HT Fuse Blowing

[TIMBO]

Hi Guys,Doing a start up on the new build and can't get past flicking the standby sw. w/o blowing the HT fuse. With amp on and tubes in all are glowing nicely (5x 12AX7 and 2x EL84 and 2x 6V6) and PT not even warm. This is as far as i can get as the fuse blows before anything gives me a sign that there is a problem. The HT fuse is 500mA and main fuse 2A (not blowing)  Thanks

[kagliostro]

I'm not sure, are you saying that the amp work fine and acting the standby switch blow the fuse ?

are you using a Fast Blow fuse ?

If the amp is fine and there are no problems try to move the switch after the 47uf cap or try to use a DPST switch between transformer and rectifier


Kagliostro

[tubeswell]

And check that you don't have an accidental B+ short to ground somewhere.

[TIMBO]

I think it is a fast blow.I had a closer look at the fuse itself and it is rated at 250vac, so i am thinking it might need to be rated at a higher voltage or it is not coping with the inrush when standby is flicked. In other builds i've had the standby switch between the PT and the rectifier.Thanks

The main fuse is OK so i would think that there is no B+ short to ground at this stage.

[DummyLoad]

what are you driving? idle current? marshall used 500mA 250V fuses in thousands of amps shipped.  does it work at all; or does it blow fuse immediately after closing SB switch?

divide and conquer.

pull VVR and chain A-B-C out of ckt. by lifting anode of 1n4007. turn on SB sw. does it blow fuse? if no, find short in VVR or B-C chain. if yes, then bypass VVR ckt. lifting anode of 1n4007 and by running OT CT to 220K/47uF - clip leads are your friend here. does it still blow fuse? if yes look for wiring fault in B-C chain or D-E-F chain. make sure FET is not going into HF/RF osc. you should use a gate stopper for VVR FET - install a 1K-10K R from the wiper of 1M to gate of FET.

--DL

[TIMBO]

Thanks DummyLoad, I redid the standby sw. and put it between the PT and the rectifier . The VVR has a 100K R between the 1Meg pot and the gate just missed it on the schem. Will try the your ideas asap. Thanks

[TIMBO]

Thanks DummyLoad, I redid the standby sw. and put it between the PT and the rectifier . The VVR has a 100K R between the 1Meg pot and the gate just missed it on the schem. Will try the your ideas asap. Thanks

The amp is a new build and i have not get it past the standby switch blowing the HT fuse.

[kagliostro]

Hi TIMBO

I was looking to your power section

did you try before your tube selector or it is a new idea ?

don't you think it will be better to connect to ground the grids of unused tubes ?

why you want to use only a kind of tube at a time, haven't considered a mixing solution ?

something like in the rebel amps .................

http://www.el34world.com/Forum/index.php?action=dlattach;topic=11436.0;attach=22915

http://www.el34world.com/Forum/index.php?topic=11436.0

Kagliostro

[RicharD]

Lamp limiter please.

[Tone Junkie]

Had a similer problem once i run my B+ wires inder the board and changed were I put a stand off without looking under the board.
I had one wire with a little to much insulation stripped off just enough to touch my al standoff. couldnt figure it out till i got mad and took the board off, had homer slaps head moment. 
Bill

[TIMBO]

Hi guys,I think i have found the problem. I mounted the mosfet of the VVR on the chassis with just the silicon grease and was unaware that the back of the mosfet is part of its circuit and i should have had a insulating washer between it and the chassis.

Kagliostro,I am not wanting to blend the EL84/6V6's and i know that some have tried it and there has not been any feedback on how this turned out. The PT i have is a standard 18W and the HT is rated at 120mA and the circuit is running at close to the max with only one pair of power tubes. The tube selector sw. disconnects the power tubes cathode from ground so that only one pair of power tubes are used. This is a circuit that ToneJunkie found and he is happy with it , i've just added a few things.

I've run out of fuses for the moment so i'll keep you posted.Thanks

[Geezer]

I've run out of fuses

Lamp limiter please.

http://www.diyguitarist.com/PDF_Files/AmpCurrentLimiter.pdf

[jjasilli]

Lamp limiter will save fuses, especially in a dead short situation. Might also be worth investing in a 500mA circuit breaker to test new builds.  Alternatively:  as a test precaution or permanently:  a 10R 1/2W resistor between ea power tube cathode and ground will act as a fuse (and also as a bias sense resistor, but with the decimal point moved 1 space to the right); Or a 250mA fuse between ea power tube cathode & ground.  So you have some options while waiting for new 500mA fuses.

[kagliostro]

The PT i have is a standard 18W and the HT is rated at 120mA and the circuit is running at close to the max with only one pair of power tubes.

OK I understand, there is a technical reason for that choice

I am not wanting to blend the EL84/6V6's and i know that some have tried it and there has not been any feedback on how this turned out.

this is from Plexi50 on this tread http://www.el34world.com/Forum/index.php?topic=11436.0

I got the dual gang 1M Mix pots wired up and it WORKS!!!  
......
The mix pots work well and does really define the amps tone between the 6V6 & EL84 tubes very nicely

I've read about this mix solution also other times, but can't give you links

Kagliostro

[TIMBO]

Thanks for all your good advice.Yes it was the VVR mosfet and it may be TOAST  I am not getting any variance when the pot is turned and at the moment it is giving full power to the power amp section, any ideas on how to check on the condition of the mosfet or other parts of the VVR circuit. Thanks

[kagliostro]

any ideas on how to check on the condition of the mosfet

Try reading here

http://www.4qdtec.com/mostest.html

and/or look here

Testing a MOSFET

Testing Mosfets

Kagliostro

[sluckey]

Yes it was the VVR mosfet and it may be TOAST 

I'd say there's a 99% chance that the source is shorted to the drain. Use your ohm meter to check.