Old amp very sensitive to single coils

[JSMac]

Hey all, I have a cheapo '70s Japanese amp the does't like sing coil pickups. As I turn the amp up there is a lot of single coil hum. Less in positions 2 and 4 but still pretty prominent.

The amp is Tempo. I haven't found much info on it. It uses a 12AX7, 6AV6 and 6AQ5. When I got it, it didn't have much volume or gain. I replaced the filter caps, upping the power supply a little. I played around with some mods to try to give more guts. Added a cathode bypass cap and upped the coupling cap values. Much beefier sounding now with probably too much gain past 12 o'clock on the volume.


Anyway, what would cause an amp to be so sensitive to getting single coils anywhere near it?

Adding on: The input area is very microphonic when I tap the wires or components around there and around V1. I swapped tubes so it's not that. I resoldered every joint from the input jack to V1. Beyond that point it is not sensitive. Any ideas what I should look for?

 

[Wildeman]

Sounds like a widow maker, try plugging it into a grounded power strip. Got any pics?

 

[cousinpaul]

12AY7 for v1?

 

[JSMac]

Sounds like a widow maker, try plugging it into a grounded power strip. Got any pics?

I put a three prong cord on it. I should have taken some pics before I put it back together.

most of the components have been replaced except for a few a couple of resistors and they speced out ok and some small caps. I should mention that the amp was noisy before I worked on it, just noisier now that there is more gain.

 

[JSMac]

12AY7 for v1?

I have one somewhere. I'll try that if I can find it or try a 12AT7 just to see if that helps.

 

[JSMac]

Hey, I do have some pics from back when I worked on it. Note where the output tranny is mounted. It was originally mounted on the original speaker with high voltage exposed and creating quite a hazard so I mounted it on the side of the cab and insulated the terminals. Could that location be causing the noise? I do recall moving it around to different areas before I mounted it and that didn't seem to make any difference.

 

[JSMac]

Here is the schematic.

 

[andrewRneumann]

Perhaps this amp could benefit from full-wave rectification and balanced heaters. Are the high voltage and heater tap returns literally combined on one lead coming out of the PT, or does it have two separate leads joined outside of the PT?

 

[dankilling]

Perhaps this amp could benefit from full-wave rectification and balanced heaters. Are the high voltage and heater tap returns literally combined on one lead coming out of the PT, or does it have two separate leads joined outside of the PT?

Watch the voltage if you go this route. 6aq5 tubes don’t like to go over 300vdc for long.

 

[Wildeman]

Cool amp, looks like you're on the right track, I'll let the real amp mechanics guide you

 

[charlie chitlin]

My first amp was a Tempo.
I don't even remember if it had tubes.
I know if you turned it up, it sounded like a buzz saw.
It was awesome.

 

[JSMac]

Watch the voltage if you go this route. 6aq5 tubes don’t like to go over 300vdc for long.

The plate voltage is high for that tube. Just over 300vdc. The data sheet states 250vdc max.

 

[robt57]

Try coily input chord maybe..

 

[andrewRneumann]

The plate voltage is high for that tube. Just over 300vdc. The data sheet states 250vdc max.

Did you change the power supply much, or do you think the original pretty much ran at 300V?

It wouldn't be hard to keep the voltage at or below 300V. Go with full-wave rectification and separate plate return and heater return current outside of the chassis. Balancing the heaters is recommended, but probably not causing buzz in the pickups.

Maybe someone can recommend some nylon/non-conductive hardware to mount those terminal strips?

 

[Burning Fingers]

Have you considered using shorting input sockets ( switched to earth with no plug inserted ) to mute the unused inputs ?

 

[dankilling]

The plate voltage is high for that tube. Just over 300vdc. The data sheet states 250vdc max.

Wall voltages are higher now than when it was built. I’ve done a few amps using 6aq5 tubes and generally try to keep them around 280vdc, so that’s probably close to what it originally was. Univox used to do some cool 6aq5 amps as well, so probably came from the same design team. In any case, there’s a lot of things that can contribute to noise in an amp, but does it behave the same way with humbucker s? Also, any pics from the front?

 

[drew1d]

If the amp was noisy before, and you increased the gain, it’s not surprising it’s noisier. Something else must be up. But before you mod again, let’s talk about a few service points. I’d start with the grounds. I see that the ground for the inputs are the chassis, did you unscrew them and clean where they mount? Also test the voltage on the chassis to see if the death cap is leaking. Stuff like that. A good servicing. Good luck.

 

[Jasonpatrick]

You said cheapo right? ……

 

[JSMac]

If the amp was noisy before, and you increased the gain, it’s not surprising it’s noisier. Something else must be up. But before you mod again, let’s talk about a few service points. I’d start with the grounds. I see that the ground for the inputs are the chassis, did you unscrew them and clean where they mount? Also test the voltage on the chassis to see if the death cap is leaking. Stuff like that. A good servicing. Good luck.

I’ll check those ground points. I did remove the death cap.

 

[Wally]

Sounds like a widow maker, try plugging it into a grounded power strip. Got any pics?

This is not a ‘widow maker’. There is a PT there, and the numbers for the heater filaments does not add up to anything close to wall voltage as they do in a widow maker typey amp.

To the noise…single coils are noisy. The noise that is eliminated in the 2&4 positions——IF you have a reverse wind/reverse polarity idle pickup——is noise that exists even with a humbucker pickup input. Unless this single-coil equipped guitar is inherently noisy, the noise is in the amp.
Turn the guitar volume to zero. What noise do you hear? Does this amp exhibit the ‘problem’ with a humbucker equipped guitar? If so, then the noise is in the amp.…or you have a noisy environment.
Any amp produces a signal with hum with single coil pickups. If one does not like that characteristic of single coil pickups, then the pickups are the problem. Does the noise over-ride the signal production when playing?