Fuse And tube question

[schenr]

Hi all, my bogner alchemist blew a fuse yesterday due to a faulty power tube, so I took it into the shop I bought it from, and the salesman swapped the power tubes (from Ruby 6l6 to Svetlana 6l6gc) and changed the fuse (from the stock ht 500mA to an ht 625mA).
I realize the amp should be rebiased but the guy was so confident that it was ok, I was wasting my breath arguing with him.
So... 2 questions:
1) does anyone know off the top of their heads the bias settings for the rubys and svetlanas? And that if I left them un-rebiased are the bias settings close enough so as to not bother rebiasing?
2) is it bad that he replaced the 500mA fuse with a 625mA fuse. I understand that it should be ok if the rating is higher, but if a situation arises where the 500mA would be blown to save the electronics of the amp, would the 625mA one blow too?
Sorry for the long winded post and thanks for any help

[Twinfan]

Simple answers from me:

a)  He should have replaced the fuse with an identical one
b)  Valves differ within a brand, let alone across brands
c)  Your "salesman" isn't an amp tech.  Find one to check the bias of your amp.

[jpfamps]

+1.

The Alchemist is fixed bias so the bias should be checked when you change power valves to ensure optimum performance.

More luck than judgement you may find that the new valves are biased appropriately, however they may also either be biased to hot (will shorten their life and potentially imperil the amp), or too cold (will not sound as good as optimally biased).

The fuse should replaced with the correct value, UNLESS there is a very good reason not too. The only good reason would be that the originally specified fuse was too small and the amp experienced blown fuses when there was no other component failure. I doubt that this is the case with your amp; in fact it looks like the fuse did it's job and blew when a valve failed.

[schenr]

Cheers for the feedback. I'm definately going to source the correct tube, but listening to how the tubes sound now, I recon it sounds better than the stock tubes, and there's no excessive heat coming off the back. But just to be safe I'm taking it to a tech today.
I'm quite frustrated at the fact that the salesman just assumed that everything would be sweet by changing things around.

[JacksonRR]

I understand your frustration, both with the new amp and the salesman. Since it has been in your possession for a very short time I would say they are obligated to pay an actual tech to bias your new valves if they refuse to complete a proper valve change. I would bring it back in and not leave until my amp was biased or I reached an agreement that resulted in my amp being biased free of charge by a reputable tech. If you still able to return the amplifier, like you have 30 days or something, then you are on the high-ground. The music store doesn't want to buy back your amp.

[schenr]

So I called up a tech who said change the fuse back to the original specs and then see how the tubes cope with the amp.
So i got the correct fuse for 1 dollar and turned on the amp. All sounds good, but the plates in the tube were glowing wayyyyyy too much and there was quite a bit of heat coming off the back of the amp.
So i called up the music store and they said theyre more than happy to exchange it for the demo model (its the last one they have) which i initially tried, and we know has no problems with it whatsoever.
So I'm happy they've shown the initiative to rectify things. Hopefully they throw in a few freebies for making me run around town so much!

[Twinfan]

They're swapping a new amp for the ex-demo one?  Not acceptable to me.  You should get a new one.

But to be honest, all they need to do is get a tech to bias yours at the cost of an hour's labour!

[JacksonRR]

Were all tubes getting to much power to the heaters or just one half?

[HTH AMPS]

They're swapping a new amp for the ex-demo one?  Not acceptable to me.  You should get a new one.

But to be honest, all they need to do is get a tech to bias yours at the cost of an hour's labour!

^ +1

you PAID for a new amp, not an ex-demo one.

tell them this and ask for cash back, an ex-demo amp should NOT cost the same as a boxed-new amp.

[Dmoney]

Were all tubes getting to much power to the heaters or just one half?

The bias won't change the heater voltage.
That is extremely likely to be the same unless there is something REALLY wrong.
Red Plating is caused by other issues.

[dave_mc]

^ +1

you PAID for a new amp, not an ex-demo one.

tell them this and ask for cash back, an ex-demo amp should NOT cost the same as a boxed-new amp.

also +1

[JacksonRR]

Were all tubes getting to much power to the heaters or just one half?

The bias won't change the heater voltage.
That is extremely likely to be the same unless there is something REALLY wrong.
Red Plating is caused by other issues.

I read it too fast and answered too fast, lol. He said the PLATES, haha.
But yeah, if the valves are biased way too hot, they could be fine for low volumes, but red plate real fast when they receive higher current from the phase inverter, which in this case could have the ability to drive the grid to 0 volts. But I'm nowhere close to that amp so I can only speculate and ask about it.

[schenr]

When I went in today they miraculously "found" a brand new one in their store room. And just to make sure they didnt just take the demo one and put it in a box, I had a look to see if the demo was still there (which it was).

So, lesson of the day: hope to hell your amp stuffs up within the warranty period, or you're screwed

[Frank]

the plates in the tube were glowing wayyyyyy too much and there was quite a bit of heat coming off the back of the amp.

The plates of the tubes shouldn't glow at all, only the heaters. Red glowing plates are a sure sign that the amp is badly underbiased. Running an amp with glowing red plates is asking for trouble, even if it sounds good to you.

The anode plates have a maximum rating for the amount of power they can dissipate as heat. Biasing is done to control the amount of current flowing through the output tubes. If the bias is too low (not sufficiently negative) then excess current will flow through the tube, the plates start to glow red hot, the tube can no longer dissipate the heat and it usually responds by failing.

The fact that the idiot salesman had to replace the fuse with a higher rated one after swapping the tubes comes as no surprise. Clearly the amp needed more bias with the new tubes and as a result was drawing far more current. His solution - swapping the fuse for a higher rated one - could have wrecked the tubes and the output transformer.

To be honest, I'd phone his manager and get him fired before he pulls that stunt on someone else.

[schenr]

To be honest, I'd phone his manager and get him fired before he pulls that stunt on someone else.

I must admit, it is quite surprising that someone of that age (the guy must have been in his 50s), who has been working with guitars for a long while (this is an assumption due to the fact that he's 50 and working as a guitar salesman), would do such a thing. Even I wouldn't recommend a customer of mine to do that, and I'm no expert on tubes or tube amps in general.
That said, I'm just happy I have a brand spanking new amp that works perfectly (for now).

But with regards to talking to the manager, I always feel terrible putting in a bad word about someone, even if they did screw up majorly, I'm just not a fan of confrontation (despite the fact I work at a liquor store and have to confront thieves trying to steal from us day in day out). So all I can hope for is that he's learned his lesson somehow.

Now on to my next project of finding some new preamp tubes that make the amp sound even more awesome than it does now!