Thinking of building a guitar amp, many questions

[Cdog]

My main question is this: Can I splice together the preamp section of one commercial amp and the poweramp of another? I'm thinking of making a 5W SE amp, but using the preamp section of a high gain amp, such as the Bogner Uberschall, Engl Fireball, or similar. Is this possible, or will it just end up a mess with too many preamp tubes for the power tube to handle? If this is not possible, then the post ends here and I'll have to reconsider what I want to do, but assuming that it is possible, I have several more questions.

If it is possible, I will surely need a larger PT than what is regular in these small amps, due to the larger number of tubes. How much would I need to uprgrade the transformer?

For the power section, is the noval 6BQ5/EL84 a good choice, or should I try an EL34, 6V6 or other power tube? How will this affect my transformer needs? And should I use tube or solid state rectification?

Those are all of my questions for now. I know it's a lot, but any advice would be greatly appreciated.

Thanks,
Clay

[black_labb]

My main question is this: Can I splice together the preamp section of one commercial amp and the poweramp of another? I'm thinking of making a 5W SE amp, but using the preamp section of a high gain amp, such as the Bogner Uberschall, Engl Fireball, or similar. Is this possible, or will it just end up a mess with too many preamp tubes for the power tube to handle? If this is not possible, then the post ends here and I'll have to reconsider what I want to do, but assuming that it is possible, I have several more questions.

If it is possible, I will surely need a larger PT than what is regular in these small amps, due to the larger number of tubes. How much would I need to uprgrade the transformer?

For the power section, is the noval 6BQ5/EL84 a good choice, or should I try an EL34, 6V6 or other power tube? How will this affect my transformer needs? And should I use tube or solid state rectification?

Those are all of my questions for now. I know it's a lot, but any advice would be greatly appreciated.

Thanks,
Clay

why is it you want to put a high gain preampinfront of a single ended output?

usually people like single ended amps because they are easy to overdrive the powertubes. people like high gain amps because they can get distortion from the preamp. people generally arent interested in distorting both, as it will end up too distorted in differrent ways, and sound mushy.

you probably wont have the headroom from the output to get a good sound.

if you want the single ended design because you wanted it to be cheaper, why not do a push pull pair with 6v6's in fixed bias? the transformer would be roughly the same price as it doesnt have to be air gapped, and you can get some reasonable headroom on it to support the high gain pre without going mushy.

[defaced]

Sure, you can do it; that's the premise of the AX84 component amps. How easy it'll be to get everything sounding right and quite, that I don't know.

[Sir Cuitous]

You might be interested in this project at:

Tone Frenzy Amp

If you link through to the PDF with Scheme you may notice the local feedback loop on the first stage and the use of a snubbing capacitor on the plate of the second stage - presumably there to control maybe too much gain.

All in all, I think I would prefer a single 6L6 in a standard champ configuration (with the correct power and OT naturally).

[tubeswell]

Angela has an interesting looking super-champy thing I'm going to have a go at one day soon.

You can sub in an octal pre-amp tube, and have different power tube/rectumfrier configurations.

http://www.angela.com/catalog/how-to/Super.SE.6V6.html

for a little xtra on the PT Ht and heater windings, you could probably have enough juice chuck in a twin-style reverb

As one of the previous posters has said, AX84 has a similar super champ with a high-gain pre-amp using two pre-amp tubes, including 1 triode Cf into a 3-way tone stack with a Master vol - enough grind to blow anyone's head off.

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

[Cdog]

I was thinking of doing a SE amp because it seemed like it would be simpler, and because it seems to make more sense to me than push-pull operation. Something about the way that flows linearly just seems right to me, while push-pull is more confusing. I've never really dealt with high gain amps, so I don't know about the design specifics, but I guess thats why most of the high-gain amps are also high wattage.

I'm interested in the AX84 SEL project, but I can't seem to find any sound clips of it, and I don't know if it will be as high-gain as I'm looking for. How difficult is it to add another gain stage after the amp is built? I'm guessing difficult, but not impossible.

Thanks for all the advice, it is greatly appreciated. I need to continue researching, but any more pointers or information are very welcome.

Thanks again,
Clay

[tubeswell]

The AX84 in the link has three triode stages and a CF stage then tone stack and output tube. Don't forget that the charm of Class A output operation is the output tube distortion when the amp is dimed. Even a SE amp with just two stages (e.g.; 1 x 12AX7) and an output tube has nice saturated sound when dimed, but doesn't get you into screaming pre-amp hi-gain. I would say three stages (e.g.; 1 and 1/2 12AX7s) gets you to a decent crunch in SE Class A

The idea of the AX84 amps is that you can change them around, so you could drop the CF/tone stack and just have four triode stages instead. That would give you plenty of crunch/sustain to play around with. You probably have to muck around more with B+ supply rail voltages, interstage attentuation/buffering and probably bypass cap/resistor, plate and coupling cap combinations to adjust the crunch to your preference.

Of course adding a 3rd 12AX7 would get you even more (and be even more challenging to get right). (Altho' you'd need a the heater winding that can handle the extra 0.3A - and you may need a slightly higher specd PT HT winding). I'd say most off-the-shelf PTs are spec'd so that it should be okay to just spring in an extra 12AX7 (Your gamble).

[Cdog]

How tough would it be to throw in a switch that takes out the tone controls and just turns it into another gain stage? I read somewhere in this book I have about switchable tone section, I just need to go searching for it, because I know I don't have the experience to throw in another 12AX7 and have it actually sound good, so I'd like to stay in the boundaries of what is already laid out. I think for my first build, I may not get exactly what I want, but I'll work towards it, and the SEL would be a good start. On to the next order of business: What are some good brands of transformers to go for? I know of hammond, but are there any gems that aren't widely known that would be good for the SEL project? Thanks for being so helpful.
Clay

[tubeswell]

Well I would probably build the SEL that's there if you want a Class A amp with heaps of pre-amp gain potential. I bet you get a decent sounding amp anyway. The tonestack will sound good with that Cathode Follower stage going into it - and you've got three conventional stages in front of it, with two gain controls in there - potential to get quite varying types of gain. Plenty of distortion potential - and an MV to cut it down to bedroom levels if you want. You'll get unreal, even and odd order harmonic distortion - it will sound fantastic, and with that KT88 you'll have plenty of volume.

As for tranny's all you need to know spec-wise is on the 3rd page.

As far as PTs go, what's important are the volts/amps specs on the 2ndary windings. 275-0-275VAC @ 125Ma on the HT/B+ winding, and 6.3VAC @ 4A on the heater winding. If you are going to get your PT wound specially, get them to chuck in a centre tap on the heater winding as well - easier for grounding. (the center tap becomes the heater-winding ground and gets hooked up to the 'top' of the cathode resistor/bypass cap (R9 and C8), instead of having that other connection - referenced as "A" - from the same location to filament pin 7 of V1.) What you are aiming to get when you switch it on with all the tubes plugged in, is something within the ballpark of those red DC voltages (measured with your DC voltmeter from each respective appropriate junction/node to the chassis ground)

As for the OT, its straightforward - just follow the specs on Page 3

The rest of the parts are listed on page 4 and 5

When you're done, if you don't like it you can change it. But personally I'd try building the amp that was there first and get it running. That way you'll come to understand the various issues and characteristics of the amp.

If you think its too hard for the first one, and would just like a simple 5W Se Class A amp with one tone control and 1 vol control, then build a 5F2A - they are neat little amps too, plenty of fun, and have stood the test of time.

[womantone]

Cdog, the switch you are referring to just bypasses the tone stack. In general a tone stack will rob some gain from the signal path but it gives nice control to the signal. A lot of people are seeking these high gain amps, when in truth a lot of the older designs already get a good amount of gain its just people can't use tone controls to shape their sound. Like for instance a good lead tone is defined by the midrange, where as chords are more defined by the ends of the spectrum with bass and treble. When building an amp you want to consider how much headroom you need, whether you want the preamp or power amp distorted. There are so many coefficients. Sounds to me like you need a marshall preamp, then modify it for cascading gain by putting a volume in front of the first preamp in order to saturate it. I would use a push pull design for the power stage. And use 6v6 or El 84 to keep the volume at a decent level.

[mooreamps]

SE are generally easier to build than P/P. No issues with PI's , bias power supplies and such, and 10 watts SE can still be pretty loud. Also, yes you need at least three gain stages to drive the power tubes. So, you could use 2 pre-amp tubes. Three gain stages for , well for gain, use the fourth stage as a cathode follower to drive the tone stack, and you're in business.

-g