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6AN8 Pre & concertina, what seems to be the problem?
6AN8 Pre & concertina, what seems to be the problem?
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Old 22nd November 2017, 03:13 PM   #21
scottpetersen is offline scottpetersen  United States
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Have these caps been replaced?
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Old 22nd November 2017, 06:45 PM   #22
el156 is offline el156  Portugal
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You need a capacitor at the input of your amplifier...
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Old 22nd November 2017, 08:09 PM   #23
petertub is offline petertub  Sweden
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I think one should take a step back and don't make any changes just yet.

The measurements seem strange, but i suspect the DMM used to load the circuits too much,
giving false readings. An VTVM or "classical oscilloscope" ( not one of the usb , that
also has too low impedance to be used on tubeamps).

A VTVM ( vacuum tube voltmeter) can be found quite cheap on ebay, same with
a classical oscilloscope. A scope is invaluable if one intend to continue working
with tubeamps.
If however the interest is not to continue working with tubeamps i recommend to
leave the amp to a technitian that has the right tools.
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Old 22nd November 2017, 08:48 PM   #24
TonyTecson is offline TonyTecson  Philippines
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6AN8 Pre & concertina, what seems to be the problem?
Quote:
Originally Posted by Junm View Post
I will check the input resistance of those dmm once home. Im on travel now.
i use a fluke 87V will have very high impedance...around 10Meg....http://www.transcat.com/media/pdf/80vUserManual.pdf

i use this to test resistors in circuit, be suspicious if readings stray off the nominal value,

i lift one side and confirm....
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Old 22nd November 2017, 10:48 PM   #25
Junm is offline Junm  Philippines
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Quote:
Originally Posted by scottpetersen View Post
Have these caps been replaced?
Yes they are already replaced.

Quote:
Originally Posted by el156 View Post
You need a capacitor at the input of your amplifier...
Yes i will try to do this

Quote:
Originally Posted by artosalo View Post
Put the grid directly to gnd (with less than 1k resistor, preferably with a jumper wire).
How are the voltages now ?
Thanks i will give you an update on this.
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Old 22nd November 2017, 11:12 PM   #26
TonyTecson is offline TonyTecson  Philippines
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6AN8 Pre & concertina, what seems to be the problem?
Quote:
Originally Posted by scottpetersen View Post
Have these caps been replaced?
those caps when leaky will cause red plating of the output tubes....
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Old 23rd November 2017, 02:14 AM   #27
Johan Potgieter is offline Johan Potgieter  South Africa
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Several matters remain contradictory. (The coupling caps of 0,3F are of secondary importance, a different matter to the 6AN8 questions.)

The low voltages on pins 2 and 3 of the triode section conforms, but is contradictory to a low voltage at pin 1 as well - that should be higher than 170V, not lower. (I note something wrong in the diagram. There is no dot (thus no connection?) at the crossing of the leads to the 150K and 15K resistors. Can one take it that as a schematic error?

Then: Is there +1,3V at pin 8 (pentode) irrespective of the position of the 500K input pot? Even when at the earthy side of said pot?

Important now, that you short pin 8 to earth and then give voltage readings again please, as asked earlier. This before any alterations.

And Junm, keep courage!
When strange voltages show up it is usually easy to locate the fault(s). It is when things do not work and no apparent discrepancy is noticable that one suffers!
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Old 23rd November 2017, 02:37 AM   #28
Johan Potgieter is offline Johan Potgieter  South Africa
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Sorry!

Missed something. Junm, in your intro post you said that the readings you found were common to both channels? The same values?? This then poses a further question, as your supply voltages do not differ by all that much.

This then takes us back to the problem of the +1,3V at pin(s) 8. That is also the same for both channels?

What am I missing?
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Old 23rd November 2017, 02:42 AM   #29
TonyTecson is offline TonyTecson  Philippines
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6AN8 Pre & concertina, what seems to be the problem?
as i understand it, these design uses the same psu for both channels...
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Old 23rd November 2017, 02:51 AM   #30
Johan Potgieter is offline Johan Potgieter  South Africa
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Exactly Tony!

Perhaps I was unclear. What amases me is then that the OP found the same erroneous independant electrode readings for both 6AN8s, if I interpret correctly. (Only the dc voltages are common.)
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