Bogen PA to Guitar Amp, few questions

[Turbo7MN]

Uh oh...

My problems just got significantly worse.

I was going to test a couple things like a guy on Ampage suggested, and after I fired it up I figured it woudln't hurt to play for a little while. There usually is a slight background hum that I verified is coming from the input (the amp is pretty much silent with no input), but suddenly the hum started getting a lot louder very fast. Right as I set down the guitar and dove for the power switch, there was a loud *bang*. this all happened in a matter of about 5-10 seconds. I shut it off within a second of it happening so I couldn't tell exactly what it did. It sounded a lot like a firecracker, my ears were ringing for a few seconds.

So I got it up on the table and cracked it open expecting to find the tattered remains of some capacitor, but I saw absolutely nothing out of order. I don't want to fire it back up, but I don't know what else could have exploded. At least it wasn't a tube .

-Darren

 

[Sherman]

Re: Uh oh...

My problems just got significantly worse.

...a slight background hum that I verified is coming from the input (the amp is pretty much silent with no input), but suddenly the hum started getting a lot louder very fast. ... there was a loud *bang*. ...

It sounded a lot like a firecracker, my ears were ringing for a few seconds.
...
-Darren

Darren,

Did the *bang* come from the speaker or from the chassis? (I know it might be hard to tell since things were happening fast and you probably weren't concentrating on where the noise was.
)

I'm not an expert but I would replace any coupling caps. Also verify the resistor values since old carbon comps can drift very badly over time.

 

[Turbo7MN]

Yeah, I haven't got the slightest idea where it came from, I just know it was loud.

 

[anatech]

Hi Darren,
The noise may have come from both the amp and the speaker. At this point, just put it aside until you get your new caps, ensure the resistors are in tolerance, 220K seem to drift the worst. Check the screen resistors if the amp has them. Consider changing the filter caps just 'cause.

Good luck, you have a project ahead.

You know, there are some 8417 stabilising mods somewhere on the net as I recall, or you may go to KT-88's and increase the drive voltage.

-Chris

 

[Turbo7MN]

I've checked the 220k resistors and they look fine. Then again, this was before my little detonation incident. I've got a little shopping list of caps now. Kinda sucks that my external can cap (filter caps, right?) will cost about $40 to replace.

I plan on installing a bias voltage doubler as well as individual bias controls to hopefully prolong the life of my 8417s. I've heard that with some mods, the EL34 can be temporarily thrown in there to check that stuff is working right. I'll pick up a pair after I get my power supply rebuilt.

In the meantime, I'm back to the old solid-state amp. *shudders*.

-Darren

 

[anatech]

Hi Darren,
Yup, filter caps.

So the 220 K resistors measured close to 220 K ?? Cool, the ones I see are more likely to be way out. All of them really, but 220 K seem to go out the furthest.

And yes, check after with inexpensive EL34's (good ones).

-Chris

 

[ethel188]

Have you tested your tubes yet?

If you have not tested your tubes yet. Where are you located in minnesota?

 

[Turbo7MN]

Nope, haven't found anyone yet. I'm in North Branch, it's a little ways north of the Twin Cities off of I-35. Can you test my tubes for me? Where are you located?

 

[ethel188]

I can, I am just south of St. Paul. If you really wanted them tested you could just mail them by usps for pritty cheap. I have been selling tubes online on ebay and tested them all. You can look at my feedback. My user ID is ethel188 for ebay. I don't have anything up for sale at the moment. It is quick and easy to test and the tester I use is a hickok 752a.

 

[ethel188]

I mean "want" not "wanted".

 

[Turbo7MN]

Alright, I'd like to work something out to bring the whole set over sometime. If you don't mind, I'd like to bring them over in person so I could see how the tester works.

I checked out a few of your recent ebay sales and I saw that you recently sold an M60A pretty much identical to mine except with dead tubes. With the totally-dead 8417 that was in that amp, did you notice anything about the tube that would show that it's dead, besides actually testing it?

 

[Turbo7MN]

Alright, so I've decided to replace pretty much all the bigger caps in there, and I've put together a little shoping list. I'm going to hold off on building a voltage doubler until I get it recapped and fired up again. I made this list just by looking at the caps in there, I still have no schematic. I'm pretty sure what most of them do though.

1 mfd = 1000 ufd, right?

I'm having some trouble finding the values I need. Digikey has pretty much nothing for axial-style electrolytics. Anyway:

1- 40/40/10 mfd 350VDC - filter cap can - *found*
1- 40 mfd 350VDC - still looking
1- 10 mfd 450VDC - still looking
1- .47 mfd 600VDC - still looking
1- 500 mfd 25VDC (ideally 50VDC with the voltage doubler) - still looking
2- .22 mfd 600VDC - still looking
2- 25 mfd 25VDC - *found*

Anyone know where I could get these?

 

[Trout]

This place Is awesome for caps of most varieties, VERY reasonable and VERY Fast LOW priced shipping.

Just Radios

I buy from this guy all the time, Never a bum part and he has everything in your list and way more. He is starting to get popular with the guitar amp guys just because his part prices and service are unmatched.

Enjoy
Trout

EDIT: He also has an ebay store that lists his entire line-up and accepts paypal, Just Radios Ebay

 

[Turbo7MN]

Whoops.

Turns out 1 MFD = 1 uF? Finding these should be no problem then, and I spend three hours looking last night for nothing. Looks like I've still got a lot to learn .

Thanks for the link, they have a lot of what I'm looking for.

 

[anatech]

Hi Darren,

Turns out 1 MFD = 1 uF?

Yup. You're on the right track now!

-Chris

 

[ethel188]

Sorry I didn't respond earlier. Unfortinitly I have a very busy schedule to meet each other to test your tubes. The bogen amp that I sold the tube that was dead you would not need a tube tester to know that it was completely dead because the tube would not fire up. Also the tube had a white film on the inside like an old flash bulb after it has been used. Some old tubes that I have seen that were really week the silver colored coating on the inside of the tube was gone and all you could see was brownish black color. One amp I fired up the 7247 tube was half bad and when you ran the amp you could here snapping and cracking sounds from the speaker. Also after turning the amp off the audio from the speaker wouldn't cut out within 10 seconds it would die out after a minute or so. After I changed that the amp worked fine. Also there is no real way to see if your smaller tubes are good just by firing up the amp or if they have any leakage unless you have no problems when you fire the amp up. Weak output tubes you will just notice how well they amplify if you had a new set to compare. Also I am going to be putting up another M60A amp that I am planning on selling it with a new set of output tubes. It is a one owner amp that someone marked up the faceplate. Internally there appears to be no rust and some oxidization. All I did with the amp was pull the original amp tubes and put some tubes in that work. I fired it up and it worked fine and I ran it for a little over an hour with no noticible changes in audio quality. Also the output tubes do normally run hot for 8417's. The MO-100A amps I had that had four output tubes got really hot. That is why if you look at the chasis cover that comes with the MO-100A it is just a screen on the top because of the amount of heat the tubes generate.

 

[Turbo7MN]

Both my 8417s look identical, so I guess I'm not really concerned about a blown tube. And at this point I just want to recap it and see what happens. That's fine about not being able to test the tubes, I think I found a place up in Harris that could do it for me, if the guy ever calls me back .

Hopefully I'll get my stuff ordered sometime this week.

 

[anatech]

Hi Darren,
The problem is that testing power tubes is best done by substitution.

If a properly woking tube tester tests your tubes as bad, they are. Good ones may not always be good. This is doubly true of power tubes.

-Chris

 

[Tchefong]

Shematic

Here is someting that can help you

http://cgi.ebay.com/Bogen-MX-60A-Ow...873409141QQcategoryZ50594QQrdZ1QQcmdZViewItem

By the way i also got a mx60a whit the mixer mxm but mine seem youger than yours.

i also got a bogen ch35b that i will use to learn about tube amp before turning my mx60a to guitar amp

 

[Turbo7MN]

My amp is an M60A, not a MX-60A. I bought the manual anyway though, seeing as the differences are certainly minor.

looks like you got quite a haul with all that Bogen stuff. I think an MXM would be fun to mess around with, lots of preamp stages .

Anyway, I've got a pretty good plan of what I'm going to do with it. First, I'm going to recap it and probably redo the bias supply at the same time. After it's running good in the original form, it's time to start modding it for guitar use.

Now, my plan was to buy all the bias stuff at the same time as the capacitors. At this point, I understand how most of the stuff in a tube amp works, but I'm in way over my head with the bias supply stuff. This is what a guy on Ampage said he did to his M60A:

The B+ supply is a half wave voltage doubler with the bias derived from a capacitive divider feed in a half wave rectifier - I simply added another half-wave doubler with a couple of 25k 20 turn pots across it to give variable, individual 8417 bias with 1 ohm, 1% 5 W cathode current sense resistors added.

I have no idea what that means. He also gave me this:

if you look there are two diodes in series in parallel with two capacitors in series. AC is fed into the the junctions of the two diodes and the two caps - B+ is taken from the cathode of the one diode and B-(at ground) is taken from the anode of the other. What you want to do is duplicate that set up with the 1/2 wave bias source. Then separate the two 22K bias feed resistors to each 8417 and connect them to the wipers of a couple of potentiometers hooked across the bias source. I believe that I used 25K 10 turn pots but I don't rememeber if I soldered in a resisistor between the + end of the pot and ground to set a minimum bias voltage. As far as current sense resistors these go between each 8417 cathode and ground - I like 1 ohm, 1% resistors but some like 10 ohm - you just have to do a bit more calculation with the higher resistance.

The only part out of that I understand is replacing the "balance" pot currently in there with the 1ohm resistors. I'm completely lost with how to do the bias supply. All I know that that right now it is set up to provide a fixed, non-adjustable -17V, and that I need to at least double that to run EL34s.

I'll provide a couple pics of the power section so you guys can get a better idea of what I'm looking at.

-Darren