Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mth281 on May 30, 2018, 07:51:26 pm
-
So i found this amp at a thrift store and paid $20 dollars for it, i knew nothing about it except it was tubed and old. After some research i discovered its actually a really good guitar amp, which came at the perfect time as i had started building guitars. Well i plugged in it and hooked up a guitar to it, was amazed when it turned on and started playing. First i noticed it had some humming in the back round, figured it needed some work and the pots were scratchy. Well i just moved to a new house and decided to plug it in again, and try playing the guitar i just finished. Well this time when i shut it off it shocked me pretty good, kinda like a car coil shocking you. I was also barefoot in the basement which didnt help. I decided its time to fix this thing up, ive always wanted to learn more about amps. Im a mechanic so i deal with dc systems all the time, ac is new to me. I got the 3 pronged cord, and the amp is apart. I have seen quite a few videos on youtube about swapping the 2 prong for the three prong. But the issue, it looks super easy on a fender, this magnatone is a different beast. I believe the two white caps(.047uf 600v) are the death caps, i would just like a second opinion before i break out the soldering guns and pliers. This amp does not have a polarity switch, so is it as simple as cutting off the deathcaps, soldering the new cables hot and common, and bolting the ground to the frame? Heres some pictures, i can upload the schematics if needed. this is not a m10a, this is the original 8inch speaker and 3inch tweeter.
-
I believe the two white caps(.047uf 600v) are the death caps, i would just like a second opinion before i break out the soldering guns and pliers. This amp does not have a polarity switch, so is it as simple as cutting off the deathcaps, soldering the new cables hot and common, and bolting the ground to the frame?
The white .047 cap in the power supply chassis is one "death" cap. Just snip it out. The white .047 cap connected to the rotary power switch is the other "death" cap. Just snip it out.
Actually, this amp does have a polarity switch. It's combined with the rotary power switch. The Antihum-1 position selects one of the death caps and the Antihum-2 position selects the other death cap. Magnatone did lot's of clever stuff like this.
Here's a larger pic of my modified power supply chassis. May give you some ideas...
I've just done a side by side comparison of the M10 and M10A power supplies. They are different enough that you can't use my power supply as a guide to work on yours. I'm removing the pic of my power supply. Sorry...
-
Lucky find - especially as its unadulterated, and with all the magic bits still intact!
It is in your amp's best interests to assess which parts really do need to be replaced asap (due to age and liklihood of collateral damage), and to confirm that all circuitry is pretty much working correctly and safely, before turning it on for actual guitar playing use. A bit of proactive effort should give you peace of mind for a while.
-
So this is a dumb question, besides polorized vs non-polarised. Does it matter what material the capacitor is made from. I’m looking for those .01uf 400v. But I’ve seen aluminum, paper in water, ext.
-
Material doesn't matter to me. The Mallory 150 series capacitors - 630 volt, would be a good choice for replacements in that amp. And they are reasonably priced. You can find them right here in Hoffman's store.
I didn't replace any caps (other than electrolytics) in my M10A. Do you suspect your caps are bad? Why? I would not replace the caps unless I knew they were bad. The point to point wiring of that amp makes it hard to change components and do a nice looking job. Lot of tedious work.
-
Those were the ones I was looking at, or those mojos. If you look at the 4th photo, in the center that blue cap is pretty much got the internally popping out. With the other ones it’s hard to see due to photo size but the caps look like they have blisters. I have some more testing to do, I tested the main caps for voltage after unpluging the amp, I hadn’t turned it on in 24 hours. I expected some residual voltage but there was none, how long do the caps usually hold voltage?
-
If you look at the 4th photo, in the center that blue cap is pretty much got the internally popping out.
That is an electrolytic. It may be a non-polarized electrolytic, hard to tell from the small pic. The value is not going to be .01 @ 400V. I would replace it.
-
That one is 25uf at 50v, there’s two of them so I planned on replacing both. The blistered ones are the ugly brown ones, those are .01uf 400v.
-
Points of maintenance on these amps:
- Clean and re-tension the molex type connectors for the umbilical wiring harness. Besides carrying various B+ supplies, ground, and bias supply the connectors carry the filament supply and 120V main supply. If these connectors are dirty or loose they'll cause problems including burning up the connector itself.
- Replacements for the odd ball power switch are nearly impossible to come by so make sure to clean the contacts as best you can with deoxit.
- If you ever need to replace the 7189A power tubes: You can't use a 7189 in place of the original 7189A unless you jumper pin 9 to pin 6 on the sockets.
- If reverb doesn't work, examine the 1uf/450V coupling cap on the driver.
It is great to hear that (1) you got a deal and (2) you dig the amp!
-
The brownish waxy covered caps (0.01uF 400V) commonly leak, but that may not lead to collateral damage depending on where it is used in the circuit. I only have a M10A schematic - is there an M10 online?
As far as protection from major damage to power and output transformer from a bad 7189A, or loss of bias, the 100 ohm 1W common cathode resistor may be a friendly poor mans fuse and fail open in time to save a winding.
Are you ok to safely check the circuit voltages shown on the M10A schematic? That doesn't cover other voltages worth measuring for nominal levels. If there was a modification to make, I'd suggest adding a resistor between each 7189A cathode at that common 100 ohm resistor - it would allow you to measure individual cathode current and therefore confirm that the 7189A's are not too unmatched, and those added resistors could be made a value and power rating to provide a better poor mans fuse in case of tube failure.
-
Well I replace the cord, removed the death caps. Replaced the bad 25uf cap. And then plugged it into a properly grounded outlet. Sounds silent(little scratchy due to knobs). No fuzz. Did notice the stereo input doesn’t seem to work right. Both individual channels seemed to work good. But then after about 10 min, the volume just died. Took it apart, no funny smells, no obvious damage. Wondering if I have bad preamp tubes. They are the original mullard tubes.
-
I only have a M10A schematic - is there an M10 online?
It's here in Hoffman's Schematic Library. Follow the link at the bottom of this page...
-
Did notice the stereo input doesn’t seem to work right.
You will need a stereo guitar with a stereo cable for that jack to work properly.
But then after about 10 min, the volume just died.
Could be almost anything. Tube substitution is an easy start if you have spares. Otherwise it's time to check voltages. I'd start in the power supply, then check voltages for all tubes (every pin). Post your findings and we'll see if there are any clues.
-
Did notice the stereo input doesn’t seem to work right.
The stereo input is a stereo jack input (tip=channel 2 ring=channel 1 sleeve=ground). you can use a mono plug, you just need to pull it back out 1/4", it'll engage both tip and ring.
Wondering if I have bad preamp tubes. They are the original mullard tubes.
seems unlikely. check voltages against the schematic. the schematic doesn't list voltages at the plate or cathode of any of the triodes, but you can verify a drop in voltage across the plate resistor. If there is no drop in voltage, there a problem at that stage.
did you clean & retention the molex plastic connector on the umbilical cord? both male and female ends? a bad connection on any of those contacts could result in an amp that makes no sound.
-
I fixed it, I made an amateur mistake, I installed the cap backwards. The old cap was marked differently than the old one, double check the schamatic sand relized I messed up. Flipped the cap and it sounds glorious.
-
It's interesting that they tweaked the varistor LFO modulation level between the M10 and the M10A, effectively increasing the vibrato intensity range, although the stage power supply levels changed a bit too.
-
Magnatone did a lot of interesting things. For example, look at the M10A schematic, in particular, Channel 2 preamp. The first triode is fixed bias (How often do you see that!) The second triode is grid leak biased. And the third triode is cathode biased. Those engineers were having fun. I wonder what the thought process was.
Then look at the power amp. It's fixed biased, but there is also an unbypassed 100Ω cathode resistor. Was that for tube protection in case of negative bias supply failure? Or just to add a bit of local NFB? Or to make it easy to check tube current with the available voltmeters of the day? Or just another 'mixed biasing scheme? I'd love to hear from the engineers on that too.
Magnatone was definitely not just cloning other amps. They had some interesting ideas.
-
I want to thank you guys for the help! Now time to do some more reading. Then build an amp in the future. May start by building a few pedals first.