212Mavguy Posted September 26, 2016 Share Posted September 26, 2016 A while ago, long enough for the thread to be archived, I shared about getting a silver face Fender Bantam Bass combo amp on eBay. It reminds me of a non-master Bassman in the controls, the preamp might be identical for all I know, it sure appears to be. It's in the same cabinet size used by the 40 watt Super Reverb combos. The amp arrived with two old stock RCA 7025's, an old stock RCA 12at7 phase inverter, an old stock GE 5U4 rectifier tube, and a pair of fresh JJ 6l6gc. It is rated for 30 watts with 6l6gc tubes. I got it for a great deal. The speaker is a freakin' freak. It's a Yamaha, trapezoidal in shape, has a cast aluminum frame, a molded styrofoam cone, a 3 inch-ish voice coil, is rated for 30 watts and has as much cone area as four ten inch speakers, as well as a wider frequency response, the Yamaha is a full range hifi speaker. A longtime friend lead guitar player/ tech not seen for 11 years was visiting, I yanked the amp out of the basement to hear the "Big" Fender amp sound that is pretty much lost today. It was still sitting in the living room two weeks later after he left when I got to looking at it kinda funny, my eyes blurred and might have reddened a bit and I went into think-land. I realized that I had never really gotten to really know that amp and since it sounded so good, I thought maybe it had untapped potential. This amp loves to sustain, sustain, sustain right from the get go. But since I wanted to , I thought about what might happen if I replaced the 5u4 with the much wimpier 5y3? I knew that the amp would have less power but would that rectifier tube be able to handle a pair of 6l6gc's? I thought about the amount of current available, compared that to what a pair of 6l6's ought to draw at idle , and came up with the idea that it might work just fine the power tubes drawing around 60-75%-ish of the 5y3's rated output. So I stuck in a vintage old stock RCA 5y3, just the regular non military type, it did not act like it was overheating and the amp sounded great, earlier breakup on the volume knob and not as loud. And now it had sag at loud volumes...awesome for earlier breakup along with enhancing that already amazing sustain. I stuck the 5u4 back in and went to check original bias setting, got 50 and 55ma current readings. Those readings seemed about 5-10 ma high for my guess, when I socketed the 5y3 current draw dropped down to three quarters-ish of the 5U4 fed numbers. From the schematic and layout I found out b+ was 440v, less than the normal 475-ish, there would be where the 30 instead of a Super Reverb's 40 watts came from, right there. So that got me thinking that why not roll some earlier, better sounding 6l6's that modern amps run too hot to use? 6l6gb or the 23watt versions of 6bg6ga's with adapters that I have a bunch of might work and sound fantastic. The lower b+ voltage in this amp dropped further by the 5y3 would then not be too far over the gb series' original operating limits... I grabbed a tightly matched pair of 50's Tung Sol 6l6WGB's, AKA 5881's, rated for 23 watts versus the GC rated JJ's 30. Stuck the 5y3 and went to the bias pot to dial in. At that time I discovered that it was a bias BALANCE, not level, pot. So they came in perfectly hot by chance, they balanced out at 46.5 each side, I'm guessing b+ would be now in the neighborhood of 400 volts instead of 440 from the 5y3 wimp factor. Bias settings are changed by swapping out resistors in this amp, just like a Mesa Boogie. It's possible to add a second, 10k bias pot for each tube to be individually adjusted without much difficulty. Just one hole...not easily seen...and this particular vintage Fender is not one of the sacred $$$$$ collectable ones. I plugged in, and started playing, I just gasped at the sounds that were coming out now. The sustain, well, no words describe how long and how the sustained notes bloomed with harmonics other than amazing... then I thought to see whether the two channels could be jumpered. In my SR's, the channels are out of phase and do not behave or sound all that wonderful jumped together. On the Bantam Bass amp, the two channels are in phase with one another and can be jumped readily. This amp sustains clean tones unusually well. That got me thinking about parallel preamps and 3d tones I get out of my Hiwatt and Harmony/Lectrolab types, noticed that the bass input on the bass channel without the deep knob engaged was actually a good bit brighter in the upper mids than the guitar channel. So I set that up to be the "bright" channel, the guitar channel was set dark and very fat like I do with the Hiwatt preamps on the "Normal" channel. I set the volume of the normal first and then added the bright until the tones start to balance, favoring the dark in volume setting overall. This amp works at dinner house volumes up to drummer equipped rockers, the speaker is such that tone controls don't need to be touched from low to high volumes. The large cone area helps bottom end nicely at low volumes. At bar gig volumes, 4-6 piece and drummer, the amp goes anywhere from clean to that old time forgotten huge Fender grind by pick attack alone. it begs to throw the pick away and just brush the strings with the fingers, the tones just gush, the harmonics multiply in the sustain like an old tube organ. It's hard to describe in words how well the amp responds and sounds now, I stuck a REAL Tung Sol 12ax7 from the 60's in the guitar channel's spot in V2. The RCA 7025 stayed in V1, now the two preamp voices are more distinct from each other. When one side is favored over the other by a slight volume knob setting the voice sounds like a different amp depending on which channel is set to dominate, you recognize each guitar's sound, within the setting differences, and the unreal, gorgeous sustain and blooming harmonic content is there regardless. Got single coils? Favor the darker volume knob a tad. Humbuckers? bring up the brighter volume knob into slight dominance instead of the dark. Am I worried about blowing the speaker? Yup. Do I think that will happen? It might. Do I care? Not particularly. I saw a bantam bass baffle and working yamaha speaker on eBay for 40 bucks plus shipping last night...And there is a nice collection of JBL and Altec 8 ohm 15's in the basement. The speaker and baffle cost less than a new baffle cut for a 15 or 4 10's, can mod the existing baffle to run a 15 very easily. When the Yamaha speaker failed, usually it was from playing a bass guitar through it with the amp set loud as possible, the styrofoam fractures. The 5y3 adds to the sustain at louder volumes, the numbers on my bias meter show that the power tubes are sucking more than what the 5y3 wimpmaster can supply without voltage drop at loud volumes, the voltage sag theory of a vacuum tube rectified guitar amp is in full glorious evidence here, and makes the amp more compressed and easier to play through at louder volume settings, along with much earlier breakup on the volume knobs. Will see whether shoving those dirty tones through the Yamaha will kill it from overheating the voice coil. Will depend on how the engineers approached cooling... Finally, I tightened all six of the screws securing the speaker baffle to the cabinet mounting cleats. The difference in sound from just doing that was surprisingly audible. The white plastic cone of the weird shaped speaker totally provides ample cheese and sleeper factor. That thing couldn't POSSIBLY... It's insane how playable and big and sweet this 22-25 watt 6l6 powered amp sounds now, stuck some effects in front, perfect, used a distortion pedal, was greeeeeeat! It's going to an open mic blues jam tonight. The boys have never seen one of these...I'll be able to do some things that a non master Bassman can't get away with and then some. Link to comment Share on other sites More sharing options...
pegleg32 Posted September 27, 2016 Share Posted September 27, 2016 Sounds like a great amp Mavguy, you did a lot of work getting it tuned up but seems like it was worth it. Would love to actually hear it. Link to comment Share on other sites More sharing options...
pressure Posted September 27, 2016 Share Posted September 27, 2016 I'm not sure what you were trying to say, could you be a little more specific... Lol. Link to comment Share on other sites More sharing options...
212Mavguy Posted September 27, 2016 Author Share Posted September 27, 2016 The jam went well, and the amp got some of the regulars playing through it, they loved it. There was a video of the amp in action but it did not come through very well on the audio, the camera was not positioned to pick up all of the instruments evenly. The tone sat in the mix nicely, was warm, fat, and welcoming, the other amp was quite a bit thinner sounding, the contrast between the two palettes was nice to hear. Link to comment Share on other sites More sharing options...
Gitfiddler Posted September 27, 2016 Share Posted September 27, 2016 Always enjoy reading about your amp adventures. That Bantam sounds like a kick to play through. I researched it and lo and behold, that goofy looking Yamaha 15" speaker was STOCK! http://ampwares.com/amplifiers/fender-bantam-bass/ Link to comment Share on other sites More sharing options...
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