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Post subject: Re: Tube swapping/substitutions?
Posted: Tue Apr 22, 2014 9:58 am
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BMW2002Ti wrote:
swerve, use either two matched 5881's or two matched 6L6GC's. Try not to mix a 5881 with a 6L6GC in the same amp. Adjust the bias appropriately.


+1

As for holding your costs to $100 for both amps, I think you'll be well served with Electro-Harmonix's 6L6EH -- plentiful, durable, affordable, and they sound good. Buy them as matched pairs or as a matched quartet and split the latter up.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Aug 31, 2014 7:28 am
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Ok, I've been a little busy lately and haven't had the time to get my Bassman back up & running. Just been playing the Bandmaster. I know, poor me...

So last week I took the Bassman in to have new power tubes put in & biased.

Since the amp tech is an independent contractor and only comes in after hours, I don't have the luxury of actually speaking to him. When I dropped the head off, I stated that I had broken a bottle and needed new power tubes and a bias job, de-ox the pots, and just a routine inspection.

I also brought the remaining 6L6-GC with me that was still good with maybe 10 hours on it. I asked if they could find a match and re-use the fairly new tube and was quickly rebuffed. "We don't have any way to check it. There's no way to match a tube, there's too many variables."

I said, "You don't have the equipment?"

"Nope, and you're talking to a guitar guy anyway. I know nothing about amps. I can write it up and you should have it back some time next week."

So I walked out thinking, 'Sounds like bullshit to me.' Here I now have a totally useless tube? That seems wasteful.

So I said, "So this tube is totally useless now?" He says, "No, I'd keep it for a spare."

A spare what? If it cant be matched to anything, what the hell am I gonna do with it? Anyway....

A week goes by and I get a call from the same 'guitar guy' who wrote up the amp saying, "Hey, your Bassman is ready to be picked up. We put new power tubes in it and we noticed you had a bunch of mis-matched pre-amp tubes in it, so we put all new matched pre-amp tubes in it. The price is 165. for it with the new pre amp tubes or 115 for just the new power tubes."

I told them I had been experimenting with pre amp tubes when I broke the 6L6 bottle and the tubes that were in there were sounding pretty good to me before I broke the 6L6. I had 3 7025's with the little u.s.a. stamp over the octagon (which from reading Beemer's post, seems to be old RCA's), and an old Sylvania yellow label 12AX7 with a fairly long plate.

Did these guys totally bullshit me and rape me for an extra 50 bucks?

I got my old tubes back but WTH?


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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Aug 31, 2014 7:59 am
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Not sure what the tech meant by mis-matched 12AX7 and 12AT7? Maybe, just the manufacturer? What brands did he put into the amp?

You can match output tubes (6L6GC) by measuring the current draw of the amp at idle. At a set plate voltage. Then, find the closest mA results amongst the tubes tested. At least, that's how I do it.


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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Aug 31, 2014 8:42 am
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Well since I never actually get to speak with the elusive amp tech, I get all my info second hand from "the guitar guy", whom is much less than helpful. (You know the kind of guy at the music store who is belittling and has seen everything and is impressed by nothing. Kind of guy who makes you feel like everything you own is $@!&). I digress.

They put wall-to-wall JJ's in it.

The only thing mismatched were the fact that there were those RCA's mixed with a Sylvania. Whether or not the amp tech actually DID have a tester and got wildly fluctuating readings from the all the 7025/12ax7's, I don't know. Maybe that's what he meant by "mismatched".

On the visit to drop it off the first time, I even mentioned to the guitar guy who told me there was no way to match power tubes, "Aren't they measured at idle by Milliamp & plate voltage?"

Blank stare, attitude and an quick tear off of the repair ticket. "Here ya go, We'll give you a call when it's ready."


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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Aug 31, 2014 2:41 pm
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JJ 12AX7/ECC83 are good tubes. Never used their 12AT7 or 12AU7. However, the old RCA 7025 will come closest to the "Fender tone." As the original amps were fine tuned to these OEM tubes.

As you prolly know by now, there aren't a lot of knowledgeable, unbiased amp techs out there. And even less sales-people behind counters.


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Post subject: Re: Tube swapping/substitutions?
Posted: Mon Sep 01, 2014 5:18 am
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Get yourself a decent bias probe to adjust bias yourself.........and start collecting preamp tubes.


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