Hilton HP-95 Square Dance sound system project

[ShoemanGB]

Yes that's right.  This rig was built by and marketed directly to the Square Dance crowd.  I bought it and it's accompanying turntable from a co-worker who's dad was a dance caller for many years all over Maine in the late 60's through the 70's.   It has not been fired up in many years.  The matching speaker cabs are gone sadly.  This is an interesting rig.  The main speaker jacks ( a pair) are not the usual 1/4". It has one internal speaker in the cab and a switch that lets you select that or a pair of external "monitor" jacks.    It has three channels, 1 phono and 2 mic. 
All the channels appear to be grid leak bias with .022MF/600V caps to the grids of the 12AX7A's and 4.4M resistors after those straight to ground from the also straight to ground cathodes.  The plates of each the Mic channels then head to to a tone pot  and volume pot that next connects to a 6C10.  That part right there could be a problem if it turns out to be bad....     That's as far as I have gotten with circuit tracing so far.   
I plan to de-tube it, plug into a lamp limiter and Variac and start measuring.  I'll proceed slowly adding tubes etc as I check for issues. 
Here's the tube compliment:
2)  12AX7A
1) 6C10
1) 7249
2) 6L6GC
1) 7027A
1) 5U4GB rectifier
It also has three cap cans, a large OT and what looks like a much smaller second OT that I think is driven by the 7027A that feeds the "monitor" speaker(s).   It looks very unmolested with lots of black Sprague plastic caps throughout.  There is an ink stamp in the interior as well from a repair shop in state dated 1974. 
My plan is to play it as is initially, then mod one of the mic channels to be the usual cathode biased stage, giving me both grid leak and more"normal" tonal options. The power and output stages will be left alone. Why reinvent that wheel.   I'll try and post some pix too. 

[ShoemanGB]

Hope this works.  Chromebooks are sort of limited in capabilities.  I apologize if the sizes are too large.

[PRR]

....all over Maine...

Blue Hill Grange?

The power and output stages will be left alone. Why reinvent that wheel.

 

[tubenit]

I think square dancing is great fun!    Hope you get it working to your liking! 

With respect, Tubenit

[DummyLoad]

interesting PS - looks it's using the odd-ball one out 7027A possibly as a regulator. the 6C10 (3x 12AX7 triodes) is an interesting touch. << OK, so i re-read the opening post - a class A power amp = a fender champ and an ampeg jet in one package! nice!

bet ya 2 bux that the 7247 is a gain-stage/cathodyne pair. 

that thing is too cool as-is to gut it. try to figure out how to use it before burying a pair of dykes in it. just my 2cents.

6C10s have become pricey and scarce, you can usually sub in a 6AC10 which are more plentiful, less scratch.



--pete

[ShoemanGB]

....all over Maine...

Blue Hill Grange?

I would not be surprised if he did. He lived in Orland.   

[ShoemanGB]

interesting PS - looks it's using the odd-ball one out 7027A possibly as a regulator. the 6C10 (3x 12AX7 triodes) is an interesting touch. << OK, so i re-read the opening post - a class A power amp = a fender champ and an ampeg jet in one package! nice!

bet ya 2 bux that the 7247 is a gain-stage/cathodyne pair. 

that thing is too cool as-is to gut it. try to figure out how to use it before burying a pair of dykes in it. just my 2cents.

6C10s have become pricey and scarce, you can usually sub in a 6AC10 which are more plentiful, less scratch.



--pete

I agree.  It's a neat looking piece.  Other than converting one input, which will be easy to reverse, this thing will remain as is.  Because it was built as a traveling system, it has a cover panel that latches over the control face too.  Google searches turn up squat on these, just a Reverb listing and maybe a few e-bay references.  The company is still in business but under newish ownership and the long time tech still does service to the newer solid state versions but this model is unsupported.  No surprise there.  I think I'll contact him though and see if I can perhaps get a schematic.  As you can see, it's a lovely point to point deal.  Makes it rugged but more of a challenge to a hobbyist like myself. 
 I had not thought about the fact that this has two different output stages in one box.  Interesting.  Even more tonal options.   And I just this morning realized that there's a choke hanging out in there too.  Talk about a bit of everything in there.     

[PRR]

You want to drain Dick Bull's brain while you can.

He won't fix a HP-95, too old and heavy and useless, and he wasn't in-house when this amp was king. But he could have seen a few repairs, and know where the notes and blueprints were last.

[ShoemanGB]

I just em'd him.  Fingers crossed.  Now back to drawing out the circuits as best as I can.

[PRR]

> 2) 6L6GC
> 1) 7027A

The funny thing is: these are nearly the same tube. Shorter glass, extra G2 pin, but suitable for all the same work.

My guess is that the 6L6GCs are a 25W-45W room amp. Then someone added a monitor function at 4W-8W. But the existing 450V supply ruled-out 6V6 and other small tubes. A 6L6/7027 working at 400V 40mA 10k load can loaf at 16Wpdiss. If you can't figure that out, in 1958 you could call/TELEX Harrison NJ and an RCA engineer would figure it for you.

[ShoemanGB]

Mr. Bull came through:
"attached is the only schematic that I have for the 95. as a tinker you know the electrolytic capacitors don't like non use. If you have a variac transformer or other way to initially power it off at a lower line voltage it will reduce the chance of failure.  Good luck."
I've only had a few minutes to really study it but I can't wait to dig in.

[PRR]

Bravo!!! All makes sense, in a 1965 kind of way.

[ShoemanGB]

I got my fine line sharpies out and dissected the schematic.  What a neat rig this is.  As you smart guys deduced right away,  this has a single ended "monitor" output stage using half a 12AX7 with cap bypassed cathode R and the 7027A with external speaker jacks and a built in 5X8 or so speaker behind the grille. That section is switchable to play either just the phono input or all three inputs.
Then there is a push-pull 6L6GC output section that is fed all three inputs as well and has a 4 or 8 ohm output selector switch to the main speaker jacks.
  Each input, 2 Mic, 1 Phono, use half a 12AX7 set up as grid-leak bias for the mics and cathode (unbypassed) bias for the phono, then go through their own tone and volume controls (simple one pot "tone" for the mic inputs and a James stack for the phono.... bonus!) then on to that 6C10 Compactron with a cap bypassed cathode design as a make up stage before hitting the 7247 and the 6L6's.
The phono stage uses a Centralab Junior Compentrol as it's volume control after the James section.  Wow.  I had to look that up to see what it does.  Fletcher-Munson correction automatically.
There is a jack labelled "Slave" on the schematic but tagged "recorder" on my specimen that looks like a pre-amp output from the 7247.   There is also a jack labelled "switch" that breaks the phono's 12AX7 cathode connection to ground.  I was able to learn that the mic's supplied with the system had a switch on them that plugged to this to let the caller silence the music if desired. 
That's a lot of stuff in one box. 

[ShoemanGB]

Here's the record player.  Dig the variable speed slider.  As my 30 year old musician son noted, it's a slow-downers dream.   Too bad the speaker cabs were long gone before I came across this stuff. 

[PRR]

That's quite a turntable.

The fully variable speed reminds me my father used to do audio for ice-dancing. Music is generally adjusted for BPM (step-rate); in competition BPM may be prescribed. When Dad got into it already we had software to split rate and pitch (no Chipmunks). But in this rig's day you could only play fast/sharp or slow/flat. And you'd probably adjust to teach novices a new step or to challenge experts.

[ShoemanGB]

I had full intentions of firing this thing up yesterday but when I sat at the bench I realized I do not have a power cord that would work.  This thing has a recessed male AC connection, and I did not get the cord in the deal.  I had intended to convert it to a modern 3 prong later in the project, so out to the shop I went to fab that and also to convert one of the 6L6 speaker jack to a 1/4" type.   A computer PSU from the junkpile was sacrificed for the power connector.  For a tidier appearance I'll move it inside the chassis instead as in the picture.   The speaker connectors are isolated from the chassis and I was able to maintain that by using some HDPE flats I have to mount the jack in and then bolt that to the chassis.  Both mods actually use the original holes which made it easy.  I still need to convert one of the monitor speaker jacks.  I also started fabbing a dummy load with two 7 ohm 50W resistors I had scrounged from left over HID headlight conversions at work.  I hate those things by the way.  But I don't mind the left over parts.

[ShoemanGB]

Got some more time in this AM.  Converted one of the external monitor speaker jacks to 1/4" so now I can use my cabs for both output systems.  I realized looking at the schematic that the AC power for the turntable was supplied though the funky 6 pin connector on the back panel.  Inside the chassis those connections were too exposed for my comfort level so they have been eliminated.  I also completed the 3 wire power connections.  Fabbed a dual 8 ohm dummy load and it was finally ready for a power up.  Variac and light bulb limiter were plugged and and with all tubes removed the initial power up was event-less.  Recto tube in and I have heater power and output negative bias voltage of -39vdc with 115v coming out of the variac.  So that all looks good and that's enough for today. I did some reading of all the advice on power ups you folks have posted and decided that what's the hurry, caps are cheap, tubes are not and my stash is dwindling.  So I'm working up an order and I'll replace all the electrolytics next. 
 On a side note, I got an EHX Attack Decay pedal yesterday so I've been chasing that Tomorrow never knows sound. Big fun!

[PRR]

Tubes are as-cheap today as when that amp was new. Gasoline was often under $0.20/gal.

We used to think along "hamburger index" but the price of food has been heavily manipulated by ill-considered policy.

[ShoemanGB]

True.  My parents bought the house I grew up in in '65, same year as the schematic I got was drawn, for $12k.   My father made something like $8k/year as an associate professor and the VW Beetle we had retailed for $1300.  It was soon replaced by a Squareback when the 3rd and last kid came along in '66 

[silverfox]

Got Variac? Bulb tester?


silverfox.

[ShoemanGB]

Yes I do, and a bag of new caps from Doug too.  Arrived Friday. Prompt service as always.  I was thinking of restuffing the 3 can caps but now I'm thinking that's a lot of time for no good reason so I'll come up with a way to just use singles as needed.  I still have terminal strips left from my Firefly build that was done using just strips.

[HotBluePlates]

If you decided you didn't need 3 separate channels, you could sell that 6C10 to some unfortunate Ampeg owner & make them very happy.  They're fetching ~$100/each (though the 6AC10 that DummyLoad suggested can be had a $8/each, at the cost of a little less gain).  I personally won't buy certain model Ampeg amps solely because of the compactron tubes and the difficulty sourcing them.

As it stands, each channel is Gain Stage (a 12AX7 section), Gain Stage (a 6C10 section; same gain as a 12AX7), Driver/Output section.  If you ditch a redundant Microphone channel, you could use a 12AX7 in place of the 6C10 for no change in performance.

... caps are cheap, tubes are not and my stash is dwindling.  ...

Tubes are as-cheap today as when that amp was new. ...

From a 1965 Tung Sol Price List:

    12AX7: $2.40
    7247: $3.20
    6C10: Maybe similar to 6D10 at $3.85?
    7027A: $6.75
    6L6GC: $4.45
    5U4GB: $1.95

From U.S. Inflation Calculator, those prices are:

    12AX7: $19.64
    7247: $26.19
    6D10: $31.51
    7027A: $55.24
    6L6GC: $36.42
    5U4GB: $15.96

A tube-set would set you back $241 in today's money.  I did a quick-look for modern production tubes, and all the same types (minus the 6C10) would run $131.65 (add $7.90 to buy a NOS 6AC10).

[DummyLoad]

Maybe similar to 6D10 at $3.85?  

6AC10 has slightly higher mu. both 6D10 and 6AC10 are socket compatible with 6C10.

another option is the 6K11/6Q11 - has 2 x 12AX7 stages and 1 x 12AU7 - may work, but may need some bias tuning to perform satisfactorily.

overall, the 6AC10 is closest sub electrically.

6U10 might make a decent 3 tube power amp - 2 x 12AU7 sections for LTPI and the single 12AX7 section for the 1st volt amp. choose your poison for the power stage.

--pete

[ShoemanGB]

All very interesting, especially the cost comparison.  Go figure they are actually less expensive today.  Probably due to the countries of origin if it's like most other manufactured goods today.  I'm hoping my compactron is still functioning, I do have a supposed NIB 6AC10 on the way from an Ebay seller just in case.
 I'd like to keep all three channels, leave one mic input as is with the grid bias, make the other mic a cathode bias per usual, and leave the cathode biased phono input as is except for some possible parts value changes after sound testing. 
3 distinctly different preamp setups and two different and selectable, hell, even blendable, output sections in one box is kind of unique.   Sadly I'm not getting anywhere near it this weekend, real life and all that ya know.

[HotBluePlates]

... I'd like to keep all three channels, leave one mic input as is with the grid bias, make the other mic a cathode bias per usual ...

You already have the 270kΩ plate load for those Mic channels.  Consider using a 2.2-3.3kΩ cathode resistor, as you'd find in a Vox AC30.

____________________________

If you're not going to be feeding your record player into this, the Phono channel input is via the cord & Jones plug on the back of the amp.

Someone really wanted to make sure the record player wasn't too loud (or the player already had its own pre-amplifier).  Between that plug and the 1st 12AX7 for the Phono channel is a voltage divider (100kΩ, 470kΩ) that knocks the signal down.  The 47kΩ plate load on this stage really slugs the gain.  And then there are Tone & Volume controls, again reducing the signal level.

[ShoemanGB]

Good to know on the phono input voltage divider and plate load.  I had not gotten to the point of looking in detail at it other than recognizing tone stack as a James, which I happen to like.    I should open up the turntable case and see if there is any sort of preamp in there.  Good catch!   It sure is a heavy beast so who knows what I'll find.   

[PRR]

Someone really wanted to make sure the record player wasn't too loud......

It is the late 1950s. Crystals were everywhere. Crystal phono needles made 1V or more. Crystal microphones, even close-talked, a fraction of a Volt.

I see:
Phono = 0.8*20*0.1 = 1.6
Mike  = 60 (to mix-tube grid)

So mike is expected to be 1/37 of phono. Or down around 27mV. ("Close mike" may not yet have had the meaning it gained from Elvis, Mick, et al.)

I don't think the "Phono" input is guitar-friendly without more gain. Small fiddles may get 2.2X more which isn't "WOW".

[DummyLoad]

you could use the phono channel with pedal(s) + EQ boost pedal - that should wake it up.

--pete

https://empresseffects.com/products/paraeq

[ShoemanGB]

After a few weeks forced delay, I was able to get almost all the re-cap work done.  All the signals are replaced, and one can cap as well.  The can cap was a challenge but after staring at the problem long enough and grabbing some zip-ties from the garage I came up with a solution.   For the other two remaining cans, which are in much tighter quarters I may try and restuff at least one of them.  I'm trying to not have to drill new holes in the chassis but it may come to that and a couple of terminal strips to get the job done.

[ShoemanGB]

I'm having a hard time with the PI stage. Could one of you please tell me what I'm looking at?  Comparing it to all my references it looks like a long tailed pair, but there are some extra bits in the schematic that clutter it up beyond what my knowledge can decipher.
 And I tried gutting one of the cap cans last night....  no thanks.  First, it stinks like a fresh railroad tie on a hot day. Even my wife commented on the stink when I came back in the house from my shop.  Second, getting all that material out of the shell and cleaning it up takes longer than, at least in this case, just figuring out how to mount the needed caps in the chassis somehow and getting it done.  Oh well. Live ans learn.
So please clue me in on the PI type, it would be much appreciated.

[sluckey]

It's an AC coupled cathodyne.

[ShoemanGB]

Thanks Sluckey!  I'll study that type and see what I can learn.  I'm trying to see how the various stages of this thing might relate to all the various amps of old and how that might guide me in later mods.

[sluckey]

Princeton Reverb uses the same PI...

     https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_reverb_aa1164.pdf

IMO little easier to see the signal flow on the fender schematic.

[eurekaiv]

And I tried gutting one of the cap cans last night....  no thanks.  First, it stinks like a fresh railroad tie on a hot day. Even my wife commented on the stink when I came back in the house from my shop.  Second, getting all that material out of the shell and cleaning it up takes longer than, at least in this case, just figuring out how to mount the needed caps in the chassis somehow and getting it done.  Oh well. Live ans learn.

They're filled with creosote, which is the same thing used on railroad ties and telephone poles. So well nosed? I did this once for a little Supro amp that didn't have a good way mount a replacement inside, and while I got it done, I wouldn't try it again. I'll just buy a Hayseed or CE can cap instead.

[ShoemanGB]

Could someone please take a look at the schematic, the PI specifically, and tell me what the 1meg from the grid on pin 2 that then connects to that half of the tubes cathode resistor does?  I don't see arrangement on any of the other PI's of this type that I've been looking at.   

Wait...I just figured it out.  It's just like the Princeton Sluckey referenced,  it's just drawn differently. I'm so used to those old Fender drawings where the flow is easier to follow.   Makes you wonder about the "styles" for lack of a better word that you typically see.  All are correct, but are some more engineer approved say perhaps for brevity? or elegance from a technical standpoint?   These are the sorts of thoughts that occur to me on Saturday mornings with coffee and schematics in hand.    

[shooter]

elegance from a technical standpoint?

B4 computer drafting, drafting was an "artistic" technical skill.

[sluckey]

It's call bootstrap biasing. Since the cathode will be at a pretty high dc voltage because of the big cathode resistor, the grid also needs a pretty high dc voltage. The bootstrap is one way to get that needed grid voltage for proper operation. Here's some more info...

     http://www.valvewizard.co.uk/cathodyne.html

[ShoemanGB]

Thanks guys.  I ran upstairs and grabbed my copy of Merlin and there it was.  Why didn't I think of that in the first place?  Looks like I'll need to add a grid stopper to pin two as well but i think I'll try it as is first so I have a baseline.

[ShoemanGB]

We have lift-off.  I spent time yesterday with the variac and limiter slowly adding tubes and checking voltages.  I still have one can cap, the first in the PS filter string, to replace and the values needed are on order.  Long story short I felt confident enough this morning that I installed the output tubes and hooked up speaker cabs and a guitar.  No meltdowns or smoke.  The mic inputs do play well with the slightly hot humbuckers.   I did mod mic 2 to have a 1.5k/22uf cathode and 1M/68K grid set up.  Mic 1 is still the grid leak and you can tell it gets pushed much sooner. The phono channel did not respond to the guitars signal.  I have to do some checking on that as the 1/4" input jack was installed prior to me and looks to be OK.  As mentioned by you guys earlier it dumps a lot of signal and lacks gain for my purposes.  It's noisy too but I can fix that.  It's been years since I played through a p-p 6L6 amp...this thing gets LOUD.   

[ShoemanGB]

One other ting I noticed, the single tone controls in the Mic channels need to be fully to the bass side to give any sort of decent guitar tone.  At 12:00 the E and A barely come through, and turned more towards the treble end it sounds like I'm playing through an AM radio like I had as a kid.  Anybody else own a Panasonic Toot-a-loop ? 
 I haven't grabbed Merlin's book yet to see if this design is mentioned but I have done some quick looking on-line today and can't, at least to me eyes, see a similar design in any common amps.  Anybody have any input? Please chime in. 

[PRR]

It's a simple "bass cut". Full cut looks like 3,570Hz. Half-turn, depending on taper, may be 1,800Hz or 360Hz.

Why? This is not for Frank Sinatra or Bing Crosby's mikes. This is for calling over loud music. You want all your speech energy in the sibilants.

For guitar you want it wide-open, full bass (may be 20Hz).

[ShoemanGB]

Thanks PRR.  So the pot allows full freq bypass of the cap at the zero resistance end of rotation and as it's turned more of the signal is forced through the cap which blocks the lows. 
 I'll have to rewire it to a conventional treble cut tone control.  I don't think bumping the cap size to pass more bass would really be useful, but it would be a worthwhile experiment first.
And I'm a Dino fan myself.  Way more fun than those other two guys.

[ShoemanGB]

I now have 3 useable channels.  Using folks observations about the Phono input stage I changed it to a 1.8k/22uf cathode bypass, deleted the voltage divider from and added a 1M/36k to the grid.  I left the plate R as is for right now. I also deleted the little mic remote switch jack from the cathode's ground path so now I have a hole just right for a mini switch on the faceplate. Maybe a switchable cathode bypass cap for one of the preamp tubes will eventually live there. I also replaced the phono imput jack's wire. The PO had about 4 extra inches of fat shielded stuff that had the shield connected on both ends.  I replaced it with some of that neat skinny stuff I got from our host and grounded it on the jack end only. 
 The tone pot on mic 2 discussed earlier was converted to a 1M pot/6.8nf cap as shown in Merlin's book.  I'm still tinkering with that and may end up with a BMP filter there but I need more studying.
 I have lot's of hiss which I suspect are 50 year old carbon comp's showing their age.  While removing one from a terminal lug yesterday one lead pulled right from the body with no real effort..
 Question,  as this rig has one PS serving both amp systems I'm thinking there is no good way to be able to be able to turn one off and use just the other, correct?  The switches required would look like something out of Dr. Frankenstein's lab I'd think.  Igor! Throw the switch!

[shooter]

looks like the original solution is easiest, voice, voice and music switch.  the PS still live, but ya can't have it all 

[PRR]

> turn one off and use just the other

Why? Save energy? Save tubes? Are you running that many hours/week to matter?

I'd pull (or break cathode) the SE 7027 without much fear. If you don't want two 6L6, maybe you want another amp?

[ShoemanGB]

Save wear on the tubes I suppose, but you are right PRR that for the amount of time this will be powered up it's a silly concern. 

[ShoemanGB]

I replaced the one remaining can cap yesterday with three individuals, and after a limiter/variac first fire up post that work with no smoke, I ran plugged straight into the power strip on the bench.  All previous testing had been through the limiter.  It sounds good and all controls respond well.  But...  after a couple of minutes a hiss starts that gets louder and seems to change pitch (?) or intensity.  I can roll the volume control back to zero and it goes deadish quiet as expected.  I switched it off very soon to avoid disaster.  I could still here the guitar just fine, but the noise floor tries to compete with it.   Any suggestions on where to look first? It seems to be thermal issue.   I think maybe pulling tubes and seeing if it disappears would be a good start.  I did have to slightly move, as in bend, the leads on some resistors in the PS during the cap change and I wonder if one or more did not like that.  They are the originals to the amp.

[shooter]

are you still configured like schematics, 2 amps, 3 inputs?
is one better than the other?, all the same?
which "path" are you using?

[ShoemanGB]

Still essentially the same as you described.  Only changes have been replacing the filter caps and modifying one of the mic inputs so it's no longer grid leak bias.   
I just spent a couple hours fooling with it and the noise issue did not appear.  One of the first things I did today was to pull the PI tube and switched it on to see if the noise was evident, figuring that would eliminate the preamp as a whole.  No hiss.  Put the tube back in, played it, and it's been fine since.  So maybe that tube socket was the culprit?   We'll see if it comes back.
I also checked voltages on the 6L6GC's.  I have 478 on the plates, 422 on pin 4 and -42 on pin 5.   The schematic I got shows a handwritten -34 coming out of the bias supply.  Guess I need to measure the resistors in that section to see if it was changed at some point.   478 seems pretty close to the allowed ceiling.... any thoughts? 
The amp sounds really good.  Clean and punchy.  I can't get it cranked up, it's way too loud to try with my wife in the house, she's recovering from stomach surgery and needs rest.  I did plug my Orman Mosfet boost in to the grid leak input and dimed the pedals knob and pushed the 12Ax7 into a very nice overdrive at bedroom volumes.  Very cool.   I'm going to need to buy a bigger power soak than my Weber Mini Mass if I'm going to get this things output section to really start singing.  My only other push-pull amp is a '65 GA5T with 2 6AQ5's which is not in the same league at all.

[ShoemanGB]

So it just occurred to me that the 7027A SE output stage is fixed bias.  I was studying the schematic and the light bulb in my head went on. No cathode resistor and a feed from the bias source to it's grid I just fired it up and found -26VDC on pin 3.   Interesting.  Not something you see very often.