Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on October 22, 2011, 04:27:08 pm
-
I have a Twin chassis on the way, the price was right.
But I already have a BF amp.
I wanna build a bass amp head out of it.
I'm thinking Ampeg since I love my Reverberocket. Any suggestions? Hiwatt? Fender? :dontknow:
-
Portaflex.
Easier than an SVT, and sounds better most of the time.
Gabriel
-
Let me check this out.......
Looks good. How difficult would it be to make it a 4-tube output? EL34 compatible??
I also have some 34s laying around but no amp at all to use them in. All my sockets are wired right but I'll need to install an extra Xformer for the heaters.
But back to the bass amp: AFAIK the 7027 operates at roughly the same specs as a 6L6GC but with different pinouts. Might I need more driver current for double the output tubes?
-
B15N
-
Freakin Ampeg. They couldn't seem to make an amp with tubes that I have.
I should clarify: I have a blackface Twin chassis coming. i.e. 4 output tubes. Should I try another design or will the B15N get me where I need to be with minor mods?
-
Freakin Ampeg. They couldn't seem to make an amp with tubes that I have.
I should clarify: I have a blackface Twin chassis coming. i.e. 4 output tubes. Should I try another design or will the B15N get me where I need to be with minor mods?
I would maybe start with Blackface Twin power amp as that just gets lots of nice clean current to the speaker, for the most part. Then mate it with whatever preamp you like. I have a Bassman Ten whose "studio" channel I'm quite partial to, personally.
Chris
-
AB165 preamp with 4x6L6. My next "bassman 100" re-build... I think this one will growl nicely.
I don't ever heard a b15N live, though, but I surely will try to build one next year.
BR,
Danskman
-
An Hiwatt with 4 el34 instead of 6 ?
Kagliostro
-
Okay, I'm nebulizing ideas....How about a Twin Reverb output but with a Marshall 100w OT? Clean is the desired effect. Also to be used as a generic huge (for me) power amp.
I imagine I could drive it w/ something like a B15S preamp- 12AX7s- I'm really liking the Ampeg sound. Tho I dig the paralleled stages of the Hiwatts! And I do have the socket spaces.
Suggestions? Misgivings?
-
You could try a Sunn 2000s with 4 6l6's instead of the 6550's.
This would give you about 100 watts and it is a simple build.
Ernie
-
That looks good too. This is gonna be some kind of hybrid of several different amps, no doubt. Simple is good.
Since I'll have the chassis space I'll make some sort of guitar channel too, with a tube-buffered effects loop.
-
What kind of Twin? I've been assuming blackface, but there are lot's of Twins--- tweed, Twin Reverb (black and silver), evil, etc. And is the chassis complete and working?
-
You could try a Sunn 2000s with 4 6l6's instead of the 6550's.
This would give you about 100 watts and it is a simple build.
Ernie
The only problem with doing a Sunn 2000S is the cost and size of the OT....which in a stock early 2000S is as big as a baby's head and amplifies cleanly down to 10 Hz! I doubt if the stock OT would fit on the Twin Reverb chassis.
I vote for the stock Twin Reverb output section, with a bigger OT since you want to use the amp for bass, and a B15N preamp in one channel and a Fender AA864 bass channel preamp in the other channel. You could stick with the Fender LTP PI or go with the Ampeg paraphase PI depending on taste. This would give you a bass amp with some versatility in the tones. I would stay away from a guitar channel in there as a bass amp design is quite a bit different than a guitar amp design and to make either work well you tend to compromise both.
Greg
-
Sluckey, it's just a chassis. "AC80" I think is the designation. Weber is selling them for $15 each, I had to bite. It looks like a BF style but I don't have exact specs; I only know I've got 4 octal spaces to work with & I suppose 6 novals. I know I'll have to drill it at least for the OT.
This will be a scratch build. When I get the chassis I'll know more.
Ernie Jr I appreciate the reality check.
My only problem w/ B15N is I want to use tubes I have on hand, 12A_7s, 12DW7 &/or 5879s. Is there an easy conversion from 6SN7s?
BTW, any thoughts on the 100w Marshall style OT Doug sells? Somewhere it seems I heard they are more finicky with matching impedances, or is that just with EL34s?
I just want to get that little nagging question out of the way.
-
a long time ago i started a similar topic where i came up with a bass amp that has 3 preamps. fender, sunn and ampeg. it's going slow (too much touring, no time in the workshop) but i now have the cabinet ready, the chassis (super twin reverb) has the PI and PA and i'm drawing the preamp layout.
-
I'm not a bass player (I don't even own a bass) so 2 preamps will suffice.
BTW any suggestions for a decent low-end bass guitar?
-
Look at Fender Squire line of basses
-
Hey FM,
How serious are you about building a bass amp?
I am a bassist, so I can tell you headroom is your friend
on the bottom end!
Since you are starting from a bare chassis, you can go in a lot of directions.
I'd start with finding how much P/S you can sqeeze in, then select output tubes
to take full advantage of that.
4 big octals;
KT88s, 6550s, maybe you could be amoung the first to see what you can squeeze
from KT120s?
Shoot for 120-200W.
More wouldn't hurt.
Pre could be B15N, Sunn 200/2000S, or Fender.
Fender?
Don't laugh, the famed Alembic preamps are nothing more than AB763 pres!
Also, you can't scimp on speakers.
I'd recommend you check out fEARfuls.
It's a roll your own design, full range, high power, no compromise.
Plan on heavy, expensive...and fun when yer done. :icon_biggrin:
-
fEARful link:
http://greenboy.us/forum/index.php?sid=771c057aa974392ea1906be93f2c0ab7 (http://greenboy.us/forum/index.php?sid=771c057aa974392ea1906be93f2c0ab7)
-
Serious as a heart attack.
Since the chassis is for a TR I was just planning on a TR PT w/ a 100w Marshall OT (twice as heavy).
I have a 3A 6.3v filament tranny I guess I could squeeze in there for bigger tubes though I hadn't really planned on that.
I was hoping to use tubes I have laying around; I have a pair of 6ca7 & a pair of EL34 I still may use but I'm still bouncing ideas around.
I'm meeting a guy on Tuesday about 2-custom 15" speaker cabs w/ no speakers. That's probably the most I will be using w/ this amp so even 4-6L6GCs would probably get the job done.
-
For good example of what you can do with 4 octals, see the Fender 300PS.
It uses an interstage trans, instead of a more common PI, but it does
squeeze 300 watts out of a quad of 6550s.
Quite convincingly, too.
There is a guy on evil bay who was an engineer at Fender, working on
that and the 400PS.
He still has a stock of left over transformers.
-
For good example of what you can do with 4 octals, see the Fender 300PS.
It uses an interstage trans, instead of a more common PI, but it does
squeeze 300 watts out of a quad of 6550s.
Quite convincingly, too.
I absolutely agree. I have a 300PS as my main bass amp. I used to play in a LOUD punk bank. We would mic the drums for shows and the guitarists has 3 100watt Marshall stacks. The guys would always ask me to turn down the 300ps because it thundered!
-
Well I'm not likely going to build a 300 or 400PS, sorry guys. That is a level of complication I don't want to take on. 100W is plenty for my needs.
But I will have a look at the schems.
-
If you only want 100 watts, a 100 watt Bassman
would be simple to build in that chassis.
If you wanted to go big, a single channel version of the 300PS
isn't that much of a stretch.
-
The 300PS looks like it has a 6V6 as a driver? is that right?
-
Yup, it sure does.
-
Yeah, that's a little over the top for me.
My current plan is to use the output section of a bassman 100, except bigger gridstoppers & screen resistors. I'll probably use a 5814 or 12AU7 instead of a 12AT7. And EL34s instead of 6L6GCs. I'll use an auxiliary filament tranny. Will I need to alter the PI circuit or power tube grid leaks?
One preamp will be AA864 Bassman.
The other is open- I'd like to do a portaflex but the B15s is the only one w/ noval preamp tubes. Everyone is pushing the B15N. Is the B15S lame or something?
I'll make the power rail as beefy as space permits.
I'm meeting some guys from craigslist next week- one has 2-custom 15" cabs that are empty, and one has a new in box 15" Peavey Black Widow for $100. So this is kind of coming together. I'll be using one of the cabs occasionally as an extension speaker w/ my Vibroverb build.
-
Everyone is pushing the B15N. Is the B15S lame or something?
They are just different than the B15N, and the B15N is one of the best sounding bass amps of all time, so people tend to look at that one only when they consider any of the Portaflex models. You could grab the normal preamp channel from an SVT...it is similar to a B15N but uses the smaller tubes instead of octals. You would have to go with something more normal for the tone stack though instead of the inductor unless you really wanted a challenge.
Greg
-
you can sub the 6SL7 tubes with 12AX7. they are not identical but a close match.
-
Look at the ampeg b26. use the 2nd channel pre amp. it has a 12ax7 and it sound great for bass. used it in one of my builds and i am using it in a current build
ernie
-
OK, I will. Back to the B15N for a second. What kind of PI is that? It's almost like a paraphase inverter but not quite. I'd just as soon use the B15 PI if that is part of the sound, but would I need to modify it for 4 EL34s?
I know I'm being needy here with perhaps obvious questions, thanx for helping me with this gentlemen!
-
It is a paraphase inverter and is probably part of the sound. I wouldn't think you would need to change anything about the PI just to use it with EL34s.
-
I meant in terms of needing to increase drive current or voltage.
-
Me too.
-
Four EL34s? Double the power tubes?
-
The heck with it. I've got enough Fender bulids.
Sunn 2000s power amp & preamp. Looks like a tetrode feeding a concertina PI, I should be able to use a 5879 to feed a triode. (BTW is it safe to leave the unused triode totally disconnected w/ the heater on?)
B15N preamp for 2nd channel.
Maybe a tremolo circuit. Why not.
-
The Sunn is a nice amp. I'd definitely keep the original 6AN8 circuit. It's a proven design. Yes, you can leave an unused triode section totally disconnected.
What parts do you have that you must use on this project? Just a chassis? The 2000S needs 6 octal sockets, a looong chassis, and has a unique layout that could be hard to achieve on your chassis even if you go SS on the power supply. Iron and tubes for the 2000S will probably set you back $400-500 and may be difficult to find. I'm not trying to say you can't or shouldn't build a 2000S in a TR chassis, just trying to get you to think about it from a logistics viewpoint. And also ask yourself, "Do I really want to put this expensive experiment in a $15 bargain bin chassis?"
Sunn 200S is the same amp but only a pair of 6550s and iron to match. It will be loud enough for most any venue except maybe Shea Stadium. And it could easily be adapted to your chassis.
If you already have a Marshall 100W OT and must use that, you may consider building a Marshall PI, power amp, and power supply. And build the preamp of your choice in front of it.
I'm just throwing out ideas since you don't seem to be locked in on anything specific yet.
-
SS bridge recto, EL34s instead of 6550s. I'd be using a 100w Marshall OT and TR PT. A 6.3v trans will power the EL34 heaters. I was really looking for an EL34 quad output I could just copy. And I like the split load driver, don't ask me why.
I'm going to try a 5879/12a_7 just to see if it sounds good.
I was going to use a regular screen supply instead of the UL taps. And I use lots of shielded wire.
The chassis is actually pretty nice. It was in Weber's bargain bin. Not perfect but almost.
So i guess it won't really be a Sunn 2000s but if it works & is clean all the way up w/ good tone, well, that's what I wanted and that's why we're here, right?
-
that's what I wanted and that's why we're here, right?
Yes. Plus, the coffee ain't bad either! :icon_biggrin:
-
The stock Sunn 200S and 2000S sound fantastic until they distort, and then they still sound fantastic, but different. They tend to distort around 5...you can control this by limiting the gain in the preamp....lower voltages on the supply can be done to accomplish this. Listen to any early Rush stuff to hear distorted Sunns with roundwound strings on a Rickenbacker or Fender Jazz. The 2nd Jimi Hendrix album "Axis: Bold as Love" is a Sunn bass amp...roundwound strings...Fender jazz bass, but it is generally cleaner recordings. A lot of America's stuff was a Sunn amp with flats on the bass. Some of the Doors later recordings were also Sunn amps. A large part of the Sunn sound though is the huge and very good quality transformers used. You won't get that sound with a Marshall transformer....just not big enough or good enough quality. I could share some pics of the stock Sunn transformers and you should shit bricks...they are HUGE, and very expensive.
Another thing you may consider....the Marshall Super Bass....basically the bass version of the plexi. That would be four EL34's and a bass amp that was often used back in the day. Another amp that was used with that tube configuation is the Vox AC100...these were used for guitar and bass without changes and even uses a floating paraphase inverter, so it is similar to what the B15N is using. I built myself a clone of an AC100 that isn't quite done yet, but haven't tried it a lot for bass. It sounds ok, but my Sunns and Ampeg SVT sound better for bass than the Vox design. The Marshall was used a lot though by a lot of bands back then and sounds good....so you may check it out.
Greg
-
Ok, thanks SG! I already ordered the transformers so let me peruse the Marshall SB.
-
My most recent build was a sunn spectrum (same as 200s but with a guitar tone stack), I used BF bassman transformers and changed a few things at the tail end of the output section to jive better with that iron as opposed to the UL baby head on the original. It's a really cool amp, clean but by no means sterile, really big and articulate sounding and definitely different from a fender. I think you'll be very happy with your build.
-
My latest plan is this: Marshall SB output, PI values for a 12AU/T7 (closer to a Bassman 100 PI).
Sunn pa for one channel & a modified B15s pa for the other channel. I chose the "s" because I do not have octal pa sockets and it's a known circuit.
The transformers arrive next week and I've procured 2 15" MDF cabs. Anybody have opinions regarding the use of folded horns (w/ crossovers) in bass cabs? I could either keep the horns or cover the openings & install ports. I'm leaning towards porting.
-
Get the best bass speakers you can and use WINisd to find the best porting for the cabs you have.
It will help if you use speakers that you know the T-S parameters of.
-
Please peruse my design, I'm sure there are some mistakes but you get the idea. It's a hybrid B25b, AA864 and big Marshall output section. If there are any glaring issues please be brutally honest.
-
Your AA864 tonestack is different from the original.
-
It's the AA864 bass channel tonestack with no changes. I increased the cathode bypass values in that and the B25 channel is a hybrid of the 2 channels found in that amp.
I'm not real worried about the preamps, but the output section is a real frankenstein of a Bassman 100 PI w/ Marshall NFB and modified Marshall power tubes, even the bias section will have to be tweaked.
I plugged my 40w TOS into the Peavey Black Widow cab, with a bass guitar, and it was immediately apparent I'm going to need a lot more power to drive this speaker.
Granted, the TOS rolls off a lot of bass. But still.
That's why I'm trying to design for EL34 and 6550.
-
It's the AA864 bass channel tonestack with no changes.
Yes but your drawing is incorrect. There are three 0.1µF caps in the tonestack. One side of each cap ties together. Then one cap goes straight to ground. Another cap goes to the Deep switch which is also connected to ground. The third cap connects to the junction of the treble and bass pots. Take a look...
-
See, I told you there'd be mistakes. But any thoughts on the power section & PI?
-
But any thoughts on the power section & PI?
Looks like it will work. But...
Why 12AU7 and 330K grid resistors? And resistors across the power supply diodes? And such a big input cap?
-
My thought was this: the 6550 max gridleak resistor is a smaller value than 6L6GC or EL34, so I reduced the value to 68k (Bassman 100). Apparently I could use up to 150k, & still might.
So I think I need smaller value plate resistors on the PI.
Which the Bassman 100 PI already has.
So I used the Bassman 100 PI, which also has 330k gridleak resistors.
12AU7s seem to like smaller plate resistors; I would have tried a 12AT7 anyway.
I modified the tail resistor and NFB to suit (I guess) EL34s or 6550s.
The big input capacitance is for what I imagine to be large current reservoir needs on the output tubes. Maybe it is too much.
The resistors across the diodes I think I got from the Torres book. If it's a waste of time I'll delete them.
Thanks for looking it over Sluckey!
-
I think you will find that the 12AU7 won't have enough gain for you. You can always use the typical 12AX7 or 12AT7 LTP circuit from Fenders or Marshalls, and adjust the value of the tail resistor if you need to reduce gain. Fenders typically use a 10k, whereas Voxes with their sensitive EL84 tubes typically use a 47k for less gain. EL34's have more gain than 6L6's or 6550's, and less gain than EL84's, but a 10k is a good starting value. I think you'll find that the 330k grid leak resistors there don't sound as good as the more typical 1M's, and the 330k's will also lose more gain than the 1M. You may try a 12AY7 as the PI....they sound really good and give a good compromise between the 12AX7's extra gain and the 12AU7's extra current drive. You should be able to stick that right into a circuit intended for a 12AX7 or 12AT7 and get good performance and sound.
Greg
-
All right SG, that is the plain talk I was looking for! I do not have the mathematical aptitude for designing circuits, yet for whatever reason can't build an existing circuit as is. There must be a sniglet for someone who has to get in over his head, again & again.
So, a stock Marshall output section. I always roll different tube types as a final tweak so I'll find the best one eventually.
-
All right SG, that is the plain talk I was looking for! I do not have the mathematical aptitude for designing circuits, yet for whatever reason can't build an existing circuit as is. There must be a sniglet for someone who has to get in over his head, again & again.
So, a stock Marshall output section. I always roll different tube types as a final tweak so I'll find the best one eventually.
Hey man, we've all been there. I'm currently in school for an EE but they don't teach tubes anymore so everything I know about them I'm self taught. Bass amps have different requirements from guitar amps and usually higher voltages both overall and in the preamp help, as do somewhat different tube choices and gain scheduling, since a bass string has more energy. But go too low on gain, and then then amp sounds and feels lifeless....so tweak away but stay within reason or you may find you are undoing a lot of stuff...haha.
greg
-
I see kagliostro has gotten some good bites for bass amp ideas too.
Looks like the Weber 6S100 has already incorporated some of the ideas I'd had for my amp, including a paralleled cathodyne 12au7. I imagine a 5879 will work about the same as an EF86.
I think the B25 preamp will work better with it too, as a 2nd channel, since it is designed to feed a pentode-->cathodyne PI.
Alas, xmas is coming & money is tight, so I'm stuck for the moment with a 1/2 finished chassis and dreams. At least the heaters are wired!
-
I see kagliostro has gotten some good bites for bass amp ideas too.
Looks like the Weber 6S100 has already incorporated some of the ideas I'd had for my amp, including a paralleled cathodyne 12au7. I imagine a 5879 will work about the same as an EF86.
I think the B25 preamp will work better with it too, as a 2nd channel, since it is designed to feed a pentode-->cathodyne PI.
Alas, xmas is coming & money is tight, so I'm stuck for the moment with a 1/2 finished chassis and dreams. At least the heaters are wired!
The 5879 sounds really good, but has less gain (still plently) and a different sound character as compared to the EF86. The EF86 is brighter overall.
Greg
-
How about this?
Some values will have to be tweaked in the real world, specifically, CS100 tone stack caps, PI grid stopper/voltage dropper and power rail resistors.
I like the paralleled concertina but the CS100 cathode and plate resistors would have made it too gainy IMO, and I may even drop them further.
Again if I am very wrong about anything here please chime in.
-
The CS100 bass pot does nothing. Maybe connect the wiper to the top side of the pot?
V4 cannot work. The cathode is tied to node C just like the plate. No current can flow thru the tube. Maybe ground the cathode resistor?
-
Duh, that was a dumb mistake. Let me work on the stack, stand by.
All right, let's try that again....
-
The transformers arrive next week and I've procured 2 15" MDF cabs. Anybody have opinions regarding the use of folded horns (w/ crossovers) in bass cabs? I could either keep the horns or cover the openings & install ports. I'm leaning towards porting.
Something isn't right in this paragraph. If you cover the horn in a folded horn enclosure then you won't get any sound. A folded horn is a physical construction built around the woofer in order to create more sound and low end extension with less power. As to using them, I did for a couple of years - big folded horn 15" cab below a regular front loaded 15" cab with two channels of a solid state amp and an active crossover. Thunderous low end.
As to normal (seperate driver) horns in bass cabs, opinions vary, it won't hurt to try them and if you don't like the sound then disconnect them and leave them there for the look. Porting will depend on the given speaker in the given enclosure and if done incorrectly could easily cause damage to the speaker.
-
It's a regular horn, not a folded one. I'm never afraid to use a term whether I know what it means or not!
-
IT'S ALIVE!!!!!!
For anyone taking notes, 4 EL34s will push a Peavey Black Widow 15" 400W speaker. It is so loud.
As soon as I get a chance I will post the semifinal schematic. I think my preamp needs just a little tweaking, as the low E string sounds pretty loose & undefined. To be absolutely sure it's not the speaker, I will have to go to Guitar Center and hook it up to some commercial cabs. Probably I'll have to do 4-10s or something.
-
Here is the schem to the best of my memory. I will fill in voltages when time permits. I was focused on the pentode feeding the cathodyne, as the stock 270k plate resistor left 40 or so volts on the plate. So a little tweaking of power supply & plate resistors seem to have fixed that, now its more like 150v. IIRC the cathodyne grid has @-8v bias to work with which is good, right?
Anyhow, the low E string sounds pretty undefined and loose at any kind of mid-to-high volume, I can't be sure if it's the amp (with a 100w Marshall OT, possibly too large tone caps, and just-kind-of-guessed PI output impedance) or the homemade sealed 15" speaker cab, or even the speaker itself. Peavey Black Widow 1508 SPS. The B25b channel is a little tighter, but lows are still loose when turned up.
So let the tweaking begin.....
-
Well I tried it again, after rolling some tubes thru the preamp I settled on a 5751 in the B25b and a 12AV7 in the 6S100. I also tried turning down the bass knobs :think1: and that really helped a lot. It sounds pretty nice.
Methinks the 100w Marshall OT just isn't going to give me a tight sound on really low frequencies, no matter how much I tweak.
I'll tighten up this speaker cabinet here over the next couple of days and call it a day for now.
I ought to keep a diary & track the times when I really want to mess around with my amps, vs the times I can't be bothered. In this 2-month episode, I built a bass amp & a Vibrochamp, and put gridstoppers on all my Vibroverb grids. I'm tired of this stuff. Tube fatigue. There's a pattern here....
-
Here are the voltages.
I disconnected the parallel cathodyne triode, and changed load resistors accordingly.
Increased the main capacitance.
The 6S100 channel was too flabby so I went to partial bypass on the V2a cathode.
Stand by for results...
-
Could be. There were quite a few possible problems, so I'm gradually changing a little at a time. If the little tweaks I just described fix the muddiness, it's all to the good.
That paralleled cathodyne was a big question mark for me so I figured I'd simplify a little.
I'll report back tomorrow night after I work it out properly.
-
Well the tweaks were beneficial. The mud is mostly washed off. The Ampeg channel, which is stock, of course sounds the best. I think I will eliminate the bypass cap on the 6S100 channel entirely as it is still too boomy & gainy, even with a 12AY7.
-
Excellent link drgonzonm.
I wonder, however, if we are comparing apples & oranges, since the Marshall & 5F6a tone stacks are both driven by cathode followers, whereas my stack is not.
OTOH, I have a free triode now, maybe I could work a CF in.