small single-ended amp with headroom

[luthierwnc]

Hi All,

One of my winter projects is going to be a little amp for portability or that I can leave in the shop without shlepping a big amp up and down the stairs. 4 to 6 watt single-ended with one or two preamp tubes and a 7868 or 7591 in the final.

Little amps like this are supposed to break-up really quickly but I'm looking for a schematic of one that holds the cleans together longer than usual. If you have experience or have heard of a little unit that doesn't turn to dirt on 1, I'd love to hear about it.

Thanks, as always,

sh

[Chuck H]

Well, how quick that sort of design breaks up is just a matter of matching the OT to the task, biasing center and not pushing the amp into distortion with too much grid signal. Those sort of amps don't break up early, per se, but since they're not unbearably loud no one can resist the urge to crank them into distorting. But the facts are the facts. Class A isn't good for sheer output volume. A pair of 6V6's in AB1 can produce over twenty watts. A single ended 6V6 will usually make about three watts. IMHE the little amps just sound so small that the clean tones lack umph. It's because they're small as watts go. Good cleans, as most guitar players percieve them, simply need more watts behind them. But much can be done to improve the typical "Champ" like circuit for clean tones. Go with a solid state rectifier so you can use bigger filters. Use a bigger OT for the bigger tube, then go a little bigger than that. Use fixed bias instead of cathode bias and make sure your center biased. Use an efficient speaker. Etc., etc... All the normal stuff you would do to make an amp louder, basically.

[tubeswell]

If you want a clean SE amp just take a normal Blackface champ design, and use higher (~350 pre-amp, ~400 6L6 plate) HT voltages through-out, and set the 2 pre-amp stages up with 100k Ra and 1k8 Rk fully bypassed, and use a 6L6 output tube with a big output transformer and a 50-100W speaker.

[Amp Kat]

I agree with both of these guys above and have designed and built one from scratch however mine is a dual channel and what they are telling you is spot on about the cleans and wattage. A single tube just doesn't push out cleans like a pair does and if you start cascading preamp stages to get amplitude it starts distorting. I did basically the Fender Deluxe AB763 for my clean channel except I added two stages for tone recovery. This goes to a single 6V6 stage Cathode biased with no negative feedback and a Mercury single ended FTPO5 that is a beautiful sounding tranny and I used the Champ output design except for the feedback elimination.

Transformer design here is a big one and imo the Hammond 169 series isn't beefy enough and just doesn't sound as good as some of the others and I ended up using a old vintage Hi-Fi tranny and taped off one of the windings and went CT to a FWB SS rectifier. Another problem you will run into is motorboating so you really need to toy with the filters and dropping resistors for the nodes however since you don't have a distortion channel that will be easier for you. This amp gets unreal clean tones and could be a little louder but will suffice for a small around the house amp. Good luck with the project.

[Steve Conner]

I agree, "small single-ended amp with headroom" is an oxymoron, like "military intelligence" or "airline food".

You can make breakup set in later on the volume control by reducing the gain of the amp. Either fewer stages, or a more complex tone stack to throw signal away, or a lower gain tube than the classic 12AX7, or a power tube that needs more drive: 6L6 instead of 6V6 or EL84.

But messing with the gain structure won't affect where breakup sets in in terms of output volume, which is what really matters. (see the Spinal Tap "Up to 11" sketch for an explanation) Only a bigger output stage or more efficient speaker will fix that. Pete's suggestion of a 6L6 and large output transformer would do nicely.

I have an apocalyptic mini-amp that uses an EL84 as a driver to push a 40W transistor output stage. Lots of headroom per pound of schleppage there.

[Jazz P Bass]

Take any 'Champ' type amplifier and hook it up to a really good speaker cabinet.
You will be amazed at the tone.

[Chuck H]

I've had a jones to build a 25W "Champ" from an 813 tube. Here's one next to a KT88. Not the pounds per watt efficiency of Steves plan. But that's not the point with this.

[echuta13]

I think Chuck H & tubeswell are right. If you keep your preamp voltages above 180v on the plates that should help with your headroom. I think the blackface preamp would be a good choice. Beef up filtering, OT, and if you run cathode bias, run it with a big bypass cap (100uf-1000uf). Running it with an efficient speaker IMO is probably one of the best things you can do. You should be looking for something with a 100db+ efficiency. Celestion Blue, Eminence Red Fang,Governor,CRex, Legend 1518 (my favorite) all fall in the 100-103db realm.

[luthierwnc]

Thanks guys. I should have stated my case better. I wasn't actually looking for clean -- just cleaner. If it is relegated to be the test bed for guitars coming through the shop, it shouldn't be a dirtbox on 2. 4 yeah but not 2. The lucky donor is a Bogen CHB10A PA with a single 7868 and a 12AX7. The schematic looks pretty usable as is but I'll probably redo the rail to get more volts on the preamp plates.

I haven't seen a NFB like this one, though. 100k coming off the 16 ohm tap to the cathode of V1b but also a 150k resistor from the cathode to the end of the rail. 470k plate resistors on both halves. The cathode is directly grounded on the front half and there is a 470R resistor on the second. I'd like to get that closer to 66:1 for cleanliness.

Haven't decided on a speaker. I've got lots to try before buying anything. echuta13, you might be on to something with the Legend.

Cheers, Skip

[Amp Kat]

I haven't seen a NFB like this one, though. 100k coming off the 16 ohm tap to the cathode of V1b but also a 150k resistor from the cathode to the end of the rail. 470k plate resistors on both halves. The cathode is directly grounded on the front half and there is a 470R resistor on the second. I'd like to get that closer to 66:1 for cleanliness.

Cheers, Skip

Why don't you just rewire that stage to BF Fender as there is really no use for a mu amp in guitar amplification or perhaps it's a direct coupled driver but still not sufficient.

[GregS]

If you have experience or have heard of a little unit that doesn't turn to dirt on 1

If it is relegated to be the test bed for guitars coming through the shop, it shouldn't be a dirtbox on 2. 4 yeah but not 2.

None of the 5W single ended amps I've had experience with broke up that quickly or easily. Perhaps whatever you've experienced was designed or modified to distort in the pre-amp and the headroom is actually not being exceeded in the power amp. You should be able to get more than enough clean volume out of a 5W amp for simply trying out guitars by themselves.

Greg

[strata]

None of the 5W single ended amps I've had experience with broke up that quickly or easily. Perhaps whatever you've experienced was designed or modified to distort in the pre-amp and the headroom is actually not being exceeded in the power amp. You should be able to get more than enough clean volume out of a 5W amp for simply trying out guitars by themselves.

Greg

ax84 p1 becomes a cleaner amp if you change the tonestack position and move it to right after the first gain stage instead of having it after the second.

[catnine]

I built a SF champ homebrew and set the cathode bias so the 6V6 runs at 14 watts and everything else on the amp is true to a SF champ spec except the PT I got from weber and it puts out a bit more voltage and has higher amp ratings 100 mA instead of the fenders 70 mA and it's got a 3 amp 5 volt . The plate runs close to 375 VDC and I also used a weber OT for a champ but it has a 4 and 8 ohm tap so I used a jensen C10R 8 ohm and a JJ 6V6s and a 5Y3 rect and it does not breakup until it's on 7 with single coil strat Pu's .

I did build a fender 5F2A and then got a larger Hammond PT and an allen amps OT that could handle a pair of 6V6's in parallel or one 6L6 GC . For the most part it being 6V6's in parallel didn't make it any louder than my stock SF champ home brew . Plus the 5F2A had a vol and tone only so that will make it break up sooner than a BF tone stack .

I ran a 73 SF champ through 2 twelves and then it was clean so the speakers do matter IMHO more than anything else . If you go with a larger OT that can handle a 6L6GC and a 10 or 12 "speaker that can handle 50 watts or more you will be clean and still get breakup yet it's still not going to do the job of a push/pull amp even at the same wattage. I like SE amps and push/pull's each has their own charactor and sound .

[Chuck H]

I built a SF champ homebrew...except the PT I got from weber and it puts out a bit more voltage...The plate runs close to 375 VDC

Huh.?. The last SF Champ I worked on had 423Vp! I think they're all over 410Vp.

I ran a 73 SF champ through 2 twelves and then it was clean so the speakers do matter IMHO more than anything else

Are you saying that the amp was CLEANER through the 2X12's? As in more clean watts, or less distortion at the same amp settings? Or are you saying that the amp was LOUDER through the 2X12's, producing more volume when running your usual clean amp settings? I think it's an important distinction since issues of power, headroom and loudness get kicked around here a lot. Often incorrectly and obviously misunderstood.

[kleuck]

I would go for a 6L6 "5F something" type of amp with a 12AY7 in the pre (or a single triode, etc) I use all the time my VHT Special 6, i mad it a crunch machine -even with 6L6- but stock with two tonestacks (a fixed "blackface" one, champion 600-like) AND a "Princeton" one, it was pretty clean, despite high loads on the triodes (220K) Actually, i would go for a 5F style amp with 6L6, lower anode loads (classic 100K) and a Bandmaster tonestack, should fill the bill.