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Thread: 5F6A Tweed Bassman Kit

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    5F6A Tweed Bassman Kit

    I'm thinking about starting in on a Weber 5F6A combo, and have been looking over the schematics. Can someone explain why the 4x10 speakers (8 ohm each) are wired in parallel to make a 2 ohm cabinet? The output transformer has taps for 2, 4, and 8 ohms. Is there some reason I shouldn't wire the speakers in series + parallel to make an 8 ohm cabinet and use the 8 ohm tap on the output transformer? Does the tap choice effect the negative feedback circuit? I'd like the flexibility to switch around between various 8 ohm cabinets with the different amps I have.

    If folks could recommend some speaker choices too, I'd appreciate the discussion.

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    2ohms parallel (just the way Fender did it up until the Blues Devilles) will sound different (airier, chimeyier) to 8ohms series-prarallel or parallel-series (thicker, gruntier), though that's no reason to stop you rewiring to 8ohms.

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    Thanks MWJB for the answer. Can you explain why it should sound different? I'm interested in learning.

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    Bent Member Chuck H's Avatar
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    In theory it makes no difference to the transformer. But I would think there is a small change in inductance, capacitance, DCR and any other thing that keeps an OT from being an "ideal" transformer when you change the turns ratio on the core. But more than that is the speakers themselves. Speakers have inductance, DCR, etc. that series and parallel the same as any other electronic componant. It's like a soup of parameters that give speakers their individual tone. If we look at just the inductance of the speakers in parallel (2 ohms) vs. series/parallel (8 ohms), the parallel arrangement has 1/4 the inductance of the series/parallel arrangement. That will certainly change the impedance curve and alter the way the same four speakers sound wired one way or the other.

    Chuck

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    I put in my order for the Weber 5F6A combo a few days ago, and now I'm thinking about tubes. According to the schematics, the plate voltage is around 432V. The Weber power transformer has two taps on the primary to adjust secondary voltage (120 and 125V). The PT also can handle 3 amps for the rectifier filament, so I think I can use a 5U4GB or a 5AR4. The original Tweed Bassman came with Tung Sol 5881 tubes. I have a good supply of 6L6 tubes and 7027A on hand, and was wondering if the 5881 tubes are necessary to get "the tweed sound", or can I use the bigger bottle 6L6 or even bigger bottle 7027A? I'm thinking the relatively low plate voltage should even allow me to try some 6V6 tubes in it. I will be putting a bias pot in the amp.

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    "According to the schematics, the plate voltage is around 432V." Ignore the schematic, typical 5F6A plate voltages, when biased up, with a GZ34 are 470vdc +/- 10V.

    Yes, you can use 5U4GB or GZ34.

    Phillips 6L6WGB can be found for semi reasonable prices, best bang for the buck...if lucky you might find a pair of match Tung Sols (New Jersey). Both the Tung Sol (Russia) and TAD 6L6WGC have their following, but ultimately it's your amp & if you can fit whatever power tube you like the sound of. Try the 6L6s you have & see how you like it.

    JJ6V6s will take 500vdc, I wouldn't advise NOS 6V6 in this amp.

    7027A seems to have a slightly different pin out? 7581A would be a drop in replacement.

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    Quote Originally Posted by MWJB View Post
    Ignore the schematic, typical 5F6A plate voltages, when biased up, with a GZ34 are 470vdc +/- 10V.

    7027A seems to have a slightly different pin out? 7581A would be a drop in replacement.
    Thanks for the info on the plate voltage. The 7027A pinout is close to a 6L6. The only difference is a 6L6 doesn't connect internally to socket pins 1 and 6. A 7027A tube connects internally pins 1 and 4, and pins 5 and 6 respectively. If you wire the socket up Ampeg-style, with no wires or components attached to pins 1 and 6, you can use either a 6L6 or 7027A tube.

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    I'm gathering more parts for the amp and came across a bunch of NOS Sprague black beauty caps and an unknown brand of oil and paper caps. Would you use these instead of the modern (and fresh) tubular polyester caps that come with the kit for the signal path? Would the vintage caps be more trouble than they're worth because of potential drifting and leakage compared to a modern cap with modern materials? I'm thinking that if I ever try to sell the amp down the road, folks may value it more with the oil and paper caps. Could it possibly sound "better" with vintage caps or is that just a bunch of cork sniffing? Is it more likely to sound worse with vintage caps? Should I just sell the vintage caps on fleabay? All opinions are appreciated.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Diablo View Post
    Could it possibly sound "better" with vintage caps or is that just a bunch of cork sniffing? Is it more likely to sound worse with vintage caps? Should I just sell the vintage caps on fleabay? All opinions are appreciated.
    Mostly it's a "bunch of cork sniffing" (love that, funny). I do hear a difference between polyester and polyprop caps but it's small, tiny really. As is the difference between any type of film cap. But the only way to know for sure is to try em. I personally use Sprague 715s because they sound good, come in all the right values and are very temperature stable. So my amps won't change tone as they warm up or when they're really being pounded on. But I think polyester has a really sweet tone for some things. And I'm one of those guys who believes he CAN hear a difference. I get poo pooed all the time for it. But I can say that there's no way I could hear a difference if you changed only one or two caps in any of my amps to a different type. Thats how small the difference is.

    As far as selling the vintage stuff. Well, if you can afford to, hold onto it right now. The economy is in the tank and collecibles are the first thing to lose value. They'll go for more later when (if?) the economy picks up and they've become even more rare.

    Just an opinion (flame avoidance)

    Chuck

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    I decided to saw apart one of the Sprague black beauties just to see the construction. I discovered that they are mylar film dielectric and aluminum foil. I'm glad that they didn't have an oiled paper dielectric, as I'm thinking that the mylar film is less likely to age poorly and leak DC. Perhaps that's wishful thinking. I'll probably give 'em a try in the amp.

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    Senior Member hasserl's Avatar
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    From what I've heard the Black Beauties are notorius for leaking DC.

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    Quote Originally Posted by hasserl View Post
    From what I've heard the Black Beauties are notorius for leaking DC.
    Is that for new-old as well as used-old? From what I've read, the foil and film caps (like these black beauties) don't have self-healing characteristics like metallized film caps. The knock on metallized film caps is that they don't sound as good as film and foil. Here's a blurb from the Sozo website. The black beauties are the same materials as the Sozo:

    Each material will interact differently with the conductors because of their dielectric constant (a measure of the extent to which the insulating material's surface interacts with the electric field set up between the charged plates). These factors and elements were considered in the building of this capacitor however, as in the original, Polyester film was chosen because of its deeper tonal qualities and the performance exceeded our objective. On a typical film capacitor the conductors are usually a metalized material bonded to the dielectric by means of vacuum impregnation. Better quality capacitors incorporate metal foils such as aluminum, tin, copper or even silver.

    The conducting material influences how the capacitor responds due to each metalís resistive properties. Different metals were considered in the construction of this capacitor. The intention was to exactly recreate the capacitorís original sound and performance; therefore aluminum foil was chosen. Other variables in building the capacitors which do not affect the measurable capacitive value, but influenced the behavior of the capacitor are the lead wires and the dimensional proportions of the cap.

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    I just received an e-mail from the shipper. My kit should be here tomorrow. So, Weber will deliver the kit in just under 4 weeks time from placing the order. That's in line with the info on their website. Time to clear a spot on the workbench.

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    Member Emeritus Forever Steve A.'s Avatar
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    Quote Originally Posted by Chuck H View Post
    ... But I think polyester has a really sweet tone for some things. And I'm one of those guys who believes he CAN hear a difference. I get poo pooed all the time for it. But I can say that there's no way I could hear a difference if you changed only one or two caps in any of my amps to a different type. Thats how small the difference is...
    Well, I can hear a difference, too, especially with the Orange Drop caps. The polypropylene seem to be a little bit harsher and brighter, while the polyester seem to be a little bit warmer "and more musical", as my old friend Dan Torres would like to say...

    So I will switch back and forth between the two when making fine-tuning tweaks and I can usually hear a difference. But if you were to have me take a blind A/B test I might not be able to tell which is which.

    I just put in one of Dan's .022uF Orange Drop caps in the Crate V18 I'm rebuilding but it seems to be microphonic so I'm going to put in one of the 630vac polyester caps I got from Mouser. Having it microphonic in an old Fender chassis made out of heavy gauge steel can make the amp sound more alive without causing any problems but it can give your amp a tinny sound in one of those cheap modern folded metal chassises. Or so it says in the Steve Ahola book of Mystical Electronics... yes, most of my ideas come to me in visions or dreams. To heck with the science, I believe in Om-m-m-m-m-m's Law.

    Steve Ahola

    Here's some links to AMPAGE ARCHIVES discussing the series/parallel issue:

    http://archive.ampage.org/threads/0/...arallel-1.html

    http://archive.ampage.org/threads/1/...peakers-1.html

    And here's a thread I found on my site but was unable to access in the archives here.

    From: PaulC
    Date: 10/20/2001 8:12 PM
    Subject: OEI 30 watter in!

    ... It has a 4ohm tap included that lets me run the blues in parallel for a clearer mid sound with a punchier low end. Has a piano clean vibe now on the
    bottom. Very cool piece of iron - will be getting many more! Try it out... Later - Paul

    ...

    From: PaulC
    Date: 10/21/2001 2:52 PM
    Subject: Re: OEI 30 watter in!

    Well - you're asking me to back up something I can hear, but not give you the math for!! It's been explained to me (but not sticking in my brain) that
    it's about a change in the backflow voltage produced by the speakers in the different hook-ups. The parallel setup increases damping. The change in
    inductance also effects the responce. Ever taken a 4x12 and wired it in parallel for 4 ohms? Big change - mids clear up and the lows punch up some.
    Ol' Jerry talks about changing the way you series/parallel the speakers in a 4x12 to get 16 ohms for different sounds. To be honest I'm not really
    hearing that one, but the 4 ohms is obvious. With my amp it just seems to add some needed attack to the bottom end and cut back a little on some
    speaker flubbing. I should get Ted to spell it all out...

    ...

    From: R.G.
    Date: 10/21/2001 4:37 PM
    Subject: Re: OEI 30 watter in!

    Paul's right about the damping. Each speaker in series acts like an impedance to spoil the damping you'd have from the output of the amp. Series
    speakers do sound different from parallel. More to the point, since speakers have different impedances at different frequencies, the damping gets
    really bad wherever the impedance(s) go high - like at the low end resonance and at the high end where the inductance of the voice coil takes over.
    This has the effect of making the tone variances more pronounced.

    The parallel connection is more tightly controlled by the amp.

    R.G.

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    Last edited by Steve A.; 10-29-2008 at 03:49 AM.

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    Bent Member Chuck H's Avatar
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    Steve, great rundown on the issues at hand. And thanks for backing me on the cap difference issue. When I said "I probably couldn't tell if you changed one or two caps" I was being liberal. I probably could actually. And I love polyester for some amps. But I don't use it because it changes tone depending on it's temperature. It's worth noting that cap construction (other than materials) can have a big impact on the tone also. So not all polyprops are created equal. Nor are the polyesters. I love Mallory 150's but I don't like the Torres Orange drop polyesters. No Offense. I like a big brash tone anyway so the 715's do a nice job and stay consistent with temperature. They're great for big overdriven Angus like tones and if used right, even sustainy leads (just consult the Trainwreck Express parts list) But for the ultra high gain SLO and Dual Recto stuff and the vintage Fender vibe polyester is king. I just can't take how an amp using them only sounds great for the first ten minutes. Or great, then OK, then like crap after you push it for one song. It's funny because when I used to play live I was using an amp I modded with polyester caps. I would actually look foreward to firing up my amp after break because I knew it would sound good again for a little while.

    No more. Nothing but polyproylene for me from now on.

    Chuck

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    The kit arrived today and everything looks great. Very nice cabinet and covering. I was worried about the chassis, since some folks have complained that the metal finishing wasn't the best quality. My chassis looks great - I'm in the metal biz and the plating looks fine. If you stick your nose up close, you can see that the steel wasn't polished perfectly smooth prior to plating. But, pull your head back a foot and it looks like a mirror. Graphics looks very good too. I upgraded the speakers to a set of Weber's Vintage Series 10A125. I also upgraded to a Heyboer multitap OT. I'd get started on the kit tonight, except I need a soldering iron tip and my Radioshack is closed for a while because of a local flood.

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    Here's a question for the experts about the Weber power transformer. The secondary has two sets of high voltage taps. One set is a pair of red/white wires - 660V. The other set is a pair of red wires - 720V. The Weber schematic uses the 720V, and shows the lower voltage not connected. Is that correct? I'd like to get the "tweed bassman" B+ of around 470V. I plan to use a 5U4GB tube rectifier instead of the Weber copper cap.

    Also, there are two taps on the primary side of the PT, one is 120V and the other is 125V. I assume this is for adjustment of the B+ to the desired range. If all goes well, this amp should be making loud noise tonight.

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    Last edited by Diablo; 11-08-2008 at 09:23 PM.

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    More questions...
    I have a Heyboer output transformer 18343 with 2, 4, 8 ohm taps and there was no wiring diagram included. The four wire colors are black, yellow, white, and green. Anyone know what's what? I've searched the web and Heyboer and Weber's sites and have come up with nothing.

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    Quote Originally Posted by Chuck H View Post
    Steve, great rundown on the issues at hand. And thanks for backing me on the cap difference issue. When I said "I probably couldn't tell if you changed one or two caps" I was being liberal. I probably could actually. And I love polyester for some amps. But I don't use it because it changes tone depending on it's temperature. It's worth noting that cap construction (other than materials) can have a big impact on the tone also. So not all polyprops are created equal. Nor are the polyesters. I love Mallory 150's but I don't like the Torres Orange drop polyesters. No Offense. I like a big brash tone anyway so the 715's do a nice job and stay consistent with temperature. They're great for big overdriven Angus like tones and if used right, even sustainy leads (just consult the Trainwreck Express parts list) But for the ultra high gain SLO and Dual Recto stuff and the vintage Fender vibe polyester is king. I just can't take how an amp using them only sounds great for the first ten minutes. Or great, then OK, then like crap after you push it for one song. It's funny because when I used to play live I was using an amp I modded with polyester caps. I would actually look foreward to firing up my amp after break because I knew it would sound good again for a little while.

    No more. Nothing but polyproylene for me from now on.

    Chuck
    Chuck,

    I'm noticing something similar (thought it was my imagination) about my 5F6A-alike....sounds great for about 10 minutes, then starts to sound overly compressed. I wonder if it's the same thing....and WHICH caps should be changed (ALL??)

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    Quote Originally Posted by Diablo View Post
    Here's a question for the experts about the Weber power transformer. The secondary has two sets of high voltage taps. One set is a pair of red/white wires - 660V. The other set is a pair of red wires - 720V. The Weber schematic uses the 720V, and shows the lower voltage not connected. Is that correct? I'd like to get the "tweed bassman" B+ of around 470V. I plan to use a 5U4GB tube rectifier instead of the Weber copper cap.

    Also, there are two taps on the primary side of the PT, one is 120V and the other is 125V. I assume this is for adjustment of the B+ to the desired range. If all goes well, this amp should be making loud noise tonight.
    Diablo,

    I built an amp that's essentially a 5F6A clone using the Weber PT you have. I used the 120V primary and the 720V Secondary wiring. With a 5U4GB I'm getting around 460-465V B+ (Tubes biased aprox 60% dissipation)

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    Thanks JoeM for the info on the PT. Your B+ is in the right neighborhood for me, so I'll hook mine up the same way. I'm still not sure about the Heyboer OT. If they used the same wiring code that Ted Weber lists for his OT, then I know what to do. But if they don't have the same color code, I may get smoke.

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    Ted Weber shot me an e-mail and gave me the OT wiring code for the Heyboer. It's the same color codes as the Weber 18343. Hooked it all up, turned it on standby and all was good for about 30 seconds, then I saw smoke! I had wired the two main filter caps to the hot side of the standby switch and at least one of them burned up. I rechecked all the wiring and found nothing unusual. I replaced all the Chinese tubes with NOS, fearing that maybe one of them was shorted. I grabbed the 36 year old capacitor can that I had replaced in my Ampeg V4, and jumpered it into the 5F6A. I put this filter cap on the switched side of the standby. I also stuck in a new fuse - the only one I had was 6 amp. Put the amp on standby and it was fine. Checked the bias voltage at -50V, took it off standby and held my breath. It worked. Plate voltage right at 462V with a 5U4GB rectifier. Checked the bias current around 40mA on the GT6L6. Played it for about a half hour and all was still well. At that point I stuck in a pair of Phillips 6L6GWB and biased them at 35mA. I tried the amp using a 12AX7 and 12AY7 in V1, and it really makes a difference in the amount of grind. Once I order and replace the main filter caps, I'll shoot some pictures.

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    Bent Member Chuck H's Avatar
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    I can't say which cap/s might be responsible. It could even be a tube or resistor. Most resistors "derate" with high temperatures. A bad one could do it at moderate temps. The effect I've heard as it applies to polyester caps is just a general dulling or top end roll off and/or the bottom end gets weaker. The compressed thing you describe sounds like a power supply or tube issue. Check your electrolytics and try a tube swap. FWIW electrolytics derate too. This could include a derate in voltage rating and the ability to deliver the current needed. This usually would result in hum or a cap failure. Do a voltage test. Check voltages and bias when you flip the off standby. Then play the hell out of it for ten or fifteen min. Check all the same voltages again. Any changes?

    Chuck

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    I finished up the amp and it sounds fantastic. Here are all the substitutions - not sure if they are an improvement over stock because I never tried it stock.

    Speakers - Weber 10A125 Alnico Vintage Series in parallel
    1.5K grid stoppers added to power tube sockets
    Victoria-style presence circuit mod, 25K pot + 4.7K resistor +.1mfd cap
    Safety diodes added to rectifier socket
    RCA 5U4GB tube rectifier
    Carbon comp resistors everywhere it made sense
    Heyboer multitap output transformer running on 2 ohm tap
    Two Illinois 100/450 caps in series for the main filters + two 220K balance
    Mostly Sprague mylar and foil caps, one Cornell Dubellier and one oil/paper
    NOS Phillips 6L6WGB, RCA 12AX7 in V2 and V3, 12AY7 in V1
    Adjustable bias - 50KL pot in series with 39K resistor
    Better chicken head knobs
    Fender style pilot lamp

    When I replaced the failed power caps, I put the new caps on the spare boards that Weber provides if you want to mount them in the Fender location. I left the two preamp caps on the mini board inside the amp.
    If I had it to do again, I would sub in better quality tube sockets. These work, but I don't think they will last forever.

    And here's the pictures.....http://s268.photobucket.com/albums/j...assman%205F6A/

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    Diablo,

    When I built my 5F6A clone, I noticed Ted had a 56K resistor in the NFB loop, where the Fender schematics (including the RI) have 27K. I played around with various values and settled on 33k. You might consider experimenting here a bit and see what you prefer.

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    You're right JoeM, thanks for pointing that out. I saw the same thing on Weber's schematic and I used the 27K resistor in the NFB loop, just like Fender. But, I forgot to list it in my mods list. This amp just begs to be played loud. It sounds like it's alive.

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    Today I hauled my amp over to two different guitar shops to get their opinions on the quality and tone. I told them I wanted to find out how it compares to other amps on the market and if I should do anything to change or improve it. The first shop sells Fenders and is the local music store that's been there for more than 3 decades. The owner is a pretty good guitar player, so I think his opinion is worth something. He liked the amp better than the original Bassmans, because it had a "tighter sound". The only issue he found was that the chassis was a little microphonic if you tapped on it while the volume was dimed....maybe the 12AY7 was microphonic? I slipped a 12AX7 in it for him to see which version he liked. He liked the 12AY7 better in V1, because it sounded warmer when cranked up. One thumbs up, and on to the Guitar Center. The GC guitar expert wasn't that much of a player, but he was very curious about what components were used in the build, type of wood, brands of tubes, types of speakers etc. After he played it for about 15 minutes, he told me it sounded very good. He wished the GC carried "high end" amps like that. He told me he thought it should be a $2200 amp. I told him I was interested in his feedback because I may want to build more to sell if they compare well to others on the market. His suggestions were to add a reverb and a master volume. I told him I didn't think the master volume was such a good idea, but have been thinking about a front end reverb unit. He liked the amp a lot, but told me he couldn't afford it. He shot some pictures of the guts with his cell phone, for his buddies. He asked if I could build other amps for his friends who wanted vintage stuff other than Fenders.....like HiWatt.

    This weekend, I'm going to haul it over to the Victoria amp dealer to see how it compares. Into the lion's den.

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    They gave the amp a test drive at the Victoria dealer. He was a pretty good guitar picker armed with a Strat. He liked the amp and tone and said if I build another, do it the same way. To him, "it sounds like a Bassman". Without A/B ing it with the Victoria, he couldn't say if it sounded any different. They were pretty busy, so there was no time to A/B the two amps.

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    1.5K grid stoppers added to power tube sockets
    Victoria-style presence circuit mod, 25K pot + 4.7K resistor +.1mfd cap
    Safety diodes added to rectifier socket
    Carbon comp resistors everywhere it made sense
    Heyboer multitap output transformer running on 2 ohm tap
    Mostly Sprague mylar and foil caps, one Cornell Dubellier and one oil/paper
    NOS Phillips 6L6WGB, RCA 12AX7 in V2 and V3, 12AY7 in V1
    Hi there!

    Think you can give me some hints on replacements for my 5F6A?
    I made the weber head model, using a homemade 2x12 with Celestion Blue's.

    Tubes? Where to get them? What to get?
    Caps? Thinking of Sprague Orange drops.
    1.5k grid stoppers? Why?
    Victoria-style presence? What does that do?
    Heyboer OT, big difference?

    //Jibe

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    Quote Originally Posted by jibe View Post
    Hi there!

    Think you can give me some hints on replacements for my 5F6A?
    I made the weber head model, using a homemade 2x12 with Celestion Blue's.

    Tubes? Where to get them? What to get?
    Caps? Thinking of Sprague Orange drops.
    1.5k grid stoppers? Why?
    Victoria-style presence? What does that do?
    Heyboer OT, big difference?

    //Jibe
    Tubes - Phillips 6L6WGB from ebay...about $45/pair. You could also try new issue Tungsol 5881 for around $30/pair. The 6L6WGB is same as 5881 designation - small bottle 23 watt. I'd get the power tubes first and see how you like 'em. Next, get a 12AY7 for V1. Try ebay..about $20.

    Caps...Sprague Orange drops or maybe try those Sozo mylar film and foil caps from Weber. I haven't tried either in the Bassman, but I do have old mylar film and foil in there, so I'd lean towards the Sozo caps.

    The 1.5K grid stoppers can prevent oscillation and shouldn't impact the tone in a negative way.

    The Victoria presence circuit just gives a quiet non-scratchy pot.

    Heyboer OT - I don't know if it's an improvement over the Weber OT, since I never tried the Weber. Heyboer builds the transformers for MOJO, general opinion is good.

    I don't want to send you on a wild goose chase. For all my changes from the stock kit, I never tried it stock, so I can't honestly tell you that these changes are an improvement. All I can say is the amp sounds very nice.

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    I don't think I'll replace the OT, not in the near future anyways.
    But tubes and caps has been on the wish list from day one, if not for anything else
    than actually finding out if it makes a difference. I'm planning on playing "stock" and
    then switch to see if I can hear it. If i'm one of "them".
    What about 12AY7? Lower gain? What does it do soundwise? Why would I wan't to lower
    the first gain stage? Apart from the fact that the original 5F6A had a 12AY7.

    I don't seem to have problem with my presence pot, if it ain't broke, dont fix it!!!?
    I had a ton of other problems, (or just one really, my broken RCA jacks), but I haven't had problem with oscillation, yet. I guess I can skip grid stoppers for now and concentrate on the bias of my tubes and getting my replacements.

    What's the difference of 5881 and 6L6, know any good webshop for getting tubes? Found a matched pair of new production Tung-sol 5881 for 39.95$ at Tubedepot.

    Power filter caps, does that affect sound as well?


    //Jibe

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    Last edited by jibe; 03-10-2009 at 10:06 AM.

  32. #32
    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by jibe View Post
    What about 12AY7? Lower gain? What does it do soundwise? Why would I wan't to lower
    the first gain stage? Apart from the fact that the original 5F6A had a 12AY7.
    You might not want to. Fender did it because it was cleaner that way. A 12AY7 also has a nice fat tone. The EH 12AY7 is surprisingly nice sounding

    Quote Originally Posted by jibe View Post
    What's the difference of 5881 and 6L6, know any good webshop for getting tubes? Found a matched pair of new production Tung-sol 5881 for 39.95$ at Tubedepot.
    5881 = 6L6GB same tube, same rating

    (However a 6L6CG has a more powerful rating)


    Quote Originally Posted by jibe View Post
    Power filter caps, does that affect sound as well?
    //Jibe
    The size of the cap will impact on the quality of the filtering. That's why lotsa peeps like Sprague Atoms (including myself)

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    Last edited by tubeswell; 03-11-2009 at 10:08 AM.
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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    The 5881 tube should break up at a little lower volume than a 6L6GC, and the original Bassmans used the 5881 tubes. My main filter caps are two 100mfd in series, so that's 50mfd vs the standard 40 mfd. Maybe it makes the bass a little tighter, maybe no one can hear the difference.... Original cap ratings are + or - 20%, so that's close enough. If you want to make the amp grind, get a y-cord and use both channels, so you use both triodes of the 12AY7. The Y cord doesn't dull the tone like normal channel jumpering.

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    I do get a little less output from the 5881, right? Do I need to make other changes in order to use 5881 instead of 6L6? It's not rated for a high plate voltage as the 6L6 and the plate voltage on mine is around 420-470v, I read somewhere that 5881 is rated for 400v. Maybe the plate voltage changes when biasing?

    The use of OT taps, does the use of either 8, 4 or 2 ohm tap change the output of the amp? I used the 8 ohm tap when I bypassed my broken RCA connector, the cab I have is a 8 ohm Cabinet. I saw a post the other day when someone stated to have 36w@4ohm and 25w@8ohm.

    Sorry for hijacking your thread but your mods made me come up with tons of questions



    //Jibe

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    Supporting Member tubeswell's Avatar
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    Like I said a 6L6GB is a straight swap for a 5881.
    If you want to put 6L6GCs in, you want to up the B+ and rebias. 6L6GCs can take more heat (in fact they sound good when they're hotter).

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    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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