Record Player Amp to Guitar Amp Input Needed

[AAVA]

Greetings, like a lot of folks out there I am currently considering the conversion of a record player tube amp into one usable for guitar. The said record player is a Hamilton Electronics 905 EJ. I was not able to find a schematic online so I traced the circuits myself and have included it in this post.
I've been reading all I can find on this site about such an endeavor and have come to the conclusion that it should be possible with the amp that I have. But I have a few questions before I go any further. As a quick background on myself I have been repairing solid state and a few tube amps for a few years now and have the necessary skills, equipment and knowledge of safety precautions to proceed. What I don't have is an in depth knowledge of tube amp design and the idiosyncrasies thereof. So I'm hoping that the tube amp guys and gals on this site can steer me in the right direction, encourage and support my love of tube amps in general.
About this amp: This record player was housed in an entirely metal case with a metal plynth! I didn't like that from the start when I first started working on bringing it back to life as a record player and when I noticed a fair amount of leakage to the case I called it quits. As of late I've decided to convert it to guitar use. The record player had a built in 8"in. 8 ohm speaker in the lid, which I plan to use. The tube complement is: 1 12AX7 and 2 50C5's. It is chassis grounded at three points. One point, of course, being AC neutral. As I've read about record player to guitar amp conversions here and on other sites I've come to the understand that the only way to make this thing truly safe is with an isolating transformer on the AC input. If there are other methods I'm all ears. If not, I'm wondering how to determine exactly what type of iso-trans to use. I'm not going to use the metal record player case but instead use a wooden enclosure of some ilk for the amp and maybe close off the back of the metal speaker "lid" since the speaker was connected by means of a 1/4" in. phono plug. Or possibly make one enclosure for both.
I believe that the amp was working when I halted initial repairs and the only thing I found necessary at the time was the replacement of a 120 Ohm 2W resistor (R11 on schematic) that was in series to the motor from the transformer secondary. All else if stock. I'm still learning about tube amp typologies so I'm not expert but looking at the schematic I've drawn is not helping me learn much more, unfortunately. So I'm not sure if this is a push-pull or single ended or some kind of hybrid design. The transformer seems unlike anything else I can find currently in a tube amp. So any input into what the deal is with this design would also be appreciated.
The record player motor was obviously removed and was measured at 46.8 Ohm. I put all the info I could think of on the schematic, which I believe to be correct.
But if anyone sees anything that looks wacky I'm happy to double check it. I could not discern the value of the ganged power switch / tone pot but will disconnect it and forward the info if necessary.
So to sum it all up; I want to make this thing safe to power up and test further and gain a little knowledge about what I'm working with.
Any and all help will be appreciated greatly.
Mark->

[mhuss]

Wow, this looks like the old-fashioned "put 120v worth of tube heaters in series and connect the AC mains across the string" approach. Since this is just a power amp, and you'd need to add another 12AX7 to get enough gain to amplify a guitar, I might just abandon this project as too complex (and potentially dangerous). The output tubes are rated for 150 plate volts max, so you probably don't have a lot of B+ available either.

[thetragichero]

at the very least you'll need an isolation transformer for safe guitar operation (they're not colloquially referred to as "widowmaker" about for nothing). you *could* potentially convert to cathodyne phase inverter with the other triode used as a gain stage. the "AA5" guitar amps certainly have a lofi charm to em
(he plays at around 37 minutes in) https://m.youtube.com/watch?v=jXf6xf7i2Es

[AAVA]

Thanks for the input "thetragichero" and "mhuss"! It's really interesting to hear what folks have to say about this amp. While waiting for comments I've begun to realize that there is quite a bit to learn from building small amps, kit's or otherwise. Trying to do a conversion on one of these things with the help of those here might be asking a little too much . Especially if what I've got to start with is kinda wacky! Someone on some forum said "first, try it and see what you get", regarding finding out what kind of output one has to start with, and that was my impetus for making this safe to power up and try out. I can certainly build a pre designed amp, trouble shoot it, tweak it in and so forth but I thought I'd just play around with what I've got on hand first. (..I use the term play loosely!) to try to learn a bit. I'm still looking forward to any and all comments from the forum.
Thanks again.
M

[Stevem]

As long as you don't mind paying a small amount for a ISO trans then that little bugger is a good platform to play with and learn why what changes / mods you make sound the way they do.

I'am all for it

The first thing that needs to be done once you have it powered safely is to rewire that 12ax7 so you have the needed 1 gain stage and then the next stage that's a combo gain and Phase splitter for guitar usage.

[sluckey]

Your output stage is not drawn correctly. The tube plates have no connection to B+ and the cathodes have no path to ground. The OT is not drawn correctly. I bet that red wire is a center tap for the primary. Probably got about 125Ω resistance between the red wire and either blue and brown wires. No idea about the motor being connected to the output transformer. That's just crazy. Every small record player I've ever seen have the motor operated directly from the AC line.

Your tone control is missing a cap. As drawn it just operates as a crossline master volume.

[AAVA]

As long as you don't mind paying a small amount for a ISO trans then that little bugger is a good platform to play with and learn why what changes / mods you make sound the way they do. I'am all for it The first thing that needs to be done once you have it powered safely is to rewire that 12ax7 so you have the needed 1 gain stage and then the next stage that's a combo gain and Phase splitter for guitar usage.

Stevem: Thanks for the input. I'm not against we wiring this guy for useful service at all. I'm just not adept enough to decide what needs to be done at this point so I appreciate the guidance. I've just ordered Designing Tube Preamps for Guitar and Bass by Blencowe, Merlin, something I've been meaning to do for a while. I'm sure that'll help break things down a bit better than skipping about on the Net! So how would one decide exactly what Iso-Trans to use? Like any other transformer I assume it'd have to take 120V on the primary but what other considerations are there?
M

[thetragichero]

please get confirmation of my math below before ordering anything!

120v:120v and then you'll need to figure out VA needed
these are all 150ma heaters (12ax7 wired without the heater center tap). 50c5 datasheet (if I'm reading it right) lists class a plate and screen current maximum of 58.5ma apiece so 117ma for power tubes and a few for preamp tube. 150ma seems like a nice round number. so 300ma total. 120v * .3a = 36VA, so anything above that should be cool. now I'm wondering if we could add another preamp tube for 50 + 50 + 12 + 12 = 124v.... closer to my wall voltage of 122v than 112v.

again, I'd want at least one other person confirming my "slight distraction from working on a project" math and reasoning here

[Stevem]

At Tubes & more site they have a ISO trans P-T169C that is rated for 100V/A which will more then cover your needs for 62 bucks.

[thetragichero]

add 27 bucks worth of parts and then it ships free, neat!

[Phil_S]

I'm all for repurposing, but $90 just to find out if/what it does seems steep to me. It's not about having the $90. It's about what value you get for it. I think it's not a good platform and a poor value.

Haunt eBay for a while. Search: (amp, amplifier) chassis. You'll find a derelict chassis with good transformers and other usable parts for $90. That would be a much better platform, IMHO. Examples:

This one looks sweet, will go for more than current bid:
https://www.ebay.com/itm/Vintage-FISHER ... SwKDlf-nFB

This is promising. May even include tubes that still work. Shipping at $30 will depress the selling price
https://www.ebay.com/itm/Tube-Amplifier ... SwnA9f8jeP

Organ chassis. Probably more 12AU7's than any guitar amp builder wants, but lots of other good stuff. Coupling caps are probably leaky, but who knows without testing them? Comes with tubes. I'm tempted!
https://www.ebay.com/itm/Used-Tube-Amp- ... SwNJFf-6cS

I like this sort of thing:
https://www.ebay.com/itm/Vintage-Tube-A ... SwK05f-45d
...and this one is even better, comes with schematic, which may help figuring out the transformers!
https://www.ebay.com/itm/Tube-Amp-Mono- ... Swp29f~5cT

Hammond AO-29 chassis, a classic conversion project:
https://www.ebay.com/itm/Hammond-H-AO-2 ... Sw~apfwYoK

[AAVA]

Your output stage is not drawn correctly. The tube plates have no connection to B+ and the cathodes have no path to ground. The OT is not drawn correctly. I bet that red wire is a center tap for the primary. Probably got about 125Ω resistance between the red wire and either blue and brown wires. No idea about the motor being connected to the output transformer. That's just crazy. Every small record player I've ever seen have the motor operated directly from the AC line. Your tone control is missing a cap. As drawn it just operates as a crossline master volume.

"slucky": Thanks for the input as well. Sorry for the delay in responding. You are correct about the B+ supply to the plates, the Red transformer lead does indeed have 130 Ohm to either Blue or Brown (Plate) leads. I also found another omission; the 120 Ohm resistor (R11) meets a ground point. I've redrawn the previously submitted schematic to reflect these changes.
In regard to the Tone capacitor it exists exactly like it's drawn in the schematic. A 0.005uF ceramic bisecting the series string of two 220K resistors and a 0.01uF ceramic connected between pin 1 of V1 and pin 2 of V2 at the point between R10 and the 0.01 cap, on it's way to pin 6 of V1. Taking your lead on the Red wire situation I measured the Violet transformer lead as well, which measured 12 Ohm to the Orange or Yellow transformer leads and over 30M to ground or elsewhere. So for now the schematic shows the Violet lead coming from the transformer with an "unknown" origin. Again the original schematic has been updated and will be updated further.
Let me know what you think when you get the time, and thanks again.
M

[thetragichero]

if you have a pickup truck, a small dolly, and the time to scour Craigslist/facebook marketplace you should be able to find a Hammond organ for free (sometimes you have to haggle or wait until nobody expresses interest in both paying and picking it up)
while it may seem daunting, a "work smart, not hard" attitude along with a dolly makes these easy to get into a pickup truck bed as one person (tip on its side into the dolly, push to the truck, turn, side it up on its back into the bed). not a lot of lifting but most of the houses in florida have tile or hardwood which helps moving on the little casters. carpet not so easy. getting it down is mostly guiding it down while gravity does the work

[AAVA]

please get confirmation of my math below before ordering anything! 120v:120v and then you'll need to figure out VA needed these are all 150ma heaters (12ax7 wired without the heater center tap). 50c5 datasheet (if I'm reading it right) lists class a plate and screen current maximum of 58.5ma apiece so 117ma for power tubes and a few for preamp tube. 150ma seems like a nice round number. so 300ma total. 120v * .3a = 36VA, so anything above that should be cool. now I'm wondering if we could add another preamp tube for 50 + 50 + 12 + 12 = 124v.... closer to my wall voltage of 122v than 112v. Again, I'd want at least one other person confirming my "slight distraction from working on a project" math and reasoning here

Thanks for the help with the Iso-Trans issue! I knew it had to be 120V & 1:1 but that's about it. I checked my GE 50C datasheet as well and your figures make sense to me. I see you summed the Plate and Screen currents to get the 58.5mA per output tube. And with 1.2 mA max plate current for a single 12AX7 I can see why you'd consider it nominal. What I don't get is that you said "117mA (58.5mA ea.) for the power tubes and a few for the preamp tube(s); 150mA seems cool. So 300mA total." 300mA? Why did you double the total current max for the existing two output tubes? Safe operating "headroom"?
Also I'm assuming that the "..50 + 50 + 12 + 12 = 124v.... closer to my wall voltage of 122v than 112v" is the total voltage/current draw of the amp?

[AAVA]

if you have a pickup truck, a small dolly, and the time to scour Craigslist/facebook marketplace you should be able to find a Hammond organ for free

I've actually got more than a bit of old iron in storage and a few chassis as well. But because I don't know much about tube amps design in general and specifically reusing old transformers I thought I'd start with something small and work my way up! But thanks, that's good advice and I've been a Hammond hunter for a while now. Like a lot of folks out there I'll see things that I might not be able to use right now but "know", or hope at least, I will in the future.
Well, the future's coming on fast!
(..I think that was in a song somewhere...)
Thanks!