rex branded Valco 6143 amp w/no power transformer

[davohilts]

Hello,
I'm trying a project that is pushing my level of understanding of tube amps. I have a Rex branded Valco 6143 amp with 8" speaker and 4 tubes (35Z5, 117Z3, 50L6, 12AX7).
I have an isolation transformer (N-68X Transformer; Isolation; Pri:115 to 230VAC, Sec:115VAC; 50/60Hz; 50VA; 1500Vrms) that I plan on installing to keep from killing myself.
What I don't understand among many things is why there are two 1/2 wave rectifiers in this circuit. I have found a schematic for the Valco 6143 that is identical except that it uses a 35W4 tube instead of the 117Z3. The 35W4 seems to be similar except it can take higher voltages.
I have seen some amp conversions where they install a full wave solid state rectifier between the isolation transformer and the 35W4 to symmetrically load the isolation transformer.
Can anyone explain to me how this amp works and why it has two 1/2 wave rectifiers and what the best way to install an isolation transformer?
Thanks,
Dave
valco-6143-amplifier-schematic.pdf

[nosaj]

Hello,
I'm trying a project that is pushing my level of understanding of tube amps. I have a Rex branded Valco 6143 amp with 8" speaker and 4 tubes (35Z5, 117Z3, 50L6, 12AX7).
I have an isolation transformer (N-68X Transformer; Isolation; Pri:115 to 230VAC, Sec:115VAC; 50/60Hz; 50VA; 1500Vrms) that I plan on installing to keep from killing myself.
What I don't understand among many things is why there are two 1/2 wave rectifiers in this circuit. I have found a schematic for the Valco 6143 that is identical except that it uses a 35W4 tube instead of the 117Z3. The 35W4 seems to be similar except it can take higher voltages.
I have seen some amp conversions where they install a full wave solid state rectifier between the isolation transformer and the 35W4 to symmetrically load the isolation transformer.
Can anyone explain to me how this amp works and why it has two 1/2 wave rectifiers and what the best way to install an isolation transformer?
Thanks,
Dave
[ATTACH]34963[/ATTACH]

35 +117 + 50 +12=214.

This is a transformerless amp that you add up the first number of each tube(denotes filament voltage) to add up the line voltage. I would say the 117 does not belong. Why do you think it is a 6143 when that schem only uses 3 tubes?
Take some pictures of the chassis top and underside and the cabinet it came in. We're all going blind on this one right now.

Thanks,
nosaj

[davohilts]

35 +117 + 50 +12=214.

This is a transformerless amp that you add up the first number of each tube(denotes filament voltage) to add up the line voltage. I would say the 117 does not belong. Why do you think it is a 6143 when that schem only uses 3 tubes?
Take some pictures of the chassis top and underside and the cabinet it came in. We're all going blind on this one right now.

Thanks,
nosaj

Thanks for the help on this one. I am assuming that this is a Valco amp because of the number stenciled on the inside of the cabinet (6143) and the metal plate nailed to the cabinet (T51406) and the Jensen speaker may not be original but I'm not complaining.
I have removed the chassis and cleaned about a 1/4 inch of greasy dust off the top and removed the OT temporarily to show the tube markings. The tube compliment is as follows (as marked on the chassis) 35Z5, 50L6, 117Z3, 12AX7. The chassis reads REX and 4 tube dual input.
Here are some additional pics.
Dave

[davohilts]

35 +117 + 50 +12=214.

This is a transformerless amp that you add up the first number of each tube(denotes filament voltage) to add up the line voltage. I would say the 117 does not belong. Why do you think it is a 6143 when that schem only uses 3 tubes?
Take some pictures of the chassis top and underside and the cabinet it came in. We're all going blind on this one right now.

Thanks,
nosaj

nosaj,
On the schematic I posted the 4th tube is a 35W4 shown below the input jacks. The schematic shows the tube connecting to points A and B of the filaments but I'm not sure of what else it connects to or why it is there.
I got the schematic here: Free Valco Diagrams, Schematics, Service Manuals :: Schematics Unlimited
Thanks,
Dave

[nosaj]

nosaj,
On the schematic I posted the 4th tube is a 35W4 shown below the input jacks. The schematic shows the tube connecting to points A and B of the filaments but I'm not sure of what else it connects to or why it is there.
I got the schematic here: Free Valco Diagrams, Schematics, Service Manuals :: Schematics Unlimited
Thanks,
Dave

Now this is just a stretch I know it says made in the USA, but I wonder if this was an export model for like overseas 220 voltages with the 117 tube in there.

My best guest would be to sub the 117 for a 35 which would bring you inline with current US voltages
35+35+50+12=132 versus 35 +117 + 50 +12=214.

Just my 2 cents, If you don't get any other info here definately try Antique Radios - The Collector's Resource They are very helpful as well.

Thanks,
nosaj

[davohilts]

thanks nosaj!
What you say makes good sense!
I appreciate you help in solving this puzzle.
Dave

[g1]

I don't think it's a 220V amp. That would put double the voltage at the plate of the rectifiers.
It may be 2 parallel strings across the 120V, one branch with the 117Z3, the other branch with the rest of the tubes and a series resistor.
You really need to draw out the circuit.
Here's a US listing for the same amp that was working, although they list an extra 35Z5 where it should be a 50L6.
https://reverb.com/item/215123-1959-...-6143-tube-amp

[52 Bill]

I don't think it's a 220V amp. That would put double the voltage at the plate of the rectifiers.
It may be 2 parallel strings across the 120V, one branch with the 117Z3, the other branch with the rest of the tubes and a series resistor.
You really need to draw out the circuit.

Agree 100%

If you look at the wiring side photo, the only thing connected to the 117Z3 are the heaters, which look to be paralleled to the other tubes which are in series.

[davohilts]

Hi 52 Bill,
So then the 117Z3 tube is just there for voltage regulation? I am going try and draw up a schematic showing the circuit and what I want to try and do with the iso transformer. Stay tuned.
Dave

[jaydeshan]

I would love to know where you, davohilts , ended up with this. I have the same amp, albeit with the 35W4. It looks almost identical to yours. Thank you for the pictures. I am currently going through a recap and am reworking the schematic in JSchem. I want to utilize a proper 3 prong grounded cord, but currently. I'm just replacing the non-polarized cord with a polarized version.

jay

[davohilts]

I put in an iso transformer and grounded power cord but the hum was so bad, I tried a few things but could never get rid of the hum so I just gave up. Sigh...