Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: yorgle on August 25, 2020, 03:21:29 pm
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In an earlier thread, https://el34world.com/Forum/index.php?topic=26367.msg288312#msg288312 I introduced my latest acquisition- a unique vintage palm tree styled SE amplifier made by a company called Regal in Chicago sometime in the late 40's to early 50's. Because there appears to be little to no information about this particular amp on the internets, I thought it might be a good idea to deposit my experiences with it here in case anyone else should happen to stumble upon one. First, some pics...
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My goal with this amp is not so much to restore it as to basically give it a deep-clean and overall freshening-up, just enough to make it, clean, stable and playable. I've only seen one other like it for sale and it's listed around $1,000.00 so I think it wise to keep this thing as original as possible- which is very difficult for me. I have a predisposition to tinkering and modifying things, so this will be a challenge. This amp appears to have died a long time ago due to a failed cathode resistor, and was obviously stored in a chicken coop, garage or barn for quite some time.
First, some specs: the tube lineup is 6SC7, 6SJ7 (or possibly 6SG7), 6SN7GT, 6V6GT and 5Y3GT for the rectifier. The first two inputs (from the left) are wired to the 6SC7, presumably for instruments, and the third input (microphone?) is wired to the 6SJ7. The left volume pot controls the two instrument inputs, the right volume pot is for the mic. The TONE pot doubles as the ON/OFF switch.
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Draw a schematic.
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Draw a schematic.
Yes, it is the first thing to do.
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After the mouse turds
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Incredible find! I hear you on the "drive to tinker." When new, that amp must have been stunning. I really want to see a new one. Maybe you could scratch build one using this as a model. And keep the original...original.
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Check this out
https://www.youtube.com/watch?v=wN4SSQjcxQs (https://www.youtube.com/watch?v=wN4SSQjcxQs)
And I also sympathize with the tinkering bit, but I would make the amp safe, and do just enough to make it sound right - keeping the cap and resister values stock. Looking at the one on Reverb, you can see that the leads from the original filter cap can have been clipped short and new caps installed. If worried about value, you could leave those leads long and heat shrink the ends. A three wire power cord has been installed, although not wired the best way, and the speaker has been reconed. I'd also test any signal caps and replace but save any I removed. Tubes, caps and resisters were never intended to last forever, but are replaceable components, so I think the whole "original" thing is kind of silly. I have a tweed Harvard and a brownface Concert; both valuable amps and I replaced filter caps, bad signal caps, and drifted resisters. They sound wonderful and if someday I decide to sell either one, and they sell for less, I won't care because I got to play through them. BTW, the asking price for that Regal on Reverb is $1K. I suspect that its a $600 amp in today's market. But who knows? It's worth what somebody is willing to pay for a cool piece of history.
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Right now, I'm drawing a schematic and when that's done, I plan to take some voltage measurements and post all of that here for wiser eyes to view. The amp words fine right now, but in all honesty, it doesn't really sound that great-- the pots are scratchy and it breaks up way too early for my taste. I'm sure that new signal caps and resistors would improve things dramatically, but then, there goes the originality thing...
I've also been working on the cabinet, gluing down some spots where the veneer had pulled loose. I'll post some progress pics tomorrow.
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Nice that its working. If not sounding great (did you listen to the one on YouTube? should sound somewhat like that) the suspects in order IMO would be:
Tubes - no originality harm there.
Filter caps - Done carefully you can return it to original.
Speaker cone - Remove and save the original and install a good speaker.
Tone caps - replace carefully only those that test bad - save the originals.
Resisters - Buy old carbon comps on Ebay to replace any that are way off spec - no harm on value.
Pots - clean 'em - probably ok.
Others may disagree - but the things that hurt amp values are drilling holes in the chassis for new sockets, switches, pots, or installing the wrong size speaker. Other than that, any change in value is very small - if any. It'd be different if the amp was pristine.
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but then, there goes the originality thing...
Let it go and fix the amp. It's very likely that many of those yellow caps are leaking dc. Replace all of them. Put them in a ziploc bag and then a safety deposit box. Measure every resistor. You will find some (many) that read higher than they are supposed to. They go in the ziploc also. Clean the pots with some deoxIT.
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safety deposit box
:laugh: very funny, and won't the bank robbers be surprised to find old capacitors in that box.
I'd love to see and hear that amp when you fix er up. Those old octal amps sound better than most people expect.
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Put them in a ziploc bag
+1
the last real repair I did was on a multi-thousand $ fender, after explaining to the client, and getting the go ahead, i put 5 parts, including a pot, in a ziploc, labeled bag, with a thumbdrive for notes and a promise if i'm still kicking n "they" want the original parts in, i'd do it for free
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And I have a good size box full of leaky caps I've removed. But I think the whole original thing is silly so I'll probably never figure out which amps I removed which caps from! I have a 66 Alfa Romeo Sprint 1300 Junior - has the original air filter and spark plugs. Runs like shit - I wonder why?
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Thanks for all of the great suggestions. I poked about with my MM tonight and indeed, many of the resistors have drifted considerably beyond their marked values. The worst is a 47k that actually measures 57k so I’ll probably change him out along with a few other of the worst offenders. I assume most or all of the caps are leaking so I’m going to have to decide what my long term plan is for this amp. Is I sell it, (which is what my wife thinks the plan is), I’m inclined to leave the re-cap job for the next guy. But if I keep it, then the temptation to get the sound Just right will eventually win out.
While I was inside, I wrote down a bunch of voltages for each tube and will add them to my rough draft schematic that I hope to post here tomorrow. The PT is a 300-0-300. I’m getting 325 at the 1st filter cap, 262 at the 6v6 plate, 145 at the screen, and 21vdc across the cathode resistor.
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Keep it! It's too cool to sell. I tell my wife that I'll sell all the amps that aren't cool. So far, that is some damn slim pickins. :laugh:
The voltages you recorded look reasonable. Over 20% drift is what I'd worry about for resisters.
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Here's my humble first stab at a schematic. I still have to fill in the values for all of the capacitors and I'm sure there are some errors to be found, so if anyone sees anything weird, please let me know.
The chassis had a bit of surface rust that I loosely wire-brushed, cleaned and then very lightly dusted with some Rustoleum dark brown spray paint-- just enough to cover the rust, but not enough to smooth out the crinkle texture of the original paint. I think it came out quite nice and retains just enough "worn in" look to match the rest of the amp. I was also able to revive the leather handle on the top by rubbing it thoroughly with petroleum jelly.
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I should mention that for the life of me, I couldn't figure out what the figure was underneath the palm tree on the right, so if anyone else is likewise challenged- its the rear view of somebody with suspenders and a big hat.
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I couldn't figure out what the figure was
:laugh:
took a drawing class once, the instructor had us "define a human" with as few lines as possible
2 is all you need :icon_biggrin: suspenders and hat might take a couple more
Nice work on the amp, quick look didn't spot anything, but i have heat stroke :icon_biggrin:
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Schematic looks believable. Voltages for the first and second tube look wrong. That will probably change when you replace those caps.
150K cathode resistor for V2 seems wrong. Look closely at that. 1K to 2K would be more likely. It's OK to measure it's value in circuit with no power on. It's connected directly to V2 pin 5 and the ground buss next to the third input jack. I can't see the color bands.
There is a capacitor from node D (90v) to ground. It's the yellow cap in the lower left corner of the chassis in your gut pic. I can't see the value but it would be a big improvement if you replaced that with a 10 or 20µF electrolytic.
Those two 47K 1/2W 5% resistors are not original.
The 19K connected to V4 pin 4 is really 18K (brown-gray-orange).
The cathode resistor for V4 is most likely 470 or 500Ω. Use a 5 or 10 watt. We talked about this in your other thread.
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Schematic looks believable. Voltages for the first and second tube look wrong. That will probably change when you replace those caps.
150K cathode resistor for V2 seems wrong. Look closely at that. 1K to 2K would be more likely. It's OK to measure it's value in circuit with no power on. It's connected directly to V2 pin 5 and the ground buss next to the third input jack. I can't see the color bands. My mistake- it's actually 1K
There is a capacitor from node D (90v) to ground. It's the yellow cap in the lower left corner of the chassis in your gut pic. I can't see the value but it would be a big improvement if you replaced that with a 10 or 20µF electrolytic. I will keep that in mind if I decide to dig in and start changing things.
Those two 47K 1/2W 5% resistors are not original. But something was there, right? Another mystery value to figure out.
The 19K connected to V4 pin 4 is really 18K (brown-gray-orange). Yup- typo on my part.
The cathode resistor for V4 is most likely 470 or 500Ω. Use a 5 or 10 watt. We talked about this in your other thread. I have a 470 in there now, but found some nos 500r KOOLOHMS on ebay that I will try when they get here.
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Those two 47K 1/2W 5% resistors are not original. But something was there, right? Another mystery value to figure out.
No mystery. They most likely replaced two 47Ks that had failed.
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Nicely done. If you want to restore the cabinet (I would.) I suggest searching for info on restoring old tube radio wooden cabinets as that's the style. Most likely a lacquer finish.
I do not know what that symbol is, but I know that Hawaiian culture is rich in symbols - you can probably find something similar. Or just claim it's proof of aliens visiting our planet.
I have an old Magna (before Magnatone) that has that tube compliment. When I got it, mice had chewed up the wires. I had a hard time figuring out how to wire it up as I never found a schematic for that particular amp. I did get it working and sounding good. I think I hand drew what I landed on. But I am up-to-camp as they say in Maine, and can look when I get home after Labor Day.
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Those two 47K 1/2W 5% resistors are not original. But something was there, right? Another mystery value to figure out.
No mystery. They most likely replaced two 47Ks that had failed.
I do tend to overthink things. By the way, I gave in to temptation and swapped that big yellow cap off the D node (it was a .1uf) with a 22uf 450v I had in my drawer. Seems much louder and cleaner now— but why?
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How much voltage do you have at node D now?
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Here's my humble first stab at a schematic. I still have to fill in the values for all of the capacitors and I'm sure there are some errors to be found, so if anyone sees anything weird, please let me know...
No filter cap shown for the D-node
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How much voltage do you have at node D now?
Slukey: 103 when I fired it up, but dropped now to 96 after 15 minutes of playing.
Tubeswell: yeah, I forgot that one. I’ll be posting a revised version tomorrow.
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96V seems better at the D-node - you probably had a bad filter cap there. (there's not a lot of practical difference between 103 and 96 for a pre-amp supply node - could be the difference between tubes warmed up properly and tubes which are a little cold at first).
What are the plate voltages like on the input stages now?
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V1 is at 20, v2 shows 5vdc.
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V1 is at 20, v2 shows 5vdc.
Oops, that's still not right. Something is pulling too much current. Pull out each tube in turn (try 6SJ7 first*) to see if its a shorted tube
*If the 6SJ7 isn't the correct tube, the pinout wiring may be up the duff (150k is waaaay wrong for a cathode resistor)
Make sure you properly discharge the filter caps first (those are metal case tubes)
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You can gently bake old carbon comp resistors. When I worked in the F18 electronics workshop we used to use 85 degrees C for at least 24 hours (sometimes 48 hours) for the old carbon comps that had sat in a warehouse for decades. It bakes the absorbed moisture out and brings the resistance back into range. We'd then use some circuit board conformal coating to seal the resistors.
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I know you want to keep this amp all original but IMO it will be worth more if it is restored to original working condition. Here's my suggestions...
Old tubes are always suspect. Get a new set.
Replace that power cord with a three conductor power cord and connect the green wire to chassis. This is a safety issue.
Replace the multi-cap can.
We need more info in order to trouble shoot. Let the amp warm up for 15 minutes and check voltages for every pin on all five tubes. It would be very helpful if you would revise your schematic and include the measured voltages on the schematic.
Measure the actual value for every resistor and put these values on the revised schematic. Indicate measured resistance (maybe with a different color font). Leave the original values as well. Be prepared to replace any that are more than 10% out of tolerance. Probably all those big resistors will be out of tolerance.
Prepare to replace every yellow cap. You can just remove the cap that's connected to the fuse. Hoffman sells some Jupiter brand caps that look a lot like your originals but they are expensive.
This is a lot to think about but the payoff is worth it IMO.
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Attached is the revised, complete schematic showing things AS ORIGINAL. It does NOT show what I've already changed, e.g., the power cord and filter caps and death cap. After listening to you guys, I agree that before I go any further, I need to get new tubes and verify the specs on all of the resistors, so I removed all the voltages except for the PT, which I'm confident is correct.
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Would you post another big gut shot now that it's cleaned up?
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...I couldn't figure out what the figure was...
Not a swim-suit and sun-hat?
A woman here, Puranjot Kaur, is swimming around the island, some 43 miles, in 24 hours. That's twice as far as swimming the English Channel or to Catalina Island.
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...I couldn't figure out what the figure was...
Not a swim-suit and sun-hat?
A woman here, Puranjot Kaur, is swimming around the island, some 43 miles, in 24 hours. That's twice as far as swimming the English Channel or to Catalina Island.
Perhaps...the broad shoulders do suggest a swimmer’s build. But I’m also thinking a cane plantation worker relaxing at the end of a hard day.
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...I couldn't figure out what the figure was...
Not a swim-suit and sun-hat?
A woman here, Puranjot Kaur, is swimming around the island, some 43 miles, in 24 hours. That's twice as far as swimming the English Channel or to Catalina Island.
Winner! Winner! Chicken dinner! :icon_biggrin:
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Cleaned up guts pic. Those new filter caps are temporary and will be replaced with axials later. I replaced one of the 47k resistors that had drifted to 57k and that bumped up the voltage at the D node from 90 to 108.
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What are the voltages at nodes A, B, and C with the new caps?
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What are the voltages at nodes A, B, and C with the new caps?
With the amp well warmed up:
A=340
B=298
C=162
D=108
6v6: plate=290; screen=162; cathode=17
6SC7: plate=26
6SJ7: plate=8; screen=15
6sn7: plate1=39; plate2=28
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Measure cathode voltages also.
All those small-tube plates are low. Forcing node C up to 280V won't make them come right. Maybe the cathode caps have gone dead-short?
What meter? A $9.99 needle-meter may seem period-appropriate, but you really want a $19 needle or a VTVM or DMM.
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I’m using a digital multimeter. The cathodes on the 6SC7 and 6SJ7 are .47 and .46 respectively. I’ll disconnect the bypass caps and see what the voltages do.
EDIT: with new 10uf bypass caps, plate voltages for v1 and v2 only went to 30 and 9.5. Now I’m getting hum where it was dead quiet before.
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> cathodes on the 6SC7 and 6SJ7
OK, and I see the 6SN7 has no cathodes. Anyway, I was trying to make things add-up, wondering WTF the plate voltages were SO low. Yes, nodes C and D are lower than we see in Fenders, but even so the plates are low. I don't think it is all un-Fendery resistor choices. But sick tubes usually run high, not low.
BTW, the 6SN7 is a "recently added type" in my 1942 manual. As all civilian production ended early 1942, this pretty much has to be post-war. The non-fieldcoil speaker also says so. But the "Hawaiian lap guitar" fad did not live long after 1947.
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Just a hunch- do the plate voltages change if you increase the volume controls? (In case it’s leaky coupling caps). You still haven’t reported whether the voltages are different if you pull the 6SC7 or the 6SJ7?
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Just a hunch- do the plate voltages change if you increase the volume controls? (In case it’s leaky coupling caps). You still haven’t reported whether the voltages are different if you pull the 6SC7 or the 6SJ7?
Neither tube’s plate voltage changes with volume. With the tubes pulled, plate voltages jump to 125 for v1 and 116 for v2. I swapped in a different v2 but still only 9 volts on the plate.
The thing is, it really doesn’t sound bad- just breaks up earlier than I prefer. I’ve ordered some new tubes so we’ll see what that does in a few days.
Madness, I know, but I put the old bypass caps back in since they obviously were not the problem.
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I would have thought the picture was two beach chairs underneath a beach umbrella. Tombstones and a satellite dish? Very nice!
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On the last schematic you posted, the mA don’t add up e.g;
1) between the B and C node you have 136v across (supposedly) 18k - this calculates at 7.5mA (Not 5.9mA) assuming 2mA is going to the 6V6 screen, there must be 5.5mA going to the preamp.
2) between the C node and Pin 2 of the 6SN7, you have 123v across 47k - this calculates at 2.6mA (not 2.2mA). 5.5mA - 2.6mA = a total of 2.9mA going to all the other preamp tubes combined.
3) Yet between the C and D node you have 54v dropped across (supposedly) 47k - this calculates at 1.1mA (not 0.8mA). Yet from 2) above, we would expect 2.9mA. Either 1.8mA is being lost somewhere, or your measurements are wrong. (The 10uF cap at the C node could be leaky)
4) From the D node to Pin 5 of the 6SN7 you have 80V across 150k - this calculates at 0.53mA (as shown)
5) From the D node to Pin 8 of the 6SJ7, you have 100v across 220k - this calculates at 0.45mA (as shown) (the plate voltage on this tube is way too low, indicating that the tube would be conducting harder than intended- a possible bias problem? or a Bad plate resistor?) The screen voltage is also low - although we would expect to see a higher plate voltage - almost at the D node voltage- if the screen voltage wasn’t high enough. Are you sure it’s supposed to be a 6SJ7 in here?
6) From the D node to Pins 5 || 2 of the 6SC7, you have 82v across 150k - this calculates at 0.54mA (as shown)
Equations in 4), 5) and 6) add up to 1.52mA, yet from 3) above, we would expect 2.9mA. So 1.38mA is missing (e.g. leaky filter cap at C node?) (or your measurements are wrong)
(Might pay room check how fresh the battery in your meter is)
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Looks like you are moving toward fixing the amp. Here's a cap can that would be perfect and look good. It will replace all four filter caps and look like it's always been there. You'll need to enlarge the existing hole. And you would also need the metal mounting plate...
https://www.tubesandmore.com/products/capacitor-ce-mfg-475v-20201010-f-electrolytic
And for ideas, here's a current discussion with a pic of a restoration job on an old square dance PA. Looks like most of the caps and resistors have been replaced. I don't like the way the caps are installed but it's certainly quicker than doing it "right".
https://el34world.com/Forum/index.php?topic=26244.msg288578#msg288578
I really would not spent much time checking voltages and troubleshooting until EVERY cap and all resistors that are out of tolerance have been replaced.
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Are you sure it’s supposed to be a 6SJ7 in here
Thinking about Tubeswell's question; this is most likely a Valco made amp. Every octal Valco I've seen, and I've seen a lot of them, that included a 6SJ7, has that tube in the V1 position. Are you sure something did not get confused at some point? I know you have the inputs going to the right tubes - but is something else mixed up?
Just a shot in the dark - I know.
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I’m 99% certain the 6SC7 is correct for the v1 position because it’s a dual triode and the socket clearly has never been wired for anything else. The v2 socket is likewise clearly original and is wired for a pentode- but I’m not as certain it has to be an sj7– the amp may have originally used a lower gain version like the 6sg7.
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maybe V1-1 and V1 -2 since it's jacked IN to both :icon_biggrin:
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6(s)J7 is THE audio pentode. Many other octal pentodes are higher gain, at high current, for radar and VHF.
This is not a Rock or even Swing amp, it is for little ukuleles, guitars, and other folk instruments. The 6SC7 is two "instrument" inputs which today we might call "lame". The 6SJ7 input is higher gain for Microphone.
It MAY indeed need "fudging" to play well with modern guitars and wild picking. But right now it looks wrong even for very mello music.
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Undoubtedly sold for and with lap steels. The only other octal pentode I've seen in use in guitar amps is the 6SH7, which is very similar to the J.
When plugged into the mic input that thing ought to wail pretty good.
Here is a nice Regal Lap steel and amp combo
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I've taken a break from the circuit for a few days and and have really been enjoying playing this thing. As I said, despite the questionable plate voltages on the preamp tubes, after implementing Slukey's filter cap suggestion, the amp has decent volume and a warm, sweet vintage sound- especially when overdriven through the mic input. So... I've decided "if it works, don't fix it" is the prudent approach and I'm going to leave the circuit as-is. Eventually, something will release its smoke and then I'll have incentive to do a proper re-cap.
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Tonight I gutted the can capacitor and stuffed the first three filter caps inside. I need to order an axial 10uf to replace that ugly radial hanging off the D node. And doesn’t that NOS 500 ohm KOOLOHM look right at home as the cathode resistor on the 6v6.
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The only hint of the can cap mod is a barely visible seam where it was glued back together.