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Old 25th August 2006, 04:30 AM   #201
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By source follower "grid" I meant "gate" of course.
Cheers,
Ian
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Old 27th August 2006, 02:08 PM   #202
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Quote:
Originally posted by gingertube
Mark,
Both the cascode diff amp front end and the direct coupling of the source follower output to the output tube grids with bias applied at the source follower grid are good ideas. In fact, if we were to get really fancy, then that bias would be applied using a bias servo. Witha "clean sheet" design all things are possible and its a "golden ruleZ" of design engineering that MKII is always better than MKI (COZ hindsight is better than foresight).

For bias servos I haven't relly seen anything simpler/better than the Norman Koren "TENA" scheme.

Cheers,
Ian
Ian, good to hear your opinion.
I'll go to Norman's site and look at the bias servo in "tena" scheme.

I've just found the Hammond 1608s, I only need some components and PS trafo before starting to build the amp.
I was thinking to draw a pcb, but in this moment it could be a limit if I'd like to try the follower/bias servo driver without rebuilding from scratch all the ampÖ

EL34: after your experience with the music angel, where you tried the el34 in triode and pentode mode, how much feedback would you use connecting the 34s in UL? Do you believe that it could work only with the shunt feedback? Maybe changing the diff amp with a cascodeÖ

Mark
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Old 28th August 2006, 10:59 PM   #203
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Mark,
With EL34 in Ultralinear you will not need any global feedback. Just the shunt feedback will be sufficient.
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Ian
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Old 2nd November 2006, 02:51 PM   #204
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I'd like to give this amp a whirl.
My last PP amp was a Dynaco ST-70 (stock, restored, and now long gone). Sound wise, do any of you know how this circuit compares to the ST-70?
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Old 8th November 2006, 04:31 AM   #205
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Adam (LowRedMoon),
Sorry its taken me a while to get around to this. In actual fact I haven't encountered any ST70s. Somehow they did'nt seem to get imported into Australia. - maybe because they were 110V AC 60Hz only, whereas Australia is 240V AC 50Hz power. What tube stuff did come into Australia was more typically British, and later on, Japanese.

Regardless of that I am quite familiar with the ST70 schematic and the usual mods that people tend to do to them.

My comments are therefore based upon my experience with other EL34 pentode mode push pull amps - not the ST70 specifically.

The EL84 version of the "BabyHuey" is lower power and if you are running very inefficient speakers (say less then 90dB/W/m) you may notice that. Sound wise it is likely to be "cleaner" (less distortion) than I would expect from an ST70 and I would expect better mids and highs and far better stereo imaging (because there is no global feedback and the local shunt feedback helps to address both the leakage inductance and leakage capacitance issues of the output transformer which normally affect the high frequency and phase response of an amp).

The 6V6 version I built (which is a glorous ROCK amp) uses Hammond 1608 and I found that they were in fact quite good. The EL84 version (which is the technically and sonically superior version) I built used some 1970's potted output transformers originally designed for a Mullard 5-10 and which are twice the physical size of the Hammonds. I would get a price for some 8K Raa Push Pull Output trannies (WITH 40% UL Taps) from some of your local "Yank" Transformer manufacturers (like Magnequest for example) and if you find that they are too expensive then stick with the Hammond. Remember that while the shunt feedback scheme imparts a whole raft of benefits, the original rationale for the shunt feedback design was to screw the absolute best out of low to moderate cost output transformers.

Explanation:
The shunt feedback lowers the output impedance (the rp) of the output tubes.
That means that the low end corner frequency which is due to rp and the primary inductance of the output transformer is moved lower
AND
the two high frequency corners due to:
1) rp and the leakage inductance of the output transformer
2) rp and the effective shunt capacitance of the output transformer
are both moved higher

Note that it is IMPORTANT to have the Ultralinear connection to the EL84 screens. The Ultralinear connection also assists in lowering tube rp and together with the shunt feedback lowers it enough that the output impeadnce of the whole amp is low enough that you don't need global feedback to get a reasonable damping factor.

In addition to that, it is instructive to look at the Total Harmonic Distortion (THD) and Intermodulation (IM) Distortion figures for the EL84 in Triode Mode, Ultralinear Mode and Pentode Mode. Unlike some other output tubes the EL84 performs best in Ultralinear Mode (For most tubes Triode Mode will be best).

Hope that answers your questions - the ones here and the ones in the PM you sent me.

Cheers,
Ian
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Old 9th November 2006, 04:34 AM   #206
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Thank you for the reply, Ian. And while I'm at it, thank you for sharing your work with us.

I think this will be a great circuit to cut my teeth on. I've long lusted after building a pair of SET amps, but the low cost associated with this amp is much more appealing to me, especially if it yields musically satisfying results. I do have plenty of moderately efficient speakers to choose from, so output power is not an issue.

I've been having some trouble sourcing apropriate PT's. I've been looking for a model with 240-260V secondaries and 5V & 6.3V filament windings, but the highest current rating I've been able to find in that range is 70mA. The closest I've found is the Hammond 270EX with 275-0-275 @ 125mA but that would require some tinkering with rectification and resistors to get the HT below 320V, n'est-ce pas?

I really want to understand what's going on in this circuit before I build it. I've been reading Morgan Jones' books when I have the time. I intend to build a thoughtfully laid out, cosmetically appealing pair of mono amps.

As preamps go, I think the Bottlehead (Extended) Foreplay would be a good bet for around $400, but I don't like the fact that I'm not forced to learn things in order to build it - the amp is already planned and laid out. That idea really turns me off. I've always been one for steep learning curves and the associated satisfaction that follows earned success. Yeah. I have problems.

How did you make out with the Hammond 1650E?

Thanks again, Ian. Is it correct to assume that you are a fellow redhead?

Ha det bra!
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Old 9th November 2006, 06:57 PM   #207
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Ian,

I built your Baby Huey as my first tube project a few months back. Iíve been using it in my regular set up. I am so happy with the result that Iím now totally hooked. In the last couple of months, Iíve tried some modifications that Iíd like to hear your thoughts on.

I found the bypass caps to be a difficult compromise. Too small, I loose bass. Too large, I loose ďairĒ. I imagine better caps will help. But avoiding them altogether seemed even more attractive. I now have the cathodes tied directly to ground and the grids fed with a fixed bias using essentially what you have in your modified Music Angel Ė I omitted the transistors, which I donít have any handy.

http://www.diyaudio.com/forums/showt...572#post907572

I used a 3 pin regulator to supply a 12V into the trim pots. I have ~310V of B+ and ~38mA of bias. I started with 470K grid leak resistors (because I had them handy). I later switched back to 270K. But the 470K seemed to be better. I now have 470K.

I think this change made a fairly significant difference, as least compared to the Xicor low ESR caps that I was using, though I havenít done an AB comparison Ė it sounded so good that I didnít feel to need to.

Iíve also connected the EL84ís in triode mode and removed the global feedback. The difference is not as big. Sometimes I liked it one way, sometimes another. Mostly personal preference. They all sounded great.


Another thing I learned was that the phase splitter balance is very critical. Even a very small drift is very noticeable. I am trying the concertina design in SYís Red Light District. It seems to be better. But this one is close enough that it really needs to be ABíed.

I might have destroyed your design intent with these changes. I hope you are not offended. As a newbie, I want to learn. Iíd appreciate your comments.


Clement
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Old 9th November 2006, 09:15 PM   #208
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Guys - I'm not at all protective of the design - hack away to your hearts content.

ctaudio - I think the change to fixed bias is probably a good idea, it should have better overload recovery than the individual bypassed current sources. If I were to go to fixed bias then I would probably try to use a bias servo to keep the "no adjustment required" philosophy of the bias set by the current sources. The bias voltages required are in a convenient range where an opamp output could be used directly.

I'm still using the current sources but found best sound with 1000uF / 25 V Blackgate NX as the bypass caps.

LowRedMoon - There was some discussion of power transformers earlier in the thread. You really need 180 to 200mA capability for 2 channels.

The Hammond 1650E did'nt make it into the EL84 Baby Huey. Its in a part built 6V6 version of the Baby Huey. This is one of my ongoing projects. The 6V6 version, using some old "coke bottle" shaped 6V6G tubes and 6SL7 as the input, is a great ROCK sound. It grunges up nicely as you push the volume BUT it doesn't do classical etc. too well.

As for the red hair - what hair is left is mostly silver grey But yeah, it was bright red in my miss spent youth.

cheers,
Ian
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Old 9th November 2006, 10:25 PM   #209
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What type of diodes are recommended after the 5V supply to the CCS's? They're not marked on the schematic.
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Old 9th November 2006, 10:29 PM   #210
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Anything Ultrafast and Soft Recovery. Use the same as the HV diodes if you are buying those or something bog standard like UF4007.
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Ian
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