tube amp, mismatched power, tube prob?

DanRJBrasil · #1 03-15-2010, 11:15 AM

I am about to buy a tube amp is a fender bassman copy point to point soldering, a very old one , is not working in the moment and I am don't want to take to a tech just yet because i don't have the money to service it at the moment.
The owner told me that till it has a fuse short the amp worked fine, as he didn't found the right fuse he let it go and never bother again, it has two 6l6 mismatched tubes, but the amp worked fine, till the bad fuse.
what kind of problems that can cause , that mismatch can fry the transformers?
thanks

Rick Auricchio · #2 03-15-2010, 11:16 AM

I would avoid it. You have no idea what might be wrong, no matter what the owner says.

**B-string** · #3 03-15-2010, 11:34 AM

One of the output tubes could have shorted, rectifier diodes shorting is the most common cause of the blown fuse. Mismatched tubes very rarely does any real damage to OT's, a shorted output tube can. It could be a find or a bitter pill!

jimbilly · #4 03-15-2010, 12:05 PM

I've never heard of 'fuse short', it sounds like the seller doesn't have a clue as to what's wrong with it and he's trying to blame it on a "bad" fuse, -odds are something fairly important shorted out causing the fuse to blow.
It needs to be cheap, and then you're going to have to spend some money to get it sorted out, it probably won't work as is, and would certainly not be reliable.

DanRJBrasil · #5 03-15-2010, 06:19 PM

the seller is a trusty friend and he stoped to use the amp when the fuse shorted and as he never found a new one ( or really cared to look for one) the amp was sitting on his basement, and he know that I like this kind of stuff he offered me the amp.
This week I 'll try to take to a good tech to see whats going on on this amp, well, just to look and give a budget at the moment

. If is just a blown tube or the caps , Ill try to service myself if not I will have to make a investment on it, and then make a offer, a point to point is not a amp to give a frown upon

the amp is a fender I think bassman (with reverb) copy from 67
it does not have the original case what is not the best thing

I tried to take it off from the sub case and it is jammed on it , I 'll have to destroy the wood sub case to take it out from it, I asked the seller and he didn't mind, so is a ok to destroy the wood sub case if need so.

DanRJBrasil · #6 03-16-2010, 04:08 PM

some pics
the tubes 6l6 2x rca 1x westinghouse
, 1x tung sol, I cannot identify the 12a's

the wood box case back of the amp

KramerBassFan · #7 03-16-2010, 04:12 PM

Well some of those tubes are worth some money, so you could recoop your losses if its a dud... but I love me a project... so thats what i'd do... fix it.

JustDavid · #8 03-16-2010, 04:16 PM

A "fuse short" is the normal condition for a fuse. Excessive current melts the inside conductor, it melts, & the fuse opens. Fuses can age & open of their own accord, but it's usually due to a defect in the circuit & the fuse serves to eliminate or minimize the damage beyond whatever is already broken.

Seems like you trust the guy, so let's keep going (normally "it works great except for the blown fuse" has me end a buying conversation). I'd take it into a good tube shop for a repair estimate, & be willing to invest a small amount of $$ for this.

Pilgrim · #9 03-16-2010, 06:02 PM

Replace fuse and try it out.

If that doesn't fix it, then the problem is not the fuse.

Step 2: find a tube tester and check all the tubes.

DanRJBrasil · #10 03-16-2010, 07:30 PM

I finally found a way to free the amp of the wooden case without destroying it,
The idea of trying the fuse first is very sound, someone knows where in the web I can find a cap for this fuse housing as the pic bellow, the prob at first look does not seems to be a bad cap as it looks that is has a cap job and none are leaking or dry they look perfect, if is just not the fuse or the cap prob a tube , I don't have a tester, but I 'll try to pay a visit to the tech and listing all that I have to do with this amp to get it and shape.

I didn't make my mind if I will or not go forward on this or not yet as I don't to expend as much as a fender 100 watt tube amp can cost even the seller giving me the old cabs and speakers with it as we had talked about.I 'll see if the cash is worth the trip

Does anyone knows how to bias this kind of amp ??

DanRJBrasil · #11 03-16-2010, 08:42 PM

Quote:

Originally Posted by JustDavid

A "fuse short" is the normal condition for a fuse. Excessive current melts the inside conductor, it melts, & the fuse opens. Fuses can age & open of their own accord, but it's usually due to a defect in the circuit & the fuse serves to eliminate or minimize the damage beyond whatever is already broken.

Seems like you trust the guy, so let's keep going (normally "it works great except for the blown fuse" has me end a buying conversation). I'd take it into a good tube shop for a repair estimate, & be willing to invest a small amount of $$ for this.

thats is a good advise , I trust him, he does not have a clue about this thing, I told him that I don't have the money now and I 'll have to take to a tech before anything and then I will decide if I'll keep or return it to him

DanRJBrasil · #12 03-16-2010, 08:53 PM

Quote:

Originally Posted by Pilgrim

Replace fuse and try it out.

If that doesn't fix it, then the problem is not the fuse.

Step 2: find a tube tester and check all the tubes.

I have to find a lid or a new housing for the fuse, maybe there's some wait to improvise a lid for the the fuse

I don't have a tester I'll take it to a tech to test it for me

Thanks a bunch you guys for the hints

**B-string** · #13 03-16-2010, 11:17 PM

Do I see a burn mark on the power supply PCB? Don't see the power supply diodes. BTW you will need a slo-blow fuse, you could probably get a fuse holder at Radio Shack.

T-Bird · #14 03-17-2010, 12:58 AM

Hi.

Cool find.

Fortunately for You and the tech, it's not PTP, just a regular turret board design. Even as simple circuit as bassman, PTP is total PITA to repair.

Is there white dust on the inside of the red (???) getter-flash tube or is it just the white background?

When powering up a probably malfunctioning tube amp, ALWAYS do it without tubes.

Be safe, and good luck, judging by Your questions an electric shock is a strong possibility.

Regards
Sam

DanRJBrasil · #15 03-17-2010, 02:56 AM

Thanks Sam
There's no white dust on the tubes maybe is just the background the pics are in 9.2 you can click on it to get a bigger one

I asked about the bias cause usually there some kind of pot that you adjust, I can't find any on here,

I dread eletric shocks if is not a simple task I rather not try to do at all, I don't wanna kill myself

Quote:

Originally Posted by T-Bird

Hi.

Cool find.

Fortunately for You and the tech, it's not PTP, just a regular turret board design. Even as simple circuit as bassman, PTP is total PITA to repair.

Is there white dust on the inside of the red (???) getter-flash tube or is it just the white background?

When powering up a probably malfunctioning tube amp, ALWAYS do it without tubes.

Be safe, and good luck, judging by Your questions an electric shock is a strong possibility.

Regards
Sam

**B-string** · #16 03-17-2010, 11:40 AM

If they fallowed Fender design the pot is a bias balance pot and bias can only be changed by changing a resistor. Since it is not a collectable amp I would have a tech install a second pot for bias adjustment or convert the balance pot to a bias adjust. Balance pots were used before tube matching to balance the tubes on output transformer taps.

KramerBassFan · #17 03-17-2010, 12:43 PM

IMHO part of the problem may be the rather dirty condition of the chassis, and maybe even shoddy wiring.

And can you read Italian?

The amp looks like its been out in salty air for QUITE awhile.

EDIT: It might be spanish.... i can't really tell.... its hard to make out a single word... aside from TREMENDAO...

i_got_a_mohawk · #18 03-17-2010, 01:20 PM

If you buy it, I'd stick some fresh tubes in, some of those power tubes are a bit leaky (the getter is oxidising).

DanRJBrasil · #19 03-20-2010, 12:44 PM

Quote:

Originally Posted by KramerBassFan

IMHO part of the problem may be the rather dirty condition of the chassis, and maybe even shoddy wiring.

And can you read Italian?

The amp looks like its been out in salty air for QUITE awhile.

EDIT: It might be spanish.... i can't really tell.... its hard to make out a single word... aside from TREMENDAO...

Actually is portuguese is a brazilian bassman copy

,
it looks like it HAS to get into a tech before anything , but I'll try to put a new fuse to see what happens.

Quote:

Originally Posted by B-string

If they fallowed Fender design the pot is a bias balance pot and bias can only be changed by changing a resistor. Since it is not a collectable amp I would have a tech install a second pot for bias adjustment or convert the balance pot to a bias adjust. Balance pots were used before tube matching to balance the tubes on output transformer taps.

I read about it, I agree with you , a pot for bias adjustment is a must mod

Quote:

Originally Posted by i_got_a_mohawk

If you buy it, I'd stick some fresh tubes in, some of those power tubes are a bit leaky (the getter is oxidising).

I would like to keep the RCA's but let see what happens after the visit to a tech. that stained in the glass of the RCA's are not oxidizing actually is a kind silver tint for the heat sink.
Maybe I'm wrong I don't know much about tubes.
Some pics of the RCA's

thank you guys for the input !!

MAMMOTHvolume · #20 03-20-2010, 01:59 PM

i'd definatly take it to a tech, homemade stuff is always very hit and miss sometimes you'll get someone who'll do a good job but most of the time there full on bodge jobs!