Single tube guitar amp, adding a transformer to a formerly transformer-less design

[MereTenacity]

Building a small practice amp from a reclaimed phono circuit.

Formerly used a dreaded selenium for 24v, I'm planning on using a transformer laying around for the heater but wanted to know if i need to consider any other factors. Added a filter cap and a fuse to the ac in. Formerly a Motorola Suitcase Phonograph, now housed in a dead ss marshall practice amp chassis (already had a 24v trans! ).

Considering a pre-amp to get some more volume and gain, i know it's not gonna be loud, just looking for unique tone and a fun amp using a single cheap valve.

I can post the schematic (original and proposed amp both) if needed.

 

[TonyTecson]

be sure to use isolation transformer, circuits that work directly from the wall are not allowed here....

 

[MereTenacity]

be sure to use isolation transformer, circuits that work directly from the wall are not allowed here....

As I will be holding the hot (guitar) end of this, I plan on doing so. What is the proper way to isolate such a circuit, just throw an iso-former on the mains? I'm using a grounded chassis with a 3-prong fyi.

 

[TonyTecson]

just make sure your circuit uses a power transformer to completely isolate the mains from the rest of the circuit...

 

[Tom Bavis]

Mouser and Allied (among others) sell a 50VA isolation transformer for about $15. Once you have a transformer you can ground the chassis for safety.

 

[DF96]

Instruments and Amps?

 

[MereTenacity]

Chassis is grounded. Looks like the reason they went with this tube is it used 110 for the plate, and a 25v heater. Looks like i'll need a 2nd transformer too as in addition to the selenium hack, they also used the phono motor as a step-down.

Am I correct in my assumption I can use 2 transfromers of the same value back to back (110vac->12vdc->110dc) to get isolated line voltage for the plate?

Yes, chassis is grounded already, just sorting out what needs to fit an where. Going to try to keep all the xformers and the 25L6GT inside the cage as it's not too hot and I'm adding a ventilation grate and some internal shielding. The chassis is an old 6" driver combo amp.

 

[MereTenacity]

Also, I pondered throwing this in the Insturments/Amps section, but it's really more relevant to the particular issues of tubes and whatnot. Consider it a 200ma line level amp if you must.

 

[kevinkr]

All musical instrument and amp threads belong in Instruments & Amps so that is where it goes forthwith.. This is a one tube guitar amplifier, and almost all threads there are about tubed guitar amps. Following forum guidelines is greatly appreciated..

 

[Eli Duttman]

MT,

Instrument amps are not my thing, but this thread has stuff of interest to me.

Start with the nnL6 data sheet and facts emerge. 1st, you will be quite OK if you bridge rectify "120" VAC to obtain the B+. 2nd, the power pentode needs several volts of drive signal.

As a 50 VA N-68X from Mouser costs $11.20, why bother with back to back filament trafos? Full wave bridge rectify the O/P of a N-68X, filter, and you've got your B+ rail. Either 4X UF4007s or 2X UF4007s and a 117Z6 (for sag) make up the bridge.

Several stages of voltage gain are needed between the guitar pickup and the power pentode. The phonograph "donor" used a crystal cartridge whose O/P was a few volts.

 

[MereTenacity]

Thanks Eli for the detailed info (and thanks mods for moving the post)

I was planning on basically integrating an overdrive (gain) pedal to drive the tube and put enough juice into the phono circuit.

I have since decided it was a bit more trouble than the tone would reward, and bought another tube project. I figured the transformers (of which I have many) might be easier.