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Old 16th April 2006, 08:35 AM   #121
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Hi planet10,

do you have a link for that 4W EL84 (triode class a) ?

regards,
Thijs
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Old 16th April 2006, 05:16 PM   #122
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Quote:
Originally posted by tschrama
do you have a link for that 4W EL84 (triode class a) ?
No exact schematic, but it is a variation on the El Cheapo (that thread is on the Decware forum but there is a souped up El Cheapo thread here) -- basically a well biased 12AT7 Long Tailed Pair driving a PP pair of trioded pentode power tubes. Baby Huey is a variation on this topology as well.

We substututed EL84s for the 12AQ5s, biased them into Class A and used iron and chassis from a Scott LK48. Power supply has all Solen caps (the only elco is the bypass in the output pair -- no room for the Solen we have to go there). The Scott had a negative bias that we used to get a minus supply for the LTP (one of the upgrades we have yet to do is to add a CCS). Another upgrade is to properly triode the output tubes (separting AC from DC conditions -- as in the PEARL triode trick or the variation Eddie Vaughn uses on the Karina. We'll also play with pentode operation (with regged screen supplies) and when we build one up with UL OPTs.

But even in its basic of states it is a really nice sounding amp.

dave
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Old 16th April 2006, 08:53 PM   #123
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Thx planet10, nice info.
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Old 17th April 2006, 04:09 AM   #124
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Quote:
Originally posted by planet10


Don't know about that, but i prefer our 4W EL84 (triode class a) vrs the 15W miniA,
15W is fine. the difference in SPL probably will not be noticeable. I hope to use it with a fullrange in a slim 12-14 liter floor standing cabinet (6"x6"x30") in a small 165-170 sq. ft room. I am just looking for a reliable schematic. My OPTs do NOT have UL capability though. I think I would try triode mode first and settle for 7-8W instead and only consider pentode if the SPLs are not adequate.

Quote:
Originally posted by planet10

possibly... but i expect the SS amp would have higher damping factor (althou neither of them is going to be what would be considered high)
This runs counter to all previous observations too. wonder why?
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Old 17th April 2006, 01:34 PM   #125
anatech is offline anatech  Canada
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Hi Navin,
Interaction between the amp and speaker can bring about interesting results some times. Synergy.

-Chris
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Old 17th April 2006, 05:16 PM   #126
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Default interesting feedback scheme

Hi, guys!

Well, it seems that I should get back to the drawing and designing board, since people are starting to catch up on me

Jokes aside, I truly hope that my RH amps and all the fuss about them that I have (tried to and) caused some time ago on these forum pages have turned some heads and ideas in the right direction.

Keep up the good work and remember good old me

Regards to all,
Alex
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Old 18th April 2006, 07:32 AM   #127
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Default back on deck

Hi all - back on deck after 2 weeks away on holidays.

Just read thru the last 4 pages of posts.

Diomedian, Giaime
- as ErikdeBest stated the current sources in the EL84 cathodes will keep bias currents close enough (balanced) that you can safely run Toroidal Output Trannies. Just use 1% matched resistors for those 16 Ohm current set resistors. I would be interested to hear how you get on.

This aim of this design was originally to maximise results from cheap output trannies of limited primary inductance and large'ish leakage inductance and capacitance BUT results are so good that its worth trying it with good output trannies.

I've started another conversion, for niece Kate this time. (aside: That will be 3 down and 3 to go - niece Kristy has her 6V6 Baby Huey based sound system and niece Laura got a vintage electric guitar and amp instead). For Kate a Chinese "Music Angel" EL34 push pull is being modified to a "Baby Huey"ish circuit. EL34s in pentode mode. It uses the MOSFET source followers DC coupled from 6SL7 diffamp, like the 6V6 version BUT has 450V rail to give the diffamp better headroom. The amp was also modified to add individual bias adjusts for each output tube. Power supply final caps were also increased from 47uF to 220uF. Will advise how it goes.

I really miss the 6V6 version which has been delivered but happy to get a "thank you" bottle of bubbly from the niece who said she was "wrapped" by the sound. I think that means it was "cool".
Technically it wasn't as good as the EL84 version but it had a gloriousness all of its own and in some ways and on some music I actually preferred it.

The other big plus from the holiday is that I drove over to Melbourne (1800 km there and back) and (amoungst other things) got back my 845 SET which had been on loan to a friend for the last 12 months - so I have another "Reference Amplifier" to compare stuff against. I'm listening to it this week while rebuilding the EL34 Music Angel.

Cheers,
Ian
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Old 18th April 2006, 08:32 AM   #128
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Default Re: back on deck

Quote:
Originally posted by gingertube
Hi all - back on deck after 2 weeks away on holidays.

Just read thru the last 4 pages of posts.

Diomedian, Giaime
- as ErikdeBest stated the current sources in the EL84 cathodes will keep bias currents close enough (balanced) that you can safely run Toroidal Output Trannies. Just use 1% matched resistors for those 16 Ohm current set resistors. I would be interested to hear how you get on.

Wow, thank you Ian!

But cheap toroids don't have ultralinear taps so?
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Old 18th April 2006, 03:38 PM   #129
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Default Cheap Output Transformers

Hi Giaime,

When I started posting about torroidial OPTs, the ones I had in mind were from this place:
http://www.engineeringvista.com/trafomatic.htm

They have (had?) an introductory offer on some "custom 10W OTPs". Pick your primary impedance with UL or not, and two output taps - US$32 or something like that. (Sorry, I hate to make this sound like an advert...) The only reservation I had about trying these was aesthetics - maybe I'll have to come up with something unique. :-)

The "standard" cheap OTP for this project that Ian probably had in mind is the Hammond 1608 - which you can find over here at least for around US$38.

Jeremy
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Old 28th April 2006, 01:58 AM   #130
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Just a hint for those building the 6V6 version with the MOSFET source followers.

I have now modified a Chinese "Music Angel" EL34 Amp (Pentode Mode) to this arrangement and have been fine tuning it over the last week (Kate's Amp).

Last night I modified the power supply to the source follower by adding a 2uF Polypropylene cap across the Mallory 40uF/500V electrolytic I used to decouple this supply - what an incredible difference, the tops became uncluttered and cleaner - the dynamic range expanded and the amp picked up rythym.

I WAS having a lot of trouble deciding between various levels of shunt feedback. Now its easy.

Rule#1 - check your power supplies FIRST.

So what about Kristy's Amp which has already been delivered?? By some fluke / good management (or whatever) I actually used a 10uF polypropylene by itself for the source follower supply decoupling in that amp.
Cheers,
Ian
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