Making my own guitar amp

[Terry Bites]

OTL with cheap triodes?

[RJSV]

Bottom hatch has the Alkaline Eveready battery,  although not so dirt resistant located there !

   As in many projects,  the uh,  'unexpected' little problem involves the ext. (external) input jack (a mini),  that regularly CROWBARs my Alcatel cellphone.  The stereo input seems to favor one side of the usual left and right audio channels.

   Roommates expressed pleasurable listening,  to my Hendrix-inspired technical limitations,  which,  actually is an accomplishment in itself.
But,  I'm no artist,  as the Microprocessor is my real,  detailed occupation.

Anything and everything electrical is the most interesting,  for myself.o

[RJSV]

Picture,  of battery hatch, on bottom.

[themadhippy]

As a starting point you could go old skool and adapt an old radio,normally you can tap into the amp stage at the volume pot.

[TimFox]

OTL with cheap triodes?

OTL with cheap triodes (no output transformer) normally requires a higher load impedance than typical guitar speakers.
Cheap triodes are typically two halves of a 6AS7 dual pass tube, or triode-connected beam power tubes, working into > 100\$\Omega\$ load impedance.
An example of a series push-pull 6AS7 working into 72\$\Omega\$, along with how to construct a 72\$\Omega\$ speaker: https://www.tubecad.com/2010/12/blog0195.htm
The classic Futterman OTL amplifiers used multiple 6AS7s in parallel to drive a 16\$\Omega\$ load.

[TimFox]

As a starting point you could go old skool and adapt an old radio,normally you can tap into the amp stage at the volume pot.

That can work, but be careful about the "hot chassis" problem encountered with many old radios.
The output transformer from an old radio could be salvaged, but most old radios did not have a power transformer.
There are signal-level transformers that can safely couple into the volume pot of an ungrounded radio.
I have used these transformers successfully in test circuits, but they are pricey:  https://www.tubesandmore.com/products/transformers_chokes?filters=2341a3027c740a91c65a740
The same vendor has power and output transformers useful for guitar amplifiers, ranging from roughly $20 to $60 ea.  https://www.tubesandmore.com/products/transformers_chokes

[Analog Kid]

An example of a series push-pull 6AS7 working into 72\$\Omega\$, along with how to construct a 72\$\Omega\$ speaker: https://www.tubecad.com/2010/12/blog0195.htm

Oy, the TubeCAD guy.
Don't get me wrong: the guy is definitely a tube-amplifier guru, knows his stuff. But my god, the gyrations and permutations he pretzels us through in his endless dissertations on almost exclusively triode-based amps. Especially his obsession with "SRPP" ...

One question: why doesn't he just use a symmetrical push-pull cathode-follower output stage? Wouldn't that have a low enough impedance to drive a speaker (8-ish ohms) directly, rather than that weird, what is it, cascode? stage he has there, with its 72 ohm output impedance? (Analogous to an emitter-follower PP output stage.)

Anyhow, OP: my suggestion, take it or leave it: forget about tubes, at least for now.
Yeah, yeah, I know: they're fascinating, fun to work with (so long as you observe reasonable safety precautions). But a solid-state first project would be a hell of a lot simpler for you. Especially when it comes just to obtaining parts, like power and output transformers, tube sockets, etc.

Once you've got that down you might attempt a 2 or 3-tube amp. (But please, please, do not attempt to use "an old radio", as you'll be dealing with a "hot chassis", which is dangerous; don't ask me how I know that!)

[themadhippy]

That can work, but be careful about the "hot chassis

Indeed,however i was thinking more along the lines of something battery power, like the ole boom boxes

[Analog Kid]

That can work, but be careful about the "hot chassis

Indeed,however i was thinking more along the lines of something battery power, like the ole boom boxes

Or the original Pignose amps, which were totally portable battery-powered ones. Plenty loud, too.

[TimFox]

An example of a series push-pull 6AS7 working into 72\$\Omega\$, along with how to construct a 72\$\Omega\$ speaker: https://www.tubecad.com/2010/12/blog0195.htm

Oy, the TubeCAD guy.
Don't get me wrong: the guy is definitely a tube-amplifier guru, knows his stuff. But my god, the gyrations and permutations he pretzels us through in his endless dissertations on almost exclusively triode-based amps. Especially his obsession with "SRPP" ...

One question: why doesn't he just use a symmetrical push-pull cathode-follower output stage? Wouldn't that have a low enough impedance to drive a speaker (8-ish ohms) directly, rather than that weird, what is it, cascode? stage he has there, with its 72 ohm output impedance? (Analogous to an emitter-follower PP output stage.)

Anyhow, OP: my suggestion, take it or leave it: forget about tubes, at least for now.
Yeah, yeah, I know: they're fascinating, fun to work with (so long as you observe reasonable safety precautions). But a solid-state first project would be a hell of a lot simpler for you. Especially when it comes just to obtaining parts, like power and output transformers, tube sockets, etc.

Once you've got that down you might attempt a 2 or 3-tube amp. (But please, please, do not attempt to use "an old radio", as you'll be dealing with a "hot chassis", which is dangerous; don't ask me how I know that!)

If you analyze a tube amp (or a solid state amp with very different supply voltage and current) you will find that, for a given set of devices and power supply, the appropriate load impedance is the same for common-cathode (normal) and common plate (cathode follower) operation.  You are confusing output (or source) impedance with load impedance (a common misunderstanding of the infamous Maximum Power Transfer Theorem).  A cathode follower will have a lower output impedance (due to negative feedback) and require more voltage drive, but can only supply the same out voltage and current as the equivalent plate output.  I cited that article as an example of why an OTL was not appropriate.
The author you criticize is straightforward and quantitative, not one who waves his hands in despair of mathematics or calculation.
My suggestion above was to try a simple single-ended beam power tube, followed by a reason not to use an OTL triode.

[Analog Kid]

The author you criticize is straightforward and quantitative, not one who waves his hands in despair of mathematics or calculation.

Well again, don't get me wrong: I'm certainly not qualified to criticize the guy. Just pointing out his output volume, which is overwhelmingly prodigious.

My suggestion above was to try a simple single-ended beam power tube, followed by a reason not to use an OTL triode.

Which would require an output transformer, right?
To me, that's a major stumbling block to playing with tube audio, since transformers are often unobtainium.

[Smokey]

If you actually want to learn stuff, get this book and read through until you understand everything.  You will learn a bunch of stuff about general analog electronics in addition to power stuff.  It's well written and has both great explanations and great fully functional example circuits.  Everything from fully discreet BJT and FET amps to integrated stuff.

https://www.amazon.com/Designing-Audio-Power-Amplifiers-Cordell/dp/1138555444/

If that book is too expensive for a highschool kid wherever you may happen to live, maybe google "Designing Audio Power Amplifiers pdf"

[TimFox]

The author you criticize is straightforward and quantitative, not one who waves his hands in despair of mathematics or calculation.

Well again, don't get me wrong: I'm certainly not qualified to criticize the guy. Just pointing out his output volume, which is overwhelmingly prodigious.

My suggestion above was to try a simple single-ended beam power tube, followed by a reason not to use an OTL triode.

Which would require an output transformer, right?
To me, that's a major stumbling block to playing with tube audio, since transformers are often unobtainium.

That goes without saying.  However, appropriate transformers are easily available to one who seeks, although more expensive than not using a transformer.
Two manufacturers of appropriate units:
Hammond   (replacement units) https://www.hammfg.com/electronics/transformers/classic/1750  (app note) https://www.hammfg.com/electronics/transformers/audio/overview?referer=968
Edcor   https://edcorusa.com/products/copy-of-cxse5-7k-5w-7k-ohms-single-ended-tube-output-transformer   and see their catalog for power transformers and push-pull transformers
A retail vendor of this kind of thing:  https://www.tubesandmore.com/products/transformers_chokes
I also posted a suggestion to salvage an output transformer from an old tube radio.
The actual wiring diagram for a simple tube amp is quite simple, and tube sockets are readily available.
For example  https://audioxpress.com/article/you-can-diy-build-a-single-ended-guitar-tube-amplifier
No PCB is required:  point-to-point wiring is still a valid technique at these frequencies.

[Analog Kid]

[Edcor   https://edcorusa.com/products/copy-of-cxse5-7k-5w-7k-ohms-single-ended-tube-output-transformer   and see their catalog for power transformers and push-pull transformers

Yikes; $44.24 for that puppy.
Obtanium, but budgetbustium.

Thanks anyhow, though: didn't know that so many transformers were still available.

[TimFox]

[Edcor   https://edcorusa.com/products/copy-of-cxse5-7k-5w-7k-ohms-single-ended-tube-output-transformer   and see their catalog for power transformers and push-pull transformers

Yikes; $44.24 for that puppy.
Obtanium, but budgetbustium.

Thanks anyhow, though: didn't know that so many transformers were still available.

 
From the retailer I cited:  https://www.tubesandmore.com/products/transformer-output-5w-5k-8k-primary-options-upgrade-fender-champ  for $20.

[themadhippy]

Yikes; $44.24 for that puppy.
Obtanium, but budgetbustium.

best you dont look at sowter https://www.sowter.co.uk/acatalog/E_A_Sowter_Ltd_SINGLE_ENDED_OUTPUT_15.html#a65

[TimFox]

Yikes; $44.24 for that puppy.
Obtanium, but budgetbustium.

best you dont look at sowter https://www.sowter.co.uk/acatalog/E_A_Sowter_Ltd_SINGLE_ENDED_OUTPUT_15.html#a65

Yes, Sowter sells to the hi-fi crowd.  Getting good bass response with a single-ended tube (DC in the primary) is expensive.