Teisco / Beltone Melody Amp Repair Issues

[Mason]

I’m trying to repair an old Teisco/Beltone Melody tube amp.  The amp is pretty simple and I’ve got it up and running, but I have a few issues I need help with.

First, there’s a loud hum when the amp is on. It doesn’t change volume when I turn the amp up, it’s always at the same level.

Second, it’s doesnt seem to be as loud as it should be. When all the way up the amp is still clean, it sounds just on the verge of breakup. It’s a small amp, but still it’s definitely not as loud as my 5F1.

Third, the tremolo doesn’t work.

So far all I’ve done to the amp is replace the preamp tubes with fresh ones, added a three prong cord, and replaced the electrolytic caps. I had to modify the cap values with what I had on hand. So in place of the 8+8+2uf cap i have just a 16+16uf cap. And in place of the 10uf cap, i have a 25uf.

I’ve checked all the resistors and they’re still good. I’ve reflowed all the solder joints I can get to. The volume pot is supposed to be 1m, but I measured it as 1.8m. Besides that, all the parts still seem to be good.

I’ve attached the schematic and picture of the amp for reference.

[Mason]

More pics

[Mason]

Last one

[shooter]

Love the old WW resistors

pull V1, still hum?
pull V2 ?
pull trem tube ?

that should get you close
verify it's not coupled from the surroundings

[Mason]

Hum is still present when any of v1-v3 is pulled.  The power tube and rectifier tube are both original, and both seem to be glowing pretty strongly. I can’t really tell if they’re red plating or not. I’ don’t have any experience with these types of tubes so i don’t know if the strong glow is normal or not.

Either way that still wouldn’t explain why i have no tremolo.

[jjasilli]

Suspect tubes first, including power tube & rectifier, and the new preamp tubes.


Is there hum when All of V1 - V3 are pulled?




New electrolytic cap could be bad; or poorly grounded.  Maybe try testing by bypassing each cap with another cap.


Do all voltage points test OK?

[shooter]

still present when any of v1-v3 is pulled

so it's down to 1/2 the amp

Measure your 1st filter cap's VAC - Not VDC, do the same to the next in line, my initial guess your rectifier might not be rectifying

[Mason]

With all tubes in on the first filter cap, I'm getting virtually no VAC, the meter moves a bit between 0-1v, I get 278 VDC.  At the second filter cap, I'm getting about 4 VAC, and 315 VDC.

[shooter]

Good numbers
I troubleshoot a little weird;
I would pull all the AV6 tubes, swap the .01uF feeding the PA tube with a new one.  Don't solder the "left side" Pin 7/100k junction, instead use a gator clip and temp ground it.  power up n listen

[Mason]

Alright, so this is a weird one. I did what you said, I’ve got a .022uf cap in place of the .01uf. All the preamp tubes removed. I’ve got the cap soldered into pin 1 of the power amp tube, and then the other side is clipped to ground.

I turned the amp on and the hum is still there, but it sounds different. There’s also a higher pitched hum that pulses in and out now. So here’s the weird part. Whenever i move my hand near the alligator clip that is connected to the cap, the hum stops. When I pull it away, it starts again. My hand never touches the clip when this happens btw.

So, any ideas?

[shooter]

the hum stops.

where did you ground the gator clip?

move it to your "main" ground stud, the one used as your main PS ground.

If that don't get it, put a steel plate over the bottom, move the whole amp to the basement, or garage, or neighbors home.

[Mason]

I grounded it to the bus wire that runs along the whole amp that it seems like everything in the amp is grounded to besides the three pronged cord that I added.

[shooter]

ohm between the buss wire and your new green AC main ground wire, whatcha get? (UNPLUGGED)

[Mason]

I get .2 ohms.

[shooter]

my hand

Are the 2 tubes that are in, harmonic? what happens when you wave around near them, tap 'em with a woodstick
plate over bottom, different environment ?

the hand trick tells me the amp is an antenna.

does your meter have a freq function?  If so measure  Fo at the speaker

[Mason]

So I’ve moved my hand around the chassis while the amps on, moved the amp around while it’s on, tried different outlets, different rooms, steel plate on the bottom, and tapped the power tube and rectifier tube. None of this changed the hum or caused additional noise. All this was done without v1-v3 in.

The .01uf cap is back in place by the way.

[shooter]

The .01uf cap is back in place by the way.

after all the tests, or during (moving, plate etc.)

without knowing freq, or scoped images, it's down to swap n see.  We're pretty sure it's "right" of the .01uF.

You have 5 Resistors, 4 Caps, 2 tubes, 2 sockets.  I would leave the left side of .01uf disconnected and grounded for testing as you swap out parts, that way any undetected gremlins to the "left" can't muck up the works.

you can even "cheat" by just disconnecting PS tap C (30k, 100k, 2uf), so that eliminates  2 R's and a cap

[Mason]

I actually figured it out.  I had mixed up some connections to the filter caps when I replaced them.  Have them wired correctly now and the hum is much quieter, and the amp seems to be as loud as it should be.  The tremolo also works. There's still some noise and hum, but I think I can take care of that. Thanks for taking the time to help!

Any idea how many watts this amp is? I'm not familiar with the 6AQ5 power tube.

[sluckey]

Any idea how many watts this amp is? I'm not familiar with the 6AQ5 power tube.

I have a similar single ended 6AQ5 and it's running at 11.9 watts.

You can figure your power as follows...

1. Measure voltage at pin 2. This is V_k. Divide V_k by 420Ω. This is I_k

2. Measure voltage at pin 5. This is V_a. Subtract V_k from V_a. This is V_ak.

3. Multiply V_ak times I_k. This is power in watts dissipated by the tube. (NOTE. Actual dissipated plate power will be a little bit less because I ignored the small screen current.)

[sluckey]

So in place of the 8+8+2uf cap i have just a 16+16uf cap.

This sounds like you replaced three filter caps with only two caps? What happened to the 2µF cap? It's very important. It supplies voltage to all three of those 6AV6s. I highly recommend you put that cap back in but replace the 2µF with a 20µF. This should help reduce hum a bit.

[shooter]

And in place of the 10uf cap, i have a 25uf.

I was guessing this was the third 

[sluckey]

And in place of the 10uf cap, i have a 25uf.

I was guessing this was the third 

I was guessing that was on the cathode of a 6AV6.

[Mason]

You're right sluckey, I had dropped the 2uf cap.  Was hoping I could get by without it in the meantime while I got the amp working.  I've found a 20uf cap to put in its place. Is that not too much of an increase? Going from 2uf to 20uf?

[sluckey]

I thought 2µF was way too small. I think 20µF is more appropriate. I would even consider 40 or 50.

How does the amp sound now? You should have noticed a big difference going from no cap to 20µF.

[Mason]

The amp sounds great now. It’s just got a faint hiss when dimed. I just have to fix the scratchy volume and tone knobs and this thing will be perfect.

By the way, here’s the numbers I got when calculating wattage. Wanted to make sure they didn’t look weird to you.

Vk = 17.47
Ik = .0415
Va = 301.8
Vak = 284.33
Wattage = 11.79

[sluckey]

Those numbers look like my numbers for L'il Smoky.   

     http://sluckeyamps.com/smoky/smoky.htm

[shooter]

Glad you're grinning!  make the neighbors happy