Gee-Tah 100 Guitar-amplifier

[tri-comp]

Hello,

It's time to get this project back on tracks. It was started about 3 years ago when the drawings for the chassis-construction were done. Some schematics were also done even though not 100% completed. In the mean time not much has happened because of one single issue: The person who promised to do the laser-cutting and subsequent bending of the steel-chassis never did so. Another opportunity presented itself recently to have this work done, even if it was at an unimaginable price. The lot set me back no less than 3 cases of beer. Mind you, Danish beer at that !
Since I didn't really build any Guitar-amplifiers in the past I'm sure there will be a lot of corrections to the schematic. I was inspired some from this project and did a little research into building guitar-amplifiers on a much smaller scale. Here's what happened at the time:
https://www.vintage-radio.net/forum/...ad.php?t=82321
The guitarist, who will eventually own this amp, has the final say in tone-, distortion- and power-matters, obviously.
For now have a look at some chasis-pictures and the schematics.
About the schematic, I'm not sure about running EL34-B G2 on UL-Tap's.
Possibly they'll end-up on around +300V instead.
I only promised the guitarist 100Watt but this has potential for a lot more if driven to the max.


rgds,

/tri-comp

That is a really nice chassis, where do I send the beer? I have yet to see a guitar amp that used UL taps.

[tri-comp]

I would wait until they've finished the ones I send.
Then they'll be more motivated for doing another deal

Anyway, this thread has a lot of pictures showing how my previous amp-chassis was cut and bended.
https://www.vintage-radio.net/forum/...ad.php?t=67511

Since I won't have unlimited access in the future to have produced whatever chassis' I fancy, I'm seriously thinking about buying a CNC-mill. It won't be anything that will handle the size of the amp-chassis in this thread (0,5m wide/2mm stainless steel) but less will do.

/tri-comp

[Diabolical Artificer]

Nice chassis as always,we've discussed using 807's a few times, have you discounted their use?

I've been looking for a 50w + guitar amp to build for a little while. I was going to build an amp built on the lines of a Fender Bassman using 6L6's/807's. Using 807's is problematic as the max G2 voltage is 300v as you know. You could use four instead of two though. But as good 807's have gone up in price recently and 6L6/El34's are about the same price what's the point?

Why use UL in a guitar amp though T, I thought the idea is to introduce distortion in a guitar amp rather than suppress it?

Will follow this thread with interest, nice to see you making amps again, Andy.

[tri-comp]

Hi Andy & All,

807's are available in the 807W/5933 variant, possibly at lower prices.
Not quite as 'nice' looking but the same 'oomph'.
I have 5 x NOS of these stashed away for future pleasures

UL for a Guitar-amp may indeed not be suitable when you expect the amp to perform under some form of distortion.
The particular guitarist this amp's built for, is a strong fan of Swedish group Spotnicks.
https://www.youtube.com/watch?v=nq2jeWr0y9U
He claims that they are still touring and are actually using a HiFi Valve-amp for a Guitar-amp on-stage.
A HiFi-amp, NOT a Guitar-amp, the point being that you may add any distortion needed, required and/or desired using external sound manipulating devices.
Hence the Send/Receive facility of this amp.
In any case I have very strong doubts of any output-stage overdrive distortion this side of 100Watts. Have a look at the Edcor xfmrs I've used.

rgds,

/tri-comp

[ColinTheAmpMan1]

Originally Posted by merlinmaxwell

I have yet to see a guitar amp that used UL taps.

Come and take a look at my two "homebrews". Both admittedly derived from amplifiers intended for record-players and the like and so UL not surprising.

I have found the Magnatone 180, Marshall 1966 (admittedly a PA amp), Marshall 1967 Lead, Selmer Zodiac Twin 30, Sound City Bass 150 and several Sunn amps having UL circuitry. There again, I am a smarty-pants with loads of amp schematics! (Actually, Aspen Pittman's "The Tube Amp Book" to be honest).

I think that it was more of a case of budget than not wanting "hi-fi" performance. An O/P transformer with UL taps would be more expensive than one without, so why bother if you can do without? There was a tendency to make guitar amplifiers have reasonable fidelity; how else would you explain the inclusion of negative feedback loops?

Indeed, the handbooks that came with many commercial guitar amplifiers (certainly Fender) suggested that the user should turn down the volume control to lessen or eliminate distortion. Those of us old enough will remember the great "clean" sound of the Shadows through their Vox AC15s. They only wanted the AC30s so that they could be louder and fill bigger auditoriums with sound, not for distortion.

It was probably in the later 60s when the benefits of a slightly "gritty" distorted guitar sound were discovered. The British Blues invasion was probably significant in popularising the distorted guitar sound, but Fender and other US amplifier manufacturers didn't catch on immediately. The almost-legendary Fender Twin Reverb (with sometimes over 100 Watts output) was one of the cleanest-sounding amps made. If you want to get distortion tones from one without a distortion effect pedal or some other add-on, you would have to turn up the volume so far that the noise would probably be heard on the far side of the next state!

[ColinTheAmpMan1]

Originally Posted by tri-comp

The particular guitarist this amp's built for, is a strong fan of Swedish group Spotnicks.

Ah, yes. I remember "Orange Blossom Special" and "Hava Nagila". I still have the sheet music of that last one. Not a group particularly known for a distorted sound. (As many others of the time).

[julie_m]

If you build an amplifier that amplifies cleanly, with an insert point, you can always add as much processing as you like to distort the signal upstream of the power stage. And you still have the versatility to produce an almost "acoustic" sound; which you might appreciate if you ever want to do something really stripped-bare.

Whereas if you rely on driving the output valves into distortion, you will be stuck with the sound of the amplifier.

[ColinTheAmpMan1]

That may be considered a bit simplistic for designers of guitar amplifiers. The notion of driving output tubes into distortion is appealing in some circumstances, but that only happens when you are putting out a lot of output Watts. Pre-amp valve distortion is of interest, too. This is where what you refer to as "processing" is of significance.

Any guitar amplifier which relies solely on output tube distortion is probably either failing in its prime intention, or deafening the audience, depending on what the output stage is designed to provide in term of Watts of acoustic power.

Don't forget, some amplifiers are louder than others, despite apparent output Wattage, because they have more efficient loudspeakers. A good example of this is the Vox AC30, which, despite only having a quoted output of 30 Watts, sounds as loud as many 100 Watt amps because of the remarkable efficiency of the Vox Bulldog speakers (Celestion made).

As an object lesson, try listening to an electric guitar played through a hi-fi system; it sounds awful, usually. A bit of distortion/non-linearity/etc. improves the sound in most cases - but I'm prepared to be surprised!

[ms660]

"As an object lesson, try listening to an electric guitar played through a hi-fi system; it sounds awful, usually. A bit of distortion/non-linearity/etc. improves the sound in most cases - but I'm prepared to be surprised! "

I'll agree with that for electric guitars, an acoustic with a pickup sounds ok through a hifi though, to me anyways.

I don't have a guitar amp these days but I do have a Korg Toneworks which seems to do the biz for effects.

I'm saving up for a National Resonator which has always been my dream.

Lawrence.

[tri-comp]

Originally Posted by ajs_derby

If you build an amplifier that amplifies cleanly, with an insert point, you can always add as much processing as you like to distort the signal upstream of the power stage. And you still have the versatility to produce an almost "acoustic" sound; which you might appreciate if you ever want to do something really stripped-bare.

This nails the idea behind my amp. quite nicely.
First be sure to get enough uncoloured and undistorted power to play with in any direction you may want.
If the pre-amp overdrive and tone-stack/presense-control won't get you there, modify using (even cheap) external devices at the send/receive point.
If the present schematic will live up to that goal remains to be seen.
Modifying the circuit won't be too much of a problem as sufficient high-mu (~65) amplifying triodes are available for most any modifications required.
The 12AC10 Compactron is just a 3/2 x 12AT7/ECC81 combo chosen for the fun of it. Well, and price ...! Sylvanias were ~$1 about 3 years ago !

/tri-comp

[tri-comp]

More work done.