Tubes for ac10

tammuz7000

Member
Messages
315
I had a fuse blow in my vox ac10. My tech is retired but he told me to replace
The power tubes and fuse so I did and it works fine with jj el84. Should I also replace the 12ax7 as well. The tubes are ecc803 and one is a groove tube premium in v2 and other is jj in v 3The amp sounds fine but was thinking while it’s apart I should do it.

He said the power tube probably caused the fuse to blow so I replaced both of them.
 

Lespaulsignature 74

Gold Supporting Member
Messages
4,903
I'm no tube expert but I would keep a spare 12AX7 around in case you should need one. There was a recent post from TGP member "Hogy" (Komet Amps) stating that people change their 12AX7's in their tube amps more often than needed.
 

Axe-Man

Member
Messages
7,556
No. If it sounds good it’ll be fine as is. Preamp tubes last thousands and thousands of hours. They can help dial an amp in slightly, but if it’s good, don’t bother. Sometimes a preamp tube change will make the amp a little too dark or bright or thin or warm and you simply may not be happy with the change once you’re used to how it sounds.
 

Julio Iguana

Member
Messages
2,739
Pre-amp tubes do not need to be changed if they’re working fine, or if you want to modify your sound. I’ve always used a high quality tube in V1 position.
 

HotBluePlates

Member
Messages
13,917
... My tech ... told me to replace the power tubes and fuse so I did and it works fine ... Should I also replace the 12ax7 as well. ... He said the power tube probably caused the fuse to blow so I replaced both of them.

No need to replace the preamp tubes. If one of them caused the problem, the fuse would have popped as soon as you turned the amp back on.
 

Laurence

Gold Supporting Member
Messages
10,110
I had a fuse blow in my vox ac10. My tech is retired but he told me to replace
The power tubes and fuse so I did and it works fine with jj el84. Should I also replace the 12ax7 as well. The tubes are ecc803 and one is a groove tube premium in v2 and other is jj in v 3The amp sounds fine but was thinking while it’s apart I should do it.

He said the power tube probably caused the fuse to blow so I replaced both of them.

No need to replace the preamp tubes. If one of them caused the problem, the fuse would have popped as soon as you turned the amp back on.

I bought one of those amps because I could get it in tan/fawn (used), and I thought it was good looking. I replaced with power tubes with NEC, and left in the original Chinese preamp tubes and speaker. Nice sounding amp.
 

Adaktug

Member
Messages
49
I had a fuse blow in my vox ac10. My tech is retired but he told me to replace
The power tubes and fuse so I did and it works fine with jj el84. Should I also replace the 12ax7 as well. The tubes are ecc803 and one is a groove tube premium in v2 and other is jj in v 3The amp sounds fine but was thinking while it’s apart I should do it.

He said the power tube probably caused the fuse to blow so I replaced both of them.
In the future do not use the STANDBY just turn of the Amplifier.
DID you use STANDBY after repairs - don-t, Leave STANDBY ON LIGHT ON...
STANDBY ON , STANDBY LIGHT ON leave on. STANDBY- done wrong by Bugera and others...

Standby:
is done by lowering the filament voltage not using the HIGH voltage.
Filament voltage is dropped by 80% for short periods of time or turned OFF
===============================================================

"standby switch usually used to warm tubes before playing, it is
safer and for longer working." Bugera

NOPE ..........................Wrong
That is
1: STANDBY OFF the high voltage is turned off while the
filaments still heat up the cathode.
2: then when turned to : STANDBY ON, LIGHT ON :
High voltage is suddenly applied to the plate and screen Grid !!!

Well not a good idea - a bad idea -
For the cathode has a large electron cloud around it when
the filaments are on ... lowering tube life / cathode wears out.
The CATHODE determines TUBE LIFE.
This floating dense cloud of electrons are IMMEDIATELY
attracted to the Screen grid and some to the plate,
Screen Grid is closer to the cathode, so WHEN the high voltage
is applied an electron - AVALANCHE - condition occurs,
the control grid cannot stop it for the CLASS A1 amp design
has a minimum Control grid voltage setting.

WE have read many years of Bugera failures - from customers found
on Ultimate Guitar forums and other forums, with faults of:

1. seeing a blue flash in the tube
2. hearing a crackle-noise, buzzing noise
3. power on lights on, but no audio out
4. channel distortions
5. weak outputs
6. channels leaking over
7. on - on etc.

This STANDBY process actually stresses the circuits due to the
sudden application of the HIGH Voltage, a major power spike. !

It is time to stop using the STANDBY procedure for high voltage.

Newtune on-off ALL on-off
Tubes warm up slowly:in Seconds. 6L6 20-15 12AX7 15-18
on: max current Sovtek 35-27
off: less than 5 mA
-------------
The Screen resistor in Bugera is 680 Ohms other manufactures use 1,000 Ohms.
These lower value resistors allow far too much current through the
Screen Grid: should be 28k ohms to 58k ohms when 2.5 - 4W Watts is max Screen power.
Also a Screen BY-pass capacitor (2nF - 4nF) is not being used
this capacitor adds to amplification stability.

Best to turn off the Amplifier to cool off the tubes and filaments / Cathode,
the filaments control the cathode current so a slow cathode current
increase happens as the filaments warm up
SAFELY setting up the circuit voltages - gently.

CJ

Tube Engineer, US NAVY
USAF F16 RADAR - retired
ENTEL-McCULLOUGH / Varian
tube manufacturer.
 




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