Beginners Guitar amp project, any suggestions?

[weatherlight]

Greetings

I recently finished my first amplifier project a few months ago. It sounds well, works well. But its kinda heavy (23kg for 40w combo guitar amp) and overdrive sounds are not as pleasing as they would be in a tube amp.

So I decided to build a tube amp. Although I'm an electronics engineer student, they dont teach us about tubes back in the univercity. I'm also a little clumsy but I think I can manage to build a simple tube amp without getting electrical shock

I came across this amplifier while browsing the forum. It seemed like a good place to start. The extreme one uses a more powerfull tube at output (or thats what I got) compared to the standart 5w version.

http://www.ax84.com/p1x.html

It has pictures, schematics and some other stuff as well. I want to build a simple tube amp (at least with eq and gain controls however. I saw some junior amps which had a vol pot only ). Any output around 8-10W is fine for me. Is this the project to build? Or are there some other alternatives which sound better and are easier to build (more pictures, schematics, building manual, guide etc)?

Thank you for your help

 

[diegot]

The P1X don't have too much overdrive. You can build the Hi-octane preamp with the P1X output stage.

I will modify my P1X with a 6EA8 for more gain and better overdrive.

 

[weatherlight]

I just checked the schematics. HO has 3 gain stages (I'm not sure what the cathode folower does, is it just for unity gain? ) where the normal p1 has 2 (which will have less overdirve for sure)

Will this 3rd gain stage add up a massive overdrive? I like to play rock and metal songs with my guitar as well and i dont like mild to low crunch tones too. However I'd like it to be as simple (and cheap as well, i'm on a tight budget ) as I can be. Will the 3rd gain stage make that much a difference?

Thanks

 

[How_Bout_It]

Take a look at this page: http://www.el34world.com/schematics.htm
About half way down is the 18 Watt Stout. Looks interesting

 

[diegot]

For sure more gain stage gives you more overdrive.

If you want be simple, go for a pentode/triode preamp.

 

[weatherlight]

I liked the preamp stage of Hioctane and SEL amplifiers.

http://www.ax84.com/classicprojects.html

problem is they offer an output of 5w (sel offers some more around 8-10 i guess) in class a config. However I could not find KT88.

I would like to use El34 (since I can find them around here) at output stage. I'm geussing it'll provide about 8-10W with class a config (P1x amp at the same site).

What I need is a SEL preamp (3 stages) with EL 34 at output (either class a or push-pull config) I noticed that the B+ voltages are different for the EL34 (p1x) and KT88 (sel). I need to place a El 34 in the sel amplifier. What values do I have to adjust (B voltages? ) to get an El34 to work nicely instead of the KT88 in there?

Also I noticed that the 18w uses 2 EL84s at output. I believe I can find EL84s as well. Can I implement this push pull power amp stage onto the sel preamp? Again what shall be the B voltages, and some other component values when i change these?

I'm new to tubes and as far as I can see, biasing and those b voltages are critical. I dont want to blow the tubes or get a distorted low output. I noticed there is a variable cathode bias mod for the amplifier. Will that do the trick? Please help.

Thank you.

 

[aletheian]

weatherlight, there is a fantastic discussion forum over at AX84 http://www.ax84.com/bbs/index.php where the project coordinators as well as hundreds of people that have built the various projects and kits hang out to discuss the amps mods and whatnot. You would probably get the answers you need over there from the people that have built them.

 

[How_Bout_It]

To change from a KT88 to an EL34 in the SEL would only need to change the cathode resistor R9 (430R 5W) to set the bias. To change to P-P would be a lot more work and parts.

 

[How_Bout_It]

Almost forgot, when switching from KT88 to EL34 you would also need to chang taps on the OT secondary.

 

[weatherlight]

Almost forgot, when switching from KT88 to EL34 you would also need to chang taps on the OT secondary.

Thanks I'll adjust the r9 for optimum bias for EL34 datasheet then. About the taps on secondary, can you explain it a little please? Do you mean the b+1 brown output for the output transformer? And how shall I adjust that ?

 

[gingertube]

weatherlight,
I see you say that you want to play rock and metal styles.
I assume you mean fairly heavy rock.

Perhaps a little counter intuitively, the way to get heavy rock and in particular metal sounds is to fairly masively overdrive the preamp section and run the power amp section quite clean.

My view is that you won't get the sound you want from a single ended power stage. You need to go to a push pull output stage. A pair of 6AQ5, 6V6 or EL84 would suit. You also need a 3 (minimum) gain stage preamp, the Hi-Octane AX84 preamp should be fine.

BTW the Cathode Follower in this circuit does 2 things. 1st it provides a high impedance load to the previous gain stage thus maximising the gain in that stage and second it provides low impedance drive of the tone control stack. This method of driving the tone stack is "Marshall'ish".

If you build the Hi Octane preamp I would play around with the values of the cathode bypass capacitors in stage 2 and stage 3. Too large and the overdrive sound can be flabby.

Cheers,
Ian

 

[weatherlight]

Unfortunately most music stores (hardware or electric stores dont have tubes ) only sell 12ax7 and el34s around here. It is very unlikely for me to get another type of tube, so I'll have to go for a dual (or single class a) el34 p-p power amp.

I'll search for a push pull el34 poweramp stage that I can directly implement over the sel preamp (I'm assuming that the cathode follower will allow me to put plenty of circuit configs after the preamp stage). Do you know any simple power amp circuits that are a pair of el34s?

Thanks

 

[gingertube]

There are lots of suitable schematics for a push pull power amp section. If you enjoyed wading through the various AX84 schematics then here is another lot in the same sort of vein.
There are a couple of suitable EL34 Push Pull Power Amp schematics

http://www.blueguitar.org/new/schem/trainwreck/wreckxpr.pdf

I built one of these myself BUT since I play Folk, Blues and Classic Rock I went for some different options and used a cathode biased EL34 push pull power amp. For Heavy Rock and Metal stick to a fixed bias power amp.

Cheers,
Ian

CAUTION: This is just one mans opinion (an educated opinion but an opinion still).

 

[chrish]

Russian 6P16P-EV (EL83) I think will work in AX84 amp, and are cheap on eBay. Russian 6P14P (EL84) are will definitely work, but not as cheap.

 

[weatherlight]

I liked the trainwreck express version A4. Power amp also features a presence which I really would like. The poweramp part (after the B+4 junction) doesn't look complicated at all. Problem is This will be my first tube amp project. I would like it to be as much by the books & schematics as it can be during the actual building. That why I choosed an ax84 project in the first place, since they even have chasis building pictures and diagrams, mounting schematics etc.

I assume I can put the poweramp seticon over the SEL preamp. What will be the b+ voltages for each amp? 325V for preamp tubes (B+5) and 390V for B+1 will be ok? What type of transformers shall I use for this hybrid combination?

 

[chrish]

I am still interested in building an amp for my son, but I am in the middle of moving house at the moment, so I am still in the planning stages. Will not have workshop set up again for a few months. I am interested in what you say Ian about the High Octane AX84 pre-amp with push pull power amp. My son wants something with rock/metal sound. Could the front end pre amp section of the Hi-Octane simply be mounted to the phase splitter and power amp of, say, the Fender Deluxe (12AX7 phase splitter and 6V6 GT Push Pull). This might be the ideal solution to my quest. I have all the parts for this except output transformer and speaker.

Sorry for any thread drift, but Ian's suggestion looks interesting!

 

[gingertube]

Guys,
We could get into a discussion of guitar amp design philosophy here but I'll try to point out the basic stuff.

In a preamp the differences largely lie in how many gain stages you have and where the tone controls lie with respect to those gain stages.

The A4 Trainwreck is what we call a 1 + 2 arrangement. It has a gain stage then a tone stack and then 2 more gain stages. Thats good for clean Fender to cranked Fender type sounds.

For the metal and heavy rock sound where you really want to overdrive the preamp section the AX84 SEL or HiOctane arrangement is better. It is what we would call a 3 + 0 arrangement. That is it has 3 gain stages before the tone stack and 0 after it - it feeds the power amp directly. This gives better control of the sound produced by 3 gain stges (at least 2 of which will be overdriven).

The 1 +2 arrangement ONLY allows you tone control from the relatively clean 1st stage and you don't have influence over the overdriven sound from the 2 post tone control gain stages.

By all means use the power section from the Trainwreck 4A BUT if you really want heavy rock / metal then the 3 +0 arrangement of the AX84 SEL or HiOctane preamps is the way to go.

Hope this made sense - basically the more you want to overdrive your preamp section the later in the chain you should have the tone controls.

BTW you also see 1 +1 , 2 + 1, etc. arrangements in the guitar preamps - all depends on the sound you want.

My home amp is a dual channel - the first channel (Clean) is a 1 +1, the second channel (Lead) is a 3 + 0.

Chris - what you suggest is entirely feasible. I'd have to look up the schematic for power amp section of the Fender you suggest but a pair of 6V6 will sound great. Once again the sound you want will dictate the design. For Blues a concertina splitter and cathode biased output tubes will be great. For a Clean Output Stage to reproduce the heavy preamp overdrive for a metal amp you are better going for the "standard" Schmidt Splitter and Fixed biased.

Once again- the example of my own home amp - I run 4 6V6G (the old coke bottle shaped ones) in parallel push pull with switching for each output tube pair between cathode or fixed bias AND between Triode and Pentode Mode. All thats a bit of a waste for me - I play Blues so that power amp stays in Triode Mode Cathode Biased 90% of the time.

I'd suggest you probably both want pentode mode fixed bias in the output stage.

Cheers,
Ian

 

[How_Bout_It]

Thanks I'll adjust the r9 for optimum bias for EL34 datasheet then. About the taps on secondary, can you explain it a little please? Do you mean the b+1 brown output for the output transformer? And how shall I adjust that ?

I've not built any SE amps so for an example I'll stick to PP but the idea is the same for either. You have a power stage with an OT having a 4K primary and three secondaries for 4, 8, and 16 ohm outputs. Your power tube expects a load of 4K, your 4, 8, and 16 ohm outputs will be used as they are. You swap a tube that expects to see an 2K load so to keep the the primary to secondary ratio the same you would use the output taps as 2, 4, 8 ohms. At least that's the way I understand it. There are many variables that affect the nominal values as stated in the specifications of tubes, transformers, and speakers. Hope that makes sense and that I didn't get that backwards

 

[Merlinb]

Also I noticed that the 18w uses 2 EL84s at output. I believe I can find EL84s as well. Can I implement this push pull power amp stage onto the sel preamp? Again what shall be the B voltages, and some other component values when i change these?

I'm new to tubes and as far as I can see, biasing and those b voltages are critical. I dont want to blow the tubes or get a distorted low output. I noticed there is a variable cathode bias mod for the amplifier. Will that do the trick? Please help.

Thank you. [/B]

Why not ask these questions on the AX84 forum??? They're actually familiar with the designs there! http://www.ax84.com/bbs/

 

[mashaffer]

Chrish;

My grandson's amp...