OTL by Tim Mellow

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ACPEN
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#1 OTL by Tim Mellow

Post by ACPEN »

Hi All,
I know we've looked at this one before, but I think it was on an 'un-related' page (?) Just that I'm looking to build one and thought a simplified sketch of the circuit might be usefull (?) If someone with the necassary CGI skills could do it properly.......... using the original for part no's and values etc.

Hope it attaches ok,

ATB,

David.
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ACPEN
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#2 OTL by Tim Mellow.

Post by ACPEN »

Aso, my suggestion of using a non centre tapped isolation transformer for the 150v +&- h.t rails was understandably queeried. Although this method (no c.t) is used by Han's and other OTL designers, it can lead to problems with h.t levels if the 6C33C's are not balanced correctly. Therefore, I'd like to suggest this compromise which uses the c.t of one of the heater transformers to provide the c.t connection. It does of course mean that the heater supply current is also carried by the h.t transformer, but I think it would be workable (?) Also, it means that the h.t transformer (large and heavy) could be positioned (on the floor) away from the amp. chassis. Only the two heater transformers being actually on the amp. chassis. :cry:

ATB,

D:/
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Nick
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#3

Post by Nick »

Cunning, but remember the first transformer will need to supply the power to the B+ and the heaters, which is another 80W for two 6c33c. Also the two transformers will each loose their bit of power so you may need 7v (or more) secondaries to get the 6.3v for the heaters.
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#4

Post by ACPEN »

Ah! spotted the possible flaw in my 'cunning' plan! Always a price to pay, but I have used this ploy before with non OTL designs, admittedly with a much lower current draw amp circuit. I initially used it to make a smaller 'visible' amp. to please my dear wife. And it worked? (Didn't notice the isolation transformer behind the settee! - well, not for some time!). I think I'll use this with the OTL I'm doing so will be able to report on how it works out.

ATB,

David.

P.S also attached a slightly better attempt at the re-drawn cct
With apologies to T.M.
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#5

Post by ACPEN »

DOH! Try again!
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Mike H
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#6

Post by Mike H »

Not wishing to throw another spanner in the works (or several) but I'm somewhat worried about anode current runaway (or cathode, whichever, same thing?)

Also, grid leak resistor (I know there aren't here) must not exceed 26 kilohms or something like that. Read it somewhere. I tihnk.

Also (yes sorry :D ) not sure if you can drive 6C33 grid capacitance adequately with something using 100k anode resistors :?:

Not that I know much about 6C33C but the more I hear about them the more I get the impression there's a can of worms waiting for the unwary



 
 
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pre65
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#7

Post by pre65 »

Mike H wrote: waiting for the unwary

That's me ! :oops:

Not tried 6C33 yet. This OTL lark is going to have to go on the back burner for now. :?
The only thing necessary for the triumph of evil is for good men to do nothing.

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#8

Post by ACPEN »

Hi All,
Progress has been slow with this, but attached shows what it might end up looking like. Yes, another Cutlery box! But, you must admit, a nice grain! There,s also my re-jigged scan of Tim Mellow's schematic - thought it might encourage any 'latent' OTL builders?

ATB,

David.
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OTL 001.jpg
OTL 002.jpg
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Nick
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#9

Post by Nick »

Looks great, very nice wood on the box.
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#10

Post by ACPEN »

Now finished! Results so far are rather good! I should mentiom though a couple of differences to the Tim Mellow design. Firstly i'm using 6C41C's in place of 6C33C's because I don't need 25W of audio (or, more importantly, the heat generated in achieving it!) This has negligible effect sonically except for an increase in output impedance and damping factor, which still betters most SE designs by 'quite a bit'! At this stage I've made no frequency response measurements,the only set up has been to the 6C41C's bias and channel balancing, which seems to be quite stable.
Having only one bias adjustment for each channel does simplify set-up considerably compared to the previous OTL I built using individual adjustments. Apart from the change of output valves, all other circuitry is as per Tim Mellow, with the following changes. I used two 2200uf 200V smoothing capacitors in the psu instead of the chain of 6800uf's plus I've uprated the voltage of C18 (100uf 250v) to 385v as it can exceed the 250v rating. Otherwise, all is as per the article. I've attached a couple of 'photo's one of which shows the inside of the amp. which is easily accessible for servicing (!) The valve heater transformers/ capacitors can be seen on the lower case and the main amp. wiring on top side. The main isolation transformer is a seperate unit and is one of the 500W bench isolation types I've mentioned previously. The beauty of this is that only a three core cable is used to connect between the amp. and psu. (Unlike my previous OTL which had multiple interconnects!). I hope this might inspire others to try this OTL design - I know it's unusual in using EF86's to drive a high gm output valve (not done!) but it works beautifully!) and because of the use of long tail pairs throughout, there is NO hum! This last point was most unexpected as my previous OTL did have a problem in this respect.

ATB,

David.
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OTL 003.jpg
OTL 004.jpg
OTL 005.jpg
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andrew Ivimey
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#11

Post by andrew Ivimey »

Looks lovely - how does it sound? What did you do with the fish knives? What does anyone do with fish knives?!

I remember playing a little with 6S41; just trying to get reasonable SE out of them. It was not a success - 'hard and glassy'

I haven't given up on 6S33 ... yet.

My OTL uses 6as7 and is pretty much a straight Futterman design - I like it!
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#12

Post by ACPEN »

I still have the knives if you're interested1 Sound is all an OTL should be. I agree regarding the 6C41 in SE but it works well in OTL. I have used 6AS7's before in SE but came to the same conclusion re. hard and glassy, Never tried them in an OTL as the 6C41C is very similar to a 6AS7 with both triodes strapped together, so saves on bases!

ATB
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#13

Post by ACPEN »

Well, 10 days later and it's still working! Infact the DC off-set has settled down to a remarkably stable '0.005V! Whether this would apply using the original valves (6C33C) I can't tell, although I'm now in the process of building one for my Son-in- Law with 6C33C's! He liked it so much, he might even replace his much loved 300B Amp! I'll let you know how it goes later. I'm very happy with the amp. as it is, as, for an OTL, it is remarkably stable regarding biasing and dc off-set. The other good thing with this little amp. is that it was very cheap to build (given that I got the mains isolation transformers for free!) Total cost including all new valves etc from Russia and Colomor (12Ax7LPS) and most other parts from RS was approx. £150! Try making a decent SE triode amp. for that!I already had the box, the metal for the chassis and nuts & bolts etc. Come on someone - build one please!

'Merry Xmas' to All!

ATB,

David.
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pre65
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#14

Post by pre65 »

Hi David, It's still on my list of things to do. :wink:

Did you get any more of those transformers in the end ?
The only thing necessary for the triumph of evil is for good men to do nothing.

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Nick
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#15

Post by Nick »

Try making a decent SE triode amp for that.
Easy if I can use the parts I already have on the shelf :-)
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