oops, I blew it up!

[chrisb]

Hi All,
I made a little boo-boo and I'm hoping you can advise me on how to proceed:
I have 3 of those record players you used to find in schools (you know the ones, they have lids that hinge off). One of them has a mic input, so I plugged my 360/12 in and it's a great little amp (6L6). The other ones have different tube configurations, so I thought I'd set about adapting them to use as guitar amps also.
Starting with the EL84 model, I decided to bypass the motor since it hummed like a beehive and didn't turn. The wiring goes AC mains>fuse>motor>trannie primary on one side, while the other side goes AC mains>power switches (1 main, 1 motor)>trannie primary. It seemed to me that bypassing the motor should be as simple as cut & splice, but in my typically rank amateur manor, I didn't count on the fact that the motor has a resistance that might be important (100K as I measure it now). So when I turned it on after wiring my bypass it popped and now won't fire up again.
Do you think I blew up the trannie, or just some section of the quad can cap?
I'd like to get it working, but I don't think it's worth a trip to the tech and I've now exceeded the boundaries of my testing knowledge... I can change the caps but the trannie is a different story.
Any advice would be greatly appreciated!
Chris

[soundmasterg]

You know, without seeing a schematic it is hard to tell you what to do here. What popped? The fuse? If so, try a new fuse. Still pops? Pull the tubes and try a new fuse. Still pops? Unsolder all the secondaries out of the power transformer, try a new fuse and see what happens. Still pops? Power transformer is most likely bad at this point.

If the electrolytic caps are more than 10 years old, then they should be replaced. There has to be a way to run the thing without the load from the record player motor in circuit, and it may involve using a different power transformer depending on how it is wired up. Its really hard to say without mor einfo about hte unit in question.

Greg

[chrisb]

All right, problem solved! It was the power switch all along (you can laugh at me now), which had apparently opened for "some reason." I simply switched the mains over to the motor's switch after disconnecting the motor, and voila!

I've also successfully converted it to guitar use, removed the cap/resistor on the input and changed the filter caps. Now it's time to set about finding the best resistor value to the plates for best gain.

[soundmasterg]

For changing the gain, you'll do better to change the dropping resistor value that supplies the preamp stages, thus changing the voltage the preamp tubes operate at, rather than changing the plate resistors.

Glad it was a simple fix.

Greg

[chrisb]

you'll do better to change the dropping resistor value that supplies the preamp stages, thus changing the voltage the preamp tubes operate at

Sorry, I didn't follow that... which pins would that be on a 6AV6, and are you saying to lower the value?

(BTW, this is a 3 tube amp: 6AV6, 6CA4, & 6BQ5.)

Thanks Greg!

[wolfgang]

hi Chris,
the 6AV6 has one triode system identical to the 12AX7 and 2 additional diodes (normally unused in amps)
Looking from below, pin1 is the grid1, pin2 is the cathode and pin7 is the plate (cw).
Changing the plate resistor makes not that big difference in gain, changing it from 100kOhms to 220kOhms gives only about 2 Decibels more gain (= 20%).
So, if it's 100kOhm or 220kOhms, it is o.k., leave it like it is.
You need (with only one triode and the EL84/8BQ5) at least 100mV at the input for full power.
Too high to achieve distortion without an extra gain booster stage.
But, if there is no electrolythic capacitor across the cathode resistor (pin2 to ground, 1KOhms-2.2kOhms),
try one (22µF/25V). this brings about 6 Decibels more gain.
The 6CA6 is the rectifier tube (EZ81)

Wolfgang

[chrisb]

Thanks, Wolfgang. I put the 22uf/25v cap in across the 1200 ohm resistor that's already there (- to pin3/ground & + to pin2) and the gain is now just right. The problem is it's now unacceptably noisy (60Hz)-- I wonder what may be the problem? Could I have gotten something too hot in soldering?

[soundmasterg]

In the power supply of the amp, there are caps to ground and resistors in between them. These are called filter caps because they filter the noisy 50/60 Hz AC out of the DC that the tubes see. If these go bad then the amp will hum, or is worst cases, become non-functional. The resistors in between these caps are the dropping resistors, and changing the one that supplies the 6AV6 will cause it's plate voltage to change. This greatly controls the gain and tone characteristics of that stage. With many old amps, they often have two or more stages sharing the same dropping resistor, so if you change the resistor value, it will affect more than just the one stage you want it to. Changing to a larger value resistor will drop more voltage, which lowers the gain, but makes the tube stage overdrive sooner, so it sounds like the amp is dirtyier. Changing the dropping resistor to a smaller value results in less voltage drop and more voltage at the plate of the tube, hence more gain and more headroom, which often sounds cleaner. You can experiment with different values to suit your taste in tone, though there are limits to how high and how long you can go. Usually the most obvious one is whatever voltages the tube needs to work correctly.

Could I have gotten something too hot in soldering?

In a word, no. If you haven't yet, replace the filter caps. If the amp is old, you can be assured they are worn out and in need of replacement. The reason you are noticing the hum now and you didn't before is most likely due to the fact that you now have more gain which means more noise. There could be some other problem with how you hooked everything up, but that is hard to say.

Greg

[chrisb]

Ah ha, this learning thing is pretty cool...
Just for kicks I bypassed the power switch and the noise greatly decreased, so I'm using it unswitched for now. I also tried other things, like routing the AC differently, that didn't help much. I wanted to try different tubes too, but I don't have another 6CA4 or 6AV6.
I did replace the filter caps a while ago, and I think I did it right.
Also, after trying things with & without the 22uf cap, I think I like the sweetness of the tone better without it. Would an 11uf (or whatever is close) give me about 1/2 the gain increase?

[wolfgang]

hi Chris,
a smaller value of the capacitor makes almost no difference. But a value as low as .68µF
or .47µF will bring you a little bass cut.
If you want only half of the additionally gain, try a 680 Ohms resistor in series with the 22µF capacitor.
Another issue to lower the hum is to find a "better", that means less noisy earthing point for the capacitor.
Sometimes it works well, most time you will hear no signifficant difference.
Wolfgang