TomCat
Ok.....I decided to start a new thread following from Karel's thread about his 5W amp project.
I've decided to go for a 8W Class A Amp head as a project.
In summary:
Channel-1:
12AU7 Pre for the clean channel (Hopefully I'll get enough gain to drive the O/P Tube properly), I/P Gain Control Only
Channel-2:
12AX7 Pre for the OD channel, Gain and Volume Control
Output Tube:
EL34, No Volume Control. Use an output attenuator - Full 8W, 4W, 1W ALA Vox AC4TV style. Loudspeaker impedance 8 / 16 ohm selectable if possible with an output attenuator.
Rectifier Tube: GZ34
Overall Bass and Treble EQ
No Reverb or Termelo
The main goal for this project is a guitar amplifier head that gives both clean as well as OD tones.
Let me know what you think and wish me luck.......
Manfred-Klose
Cool project.
I think a small head with a wood look will be pretty cool.
Is there any specific replica you are building, or just throwing some designs together and making your own?
TomCat
Its a kind of Fender Princeton meets Vox AC4TV with a slightly beefed up output to 8 to 10W and a Laney VC15 dirty channel added for good luck........in short......my own design with outside inspiration.
I ike the idea of the nice timber head look. Maybe some nice expanded metal inlays for ventilation. Could even go for a vintage clear sunburst type finish......
Gearhead
Correct me if I have this wrong but imho you're going to have a hard time getting 8W out of an EL84 in class A. In a one tube design class A is all you can achieve since tubes can only do positive signals. Two tubes can be combined to do anything from class A to class B which is how two EL84's can push out a maximum of 18-20W in class B, with a short life and massive distortion.
If you want to use a single EL84 you're looking at 6W max (which is enough when using a Celestion Blue like you suggested elsewhere, it's 95dB per Watt) but you will not have the crossover distortion that is so typically rock. If you want more out of single ended you go to KT66, 6L6 and other harder-to-get types.
matta
Gearhead wrote:
If you want to use a single EL84 you're looking at 6W max (which is enough when using a Celestion Blue like you suggested elsewhere, it's 95dB per Watt) but you will not have the crossover distortion that is so typically rock. If you want more out of single ended you go to KT66, 6L6 and other harder-to-get types.
Some EL84's in full pentode mode have a max output of 8W, so it is possible, but living on the edge... the REAL edge ?
I'm just interested to know what you settled on 8W TC? Why not 5W (like many great amps from the Champ to the GA-5)? What are you hoping to achieve by the extra wattage, 5W is pretty darn loud at full tilt! I think if you do want to go bigger, maybe try a 6V6 or 6L6.
Cheers
Matt
Gearhead
matta wrote:
it is possible, but living on the edge... the REAL edge
I stand corrected. The trouble with getting to that maximum power is that you need to get everything right - power supply must do exactly max. voltage at the ideal bias. Not something easy to do, especially since trial and error might lead to rapid failure.
matta wrote:
maybe try a 6V6 or 6L6
Ditto about the 6L6 but the way I read the data sheets you're not getting more out of a 6V6 than of an EL84?
On that note: my home brew P/P amp switches between EL84 and 6V6. In 6V6 mode I get audibly less power than in EL84 mode. Should I go and tinker with the output circuit because 6V6 should be able to do the same power or should I look at the phase splitter to remedy?
AlanRatcliffe
How feasible is it to design a small amp with an output valve you can swap for others? Something like the Cornford Carrera which allows you to use any octal or nine pin power valve. It also has three preamp valves (single channel) which can be swapped with any combination of 12A_7 valves. Always thought that was nifty - the ultimate low powered 12" studio amp.
Gearhead
I'd say it's very feasible depending on your requirements. The tubes are not all going to work at their optimal (... fill in whichever criterion you want: power, distortion, bias) unless you add some switching matrix for biasing, plate voltage, output impedance etcetera. The power supply can easily be dimensioned for the most hungry tube in the range but such an amp would lack sag on the others (if you're interested in sag).
My switching between EL84 and 6V6 was inspired by the text of Randall Smith's patent for tube switching which quickly leads one to find out that electrically those tubes are very similar. They require little compromise when combining them into one amp. Your all-tube concept would either require many compromises or quite a bit of switching.
TomCat
Interesting debate etc.......
I'm looking at the EL Thirty Four, not the Eighty Four for the O/P.
For me optimal will be 10W but I'll settle for 8W. my thought for the 8 -10w amp is that I'll get a little more clean gain at higher volumes that the 4-5W. I know it's only a small gain, but every little bit helps.
BTW I'm also looking at the Eminence Red Fang 10" as an alternative to the Blue. Has a whopping 102dB efficiency so it'll really shout at 8-10W.
http://www.eminence.com/guitar_speaker_detail.asp?model=redfang10&speaker_size=10&SUB_CAT_ID=4
Nobody is bringing them in so I'll have to get one direct from the "States".
For the first build I'm also considering scrapping the output attenuator until the amp is working 100%.
AlanRatcliffe
Damn, that's efficient for a 10".
The Emi 10" Ragin' Cajun has been getting a lot of rave reviews from guys upgrading their stock 10s in smaller amps like the Princeton. More of an American voice though. Both have good low end for a small amp.
If you really want the extra headroom, wouldn't it make more sense to build a 15W amp?
Manfred-Klose
For the first build
seems like the bug bit you ? big time, so there will be more TC amps? ?
TomCat
Alan Ratcliffe wrote:
Damn, that's efficient for a 10".
The Emi 10" Ragin' Cajun has been getting a lot of rave reviews from guys upgrading their stock 10s in smaller amps like the Princeton. More of an American voice though. Both have good low end for a small amp.
If you really want the extra headroom, wouldn't it make more sense to build a 15W amp?
I thought about going 15W with dual OP tubes etc......I will probably go that route as a second build as a compact combo later.....add a reverb tank and all......
I like the idea of a single OP design for now as the build will be simpler....This being my first journey into the tube amp world....
Heath
have a look at the AX84 project on the net , seems to be heading in your direction . www.ax84.com , and the guys are extremely heelpful
Karel-Mars
Alan Ratcliffe wrote:
How feasible is it to design a small amp with an output valve you can swap for others? Something like the Cornford Carrera which allows you to use any octal or nine pin power valve. It also has three preamp valves (single channel) which can be swapped with any combination of 12A_7 valves. Always thought that was nifty - the ultimate low powered 12" studio amp.
I looked at the Carrera and I think an ultimate low powered studio amp is worthwhile prototyping. I'll be tempted to leave the reverb and the fx loop out and use only two 12A_7's for single channel pre-amp with master volume... at least for starters.
The AX84 guys have a Single Ended Lead amp that seems to be the one called for when you want Marshall style tone controls.
TomCat
@ Matta and Karel.......PM'd Ver-1 of the schematic to you.
Simplified things somewhat in that it is now only single channel. Took out the OD channel for now.
In Summary:
> Single Clean Channel with single I/P
> Bass / Treble Tone Controls
> Volume Control
> 1 x 12AX7 Pre Tube
> 1 x EL34 Power Tube
> 1 x GZ34 / 5AR4 Rectifier tube
Looking to build as a small head with 8 & 16 ohm O/P.
Will keep you all posted on progress......
Gearhead
Sorry TomCat about the confusion 84/34.
If I can make some unsolicited advice: go for a different rectifier tube. Your EL34 and 12AX7 combined will eat 90-110mA while the GZ34 outputs 160. Compare the GZ34 to for instance the EZ40: The heater requirements of the GZ34 will consume twice as much energy and (since it is directly heated) will require a 5V of the power supply. The EZ, as the name suggests, can be heated from the same 6.3V as your other tubes which makes the amp simpler and the power tranny cheaper. It will also have more sag which will make for a spongier sound.
If you have any trouble sourcing EZ40 locally, I can quite possibly help.
For OPT I would go for the Hammond 125DSE. At 2500Ohm you will get 'only' 9% distortion on full input swing, is going to sound goooooood ?
TomCat
Hey Gearhead....No prob..... 8) 8)
Thanks for the advice on the rectifier tube.....definitely worth some serious consideration.
I want something that will sag nicely....as you say to get lekker spongy.....
Also thanks for the heads up on the O/P tranny as well....I presume Mr Valve will have that.
TomCat
@ Gearhead
Just had a look at the specs on the EZ40, which seems to be 90mA. I see there is also an EZ81 which is much beefier.....but maybe not desirable.....apparently very scarce.....
Just a question........Will the 90mA limit of the EZ40 contribute to the sag....and therefore the songy sound you are referring to?
Gearhead
The sag in any recto tube is a function of the output power. Some tubes sag more than others, so you will find a graph of the voltage versus current in most data sheets.
Now that I think about it: your amp will have no sag, being all class A. My mistake :-[ (although a common one). If you want sponginess, go class A/B or even B for which you will need push/pull.
TomCat
My main goal with the amp is to get reasonable clean gain at practice and mic'ed church stage levels.
Had a Peinceton on stage at church (an early one...Silverface) last week and seemed to "not quite make" it with clean gain on stage with a full band....there was a little....although not unpleasant amp breakup....
Based on my requirement, will the EZ40 still make it with the EL34 and the 12AX7 running off it....
