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sickss 3rd March 2005 05:09 PM

833 push pull design help
 
As some of you may have seen, I am looking to build a subwoofer amp to replace my current s/s piece. I am strongly considering a pair of 833's in p-p config. Sounds good on paper...but one BIG problem...TRANSFO'S !

I think I can come up with b+ from some older RF amp parts...I'll have to dig... but output is going to be a problem!

Any work arounds? Any alternatives?

Also...concerning a project like this (I have build a good deal of RF amps...and no what they cost..but am not familiar to much with AF). Is a $5-7k budget realistic for something like this?

Thanks
Dennis

aurt 3rd March 2005 09:23 PM

A friend of mine had a custom xformer much like what you're looking for made by Peter Dahl.
Plitron also was in the running for that job, and seemed perfectly willing and able to supply one fairly reasonably. Apparently this is about the size of unit where it starts to make sense to use a toroid from an expense point of view, the savings from raw materials beginning to counteract the cost of thre complex winding.
You just have to love the 833s.
By the way, how are you planning on driving them?

BHD 3rd March 2005 11:49 PM

Although their site doesn't list any P-P circuits for the 833, Lancroft used to offer push pull transformers for the tube. Currently their site is "under construction" but still offers some S.E. 833 circuits (among others).

http://www.users.bigpond.com/triode/

There was a company a while back called Shoreline (I think) that offered 833 monoblocks for some ridiculous amount, around 40K.

Personally, going through all the trouble of dealing with potentially lethal voltages and expense seems a bit extreme for a subwoofer amp, but hey, bragging rights don't come cheap!

:)

rcavictim 4th March 2005 04:44 AM

I think I would be inclined to try cathode coupling the output tubes. The driver requirements would be extreme but could be handled by an interstage xfmer also used as phase splitter driven by a conventional P-P power amp. The benefit from throwing away voltage gain in the output stage would be in greatly simplifying the requirements of the output transformer. No high voltage on it and the turns ratio would be small so good HF response is possible at low cost. Output Z would be low even before negative FB was applied for good driver control. Heck, since you don't need HF response you might find a power transformer that would work as an output transformer. A toroidal power xfmer could give you full range fidelity and I might even have one.

Thanx for the inspiration to come up with this idea! ;) That can be project number 4,018.

Brett 4th March 2005 04:57 AM

Quote:

Originally posted by BHD
Although their site doesn't list any P-P circuits for the 833, Lancroft used to offer push pull transformers for the tube. Currently their site is "under construction" but still offers some S.E. 833 circuits (among others).

http://www.users.bigpond.com/triode/

I emailled Bob Sugden of Lancroft recently, and he's stopped winding transformers as of about a year ago (early 2004). Great pity for us as his iron was excellent.

Brett 4th March 2005 05:01 AM

Re: 833 push pull design help
 
Quote:

Originally posted by sickss
As some of you may have seen, I am looking to build a subwoofer amp to replace my current s/s piece. I am strongly considering a pair of 833's in p-p config. Sounds good on paper...but one BIG problem...TRANSFO'S !
This is an interesting intellectual exercise, but why in heaven go to all the effort of building a massive inefficient tube amp when a good sand amp does very well as a low frequency amp and costs peanuts per watt?

sickss 4th March 2005 12:21 PM

Everything I currently am working on is tube. It seems "out of place" to have a solid state fired sub. Also....believe it or not, after hearing a larger vtl piece firing a pair of Adire's...I am convinced...by all means that was the most acurate reproduction of low frequency I have every heard.

I have also played around with the idea of OTL. The biggest problem with the circuit design though is going to be compensating for the great impedence change on the newer large x-max pieces.

The driver stage is not of concern currently. But I would like to use RF tubes to stay with the theme...maybe some 811's or 813's... Only because this is what I have.

Thanks
Dennis

bear 4th March 2005 04:06 PM

A few preliminary questions need to be answered:

- feedback?
- class of operation?
- power output?

Then you can start to consider the yin/yang part.

Since this is a SUB amp only, you can forget about the HF response from your output iron. Pretty much. You want major inductance for LF capability. Big *** core...

Cathode coupling the output stage is a definite possibility if only because you might find suitable iron more readily it makes it a plus.

The B+ required on an 833 is *very high.*

The 813 is a better choice for a big tube - as are 845s and 211s, both of which can be run in Class AB with appropriate bias. 811 will do ~150watts for a pair no problem and drive pretty easy.

Build a bridged pair of 811s and you've got a ton of power without too much big iron and too much expense, modest B+ requirements.

Not as much sex as a bigger bottle... but still it is a DHT!

Keep in mind that a non feedback tube amp will sound rather different than a feedback tube amp in the bass region...

I'd stay far away from Plitron for output iron in general - they're not so good with DC imbalances, eh? Just my observation of their specs...

Oh, if you use LP as a source, definitely HP the amp's stages @ ~8-10Hz... keep the subsonics out of the iron. :)

_-_-bear :Pawprint:

ReinoutdV 4th March 2005 07:25 PM

transformer availability
 
2 Attachment(s)
Hi Sickss,

last year i finished my 833-amp. Not PP but SE. With a nice twist: ultra-parafeed.....in order to get rid of the massive amount of saturating DC in the OPT.
It is a full-range amp, and not just for the bass-region. Although the bass-output is immense. Talk about real authority here !

Jusy like you i was looking for a good transformer-supplier; if possible in the Netherlands so i good really SEE what the quality was. I had good experience with Heart (established Dutch firm.....doing this business now for 3 generations....very low profile).

They did almost all my transformers/coils for the project. And that's more than 30 pieces.
The output is an EI150-affair weighing in at 12 kg. Not because i needed that huge core (ultra-parafeed....no DC), but because i wanted a very well designed coupling between the 7 primaries/6 secondaries. With my 1400 V/200 mA that means large diameter copper.....large transformer.

Just for fun i inquired what a huge 833 PP-transformer will do:
- custom designed PP833;
- EI-180 transformer;
- stacked height=100mm;
- Armco 0.35 iron;
- combined 7-layer model (just like mine);
- isolation class 4 (kapton foil + Kraft-paper + vacuum impregnated;
- secondaries as 30 cm leads, etc, etc.
====== 369 a piece ex VAT/shipping/etc.
Certainly not cheap.....but i can assure you that this is real quality.

I'll enclose some pictures.
About the rest of my design: A2-class (positive grid driven), 1400 V on plate, 5 K OPT, 100 Watt output, no feedback whatsoever.

The high voltage is not the most essential part.....take care of the 20 Ampere inrush for the filaments ! Seconds later it will stabilise to just 10 Ampere.......

And it does sound great !

Regards,
Reinout

ReinoutdV 4th March 2005 07:28 PM

finished amp
 
2 Attachment(s)
Hi Sickss,

As promised a picture of my 833 amp. Not for only bass-output.

Your design: PP 833......what is the speaker you're going to drive. Or are you planning to power up a stadium ?!

Picture only one channel: upper part audioamp. Lower part the external power supply.

Reinout


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