1959 Fender Tween Twin 5F8-A Power Supply Questions

[keithb7]

I recently took possession of a 1959 Fender Twin, 5F8-A. It is mostly original. It has been in storage for years.

Before plugging it in I cleaned it and made a few checks.
I replaced two failed bias caps, and plugged amp into light bulb limiter to see what if it would work and what voltages are present .
No sound from amp so I started probing around.

As I have limited formal electronics training I am hoping I can post a few questions and those in the know can help me along.
Schematic and layout here for reference:





Here is what I have found so far in my Twin:

Red wires into rectifier, pins 4 & 6 measure 416V AC. Yellow wires going into rectifier tube 5 volts AC.
DC voltage coming out of rectifier tube, pin 8, that goes to stand-by switch reads 197.5 VDC. Measured at the connection on the standby switch. Found a bad standby switch. I don't have a spare, so I wired removed the switch temporarily and soldered wires directly together, eliminating switch.

Powered up amp. DC voltage on first two B+ caps climbs as high as a meagre 10V DC, then starts dropping and stabilizes at about 7 VDC. These are the original B+ caps still in there for now. Unsure if these are a problem yet at this point. I did measure and record original cap readings, read with a digital ESR meter. Here were my findings:

1959 Caps:
23.1uF 1.99 ohms
24.39uF 2.70 ohms
16.21uF 5.2 ohms
16.74uF 3.2 ohms

Next item in the power supply, I turned to the choke. I Took an resistance reading between the two black wires going in and out of it. I get 99 ohms. According to a Classictone replacement choke, the resistance is about 105 ohms. So I think my choke should be ok.

I am a unsure why the rectifier tube is putting out 197.5 VDC. I have a new Sovtek 5AR4 that I also put in there. Same results measured as vintage tube. Any ideas on the power supply?
Before I go ahead and replace the original power supply caps.? I agree they are old and should be changed anyway, before I do, I am wondering if they may be part of the symptoms I am seeing so far. Thanks.

[g1]

Are those readings while on the bulb limiter?

[keithb7]

Yes they are. The bulb is bright when the amp is first fired up. Then dims down very dull. I was thinking if the amp needed more power it would pull it and the bulb would be brighter, no?

I just measured again with bulb limiter out of the way. 575V AC off both red wires of on the rectifier tube.

[52 Bill]

The voltages you are reading may be lower due to the limiter. The really low voltages once the standby was jumpered can be caused by a shorted cap or even the choke.

Remember that cap testers do not test at high voltage and that one may fail once the voltage across it reaches a certain level. While the resistance of the winding may be okay, did you test to see if the winding is shorted to the core?

Try removing all of the tubes and see what happens to the voltage levels. Keep the limiter on for now.

[The Dude]

I would pull the tubes until you verify "normal" power supply voltages. You could have bad tubes loading the supply down.

Edit: Dammit, Bill beat me to it.

[keithb7]

Live updates here as I read your responses. Thanks folks...
I pulled the light bulb limiter out of the test. Plugged straight in to the wall.
15 amps on tap. Oh boy here we go.

Read 575V off both red wires at rectifier.
Got up to 128V DC on the first B+ caps and some crackling.
I plugged in a guitar and we have a signal. That's exciting. It's a weak signal but it's there.
Old caps coming out, F&T's going in then another update.

[Enzo]

before you start changing a lot of stuff, do what Dude said, pull all the tubes and see if the power supplies are all healthy.

The choke is probably fine, but your measurement wasn;t the right one. You needed to measure resistance from either wire to its frame. That is what could load down a supply. If the choke has a short among the windings, it just turns into a wire. But a short to frame shorts your B+.

A loss of filtration in the filter caps results in a HUGE ripple, and thus B+ at maybe half its level. When you measure at the standby switch in stanby mode, the only filter you have is that tiny 0.05uf cap, so that would explain the very low reading.

But when standby switch is in operate mode (or just wired across), you get only 7-10v DC????? That right there is a big problem. No point in going further until you correct that. Either there is a short on the supply loading it down - bad cap, bad output transformer, bad tube, or there is an open connection. That is one reason we want to start with no tubes, then with the rectifier ONLY.

Oh, I see you ditched the bulb. Good. The bulb is great for protecting the amp if it were blowing fuses. Once we know the thing won;t try to melt down, we get rid of the bulb. Your wall outlet makes 120v and plenty of current. When you amp is on the bulb, the brighter the bulb is means the bulb is getting more and more of that 120v. And that means the amp has less and less of it, not more.

[keithb7]

All tubes out, original caps, power supply readings:

Red wires on rectifier tube: 281.9 VAC, each wire.
Yellow wires 5.836 VAC

Plugged in rectifier tube only, all other tubes out. Measured DC voltage on B+ cap 1. Watched voltage steadily, somewhat slowly climb up to 200V DC. Turned off amp and replaced B+ caps.

New B+ caps installed. All tubes out except rectifier. I measured B+ voltage on cap 1. Similar results, somewhat slowly watched b+ rise to about 320 VDC. It does appear something
in the circuit is letting B+ power go to ground. I'll start checking other parts and pieces in the circuit. I can't seem to get a resistance reading off the choke wire to it's frame. Hmm...

[g1]

There is a 4700R after the choke. You can measure the voltage across it to see if the cap on the other side of it is leaky (or anything else down the line).
With all tubes aside from the rectifier removed, there aren't many other places that could be loading down the supply.
You might also want to try disconnecting the center tap of the OT primary.

[keithb7]

Thanks for you replies.

I tried to remove the B+ feed from pin 8, all tubes about except rectifier tube, no current limiter, I measure 263V DC off each rectifier pin 8 and pin 2.
Now I am getting about 268 VAC on red wires on pins 4 and 6. I was getting a little higher reading yesterday. Re-installed B+ wire.

Ok, summary of where I am at:

Light bulb limiter is out of all testing going forward. I no longer need it as the circuit seems stable. The previous measurements in post #8 was also without the light bulb limiter. Going forward, it will be out too, so pretend I don't have one.

The initial B+ measurements were low, as I was using the current limiter. Once I removed it I saw some B+ voltage improvements. Then I replaced the B+ caps and saw some more improvement. I changed the two electrolytic caps on the board too. The ones off V1 and V2. Again some improvement I think, however only with all tubes out except rectifier tube. With tubes out I can now get the B+ up to 378V DC.

Measured to ground from each individual lead on choke to check and see if maybe there was an internal short. None measured, it appears good. No connection from either choke lead to ground measured. Impedance through choke is about 99 ohms. A new Classictone shows about 110 ohms according to their specs. I am thinking the choke is probably fine.

I pulled all tubes and checked for internal shorts with my tube tester. Found 1 6L6 with an internal short. Replaced a pair with known good tubes. Flashed up amp again with all tubes in.
B+ voltage on cap 1 climbs to, and maxes out at 180V DC and then starts to drop, settling at 146V DC. No guitar sound at all now thru amp.
Also lifted red OT centre tap wire off board to see if there would be any differences in B+ voltages. No difference still stuck at 146V DC B+.

Pulling out only all 3 pre-amp tubes, B+ will steadily climb to 284 VDC and stay there. Re-inserting V1 12AY7 only, voltage tops out at 228.6 VDC. Pulling V1 out and inserting V2 only voltage reaches 228 VDC also. Pulling V2 and only installing V3 yields 235 VDC B+.

My head hurts. Time for a break. Then I'll probably trace every wire in the circuit and compare it to the layout and schematic. See if anything pops out.

[Enzo]

When you say first cap, do you mean that film cap before the stdby switch or do you mean the first filter cap, that pair of 20uf?

Power off, you took AC voltage readings at the transformer, but do me a favor, check resistance. DO you get continuity to ground from either red wire? An open or problematic center tap could be involved.

[keithb7]

I have been taking B+ readings at several locations including at the first pair of 20uF caps. Also at the choke leads and other 20uF B+ caps.
Is the first cap off the standby switch, a cap to prevent popping noise when the standby switch is enabled? Is it necessary? I was thinking it was some sort of death cap. When that cap fails it could allow 400V DC right into the chassis, no? I was planning to clip it out and remove it.

From the red power supply wires to ground, I get 3.5 M ohms of resistance. From yellow wires to ground I get 9.89 M ohms.

[Enzo]

No. If it shorts, it grounds the B+ and blows a fuse. Death caps refer to caps that connect the AC mains to your chassis. leave it there, just don;t expect any filtration from it. That is what the electrolytics are for.

OK, I think that is your problem. Measure red wire to red wire resistance with power off of course. I don't know what it will be, but I will expect maybe 200-400 ohms or something. Look at your schematic, those wires are the ends of the high voltage winding, and the center tap of that winding SHOULD be grounded. That would mean from either red wire we should see about half the red to red resistance to ground. SO if the red to red was 300 ohms, I'd expect about 150 ohms to ground from either red. it looks like your center tap is either open of not well grounded.

[keithb7]

To wrap up this thread...My 59 Twin is singing again. I changed out only what I had to. The Power Transformer was my problem. I installed a new Mercury Magnetics ToneClone for this Twin.
Thanks for your help once again. Here's everything that I had to replace from this stock circuit 1959 Twin. I'm happy, it sounds so great!