Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tdvt on January 28, 2012, 11:10:21 am
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I've been thinking about trying to build a simple original design & came up with this. It's primarily intended as a harp amp & has a few features to that end (I hope).
I wanted it to have fatter sound & have extra bottom end available, so I paralleled the input stage (single/parallel jacks) & use the James-style tone control (variation on a lower-loss version I found values for) driven by a cathode follower.
I also paralleled the output stage for more watts, though it will probably still need a line into the PA. I don't play harp (in public) but play with a harp player on a regular basis & talked a little about it with him.
I chose to put the tone section before the last gain stage where alot of designs I looked at fed the power section directly from the tone circuit, but that didn't make sense to me. Seemed better to run the tweaked signal through a gain stage before the output. Advantages/disadvantages?
I'm afraid I'm still a "cut & paste" designer, so the values used are fairly standard from all the schematics I've amassed of similar circuits. The tone circuit was modeled in TSC so I'm pretty sure those should be OK.
Things I'm unsure of:
Coupling cap values (placement OK?)
Screen resistor & voltage?
OT impedance with parallel output tubes?
Implementing a line-out (possible to include the power section?)
I have not finished designing/drawing the power supply yet (this was my first drawing in ExpressSCH), I was thinking around 350V for B+ & also want to incorporate VVR on B+4. The guy I play with has a modded PR with lowered pre voltage & likes the sound. I'll probably go with a SS rectifier to start.
Anyway, there it is any/all advice & insights are appreciated.
Thanks, TD
(http://i47.photobucket.com/albums/f155/tdgrvt/TDSEamp.jpg)
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"I chose to put the tone section before the last gain stage where alot of designs I looked at fed the power section directly from the tone circuit, but that didn't make sense to me. Seemed better to run the tweaked signal through a gain stage before the output. Advantages/disadvantages?"
I think that's a great idea.
OT for 2 tubes in parallel should be half of what's used for one.
You forgot to add the coupling cap between last gain stages plate and volume pot to block DC.
Last gain stages grid needs a grid leak resistor. If you eliminate the .022 connected to it's grid it will have a path to ground through the 680K and bass pot. Or you could add a 1 meg after the .022(from grid to ground) if you wanted to keep the .022 but I don't think it's neccessary.
They say that it's bad to feed the power tubes with a pot only, because if it's dirty and loses contact the power tubes have no grid leak resistor. Some put a resistor from wiper to ground just in case.(I don't know why it's ok for 12ax7s but...)
I like the seperate cathode resistors you could pull a tube and switch to the next OT tap if you wanted to.
(you could even use a DPDT switch that would lift one's cathode and switch OT taps)
That's my $.02, Hope it sounds good. Good luck.
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There's usually a resistor on the top and bottom of the bass pot in that tonestack.
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There's no way to balance the two inputs.
That type plate-mixer has low gain and overloads very easily. Be prepared to separate the plate loads and use mix resistors.
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The 0.022uF coupling cap from the Treble pot wiper seems unnecessary; d.c. is blocked by the 0.022uF cap from the output of the cathode follower.
The "best place" to implement a line out would seem to be from the speaker output. Hammond makes a line-out transformer (the 109T) which has pins for pcb mounting (with proper technique, you could solder that on eyelets or turrets). This transformer is theoretically a 10k:10k transformer; place the primary across the leads of the OT secondary, and place a 25k pot across the 109T secondary. Use the wiper of the 25k pot as your line out. The pot causes the 109T to reflect a high enough impedance to be reflected in the primary that the addition of the line out has very little effect on the output stage.
What output level were you shooting for from your amp? Any reasons for the SE output? I'm guessing your harp-playing friend expressed a preference for SE. Do you really need two inputs? I know I'm moving away from any extra input jacks, as they never seem to be used; will your friend ever have a second player playing through his amp at the same time?
Do you have a Champ handy? What does your friend think about the overall sound of that amp as-is? What preferred speaker are you looking to use (it makes a huge difference)?
I ask because I wonder about the size of the coupling caps. I don't play harp, but some players seem to look for more low-end, while others might feel adding lows lends to feedback (not good with the microphone). I'd defer to real experience from players than my guessing with no experience.
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Thanks to all of you for taking the time to look at the schematic, your expertise is greatly appreciated.
The thread was meant to be called "FIRST original design" as I haven't tried this before, not much new about it as it's mostly borrowed bits.
All of your suggestions/corrections now seem fairly obvious, I will try to revise my drawing later today & repost.
As far as doing an SE amp; seemed like a simpler first project (sort of), definitely seems to be preferred for harp & none of the guys I play with have an SE amp (no Champ) so it was something different.
I was thinking of 2-10's for this, we play fairly small rooms & the harp through a SFPR w/a pre out, is usually just padded with the PA. It's almost loud enough on it's own, my hope was that this design would be just a little more ballsy but still be SE. But that might not be workable, hence the line-out on this one too.
Also, I might have the jacks drawn wrong but the two inputs are meant to be single or parallel stages on V1.
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Zero-in on your desired output power first; what amps are your buddies typically using for harp? Are we thinking 10w or 60w is the goal?
I ask because doing 2x6L6 SE is a big, heavy and expensive OT, depending on how far you need the bass response to extend.
A good working amp design should start with the output stage (with consideration with available power transformers) to target the power level needed. Then refine the skeleton with the speakers desired and the range/bass extension needed. Figuring these parts out dictates what the preamp must deliver to drive the output stage, and makes the design details come together better.
If you design "cool preamp" first, it may have too-much (or more often, not-enough) to mate well with the output section.
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Zero-in on your desired output power first
Actually that's how I ended up with this layout.
I knew I wanted to try a single-ended amp (tone-wise), but ideally/hopefully around 20-25W & that seemed doable with parallel output so that is where I started.
I was then initially trying to have just 1 pre-amp tube, a 12ax7 with a Fender stack between the sections, then I came across the James-style tone controls & liked the flexibility it had. But the information I was finding was that the James-style had alot of loss, so it seemed like a good idea to use a cathode follower in front of it, which would require another tube. At the same time I had seen alot of paralleled input circuits & from what I was reading they were fatter sounding & lower noise which were both appealing features & that would tidy up the package, using all sections of two tubes.
I have revised the schematic, incorporating suggestion & corrections. I moved the volume to the other side of the last gain stage though I went back through alot of other designs & immediately in front of the power section seems to be pretty popular. I think this is close.....
Thanks, again, TD
(http://i47.photobucket.com/albums/f155/tdgrvt/TDSEamprevB.jpg)
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> single-ended amp ... around 20-25W & that seemed doable....
The number of honestly 25 Watt Single-ended amps in the world can be counted on the remaining finger of roadies who have carried one.
OK, that's not quite true. However they are very few, and some which claim 25W may be fooling themselves.
Since you admit that PA may be needed, stick with a single 6L6 and a happy 9 Watts.
> two inputs are meant to be single or parallel stages on V1
If this were guitar, then it _would_ overload in the 3rd stage, before the volume control.
I don't think a cupped Bullet is weaker than guitar, though I do not know.
In general having the only volume control at the power tube is a poor idea. You "can't turn down". 6L6 was designed to "drive easy" from driver of similar B+... and no loss between. Turn down more than a trace, and the driver runs out of headroom before the output does. (And considering the cost/weight of the proposed 25W SE, you sure want ALL the goodness you paid for.)
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Looking at your tone stack, I believe the caps on the bass pot should go from outer lugs to wiper, in other words "in series" with the resistors instead of parallel. They're not actually in series, but I can't describe it right.
Just a thought on the wattage output: you'll be surprised just how loud 8/9 watts can be through a decent speaker.
Hope this helps! I'm still a new guy at this too, so I always follow new projects with much interest. I just installed the James stack in my build (switched from the FMV) and really like it a lot, FWIW. Cheers!
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Looking at your tone stack, I believe the caps on the bass pot should go from outer lugs to wiper
I agree.
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Wattage = Volume
that equation is only true if all else remains the same.
You can see TREMENDOUS gains in volume by using a high sensitivity speaker like the Eminence Wizard.
http://www.eminence.com/speakers/speaker-detail/?model=Wizard (http://www.eminence.com/speakers/speaker-detail/?model=Wizard)
You can plug in a few numbers here :http://www.sengpielaudio.com/calculator-efficiency.htm
again all things being equal, you won't find too much difference in perceived volume between a 12 and 25W amp.
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Looking at your tone stack, I believe the caps on the bass pot should go from outer lugs to wiper
I agree.
OOPS, definitely put those in the wrong spot. Had my hand drawn one correct but didn't translate over.
Seems like a single output tube is a much more realistic design, so I have revised the schematic yet again,
Thanks for looking & all the input,
TD
(http://i47.photobucket.com/albums/f155/tdgrvt/TDSEamprevC.jpg)