Evaluating unknown guitar amp transformers at a glance

[Zelbrek]

Hi all.

When evaluating some used piece of equipment like a "dead" guitar amp, or some other device like an old radio or PA amp, are there good rules of thumb that will allow me to make a snap judgment about suitability for reuse if I have limited visual access because I'm shopping online or at a yard sale where the owners won't let me "peek"?

Seems to me that the "bigger is always better" argument is kinda useful but has serious limitations. Number one, transformers serve many functions inside a guitar amp: They deliver power to the rectifier(s) and match the output of the power tubes to the speaker(s) or other loads, but they also may find use as power chokes(?) or in between tubes (phase inversion?) and as drivers for reverb tanks. Going further afield, as in the case of the chassis shown below which came from an unknown (Conn?) electronic organ, I've seen little gizmos that look like transformers but apparently serve some other function: In tone generation circuits for organ voices or percussion sounds, i.e. oscillators?

One tip I've picked up is to count the tubes, look up their current and voltage specs, and--at least in terms of figuring out what kind of power transformer they use--make a quantitative judgment about the PT based on that information alone.

For instance: This little baby, whose innards I've carefully gutted so as to allow the power connections to survive untainted, sports a massive brick of a power transformer that obviously was at one time capable of safely driving all of the following tubes: (1) 5U4, (2) 6AU6's, (1) 6BA6, (2) 6C4's, (2) 6V6's, (2) 12AU7's and (1) 12AX7. Assuming the tranny's still OK, which any idiot with an Ohmmeter can figure out, then I've got the basis for an amplifier that will "tumble the walls", right? Or, do I need to be more conservative, because guitar amps tend to push their power transformers a little harder than other devices, such as the putative Conn this chassis came from?

More vexing is the issue of output transformers. Figuring out how they function in a guitar amp seems to require some deep knowledge of weird stuff like cube roots, vectors and transfer functions.... Which are utter Greek to me. But in the case at hand (and based on a tip I received here in response to another post), I'm making the call that this organ's tiny output transformer won't end up being deployed in the Walls of Jericho Amp, because the organ used it in a power amp that formed a sub-circuit within a larger master circuit. IOW, since I know that some of the tubes were used as oscillators or whatever, I can reason that the actual power amp was built around some subset of the overall tube complement--maybe two or three 6- or 12A_whatevers in the preamplifier section, another smallish tube for phase inversion, plus the two 6V6's in push-pull mode. Right? So, if and when I get around to building a big 'n bad amp, I may be able to us this PT, but will definitely need to find a bigger OT.

No worries. Since all I'm interested in building at present are low-wattage gems with killer tone, maybe I can use the organ's OT in a cute little bug that lives inside a toaster oven, like the "Li'l Smokey" that was recommended to me as a first foray into amp building? I've gotten to the point where I sorta understand the "Put so many volts AC in one side, measure what comes out of the other side, use my slide rule to do something or other, and end up with a chart that says something like 'One 6V6 in Class A driving one 8-Ohm speaker will see an input impedance from the OT that'll make it feel warm 'n cuddly, not freezing to death or having hot flashes.'" approach, but I know and understand that grownups with math skills and engineering backgrounds approach output transformer selection in a far more sophisticated manner, and it is this knowledge that I aim to eventually work out for myself, even if I have to re-carve some neural pathways in my brain that seem to have burned out upon orbital reentry after my last space walk.

Here's da pix:

[sluckey]

That PT and OT are a good match for any 2x6V6 20 watt amp. Think Deluxe Reverb, Plexi6V6, etc. Lot's of other choices to pick from.

[kagliostro]

Ciao Zelbrek

When you recover an amp or one other amplifier part of an organ or old radio etc. etc., and you decide to build from it a guitar amp, is good practice to maintain the original Power Transformer, Power Supply circuit (at least the part that supply the power tubes), Power Tubes (if present) and Output Transformer

On the original circuit those parts are planned to work together in a good way, so, if you aren't more than skilled building amps, you can take advantage of that and plan only a preamp adequate to your purpose and, may be, some minor Power Supply mod to the part that feed the Power Tubes

I don't mean you must maintain the original layout for the part of the PS you maintain, you can rebuild it in a new turret or eyelet board instead of Point to Point Layout, but to maintain the Power Supply schematic

--

The images you posted are too small and so is not possible to see details

if you have a large image that didn't fit on the forum posts, you can upload it (for free and without registration) on IMGUR (https://imgur.com) then put the image link on the post using the IMAGE TAG (the small button showing Mona Lisa)

Franco

[PRR]

> (1) 5U4, (2) 6AU6's, (1) 6BA6, (2) 6C4's, (2) 6V6's, (2) 12AU7's and (1) 12AX7.

We note the rectifier and the two 6V6, also what seems to be an output transformer. It seems this was an Audio Amplifier in, indeed, the "Deluxe Reverb, Plexi6V6" class. This iron and the 6V6es worked together for some audio purpose in 1959; and probably today too. Don't over-think.

There's no point listing the little tubes. They rarely make a difference to a stage amp, they don't eat enough to upset our plans.

I'm about to go out for a big beefsteak and side vegetable. It's not about the asparagus.

[Zelbrek]

Thanks, all!

For all the long verbiage in my OP, I was really only concerned with teasing out two points that I think will be useful for newbies (like me) who lack a facility with complex mathematical jargon:

1) BOTH power and output transformers have ideal and actual ratings which must be taken into account when making a determination of suitability for use in a new amp build, but we can make educated guesses just by looking closely at the circuit(s) they came from. The great replies I received to my dumbass question tell you all you really need to know IF your trannies were pulled from existing circuits: Let's say that Device A (old organ, PA amp, etc.) had two 6V6's and three 12A_7's; ergo the power transformer can be used for an amp of your own design which uses the same tube complement. Device B (some other yard-sale find) had two 6V6's in push/pull driving an 8-Ohm speaker; ergo your custom-built amp, which also has this combination of tubes & speaker, can safely use the OT from Device B.

OTOH, if you take the smarter approach and order your trannies brand new, these same rules also apply. Example: your ClassicTone 40-180XX output transformer is marketed as being suitable for use in a Fender Princeton, Champ or whatever, so if your custom amp build uses the same output section and speaker rating as those Fender models, then this OT will work just fine in your new build. And the same goes for power transformers.

2) Calculating impedance involves higher-order math skills that even many engineers may struggle with, but they know that basic rules of thumb, or "first approximations", are often sufficient, so they, too may take the simple approach outlined above.

Just in case readers wonder what all the fuss is about with impedance, here is an article from Wikipedia. Trigger warning: There are a lot of weird Greek symbols and crazy stuff like "equations" or whatever that you'll need to understand if you intend to follow along at home. Set your phasors on "stun", and when it comes to the numbers, feel free to use your imagination, 'cuz some of the numbers are imaginary....

https://en.wikipedia.org/wiki/Electrical_impedance