HD-150 sounded "Lifeless & Wimpy"

I recently repaired a 212HD-150 combo for a friend of mine, Country singer/songwriter Tom Mann:

Tom Mann's MySpace Music page

Tom complained that the amp no longer sounded as full and ballsy as he remembered it.



Since the test date (hand-written on the PCB in black marker pen) was in 1980 and the amp had all its original electrolytic capacitors a cap job was at the top of my "to do" list. However, I found several other problems.

At one time this amp had a catastrophic failure of an output tube which caused a grid resistor to smoke (as evidenced by remnants of a black smokey deposit on the remaining components). Also, multiple types of output tubes were evident (with one tube clearly lacking in the vaccuum department). Even though the amp might have worked with these tubes, they "looked" so tired so I opted to replace the entire set with a matched quad of JJ 6L6GTs.



When I benched the amp I found that it had only 83 watts of output on the High Power setting with severe ripple of the High Voltage power supply at clipping. I also found an instability that caused the amp the intermittently chirp and whine at or near clipping. It was obvious that this amp wasn't healthy and confirmed that the amp couldn't sound right!

First, I installed the output tube stability mod for the 75 & HD150 models as outlined in the Music Man Factory Service Bulletins. This factory mod is an added resistor (per pair) to stabilize the output stage when a non-Sylvania 6L6GT tube is employed. The previously noted instability disappeared nicely!

I also found that the 22 Vdc Screen Grid supply was at ground potential(0Vdc)and then the light-bulb illuminated above my head. Ahaaa! When Zener diodes fail they invariably short circuit and this is precisely what I found... the 22V Zener was shorted. Since I didn't have a 22V Zener diode handy, I "totem-poled" (stacked in series) a 15V Zener and a 6.2V Zener for a "close-enough" 21.2Vdc.

So after adding the stability mod, fixing the screen supply, cleaning/treating all pots & jacks, performing a complete "cap job" and installing/biasing a new matched set of output tubes the amp was stable and delivered 156 watts RMS into 8-ohms at 120Vac input.

The preamp wasn't noisy, the reverb and Phasor circuits worked as advertised, all the controls and switches performed their functions correctly and the amp sounded full-bodied and authoritative & ballsy like an MM HD150 should!

Total parts cost at around $135.00. Now she's ready for another 30 years of service!


Steve

Nice catch. Glad to see that it survived. But MY GAWD-156 watts? Who could turn that mutha up!

Hi Mike,
Hey buddy, Spinal Tap would..!! (Their amps go up to 11)

Huh? What did you say? Can't hear you!

My HD130 intermittently blowns a fuse when I turn it on low power. I normally use it to play my bass through it in a band and switch it on using the normal high power and its fine.(has not missed a beat in 10 years).I have just started using it a home playing a Telecaster through it