I have no use for a higher power amp. I have a 68 Bassman sitting around needing a cap job. The 25W Wurlitzer OT's I have are more than these poor old ears can handle now days. And a boat anchor of a power supply. Bits and pieces from a Hammond tone cabinet. The left two hunks of iron on the left are the OT's. Two chokes and the power transformer. A big can of from what I am told is oil filled capacitors.
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(Aside: I'm continually amazed by your vintage electronics finds. Wow!)
For some time now, I've been using a household vacuum cleaner as a mental benchmark for maximum acceptable apartment loudness. I figure the neighbours will put up with a vacuum cleaner, so they might put up with guitar noises that aren't any louder than that. Maybe.
So we might ask ourselves: How many vacuum cleaners would it take to be as loud as a 40 watt guitar amp?
We need some guesstimates. Several online noise tables suggest that a household vacuum cleaner produces about 70 - 75 dB SPL. They never list the distance from the vacuum cleaner, so I'm assuming this is in a typical small room where there are enough room reflections to make the loudness fairly uniform once you get beyond a metre or two from the vacuum cleaner.
We also need to guess at speaker sensitivity. Those vintage alnico speakers in the photo may be around 90 dB@1W@1m, I'm guessing.
So what happens if you put 40 watts into a speaker with 90 dB sensitivity? You get 106 dB (!!) SPL at 1 metre. And that is equivalent to one thousand, two hundred, and sixty five typical vacuum cleaners, each putting out 75 dB!
One day, I still want to build a too-loud-to-use guitar amp, simply because I've never built a powerful valve amp. But to make it usable, the preamp will have to generate all the "tone", so you can turn the thing down to one-vacuum-cleaner volume!
-Gnobuddy
i put out with the dimebag wah that have two isolated outputs into that blackheart combo (1x12) thats damn loud now, plus a 18w plexi in a cab loaded with two speakers... one 10' another 12' from giannini tremendão era (loud!)
The reason I decided to pick it up is the smallest transformer on the top side and underneath you can see the reverb transformer at the top left hand corner. Even the little choke is more than I need in a 6V6 Bassman. Speaking of 6V6's, that is what drives the two output transformers. The bigger bass transformer going to the 10's has four 6V6's. The smaller treble transformer went to two 12's (which the guy kept for himself) and it is also driven by four 6V6's. I am assuming the speakers added up to 8 ohms although I have to check to make sure.
There was an amp for sale, I didn't know much about the manufacturer but looked up the amp and what it had for speakers. The ad said it stopped working but I was only concerned about the speakers. A pair of G12P-80 Celestions, 98 dB spl so 1W 101 dB. 200 W on tap, must be one mother of an amp. I popped the chassis out, voltage to the rectifiers, the preamp board lights up. Started measuring voltages, nothing, nothing, where is the problem? Must have moved something as I could now hear noise from the speakers. Plugged the guitar in and it made guitar noises.
Button her up again (short leads on the speaker and it was not a friendly testing arrangement) and plug it in. Clean channel, not bad sounding amp. Turn it up and OH MY!! Way too much power for a living room, the amp is a Randall RG200, built for the drop-D metal kids. Turned down the master, went to the distortion channel. Nothing. Must be another bad connection, not a big concern for now. Thought one of the speakers would be a good match for a Peavey Bandit amp I was given. But now with most of the amp working not sure what to do with it. Almost as heavy as any Fender Twin I moved. Large magnets, big power transformer, whatever pressed sawdust board they used for the cabinet. Not an old man's amp at all. But for $40?
The 40W Hammond output transformer is huge beside a Bassman OT, about the size of the Bassmam's power transformer.
A Hammond Fender Deluxe replacement output transformer beside the high frequency output transformer from the Hammond tone cabinet. Should get a 15W amp out of it. The reverb transformer from the Hammond is built on the lower right. Wondering about the frequency reponse, not that you need a lot for reverb. Makes me wonder about the 6-12V power transformer with dual 120V primaries. Might try it as an OT just for kicks.
A Hammond Fender Deluxe replacement output transformer beside the high frequency output transformer from the Hammond tone cabinet. Should get a 15W amp out of it. The reverb transformer from the Hammond is built on the lower right. Wondering about the frequency reponse, not that you need a lot for reverb. Makes me wonder about the 6-12V power transformer with dual 120V primaries. Might try it as an OT just for kicks.
I would be interested in your findings.
I've read that this particular transformer design ("flatpack") is relatively inefficient (higher leakage flux is the price to pay for the lower profile and easy PCB mounting.) Somebody tried to use back-to-back flatpack transformers like these in a DIY valve guitar amp to generate both heater voltage and B+, and because of their poor efficiency, these transformers were much more prone to just overheat due to the high magnetization current of the second transformer.
On the other hand, a couple of decibels loss in a low-power guitar amp OT may not matter much, so, who knows, they might work just fine there, if you can find a steep enough winding ratio for your needs.
-Gnobuddy
IIRC George managed to push 30+ watts through a $5, 10-watt, 70V audio line transformer, amazingly without blowing it up. So who knows, perhaps your flatpack transformer will do even better than that!
In the past I've also read claims that power transformers don't have enough HF bandwidth, but I'm beginning to suspect that good guitar tone requires surprisingly little bandwidth, much less even than the 5 kHz that's frequently mentioned.
It would be nice if the transformer works out...another source for affordable guitar amp OPTs would be wonderful!
-Gnobuddy
It would be unlikely for a transformer in audio duty to "blow up". The actual stress is quite low when THD gets gross. Tubes, especially hard-clipped, *could* over-volt it.
There's several cats to skin the ways.
The simplest power transformer is one winding over the other. This can give full audio bandwidth at low to medium impedances, though not the controlled supersonic response needed for heavy hi-fi NFB.
An alternative is each winding in a separate bobbin on separate legs of an O-core. Now we have terrible high frequency coupling and commonly have roll-off by 400Hz.
Of course the other extreme starts with slicing-up each winding and layering them P-S-P-S-P-S-P for more intimate coupling. Williamson, Dynaco, everybody in classic hi-fi. (This goes on to fully bi-filar and many-filar which can give response from bass to MHz especially at lower impedances.)
Yes, I squeezed over 30 watts through one of these at 1 KHz. I needed 350 or so volts of B+ to do it. That means that the OPT sees maybe 650 volts P-P.
Saturation limits the fun to about 10 watts at 80Hz, so you might get 15 or more with some under sized coupling caps in a guitar amp........ You might also get smoke.
I did not try to play guitar through it at that power level. They work just fine in my little 4 watt amps. 10 watts maybe, but I haven't tested that yet. There could be some real nasty voltage peaks when the amp is driven to clipping around the speaker's resonant frequency.
These things are made, and insulated for, 70 volt lines. I'm thinking that I got lucky and the next one might turn into a nuclear fireball!
70V 10W Speaker Line Matching Transformer
When I was a kid operating on zero budget I often used medium sized power transformers for OPT's. Drive the HV windings in P-P and wire all the filament windings in series to feed the speaker's. I didn't understand things like impedance yet, but even a dumb blonde kid can figure out that the more speakers you hook up, the louder it gets. All speakers were wired in parallel. Some of my contraptions used lots of speakers, and some of those speakers came from similar tone cabinets.
A conventional power transformer from a B&W TV set worked reasonably well in a guitar amp, and sounded nice enough that people actually bought some of my amps.
Some time ago I looked online for FFTs of electric guitar notes, and found several. The fundamental frequency was never the strongest, not even close. Sometimes the strongest spike in the FFT was the 2nd harmonic, sometimes the 3rd, sometimes the 4th (presumably depending on picking location, force, pick shape, etc.)
So maybe we don't really need -3 dB power response to 80 Hz. Maybe very little will be audibly missing if you are down to 10 watts (-5 dB from 30 watts) at 80 Hz?
I first encountered the idea of using audio line transformers for valve guitar OPTs on an Australian guitar forum. Valve OPTs are very expensive in Australia, and as so often happens, necessity was the mother of invention.
At any rate, I did read some complaints about thin bass from people who were using their first 100 V audio line transformer as a guitar amp OPT, but there also seemed to be a fair number of people who were quite happy with these.
Maybe even more surprisingly, I don't remember reading a single report of one of these transformers flashing over. Sure, the Aussie audio line transformers are designed for 100 V RMS, so they are supposed to be happy with 140 volt peaks, but they were being asked to handle four times that much voltage.
All the audio line transformers I've found here in North America are of the "70 V" type, so they are designed for peak voltages of 100 volts. So we're talking about applying five to six times the rated primary voltage.
Good to know, thanks for that!
-Gnobuddy
No; because when lines are run VERY far there is a good chance the amplifier "ground" and speaker "ground" are not the same.
(Speakers are often run ungrounded, but strict electrical protocol would require it.)
Normally all buildings in the same complex are bonded together. Where I used to work there was 3V AC difference upstairs to downstairs, trivial, but "ground is not ground"! And faults happen: I recall a lost Neutral throwing an extra 100V around. In factories it may be some fraction of 440V or higher distribution voltage.
Against this: the standard insulation is fancy brown-bag (kraft) paper which is pretty reliable to 500V with some higher surges. So the insulation is probably "OK" to hundreds of volts just because that's what the cheapest stuff can stand.
And the beginning of the winding is not adjacent to the end of the winding. Well, maybe where the wire of the inner winding is brought out of the coil. We have a couple dozen buildings, multiple distributions in some buildings, regular power and emergency feeds. Ground loops, induced voltage, the can mess up our building controls.
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