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Post by thetragichero on Nov 6, 2019 16:29:20 GMT -5
HEAT GUN FAIL $20 in grill cloth down the drain, and of course i ordered just enough new one on order, needed some parts for this badboy anyway 1950s dukane intercom. 2x 6v6. hoping the pt has enough heater current for some 12ax7 type dual triodes because the original tubes were relatively more obscure need to think of something inventive for all those front (i think) spdt switches. only one can be pressed at a time from my limited testing so i gotta get to brainstorming. thinking a 2x10+2x12 alnico speaker cab build to go with it... make it an A type cabinet with the 10s on the slanted part
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Post by reTrEaD on Nov 7, 2019 2:44:10 GMT -5
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Post by sumgai on Nov 7, 2019 2:48:24 GMT -5
trag, You might want to double-check that output transformer.... my gut feeling is that nearly all Intercom systems of those days (and indeed, most PA systems as well) used 25 or 70 volt output systems, not so many watts of power. This was because of the fact that when you have scads of speakers hooked up, your impedance is liable to be all over the map.... meaning, if a teacher in a room has turned off the speaker, then the load presented to the amp will not be the same as usual. And if several teachers do the same thing.... In a voltage driven setup (often called "line powered speakers"), the impedance is of almost Zero importance - we're not dependent on gobs of current, but instead we're dependent on voltage. You'll recall from Ohm's Law that voltage at very low amperage requires little resistance in order to work. Thus, varying load conditions don't affect the amp (specifically, the OT and tubes) nearly so much as would happen were a current amplifier stage to be used in these situations. Without a specific model number, I had zero results trying to find a schematic. Hopefully you can do better. HTH sumgai
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Post by thetragichero on Nov 7, 2019 9:57:52 GMT -5
i did some googling prior to purchasing and found a thread with someone having the same conversion plans. somebody walked him through calculating the transformer's impedance and other such maths + hooking up to mains. it was determined that for an 8ohm secondary load it would have a primary impedance of 5k... a little lower than i think the 6k6 I'd find on a 5e3-type transformer but something that can be designed around if all else fails i have the stock transformer out of my pawn shop excelsior that i could use. certainly not ideal because it's undersized but at least a backup option
the model number is 4b100b. there's a free schematic (ain't paying 15 bucks for a pdf) i found for the 4b100a and apparently the only difference is centered around the first preamp stage
i had the thought of trying to make this a fixed bias 6v6 just because they're not nearly as common and I've only really messed with cathode bias amps have to see if i have a 12at7. was thinking ltp pi and based on an online calculator this tube would strike the perfect balance between voltage gain of a 12ax7 but with more current driving the power tubes and possibly more clean headroom
so I've confirmed that the buttons along the bottom can be pressed at the same time i figure the first couple could adjust the first stage cathode bypass cap. not sure of specifics yet but i figure a "tight" switch would make that value say 100nf (i like that in my last/current build) whereas another adds 22uf. maybe later in alter tone stack values, etc. necessary? no, but if they're there might as well make em useful. once i start my disassembly process I'll get the multimeter and confirm that they're spdt as they appear
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Post by thetragichero on Nov 9, 2019 14:55:52 GMT -5
responded to Craigslist ad for an organ. picked it up sight unseen dusty but......... it's another 50s (1956 judging by speaker code) Hammond L112! it even has the tube of Hammond-branded oil for the tone wheels, which makes me question how they were maintained but they'd neither here nor there... same ao-43 chassis as the previous one so i know i can reuse much of the power supply (i enjoyed the reverb driver choke placed after the first filter cap). there's enough heater current to run four additional tubes that are on separate chassis (for the vibrato and percussion) so I'm wondering if i should get cute and try dc heaters possibly will do this one fixed bias, maybe a higher gain build anyway, yay!
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Post by newey on Nov 9, 2019 20:29:16 GMT -5
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Post by reTrEaD on Nov 10, 2019 1:28:57 GMT -5
so I'm wondering if i should get cute and try dc heaters possibly will do this one fixed bias, maybe a higher gain build anyway, yay! DC heaters would be quite nice for the first stages in a high gain amp. The PI and output stages, really don't see much benefit. And the more current you need to supply, the less effective the same filter caps would be. EL84s (6BQ5s) require a lot more heater current than most preamp tube so you'd be farther ahead if you use AC at the back end and DC up front. JMO.
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Post by sumgai on Nov 10, 2019 21:18:24 GMT -5
reTrEaD nails it. In my experience, DC is beneficial for quieting hum, and not much else. After all, heat is heat, and it takes a certain amount of amperage to generate that heat, regardless of it being AC or DC. When you consider the added expense of low-power-value components to make DC available for noise-sensitive pre-amp tubes, the benefit is obvious. Doing the same for power-hungary output tubes doesn't make as much sense, due to the cost of the power-capable parts needed to do the job.
In many cases, the PI (phase inverter) is also a pre-amp tube, so I'd put the DC there as well, even if the hum has been somewhat suppressed by earlier stages. But as usual, YMMV.
HTH
sumgai
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Post by thetragichero on Nov 10, 2019 23:44:51 GMT -5
fiddling with analog octave up circuits and an mxr distortion plus clone so far the best sounding is a single transistor to full wave rectifier I scribbled in my notebook but didn't attribute to anything.... did a bunch of googling transformer is from a take on the tycobrahe octavia called the 'bobtavia' which is basically an lm386 driving the transformer followed by the full wave rectifier. sounds okay but noisy wasn't able to get much out of tim escobedo's push me pull you next up to try is the green ringer while at the end of a distortion is nice my ultimate goal is to have the octave up in the feedback loop of a simple pt2399 delay. i figure it could either be neat/weird or a frustrating waste of time. right up my alley!
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Post by thetragichero on Nov 11, 2019 16:07:39 GMT -5
tried the octave portion of the maestro brassmaster. definitely the best (trashy) octave effect (which is good since i think I'll need the lm386 for a battery-powered cigar box amp for the old man... he built a cigar box guitar and wants a matching amp) so the bobtavia goes bye bye but the bjt phase splitter to full wave rectifier sounds the most useful after the distortion+... i plan to use a dpdt switch between octave and standard si clipping diodes, and I'm hoping i can fit it into a 1590a enclosure
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Post by thetragichero on Nov 15, 2019 3:02:18 GMT -5
glutton for punishment so i built this distortion plus clone (with a dpdt to select between the stock clipping diodes and an analog octave) in a 1590a enclosure. with a little pressure on the bottom until the screws bit i was able to fit all 10lbs in the 5lb bag also clears one of the breadboards for a big muff clone built around a uln2001 transistor array chip. that should fit in a 1590a to once i get my teeny board mount pots in
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Post by newey on Nov 16, 2019 9:18:04 GMT -5
Tidy bit of packaging there! 1+!
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Post by thetragichero on Nov 21, 2019 0:20:04 GMT -5
got my first ever delay build working, packed up, and shipped off for a pedal exchange I'm a part of now i am stripping the chassis of this intercom so i can build a marshall-esque 2x6v6 amp in here (stripped the Hammond amp, vibrato, and percussion chassis earlier). awaiting filter caps and new tube sockets
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Post by reTrEaD on Nov 21, 2019 9:55:01 GMT -5
If I was going to build around a pair of 6V6s, I might go with something like a Trainwreck Express Clone, which is essentially a Fender Deluxe with a single input channel, an extra stage of gain, and a solid state rectifier. Although I might consider using a tube rectifier.
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Post by thetragichero on Nov 22, 2019 0:25:17 GMT -5
had never considered something like that but i am intrigued. especially since i wanted something fixed bias with a pair of 6v6 stripped as much as i could without busting out the power tools (drill out rivets and use wire wheel to remove rust) and drawing the ire of my sleeping better half next up is figuring out why this caught on fire first thing i notice is that whenever the owner replaced the power tubes (EIGHT el84) he did not realize that they're blocking the cooling fan from spinning poor design or something else dumb? i am intrigued
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Post by thetragichero on Nov 22, 2019 0:45:52 GMT -5
well that would do it!
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Post by reTrEaD on Nov 23, 2019 14:03:46 GMT -5
Oh my! Eight EL84s? That suggests a very husky PT and I would guess the primary winding of the OT is such that it presents a very low impedance when the output windings are loaded as-advertised. The PT should be no problem but you might want to consider loading the output windings of the OT with a lighter load if you design around a pair of 6V6s.
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Post by thetragichero on Nov 23, 2019 15:18:41 GMT -5
oh this is a stereo power amp i got in for repair
i DO have my eyes on an organ that has between 6 and 8 el84s in it when we go to see the future in laws next weekend. would require driving my truck but from the way it's setup I'd be able to make two amps after purchasing a second power transformer... just waiting for a response to my email...
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Post by thetragichero on Nov 28, 2019 11:22:45 GMT -5
breadboarded last night but unable to test until this morning. glitchy sub octave fuzz. using cmos chips
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Post by thetragichero on Nov 29, 2019 17:45:18 GMT -5
sloppy schematic. used 1M trimpot on the breadboard for sub volume. will experiment with parallel resistors on the outer lugs to see if a smaller pot will tame it a bit... very easy to dial out the 'muff' sound. I'm sure the taper on the pot will help the other half of the 4013 could allow for two octaves down but it was too much. I'm contemplating a guitar synth build with controls for one and two octaves down (I've seen some folks shape the waveforms into things beyond the square wave the chips output) along with one octave up anyway i am going to try to fit this into a 1590a. using 6mm pots and only one side of the 4013 should help. i keep getting a ton of these logic chips out of organs
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Post by thetragichero on Dec 12, 2019 3:00:33 GMT -5
one of the better sounding Marshalls but also apparently one of the least reliable: jtm60 this one worked great until it didn't. wasn't able to get enough negative voltage on the bias trimmer so it smoked a couple power tubes (what's weird is that it worked great until it didn't) replaced the electro caps in the bias circuit, replaced r147 (which is just a jumper even on the factory schematic) with a 47k resistor to give the bias trimmer more usable range, replaced the coupling caps from the phase inverter in case they were leaking dc. finally removed the 51v zener diode (there so that the maximum negative voltage isn't more than -51v, but i wasn't getting more than -20v) and threw some small caps in parallel with the 22uf electro non polar caps from the transformer as i was getting pretty large resistance readings but didn't have replacements. biased up to -42v at the junction of r110 and r111 as the manufacturer recommends (according to some googling) time for bed
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Post by reTrEaD on Dec 13, 2019 1:30:56 GMT -5
one of the better sounding Marshalls but also apparently one of the least reliable: jtm60 this one worked great until it didn't. wasn't able to get enough negative voltage on the bias trimmer so it smoked a couple power tubes (what's weird is that it worked great until it didn't) Reading up on this, it seem this particular design is prone to a lot of problems. Solid state components mostly for relays that tend to fail because the ventilation is poor and heat causes issues. Also the jacks are reportedly of very poor quality. The high idle current in the output tubes due to insufficient bias on the grids won't sound bad, although you won't be able to generate near as much power output that way. However the power dissipated within the tubes themselves could be off the charts. And that will compound the heat issue associated with the poor ventilation of this model. After you get the circuit issues handled you might want to look at ways to improve the cooling. Maybe a small fan? idk.
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Post by thetragichero on Dec 14, 2019 23:32:52 GMT -5
power amp is working on the Marshall. somewhere in the first couple of tubes there's an issue. no guitar, just some squealing noise. tone controls affect the noise so it's upstream of that. plugging into effects return there is guitar. will check further sometime this well, after i make my weekly call to antique electronic supply checking on the 9" creme replacement logo from Marshall have been avoiding starting to fit my hex inverter muff + cmos sub octave onto perf to fit the 1590a enclosure. sub octave part and power section are on it. will end the night watching some ncaa basketball before calling it a night. playing guitar at church in the morning
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Post by thetragichero on Dec 17, 2019 23:36:36 GMT -5
somebody on the bass forum mentioned a desire for a pedal that clones the distortion from the peavey centurion/260 bass amps. those are based on several op amps and a bit too complex for anything but my big breadboard (which is in use), so I'm taking a stab at the earlier peavey century preamp which is all discrete
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Post by thetragichero on Dec 21, 2019 8:34:26 GMT -5
this is from right before i knocked off for the night so far the box still closes which is important volume and tone knobs and a nfb switch will go to the left of the tubes. power switch and indicator led will be front and center. output jack on the left side behind the ot still dreaming up the best place for the input jack. trying to keep it away from power but also remove the ability for the amp to be usable with the lid closed. somewhere by the controls would be nice but i have to do some depth checks
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Post by blademaster2 on Dec 21, 2019 8:48:11 GMT -5
I am impressed! I have heard of 'cigar-box ukeleles' before, but never a tube amp. I gather this is an effect unit or preamplifier, since it does not have an output stage for a speaker?
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Post by thetragichero on Dec 21, 2019 9:57:17 GMT -5
that is preamp and power amp. half of the 12ax7 is a preamp stage. that goes into a tone control and volume control. next is the second half of the 12ax7 as a driver stage going into a 12bh7 (12au7 would work but I've got these from organ pulls so i might as well enjoy the increased output) with both triodes in parallel running into a single ended output transformer (both transformers are what you would find in a stand alone reverb unit) my old man has become a decent (albeit slow) luthier in his retirement. having taught shop for many years and taken a course up in easton (exton? i don't know), pa anyway he had the idea to make an actual 6-string playable cigar box guitar. the neck was purchased off eBay but he did the cutout at the top to match the logo and the gold flake paint what he wants us a battery powered amp that's fully contained (speaker and all) in cigar box. still working on that (have suitable speakers and some ideas, gotta build on breadboard and measure current draw so that we can figure out battery requirements - that will be the hardest part as most of my ideas ended up needing too much current... i think a lm386 or two will be okay) for now I'm knocking out the easier stuff: tube amp and a semi-matching speaker cabinet
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Post by reTrEaD on Dec 21, 2019 10:24:41 GMT -5
remove the ability for the amp to be usable with the lid closed. Putting the input jack on a bracket so the lid must be open to access it, would be a very good idea. Operating this with the lid closed would be a very bad idea. With the lid closed (no ventilation) a LOT of heat would build up. The results could be ... tragic.
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Post by thetragichero on Dec 21, 2019 11:07:15 GMT -5
not only that: I'm making sure that it can close but there's a very good chance a tube would be in direct contact with a wire or other meltable bit also trying to keep noisy power stuff as far from the input,pots, and tubes as possible having solved a good number of the fitment issues i expected to encounter I'm certain something will come to me
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Post by thetragichero on Dec 21, 2019 22:11:16 GMT -5
power supply (besides mains switch and heater artificial center tap) completed. heater wires ran. been out of the house most of the day so I'll take the little bit of progress
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