


yes, specially for torroids, since they operate at higher flux density than mos EI's, besides EI's have built in air gaps which acts as "vent valves" and counters too much flux build ups...

isolation transformers can help here....
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