Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on April 25, 2012, 01:45:34 am
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Hi guy's, I don't have a heater tap on the tranny i have so a second transformer is required but the ones i can buy local that have a 6v tap are only rated to 2A and this fall short for my requirements :sad2: but i can get another that has dual 12v with 9v taps rated @ 5A.Am i able to do what i have drawn in the schem and maintain the 5As. Thanks
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Hi TIMBO
As you have draw the connection it seems to me you obtain a 12v output because on the bottom winding you are connected between 9v and 12v, so you have a 3v output, in the upper winding you are connected between 0v and 9v so you have a 9v output --- 3v + 9v = 12v total output
I've drawn what seems to me the right connection to do
Kagliostro
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Thanks Franco, Your spot on. :worthy1: and this will still give me 5A????
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The 5A depends on the transformer
you wrote that the transformer is a 60VA
to have 5A you must put the output windings in parallel (12v || 12v = 12v @ 5A) 60VA / 12v = 5A
must consider that you have two 12v windings so 12v + 12v = 24v
60VA / 24v = 2.5A (the thickness of the winding can afford 2.5A)
so if you connect the transformer as to have 3v from each winding and then in series to have 6v you obtain a 6v 2.5A not 5A
other way is to put the 9v connections in parallel than use an LM338T so you can have 6.3v @ 5A
consider that the LM338T is rated for 5A continuous, 7A peak
how much current your project needs ?
if you can use the 12v instead of the 6v (tubes in series), parallel the 12v windings (you get 5A) and you can omit the use of a regulator
Kagliostro
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Which tubes do you have to heat? How many?
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Hi guy's, This has been a real pain, The build has 5 X 12AX7 and 2 X ECL84 which should give me about 3A but i was wanting to have a bit more in case the ECL84 aren't up to the job and i was thinking of using a pair of 6K6 which will require another 12AX7 for the PI.
At the moment i have all tubes wired in parallel except V1, i have it wired for DC supply.
The transformer i have is rated @ 2A max off the 6v tap.
At a pinch i could leave V1 on DC, 4 x 12AX7 in series and 2 X ECL84 in parallel ( i'm not sure if the ECL84 can be run in series as no info was found) should give me about 2.3A still over my tranny of 2A
With the 6K6s and PI will still give just over 2A. Thanks
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Why not run the 12AX7's at 12V and run the 2 ECL84's in series to get to 12V's?
Dunno if you've seen this, might be helpfull;
http://www.freewebs.com/valvewizard/heater.html (http://www.freewebs.com/valvewizard/heater.html)
Brad :icon_biggrin:
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12ax7 tubes can be used at 12v and final tubes don't worry if they are connected in series
someone point that isn't a good thing for preamp tubes heaters to be connected in series, but I think that is most important for HiFi guys than for guitar amp applications
however that isn't a problem for you as 12ax7 has a 12.6v connection (pin 5 & 4 - fine with 12v) and you must consider also that connected to 12v the consumption is halved to 0.15A instead of 0.3A
so your amp require
if 5 x 12ax7 (in parallel @ 12v - 0.15A each = 0.75A) + 2 x ecl84 (in series @ 12v - 0.72A each = 1.44A) you need ~2.19A
if 6 x 12ax7 (in parallel @ 12v - 0.15A each = 0.9A) + 2 x 6k6 (in series @ 12v - 0.4A each = 0.8A) you need ~1.7A
you can connect the preamp heaters to one of the 12v winding of your transformer and the final tubes to the other 12v
Ciao
Franco
I forgot: if you want you can have also V1 with DC (follow the Merlin's 7805 power supply on the link posted by Willabe - you don't need the MJ2500 if you want DC only for V1)
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Hi guy's, I think that if i run everything in series with 12v @2A should be OK
5 X 12AX7 @ .15A = .75A
2 X ECL84 @ .36A = .72A
TOTAL = 1.47A
I posted this at the same time. I though that if the ECL84s are in series that the total current would be .72A :dontknow:
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This is what i was thinking with the transformer i have now
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I think that 2 x ecl84 in series need 1.44A, not 0.72A
Kagliostro
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I think that 2 x ecl84 in series need 1.44A, not 0.72A
Two ECL84 in series will draw 0.72A from a 12.6VAC supply.
Two ECL84 in parallel will draw 1.44A from a 6.3VAC supply.
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Thanks Darryl
was my fault
Kagliostro
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THANKS GUYS :thumbsup: :thumbsup: