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PostPosted: 17 Apr 2011, 20:10 
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Over on the KT120 Oddblock thread came this little statement:
poty wrote:
Suncalc wrote:
... you can put together a really inexpensive amp that actually has pretty good sound. Two channels, two tubes (plus a rectifier for purists)....
If It would be possible to find inexpensive and small DIY acoustic systems anywhere - I would make the amp for my office desktop computer for sure.

So I decided to take up the challenge. I've had a little design for a 6CY7 amp bouncing around in my head for a while and this afternoon I decided to try it out. Now the 6CY7 is of that peculiar class of dissimilar triode tubes which were manufactured for television sets. This particular one was supposed to be a combined vertical oscillator and vertical deflection amplifier. Well, driving a vertical deflection coil is just like driving an audio output transformer and oscillator triodes are generally pretty linear and have good gain. So this tube gives you both a preamp and output stage in one small 9-pin package. Now I could bore you with load lines and such but instead I'll just jump right to the schematics.

This is a pretty simple circuit with very few extraneous components. The preamp has a gain of almost 25 and the output stage is biased at about 29v so you can drive this amp to full power with a 1.2v peak input voltage. This is good for most portable CD players, iPods, and computer sound cards. Here is the main amp schematic.
Attachment:
schematic.jpg

And the power supply is also very simple.
Attachment:
ps_schematic.jpg

So I decided to prototype up the power supply and a single channel. I noticed a few months ago that Antique Electronic Supply had a no name 5kΩ:8Ω 8W output transformer for $13.95 each so I ordered a couple just to try them out. Here is the prototype with the inexpensive output transformer.
Attachment:
Amp_1.jpg

The amp faithfully produced a nice 1.5W peak power output into an 8Ω load. Now I will say that the output transformer (when driven with a pure sine wave) seemed to kind of fall apart below about 90Hz. However, when driven with normal music (I used Holst's "Planets" suite) it sounded much better than I would have thought given the single tone performance of the transformer. I also tried out the exact same amp with an Edcor GXSE10-8-5K just to compare.
Attachment:
Amp_2.jpg

Now I'm not going to say the amp didn't sound better with the Edcors. In fact, the bass was somewhat tighter and more controlled. But since the Edcors are twice the price of the little AES transformer, this really shouldn't be much of a surprise.

Now the name of this thread is "Inexpensive Dissimilar Triode Amps" and we need to address that whole affordability issue. One of my intents was to see if I could put together a nice little stereo amp for not too much money. Now, I don't know how to define "affordable" with respect to DIY tube amps. Mark's little 300B amp over on the 300B SET Design Project already has over $4000 sunk in parts so I know that that's not it, but I would be happy if I could bring this one in for around $100 (American). So, given that I had already chosen some output transformers from AES, I thought I'd see what it would cost to buy everything new from them for this stereo amp. Here's the damage...
Attachment:
20110417 Price List.png

So at $147.18 I missed my target by more than a little. However, it's still not too bad for a complete stereo amp. The big ticket items in this list are the power transformer, the filter choke (I refuse to build a power supply without one), the output transformers, and the amp chassis. This parts list also does not allow for a volume pot but you could throw one in if you saw fit. It also is a little more expensive than it could be because I stuck with all new components from one supplier. If one had a decent parts bin you could probably eliminate the binding posts, phono jacks, fuse holder, switch, sockets, and most of the piece part components. Diodes are also cheaper then the 6AC4 but you'd need to add a standby switch. If you just purchased the tubes and the iron you would pay just $102.45 (pretty close to my target).

There you go. One relatively inexpensive 1.5W/channel tube amp for use with a computer or iPod. It may not have a lot of volume or headroom, but it's a fun little project to throw together. :up:

Does anyone else have an example of something like this? Maybe something with a nice octal 6EM7?


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Last edited by Suncalc on 17 Apr 2011, 21:26, edited 1 time in total.

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PostPosted: 17 Apr 2011, 21:07 
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I love dual dissimilar triodes for spud amps :thumbsup:

I use a 6FD7 in my workhorse. 6JT8 I use too in reverb drivers, but work as little spuds.

Cheers!

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PostPosted: 18 Apr 2011, 07:51 
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Location: Lilburn, Georgia USA
Hello, I found the 13EM7 design at www.pmillett.com He has some interesting designs for lots of tubes. Even has high quality circuit boards for them too. The 13EM7 amp costs around $200 or so including circuit board and he gives parts lists with numbers for Mouser, Allied, Parts Connexion, etc. Also he gives the plans for where to drill the holes for mounting everything. Good site! Thanks, Robert P


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PostPosted: 18 Apr 2011, 14:36 
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Cool, :thumbsup: I believe there are a few changes that some folk might find useful in some uses. For more gain, bypass the cathode resistor in the first stage. 100uf would probably be sufficient. Second for even higher quality sound, a minimal NFB loop from the output to the cathode of the first stage would probably clean up the sound even more and can be used to tailor the sound to overcome some of the problems of the transformer. I would keep it under 6 db as you will both lose gain and I generally figure the less NFB the better. In the circuit I mention later, the NFB really made the little amp sing. Extremely clean and extended response. My HP distortion analyzer showed a reduction in distortion from nearly 6-10% (frequency dependent) to under 2%.
I expect to post a circuit with similar aims but with a triode-pentode as soon as I can. I called it the ugly little amp. Russian 6GV8s for about 1-2 watts. I used the Edcor GXPPs as well and for anyone considering a small amp like these they are super, just as noted a bit more costly. :$: :$:

Good listening
Bruce

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PostPosted: 19 Apr 2011, 06:00 
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OK! I understand cheep tube project,....but I don't understand use cheep output transformers, that will decrease quality of sound! I very doubt that trani for 13 $ will sound like EDCOR, Better pay 10-15$ more, but sounding will be real !! Use resistors for 20-30 cents, IMO, is not good idea....Resistors sound very different. Better use in signal paths res. /maybe 5-6 res./ for 1 $, like tantalum and You will feel very big difference in Your cheep project Better forget for cap shunts about 25 cents, sound will very far from real, and use caps for 3-4 $ like Nichicon, Rubycon /it will cost total for You about 40-50 $ more/.
Interstage caps 0.01 uF will be, IMO, 0.1-0.2 uF . /Bass will lose with 0.01 uF/
And people that will come in Your office will really like Your music.
/IMO,...if apl. will very cheep, the sound will....cheep.../.

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Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
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PostPosted: 19 Apr 2011, 09:29 
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I just wanted to make a few points.

Bruce is correct. By bypassing the cathode in the first stage and then providing some feedback I could probably cleanup the sound. I usually avoid feedback because of it's effect on higher order harmonics. However I'm usually using quality audio tubes in carefully designed output stages. When "shooting low" with tubes like these (6CY7, 6EM7, etc), using some moderate feedback to cleanup the sound is probably a better approach.

As for the output transformers, this was just a test. After comparing the response to the Edcor GXSE series, I don't believe I would ever use these little AES units in anything but the simplest of amplifiers. However I don't necessarily agree with Azazello about the resistors. The ones specified are all 1% metal film resistors with which I have had excellent results in the past. I don't believe that you need to pay much for good quality resistors. Replacing the two audio caps may make some improvements but when starting with this tube, I'm not sure it's worth the money.

My entire point in this post was not to produce a very high fidelity amp with this tube, it was to see if I could come in around $100 for parts and see if I could still get decent quality sound. I believe I was successful in my attempt. And I believe that I answered Poty's implied challenge from over on the KT120 Oddblock thread. Now maybe if we can redefine "affordable" to about $250 to $300 then I believe that I could produce a really fine quality amplifier for that sum.

As always, comments are more than welcome. :D

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PostPosted: 19 Apr 2011, 13:48 
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Hi, No sales pitch here, but the ugly little stereo amp (project to be posted when I can get to write it up) will shortly become the "Alpha" and in kit form ought go for about $200-250 (still sourcing a few parts). Unfortunately tube amps will never again be cheaper than SS ones. Some parts that they need (high voltage PS, trannies) will always add up to $$$. I agree on using good trannies. I have a box full of junkers. They are fairly good for paper weights and perhaps fishing lure weights. I actually had Edcor design some trannies for the little amps. U/L with super specs. For the diy community, the GXPP series are hard to beat. I consider the transformers the most important part of any tube power amp. Everything else can be made to perform pretty well with proper design. You can do work arounds on the trannies, but if they are junk, :eek: then the end product will never be great. For someone who wants a modest amp (pp) a GXPP10 at 10K and a pair of Russian 6GV8 replacements can make a cute 8-) and very excellent sounding baby oddwatt and the cost is really low. I would think that you could build one for under $200 in parts easily. Maybe even under $150. About 3 watts output with lots of gain.

BTW, there is no reason that the OW style circuit can't be used on triodes. I have considered using a SRPP 12AU7 to drive a 6CG7 (PP OW) as a really viable design with about 1 -1.5 watts of output. Might be a really sweet amp.

Good listening
Bruce

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PostPosted: 20 Apr 2011, 09:33 
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It's not a dissimilar triode but it's even simpler.
http://amps.abraxasaudio.net/projects/6197-spud1/

I've built 4 of them over the years and you can build one for about $150. Also I have a 6V6 amp schematic that can be build for $150 and it sounds great! Both amps give you about 2.5W out.


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PostPosted: 20 Apr 2011, 10:24 
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Cheep ampl. with good sound!
http://www.autocostruire.it/old-site/trials/ella.htm

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Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
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PostPosted: 20 Apr 2011, 15:36 
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Hi.
Suncalc wrote:
There you go. One relatively inexpensive 1.5W/channel tube amp for use with a computer or iPod. Does anyone else have an example of something like this?

If I were to build a cheapie amp to drive an iPod, I'd save more with much more simple & cheaper designs.

Here you go (I already posted here before I built one dirt cheap, anyway):-

A stereo power amp with 1/2 12AX7 driving a 50C5, delivering max 2.5W.
Heaters (2x50C5+1x12AX7) string up to 112.6V to save the cost of transformer as 50C5s are also powered with line voltage direct (after a half-wave rectifier).
The next most 'costly' item will be the 2 O/P irons. We can try a small 2x110V:6V power iron, a few buck a piece.
For 8R loudspeakers, working back using only one half of the primary winding. 110V/6V=18.333
Sq of Z ratio = sq of 18.333=336.11 So the O/P Z reversed will be 336.11x8=2.68KR.
Comparing 50C5 pentode load (2.5KR) is close enough.
If you want to triode-strap 50C5 like I always do with any pentodes to get much better sound, the O/P power to go down about 1.5W or less. Then 2.68KR O/P Z for lthe converted power iron will be OK as triode is much more forgiving in its O/P Z demand.

With the huge saving on the irons, the other parts will be of peanut cost. Way way less than 100 buck budget. For iPod it should be OK.

FYI, The 50C5 (triode-strapped) power amp (no global NFB) I built years back. driven by CD player direct sounded pretty nice & believe it or not, could drive a pair of costly brandname studo monitors of 91dB/W/M sensitivity nicely in a small listening room.

c-J

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