Missing grid leak? ( Echolette m40 )

Discussion in 'Shock Brother's DIY Amps' started by PelleOter, Sep 9, 2017.

  1. PelleOter

    PelleOter Tele-Meister

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    IMG_2255.JPG Hi all!

    I am servicing / restoring an old german amplifier - Klemt Echolette m40, 1959/1960. Really cool and compact little amp-head with 5 x 12ax7s + 4 x EL84.

    I am working of this schematic, and I have a question. If you look into the second grid of tube three (marked rö.3) - I cannot see any DC-path to ground for the grid, if all the switches off of the input stage are open? Is there something I don't see here? The switches are pull-pot switches on each of the four input's gain controls - and their purpose are to send signal (via the 5-pin DIN socket - "Ausgang lll") to an external tape echo device. But for normal use the amp will be operating alone, with the switches open - and it don't seem like a good idea to leave grid V3b floating.. Should I add a resistor to ground straight from the grid?

    And never mind the blue line editing in the output section.. I am keeping the original UL setup.

    Ole P
     
  2. PelleOter

    PelleOter Tele-Meister

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    IMG_2233.JPG IMG_2234.JPG

    For those of you that might be interested, here are some pictures of this thing. (Yes, I've got three of them..)

    The interior was pretty scruffy - all sixty year old components, and a lot of dirt! The chassis is perforated to aid cooling, but that also leaves the circuit somewhat exposed to dust and dirt from the outside world. Almost all of the capacitors where leaking and/or off spec, and many of the old cc-resistors looked fried too. Plus, all the wiring was brittle as crackers - therefore I decided that the better solution for making this amp reliable and sounding good, was to gut the whole thing and do a total rebuild. Just about the only things that still worked in this amp was the transformers.. (luckily!)

    I am intending to stay faithful to the original circuit and see how it performs. I am adding adjustable bias, and replacing all the weird DIN sockets with 1/4" jacks.
     
  3. PelleOter

    PelleOter Tele-Meister

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    IMG_2319.JPG ..and here's how it stands now
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    How about through R95 & R96? (due north of the tube)
     
  5. PelleOter

    PelleOter Tele-Meister

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    Hi Rob - and thanks for your reply!

    Yes I see R95 and R96, and they do indeed provide the grid leak for V3a (together with the other 1M resistors and 500k pots from the network off of each channel volume/tone network) In total it should be around 200k-ish to ground from V3a grid.

    If not my eyes and head is confusing me - which they very well can:) then I cannot see any connection between V3b grid and these resistors (R95 R96) If one or more of the "send to Ausgang" switches were closed, then the grid would be grounded through the same tone/volume stack.

    But for normal use, without pulling the pots, It still seems to me that the V3b grid would be left floating..

    Hmmm?
     
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Oh, sorry, I though you were talking about the left triode. The right grid leak must flow through that ausgang thing.
     
  7. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    Wow, that thing is built like a piece of military gear!
     
  8. PelleOter

    PelleOter Tele-Meister

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    Okay, then we see the same thing it seems. Would you then agree that this must be a design flaw - and that it would be preferable to add a new resistor directly from grid to ground on V3b? I guess it should be a fairly high value, in order to not load the input down too much? Can I use a 10meg even?? (If I see this correctly, the "new" grid leak resistor would parallel with the 1megs and 500k pots from the previous stage whenever one or more of the "send to Ausgang"-switches are closed.) And I don't want to offset the input impedance of v3b more than necessary, right? In case of that input/output DIN being used, that is.

    I am having a bit trouble understanding how this Ausgang-thing is supposed to work actually, but it seems to be both a preamp output and a return to mixer stage. And I don't think that v3b is even in the signal path as long as all four switches are open. Maybe it is not so bad for grid to be left floating - I mean , it would be grounded the moment you decide to connect an external device.

    Haha, yes it is! Every wire carefully wrapped with cotton string, all stainless steel chassis, nice heavy transformers too. I have never worked on a true point to point amp before, and I must say it's quite a challenge to organize everything in a neat way. It is not more room in this chassis than it have to be, to say the least! Hopefully it will turn out good

    And sorry for any writing/grammar errors in my posts - English isn't my best language!:)
     
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  9. robrob

    robrob Poster Extraordinaire Ad Free Member

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    If the grid leak was a problem there'd be a crap ton of hum so I wouldn't worry about adding a secondary grid leak.
     
  10. PelleOter

    PelleOter Tele-Meister

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    Okay, thanks again for your input.

    It wouldn't potentially damage the tube in any way either? In previous tests (before I stripped out all components) - I measured a few volts DC on the grid. My concern is if not this charge building up on the grid, will alter the bias point, and eventually fry the tube?

    Maybe I'm over-thinking this
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Normally negatively charged electrons build up on the grid so they actually slow the flow of electrons through the tube to cool the bias. As long as hum isn't a problem you're ok.
     
  12. PelleOter

    PelleOter Tele-Meister

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    Sorry, I forgot to answer! Okay - I've had no hum problems, so as long as the tube can not go into meltdown-mode, I will wire it up per the schematic - without a grid leak I also crossed checked with another amp of the same type, which is still all original. And sure enough, it too reads no continuety from grid to ground, unless I pull one of the pots, so I guess this is the way they were supposed to be.

    Thanks for exellent help, always something to learn!

    Cheers.
     
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  13. SinP

    SinP NEW MEMBER!

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    Hi did you finish your Echolette M 40? Can we see some more photos I like your rebuild work. Regards from Canada
     
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