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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 11 May 2010, 10:13 
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Hi all.

I have a desire to construct a small stereo class A push pull amp using the ECL85 (6F5P) that is essentially a clone of the oddwatt circuit. The goal is to power a pair of bookshelf speakers with an amp that can be afforded by almost anyone and is super easy to build. Each channel would use the triodes as a Shunt-Regulated Push-Pull (SRPP) amp, and the pentodes would be configured as a Self Inverting Push-Pull (SIPP) stage using a CCS, identical to Bruce's circuit.

The OTs will be off the shelf inexpensive 70 volt line transformers with selectable taps. The power transformers are off the shelf dual 120/240 v primaries and 12.6 v center tapped secondaries to both power the heaters and a second step up power transformer to provide B+. The OTs have no UL tap so the tubes will be run in either pentode or triode mode (the only planned difference between the topology of the oddwatt and this amp).

I've been told this has already been done but can find no information or component values. Does anyone have any experience with using the ECL85 in this way? Did it sound good? Any recommendations for OTs? I'm not looking for 20-100kHz response here, just something for casual moderate volume non-critical listening. I've got $5 Speco transformers good for 10 watts and with an 8K primary although I suspect the bass will be lacking. I'm looking into some others as well.

Any ideas would be appreciated.

Thanks.

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PostPosted: 11 May 2010, 12:51 
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The initial ECL85 (6F5P) schematic "stolen" from Bruce:

Image

Now I just need to find the right passive component values.

Thanks.

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PostPosted: 11 May 2010, 22:13 
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Interesting tube choice. Worth trying with the el-cheapo transformer to see if it is worth pursuing further.

There is some SE info here: viewtopic.php?f=9&t=1697
but I have not seen one in a SIPP.
Cheers

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PostPosted: 11 May 2010, 22:18 
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Hallo everybody,

I did try that on a breadboard, it works and sounds decent. I didn't triode strap the pentodes, but had it run in pentode mode, sounds better to me. Is definitely worth the build, should give you close to 10 watts, maybe. Have fun.

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Greetings from Holland, Ben

My DIY Audio Projects:
- Single-Ended 12B4A tube amp with ECC802S driver
- DIY Push-Pull KT88 tube amp (OddWatt amp from scratch)
- 832 / GU32 tube push-pull amplifier project


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PostPosted: 11 May 2010, 22:45 
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Gio wrote:
Interesting tube choice. Worth trying with the el-cheapo transformer to see if it is worth pursuing further.

There is some SE info here: viewtopic.php?f=9&t=1697
but I have not seen one in a SIPP.

Gio,

Check out these as OTs (pseudo OTs). They're Edcors, and less than $12 each. Specs say 40Hz - 20kHz (-1dB), 8 watts, and I calculate the impedance as either 10K or 5K depending on the tap used for push pull.

http://www.edcorusa.com/Products/ShowPr ... spx?ID=220

What do you think? Less than $24 for stereo output iron with 40-20kHz -1dB with 8 watts per channel. Darn cheap, and it's Edcor.

Yeah I saw Ben's amp. Very nice. It was Ben who inspired me to try this small amp and also gave me the idea for using this tube. There are some good Russian tubes out there if you dig around. I almost went for Ben's design but wanted to try a "Mini-Oddwatt"! So far the parts cost without a chassis and not counting shipping is less than $100 for the entire stereo amp!

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PostPosted: 12 May 2010, 12:15 
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Bennybuis wrote:
I did try that on a breadboard, it works and sounds decent. I didn't triode strap the pentodes, but had it run in pentode mode, sounds better to me. Is definitely worth the build, should give you close to 10 watts, maybe. Have fun.

Hi Ben, did you try this SE or SIPP (OddWatt)?
sampleaccurate wrote:
Check out these as OTs (pseudo OTs). They're Edcors, and less than $12 each. Specs say 40Hz - 20kHz (-1dB), 8 watts, and I calculate the impedance as either 10K or 5K depending on the tap used for push pull.

http://www.edcorusa.com/Products/ShowPr ... spx?ID=220

What do you think? Less than $24 for stereo output iron with 40-20kHz -1dB with 8 watts per channel. Darn cheap, and it's Edcor.

My temptation would be to spend an extra $15 and get a Edcor XPP10 (they have 8k and 10k) rather than take the chance on an audio distribution transformer.
Cheers

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PostPosted: 12 May 2010, 14:22 
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Gio wrote:
My temptation would be to spend an extra $15 and get a Edcor XPP10 (they have 8k and 10k) rather than take the chance on an audio distribution transformer.

Interesting. I think I agree, and it's only about $11 more for a pair. The XPP10 quotes 40-18K, but it's significantly heavier than the WS8-140, although the WS8-140 quaotes specs of 40-20K.

I might try both and sell the ones I don't use. The WS8-140 has secondary taps that could push the primary impedance up to 20K if desired. Tough call. Does anybody know how the construction differs? Same steel, same wire, same type bobbin, same specs all around almost except the wattage and secondary taps.

Thanks for the suggestion.

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PostPosted: 12 May 2010, 15:15 
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Hi All, Certainly a workable plan. The circuit is rather tube friendly. I would opt for the pentode connection and expect about 5-7 clean watts. Maybe as much as 10 if you can live with distortion. The Edcor iron is of course my preference as they are proven perfomers, but other trannies will work. The 10 watt GXPP 10k would be my choice. I use them in similar amps and they sound rather nice. The 6 ohm output works well with nearly any speaker system.

Good listening
Bruce

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PostPosted: 12 May 2010, 15:22 
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gofar99 wrote:
Hi All, Certainly a workable plan. The circuit is rather tube friendly. I would opt for the pentode connection and expect about 5-7 clean watts. Maybe as much as 10 if you can live with distortion. The Edcor iron is of course my preference as they are proven perfomers, but other trannies will work. The 10 watt GXPP 10k would be my choice. I use them in similar amps and they sound rather nice. The 6 ohm output works well with nearly any speaker system.

The GXPP 10K is 2 lbs - twice the weight of the XPP at the same wattage, but with almost identical specs. I have to believe the heavier iron will behave better at low frequencies.

This is a tough call. It's almost like buying a car. How much do I want to spend to "upgrade" to the next level.

One main goal is keeping the cost to a minimum. Going with the upgraded trannys will push the cost over $100 which was my target.

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PostPosted: 13 May 2010, 12:49 
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The WS8-140 are likely rated for use as audio distribution which may be different than how ratings are determined for tube output transformers.

sampleaccurate wrote:
The GXPP 10K is 2 lbs - twice the weight of the XPP at the same wattage, but with almost identical specs. I have to believe the heavier iron will behave better at low frequencies.

One main goal is keeping the cost to a minimum. Going with the upgraded trannys will push the cost over $100 which was my target.

The XPP ($17) is rated at 70 to 18K Hz and the GXPP ($26) 40 to 18K Hz.

Bruce, on your push-pull EL84 tube amp builds, did you get a chance to compare the frequency response of the XPP vs the GXPP?

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