I have a Voice Of Music single-ended 6BQ5 amp that hums a bit...but I found the hum goes away when the bass is turned all the way up.

http://forums.pelicanparts.com/uploads11/vom+small1192864136.jpg

Is this likely a grounding issue...or simply clean the bass pot...or something else like a bad cap?

Hi, Revises filtering capacitors, Good Luck,Martin

Nice little amp. If you want to keep and use it, cleaning the pots, tube pins and sockets is always a good idea. I also would look at replacing the electrolytic caps in the amp since electrolytics are supposed to be good for about 12 years and these are 40+ if they haven't already been replaced. If you aren't going to change the electrolytics, then definitely don't leave the amp running when you aren't around to monitor it.

Hi Tim,

It doesn't hurt to check the resistors either. Measure the resistance of the bass pot/pots, and see if it's still within value. It could a capacitor or resistor in the tone compensation network, or any number of things really.

Replacing the electrolytics and coupling caps are a easy way to solve a lot of problems right from the start.

This is how far I got with the little Magnavox SEP 6BQ5 amplifier you sold me. :worried:
I just need to pony up for the parts I want to use. Got sidetracked with other projects......

I should get it finished so I can have two of the same kind/type of amplifier for tying a bi-amped speaker configuration.

Mike


http://www.audiokarma.org/forums/attachment.php?attachmentid=62935&d=1190750739

Well I took off the bottom plate and here's the amp's guts.

http://forums.pelicanparts.com/uploads11/underside1192919209.jpg
http://forums.pelicanparts.com/uploads11/pots1192919230.jpg
http://forums.pelicanparts.com/uploads11/caps1192919245.jpg
http://forums.pelicanparts.com/uploads11/capss1192919263.jpg

I don't know which caps are the coupling ones, and I don't see any electrolytics except for the big can. Ideas?

Mike the amp looks great! A far cry from the one you took home eh? How does it sound?

If I were you, I would not chase ghost.

The problem you described is a result of this particular type of amp design. This VM amp is designed to be used with a record changer. The ceramic cartridge used in the changer requires a very high impedance load (1-2 Mohm), and some manufacturers use a variable load as bass tone control. The high impedance in the input tube grid is the reason for the amp to hum a bit. The high impedance is similar to a open ground, but in a much lesser degree. "When the bass is turned all the way up", the grid sees the much lower source impedance through top end of bass control pot, so the hum goes away. For the same reason, the grid sees much lower impedance to ground when volume pot is at low position (through lower portion of the pot, obviously). Depending on the design, some amps of this type have highest hum volume when volume pot is at middle position where impedance to ground through either upper or lower part of the pot is at maximum.

The solution is to bypass the bass tone control (it is not very effective with low impedance source anyway). You can also replace volume pot with a lower value, such as 500K, one to get even lower noise. If the grid doesn't have a "grid leak" resistor to ground, you can add a 1Mohm one.

Ultralc, you make too much sense, thanks! It does make sense for me to bypass ALL the pot controls, as if I can get it to work without hum, it will be hooked up to a preamp.

I have never done a bypass...all of the pots have 6 legs, except the balance , which has 3.

http://forums.pelicanparts.com/uploads11/pot1193092534.jpg

Thanks for the help!

the pots are double gang, one side is for left, one for right.

To do this the only way I know how, it looks like it would be a matter of running a resistor from #2 to #3 and another from #5 to #6. If you want the bass at full, just use a jumper wire. If you want somewhat less bass, I'd do it the easy way, adjust the pot to where you want it, carefully remove it from the board without turning it, and measure the resistance across the legs. Install a fixed resistor of the same value and you're done.

Possibly someone more experienced in this sort of thing could add some insight on how to completely remove any tone compensation from this.

There are several ways of implementing tone/balance/volume controls in this type of amp, so bypassing method varies. Based on the photos of your amp, it appears that if you just simply jumper "2 and 3" and "5 and 6", you will bypass bass pot and it's parallel caps. As gadget73 suggested, only do this if you like what you hear with bass pot at maximum position.

For other stuff on the tone control board, I would start by removing loudness circuit parts connected to volume pot, and then try to find a way to bypass treble control without affecting balance. Don't remove any parts connected to the preamp tube. Do not try to remove pots from PC board if you are not experienced in desoldering.

If you have a schematic, people here can provide more specific suggestions.

It's not terribly difficult to draw up a schematic by yourself.

If you know the basic circuit elements of a tube amp, you can trace things out.
It just takes time.

A point to point wired amp is a little easier to trace out than the traces on a printed circuit board. But the values are listed under the parts, and the PCB is simple enough to trace the traces.

I could never deal with the tone pots on vintage tube amps. I like what they do soundwise. But they always do such a crummy job at it. And IMO, everything seems to sound better once things are simplified into a power amp and the pots are gone.

I'd sit there with a pencil and paper, and trace it out.

Or just run a right and left input right to the driver tube grids, and use a 1 meg grid to ground resistor like mentioned.
1500 ohms or so for each cathode of the 12AX7, (if that's what it is)
No need to bypass the cathode R's with a 12AX7 with the gain it has.
Use a 047uF or 0.1uF from the 12AX7 plates to the 6BQ5 grids, with a 470K grid to ground resistor.
I don't know if the cathodes of the output tubes are sharing a resistor, you could either use two, or stick with the one.
The cathode bypass cap is probably a section of the aluminum can cap. You can use a axial cap at 50-100 or whatever microfarad across the cathode resistor/resistors.

A nice option is the capacitor can is held in with a clip. You could swap out that can rather easy with a Black JJ multi-capacitor. You may have to bend the clip a little, but it would be easy. The original capacitor can may be fine for all I know....
You'll have to figure out if the amp has feedback, and how it is set-up in the circuit. It could be just a loop from the secondary lead to the driver tube cathode, or tied in with the tone controls pots somehow. I dunno....they have many different ways of using feedback.

Some tag strips, and a little time, you could probably PTP wire the amp as a straight power amp and throw away the PCB.

The little Magnavox amp I bought from you is pretty much a dummy amp with no guts. :grumpy:
I need to buy parts, but a Klipsch Lascala speaker project has kinda taken presidence over the amp.

You oughta hear my little Maggotbox on these Beasts. BIG sound from a little amp.

""For other stuff on the tone control board, I would start by removing loudness circuit parts connected to volume pot, and then try to find a way to bypass treble control without affecting balance. Don't remove any parts connected to the preamp tube. Do not try to remove pots from PC board if you are not experienced in desoldering.""

This would be about the easiest way to go about it. Go part by part. I tend to go overboard......

Mike

This is sure a learning experience.

Thanks for all the suggestions. I was going to put resistors in place of the pots, but pots are just variable resistors and it seems that alone probably won't decrease the hum. I think the solution is to take the pots out of the circuit...but I'm not ready to undertake this yet. Mike, yeah I know, it's probably easier than I'm thinking...maybe I'll sell you another amp soon and you can chech this amp out. It uses our familiar 6EU7 instead of a 12AX7 btw.

Thanks again!