ideas about single ended amp design - new to amps

[black_labb]

hi guys,
ive recently become very interested in building a guitar amp, as im sure you all were at some stage (which is why you are her i believe). i bought a book on the subject (The Guitar Amp Handbook: Understanding Tube Amplifiers and Getting Great Sounds by dave hunter) and have been reading it alot.

I have been getting some ideas for a design, which will be based on a princeton circuit due to the simplicity . there will be some minor changes (alot of the changes are just putting in some switches such as a bass/normal/treble bypass switch on a on-off-on switch and other similar ideas)

i have been thinking of using a larger output tube such as a 6L6 as opposed to a 6v6 for a bit more output, as id like to be able to use the amp comfortably at practices and gigs, and the 12 watt or so of the 6v6 may not be fully up to the job. would changing this simply require a larger output transformer, and a power transformer with more current available? (aswell as adjusting the biasing and such)

another question, how would one use multiple output tubes in a non push pull configuration? you cant just put them in series or the second one would have too hot an input, would it be a matter of splitting the signal after the preamp and having them run in paralell, and then join them again to go into the output transformer? if this is the case, would it be easy to shut one half down to reduce the output (or even have separate volume controls for each half) for use for just bedroom play? would it be adviseable to have an additional preamp triode to boost the signal again after the splitting (or an additional preamp triode for each signal path). would this be basically a push/pull design without a Phase inverter making it a class a design (this is my beefed up princeon design still...) would this still be considered single ended as far as the ot is concerned? i know this might be a bit advanced of a circuit for a new builder, but i think that it is quite basic and just a second doing one section of the schematic twice, with a possible extra preamp tube.

now a couple things i would like to check my understanding on.

output transformers should be a level above the expected wattage of the tubes being used. is there anything wrong with using for example a ot rated for 75 watts in a champ or other low powered amp? this just means the ot doesnt run very hard.

phase inverters are only used in push pull amps. correct?



thanks for any help given

[Rob Mercure]

BL.

You might want to first build a Champ/Princeton "stock" just to get your feet wet in construction. To answer your other questions, output tubes can be put in parallel - in fact different tubes can be put in parallel and biased so that, for example, a 6V6 conducts at lower signal levels and a 6L6 at higher levels. But let's stick with two 6L6s for simpliciity - if you have the amp cathode biased you have to have twice the resistance for one tube (or half for two tubes depending on how you look at it). Also your output tranny's primary impedance will have to be halved for two tubes - this can easily be accomplished by moving the speaker to different impedance taps on the secondary to reflect a different primary impedance - so you will need an OT with a tapped secondary - there might also be some issues with altering NFB resistors if you use it. A 12AX7 or such should easily drive two paralleled 6L6GCs in class A. Your 75 watt OT is probably made for PP and wouldn't be properly gapped for single ended operation - you'll need a tranny designed for SE operation.

And, hope I've hit most of it, even a single 6V6 can be awfully loud for "bedroom" use.

Rob

[MWJB]

Phew ...that's a lot of Q's...

"i have been thinking of using a larger output tube such as a 6L6 as opposed to a 6v6 for a bit more output, as id like to be able to use the amp comfortably at practices and gigs, and the 12 watt or so of the 6v6 may not be fully up to the job. would changing this simply require a larger output transformer, and a power transformer with more current available? (aswell as adjusting the biasing and such)"...You'll be lucky to realise much more than 10W from a SE 6L6, gigging is still going to be a tall order (if you're serious about gigging it consider push pull). Nevertheless, if you buy an OT rated at 90mA this should be enough to handle a 6L6. A 6L6 draws twice as much filament current as a 6V6 so bear this in mind also when selecting a PT, 100mA B+ secondary rating should be OK. A BF Champ/Princeton style power transformer would do the job. Use a 10W cathode resistor for the 6L6 and a 100v cathode bypass cap.

Personally I would set up the bias of a single tube SE amp so that the 6V6 is running 100% dissipation (or maybe a shade more), then when you sub out for a 6L6 it'll be running 80%-ish. You could play with rectifiers to get a better sweep (e.g. SS with the 6L6). Of course, you could always switch in/out cathode resistors to optimise?

Run multiple power tubes in parallel SE, see the Gibson GA8 schem at the Free Info Society website, under electronics, audio.

"if this is the case, would it be easy to shut one half down to reduce the output (or even have separate volume controls for each half) for use for just bedroom play?" Give each socket it's own cathode resistor & cap and you can just pull a tube at will, if you want to run 2x6L6 in SE you'll need a relatively hefty PT or just swap 2x6V6, 1x 6V6, 1x 6L6.

A 75W OT in a Champ will probably quadruple the weight of the amp, cost an arm & a leg and sound sterile...10-15W would be ideal.

A SE amp doesn't need a phase inverter.

"i know this might be a bit advanced of a circuit for a new builder, but i think that it is quite basic and just a second doing one section of the schematic twice, with a possible extra preamp tube."...there's really enough to go wrong with your first build - layout & grounding errors, eliminating hum/unwanted noise etc. I'd suggest that you keep it relatively simply for your first attempt (say 5F2A style with 6L6 capability, possibly parallel SE if you like). When that's built, runs quietly and sounds great (when A/B'd against other SE amps) THEN start re-inventing the wheel.

[black_labb]

ok, im glad i seem to have the understanding i thought i had. what ive been thinking is to make the simple princeton circuit but leave points where i can add the second output tube in the box and leave the biasing in a way that it would be wasy to work on, and then start on the extra parts to the amp once im happy with how it would be running (and can live using my solidstate again).

what would your estimates be for the wattage for 2 6l6's in paralell? someone mentioned that getting 10w would be a bit of a stretch, does that mean the wattage would be around 16-18 watts? or do tubes in paralell not simply add together in this way?

thanks for your help guys,

[MWJB]

I wouldn't get too hung up on a W figure. Your amp is going to be class A, SE and in reality may struggle against PP fixed bias amps with an apparently louder voicing.

Pay more attention to finding a PT with a reasonable spec for running 2x6L6 in SE cathode bias. If you want to drop in 6V6s then you won't want much more than 330-0-330VAc for the B+ secondary, you might be running the 6L6s at 70+mA, so you'll want AT LEAST 170mA rating for the B+ secondary, preferably more. 4A+ would be nice for heater current, (5A+ if you want to try 6550 type tubes).

Likewise the OT will have to cope with perhaps 150mA, probably be in the 30W rating region.

As Rob says, a SE 6V6 amp can be very loud for home use (a pal of mine can be heard in the shop over the road when playing hs 5F2A indoors with doors & windows closed!). If you max this thing out you will probably have something too loud for home, but not loud enough to gig. Think hard about how you're going to use it most, a good 4-5W amp will always have a use (practice, recording, line it out for more volume).

[black_labb]

im still a ways away from making it happen, today i decided that the best way to do this would be to have the 2 output tubes each with their separate volume controls, to allow adjustment of each. if i want a dirtier sound at lower levels, turn one off and the other up, if i want a cleaner sound at higher levels keep them both relatively close together.

weber has a 25 watt single ended transformer which seems to be a good option if i do go ahead with the design ( https://taweber.powweb.com/store/magnetic.htm ). i dont know if this really would be too quiet for gigging, as it would only be small venues with adequate pa systems. i cant see something around 20 watts of output being too small. either way i will be deciding on the design further along the track, as ill probably learn a fair bit along the way.

thanks for the help

[MWJB]

As well as wattage you need to consider "envelope " and voicing. You can only really get an idea of wattage when comparing like amps. If you end up with say 15W your amp will only be 10-15% louder than a single 6L6 in SE, cathode bias will take the front off the note, your amp will be blown away by 15W fixed bias Pro Jrs & push pull Princetons on a stage. Maybe OK if your drummer plays quietly with brushes, or you don't have a drummer. If you're going to line out thru the PA and use a monitor then 5W will be as good as 10W or 15W. I know a guy who used to get a killer sound with a '56 Princeton mic'd up. Not necessarily trying to dissuade you, just highlighting the dynamics & pitfalls.

Have you thought about stereo SE with Left, Right and "Both" inputs, or patching accross when you want to use both 6L6s?

[black_labb]

thanks for that, ill do some more research, i never considered that an amp wont be as loud just because ofthe construction techniques. i was just going on the fact that an amp with 10 times the wattage is still only double the volume (because i dont need half the volume of my 100w fender solidstate, not too bad an amp for the money though). i didnt even consider that the design would have such a big impact on volume, thought it was just a tone thing.

ill check out what is nessesary for a amp that will be loud enough. might make a princeton copy (with some extra controls mentioned in the first post)and then get some ideas for a second amp.

[stokes]

Speaker choice will make a big difference in how much of the out put you hear.I have a 5E3 clone that had a decent Eminence speaker in it.It wasnt loud enough to use with a drummer,but when I put the EV12L in it,it is more than capable.I also built a 2X6L6GC SE and used a Hammond 1627SEA 30watt OT,tried different preamps and settled on a T-wreck type pre,it is loud.I run it thru a cab with a 15" Weber and a 12" Weber in a home made Leslie cab at the same time and it is more than able to keep up with a drummer.

[catnine]

I built a 5F2A and then changed the PT to a larger hammond and the OT to one Allen Amps sells and did a parallel 6V6 and if I pulled both 6V6's and installed one 6L6GC the OT had taps so I could do this. I had a 73 SF champ at the same time and all I can say is full up the parallel 6V6 or one 6L6GC was not that much louder if it was louder at all . With parallel 6V6's or one 6L6 I could let the bias alone .

I really thought it was a waste of time because the end result was not what I expected at all . It sounded good but not really better that a SF champ with one 6V6 . Yes the 6L6 was cleaner at higher levels and I used the same speaker for both just to see what the end result might be. You can build a 6V6 push/pull a lot cheaper and then you will have more to work with and it will be louder.

This is not to say you should not go with what you want , it was just my experience after lots of work . I also found that even with a larger PT and OT needed to power a parallel 6V6 which run at 100% the PT as well as the two tubes generated a lot more heat so I needed a fan to keep it cool. Two 6L6's in parallel will generate even more heat . I didn't skimp on tranny's either .

Gibson goldtones had two SE parallel EL84's for stereo to put out 30 watts yet I feel most of the loud came from using two heavy speakers it was like two SE amps one to each speaker using the same preamp section and a PI so it was really a push/pull with the PI running the two se sections .

[Austin]

I have gigged with a full band with my THD Univalve and a single 6L6 several times and been plenty loud even without being miced through the p.a. The funny thing is, a lower wattage tube is almost as loud as a bigger tube but the bigger tube is cleaner and bassier and harder to get the sweet harmonics like a 6v6 or 6k6. Most places I have ever played want to mic your amp anyways, so they can set the mix from the back and in that case it matters less the db of your amp as much as the tone so if you go cathode bias you could be able to try out several tubes to get the sound you like and then mic up. For instance sticking a 6w6 in an amp cathode biased for a 6l6 makes some super gain.. Even a little 6g6 or a 6v6 is plenty loud enough to make my ears ring for a day or two if I over do the volume with the right speaker. I have a celestion gold in a 1x12 closed box cab and it is plenty loud enogh for open blues jams, almost too loud really. I prefer to run through a full stack in a gig because though it might look more intimidating it is actually easier on m ears because the sound is a bit more diffused and with more presence. You can get about 20 watts (I know some may doubt this) safely from one el34 or 6l6gc if you really push it and have a big enough output transformer and power supply, with volume cranked all the way up into distortion... Some of these guitar speakers will deliver 100db with ONE WATT and that is enough to hurt your ears. More watts is more bass, that is the main diff.