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Rebuilding my Tube Sockets on my MM210/65

Hello, I have a 1977 MM210/65 and I have just replaced all of the electrolytic caps
in the amp.

Now I would like to re-build my 2 tube sockets but I have no idea what value the diodes
are, there is one diode per tube socket. These are black with a silver ring around one end.
They look like the size of a small resistor ?

Also, there are 2 brown Mallory axial leads, capacitors, one per tube.
The spec reading I got off of the original ones read: .001-2000V

Then there are the resistors on each tube socket, I can read their color coding alright
but I don't know their wattage rating ?

And lastly, I'd like to replace the large cement 10 Ohm / 7watt resistor. I have found the
resistor, but it's rated at 10 watts, can I use it ?

Yes I printed out the schematic for this amp off of the web, but the actual drawing/artwork is
simply horrible, I struggle with eye sight issues anyways, but this schematic art is really bad.

I can't begin to tell you in advance how much I would appreciate some help with this stuff.
Thank You for gracious help.

Peace & Happy Holidays, Charles

mm210's picture


Here are scans of an original schem from my files. Have to leave now but I'll get back tomorrow and try to look up the rest of the stuff. Mike

PDF icon MM-65-orig.pdf704.17 KB

Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Hi Mike, I wrote you back thanking you for forwarding the better drawing/art work of the schematic
for my 210/65 amp.

So today I found some information on the Zener Diodes used as protection devices on these tube sockets. Apparently these Diodes are often referred to as Fly Back Diodes.

Zener Diodes from Mouser: UF-5408 rated at 3amps / 1000kv PRICE: .25 cents each
Also from Mouser and closer to spec than the one above is part number: 583-R5000 F-T rated at 200mA @ 5000kv
PRICE .76 cents each.

I still haven't figured out what the value is for the two capacitors, each tube has one of these.
The Mallory in my amp reads .001-2000v
So my question is .001 what ? pico fared or micro fareds or ???

Are the resistors on the tube sockets, called grid resistors ? Please tell me what they are rated at and what wattage
are they ?

I have done repairs on my amps for years, but I'm terrible at reading a schematic. I usually end up asking for help in determining what is causing my problem and then after finding out what I need to do to fix the problem I just in, never had a bad experience yet.

I hope things go as smoothly on this task.

Peace & Happy Holidays and thanks to all you guys who take the time to help guys like me, the world is a better place because of kind folks like yourself.

mm210's picture


Haven't had time to look up everything but those are NOT Zener Diodes. If you are describing theebig ones on the sockets, they are 5000v at 250mA, not zener. There ARE zeners in there but in the low voltage section. There was a discussion here about the replacements for them and I believe Lars found the ones they use in peavey amps for the same application to be 3000v. Search maybe for 5000v diodes and see what you get on this site. Mike.

PS-those are .001 uF@2000v.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Hi Mike and again THANK YOU for your help. As for the diodes...I'm referring to ones that are on
the sockets themselves. They are black with a small silver ring around one end and they are about
the size of a small resistor, ( actually about the same size as the resistors that are also on the tube

So then these aren't called Zener Diodes ? Ok, what is the proper name for these ? I understand that
these are used as a protection device from power spikes, is this correct ?

What I'm hoping to do is place an order soon for all of these parts with Mouser so that I can get them all from the
same source.

Also, do you agree that after 37 years of use and the amount of heat that these amps can generate that it would
seem logical to want to replace not only all the electrolytic caps, but the brown Mallory caps as well. Most of them
are all the same .047@600v.

Of course the 2 on the tube sockets are the exception as you so kindly noted, .001uF@2000volts.

BTW, did you see the pictures of my amp that I posted ? One totally custom & cool MM210/65 if I do
say so myself.

Peace & Happy Holidays, Charles

mm210's picture

What pics? Wouldn't change

What pics? Wouldn't change any caps unless they are electrolytics or they are bad.

mm210's picture

Search for 5KV diodes here.

Search for 5KV diodes here. They will answer some of your questions first.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Pictures of my 1977 MM210/65 Cool Custom if I do say so myself.

mm210's picture


Still no pics showing. Are you trying to attach them too big?


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Hi Mike, yes I believe the files are to big, but this says they were resized in order to post, so
I just hit the save box and figure that they were up-loaded ?

Peace & Happy Holidays, Charles

mm210's picture

I think the way the software

I think the way the software is working is that it resizes the pics and they are saved in the same place with the same file extension but they are NOT uploaded. You then have to go browse, pic them again and THEN upload. I THINK. This is all new software so we're all learning as we go.


A tip for tube sockets

Charles, this is a little off-topic but regarding rebuilding and tube sockets, I've found that cleaning the tube pins AND the receiver clips in the sockets themselves with Deoxit Gold (or similar product) facilitates a huge sonic improvement in my MM amp. It's about a non-oxidized metal-to-metal connection but usually we only think of plugs and jacks. I have a small brush that fits down inside the socket receivers so I can scrub the oxidation off in there and I wipe the tube pins with q-tips. This alone really changed my sound a lot, fattened it up and quieted it down. Hope this helps.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Thank you for the tip on cleaning the sockets. As it turns out I just did this very thing this weekend.

However, I still find it hard to believe that after 37 years of heat and use, that the .001@2000V capacitor, the 1500ohm resistor the MR250-5 diode, the 10watt ceramic resistor and the large gray round resistor going to a terminal strip don't need to replaced.
700volts of current and enough heat from the 6CA7 power tubes to fry eggs, it just seems like a good thing to do, maybe just as a preventive maintenance kind of thing ?

Does anyone agree or am just being paranoid ?

Peace & Happy Holidays, Charles


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

OK, can somebody PLEASE verify that I'm ordering the correct parts to rebuild my tube sockets on my MM 210/65 amp.
I'm FREAK'N out that I might blow the amp and myself up if I install a wrong part , yikes !

Carbon Resistors: 1/2 watt rated at 1500 ohms ( I want to stay original with the carbon type)
Capacitors: .001uF@2000volts (one on each tube socket)
Long round Grayish resistor, looks like some kind of ceramic coating: 1.5K ohms @ 5% NOTE: Isn't there a wattage requirement for this resistor ?
Large long rectangle shape White Cement resistor: 10ohms / 5watts / 10%
Diodes (one on each tube socket) 200 to 250mA @ 5000volts.

NOTE: all of these parts are available from Mouser.com

Peace & Happy Holidays, Charles

mm210's picture


DON'T use carbon. Use metal oxide on tube sockets. Replace what you want-it's your amp! The big 1.5k's are 10 watt. Everything else OK. Mike.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Thanks Mike for verifying the parts list for rebuilding the tube sockets.

In the beginning of all this, I failed to state why I thought it might be a good idea to do this rebuild.

When I bought the amp it was a complete mess. I immediately stripped it down in order to remove the ton of dust and crap that was in and on the amp. What I didn't notice was that somebody had broken off one of the tube's guide pins and when I went to reinstall them I assumed I had the tube in it's proper alignment with the socket since it went in so easily.

As it turns out I did not have the tube inserted correctly and within a few seconds I started seeing smoke.

I immediately shut the power down and pulled the chassis back out, but I really couldn't tell what was smoking, only that the smoke had come from that particular area where the first tube is located.

This is why I'm concerned about replacing the very few parts on the sockets and this is why I was trying to get your's or anybody else's opinion on this. I just feel that something (s) have been compromised ?

Peace & Happy Holidays, Charles

bill_moore's picture

Cvalona, I did have a shorted

Cvalona, I did have a shorted diode when I restored my RD100. The other tested fine, you might test yours before replacing.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Hi bill_moore, hey can you please get back to me on how to test the diodes ? I'd really like to check mine out before removing them.

Peace & Happy Holidays, Charles

mm210's picture

If they are shorted, they

If they are shorted, they will pass DC resistance test in both directions. Should be only one.


What those plate diodes do.


The purpose of those diodes on the tube sockets is fault protection. When the amp functions properly, even at Hi Power setting,, those diodes are reverse biased (inactive) and do nothing. They come into play only if there is a catastrophic failure of the HV power supply or the output transformer or the speaker. Any of these type of failures can cause a huge negative voltage spike to appear at the Tube Plates. At that point, the diodes become forward biased (active) and conduct those negative spikes to ground. So what is the correct value for those diodes? Anything greater than 3 to 4 times the maximum plate voltage should be more than adequate. 700VDC x 4 is 2.8kVDC. Lars picked a good number at 3kV Peak Inverse Voltage. The next obvious question is why did MM choose 5kV PIV? Who knows? They probably got a really good deal on 5kV PIV diodes or it's what they had on hand in lab stock when the engineers designed the circuitry. In short, any diode rated at 3kV PIV at 250mA or greater should work fine. Make sure to note the polarity of the diodes when you change them out. If you accidentally put them in backwards, they will short out your HV Power Supply as soon as you switch on the power causing the smoke to get out. The cathode (the side with a line) of the diode must connect to the tube plates and the anode must connect to ground.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Hello mm, THANK YOU for getting back to me, however PLEASE tell me how to to run this test ?
I have no electrical engineering knowledge as to how to make this test. I have a DMM...

I have some basic electronic repair skills, but testing a diode isn't one of them.

Also, do I need to remove the diode(s) before testing them or can I test them while they are on the tube sockets ?

Also, if one or both are shorted, does the amp still function properly, but it does so without any protection that these diodes are suppose to offer ?

Merry Christmas mm, Charles


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

Hello mgriffin155 and THANK YOU so very much for the information regarding the diodes.

So Mouser sent me 2 different ones: 1st) 200mA@5000kV 2nd) 250mA@4kV

I spoke with directly with Kevin Nelson and he confirmed either of these 2 values would work as replacements.
I'll most likely go with the 2nd one that I have listed above.

So inclusion, it appears that these diodes really don't have a way of failing unless somebody puts one in backwards ?

As I understood your narrative above, the diode unless damaged by incorrect installation functions or it doesn't ?
If my amp is working and it is, I'm concerned about whether or not I "need" to change the original diodes out ?

Some local tech here told me that the diodes either work or they don't, but what he didn't say is whether the amp would work
or not if the diodes(s) went bad ?

Once again thanks to everyone who has helped me with this stuff, I can't begin to tell you how much I appreciate your efforts.

Merry Christmas, Peace & Best Wishes for the coming new year, Charles


To change or not to change

Hi cvalona,

Since the amp is working, I recommend you leave the diodes alone. Those protection diodes rarely fail due to old age. Their only job is to protect the tubes and adjacent circuitry if the HV Power Supply and/or output transformer and/or speaker catastrophically fails. When I restored my 1981 MM112-65RD, I left the original diodes in. I did test them as described earlier by one of the contributors and both tested good.

Let's go over "How to test solid state diodes". Switch the amp off and unplug from power. Wait an hour so the Hi Voltage Power Supply can completely bleed off. There is very hi voltage where you're getting ready to work and you do not want to get shocked. Get your DMM and set it to Ohms on the 10MegOhm range. Put the red lead in + and the black lead in - on the DMM. Connect the red lead to the diode on the end with the silver ring. Connect the black lead to the other end of the diode. The reading should be very hi resistance on the DMM for a good diode. If the reading goes to near zero, the diode is internally shorted and must be replaced. Now connect the black lead to the end with the silver ring and the red lead to the other end. You should read a very low resistance for a good diode. If the reading is very hi resistance, the diode has opened internally and should be replaced. To summarize, hi resistance in one direction and low resistance in the other direction is good. Hi resistance or low resistance in both directions is bad.

Will the amp work if the diodes are bad? It depends on the failure mode. If the diode has "opened internally", that is it will not conduct current in either direction (very hi resistance), the amp will operate properly but will be unprotected. If the diode has "shorted out" internally, that is it will conduct current in both directions (very low resistance), the amp will not work since the tube plates will be shorted to ground.

Hope this helps.


Rebuilding my Tube sockets on Musicman 210/65 with 12AX7 Driver

mgriffin 155, Holy gollie wow, THANK YOU so much for the help ! This answers all my questions and concerns about the diodes.

As I have stated in an earlier thread, the whole reason for any of this to be taking place is because I accidentally installed one of the 6CA7 power tubes with a broken off guide pin into tube socket number 1 and I saw smoke in a matter of seconds, but luckily I was able to shut the amp down quickly.

I then pulled the chassis back and examined the components on that tube socket and really didn't see any physical of damage
from over heating except the 1.5k ohm ceramic wire wound resistor that connects to pin #4 and to a small chassis terminal strip. It looked very strange and had bubble like imperfections on it.

I also want to point out that the amp still worked great to my ears, clean, bold and plenty of punch. I even did a gig the same night with it. NOTE: I figured out how to install the tube with the missing guide pin into the correct socket orientation so I was able to use the existing tubes, thus saving my new JJ's until I could have a deeper look.

Having bought all of the necessary parts from Mouser.com a few days ago I decided to take another look at these tube socket
components. However, before I removed anything I got my Bias tester out, it's made by Amp Head Inc. and I decided to take a current reading on both tubes.

Tube one where the broken tube was installed read 34mA. The other tube read 29mA.

I turned the Bias adjustment pot/control, the pot nearest the power transformer just a wee bit in both directions trying to get both tubes to read the same. No matter what I did I couldn't get the tubes to balance so I left the control set to where it was
29 / 34.

Remember now, I just played a 4 hour gig with this amp and it sounded great.

So back to today...I decided to remove the 1.5k Ohm resistor that I mentioned above and install a nice new shinny ceramic model made by Ohmite. I didn't replace anything else, not even the diode.

After I installed the resistor, I hooked up the Bias tester again, connected the amp to the speakers, turned the volume on both channels to zero and let the amp warm up for 15 minutes.

Took another reading heres what I got: Tube #1 (where the bad tube had been and where I saw smoke) read 29.8mA and tube #2 read 29.2mA. I'd say this is plenty close enough based on what I've read.

Buttoned the amp back up and fired it up, Fantastic !

I want to apologize for seeming so long winded about all this, but I truly believe that being thorough in explaining ones trouble shooting scenario that it might also help others who like me, love to keep their gear in tip top condition, but need as much clarity about performing these kinds of tasks as possible.

You my friend have done an outstanding job with your explanation and attention to detail, THANK YOU once again.

Merry Christmas, Peace & Best Wishes to you and yours in the coming new year, Charles

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