Selenium Bridge Rectifier On Geloso Tube Amp

[Phoenix]

I was always curious about how we can get a signal out from a tube amp. Understand that getting a pre-out is completely different than getting a whole circuit line out.
Was looking smthng like in 5e3 line out.



Could i do something similar here getting a "full" out?

A line-out from the output transformer is quite simple, but remember that the amp must always have a load to avoid damage.

If you're happy with the amp still making its own sound this can be a speaker - and has the advantage that the reactive load will help the line out sound sound more "correct" than a plain resistive load - thanks to the low frequency resonant peak (though you'll probably still want some high and low pass filtering). You could take the line out from a DI box.

If you want silent recording you'll need to add some other type of load - this can be plain resistive (in which case you'll need some sort of speaker emulation - be it analog or something like impulse responses - to mimic the tonal shaping a speaker/cabinet does), reactive, speaker motors, etc. Many speaker attenuators (but not all) can act as a dummy load and often have a line-out built in.

Once you have a load sorted out, if you don't already have a line-out built-in to the load or DI box, you can add it to your amp. First calculate the nominal output voltage of your output transformer's secondary. You've wired yours/will be using it at 8ohms, right?
V=SQRT(P*R) or SQRT(30W*8ohms)=~15.5V. You then need to scale that to something appropriately line-level.

I like to use a resistor in series with a pot so that I can adjust the output level to suit how much the amp is being driven - it's nice to be able to get a healthy line level out even if the amp is putting out less than max power, and it can always be turned down to avoid clipping the input of whatever you're running it into.
I typically aim for around 2V max output from the pot at rated power. A target of about 10k total for the voltage divider/attenuator (resistor and pot) is a good idea, it's a very small load on the amp so won't rob power, but low enough to keep output impedance sensible.
We need to divide ~15.5V down to ~2V, or 7.75 times. As we have more choice in resistor than pot values, we should start by choosing that, and select a resistor to match. 1kA is a convenient value, and a 6k8 resistor in series will get us close to our 2V max level and 10k divider resistance at 7k8 total.

Note the above is mostly rule-of-thumb, these are rough targets that have always worked well for me, but you do not need to rigidly stick to them.

Hope that helps.

[sarakisof]

Some pretty helpful advice here Phoenix. But before doing so, i am wondering if it worths the effort.
So first just a general thought: apart from using this nice amp by the 'normal' way feeding speakers, micing and recording any instrument, would it worth the effort to also use it as a preamp output / DI kind of for recording bass or a combo organ (those with line pre out, like Farfisa's, Philicorda etc) or line output like you described above into mixer / outboard gear? I am also in the middle of a project for Gyraf G9 tube preamp / instr line DI (not point to point). All i am wondering if this Geloso true p2p tube circuitry could give me a bit 'more' to my sound. But if it cannot be done like this considering it is a 'power amp and works perfect like that' then better leave it and use the Gyraf G9 for this purpose only. In simple words, by the way described above could i get a decent, studio usable signal and/or how could i use the preamp section only for bass when i want, completely isolating the power tubes so they don't waste life?

[amptramp]

It is possible to shut off the power tubes by switching the heaters and B+ off including both plate and screens.  It may be easier to switch the cathodes off.  You may think only the heaters need to be off but there are some bad cathode stripping effects from not switching the B+ as well.  Be prepared for the B+ to rise significantly with the outputs off since the supply will only have a few milliamps of current drain.  The output from any of the 12AX7 stages would be high-impedance so it might not be compatible with your mixer board or whatever else you are connecting to.  Take the output from the first 12AT7 cathode, the feedback network will not interfere with anything.

[Phoenix]

So first just a general thought: apart from using this nice amp by the 'normal' way feeding speakers, micing and recording any instrument, would it worth the effort to also use it as a preamp output / DI kind of for recording bass or a combo organ (those with line pre out, like Farfisa's, Philicorda etc) or line output like you described above into mixer / outboard gear?

I guess what you're asking here is if it would be worth running a bass or organ into the amp, then taking a line out from the output transformer into a mixer/PA for sound reinforcement, or for direct recording?

Let me answer that in stages. Yes, sometimes a line-out is more convenient for sound reinforcement than miking up a cab - no mic required, no mic preamp required, less potential for feedback, often simplifies using extra effects processing like reverb/compression (I guess that's what you meant by outboard gear?) as there's less chance of interaction/feedback than with a mic. But, the sound is usually considered "inferior" to miking a cab, as you don't get all the tonal shaping that a guitar cab or mic placement does. Various types of speaker emulation can help mitigate that criticism.

For direct recording - it's another tool. If you consider the amp as an effects processor (which it is - it doesn't just make the input signal louder, but also alters the tone), then it's a tool that you can use to shape your sound. How useful a tool it is depends on your preferences. Again, same issues with losing the tonal contributions of cab and mic placement exist - but this only a positive or negative depending on your perspective. Results are *different*, not necessarily good or bad, it depends on what you want.

I am also in the middle of a project for Gyraf G9 tube preamp / instr line DI (not point to point). All i am wondering if this Geloso true p2p tube circuitry could give me a bit 'more' to my sound.

All construction methods have merit, no one is superior to another and one will not give you "more" than another if both are *properly* executed. Point-to-point/dead bug does have a specially reserved place for ultra-high impedance circuits, but that's not what you're dealing with here. Use whichever construction method suits the task, but "tone" should not be considered a metric here.

But if it cannot be done like this considering it is a 'power amp and works perfect like that' then better leave it and use the Gyraf G9 for this purpose only. In simple words, by the way described above could i get a decent, studio usable signal and/or how could i use the preamp section only for bass when i want, completely isolating the power tubes so they don't waste life?

They're different tools. An axe, a chisel and a saw all cut wood, but they suit different purposes. Same with preamps vs poweramps, or just two different models of amp. If all overdrive stompboxes do basically the same thing, why are there so many different ones? Every part of the signal chain has some greater-or-lesser influence on the tone. Some influences are small enough to ignore, others will be drastic. You'll get different results taking signal from different parts of a circuit - taking a line out from the preamp will sound different than a line out from the output transformer because they've had different amounts of processing. Which one suits your task is up to your requirements and preferences.

I'll give you some examples of how I've made use of a line out from an output transformer. I tend to add a line out to small tube amps I build, like 5W and under, as the low power output makes it easy to include a built-in dummy load. Sometimes I will run one of these small amps into another, larger guitar amp so I can get more volume.
Sometimes I will overdrive the small amp so I can "crank" it and get output tube distortion, then have complete control over the volume by using another amp (sometimes I want it louder, sometimes quieter).
I prefer the results I get from running an amp into a resistive load and then into another guitar amp and cabinet (the resistive load works fine here as the signal still ultimately goes through a speaker cabinet, so no need for emulation) for volume control over a speaker attenuator, YMMV.
I also prefer the results of running a big amp into a dummy load and sending the line out into a smaller amp for "bedroom" volumes over the results I've gotten with speaker attenuators. Again, YMMV.
Sometimes I'll use it to add effects on the line out, for "studio-like" results live - reverb/delay/tremolo/etc on *top* of power tube distortion, something that normally would be done in post-processing and not possible live with a single conventional amp.
You've mentioned you mostly play clean, so these are probably not so applicable to you, but it should get you thinking about the creative potential available.

In simple words, by the way described above could i get a decent, studio usable signal and/or how could i use the preamp section only for bass when i want, completely isolating the power tubes so they don't waste life?

Yes, you can take a line out from the preamp and avoid the need for a dummy load by turning off/disabling the power amp, like Ron suggests. But the results will be different. Good/bad/indifferent is up to you.
Switching off the heaters to the power tubes is the simplest way to disable the power amp. Don't be tempted to add a switch to disconnect the power tube cathodes (something I've seen done before), as that strains Vhk ratings. Turning off Vg2 for pentodes/tetrodes can also work, but can sometimes leak small amounts of sound and requires switching high voltage DC - something to be avoided (see also: standby switches).

You may think only the heaters need to be off but there are some bad cathode stripping effects from not switching the B+ as well.

You've got that backwards Ron, leaving heaters running without B+ causes interface resistance to develop on the cathode, but B+ with no heaters has no ill-effects on most tubes (see above link on standby switches).

[ThermionicScott]

If you just want a tube preamp for those occasions, that's a simple build.   

[amptramp]

@ Phoenix, that is a great article on standby switches.

I had seen them used in series with the centre tap of the power transformer so they would be nominally at ground potential rather than at high voltage when switched on.  I have a number of tube receivers that use power switching to the plate supply of various tubes that I am going to change over to have some B+ on the tube at all times.  Thanks for clearing up the confusion.

[Phoenix]

@ Phoenix, that is a great article on standby switches.

I had seen them used in series with the centre tap of the power transformer so they would be nominally at ground potential rather than at high voltage when switched on.  I have a number of tube receivers that use power switching to the plate supply of various tubes that I am going to change over to have some B+ on the tube at all times.  Thanks for clearing up the confusion.

Yep, quite a few examples of standby switches on the ground-side. Unfortunately it doesn't solve the problem of exceeding the max switching rating of the typically only 250VAC rated switches. If a nice Carling 110 series switch is used it'll also have a 250VDC rating, but more often than not they're only rated for 30VDC, which is the most common rating for typical switches. It's remarkable they don't fail more often. Switching double or often even over an order of magnitude above their ratings isn't kind on them.
By moving it from the high-side to low-side you're just changing it from high-voltage connecting to ground, to ground connecting to high voltage - the uncharged caps are always at a different potential than the other side of the switch, and that can lead to arcing/pitting/welding. Having it on the low-side does avoid stressing the insulation while in operation (does nothing to help while in standby though), but personally I've never encountered a standby switch with insulation breakdown (and a properly earthed chassis will protect you anyway), but quite a few with pitted/welded contacts. Really I think high vs low side switching is just a matter of layout convenience, and doesn't address the problems standby switches actually exhibit.