Tube amp impedance question

[Guitarzan]

i've searched the board for the answer to this but no luck.
I've been laid up again which explains why I've MIA for so long. Anyway, I can sit up in my adjustable bed, and have been playing around with a Korg X50 Synthesizer, with a Shure Srh240 headphone set plugged directly into the main mono out. With the volume on the keyboard up all the way I can hear it but it's pretty quiet.


So I wanted to know if I can plug that headset directly into the speaker output (8-ohm tap) of my homemade tweed deluxe? is it safe for the OT on the amp to do this? The speaker would then be unplugged so as not to bother Jane and the Monkey, so just the 38-ohm headphones would be the load on the OT. I'd only be using it at very low volumes more than likely, or... if it's better to do it this way, I could turn the amp up some and cut the volume slider on the keyboard way back.


I know it's really basic stuff but again, been laid up and am a little confused.
Thanks in advance for any anticipated answers.

[HotBluePlates]

I'll assume you can alter your amp while you're in this time of wanting to use the headphones.

You can do it. In fact, we could prove there are frequencies at which your speaker looks like a much greater impedance than what these headphones will be.

BUT... The max input level to the headphones is 1/2w. You run the risk of blowing the headphones if you're not ultra-careful turning up the volume. And do start with them NOT on your head when you turn up, as you risk blowing out your eardrums.

I'd suggest amp volume on zero, guitar volume on 0-1, and strum/hold a chord while carefully turning up the amp volume. You'll need to get just past the bit of rotation at the bottom where the amp goes from no-volume to some volume. Once you have a little something coming through, use only your guitar's volume control from there out to increase volume.

Ideally, you'd create a line-out with a resistive divider across the speaker jack, feeding a dedicated "headphone jack".

Let's call your Deluxe "18w". Speaker jack voltage for that power level (which could be exceeded under distortion) is Volts = √(18w*8Ω) = 12v RMS. Your headphone never want more than 1/2w into 38Ω or Volts = √(0.5w*38Ω) = 4.3v RMS. The division ratio would be a max of ~2.8:1. A divider made of 68Ω and 38Ω in series would yield a total resistance of 106Ω and provide the correct division to stay under 1/2w out with 18w output from the amp.

That said, a 2kΩ resistor in series with a 1kΩ pot comes close enough to correct voltage division figured above (but rounds downward in a safe direction), and uses values more likely to be available. Ideally, all of that is across an 8Ω load resistor (rated for probably 50w and bolted in the chassis) which is engaged when you pull the speaker from the jack... Probably by way of a switching jack like a Switchcraft 12A, where the normal OT secondary hot goes to the tip lug, one end of the 8Ω resistor goes to the switch contact lug, and the normal OT secondary ground and other lead of the 8Ω go to the sleeve lug. Then your 1kΩ connects to the switched contact lug, runs to the top of a 1kΩ audio pot whose wiper goes to the tip jack of your headphone out. And headphone sleeve to speaker jack sleeve.


But like I said, barring all of the above, be very careful! Both for your headphones & your ears.

[Guitarzan]

That very much man,
I knew someone would give me not only the answer but remind me what my question was missing in the process. Blowing my ears and/or headset is something I should, have would have known a year ago. I fear I'm losing my mind too. BTW it's a Korg X50 that I can sit up and fool around with. I used to play guitar and keys and while the guitar mood hasn't struck me lately I did just 2 days ago get the Monkey to pull the Korg out of mothballs for me.
I don't remember if the Deluxe which was made from a kit had a second speaker out that disconnects the internal speaker. I think it does, If I get to feeling any better, maybe I could make a box that does as you describe and plug into that external speaker jack, then plug the keyboard into the box.

[HotBluePlates]

The original Deluxe Reverb had a 2nd speaker jack, but it was simply in parallel with the first.

The main speaker jack has a switched contact, but it just shorts the hot to ground on that jack if the speaker is unplugged. It also confuses people who build from a kit, because they sometimes plug into the 2nd jack instead of the main jack, and get a very weak, distorted output. Yet the amp is just fine.

[Guitarzan]

While I didn't have any issues like that, I was under the misconception that the ext sp jack, I having trouble remembering now, but I was thinking it actually disconnected the internal speaker. That would be a huge mistake then since it could leave you in a situation with no load on the OT. So yeah, the shorting jack makes more sense.


What I never liked about that amp it was the way the volume controls have an effect on one another. What's up with that? I've seen mods on this board that you can do to "fix" that but what was Leo thinking?

[HotBluePlates]

... What I never liked about that amp it was the way the volume controls have an effect on one another. What's up with that? ...

To me it's actually a plus, because you can use the interactive volume controls as a "tone control" to get sounds you can't otherwise get with the stock tone control. The downside is you need to be able to let the amp crank out its max volume to make use of it.

You put one volume control at max and the other at half to use the interaction. Then you need something like an external A/B switch to switch between channels. One channel is very distorted and mid-heavy (it has max mids when the unused channel is at 1/2). The other channel is cleaner and has scooped mids (mids are at a minimum when the unused channel is full-up).

But you might need something like a smaller output stage or a VVR/power-scale control to get less total output if the full output of the Deluxe is too much for your playing situation.

... I've seen mods on this board that you can do to "fix" that but what was Leo thinking?

You've gotta put yourself in the mindset of the time. I don't know if it's an accurate gauge of cost of living in the old days, but I plugged the $129.50 list price of a 1957 Deluxe into an inflation calculator and got a 2016 value of ~$1097. It was not unusual to have more than one player plugged into a single amplifier, hence Fender amps into the late 60's with multiple channels that aren't easily/natively switched for use by a single player, and the 4 input jacks which are silly in a modern context.

Fender needed an easy way to mix 2 channels. The mods you've probably seen would swap the volume pot input/output connections around, and add 2x high-valued mix resistors. Fender's way saved the cost of the 2 resistors. It's probably as simple as that.

Apparently Valco had the same mindset when they saved the cost of a single resistor by using a grid-leak biased split-load phase inverter in an amp.


The approach was not unheard of at the time, with Madman Muntz doing something similar as much as a decade before with TV's to compete with larger manufacturers.

[Guitarzan]

Wow man, that is THE single best explanation of the Deluxe controls that I've ever read! Well done.
I've heard Leo was prone to doing all sorts of money saving things like using up old parts in new design, or maybe it's put better to say designed a new one around existing stock, before going full on change in models.


So to save a couple of pennies on some resistors sounds entirely plausible given Leo's reputation for being....shall we say frugal?


When I get outta bed, I'm going to have to revisit that Deluxe. I haven't even plugged it in for a good 3 years. After building it, plus a 5F1 Champ, I found the Champ to be the better of the two at least for my ear and how I used it. I was jamming with a drummer at the time and the little, easy to carry Champ kept up with him without any problems at all. That little amp is almost frighteningly loud for what it is.


When I was still gigging, I had the Ampeg V-4 half stack I bought new in '77 or '78. Now that thing was loud and a ball buster. Probably one of the contributors to my 3 back surgeries and still counting. But the little champ gets a way better growly sound at volumes that don't bring the cops but are still loud enough to satisfy me in any circumstances these days. Especially considering that everyone runs through the P.A. these days anyway so if you find yourself in a situation where the Champ just can't keep up, and for me they are very rare, throw an SM57 in front of that little 8" speaker and run it thru the PA and your still golden. Plus it takes the few pedals I use really well. I absolutely LOVE that amp.

[HotBluePlates]

... When I was still gigging, I had the Ampeg V-4 half stack ... that thing was loud and a ball buster. ... everyone runs through the P.A. these days anyway so if you find yourself in a situation where the Champ just can't keep up, and for me they are very rare, throw an SM57 in front of that little 8" speaker and run it thru the PA ...

What I've read is the race for guitar amp power was all related to not having great PA's widely available. Now, you don't need to fill the venue from your guitar amp on stage, and I for one would like to preserve my hearing as long as possible. Stage volume has dropped over time. And many acts (pro, at least) use in-ear monitors, so you don't even have monitor wedges adding to stage volume.

It's a whole other ball game than what it used to be.

[Guitarzan]

That's for sure. My neighbor across the street was my drummer for about a year back in the '70s. He's since taken up harp and fronts a local cover band and when he tried my champ he had to have one. He was using one of those Peavey Classic 30 amps but after trying out my Champ he was immediately sold so I built him one and he's been using it for about 3 years now. Never had any problems with it, plays maybe 60-80 gigs a year with it and only really needs to run it thru the PA when it's an outdoor gig. They still use stage monitors but now they're the size of a shoe box and sound amazing. Plus they fit on stands which gets 'em up off the floor. Not sure what the advantage is there, but I suppose I would if I was still in it. Anyway the big wedges are definitely gone. these guys don't like to spend too much money on new stuff unless it really necessary so I assume the shoe boxes are a necessity these days. One of the advantages is that they can fit in spots that we NEVER could have squeezed into all those years ago.  He calls his Champ his lunchbox after lugging that Peavey around for all those years.


Yeah, that Ampeg was a beast. I'll bet the head weighed 70 or 80 pounds and bottom was just as bad. It was loaded with I believe they were CTS? Four 12" speakers with massive square magnets that weighed a ton, plus the cabinet was build like Hitler's bunker.


Gee i'd forgotten how enjoyable it was to chat with the folks who populate this board. You're always so helpful, knowledgeable, and non-judgmental with guys who are as quick on the uptake with the finer aspects of the innards of all these devices. You learn a little building a kit but it's really a matter of painting by numbers so I've gotten just heaps of assistance from this board when I've had to modify an eBay find or something from the flea market for use with guitar. You know, old audio stuff. Thanks very much for all the help as well as the diversion from being laid up. You have helped me pass the time very enjoyably with these posts. If you ever find yourself in S.E. Wisconsin get in touch and I'll buy us a couple rounds. 

[PRR]

The real answer to your situation is to get a Rolls/BOSS/Beringer headphone amp. $50-$20 at the rock shop; maybe less if you have GAS musician friends. It takes a Line Input from the synth and outputs headphone level power, with a knob. It can maybe blow your ears or your phones but not as quick as a Champ in your ear. I suspect the stock Champ would have to be turned way-way-down to give a rational listen level, which makes it tough to adjust "just a little" without surprise.

Maybe more than one input, since by the time they fold a box and wall-wart it costs little to add a few knobs and jacks. We used to use a ROLLS mini-mix headphone amp, about the size of a 3x5 card, 3 or 4 stereo line ins with auto mono switching, and I don't think it was $30 each (we did buy a dozen).

For that matter, the $99 mini-mixers will have line-ins and headphone out. You or a friend may have one for sub-mixing a synth or drum-rack. Ignore the other 7 ins and 4 outs and meters and suchlike.

I understand that you might not trust the Monkey to go to Sam Adams Music with your credit card or piggy-bank. Too many toys, and most of them not what you want now.

[HotBluePlates]

The real answer to your situation is to get a Rolls/BOSS/Beringer headphone amp. $50-$20 at the rock shop; maybe less if you have GAS musician friends. It takes a Line Input from the synth and outputs headphone level power, with a knob. It can maybe blow your ears or your phones but not as quick as a Champ in your ear. I suspect the stock Champ would have to be turned way-way-down to give a rational listen level, which makes it tough to adjust "just a little" without surprise.

Maybe more than one input, since by the time they fold a box and wall-wart it costs little to add a few knobs and jacks. We used to use a ROLLS mini-mix headphone amp, about the size of a 3x5 card, 3 or 4 stereo line ins with auto mono switching, and I don't think it was $30 each (we did buy a dozen). ...

Yes, this is the real answer.

I have one of those Rolls headphone amps somewhere. I used to use it back in the day with a portable CD player to be able to drive a set of decent AKG headphones.

Or for a bit more money (and a couple days wait for the mail), you could get a Schiit Magni. You will need a 1/4"-to-RCA adapter (or whatever size is smaller for the Korg's jacks). You will not need another headphone amp, even if you get planar magnetic headphones. Unless you get some seriously expensive headphones and an undying love of listening to music through them... I manage just fine with my Magni and the planar magnetics.

[Guitarzan]

Well I got to thinking about it and once upon a time, I had a NIB Kustom tube 12 that I made a talk box out of by installing a horn driver and some plumbing parts for the tubing to connect to and it actually worked great. Just as good as the Heil Talk Box I had. So I sold the Heil, and played with the home brewed job until I got tired of it and then shelved it.


I could see a headphone jack on the control panel of that little amp in my mind's eye but wasn't sure so I actually got OUT OF BED...hobbled down to the basement and whaddya know? right there on the shelf where I lect it, complete with tubing all coiled up and tucked into a bag was that amp and YES!! It does have a headphone out right next to the blue pilot light.


The monkey would have been okay with my card as long as he didn't pass any drive-thru fast food places along the way. He can't resist hitting a drive thru every time he leaves the tree house. I suppose I could have worse problems. At least he doesn't hit the bars, or other cars, fire plugs, light poles, etc.


I think I would like to have a dedicated headphone amp though. I like the sound of $20-$50  lot more than a C-not tho. Does Hoffman sell one? You guys have really done more than your share on this already so I'll search the site myself. I'd prefer not to have to do adapters to make it work. I have a laptop bag with a bunch of little compartments and whatnot that's dedicated to adapters. Over the years, I've collected the adapters that Hell won't have. I'll bet I have 6-8 pounds worth of them and all sorts from Gender benders to adapters. Some pretty odd ones too IIRC. Haven't looked in that bag for quite some time.So it's not a matter of having to buy 'em or laziness (in this case anyway, cuz I really am pretty lazy) but they usually end up giving me trouble one way or another. the fewer connections in the signal chain the better IMO.


The Shure SRH240 headphones sound OK to me. They are old school inasmuch as they're big honkin' coconut halves with big padded thing that holds 'em together & rests on the top of your head. But anything smaller tends to walk when you had 3 teenagers living at home. So I bough something I figured they couldn't stand to be caught dead with and I've managed to hold onto them for a good number of years. The oldest is is closer to 30 than he is to 20 so now that they've all grown they buy their own stuff. There was a good decade there tho when I couldn't keep a cell phone charger to save my life.


That Rolls Mini-mix sound like it might be just about my speed if it's got 1/4" inputs. The Korg has a Mono out but it also has L & R Stereo outs. It probably sound pretty good in stereo.


It pretty amazing how much you're missing when you just plug headphones into the mono output. With an amp, you really hear all of the little nuances of the great many voices this thing has. I took it in a trade along with an American Strat from the mid '90s wOHSC locking tuners, lace sensor pups, rosewood fret board, Black MOTS pickguard and nice dark blue finish.I found a collector a '68 Tele that was one owner, W/OHSC for an insane price. The Korg and the Strat were my finder's fee.

[Guitarzan]

My neighbor Jesse the harp player who used to be my drummer in the '70s is having a party at the end of July. So they play a set to kick things off then let everyone mix it up in a jam session that goes until we feel like the neighbors have had enough or the cops show up. Most of the neighbors will be there, and the way the neighborhood is laid out, there's a school yard behind Jesse's house, and beyond the school yard there's a deep valley that's about 1.5 miles wide. It's perfect to sort of aim the band that way and the only neighbors that could be impacted are right across the alley and they'll be with us anyway. It's been 4 year, I think...yeah, at least 4 years since the last one of these. The neighborhood is relatively stable tho so not a lot of people have changed around us aside from the little ones becoming teenagers and the teens moving away. Our neighborhood is at least 90% owner occupied. The others have landlords that don't wanna see some A-hole louse up his property so they keep pretty close tabs on renters.


So my motivation in hauling this stuff out and trying to bone up is to have one last jam, and with hope, make  decent showing of myself. It'll be the last one for me, so I'm shooting for half a dozen tunes that I know I can pull off instrumentally a few on Keys and a few on guitar. Or I guess I should say I'm pretty sure I can pull 'em off. I really haven't played at all to speak of in 3 years. I was playing regularly prior to that but it's amazing how fast you lose the chops if you don't keep 'em up. After the party, barring some miracle where I lose about 15 years of age & all this hardware they've been installing in my body bit by bit, I'm calling it quits for good and everything but my 1st Les Paul, and my Champ will be for sale.


I've got some oddball axes, Teisco I think, no headstok logo on most of 'em, some NOS Cort guitars that aren't worth much but would make a nice entry level instrument for a kid or something like that. A couple Strats, and bunch of amps that all have to go. I haven't got it in me to work on the stuff that needs it Some of it is simply in need of a setup) and it's just not right to let the stuff be crammed into closets, under the bed, and shelved in the basement. It should be in the hands of people who want it to play. The stuff isn't doing me or anyone else any good anymore and I intend to remedy that before fall. It's my hope to sell everything locally. That way the buyer will get the most for his money, not having shipping charges to deal with. I will ship something out tho if someone reads of my "Musician's rummage sale" and simply has to have something that I have.


I'll probably post a list of everything, (and there is a lot once you figure in Mics, Cables, stands, and all of that kind of crap) on the appropriate portion of this forum in case any of you guys are interested in any of it. I'll try to make allowance for the shipping as best I can. Or if you intend to be in the Milwaukee area this summer get in touch with me and we can work out a meeting. Come at the end of July and you can join the party!

[HotBluePlates]

... I think I would like to have a dedicated headphone amp though. I like the sound of $20-$50  lot more than a C-not tho. Does Hoffman sell one? You guys have really done more than your share on this already so I'll search the site myself. I'd prefer not to have to do adapters to make it work. ...

I think the Rolls that PRR had in mind was the HA43. I had one like that, but probably 3 generations older. Yes, still 4 channels even though I only needed one (not sure they make just a single-channel headphone amp).

[Guitarzan]

I found a Rolls HA43 online for $50.00 OBO. Snagged it for 25 & free shipping.
It looks great! Can't wait for it to arrive now. I'm looking forward to playing with it now.