Hybrid Music Man 212-65 1980 Build - Learning As I Go, Some Questions

[keithb7]

I recently bought a 1980 Music Man 212-65 amp.
All the parts in the circuit appeared to be original. I replaced all the e-caps and just about all the diodes. I replaced the two bipolar BJTs in the phase inverter. The new BJTs contributed a massive improvement in the amp's tone! I picked up 5 new LM307H op-amps (round form-factor).

Through trial and error I have confirmed there are some contact issues within the op-amp sockets. Oxidation likely I assume. I tried spray-cleaning the contacts in the op-amp sockets. The sockets are very small. The spray I used doesn't seem to be having an effect. Is there a contact spray you recommend? Deoxit maybe? Alternatively, anyone know where I could source replacement op-amp sockets? Searching the internet I have not had any luck with the round sockets needed.

I have set the power tube bias to 25 mV as per the Music Man instructions. The amp is really sounding great.

[g1]

The bias is approx. 6.5mA per tube. That Musicman spec. is 25mV across each of the 3.9 ohm resistors. (see attached)
Changing the transistors should not have made a difference to the tone. That is why I'm wondering if maybe you used a bias probe or something and set for 25mA per tube? If biased that hot, the tubes will not last. The very cold bias is how they enable the tubes to withstand the 700 volts on the plates, even in the models with 6L6's.
If you set for 25mV across those 3.9 ohm resistors and still found a massive difference in tone compared to the old transistors, I would think they must have been defective.

Deoxit is probably the best bet for the IC sockets.

[keithb7]

Thanks g1. I did set the bias at 25 mV at each resistor as per the Music Man instructions. I did not use a bias probe and set it to 25 mA. I did my various repairs in stages. Set the bias, then stopped to sound check the amp. Then changed the BJTs and again sound tested the amp. The BJTs did offer a much improved sound. I assume they were worn out, burnt out I guess as they were original from 1980.

[g1]

That is interesting that they were functional but had poor sound.
Good luck with cleaning those can IC sockets, it may be hard to find replacements. I don't think I have ever replaced one, they were already on the way out when I got started.

[Tassieviking]

If you do a search for TO5 8 pin socket or TO5-8 socket you will find some, just check the measurements if it fits your board.
Otherwise Digikey has some :https://www.digikey.com/en/products/...005000/1212172
Mouser : https://www.mouser.com/c/connectors/...stor%20Sockets

I would just use some gold plated 2.54mm SIP sockets and use one pin at a go, they come in long strips and you just cut out as many as you want.
https://www.taydaelectronics.com/con...ingle-row.html

[keithb7]

Latest update on my Music Man amp:

I’m looking for ideas on where to go next with my 212 Sixty-Five. Its a 1980 with solid state phase inverter. I’ve studied the schematic extensively, 2475-65.

I believe I have the circuit sounding very close to as it did new. Yet I’ve no other MM amps here to listen to and compare. I’m getting un-appealing break up at low volume. As low as 1-1.5 on volume knob on either channel. I have replaced a lot of the typical wearing & aged parts and more. I’ll provide a list of what I have replaced. Everything I’ve replaced seems to have improved the amp with each step.

I sourced 5 new round op-amps from China. I subbed them in where needed. I swapped around op-amp positions multiple times to ensure all are working properly. All pre-amp functions seem to work nicely. I checked and tested resistors and capacitors around each op-amp. Measured good.

Replaced every e-cap in the amp including main tube power supply caps, all e-caps on main board, bias board and phase inverter board.

Replaced almost every diode with brand new. Including diodes in main board, bias board and phase inverter board.

Swapped in new EL-34 power tubes and set bias to 25 mA.

Replaced both Bi-polar PNP chips on the PI board heat sinks.

Cleaned all pots and op-amp sockets with de-ox-it. I replaced channel 2 dual-gang volume pot. Replaced the master volume pot. I inspected all connections with a light and magnifying glass.Touched up any that were suspect. I hooked up a different speak cab to try other speakers. No improvement.

I’m leading toward the issue being after the pre-amp circuit as the break up occurs no matter what channel I use.

Measuring the b+ voltage:
Low position 498V
Hi position 746V
No apparent difference in sound between hi and low settings.

Pre-amp voltage is nicely sitting at 16V.

I have not yet replaced the dual op-amp LM1458 on the PI board.

I’ve experimented with various single coil and hum-bucker guitars. Trying both inputs. in both channels. No improvement noted.

A bit about my experience:
I am an amp electronics hobbyist. I have some tools and decent tube amp experience. I’m self taught. I read books and am always learning. I’ve successfully built about 10 tube amps. Repaired many others.

Perhaps my next step is to again insert a sine wave signal and try tracing it with my cathode-ray oscilloscope. Also replace the dual op-amp on the PI board. Maybe OT is weak or saturating? I am unsure if I can test an output
transformer with a DMM.

Any other ideas on where to go next is greatly appreciated.
Thanks in advance.

Keith

[g1]

Can op amps are obsolete. Any coming from China will be fakes. Did you have the same issue with the originals? The ones from China may work, but they are not 'real' replacements and you can't trust them for sound quality or reliability. So you really can't count those out unless you can inject signal after those. Maybe lift the wire at the master wiper and put signal in there?

When you say 25mA bias, do you mean the 25mV across the emitter resistors (approx. 6mA per tube)?

[keithb7]

When you say 25mA bias, do you mean the 25mV across the emitter resistors (approx. 6mA per tube)?

Sorry I keep saying mA in error. Yes 25 mV. I agree with your point on the op-amps from China. I'm not sure where I am going to get proper replacements.
I seem to recall hearing about an adaptor. To solder into the board. TO adapt from round to newer square op-amps. I'll take some time to review that option.
Thanks.

[keithb7]

I think this adaptor is the one I would need. Confirming I solder-suck the bottom of the board at each current vintage op-amp socket. Remove each socket. Install these adaptors & solder, then update to square op-amps.

https://www.cimarrontechnology.com/p...o-to99-adapter

[The Dude]

......Perhaps my next step is to again insert a sine wave signal and try tracing it with my cathode-ray oscilloscope......

Yes, that.

I'm also not clear on the op amp thing. Did you get op amps from China or just op amp sockets? If you just got sockets, it shouldn't be a problem if the legs are fitting tightly into the sockets and making good connection. If the original op amps are good and you just have socket issues, you could also just toss the sockets and solder the op amps directly into the board, which would be more reliable anyway.

[Helmholtz]

I understand the only problem left is "break up" at low volume, otherwise sounding fine?
What exactly do you mean with "break up?

Get out your scope and measure power output into dummy resistor.

BTW, which of the schematics out of the list applies?

[g1]

Part of the problem with newer type op-amps is they might be too good. That sounds counter-intuitive but some older equip. will sound harsh with faster op amps that also have higher bandwidth. So the Chinese parts could actually be modern spec in an old style case.
There are NOS parts available from US sellers in the 5 to 10 $ each range.

[keithb7]

The early harsh sounding break-up is the main problem. I agree, it is likely due to the imitation op-amps I sourced from China. I'd be interested in learning about NOS sellers who are offering originals for sale. I spent some time searching on-line, I was not able to find any. To clarify, I did not replace any op-amp sockets. I cleaned them all up with de-ox-it. I did replace some op-amps with the Chinese ones I found.

I did find some posts where Music Man amp owners adapted up to 8 dip modern square op-amps. They experimented and had good results with various op-amps.

I don't have a large 4 ohm dummy load resistor to test the amp's out put power. I'll look into that and see if I can learn how to do that.
Thanks for all your input so far. I have some ideas and some needed support. Thank you.

[g1]

I'd be interested in learning about NOS sellers who are offering originals for sale. I spent some time searching on-line, I was not able to find any.

On ebay I searched for LM307H. Scrolling through the list I ignore anything shipping from China and check the ones from US sellers. From there you can get a pretty good idea of which are NOS parts. Depending where you are, if you can drive across the line for parcel pickup, you can save a lot of the shipping costs.

If these links work, they are a couple examples of what I mean:

https://www.ebay.com/itm/323560802524
https://www.ebay.com/itm/293655065618

[Tassieviking]

I have used this seller once before for hard to get parts, and I got what I was after.
He has the LM307N which I believe is the DIL version so you will have to get the adapter boards, or add copper leads to an 8 pin DIL socket and make your own adaptor.
http://www.worldwideelectroniccompon...EC_CATALOG.pdf
http://www.worldwideelectroniccomponents.com.au/


Here is a USA company that are supposed to have 53 LM307H in stock but you have to get a quote from them first:
https://www.bristolelectronics.com/lm307h-nsc