Hello.
New member, first time posting.
I recently acquired a MM 410-65 for fairly cheap. It's in very good cosmetic condition for 40 y/o amp but has several functional issues. It appears to have had the 12ax7 phase inverter at one time, but has since been removed. I'm guessing that means it's either an early one that had been factory serviced or a transitional one where they were using leftover chassis from the tube version. Also, has a set of Groove Tubes which I'm guessing aren't original.
What's wrong with it:
It appears that only 2 of the 4 speakers produce sound when plugged into the amp. I tested each speaker individually and 3 of the 4 work. Does the wiring schematic and each speakers' place in the signal chain affect that? The bad speaker is upper left when looking at the amp. The speakers that produce sound when connect to the amp are upper right and lower left.
The reverb doesn't appear to work. Tank is connected, but the knob does nothing. The foot switch is busted as I can't get it turn vibrato or reverb on or off even when I swap the leads. What's likely the culprit with the reverb?
Amp just sounds overall a bit muddy even with the treble cranked up all the way, both channels. Is this something a basic service can fix?
A couple general questions about it:
-What was the make of the speakers and their specs? They're black on the rear with no indication of wattage or impedance.
-How were the speakers wired (e.g. parallel, series or s/p)?
-If I were to replace the speakers, what's a comparable replacement? I like bright and chimy, and would be happy if it sounded like a Super Reverb.
I've heard this amp running through an Echoplex (just for the preamp) and it sounded phenomenal. I know my amp has that potential with a little work/investment.
Thanks.
-Doug
Series-parallel
Hi Doug,
since you get sound out of one pair of speakers, it means they are wired series-parallel. Two pairs of series-connected speakers are run in parallel. This will yield an 8 ohm load for the amp if all units are working. The original speaker in this model is an Eminence 25W type with a ferrite magnet. Since you are only putting out ¼ of 65W = 16¼W into each speaker at the most, you don't have to spend a ton of money on each unit. Might be worth it to just have all the originals redone for a tighter sound. From a personal perspective I've had good results with Eminence speakers - such as The Copperhead. If you get a slightly higher powered speaker you can run the amp a bit hotter without it being too loud.
PS - the tube socket with the plate over it persisted in later amp models using the two-channel amp chassis, even if that amp model (e.g. Seventy-Five) never had a tube phase splitter.
Good luck
Cheers
Lars Verholt
Muddy
Lars has you pointed in the right direction for speakers. If the amp sounds muddy or thin, it may be ready for new electrolytic caps and fresh zener diodes. They get tired with age and make the amp sound weak. Have all of the power supply voltages checked and electrolytic caps inspected by a qualified tech. Fix any thing out of spec or that looks fried. The reverb problem may be a broken tank and the tremelo issue may be a bad FET. Get all the power supplies in good shape first and then troubleshoot the tremolo and reverb. The schematic for your amp can be found in the DOWNLOADS AND RESOURCES section at the top of the page,
Good Luck,
-mgriffin
Thanks, Update
Thanks for the replies. I used a multimeter to find the fault in the reverb tank. Ordered a replacement that's supposed to be very similar to the original. Also, bought a used Jensen C10R (Ceramic/ferrite, 25 Watts, 8Ohms) to replace the blown speaker. Just based on my research and what Lars wrote it seemed very similar in spec to the original. Thoughts? I would love to upgrade the speakers at some point but just wanted to get a baseline for now. Once I get the amp fully functional again, I'll see how it sounds and take it to a tech to have thoroughly checked out.
Thanks again.
-Doug
410 65 Speakers and Reverb
I have a pair of the Ferrite FA-10-GA speakers that I bought a while ago but never used. If you are interested, I would be willing to sell them to you. For the Reverb, Amplified Parts is a good source for MOD replacements for the original Accutronics reverb units. https://www.amplifiedparts.com/search/node/music%20man?cats%5B0%5D=260 You may be able to refurbish the original tank by desoldering the joints.
Ewan
FA-10-GA
Ewan,
Thanks for the offer and information. I looked up the FA-10-GA speakers you mentioned but couldn’t find anything. What can you tell me about them? Are they similar to the F10G speaker that I read originally came in the 410-65? Model numbers seem very similar. Let me know.
Thanks.
-Doug
Ferrite Speakers
Doug, Sorry for the confusion. The part number that I quoted was for the replacement speaker from the 1975 Music Man Price List. In 1976 the number was changed to F10G. Regardless, these speakers were repainted at some point so I’d have to scrape some paint off to find the part number and date code. It was an eBay purchase and they were reportedly pulled from a 410-65. They look exactly like the other Music Man 10” speakers that I have, except they have ferrite magnets. I had planned to build a 210 cabinet with them but they may be of more use to you. If you want pictures email me at emtytler@gmail.com
Good luck on your project.
Ewan
Speakers Fixed...Sort of
I put in the Jensen C10R today and all four speakers are working again. Only issue is that the top two (Jensen and original) are like 10+ dB louder than both the speakers in the bottom row (both originals). I dB metered each speaker separately so it’s not that the new speaker is louder. It’s basically the same output as the original next to it. Any thoughts about what could be going on?
Thanks.
-Doug
Impedance and resistance
Doug,
This could be an impedance and resistance issue. Are the pairs of speakers wired in parallel and the pairs are then wired in series? If one pair has a lower combined impedance than the other pair, it will pass more current and be louder. I checked the specs on the C10R and it is nominally 8 ohms but the DC resistance is given as 6.7 ohms. If the stock F10G speakers have a higher resistance, say in the 7.0-7.2 range that could explain this, To test this, measure all the DC resistances and try to make the total DC resistance of each pair as close as possible. If that balances everything, then that was the problem. Otherwise, make sure that all the connections are good on the quieter pair.
Ewan