A long time ago I bought two SS amp heads with 12ax7 preamp, faulty (blown fuse ha ha ) Toroid transformer with lots of taps. 250 - 0. 12-0. 17-0-17. very low I for the 250V. .1amp. but should be enough.
I made a AA1164 princeton reverb in the chassis using EL91 pentodes instead of 6V6. good for 5 watts? They are biased at _19v. EL91 valves are like tiny 7 pin 6V6. I have used 6SL7 triodes in low watt amps but didnt love the sound. The only changes to the schematic other than the voltages (250 not 400) are the dropper resistors (They are 4k7) and the output valves. so, the reverb is there and ok but not over whelming. Like wise the tremelo. There is something wrong tho. No clean tone. Some notes loud some not. No sustain, notes die out quick.
I suspect some oscillating.Grid stoppers the same as the original schematic.... Waiting for new probes today. Sorry no schematic, google princeton reverb AA1164 redrawn brings up a nice schematic love some ideas
I made a AA1164 princeton reverb in the chassis using EL91 pentodes instead of 6V6. good for 5 watts? They are biased at _19v. EL91 valves are like tiny 7 pin 6V6. I have used 6SL7 triodes in low watt amps but didnt love the sound. The only changes to the schematic other than the voltages (250 not 400) are the dropper resistors (They are 4k7) and the output valves. so, the reverb is there and ok but not over whelming. Like wise the tremelo. There is something wrong tho. No clean tone. Some notes loud some not. No sustain, notes die out quick.
I suspect some oscillating.Grid stoppers the same as the original schematic.... Waiting for new probes today. Sorry no schematic, google princeton reverb AA1164 redrawn brings up a nice schematic love some ideas
I will try to post pictures. I have tried but maybe to big. keep getting "waiting for diyaudio"
Anyway, all good, just needed to reference the negative bias to the cathode. They are on separate winding's, never occurred to me until I hooked the scope probe between signal and cathode and it sounded good.
I will post some pictures, I am quite pleased with myself!!
http://ampwares.com/schematics/princeton_rev_gz34_aa1164.pdf
Anyway, all good, just needed to reference the negative bias to the cathode. They are on separate winding's, never occurred to me until I hooked the scope probe between signal and cathode and it sounded good.
I will post some pictures, I am quite pleased with myself!!
http://ampwares.com/schematics/princeton_rev_gz34_aa1164.pdf
That HV winding was designed to feed a single 12AX7 preamp and in no way can handle a Tube power amp, period.
Wire must be hair thin, and in any case transformer window must be 90% or more filled with 17+17VAC wire for the SS power amp.
Your power amp may work (poorly), and in no time burn the 250V winding.
That IF they are fed proper voltage and current, which they are not.
Why am I not surprised?
I suggest before definitely burning that transformer you make any reasonable Tube preamp you like, and feed an SS power amp happy with +/-24V.
plate current for a 12ax7 is 6ma. 4 triodes is 24? that I assume is running flat out and at higher voltages than what I have EL91 is 18ma x2 is 36ma. Running flat out. transformer is 100ma. Am i missing anything. If wouldnt be the first time. Surely the physical size of the transformer is also relevant? the reason it sounded like **** was because the negative bias wasnt referenced to the rest of the ground. Read the post I didnt want a solid state amplifier, already have one. In any case, its sounds good now.maybe halve the 1meg of pin 0ne on the tremelo for a bit more signal. thanks for your helpful criticism, it is much appreciated
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In most preamp circuits I've seen, plate current for a 12AX7 is about 1 mA per triode, 2 mA for both. Since preamp stages are almost invariably operating in class A, that current doesn't change much with signal level.
Keep an eye on the transformer temperature. Check every few minutes for an hour or so after power-up. If it doesn't heat up excessively during that time, I would guess it's okay.
Voltage drop under load is another clue, but an indirect one compared to temperature. Drastic voltage drop equals too-heavily-loaded transformer.
Congratulations!
-Gnobuddy
transformer seems ok, time will tell
The tremelo isnt quite what I want, . Going for that jonny marr "how soon is now" type thing. Its not that. It seems EL91 can be -17 to -25 so i am messing with that. othet than the tremelo its a nice sounding amp. much like the real thing but not so loud
some tremelo help would be good
The tremelo isnt quite what I want, . Going for that jonny marr "how soon is now" type thing. Its not that. It seems EL91 can be -17 to -25 so i am messing with that. othet than the tremelo its a nice sounding amp. much like the real thing but not so loud
some tremelo help would be good
Johnny Marr / the Smiths) wasn't ever on my radar, but Google turned up this clip: YouTube
What is your tremolo doing (or not doing) compared to the sound you want? Frequency not right? Wobble not deep enough?
I have zero experience with tremolo, but I'm attaching a cleaned up and rotated right-side-up version of the schematic in PDF and PNG formats for easier reading, in the hope someone else may chime in.
-Gnobuddy
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The finished item. The tremelo isn't as deep as I want.tried all suggested mods. Led bias. Lower buffer. Different bias Tec.
I get a lovely sine between pin 1 v4 and the output cathode but after that it's an ugly squiggle. Most likely my lack of scope skills/understanding.
The amplifier sounds good. Like a small PR. I like these EL91 pentodes. Also love the paint on tolex. This amp will honestly never leave the bedroom. Its cheap and looks great
Transformer still hasn't melted......
I get a lovely sine between pin 1 v4 and the output cathode but after that it's an ugly squiggle. Most likely my lack of scope skills/understanding.
The amplifier sounds good. Like a small PR. I like these EL91 pentodes. Also love the paint on tolex. This amp will honestly never leave the bedroom. Its cheap and looks great
Transformer still hasn't melted......
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Have you tried lowering the value of the 1 meg resistor (the one that feeds the tremolo signal to the 250k "intensity" pot)? That should strengthen the signal across the intensity pot, and allow you to dial in more "wobble" when you want to.
Speaking of the intensity pot: as the pot ages, one day the pot wiper will lose contact with the resistive track inside the pot. When that happens, the two 6V6s lose their negative grid bias, and both go to zero bias, i.e., maximum current. Minutes after that you have two red-plated and destroyed 6V6 valves, and perhaps ancillary damage to the output transformer, power transformer, and rectifier valve if you're using one.
I think there's a simple fix, shown in green. Add the fixed resistor so that when the pot fails, the 6V6 control grids are still biased negative.
The output cathodes are grounded in this circuit, so I think you're saying you got a lovely sine between V4 pin 1 and ground. Which is good!
The next place I would try is between ground and the control grids of the 6V6s, one at a time. Try using "AC coupling" on the 'scope if necessary. You should be able to see a clear low-frequency sine from the trem oscillator at the 6V6 control grids.
Got a link to share?
I found a product like that several years ago, but it was banned shortly after due to its toxicity.
-Gnobuddy
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Gnobuddy, thanks. Tried the 1meg. Its 270k at present. Its good "down on the bayou" but not "how soon is now"
please note the output valves are EL91 pentodes. Like small 6v6's. 250v and a tiny current draw but a real pentode sound. I am very impressed. There is also a fantastic SLO build by Bancika using them.
the paint. Home - Acry-Tech Coatings.
duratex
i think for "how soon is now" by the smiths, there were two fender tremolux???
any way two amps with two people sweeping the intensity into one microphone. and jonny marr playing a chord
down on the bayopu is enough
please note the output valves are EL91 pentodes. Like small 6v6's. 250v and a tiny current draw but a real pentode sound. I am very impressed. There is also a fantastic SLO build by Bancika using them.
the paint. Home - Acry-Tech Coatings.
duratex
i think for "how soon is now" by the smiths, there were two fender tremolux???
any way two amps with two people sweeping the intensity into one microphone. and jonny marr playing a chord
down on the bayopu is enough
Then it's time to try 120k instead.Tried the 1meg. Its 270k at present. Its good "down on the bayou" but not "how soon is now"
When I have no way to calculate the proper value of a component, I often just repeatedly halve or double the value as appropriate, until it works the way I want it to. In the E12 resistor series, 120k is the closest to half of 270 k.
Sounds like your project really worked out for you. Congratulations!
Now, how about a proper schematic, showing what you actually built instead of what Leo Fender built decades ago?
(If nothing else, you will need it yourself some day, when you need to repair or modify the amplifier.)
A few years ago I built a push-pull guitar amp with a pair of tiddly little 6AK6 pentodes, even lower-rated than your EL91 (6AM5 to us in North America.) The 6AK6 are only rated for 2.7 watts anode dissipation, versus 4 watts for the EL91.
-Gnobuddy
That might be wise. If you take it too far, the output valves will go all the way into cut-off on negative half-cycles of the tremolo signal, and then you will hear thumping sounds in the speaker....down on the bayou is enough
But there is always the Colin Chapman approach: fiddle with it until you've taken it one step too far, then back off just that last step. (Supposedly that was how he decided when his racecar chassis' were just barely strong enough to survive the race, meaning they were also as light as they could possibly be.)
-Gnobuddy
Makes you wonder how his race drivers felt about this particular bit of engineering philosophy, no?
-Gnobuddy
Many writers commentated on his "tongue in cheek" attitude to many things. Posthumously, the judge took a rather dimmer view.
And he's quoted as saying "Money is how we keep the score in motor racing nowadays."
And yet, after all that, I'm still a Lotus fan.
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