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22nd Feb 2011, 4:55 pm | #1 |
Dekatron
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Mullard 5-10 homebuild project
I,m re-building a couple of Mullard 5-10 from the bottom up as it were.As I said in a previous post,Ive decided to put the two monoblocks in one housing/unit.But not as a stereo setup.I plan to use a seperate pre-amp to "control" them.
My first query is about the positioning of the mains transformers(see pics).As you see I have them at one end;they are 2" apart.Will this be ok? Secondly,I have managed to lay my hands on two VVT,VTP204200 OP transformers.I,m worried that they are the wrong type for the low loading recommended in the 5-10 plans.In the bumf it says to use low loading for speech/music.As I understand it low loading is for speech etc,normal loading for testing up to full output power.Still foggy on what distrebuted loading is for.Could anyone clarify this part of the amp. In the tech specs of the OP transformers I have,it says the primary off G2 from the EL84,s can be run at 43 or 20%.So I have I the wrong OP transformers for low loading? Any clarification on this would be most appreciated,Andy
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22nd Feb 2011, 7:18 pm | #2 |
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Re: Mullard 5-10 homebuild project
Distributed loading is another name for ultra-linear i.e. the screen grid goes to a transformer tap. 20% or 43% represent different compromises between power and distortion, if I remember correctly.
Low loading and normal loading is really about bias, but there may be a knock-on effect on anode load. During loud signals, cathode resistor bias will shift to a higher cathode voltage - equivalent to a more negative grid voltage. This can mean the valves are biassed too near cutoff so you would get distortion. To avoid this the valved are biassed at a higher current than optimum, so that there is scope for change. This is normal loading. Low loading means less quiescent current, but therefore less scope for bias shift. The change in bias takes time to take effect, so brief transients are OK. The Mullard book does not explain this particularly well. |
22nd Feb 2011, 9:43 pm | #3 |
Dekatron
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Re: Mullard 5-10 homebuild project
Hi Andy, if you can get the Mullard book on the 5-10 etc amps its all explained in there.
Ed |
22nd Feb 2011, 9:49 pm | #4 |
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Re: Mullard 5-10 homebuild project
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23rd Feb 2011, 4:24 pm | #5 |
Dekatron
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Re: Mullard 5-10 homebuild project
Thankyou all,Ive read the book/plans,but I,m still a little uncertain.As I understand it,I need to set up the amps for low loading.I want to be certain wether the OP transformers that I have,are suitable for this.They are VVT204200 UL Output-6600 ohms.In the specs it states,Primary Circuit type-Ulra PP univ,Sec Cir type-Univ LS,Valve Res Load Anode 6600 ohms,Output 14w.
In the 5-10 book/plans it says-OP trans,Low loading-6k,Normal-8k,dist loading-6.6k for 20% sreen grid taps,8k for 43% screen grid taps. So,I have the wrong transformers for low loading.Do you use a different OP transformer for each type of loading? In another post on the forum,someone said to be carefull of the positioning of transformers,as the magnetic field from one,can effect the other.And if close together,to put them 90 degrees to each other.However the mains transformers I have,the leaves of the transformers lie paralell to the top of the chassis as opposed to at 90 degrees to the horizontal..They are 2" apart.(see pics in oriinal post)So as I have them positioned in the pic,will in your opinion be a problem? I apologise if I,m being dense,just cant get my head around this loading/OP transformer problem.Thanks for bearing with me,Andy
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23rd Feb 2011, 5:09 pm | #6 |
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Re: Mullard 5-10 homebuild project
Given the variation of speaker impedance with frequency there is little practical difference between 6.6k and 8k loading. You may notice the difference in power and distortion into a dummy load, but with real music fed to a real speaker it will be fine.
Given your transformer, it looks like if you want to be reasonably precise you can either do pentode connection with low loading or UL connection with normal bias. If you get it wrong it won't damage anything, you will just get a bit less power or a bit more distortion. Regarding orientation, the thing to worry about is hum pickup from the mains transformer not interchannel crosstalk. Almost all amplifiers have the output transformers round the same way, but perhaps different from the mains transformer. |
23rd Feb 2011, 5:38 pm | #7 |
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Re: Mullard 5-10 homebuild project
Apologies for barging in on this thread but I've just found:
Stern's complete assembly data and diagrams for the construction of the Mullard 5-10 main amplifier only together with the Mullard 5-10 incorporating control unit About 16 sides of foolscap. From Stern Radio, Fleet Street, original cost 2/- If the OP or anyone else wants this it's FOC. |
27th Feb 2011, 1:27 am | #8 | |
Hexode
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Re: Mullard 5-10 homebuild project
Quote:
The best results are obtained with each half of the primary winding tapped so that about 20% of the turns are common to the anode and screen-grid circuits, when the anode-to-anode loading should be 6.6 kΩ. With distributed loading, total distortion is reduced very much compared with normal loading. The rated power reserve of 10 W is maintained although overloading now occurs at 11 W instead of l4 .W. However, the rate at which distortion increases with output power after overloading is less with distributed than with normal loading. The frequency response of the amplifier is the same for either method of loading, although the stability is improved with distributed loading because the lower level of distortion is obtained with reduced loop gain. " The transformers you have are fine. I'm building the 5-20 with similar (but higher power) trannies from VVT.
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4th Apr 2011, 2:29 pm | #9 |
Dekatron
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Re: Mullard 5-10 homebuild project
Heres where Ive got so far.Ive made a wooden box 15" x 15" x 4" ,but with a steel top (see pics)Ive put the two transformers I had at the back,drilled the holes for the valve seats,put an on/off switch at the front and connected the two transformers to an old computor IEC socket via two old fuse holders that came from my old caravan.
I,m not one for planning every detail before hand.I,m more "I,ll plonk this here and see how I go" .Which means I make mistakes,feeling my way as I go. So,in a nutshell,I,m building two Mullard 5-10,s in one unit,and am toying with putting a pre-amp in the same box/unit.There are lots of issues I have to work out;like where everything goes,while at the same time making sure that the siting of components,doesnt "infect" the input signal. I may have a problem with one of the mains transformers though.(the left one in the pictures)It looks older than the other,has no screen,no center tap on the 6.5v heater supply,and only 5.1v supply for the rectifier.I,m not sure as to the current it can supply either.(The other is 120mA). I have a few questions that maybe some of you may be able to help with. 1)Can I supply the two EZ81,s heater supply from the right transformer,its rated at 1A? 2)Is the pre-amps HT supply able to be driven from one transformer? 3)I,m having a bit of trouble trying to fix the value of the resistors that come before the rectifier.Are they there to limit the voltage/ current? Its tricky because I dont know the specs of the two RS transformers I have. There are lots of other things I have to suss out;the Mullard 5-10 plans arnt specific on lots of points.Whereas the Stern plans I have (thanks Jeffery) are perhaps more explicit. In conclusion,Ive a long way to go,this project has taught me alot already. Andy ________________________________________________
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Curiosity hasn't killed this cat...so far. Last edited by Diabolical Artificer; 4th Apr 2011 at 2:32 pm. Reason: Heater supply |
4th Apr 2011, 4:12 pm | #10 |
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Re: Mullard 5-10 homebuild project
EZ81 heater is isolated from its cathode, so you should be able to drive two rectifiers from one heater secondary provided the total current is OK.
Measure the HT secondary resistance, and the primary resistance. Add them together (slight cheat but should be close enough). If this is less than the minimum resistance for an EZ81, then add a wirewound resistor in series to make up the difference. It doesn't have to be exact. |
4th Apr 2011, 4:54 pm | #11 |
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Re: Mullard 5-10 homebuild project
Thanks Dave,that gives me some idea on one of my queries.By minimum R of EZ81 what do you mean? Andy
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4th Apr 2011, 6:07 pm | #12 |
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Re: Mullard 5-10 homebuild project
The datasheet will specify a minimum series resistance. This limits peak current to what the EZ81 can handle. Most of the resistance will be provided by the mains transformer.
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4th Apr 2011, 9:08 pm | #13 |
Dekatron
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Re: Mullard 5-10 homebuild project
Now here's a challenge for the best of anoraks.....
Can we identify the Radiospares transformers for Andy? It would probably be helpful to his PSU design to know what they are supposed to be capable of delivering. Anyone got some suitably ancient catalogues? I would guess at mid/late '50s.
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5th Apr 2011, 6:36 pm | #14 |
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Re: Mullard 5-10 homebuild project
Thanks,still a little worried about using the transformer that has no screen (to stop interference?) and no CT on the 6.5v HTR supply.The other transformer is fine.It has labels on each lead with specs,Andy.
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5th Apr 2011, 7:45 pm | #15 |
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Re: Mullard 5-10 homebuild project
Hi, To add my 2pworth, Transformer orientation, its the axis of the coil formers that is important i.e. they should be at right angles. From your picture in post 9 , the OP transformer axis is up and down in the picture but its hard to see the mains TX axis. Having said that, the trannys are so far apart that induced hum would be very low anyway. EZ81s take 1amp for each heater so would probably o/load the transformer. EZ80s only take .6A but might be a bit light on HT current, but both could be driven from the 6.3v sec. A transformer screen is useful but not vital. A centre tap can be simulated by 2 200ohm resistors connected across the 6.3v with the centre point earthed, or you could use a 500ohm pot with the wiper earthed, an old idea, handy for "nulling out" any hum!. By the way, where are you going to put the smoothing caps ? I see you dilemna about the transformer, how about using the 5.1v sec for the other EZ81? it would be underrun but might be OK. Your right about the badly written bit in the Mullard book about low loading etc. I never understood it but the 5/10s sounded good !.
Peter |
7th Apr 2011, 2:48 pm | #16 | |
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Re: Mullard 5-10 homebuild project
Quote:
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7th Apr 2011, 4:17 pm | #17 |
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Re: Mullard 5-10 homebuild project
Hi Magnetic and dickie, to clarify, I dont think leakage between op transformers would cause problems, only give very slight crosstalk. What I was concerned about was 50Hz leakage from mains trans to OP trans. An easy test would be to connect mains to both mains trannys and speakers to OP trannys, with nothing else connected. If no hum is heard all,s well!.
Peter |
8th Apr 2011, 2:05 pm | #18 |
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Re: Mullard 5-10 homebuild project
At the moment the siting of O/P transformers as in pics,is not set in stone.If I do site them there,I would have to make some sort of casings.They,ll look tatty else.As it is I may put them inside,and put the four EF86,s of the pre amp in their space.As I mentioned in earlier posts,I,m still winging it.
Re the preamp,I,m going to have to re-jig it,re the inputs,changing DIN sockets for RCA,and updating the ins;IE deck,computor,mixing desk(maybe),R2R,CD player etc.I have plenty of space to pop the preamp in the same box.Just lots of factors to consider.Keeping the signal hum free etc etc. I,m taking it very slowly.Once Ive sorted the power supply out,I,ll have a better idea,thanks for all your help,Andy.
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10th Apr 2011, 6:32 pm | #19 |
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Re: Mullard 5-10 homebuild project
The VVT output transformers are 'universal'. This means that they are a compromise between the various possible options, e,g., 43%/20%/triode/pentode connection and 3/8/16 ohms output.
In listening tests under most domestic conditions, the loudspeaker causes more distortion/frequency response problems than the transformer. The transformer is normally within the feedback loop which reduces it's effects. VVT is about to produce individual variants to optimise each of the options. These will be done on request. |
30th Apr 2011, 12:17 pm | #20 |
Dekatron
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Re: Mullard 5-10 homebuild project
Ive just about sorted out where everything goes (see pics)I,m going to follow the Sterns layout-ish,rather than the Mullard original.As a first time builder,the original Mullard design/specs is a little vague in some respects.
As you can see in the pics,Ive put the preamp at the front.Ive had to jiggle things around a little to get it to fit.Im going to have to move the position of the EF86,s into the centre,inline of the aliminium box,as where they are at present. As to the power supply,the bigger transformer is ok,but lacks another 6.3v heater supply for the preamp tubes.The smaller transformer still leaves me with a problem,as the rectifier heater supply is at 5v.Ive been looking for another RS transformer,the only one Ive found is going for £750 on Ebay.Its either that or use a GZ34/5.Or use a seperate transformer for just the heater supplys.I saw some on an old Ebay listing.In the meen time Im going to consentrate on getting one 5-10 working.I have one 50/50uf F+T cap,and some more on order.One thing I noticed is that the original Mullard design uses two 50/50uf caps,but the Sterns uses a 16/16uf for the second smoothing cap.Not sure why.I,m a little wiser as to what value resistor to use on the EZ81 anodes(the Stern uses 2 300R,s)Am I right in thinking as my transformer is a tad over 300v per anode that that value should be ok.Sorry still a bit confused about this.Dave G8 etc says to add primary and secondry resistance,then make up the difference.Therefor that gives me 14 ohms + 168 ohms =182ohms.Datasheet says 240ohms @300v =72 ohms,doesnt seem right.Lastly,Ive ordered a couple of NOS Brimar EZ81,s off Ebay,they look far the best Ive seen. Ive still lots to figure out,especially regards the preamp.In the Mullard design the main selector switch has four wafers,mine has two.Also I have to figure out the correct input impedances for a CD player,computor and what not. Anyway slowly getting there,am limited by lack of money,but have been saving up and some resistors came this morning,resevoir/smoothing caps coming soon.Any feed back much appreciated,Andy.
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