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Post subject: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 8:17 am
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I'm searching the forum for information on the various nomenclature given to various tubes and I'm pretty confused.

My issue started with trying to get a '64 blonde 6G6-b up & running.

The original tube chart was/is peeled off but the schematic I found said it takes (2) 5881 tubes and (4) 7025 tubes.

When I got it, there were no power tubes in it and 4 mismatched 7025 tubes.

The tech installed (2) 6L6-GC power tubes and left the 7025's in it at my request until I could decipher what they were and if they needed to be changed.

So it seems that 5881 tubes are no longer in favor and 6L6-?? whatever can be substituted for them. Good or bad I'm not sure.

Same thing for the 7025 tubes. It seems everything I read says 7025 tubes are increasingly hard to find (at a decent price) and instead are being replaced with a 12ax7 "type" tube.

I'm not a smart man, but I can read enough to know that a 12ax7 and a 7025 have different characteristics and they were made that way for a reason.

So bottom line for me is what the heck is recommended to go in there? I talked to 1 guy who said, "you gotta remember when these amps were built and heavily in use, back then, guys would just jam anything into the socket that would fit."

When replacing one, should I replace them all? Can they be mixed & matched? (7025's and 12ax7, 12at7,12au7?

Thanks


I'm not even talking about brand names or any of that yet, that's a whole-nuther animal.


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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 8:40 am
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5881s are functionally equivalent to 6L6GWB/6L6GC tubes. They were originally built as a heavy duty, Mil spec tube. A 6L6GC is a 30 watt tube while a 5881/6L6GWB is a 23 or 25 watt tube (depending on who you talk to). Any of the three can be used if properly biased, although the modern 6L6GC will handle higher plate voltages than a 5881/6L6GWB.

http://www.kcanostubes.com/catalog/28

http://www.kcanostubes.com/catalog/5

7025s are low noise, high performance versions of 12AX7s. 12AX7s can be used in place of 7025s with no issues.

http://www.kcanostubes.com/catalog/4

12AT7s/12AU7s are different from 12AX7s with lower gain and may be used for different functions. They can sometimes be swapped for 12AX7s/7025s but not always.

You can use modern 6L6GCs and 12AX7s in your amp without issue if you don't want to pay the price for vintage tubes.

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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 10:01 am
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swerve wrote:
......and 4 mismatched 7025 tubes.


"Mismatched" in what manner? Brand? Date?

Pre-amp tubes do not require strict performance matching as do the bottles in a push-pull output stage. The only caveat would be the 7025 used in the V4 phase-inverter position which ideally should have balanced transconductance between the two halves. Absent a demonstrable reason to replace, I'd leave those 7025's in situ and concentrate the servicing efforts into ensuring a stable power supply and a correctly-biased pair of 6L6's (or 5881's if you can find a set you prefer). Follow bluesky's recommendations and you should have a beautiful sounding Bassman.

Arjay

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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 2:48 pm
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So with the 6L6-gc being a 30 watt and the 5881 being more in the 25 watt range, it sounds to me that I would be driving a 5881 harder to achieve the same relative volume as a 6L6-gc? And as a result wouldn't that alter the headroom/break-up characteristics and tone?

As for the mismatched 7025's, (2) of them had "7025" in white and "u.s.a." stamped on them.
One of them has no markings at all and the last one says "Made in Hungary"

I don't have any other info on them and so was curious as to how large a part they play in tone.
It's not that it sounds bad, I just want to explore the options and alternatives.
Thanks for sharing the knowledge.


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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 3:05 pm
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swerve wrote:
So with the 6L6-gc being a 30 watt and the 5881 being more in the 25 watt range, it sounds to me that I would be driving a 5881 harder to achieve the same relative volume as a 6L6-gc? And as a result wouldn't that alter the headroom/break-up characteristics and tone?

As for the mismatched 7025's, (2) of them had "7025" in white and "u.s.a." stamped on them.
One of them has no markings at all and the last one says "Made in Hungary"

I don't have any other info on them and so was curious as to how large a part they play in tone.
It's not that it sounds bad, I just want to explore the options and alternatives.
Thanks for sharing the knowledge.


The difference in output volume between a 30 watt and 25 watt tube is inconsequential. The 5881 may break up before the 6L6GC due to different bias requirements.

Hard to say what the other two tubes are without testing them to determine their electrical characteristics. If you post a photo of them, someone here may be able to tell you what they really are. All of the tubes play a large part in the amp tone.

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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 6:37 pm
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bluesky636 wrote:
All of the tubes play a large part in the amp tone.


+1

And in the case of your specific Bassman, it really is using all of the tubes. The 6G6 (and sub-variants) are unique among Leo's designs insofar as every tube is utilized to the maximum extent possible. The bass instrument channel features four gain stages (vs three in an AB864, AA165, AB165, and follow-on revisions). It's the ballsiest-sounding Fender Bassman ever made. And Bill is correct in that you'd be hard-pressed to detect a five or even ten watt difference between output-tube types. Comparing vintage bottles only, the 5881 is likely to compress and saturate more quickly than a 6L6GC, which will enrich the harmonic content when the amp is pushed into overdrive. Either tube will sound great though so long as the idle bias is optimized for them.

Arjay

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Post subject: Re: Tube swapping/substitutions?
Posted: Sun Apr 06, 2014 7:11 pm
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swerve wrote:
So with the 6L6-gc being a 30 watt and the 5881 being more in the 25 watt range, it sounds to me that I would be driving a 5881 harder to achieve the same relative volume as a 6L6-gc? And as a result wouldn't that alter the headroom/break-up characteristics and tone?

As for the mismatched 7025's, (2) of them had "7025" in white and "u.s.a." stamped on them.
One of them has no markings at all and the last one says "Made in Hungary"

I don't have any other info on them and so was curious as to how large a part they play in tone.
It's not that it sounds bad, I just want to explore the options and alternatives.
Thanks for sharing the knowledge.



Welcome to the Forum! Who made those 5881's? Are they "Made in USA" Tungsols? These tubes are often re-labeled RCA, Raytheon, GE, etc. Also 6L6WGB. Most are Tungsol made. These are very good tubes. Just bias the amp to about 60-65% of the 23-25 watt max and you'll be ok. 5881's have a different tone than 6L6GC. They compress more than the usual 6LGC at high volumes. I doubt that you'll hear any distinct volume differences between a healthy, strong testing 5881 and the 6L6GC. Tonal differences... yes. Overall volume differences... no.

As for the 7025. They have specs identical to the 12AX7. The original US made ones (and ONLY the original US labeled 7025) had spiral wound heater line within the tube. These tubes help to reduce noise by cancelling some of the hum, similar to how wrapping external AC heater lines in the amp helps to cancel out hum. The spiral wound filament cancels the hum induction within each tube. There are several new made "7025" labeled tubes. None have this wound spiral heater line. Only the Sovtek 12AX7LPS has this arrangement with the heater line. The other 12AX7 type that has spiral wound heater line are US made, old-stock 12AX7A mil spec tubes. The 1950-60's Raytheon 12AX7A being a highly sought after tube. For both quiet use and tone.

All 12AX7 subtypes (including the famous US made 5751) are interchangeable. The Sovtek 12AX7LPS is a good tube. But must be selected for minimum microphony and low hiss. Tube hiss is caused by plate and grid issues and not hum from the heater line.

Now, that Made in Hungary tube is prolly a Tungsram. One of the finest 12AX7's ever made. Good NOS Tungsrams are harder to find than NOS RCA 7025. The Tungsram does not have a spiral wound heater filament. What most of them do have is this very distinct overdrive tone. Also, ALL Tungsram signal tubes had a small aluminum plate attached to the top mica disc, inside the tube. This aluminum plate has a two-digit number or a number and letter stamped onto it. This signifies the partial lot number of date of manufacturing. See the small plate in the tube on the right in the photo?

Tungsram ECC83/12AX7 have an aggressive and dark tone. Lots of "bite" to the harmonics. I have a few 1970-80's made Tungsram ECC83 (Euro designation for the 12AX7). They are waiting for good SE amp rebuilds. In SE amps (like the Champ), the Tungsram ECC83/12AX7 gives a unique, powerful tone to the amp. Many of these SE amps tend to be a bit too much midrange warmth. The Tungsram adds a mid-high "edge" to the OD tone. Results in a smooth and aggressive overall tone.

I can see why someone put it into a 6G6-B Bassman. As transition Fender amps (Brownface and Blondes) tend to have a tone between the Tweeds and the Blackfaces. A Tungsram ECC83/12AX7 would give this section of the amp, it's plugged into, more of a Blackface tone. All other parts being somewhat equal.

Enjoy the amp! Pictures would be nice.

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Post subject: Re: Tube swapping/substitutions?
Posted: Mon Apr 07, 2014 1:57 pm
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I also have a '65 bandmaster that I'm going through the same process with.
I pulled out the tubes that are in where the 7025's are supposed to be and there are (2) more of the Tungsram ECC83's in there with the yellow print, 'Made in Hungary"

So I put them in to the Bassman. making (3) of those Tungsram ECC83's and (1) of the "USA" 7025's in the pre-amp section.

I think I can stop thinking and just play.

Thanks for the information, you guys know your Shiite.

P.s. WTF is a "getter" and what does that have to do with anything?


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Post subject: Re: Tube swapping/substitutions?
Posted: Mon Apr 07, 2014 3:08 pm
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A getter is a ring, cup, or square wire, usually at the top or bottom of the tube. It holds a metal that is "blasted" by heat or electricity onto the inside of the glass envelope, during the final stages of tube assembly. The deposit is the shiny metal deposit you see at the top, bottom, or sides of the tube. This blast absorbs any oxygen left in the tube after a mechanical pump draws out most of the air.

Ppl use the shape and position of the getter to ID subtypes of tubes. And the era when the tube was made.

http://en.wikipedia.org/wiki/Getter


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Post subject: Re: Tube swapping/substitutions?
Posted: Mon Apr 07, 2014 4:15 pm
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Thank you, I've got alot of reading to do but you've helped a great deal.


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Post subject: Re: Tube swapping/substitutions?
Posted: Mon Apr 07, 2014 9:41 pm
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swerve wrote:
I also have a '65 bandmaster that I'm going through the same process with.
I pulled out the tubes that are in where the 7025's are supposed to be and there are (2) more of the Tungsram ECC83's in there with the yellow print, 'Made in Hungary"

So I put them in to the Bassman. making (3) of those Tungsram ECC83's and (1) of the "USA" 7025's in the pre-amp section.

I think I can stop thinking and just play.

Thanks for the information, you guys know your Shiite.

P.s. WTF is a "getter" and what does that have to do with anything?


Just remember that you need a 12AT7 in the Bandmaster's V4 socket.

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: Tue Apr 08, 2014 5:20 am
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I think I get it. The bias setting is the key.

This is what I was going to go with but that Sylvania tube is also marked 7025.

Have I messed anything up by running that 7025 in the V4 position?



https://lh5.googleusercontent.com/-Sxdy ... /photo.JPG


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Post subject: Re: Tube swapping/substitutions?
Posted: Tue Apr 08, 2014 8:23 am
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No problem, in the 6G6-B Bassman, using a 7025 or 12AX7A in V4. This amp with equipped from the factory with four 7025 (V1 through V4). And two 5881 output tubes (V5 and V6).

BTW... this Bassman has a non-adjustable bias supply. To alter the idle bias of the output tubes, you'd need to juggle the values of either the 1k-ohm, 1-watt dropping resistor. Or the 27k-ohm, 1-watt circuit to ground resistor. I usually change the 27k-ohm one, since this gives you a greater range of values to choose from.

Going to a small resistor value (like 22k-ohm) = hotter idle bias. Going to a larger value (like 33k-ohm) = colder idle bias.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf

_ _ _ _ _


If you are talking about the 1965 Bandmaster. The tube closest to the 6L6GC output tubes (the phase inverter) should be a 12AT7. The Bandmaster has an adjustable bias pot.

http://bmamps.com/Schematics/fender/ban ... 63_sch.pdf


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Post subject: Re: Tube swapping/substitutions?
Posted: Tue Apr 22, 2014 7:14 am
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So...
now that I have to re-power tube both my amps. what would the preference be for each?
The Bassman calls for 2 5881's and the Bandmaster calls for 2 6L6-GC's.

I realize that I could use 6L6-GC's in both and may end up doing just that but I was curious what your suggestions might be.

Lets keep a budget under $100 total for all 4 tubes.

Are single tubes of any use? I occasionally run in to a single 5881 or 6L6 but I hear so much about "matched" sets I don't know if I can do anything with them since I don't have a tester.


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Post subject: Re: Tube swapping/substitutions?
Posted: Tue Apr 22, 2014 7:21 am
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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.


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