I’ve been looking for a PA amp to convert into a 1x12 combo guitar cab for a couple of months now, and finally found what appears to be an ideal candidate for a conversion.
This amp was made by Viking of Minneapolis, and the model number is PA-93.
The specs: 12AX7+12AU7 preamp tubes, 12AX7 PI, push-pull EL84 output. Actual wattage is unknown, since I can’t find a single mention of this amp anywhere, except for a brief video on YouTube that wasn’t very informative. Viking of Minneapolis was well-known for their reel-to-reel decks, but apparently not their tube amps. The PT and OT are beefy, but small, so I’m expecting this to likely be in the 13 watt range.
The controls on the front appear to be a rotary on/off switch on the left, tone, then two volume controls for the 1/4” mic input on the front and the rear phono input. The pot all the way to the left appears to be a bias adjustment of some kind. Maybe hum cancellation?
Here’s the gut shot.
All the date codes point to a manufacturing year of 1960. The latest code I’ve seen is the 36th week, on that cap just to the right of center. It has diode rectification, though I haven’t been able to identify those either. The diodes have “IR” on one side with a diode symbol in between, and the other side says “66” and right below it says “4241.”
Here’s some other close up shots of the interior.
12AX7 input:
12AU7 second stage:
12AX7 PI:
EL84 power tubes:
The blue wire coming off the 8ohm OT tap here appears to be negative feedback of some kind. It connects to the resistor on the terminal strip and then back to the 12AU7. The OT has 70V, 8 ohm and 4 ohm taps:
The kinda cool part is this came complete with tubes of unknown origin, and they ended up being a Telefunken 12AX7 on the input, and a pair of Mullard rX1 B0K EL84 power tubes, complete with soot. The other two tubes were just an RCA and a Sylvania, nothing special:
I’m not sure what state they’re in, since I don’t have a tube tester, but needless to say I’m buying replacement tubes to use while testing the circuitry, lol. Preliminary checks so far are very promising. The resistors haven’t drifted out of spec on all that I’ve measured, and even the caps all test good, which was the biggest surprise so far. We’ll have to see what happens when they get a few hundred volts through them.
I’ll probably end up having to trace out a schematic myself, but I was hoping somebody might know something more about Viking of Minneapolis, or about any other amp that used this circuit, that might point me in the right direction. Some of these old manufacturers are really tough to find info on.
Thanks!
This amp was made by Viking of Minneapolis, and the model number is PA-93.
The specs: 12AX7+12AU7 preamp tubes, 12AX7 PI, push-pull EL84 output. Actual wattage is unknown, since I can’t find a single mention of this amp anywhere, except for a brief video on YouTube that wasn’t very informative. Viking of Minneapolis was well-known for their reel-to-reel decks, but apparently not their tube amps. The PT and OT are beefy, but small, so I’m expecting this to likely be in the 13 watt range.
The controls on the front appear to be a rotary on/off switch on the left, tone, then two volume controls for the 1/4” mic input on the front and the rear phono input. The pot all the way to the left appears to be a bias adjustment of some kind. Maybe hum cancellation?
Here’s the gut shot.
All the date codes point to a manufacturing year of 1960. The latest code I’ve seen is the 36th week, on that cap just to the right of center. It has diode rectification, though I haven’t been able to identify those either. The diodes have “IR” on one side with a diode symbol in between, and the other side says “66” and right below it says “4241.”
Here’s some other close up shots of the interior.
12AX7 input:
12AU7 second stage:
12AX7 PI:
EL84 power tubes:
The blue wire coming off the 8ohm OT tap here appears to be negative feedback of some kind. It connects to the resistor on the terminal strip and then back to the 12AU7. The OT has 70V, 8 ohm and 4 ohm taps:
The kinda cool part is this came complete with tubes of unknown origin, and they ended up being a Telefunken 12AX7 on the input, and a pair of Mullard rX1 B0K EL84 power tubes, complete with soot. The other two tubes were just an RCA and a Sylvania, nothing special:
I’m not sure what state they’re in, since I don’t have a tube tester, but needless to say I’m buying replacement tubes to use while testing the circuitry, lol. Preliminary checks so far are very promising. The resistors haven’t drifted out of spec on all that I’ve measured, and even the caps all test good, which was the biggest surprise so far. We’ll have to see what happens when they get a few hundred volts through them.
I’ll probably end up having to trace out a schematic myself, but I was hoping somebody might know something more about Viking of Minneapolis, or about any other amp that used this circuit, that might point me in the right direction. Some of these old manufacturers are really tough to find info on.
Thanks!
Gut it. Much easier. All those caps will be bad, if you tested them with high voltage you will see. Transformers are the correct size or even large for a guitar amp, leave them, they are fine. You can probably drop down to 125 ohms for the output cathode resistor, add a cap if there is none. Chances are the tubes are fine also, why buy replacements. If it has 70v line outputs, it was a PA amp of some sort.
Thank you for the tips and help with the ID. Replacing all the caps is definitely on the agenda, I just want to hit the old ones with some high voltage first and see what happens. My preliminary testing was more for curiosity’s sake than anything scientific.
I bought some replacement tubes for testing/building because those OG Mullard EL84s sell for between $100 to $150 a pair, and the Telefunken 12AX7 is about $75 too. I didn’t want to risk a small fortune in tubes while I mess around, so I bought some cheap tubes to use while getting the circuit back in order.
I’ll definitely use the originals, and have the others as backups once it’s finished. I’m not sure how much life is left in the originals too, so having backup tubes couldn’t hurt. I did manage to pick up a barely used RCA cleartop side-getter 12AU7A for $1.25, so I’ve got that going for me anyway.
I bought some replacement tubes for testing/building because those OG Mullard EL84s sell for between $100 to $150 a pair, and the Telefunken 12AX7 is about $75 too. I didn’t want to risk a small fortune in tubes while I mess around, so I bought some cheap tubes to use while getting the circuit back in order.
I’ll definitely use the originals, and have the others as backups once it’s finished. I’m not sure how much life is left in the originals too, so having backup tubes couldn’t hurt. I did manage to pick up a barely used RCA cleartop side-getter 12AU7A for $1.25, so I’ve got that going for me anyway.
If it were mine, I'd definitely trace down the schematics first. Then look which geetaar amp it is comparable to. Then plug in a geetaar, a speaker and play. If I'd not like the sound, I'd tear it down and replace the circuitry by some I'd find in the WWW. There are many around, using a pair of EL84's/6BQ5's.
And yes, that pot with the knurled knob is dedicated to minimize hum. Get it completely into the chassis, 'cause you don't need to operate it once you've found the right spot. Or even replace it by two resistors.
Best regards!
And yes, that pot with the knurled knob is dedicated to minimize hum. Get it completely into the chassis, 'cause you don't need to operate it once you've found the right spot. Or even replace it by two resistors.
Best regards!
I would too if it were mine.
For what it's worth: The "B" in de second line close to the base of the EL84 in one of your pictures indicates that it was manufactured by Mullard, Blackburn.
For what it's worth: The "B" in de second line close to the base of the EL84 in one of your pictures indicates that it was manufactured by Mullard, Blackburn.
Thank you both for your advice.
I’m definitely planning on replacing all the caps, since those green Micamold caps are on par with Sangamos as far as quality goes. It’s not a question of if they’ll go bad, it’s when. Plus I want to put some nice film caps between stages to improve the sound quality.
I’ve had a PA amp converted to guitar use once before, a Bell Pacemaker 33 that had push-pull RCA blackplate 6L6’s putting out 33 watts, and that thing was a monster. I got real lucky on that one because I was so clueless I just plugged it in and played my guitar through it, but it sounded great. It had a bit of hum since it probably needed caps (which I now know in hindsight), but otherwise it didn’t have any problems.
On this one I’m trying to be more methodical, lol. The EL84 is an early Marshall / Vox sound, so I’m hoping that’s what I end up getting out of this. I’ve already bought the speaker for the combo, a Jensen Jet Blackbird 40 12”. An alnico speaker with a bit higher power handling, but it should still have that classic late 50s / early 60s alnico voicing and compression.
I’m definitely planning on replacing all the caps, since those green Micamold caps are on par with Sangamos as far as quality goes. It’s not a question of if they’ll go bad, it’s when. Plus I want to put some nice film caps between stages to improve the sound quality.
I’ve had a PA amp converted to guitar use once before, a Bell Pacemaker 33 that had push-pull RCA blackplate 6L6’s putting out 33 watts, and that thing was a monster. I got real lucky on that one because I was so clueless I just plugged it in and played my guitar through it, but it sounded great. It had a bit of hum since it probably needed caps (which I now know in hindsight), but otherwise it didn’t have any problems.
On this one I’m trying to be more methodical, lol. The EL84 is an early Marshall / Vox sound, so I’m hoping that’s what I end up getting out of this. I’ve already bought the speaker for the combo, a Jensen Jet Blackbird 40 12”. An alnico speaker with a bit higher power handling, but it should still have that classic late 50s / early 60s alnico voicing and compression.
I'd second taking the time to draw out the schematic; the pics of each stage are nice, but, you'll have to do the physical to drawing translation yourself.
The bottom side looks in ok shape and your plan to replace everything with film will take it to 100 years old and still working! easily; not sure if I'd "gut it". There is the matter of the B+ filter caps, which may / may not have survived. Hope you have a Variac and brought it up to working voltage slowly... Especially with those SS rectifiers.
I assume there's Bass and Treble controls, which you can replace with a more guitar-ish tone stack circuit - or just change the frequency of operation on the existing bits to be more suitable for guitar; I'm sure by now someone, somewhere has already done it; guessing a "Baxandall for guitar" Google search will turn up a ton of info on that 🙂
The bottom side looks in ok shape and your plan to replace everything with film will take it to 100 years old and still working! easily; not sure if I'd "gut it". There is the matter of the B+ filter caps, which may / may not have survived. Hope you have a Variac and brought it up to working voltage slowly... Especially with those SS rectifiers.
I assume there's Bass and Treble controls, which you can replace with a more guitar-ish tone stack circuit - or just change the frequency of operation on the existing bits to be more suitable for guitar; I'm sure by now someone, somewhere has already done it; guessing a "Baxandall for guitar" Google search will turn up a ton of info on that 🙂
Thank you also for your reply.
There seems to be some lingering confusion about my post here. I only posted it with the hope that somebody might have either an old schematic for this amp (Viking of Minneapolis PA-93), more information on the company that made this amp (so I could track down the info myself), or possibly they knew of another PA amp in this era using this tube layout that I could reference.
The close up shots were just so people could see the layout more closely, for the last reason I mentioned. I fully plan on tracing out the schematic myself, making the necessary circuit modifications (like adjustable NFB and a more appropriate tone stack), and doing this the proper way. I haven’t applied power to it yet, but from the looks of things internally it should only need to be hit with the multimeter, recapped, and brought back up to spec where necessary.
I don’t currently have a variac, but they don’t seem to be very expensive, so one is now on my short list of tools to acquire. Thanks for the suggestion.
There seems to be some lingering confusion about my post here. I only posted it with the hope that somebody might have either an old schematic for this amp (Viking of Minneapolis PA-93), more information on the company that made this amp (so I could track down the info myself), or possibly they knew of another PA amp in this era using this tube layout that I could reference.
The close up shots were just so people could see the layout more closely, for the last reason I mentioned. I fully plan on tracing out the schematic myself, making the necessary circuit modifications (like adjustable NFB and a more appropriate tone stack), and doing this the proper way. I haven’t applied power to it yet, but from the looks of things internally it should only need to be hit with the multimeter, recapped, and brought back up to spec where necessary.
I don’t currently have a variac, but they don’t seem to be very expensive, so one is now on my short list of tools to acquire. Thanks for the suggestion.
Usually I also do not use a variac to power old equipment the first time. Just pull the tubes, put a mains bulb tester in series with the power tranny, throw the switch and see what happens. The bulb should flash, then go dim. If this is ok, I'd measure the voltages at all filter caps to exclude shorts. Let is settle for some hours to get the electrolytics reformed. Then get the valves in and do the same procedure. Anyway, due to the heater power consumption, the bulb won't completely get dark then. If there's a schematics available (I know there isn't in your case...), I'd measure the voltages and compare with the schematics. Then it's time to remove/short the MBT.
Best regards!
Best regards!
There used to be a HUGE and very well made documentation source dedicated to Service Techs called Sams Photofacts, chock full of schematics and service manuals for amps such as yours (and TVs, Radios, vinyl disk players, etc.)
I am certain one includes yours and many similar ones.
I guess some site dedicated to old radios and such will have some downloadable copy.
Just to start with something:
https://sfpl.org/locations/main-lib...ctronics-repair-resources/how-find-schematics
https://theschematicman.com/
Maybe not exactly yours but browsing "60´s PA amps" will quickly find a "brother", there´s not many ways to skin a cat, and those were quite generic.
Thank you both for your replies.
I’ve seen the light bulb current limiter on Uncle Doug’s channel, so I might try that route too. I’m still in the planning and acquiring stage for this project, and I’m not in a huge rush to get it done. I just want to do it right.
I did download the archive of “Most Often Needed Radio Diagrams” for all years, but was unable to find this amplifier in any of the manuals. I hadn’t tried Sam’s Photofacts yet, so thanks for reminding me about those. I’ll try to track some down and see what I can find.
I’m still open to any and all advice, and so far you all have been awesome with your tips and suggestions. Thank you!
I’ve seen the light bulb current limiter on Uncle Doug’s channel, so I might try that route too. I’m still in the planning and acquiring stage for this project, and I’m not in a huge rush to get it done. I just want to do it right.
I did download the archive of “Most Often Needed Radio Diagrams” for all years, but was unable to find this amplifier in any of the manuals. I hadn’t tried Sam’s Photofacts yet, so thanks for reminding me about those. I’ll try to track some down and see what I can find.
I’m still open to any and all advice, and so far you all have been awesome with your tips and suggestions. Thank you!
Want my honest opinion? Ditch it.
I've wasted quite some money and time in two similar projects in the past.
This thingy is from ~1960 and it is quite likely all parts have deteriorated significantly. All e-caps will be bad, most of the other caps as well might have gone leaky (may even pose security risks). The carbon-composition resistors will have increased in value and become unstable. The transformer wire isolation may have gone bad as well imposing security threats, plus there is high risk of partial winding shorts which are not so easy to detect when they develop (from heat stress and vibration). Besides that, the output tranny looks undersized for guitar amp duty (assuming you want to use it for live gig levels, even overdriving the output stage). The pots will likely be bad as well... it looks that some of them have been replaced in past, though. Then there is that 500Ohms cast cement resistor which also must have been installed not too long ago...
The stock circuit will be not suited for guitar duty anyway so you will need to do a complete rebuild based on a proper schematic (with proper safety earth grounding of the chassis and all) to get some usable guitar tone from it, assuming that the transformers are OK and the circuit is adapted to their properties (like rectifing circuit details). That's going to be a long and costly effort.
I think you'd be better off buying something like a used Jet City 20 Head or Combo and mod the hell out of the well documented circuit to fit to taste... at least that's what I did.
Sounds like fun! 🙂 (Always assuming you've informed yourself of all the necessary safety practices, and have appropriate safety gear as well. I recommend safety-goggles and a pair of class 0 electricians gloves in your size.)
Your PA may or may not sound great with guitar as-is. But I think you're right, it's an ideal candidate for conversion.
At the very least, you have a pre-drilled chassis, and a suitable pair of transformers. Depending on condition, you may also have a bunch of other usable parts (pots, tag strips, tube sockets, etc).
My suggestion is to start with the most important thing of all - safety. That means adding an IEC inlet, and a proper three-pin, three-wire power cord, making sure the metal chassis is connected to (mains) safety earth.
This part is non-negotiable. The metal chassis needs to be safely grounded any time the amplifier is powered on. Otherwise, you risk electrocution via the guitar cord and guitar strings. This is not an option.
If your experience is anything like mine, the worst part of the project will be trying to cut the hole for the IEC inlet into the metal chassis. It literally cost me both blood and sweat, but with a Dremel, files, and lots of time, I got it done.
Some IEC inlets only need a rectangular hole, and this will be easier than the six-sided hole I had to cut. Also, you can buy an IEC inlet that includes both an on-off switch, and a fuse. You'll need to cut a bigger hole in the chassis to use one of these, but IMO it's worth it - you'll have less wiring to do, and you won't have to buy a separate power switch, fuse holder, etc.
The entire power amplifier section of your little PA (cathodyne phase splitter, push-pull EL84 pair) will probably work just fine for guitar, with few or no changes, other than replacing any damaged components.
Worst case, you may have to add grid stoppers, or increase their value if they're already present. Oh yeah, you will need to add screen grid resistors for the EL84s if they aren't already there.
Chances are you'll have to change the preamp significantly to make it sound good with guitar. I would be tempted to remove most of the preamp components, and start over. But before you go that far, trace out the schematic. You may not need to change everything - maybe only the tone controls, values of coupling and bypass caps, and so on.
There are so many schematics for vintage tube guitar amps available on the 'Web - I don't think you'll have too much trouble.
What's not to like? 🙂
Don't throw in the towel - and keep us posted!
-Gnobuddy
I sure hope the OP does not take your recommendations, I think your limited experience may not be relevant.
That's it! Tube powered PA amplifiers or amps from worthless organs are the cheapest source for valuable components by far, especially the »iron«, tube holders and even eletrolytics (they need to be tested anyway). Here's what I did with the chassis and iron of a Hammond AO-64/68 combo:
Best regards!
Best regards!
Actually it's a L profile that I chose in order to hide the holes in the original chassis and to gain space for the potentiometers 😉 .
Best regards!
Best regards!
And if my brain had been functioning this morning, I would have noticed that, since it's very clear in one of your photos! 🙂
-Gnobuddy
I'm restoring the same amp and need to identify the resistor that's sitting directly under the 500Ω 10W resistor under the can cap.
I put a 1K in there, but the audio is distorted. Can you help?
Sure would like to locate a schematic for it!
I put a 1K in there, but the audio is distorted. Can you help?
Sure would like to locate a schematic for it!