Fender Bassman 100T Chapter 3

[GG7]

I also noticed the active channel has a bit lower gain. That really doesn't mean anything I don't think. Different preamp designs, different gains. Don't remember mine being quite as radical as 2 notches on the gain. Seemed more like 4 on the passive channel equaled 5 on the active channel. I'll recheck and let you know how mine works.
1/2 of 12AX7 V1 is the first gain stage for both channels (Brings the instrument level up to line level). The second half of V1 is used in the Bright circuit (pull treble eq knob) for the vintage channel. Both halves of V2 are used in the active channel's overdrive circuit.

Have you tested your preamp tubes to see how balanced your T1 and T2 is on them and how the V1's triodes compare to the V2's?

The gain on my two channels are pretty much the same volume, with the active side actually being the side that is a bit hotter than the vintage side with everything set flat.

My AX7s are about as close to equal GM on T1 and T2 as you can realistically get. One of the criteria for choosing the tubes I ended up going with was how even the T1 and T2 was on the AX7s and especially the AT7. I also picked two AX7s of the same brand that tested close to the same levels as each other. That could be whats behind your different channel volumes. The triodes on the GT AX7s that came in my amp were definitely NOT the same on both sides, and definitely not the same between the two tubes themselves.

 

[GG7]

Thought I would post some info about tube testers in this thread for those who may be considering one.

Having a tube tester for checking your tubes is a must have in my opinion. While you don't have to spend the big bucks on the top end models like I use, I would at least suggest picking up a little B&K or Sencore Emissions Tester, so you can at least see if a tube is going bad, or a new tube is clean and free of shorts and whatnot that can ruin your day in a hurry!

Here are the vintage tube testers I use for checking all my tube stock before I actually put anything in one of my amps. In addition to these two, I also own a several Hickoks including a 539B, two Weston 981s and a Heathkit TT1A. Basically, all the testers I bought over the years as I moved up towards the 3444. They all still work great even though the youngest of the Hickoks was made in 1966!

The undisputed holy grail of portable tube testers is the Triplett 3444 introduced in 1959. This tester runs the tubes at very close to the actual plate voltage the 100t runs them at, so it is very effective at detecting shorts, gas and cathode and heater abnormalities that don't always show up at typical tube tester testing voltages. There has never been a tube that this unit has identified as bad that was not ready for the ash can when tested in my guniea pig tube amp.

TRIPLETT 3444 (1959 Production)

TV-7 B/U Military Tube Tester (1957 by Foray Industries)

I use this tester as s second opinion, mainly because these are very common with vintage tube sellers in the NOS tube resale market, and buyers tend to be more familiar and comfortable with the % test results than they would be with those from the 3444. Although the Triplett is accurate to +/- 5% of actual GM, and the TV-7 is double that at +/- 10% of actual GM.

Testing a vintage 12AX7

This unit was made in 1957 by Foray Industries, which is most commonly associated with the D/U series produced from 1958 until 1982. Only a thousand of these Forway B/U models were produced before production was shifted over to D/U production, so this tester is considered highly collectible.

I got it for $25.00 from a surplus electronics store in Fresno because it was dead as a door nail and wasn't powering up. It looked to be in great shape inside and out, so I took a chance that it could be fixed. I sent it down to Dan Nelson who is considered the TV-7 restoration Guru, and he had it restored and calibrated in no time! The restoration cost me $260.00, which is nothing considered that the unit now is worth close to 2 grand!

One of the cool things about this tester is that it has all porcelan switch plates and sockets.

TV-7 Interior

When I saw how clean this thing was inside, I knew it could be saved! to think, if I hadn't found this, it could have wound up at the dump and lost forever like so many of them were!

And it also has the super fast responding Phastron output meter!

Testing the GE 6L6 GC
If the tube passes all the rigorous short tests on the 3444, it moves on to the TV-7 for a comparison test of GM.

Excellent result!

Anything over 41 on this tester is considered new old stock results. This one is well over that number. Each notch on that meter scale counts for "2" so the result for this tube would be 47. The average I get for a brand new GE 6L6 GC is 45, so this one would make a great 100t tube!

So, these are the tools I am using when testing my amplifier tubes, and making comparisons of current production Russian product and the NOS vintage stuff from the US and Europe made back in the golden age of vacuum tube technology!

 

[GG7]

The first track from my new band is online.

In the studio, I used my Spector 435LX (strung to C standard tuning) into 2 channels: A clean signal through and Avalon U5 and we mic'd my 100T through a vintage Ampeg 810. I used the overdrive channel with a fair amount of grit dialed up. This was before I switched to the Winged C power tubes, so it was completely stock.

http://chewbacchus.bandcamp.com/track/asphyxiation

Hope you like it.

Cool! Really good recording quality there! Sounded nice and full on my laptop speakers which is saying something, since few things do! I could hear everything in the mix, and the bass sounded nice and fat.

Nice work!

 

[rodl2005]

Great post GG7 thanks!!!

 

[NOLA Bass]

Cool! Really good recording quality there! Sounded nice and full on my laptop speakers which is saying something, since few things do! I could hear everything in the mix, and the bass sounded nice and fat.

Nice work!

Thanks very much for the kind words and all that killer info on the tube testing. However, now you have me wanting to get a whole new piece of gear I'd never given a thought too.

 

[GG7]

Thanks very much for the kind words and all that killer info on the tube testing. However, now you have me wanting to get a whole new piece of gear I'd never given a thought too.

You're welcome! I don't really listen to that style of music much these days, but I was a huge Black Sabbath fan when I was a kid, and in the 80s I really enjoyed Metallica! So, I can appreciate what you guys are doing with your material, and I liked what I heard on that track.

Regarding the tube stuff...

Thought you guys would appreciate seeing what a serious NOS tube collector uses to test his amp tubes before he installs them in an amp, or sells them to other musicians or Hi-fi buffs.

Speaking of which... I have decided to sell off a portion of my large tube collection later this year on eBay. While I am not ready to start doing that yet, I would consider a few limited sales to folks here on the TB forums ahead of my official eBay store offering. So anyone interested should PM me and I can give you an idea of what kind of $$ we are talking about. These will of course be more expensive than a full 100t set composed of new Russian tubes, which I have seen using Winged "C" power tubes and reissue Mullards going for around $180.00 I can actually get close to that price using a quad of Winged "C" 6L6s but with vintage GE, Sylvania, Jan Philips or Raytheon 12AX7s and either a Sylvania or GE AT7.

A full NOS set will be more than that of course.

What I would call my KILLER set would consist of FOUR GE 6L6 GC power tubes, 50s or 60s vintage Amperex, Mullard or RCA 12AX7s, and an RCA 12AT7. All performance matched and with balanced triodes on all the preamp tubes.

Anything I sell will be performance matched and double tested in my Triplett 3444 and TV-7. One major advantage of buying any set from me in addition to me owning the same amp and having tested most of these brands in this amp for tonal characteristics is that I will be setting you up with a closely balanced AT7 tube, which is very important. People always talk about matching the T1 and T2 of a 12AX7 for V1 and V2, but in reality, the AT7 in the Phase Inverter position is really the one that benefits from balanced triodes the most!

If you do PM me, be sure to let me know what kind of music you play, what kind of basses you use with your amp, and what kind of playing environment your amp is used in most of the time. Some of these vintage tubes are more finicky than others about operating temps etc. so all this info would help me steer you towards the best brand combo for what you are looking to do with your 100t!

 

[Freight Train]

Have you tested your preamp tubes to see how balanced your T1 and T2 is on them and how the V1's triodes compare to the V2's?

The gain on my two channels are pretty much the same volume, with the active side actually being the side that is a bit hotter than the vintage side with everything set flat.

My AX7s are about as close to equal GM on T1 and T2 as you can realistically get. One of the criteria for choosing the tubes I ended up going with was how even the T1 and T2 was on the AX7s and especially the AT7. I also picked two AX7s of the same brand that tested close to the same levels as each other. That could be whats behind your different channel volumes. The triodes on the GT AX7s that came in my amp were definitely NOT the same on both sides, and definitely not the same between the two tubes themselves.

Yeah, now that I look back on the tests I did with the cab I see that I measured the active channel to be a notch hotter gain. 4 on the passive channel equaled 3 on the active channel gain. I'm gong to check mine again after this weekend and compare it to the old results to see if anything's changed.

 

[GG7]

Yeah, now that I look back on the tests I did with the cab I see that I measured the active channel to be a notch hotter gain. 4 on the passive channel equaled 3 on the active channel gain. I'm gong to check mine again after this weekend and compare it to the old results to see if anything's changed.

Sounds like a plan. If your factory GT pre tubes are anything like mine were when I tested them on the 3444, there is a very good chance that they are not balanced with themselves or each other.

At least you know that a retube with a known balanced pair of AX7s would likely even things up.

 

[Freight Train]

Sounds like a plan. If your factory GT pre tubes are anything like mine were when I tested them on the 3444, there is a very good chance that they are not balanced with themselves or each other.

At least you know that a retube with a known balanced pair of AX7s would likely even things up.

Matched 12AX7's will have nothing to do with the gain of the channels. V1 provides the gain for BOTH channels. If the blend control on the active channel is at 1, then V2 is not even in the picture. V2 is strictly the overdrive tube in the active channel. When blend is on 1, the signal does not pass through V2. The fact that the channel gains may be somewhat off means nothing. They are completely different circuit designs so there's no reason to expect they would have exactly the same gain. One possible cause of the active channel having a slightly lower gain is that after V1, which again is the preamp tube for BOTH channels, there is a buffer in the leg that feeds the active channel, as that leg is split to feed the DI when in "pre" mode. That buffer could very well account for the -1dB loss. The vintage channel is fed directly from V! with no buffer in that leg. But regardless, the fact that they exhibit different gains means nothing.

 

[Tim1]

Well, despite some of this technical talk being over my head, I am delighted that we finally seem to be over all the autobias stressing. Hopefully those heads with issues have been replaced or fixed by now and Fender have fixed the problem in manufacture. We can focus on the (many) positives again. Yay! Off to band practice shortly with the 100T and Berg BV115

 

[UpwardGroove]

Off to band practice shortly with the 100T and Berg BV115

Did you ever get around to posting a picture of this rig?

 

[JimmyM]

Would love a tube tester, but I have learned to rely on my ears for it instead. I can always hear when a tube has bought the farm, and I just swap tubes or tap them with a stick till I find a culprit. if I don't hear anything wrong, I just leave it.

 

[60bass]

Would love a tube tester, but I have learned to rely on my ears for it instead. I can always hear when a tube has bought the farm, and I just swap tubes or tap them with a stick till I find a culprit. if I don't hear anything wrong, I just leave it.

:thumbup: back when I gigged regularly with Showman heads (wish I still had one). I always had a spare set of tubes, fuses, and a "chopstick" tube tester

 

[Tim1]

Here you are, Upward Groove. A bit of bass porn for ya. 100T sitting on Bergantino NV115, Super Bassman 300 on Berg NV215 and Bassman 610 neo all alone. Custom Shop 59 Precision, 68 Jazz and Sadowsky Metro MV4. I could have sat my backup Markbass on the 610, but somehow it just didn't feel right . As you can see, the 100T sits comfortably on the NV115.

 

[UpwardGroove]

Here you are, Upward Groove. A bit of bass porn for ya. 100T sitting on Bergantino NV115, Super Bassman 300 on Berg NV215 and Bassman 610 neo all alone. Custom Shop 59 Precision, 68 Jazz and Sadowsky Metro MV4. I could have sat my backup Markbass on the 610, but somehow it just didn't feel right . As you can see, the 100T sits comfortably on the NV115.

Thanks, Tim. Looks great. I bet it sounds even better! Love that CS '59 P.

 

[idoru]

Got through last night's gig on the replacement head without a hitch. Man, I love these amps.

 

[60bass]

Got through last night's gig on the replacement head without a hitch. Man, I love these amps.

That's good news ! Looks like the replacement is going to be a winner for you .

 

[GG7]

Would love a tube tester, but I have learned to rely on my ears for it instead. I can always hear when a tube has bought the farm, and I just swap tubes or tap them with a stick till I find a culprit. if I don't hear anything wrong, I just leave it.

Its not so much for detecting a tube that is going to fail soon. It is to prevent putting one in your amp that has a dead short or some other nasty going on. My Triplett can find problems that most tube testers will miss, but a power amp running that tube at full plate voltage will certainly find it, and sometimes the results are not pretty!

Some of the garbage being sold on eBay these days can certainly ruin your day! ;o)

As long as you buy your vintage tubes from someone you can trust, then the need for a tube tester isn't as crucial. All the tubes I ever sell are thoroughly tested and both preamp and power tubes are matched to provide the best performance. Having balanced triodes on your AT7 for example can have a real impact on your entire tube set.

You can't test the balance of a dual triode tube like an AX7 or AT7 with your ears.

 

[Freight Train]

For those into tube rolling, it's occurred to me what I think could be the best way to approach the preamp tubes in the 100T. V1 is the preamp tube for both channels. So that tube you should choose for your preferred clean tone; the one GG7's recommended sounds fantastic. That'll establish your clean tone on both channels. Now for V2, that is in the overdrive circuit of the active channel, and is only affecting the sound when you begin to dial in the blend control.
So here's the deal: if you use a lot of overdrive, then V2 should be chosen for its distortion characteristics, as opposed to choosing V1 for its clean characteristics. I'm sure GG7 could help us with that. Doubtless some 12AX7's distort easier, some may have a mellower distortion, some grittier and more aggressive, so you could choose V2 for your particular taste in distortion.
BUT, here's the cool part that occurred to me. If you don't like or use overdrive, then you can use V2 to give a completely different clean tone to the amp when you switch to the active channel, if you choose a different tube from V1 with a different clean tonal quality. If you set the active channel overdrive knob on 1, and blend on 10, then you are passing the entire signal through V2, but with no distortion. V1 is still affecting the sound, as it is the preamp tube for the active as well as the passive channel, but now V2 is adding its characteristics to the sound as well. And if you dial the blend back from 10, then you'd be adding more of the characteristic of V1 and less of V2.
I'm sure the V2 approach I'm describing for the overdrive fan is the best, but using it for a different clean characteristic I'm just theorizing on. Maybe GG7 could do some experimenting with that and let us know if it's a viable idea. It sure looks good on paper, but ears are the final judge. It could be that combining the different clean characteristics of another tube just makes a mess, but if it works it could allow you to switch between two different clean tube tones.

 

[GG7]

For those into tube rolling, it's occurred to me what I think could be the best way to approach the preamp tubes in the 100T. V1 is the preamp tube for both channels. So that tube you should choose for your preferred clean tone; the one GG7's recommended sounds fantastic. That'll establish your clean tone on both channels. Now for V2, that is in the overdrive circuit of the active channel, and is only affecting the sound when you begin to dial in the blend control.
So here's the deal: if you use a lot of overdrive, then V2 should be chosen for its distortion characteristics, as opposed to choosing V1 for its clean characteristics. I'm sure GG7 could help us with that. Doubtless some 12AX7's distort easier, some may have a mellower distortion, some grittier and more aggressive, so you could choose V2 for your particular taste in distortion.
BUT, here's the cool part that occurred to me. If you don't like or use overdrive, then you can use V2 to give a completely different clean tone to the amp when you switch to the active channel, if you choose a different tube from V1 with a different clean tonal quality. If you set the active channel overdrive knob on 1, and blend on 10, then you are passing the entire signal through V2, but with no distortion. V1 is still affecting the sound, as it is the preamp tube for the active as well as the passive channel, but now V2 is adding its characteristics to the sound as well. And if you dial the blend back from 10, then you'd be adding more of the characteristic of V1 and less of V2.
I'm sure the V2 approach I'm describing for the overdrive fan is the best, but using it for a different clean characteristic I'm just theorizing on. Maybe GG7 could do some experimenting with that and let us know if it's a viable idea. It sure looks good on paper, but ears are the final judge. It could be that combining the different clean characteristics of another tube just makes a mess, but if it works it could allow you to switch between two different clean tube tones.

Would be happy to advise you guys on the best thing to shoot for in each of those applications.

Can't say that I ever did more than just assume that the V1 and V2 on this amp were similar to other dual AX7 builds out there like my VOX AC15 twin.

Freight Train has done his homework on everything else related to this amp, so I see no reason to doubt him on this.

If the V2 is the distortion tube and nothing else, this does't open up some cool possibilities for playing around!

For one thing, running a 5751 in there ala Stevie Ray Vaughan would be a fun thing to try!

The tubes I have in mine right now are a matched set of Mullard Blackburns. These have about the nicest clean to distortion personality of anything every made, but if the V2 is just for distortion, then there are actually three or four other brands and types I think would work better in there!

When pushed into distortion a mid level testing but balanced Amperex "Made in Holland" from the 60s definitely sound killer in my VOX AC15's V1 and V2.

Thanks for this info! I think I will have to do some messing with just the V2 position and see what I can come up with as the best recommendation!

I can safely recommend those vintage Amperex or Telefunken for a nice warm tube distortion in the preamp section. Amperex are not as over the top expensive as the Telefunkens YET, but it seems that folks are starting to discover them lately based on rising prices I have seen on auction sites etc.

Cheers. )