sound after picking and disappear slowly

[reroe]

hi guys
i need some help.
i've just finished my 2nd job, tiny terror clone.
i started up with bulb limitter, checked lead dress and some voltage values like...
1) heater filament : ~6V AC for 12ax7, el84
2) plate voltage : ~250V DC for 12ax7, ~300V DC el84

fortunately, it has no special hum or hiss.
but when i plugged my guitar and turned volume up
it sounded a bit later, after picking and soon it disappeared gradually.
it's likely that i use my volume pedal with delay.

i can't find any clue about this symptom.
would you give me some help?

[M Fowler]

So you plugged into the amp, then strummed the strings, and the sound was delayed?

I am trying to get to your question is why I asked.

[Alexo]

Sounds kinda like what would happen without a ground reference for one of your tube grids. Use your VOM and check for resistance between pin 2 and ground and pin 7 and ground on all your 12AX7's. If any of those reads open, there you go.

Otherwise, could be an ultrasonic oscillation.

[reroe]

thank you all.

m fowler > yes, what you asked is the problem i have. please help me~

alexo > i'll check my grid resistance like you said.
do you have any suggestion or way to check
if i have ultrasonic oscillation-like problem?

[C Moore]

Not to sound idiotic, are you sure it is the amp?
Only reason I ask is because a speaker with a bad coil will often make that sound of decay/delay.
Good Luck

[Structo]

Can you take some pictures and post them here?

I had a bad grid leak resistor do that sort of thing before.

[guitarmike2107]

unplug your guitar from your volume and delay pedal.. Solved

sorry not much help

the grid refernce issue sounds like a good idea, without them your valves could be baised just about anywhere.

[reroe]

i checked grid resistance and there's no open (can read specific resistances.) also it's not a speaker problem (works well with another amp)

more specifically my amp sounds like a "tremolo"
sound appears and disappears repeatedly.
it is likely that there's some destructive and constructive interference with singal.

...anyway while checking the amp i found a silly thing
there's 100pF cap connected to gain pot. i wired just a 10pF cap by mistake. so to fix it, i added more 56pF cap in parallel with 10pF cap
(now i don't have an exact 100pF one)

of course i'm not sure this thing is related to my problem (my ignorance about electronics and circuits thing) but now i can hear better, non-fluctuating sound.

can it be happened? is it just my feeling?
or if it can be, what is the principle?

[brain_lp]

hi ,, im indonesian

sorry 4 the bad english
i have got problem like u, n i solved by checking all of the solder component connection in the pre amp section.

sometimes its caused by value resistor after coupling caps in every stage
so please check again

hope its help

[kd]

Check your solder joints at the speaker output jack. A bad joint here can cause strange things to happen.

Ken