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Author
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Topic: 59 bassman RI mods needed for more clean tone
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Steve Walz
Member From: USA
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posted 11 May 2005 11:48 AM
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I need to change a few things on my amp to get a better sound and cleaner headroom. I searched the web for mods and there is some good info on hot rod delux mods and tune up stuff but not much on the 59 bassman.I'm going to change out the 12AX7's and put in 12AT's for a cleaner sound, any advise or suggestions on tube choices for that tube? Should I change all three or just the first one. Should I bias those? Do they need to be matched like the power tubes? I have Ruby tube 6L6GC I think. Is that brand good? Would it be worth $70 or $100 for better quality mathced tubes. I have read that 7581's or 7581A's will add more clean headroom. I've added a fender 15" inch speaker a while back. I have read that a closed back will push more bass with out using so much power because you can turn the bass EQ down a bit. Any thoughts on enclosing the speaker in it's own section of the cabinet and leaving the tubes exposed for heat? I'm using the non tube rectifier now.
Thanks,
Steve |
Jon Light
Member From: Brooklyn, NY
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posted 11 May 2005 12:57 PM
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To several of your points and questions:---Yes, changing V1 and V2 (for the second channel) to AT7s is a good idea. No, they don't need to be matched or biased.
Unless the Ruby Tubes 6L6's are old, they wouldn't likely be the cause of major headroom issues.
For new tubes, both preamp and power tubes, I like JJ Teslas and I get them from Bob at http://www.eurotubes.com
who I like doing business with. Very reasonably priced, too. Definitely leave good ventilation for your tubes. All tube amps that drive closed cabinets use separate enclosures for the speakers--there is no getting around the need for tube heat dissipation. I see no reason why creating that enlosure within one cabinet would be a problem. Except for the weight. I am not experienced with the 7581's to know if that would have a significant effect on headroom. And one last thought--if you are using humbucker pickups, try using the second jack of either channel--it is attenuated and will reduce how hard you hit the preamp tubes. [This message was edited by Jon Light on 11 May 2005 at 01:00 PM.] |
Brad Sarno
Member From: St. Louis, MO USA
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posted 11 May 2005 03:10 PM
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I like the JJ's too. They make a nice 6L6. I'm generally not a fan of Ruby tubes, but that's only based on a single experience I once had. The easiest way to nearly double your clean loudness is to put a JBL D-130 4-ohm in there. Speaker efficiency goes a LONG way when trying to get more volume. Those Fender speakers aren't nearly as efficient. You may gain a few watts with some tweaks, but you'll be amazed at how much real volume you gain with a JBL. Also, I much prefer the open back on that amp, for steel at least. Brad Sarno |
J Fletcher
Member From: London,Ont,Canada
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posted 11 May 2005 05:16 PM
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If you are only using a single 15" speaker, you likely have a impedance mismatch as the output transformer needs a 2 ohm load for best power transfer. Unless you have a 2 ohm 15" speaker. You will get the most clean power by matching the speaker to the output transformer....Jerry |
Steve Walz
Member From: USA
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posted 11 May 2005 05:23 PM
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Thanks for the suggestions, they are helping. I already emailed eurotube. But the more ideas the better. What about using a path cable from one input to the other and which way do you do that?I'm fairly sure that my 15" is a 4ohm speaker. I was told that it was close enough to not hurt it, but not perfect. Steve |
Jon Light
Member From: Brooklyn, NY
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posted 11 May 2005 05:33 PM
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I agree that you will not hurt it (at least not in a vintage Fender--I don't know how hardy the transformers are in the reissues) but since you will have lower output because of the mismatch, you will be cranking it higher to get equivalent volume levels so that could be a factor in the early break up that you are trying to remedy. Of course, remember that the tweed era circuitry is all about sweet breakup, compared with the blackface circuits.
The channel patch would be done by pluggung guitar into Ch 1, jack 1, and running a cable from Ch 1 jack 2 into Ch 2 jack 1.
I'm not aware any advantage to doing this, though, other than more control over tone color. |
J Fletcher
Member From: London,Ont,Canada
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posted 11 May 2005 05:40 PM
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By mismatching the speaker impedance, you're losing approx 25% of your power. If you want a bit more headroom in the preamp section, you could remove the 250uf cap across the first tube's cathode resistor, or replace with a lower value, say 5uf or so. You will lose some gain, but can compensate with the volume control...Jerry |
Brad Sarno
Member From: St. Louis, MO USA
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posted 11 May 2005 06:15 PM
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I thought the reissues ran at 4 ohms with 4 - 16 ohm speakers. Not sure though.Brad |
Ken Fox
Member From: Ray City, GA USA
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posted 13 May 2005 05:36 AM
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Not sure, but if you have a rectifier tube in there, change it to a Ted Weber, Copper Cap rectifier and rebias the amp. JJ tubes 6L6GC tubes really maximize headroom as well. Keep the bias at 32 ma or hotter to. Not unusual to see JJ tubes running fines at well over 40ma bias current. |
David Doggett
Member From: Philadelphia, Pennsylvania, USA
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posted 13 May 2005 07:28 AM
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Here's a link to the schematic for your amp over on the Fender.com site: http://www.fender.com/support/amp_schematics/pdfs/59_Bassman_Schematic.pdf I agree with others above that the most important thing is to match the speaker load to the 2 ohms the amp was built for (four 10s, 8 ohms each). I don't know where to find a 2 ohm 15" speaker. Most of them are 8 ohms, but Weber allows you to select 4 ohms (not sure if they provide one at 2 ohms). If you run two 4 ohm speakers in parallel, that will be 2 ohms, and that may be the easiest thing for you to do. I don't know if that cabinet will hold two 12s. For 15s (which I would prefer), you could get another 15 in an extension cabinet, and run that parallel with the internal speaker (I don't know if the extension speaker jack puts the extension in parallel or series). You could get the extension cabinet as a closed back ported design and leave the Bassman cabinet open backed. In addition to keeping the tubes open, another reason for using open backs on combo amps is that a closed back cabinet has to absorb the heavy blows of the low bass notes. That constant vibration is not good for tubes. This would be more a factor if you played bass through the cabinet. The tubes would not be in the closed area of the cabinet that absorbs the main blow, but some of the vibration might be transferred to the whole cabinet. You could also put your amp chassis in a head cabinet and run two closed-back extension speakers in parallel. |
Steve Walz
Member From: USA
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posted 13 May 2005 10:06 AM
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fantastic advise! I'm not in love with the 15" speaker that I have since it is just a basic fender speaker and if I add another 4ohm speaker I could spend a little more on a higher quality speaker and use that one as the miked speaker through the house. I can build a closed cabinet for a different sound as well. Thanks for all the help again, Steve |
J Fletcher
Member From: London,Ont,Canada
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posted 13 May 2005 12:27 PM
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You could replace the output transformer with one that has 2, 4, and 8 ohm taps, available from New Sensor (around $50) and other vendors, and install a nice JBL D130F as was suggested by Brad Sarno, using the tap that is appropriate for the speaker's impedance. A solid state rectifier will give you a bit more headroom, and the JJ6L6GC's seem to be a bit more efficient than some other currently available tubes. Do all these and the amp will be noticeably cleaner and louder. But it will still be a 50 watt amp. Is 50 watts enough for what you do? It's usually enough for me....Jerry |
Brad Sarno
Member From: St. Louis, MO USA
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posted 13 May 2005 12:50 PM
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I just finished servicing Tom Brumley's '59 Bassman that has a JBL D130. It's Brumley's main amp. It's the "Together Again" amp although that song was recorded in early '64 when it was still a 4-10" amp. Later that year Leo Fender changed it over to a 1-15". Leo changed the baffle, tolexed the whole thing in black, and switched out the transformer to the 4-ohm Bandmaster/Pro type transformer. I think it's a 35-40 watt transformer. That amp is loud and fat. Tom swears it's always been plenty clean and plenty loud and reliable in the 40 years that he's been using it that way. Later he had reverb added. It's a cool amp. Not too heavy at all, and very fat and loud and clean sounding. If I were to put a 4-ohm transformer in a Bassman RI, I'd go for something that could cleanly handle 50-60 watts. I think Tom's transformer may be a little on the edge of maxing out its power/headroom. Maybe that's part of the sound. Isn't a Super Reverb a 4 ohm amp? Maybe that would be a good tranny. Regarding power and headroom, last week I played steel thru my '68 Fender Deluxe Reverb at a big outdoor festival. It was enough power for my situation. I sat pretty close to it and let the soundman do what he needed. That's about 24 watts. The JBL D120 makes that a very loud amp. That Bassman RI may need some better caps in the signal path to help clean it up. I bet Fender isn't using the best parts in there. Orange Drops 716p series are pretty nice sounding. Film/Foil type. Brad Sarno |
Scott Appleton
Member From: Half Moon Bay, California, USA
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posted 14 May 2005 09:12 AM
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When I was rebuilding tube amps I used shielded
resistors in the input chain along with the upgrade int the capacitors. This helps the preamp tubes hear cleaner.
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Mullen S12 Almost Mooney
71 Tele, Regal 45
Sho Bud S10 NP
Line 6 Flextone 3 + JBL D130, Acoustic 100 W all tube, Nash 112
digitech 2101 FX |
Steve Walz
Member From: USA
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posted 18 May 2005 03:46 PM
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So I have a plan now on that 59 reissue. I don't want to drag around another 15" speaker inorder tocorrect the mismatched ohms, so I think I'm going to replace the output transformer with one that has multiple ohm choices so I can put the old speakers back if needed. It was suggested to me to use the Marshall 50 transformer due to the mutilple taps and due to the quality.Is there a difference between brands of transformers? Is it worth it to spend more than $50? Is there a tone difference? It seems people think that a high quality transformer has a lot to some difference in tone. I just don't want o do what I did when I bought the less than high end speaker that I put in originally. There is a muti Tap output on tubeampparts.com for $75, and a $105 50 watt marshall at mercurymagnetics, and $70 one for a pro reverb which is 4 ohms. Any advice? I also noticed an impedance converter transformer at 50 watss for mutiple ohms for $25 on weberspeakercom.secure. Would that work instead? I ordered 12AT's and new JJ 6L6's and will have those biased. And I'm tooking into a altec speaker. Thanks for the help again, Steve |
Brad Sarno
Member From: St. Louis, MO USA
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posted 18 May 2005 04:12 PM
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Steve, I've got an old alnico Altec 15" speaker that's 8 ohms. If you get the multi-out transformer, then you could use it. Let me know if you're interested.Brad |
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