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Old 11th August 2009, 07:28 AM   #821
dupyv is offline dupyv  Luxembourg
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Thanks Ian for your response.

I'm beginning to understand why the zobels were fitted in the classic UL-tube amp designs. So far I never needed them in the amps i built.
Here in the Baby Huey UL-feedback case they are likely to be required. Ultralinear in general is a bit more critical than I always thought.

I already increased the screen grid-stoppers to 1k with no noticeable effect. I'll try the zobels on the anode-screen to see if it cleans up the response.

About your Maida-reg experiments: Sounds interesting. Recently I use the classic TL783 HV regulators, they withstand a delta of 125V over the chip. Easy to put into operation for quick checks, reliable and stable.
The more you add, the more you loose of the simplicity of the original BabyHuey (a bit my concern when tweaking it).

YvesM: I know you're negatively biased concerning UL. I'll take your point and keep it in the back of my head.

Kind regards,

Yves
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Old 11th August 2009, 07:43 AM   #822
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Location: Adelaide South Oz
Yves,
"No worries mate" as we say in Oz.

For teh anode screen tap zobels - these should go direct across the transformer wires.
anode to screen is 0.3 of total pimary or 0.09 of impedance - so with 8K total primary that gives 720 Ohms impedance of one anode to screen winding.

Zobel traditionally use a resistor value of 1.4 to 2 times the impedance they are in parallel with so 1K to 1K5 for the resistor. 2W Metal Film resistors. They can dump a lot of HF power.

With 1K then 1nF will have roll off above 159kHz, probably too high.
That suggests to me that 2n2 would be better, then roll off is above 72kHz.

I know that the little Hammond 1608 had resonances at about 68kHz

Cheers,
Ian
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Old 11th August 2009, 04:18 PM   #823
netbug is offline netbug  United States
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Quote:
Originally posted by gingertube
...
Additionally g2 stoppers in the screen grid connection were often required and counter intuitively it seemed that the better the output transformer the more the g2 stoppers (higher value resistor) were required.
...
[/B]
total noob question..

Are the g2 stoppers the 33R 2W resisters off the UL taps on the output tranny?
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Old 12th August 2009, 10:44 PM   #824
dupyv is offline dupyv  Luxembourg
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Default Success

Ian : Feedback about zobels

The 2n2-1k zobels across anode and screen tap took care
of the oscillations completely. See attached trace measured under
the same circumstances.
Even the one channel which was only marginally stable is cured completely.

The signal now drops in a linear way at 20kHz way down to
60kHz. Without the zobels there were several rises/drops in the 30kHz..60kHz band, although according to the spec. sheet the resonance of the James 6228 is at around 100kHz.

Time for a listening session...

PS: I've found one nice substitute for the EL84: The E80L (with lower power 9W@250V). A bit less gm, but quite comparable. You've to rewire the g2 and connect the g3, run it at 250V-24mA. I didn't do a full listening test but noticed the balanced sound of this tube. The build quality is exceptional as it is an industrial variant with gold pins.
Another tube that could do is the EL821, a video pentode with a higher gm and about 9W. The STC (Australia!) spec sheet says
that higher distortion is to be expected in audio service. I'll check
this one too if time permits.

Bye

Yves
Attached Images
File Type: jpg bh-fixed-ul-zobel-2n2-1k.jpg (28.0 KB, 744 views)
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Old 12th August 2009, 10:52 PM   #825
dupyv is offline dupyv  Luxembourg
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Netbug,
>Are the g2 stoppers the 33R 2W resisters off the UL taps on the output tranny?

Yes,that's right.
In the later schematic of the fixed bias Baby Huey they are 150R.
100..150R is a common value at this position if you look at the
classic schematics.

Yves
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Old 12th August 2009, 10:55 PM   #826
netbug is offline netbug  United States
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Quote:
Originally posted by dupyv
Netbug,
>Are the g2 stoppers the 33R 2W resisters off the UL taps on the output tranny?

Yes,that's right.
In the later schematic of the fixed bias Baby Huey they are 150R.
100..150R is a common value at this position if you look at the
classic schematics.

Yves
Any opinions on if it is worth changing out the 33R for 150R?
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Old 21st August 2009, 01:20 AM   #827
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Default UL Zobels

Yves (dupyv),
I implemented the anode to screen zobels on my prototype BH and thought they were very worthwhile. I was a bit concerned about getting these zobels "right" as a too agressive zobel will kill the top end.

I think I may have made a "blue" (OZ vernacular for "screw up") BUT I did it this way.
I set feedback to zero by shorting across the shunt feedback set resistor. I then adjusted the Cap value (R value left at 1k) until there was just the slightest overshoot but no more than about a half cycle of damped ring with a 10kHz square wave into a pure resistive dummy load.
I came up with a value of 820pF for the series cap. I'm wondering if the 2n2 I originally suggested and you implemented might be too much. I'm also wondering if I should have set the zobel with the normal level of feedback in place.
I have tried to do this (setting of anode to screen zobels) "scientifically" in the past using this article from VoltSecond as guidance.
http://www.siteswithstyle.com/VoltSe..._in_xfmrs.html
I had very limited success. At that time I was applying this to a Hammond 1650T Output Tranny driven by a quad (PPP) of KT88 in Ultralinear Mode. The Push Pull sides seem to want different zobels
BUT this was probably just an artifact of their interaction. This is such a fundamental thing that I may do some more work on it. Getting the series R in the screen and the R and C values between anode and screen taps should be amenable to proper test/analysis (if ONLY I had a better understanding of what I'm doing).

Netbug,
Yes change the series screen resistors to a higher (than 33 Ohms) value. 150 Ohms would be minimum I would use with B+ of 340V. I'd be inclined to go to 270 Ohms (use 2W Metal Films).

Cheers,
Ian

Last edited by gingertube; 21st August 2009 at 01:59 AM.
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Old 22nd August 2009, 12:22 AM   #828
dupyv is offline dupyv  Luxembourg
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Ian (Gingertube),

On the UL-feedback variant, I lowered the C in the zobel to 1n5 to get back a minimal overshoot on the square-wave. Right now it's not over- or underdamped, it's on spot.

I went back to anode-feedback on my current build of the baby huey.
With UL-feedback, I was unable to get the same focus and soundstage.
I went though several values of the feedback-resistor but each time voices
remained dull and flat. I tested this on two Fostex fullrange speakers/horns, with identical results. I suspect that 2way/3way speakers might respond
in a different way to UL-feedback, hence the better results you obtained
with UL-feedback.

A a test-record i used the album "Cewbeagappic" from Beady Belle, excellent jazz with lots of spatial information and tight focus. Play track 6-Shadow and in 5 seconds you know if the amp sounds right or not.

I did some tube rolling in the baby huey (anode-feedback version ):
- EL821/6CH6 (12watts): nothing exceptional, better than some russian EL84's, quite laid back.
- E80L /6227 (9watts): 250V - 24ma idle, feedback resistor set to 14.1k, nice voices & bass, better than most standard EL84's. Has a lower power
rating than the EL84. I wouldn't push it to 300V, you'll have to live with around 10W power output
Comes on place 2 of my favorite tubes for the baby huey, the 1st being a Tungsram EL84.

Cheers,

Yves
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Old 24th August 2009, 10:42 AM   #829
otanica is offline otanica  Australia
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Hi
I want this amp to be my first big project. Can anyone make a list of parts and manufactures etc.

Thanks
Brad
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Old 24th August 2009, 11:36 AM   #830
otanica is offline otanica  Australia
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Hi
I would like to try making the baby huey as in post
http://www.diyaudio.com/forums/showp...&postcount=604
Does anyone have a list of recommended parts. I dont want to spend millions but dont want to skimp either.

Thanks
Brad
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