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craig merluzzi

Tremolo Problem HD130

While biasing my 1974 MM 212 HD130 after new tubes, I mistakenly tweaked on the tremolo trim pot and now the tremolo circuit isn't functioning correctly. The problem seems to be more with the intensity rather then speed although both adjustments aren't working correctly when tremolo is switched on. All the caps and resistors appear intact, so I'm thinking I may have damaged the TR1 or TR2?

I'd like to try replacing TR1 & TR2 to eliminate those components, but don't know how to identify them? I'm surprised at how little component information is available. I'm looking for a component layout diagram for a HD130 amp and associated component parts list - specifically TR1 and 2. Any help would be greatly appreciated.


Troubleshooting Tremolo

There should be two small round blue trimpots mounted flat on the main board on the right side of your chassis, viewing the amp with its control knobs facing you.

The one on the far right is for adjusting the amps power tube bias, the more inboard one for adjusting the tremolo.

The tremolo trimpot only works within a narrow range of its rotation, so I would first see if you can dial it back in. There is a very specific technical way to do it requiring an oscilloscope, I just do it by ear... Turn the amp on, plug in a guitar, set the Volume low, Speed to 3 and Intensity to 5. Using an insulated screwdriver that fits the trimpots slot, rotate it fully counterclockwise, then slowly back clockwise, while hitting the strings of the guitar. When you hear strong tremolo, you are in the right area. I generally set it so that from 1 - 5 is "normal" tremolo, 6 and higher produces the doubling tremolo unique to MM amps. Basically set the trimpot where you like the range of sounds you get throughout the the rotation of the Intensity control.

Probably a good idea to make sure your amp is properly biased first, that seems to make a difference as well.

If you still have no tremolo, I would suggest testing the trim pot. It's a 250K pot, set your multimeter to Ohms and measure across the trimpot's contact points at various pot rotation spots to see if it varies from low to high resistance.

If the pot is good, one or both tremolo transistors (2N3391, 2N4091) may be shot. You can get them at Mouser or Newark electronics, possibly Reverb or eBay. I may even have a few spare.

As always, be careful and good luck!


Hi, I have the same problem

Hi, I have the same problem as you. Tremolo is not working and there is a low
volume distortion sound on background of channel 2 even with the volume on 0.
I checked the potentiometers and they are OK. The orig. transistors are expensive on eBay, is there any correct replacement for the 2N4091 or 2N3391? Or voltages on them please, that would be excellent.



First things first

Have you cleaned all the pots, jacks, tube and IC sockets with electronic contact cleaner? Biased the amp correctly? Replaced all electrolytic capacitors? Do the cleaning and biasing first and see if the trem works after. If not, do the recap. If still no tremolo, replace the transistors.

Mouser has the 2N3391A for $.066 and the 2N4091 for $11.92, minimum of one each. Can find the 2N4091 for less on eBay.

Bill Roberts

Tremolo Problem

If you are getting oscillation at all then the transistors are fine the distortion and or no tremolo is most likely from leaking capacitors. They may still show a correct value even if you pull them and test them on a capacitance meter but they are probably bad because of age - leaking DC. You will need a capacitor tester that tests for leakage. Personally I wouldn't go through the trouble and just replace the 5 caps that are in the tremolo circuit. C17, C25, C28, C29, C30 on the schematic. 1 x .1uF 2 x .22uF 2 x .33uF. would recommend Illinois Capacitors Yellow Polypropylene Film less than $10 in parts.


Hi, yes, all cleaned, recaped

Hi, yes, all cleaned, recaped, biased. The channel 1 works excellent. ICs work, reverb works. 2nd channel has a low volume distortion on the background.

I am from EU /Czech republic/ and there is 20 USD shipping from Mouser. I would try eBay rather but you have no guarantee from receiving fake transistors. If you have the voltages that would help me a lot. Thank you.


Can put a meter on mine tonight

The schematic shows 16vdc feeding each transistor, can measure the actual voltage I am getting on a 130 I have on hand tonight. Meanwhile, are all the other components in that part of the circuit in spec?


Just to be complete...

...pull the two tremolo transistors out and see if it impacts the background distortion.


I measured the resistors, but

I measured the resistors, but only in PCB. Potentiometers checked with unsoldered leads. Capacitors replaced.


If you need parts


Lars Verholt

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