Record Player Amp to Guitar Amp Input Needed

[AAVA]

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

I ordered a couple as well. I think I'm hooked!

[Phil_S]

Yeah, it's the old free smak on the playground trick. I built about 14 amps over about 15 years. The last one is about 90% done and is awaiting the finishing touches...for over 3 years. I even built a nice cherry cab for it. What broke the addiction was that I got a job! Before that, I was a consultant and had enough free time to do this. I retired a few weeks ago and that unfinished amp is headed to the front burner. Maybe next month.

The other problem was the cherry cab. After I built that I started building end tables and that's addictive, too! The woodworking time cut into the amp building time. Choices to be made.

Good luck to you!

[AAVA]

Yeah, it's the old free smak on the playground trick. I built about 14 amps over about 15 years. The last one is about 90% done and is awaiting the finishing touches...for over 3 years. I even built a nice cherry cab for it. What broke the addiction was that I got a job! Before that, I was a consultant and had enough free time to do this. I retired a few weeks ago and that unfinished amp is headed to the front burner. Maybe next month. The other problem was the cherry cab. After I built that I started building end tables and that's addictive, too! The woodworking time cut into the amp building time. Choices to be made. Good luck to you!

It's funny, the commonalities a lot of us probably have. I used to do woodworking as well. And yea, it's addicting too. After this amp is finished I'm probably going to build another small wattage amp and THEN maybe go for a bigger one. I've got a lot of iron and stuff waiting in storage and for a while thought I'd never get to it.
But for now, "they tried to make me go to rehab and I said, no no no!"

[AAVA]

Error. Duplicate of above. Apologies.

[AAVA]

Ditto.

[AAVA]

Well, back at it! The Triad N-68X Iso-Trans is here and I've uploaded the spec sheet. The trans hook up looks simple enough: Blk/Yel-Blk & Rd-Blk /Grn-Blk are the primary hookup and Red - Red are the secondary but I'm not familiar with the "Shield" lead (white). Is it usable / necessary for what I'm doing?
When I wire the primary a three prong cord will be used with ground connected to the plate mounted on the bottom of the trans and a lead mounted to the amp chassis. I'm currently working out how and what this is going to be mounted in so I"m wondering about the placement of the iso trans. I'm pretty sure it should be away from the output transformer but the ground still needs to be connected to both. So would one plate connected to both be the best idea or separate plates with a grounding wire connected to both? Note: the solder point you can see in one of the secondary wires is covered in clear heat shrink tubing. ; )

[AAVA]

I just ran across the image below on a site online explaining how rewiring the switch keeps the primary of the transformer
from being powered all the time it's plugged in. I get it. But to do this with my amp it looks like I'll have to do a little modification to the way things are set up since R1 (33 ohm) and D1 (green wire) come right off of the switch. They'll need a separate secondary connection point of their own I believe. Am I on the right track?

[Phil_S]

Look at the original schematic. It shows the heater string and the amplifier circuit wired in parallel. I think you just connect the iso trans in place of the original wall plug, but without the switch. The switch and fuse should be on the hot leg of the iso trans primary. I can't say for sure that's it. It would be better if someone else addresses this, too.

[AAVA]

Look at the original schematic. It shows the heater string and the amplifier circuit wired in parallel. I think you just connect the iso trans in place of the original wall plug, but without the switch. The switch and fuse should be on the hot leg of the iso trans primary. I can't say for sure that's it. It would be better if someone else addresses this, too.

Hey Phil, I'm not sure if I'm "picking up what you're laying down", as they say : ) On my amp the AC power cord was connected directly to one side of the switch and the other side to "ground". The schematic I posted with the intended switch mod is for interrupting and fusing the AC from the wall and the primary side of the Iso-Trans. You're right about the "hot leg" of the primary getting the fuse but not sure what this has to do with the secondary other than the fact that the connection to R1 & D1 which were previously connected to the second side of the switch would need a separate terminal connection so that the secondary's connections are post primary and post switch instead of actually connected to the switch.
I'm trying to figure out the correct size of fuse to use right now; the total current draw of the amp is .300mA and I suppose that headroom is necessary so I'm thinking that 1/2 amp, or there and abouts, would be right. All input is appreciated!
More to follow tomorrow..

[thetragichero]

diagram you posted should be good, although I'd place the fuse before the switch myself

[martin manning]

I think I'd just reroute that 33Ω over to the rectifier and tie in the iso transformer as shown below.

[AAVA]

I think I'd just reroute that 33Ω over to the rectifier and tie in the iso transformer as shown below.

Hi Martin, Thanks for the input. Were you recommending moving R1 (33ohm) in series with D1? The way it's shown in the schematic you've posted is exactly how it's currently wired. A stand-off was installed to facilitate putting the switch in series with the hot line and fuse. On which are R1 and the dark green wire which leads to D1. When all is said and done I'll create two versions of the schematic, one I'll leave as the original design and the other will be of the additions and mods.
Thanks again Martin

[martin manning]

Were you recommending moving R1 (33ohm) in series with D1? The way it's shown in the schematic you've posted is exactly how it's currently wired. A stand-off was installed to facilitate putting the switch in series with the hot line and fuse. On which are R1 and the dark green wire which leads to D1. When all is said and done I'll create two versions of the schematic, one I'll leave as the original design and the other will be of the additions and mods.

The circuit is the same. I was just looking to avoid adding an extra terminal strip by routing the 33Ω directly to the diode. Regarding what you have there now, I would run the ground side of the iso transformer secondary directly to the main filter ground.

[AAVA]

The circuit is the same. I was just looking to avoid adding an extra terminal strip by routing the 33Ω directly to the diode. Regarding what you have there now, I would run the ground side of the iso transformer secondary directly to the main filter ground.

Got it. Will do. With only the speaker connected I powered it up earlier this afternoon and there was surprisingly little hum. Encouraging!
I'm making the suggested shield connection now and after that take some voltage readings. I'll report back later.
Thanks Martin.

[AAVA]

OK, here are the first measurements on the amp: [zero signal]
AC source: BK 1655 AC power supply. [Set to 120 VAC]
Iso Trans Primary = 120VAC
Iso Trans Secondary = 125VAC
Heater string: [AC]
V3 (50C5) pin 4: 118 VAC - Pin 3: 65 VAC
V2 (50C5) Pin 3: 65 VAC - PIn 4: 14 VAC
V1 (12AX7) Pin Pin 4: 14 VAC - Pin 5: 0.023mV (chassis ground)

DC Measurements:
C3 Multi cap: C3-A: +148 VDC - C3-B: +143 VDC - C3-C: +136 VDC

V1 (12AX7) Pin 1 (plate): +68.1 VDC - Pin 6 (plate): +67.5 VDC - Pins 8/3 (shared cathodes): +0.794 VDC

V2 (50C5) Pin 7 (plate): +135.4 VDC - Pin 6 (screen): +143.3 VDC - Pin 2 (grid): +0.025 VDC - Pin 1 (cathode/sup grid): +11.5 VDC

V3 (50C5) Pin 7 (plate): +135.4 VDC - Pin 6 (screen): +143.5 VDC - Pin 2 (grid): +0.025 VDC - Pin 1 (cathode/sup grid): +11.5 VDC

The choice of 120 was just for measurement purposes since that's what I'd consider the un-usual high I'd normally run into. I'd imagine this amp was designed when 110 to 117 VAC max, was the norm. The question arises as; if these voltages are dropped what values will I have at lesser/different wall voltages?
And how does one choose where the operation point will be? I can't have voltages this high and I suppose a voltage point too low exists as well.
When AC voltages change from place to place, so to speak, there's seems to be a problem in search of a vintage solution here. One which I'm sure exists.
But as of now my thinking is that the 33 ohm resistor isn't high enough because my heater voltages are too high; +65 VAC across the 50C5's and 14 across the 12AX7. (..after looking at this I'm not so sure my logic is correct for voltage drops across the heaters; would the voltage drops be: 53, 51 & 14?).. and after D1 I've got almost 150 VAC on the multi-cap which is rated 150 V Not good! I thought I'd need a bench-mark (120 VAC) for worse case scenario but both voltages have gotta come down I'd say. There might be a picture of this in the dictionary under " how to go though tubes quickly and blow up caps!"
I've modified the schematic of the current configuration of this amp and have posted it below. I'm thinking hard over here so all input is appreciated.