IT stage microphony

[claudiomas]

hello,
sowter 8424 IT can be wired with primary series 20Kohm 70H 30mA
or parallel 5K 18H 60/70mA or little more.
problem in series mode 20K and EL34 +B350V,sound very sweet but high microphony !!!
same circuit parallel 5Kohm no problems.
same problem with Hammond 126C 10Kohm 10mA
tube 6E6P-E +B150V sound so nice open detailed but High microphony
with Hammond 126B 5Kohm no problem.
power supply its smps but CLC filters
WAY???

 

[Osvaldo de Banfield]

Microphony and PS are different problems.

 

[claudiomas]

Microphony and PS are different problems.

wath you means???
the question its : way microphony with higer impedance IT wen low impedance its OK same tubes same power supply ?

 

[Osvaldo de Banfield]

Let me explain. Microphony is an audio modulation caused by mechanical vibration of parts conducting electric currents, or generating them by reaction, for example, when moving in a strong magnetic field. This fact doesn't depend upon impedances of output transformers, nor output tubes, because the gain from there is very low. Microphony usually affects firsts or earlier stages in an amplifier.

 

[claudiomas]

Let me explain. Microphony is an audio modulation caused by mechanical vibration of parts conducting electric currents, or generating them by reaction, for example, when moving in a strong magnetic field. This fact doesn't depend upon impedances of output transformers, nor output tubes, because the gain from there is very low. Microphony usually affects firsts or earlier stages in an amplifier.

so you means i'm dreaming?
well no the amplifier its on my bench
i'm building a GM70PP and if i wire the Sowter 8424 in series 20K the EL34 become very microphonics and if wired in parallel 5K no more microphony
same thing for firsty stage,6E6P-E IT Hammond 126C 10K get microphonic but with 126B 5K no microphony.
so theory cannot explain let experienceds diyers answer if they get this problem in the past
tanks for trying aniway

 

[TheGimp]

The first question is "What is microphonic?"

Have you tried lightly tapping on the tubes with a pencil to see if they are sensitive?

Try tapping on the transformer to see if it truly is contributing.

Try isolating the transformer from the tubes with a shield to determine if magnetic interference is occurring?

 

[claudiomas]

The first question is "What is microphonic?"

Have you tried lightly tapping on the tubes with a pencil to see if they are sensitive?

Try tapping on the transformer to see if it truly is contributing.

Try isolating the transformer from the tubes with a shield to determine if magnetic interference is occurring?

yes, microphonic microphonic microphonic touching the bulb even with the little finger can by heard in the speacker
tapping the transformer or chassis
this PP its a no feedback amp no signal capacitors and no oscillation
input tranbsformer to 6E6P-E IT/ EL34 IT-splitter /GM70PP
i do not smoke ganja and no lsd25 no peyote no mescalyn no benzo
this its not the firs amp i build so try to understand i now wath means microphony

 

[Osvaldo de Banfield]

Do you have an oscilloscope? Perhaps the amp oscillates and then becomes microphonic.

Please, I'm trying to help you, don't make fun of me. Many thanks.

 

[claudiomas]

Do you have an oscilloscope? Perhaps the amp oscillates and then becomes microphonic.

Please, I'm trying to help you, don't make fun of me. Many thanks.

of course i have some oscilloscope and much more
professional repair from 40years
forget it i give up
5K its perfect

 

[trobbins]

Claudiomas, I know you have moved on, but your first post identifies an SE output stage change (20k to 5k impedance) - did you change the bias current? If not, then the microphony could have been related to the bias current setting for 20k ohm configuration, and possibly the output stage was operating in to an impedance and winding that was starting to saturate whenever any signal was applied - i.e. tapping the EL34.

 

[claudiomas]

Claudiomas, I know you have moved on, but your first post identifies an SE output stage change (20k to 5k impedance) - did you change the bias current? If not, then the microphony could have been related to the bias current setting for 20k ohm configuration, and possibly the output stage was operating in to an impedance and winding that was starting to saturate whenever any signal was applied - i.e. tapping the EL34.

for EL34 the bias its 50mA for 5K and 30mA for 20K,
for 6E6P-E 14mA for 5K and 11mA for 10K, i try +- bias, no change.
and belive my no saturation, the sound its more open and detailed very sweet.
the coànbinatiuon of the firs and second stage in higer impedence sound betterfor female voice cello sax alto etc.. taths all.
if 5Kfirs stage and 5K second stage the sound its also good, just more compressed more punch and dinamyc but little less sweet but no microphony

if only second stage hig windings only EL34 microphony
if only first stage hig windings the 6E6P-E... and also EL34 microphony!