Record Player Amp to Guitar Amp Input Needed

[AAVA]

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.

Thanks Stevem, I'll check it out! I've also got some Iso-transformers in storage that I was checking out the other day but wasn't sure how to determine whether they'd work or not. They're Triad N68X, but it looks like they're only 50VA. I've surely got some AC transformers and output transformers as well but I'm trying to not put the horse before the carriage. Even if amps are designed from the output back, so to speak.

[AAVA]

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: ..(See original post)

Thanks Phil, this approach, I would suspect, is definitely in my future. As I was telling "stevem" I've got a few chassis and a bunch of transformers in storage but I thought I'd start out small and with what I had on hand (literally, right behind me). The thought of dealing with old transformers, that I could hopefully find spec sheets for, at this stage feels like it'd put me a bit out "over my skis"! But I do appreciate the links and the thought. I'll be keeping my eyes open regardless of and in addition to what I've got in storage! And when I do get to that point you can bet your boots I'll come a'callin'!

[sluckey]

In regard to the Tone capacitor it exists exactly like it's drawn in the schematic.

Not so!

Your schematic shows that C5, C6, and R10 all connect to the tone pot. However, the photo clearly shows that C5 is the ***ONLY*** component that connects to the tone pot.

Additionally, V1 pin 6 no longer has a path to B+. Cannot work as drawn.

[AAVA]

In regard to the Tone capacitor it exists exactly like it's drawn in the schematic.

Not so! Your schematic shows that C5, C6, and R10 all connect to the tone pot. However, the photo clearly shows that C5 is the ***ONLY*** component that connects to the tone pot. Additionally, V1 pin 6 no longer has a path to B+. Cannot work as drawn.

I believe I understand, and you are correct, Sir. The C5 cap was on the wrong side of R10 on the drawing. As for "..pin 6 no longer has path to B+".
I take that to mean that with C5 where it was there would be no path to B+. My bad. Should have seen that. "Decoupling, it would be!" [Yoda voice]
One other issue I'm currently pondering is that the "Tone pot" is, in reality, a dual AC switch/Variable resistor type. The first two terminals of the Pot are soldered together. The third connection is where C5 is connected. I have updated the schematic again to show the bridging of terminals 1 and 2 of the Tone pot.
I think this is how it should be represented. Thanks again for taking the time to help.
M

[Phil_S]

The thought of dealing with old transformers...

Chances are less than 50/50 you'll find spec sheets. There are other ways to guess and test to some extent. Part of the fun is not letting smoke out. As I expect you know, once you let the smoke out, it won't go back in.

[AAVA]

The thought of dealing with old transformers...

Chances are less than 50/50 you'll find spec sheets. There are other ways to guess and test to some extent. Part of the fun is not letting smoke out. As I expect you know, once you let the smoke out, it won't go back in.

It sure won't! Luckily I haven't done that much at all. Maybe I'm not trying hard enough! I've just been focused on fixing things and not hardly at all on making anything. Let alone repurposing stuff. There are definitely mistkaes to be made in the future!
I'm sure there's plenty here on dealing with old trannys too. I'll cross that bridge when I come to it!

[sluckey]

Tone pot now looks correct. Vox calls it a cut control. Redraw T1 like this and you'll have a working output stage.

[AAVA]

Tone pot now looks correct. Vox calls it a cut control. Redraw T1 like this and you'll have a working output stage.

With "slucky"'s kind help the schematic for this amp, found below and in the first post, are now correct.
I'm going to take a break for a minute and then get back to the question of the obvious and not so obvious additions that this amp would need to be any kind of viable guitar amp. First up would be the isolation transformer and then as "mhuss" suggested, the addition of another gain stage. Merlin Blencowe's "Designing Tube Preamps for Guitar and Bass" just arrived yesterday and I'm into it. If anyone has any impressions on what I've got going on here I'm still all ears. I appreciate greatly the time folks are taking to offer their help and guidance. More to follow...

[Stevem]

When you get it up and running one of the first things I would audition would be placing a 50uf 50 volt cap across the 120 ohm bias resistor.

[Phil_S]

I'm going to try one more time to discourage you on this project. I see that I'm in the minority, and, of course, you should do as you like. This is a practical consideration. Where do you plan to mount the iso trans? That is a very small chassis. It would be better if you worked this out before you take the plunge. Good luck!

[AAVA]

When you get it up and running one of the first things I would audition would be placing a 50uf 50 volt cap across the 120 ohm bias resistor.

I will put your suggestion on the list I've got going for just such recommendations. Thank you!

[AAVA]

I'm going to try one more time to discourage you on this project. I see that I'm in the minority, and, of course, you should do as you like. This is a practical consideration. Where do you plan to mount the iso trans? That is a very small chassis. It would be better if you worked this out before you take the plunge. Good luck!

Thanks again for your comments, Phil. Minority or not your opinion is always welcome and I've been thinking of exactly that, "how would this all go together" and "at what cost". My thoughts so far are that the iso-trans isn't that expensive and I've got 9 pin sockets, wire and more 12AX7's so no cost there. The stock speaker is an 8" in. speaker so in using that the enclosure would have to be reasonably sized. So my first thought was using a small additional plate to mount the iso-trans and the amp chassis together on. Or should I not mount the chassis and the iso-trans together on a "common" plate? Then mount into an enclosure.
Should be room I think. I PM'd "thetragichero" about something he said earlier in the thread and when I hear from him I'll continue. Back soon...and thanks again.
M

[thetragichero]

Hey there! Since nobody responded to your request for a check on your "math and reasoning" I thought I'd shoot you this PM and question. I also responded to this message earlier in the thread but I'm not sure if you've had the time to check it out or otherwise. So I'm PM'ing you the question. Not to be pushy but I do want to understand what you posted. So here is my earlier reply, in part, to your post.

".... 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?"

Please let me know when you get the time, and thanks again for your input.

quoted pm because it should allow others to correct my understanding where it may be incorrect along with add pertinent information for future Google searchers...

so this was meant to have the heaters run straight off the wall voltage. the way i understand series string heaters is that they all need to be the same ma current draw (150ma in your case) and then each tube gets the proper voltage to heat it. you'll notice you have 50v heaters on the power tubes and 12v heater(s) for the little bottle(s). the total heater voltage is meant to (more or less) equal the wall voltage (some aa5 amps had a small value resistor on the heater string because 12v preamp 35v rectifier 50v power tube is slightly less than the 110v or whatever wall voltage). so the tubesl heaters in your record player add up to 112v (50 + 50 + 12), on a (nominal) 120v line voltage, line supplying 7% more vintage. if you added a second 12ax7 (which would conveniently be used to supply a preamp for the guitar) that would put the heater string at 124v, line supplying 4% less voltage. the outlet on my bench supplies about 122v regularly anyway

back to current draw.... we have roughly 150ma (including whatever margin of error plus round number for easy computation) for the current drawn by plates and screens PLUS the 150ma for the heater string. added together that's 300ma

how do we determine VA (volt amps) needed in the isolation transformer? multiply the voltage (120v) by the current draw (300ma) to get 36VA.
this transformer looks promising, some reviews mention using in something similar to what you're after (in fact i have one of these sitting in my saved cart for whenever i order parts again): https://www.tubesandmore.com/products/t ... a-2-x-115v

hope you don't mind me replying here. this is as much to confirm my reasoning as it is to help you out

[AAVA]

"thetragichero" I don't mind in the least your replying on the forum. I appreciate it greatly. I've left out the quotes from this reply because they're directly above.
Though I'll not relinquish my responsibility to ever understand electronics better it is invaluable to have someone explain their reasoning when assisting folks like me trying to get their head around this stuff. I know it'll take time and I appreciate your taking the time to explain it a little better. And your explanation answered all my questions. I'll be ordering the transformer you linked to your reply as well. Because I have a bench Isolation Transformer AC source [BK TR110] I decided to plug this guy in and see what's going on at this point. With guitar plugged in I've got audio passing and the apparent volume at the mid point is more than I expected but there is a fair amount of hum. So now I'm considering grounding issues. I'm off to a bit more reading so I'll return soon.
Thanks again,
M

[thetragichero]

i ordered one of those iso transformers as well. I've got a box full of 25l6 so why the heck not? 4 of em plus a few little bottles. i love quirky little amps