Music Man HD-130 distorting at low volume

[ahdw]

Hey all! Was excited to recently find one of these old MM HD 130 heads locally at a good price. I have wanted one for a while to compare to my V4B reissue for practice/small gig duty. I play in an indie rock band that can get quite loud with two Twin Reverbs and a very forceful drummer, and the V4 just baaarely runs out of headroom/muscle in most of the scenarios that I try to use it in. I have heard the 130s are all about being loud and clean which sounds like the ticket.

Anyway, the one I found is an "HD-130 Reverb" model that was originally aimed at guitarists but has an identical power stage to the non-Reverb model as I understand. It has EL-34s installed. I tried to use it at practice the other night and followed the trick that I've heard repeated for these amps of diming the master volume and using the gain knob to control the overall volume via the preamp, in order to avoid the nasty preamp distortion (which I can confirm sounds quite trash.) With the master at 10 and the gain knob right around 2-3 I had more than enough volume, but I was getting a fart-y, strangely compressed sounding distortion when I dug into notes a little bit. I tested it afterward with my passive MIJ Mustang bass, no pedals, and was still able to get this strange distortion just from playing heavily at a quite low gain. It does *not* sound like the bad preamp distortion, it's more like a "running out of headroom with your cab/speakers" sort of sound even though I know I'm not anywhere near the limit of the amp or the cab.

I am going to have it benched by my tech as soon as I can but in the interest of trying to better phrase my observations to him, I was wondering if anyone here has experienced this on the 130. I am fairly new to tube amps and I feel like it could be something as simple as needing a re-bias, or as complex as a full cap job or similar since it is a half-century old amp at this point.

 

[Mike Caldwell]

Just for the sake of double checking try another amp with the cabinet one more time.

 

[ahdw]

Just for the sake of double checking try another amp with the cabinet one more time.

Done that and tried the amp with a different cabinet as well.

 

[ajkula66]

If it's a piece from the '70s, it was originally spec'd with 6CA7 tubes which are very similar to EL34 but not quite the same. I'd start with checking the tube condition and bias, but I'm not a tech, so take this with a grain of salt.

Good luck.

 

[Wasnex]

The problem could be caused by lot's of things. Your tech will need to troubleshoot.

These amps run unusually high plate voltage and are said to be hard on tubes.

I wouldn't expect it to be much louder than your V4B. BGM Amp Lab measured the reissue V4B at 111W clean, but said it does not have much dynamic headroom above this.

Ref: https://www.bassgearmag.com/amp-lab-ampeg-v-4b/

I found one claim of an HD130 making 140W clean. 111W to 140W is 1dB. It's possible the HD130 has more dynamic headroom.

I do think the HD130 will have noticeably different tone and OD characteristics. AFAIK, the HD130 is probably better for clean than output drive.

 

[agedhorse]

The drive circuit on the 130HD uses a unique cathode pull-down drive circuit with the grid undriven except for POSITIVE DC bias. The current flow through the cathode raises the cathode voltage across the driver transistor. The grid then becomes negative with respect to the cathode. The transistor functions as a current source.

I would expect the real world power to be around 100W (maybe a little more) but the overdriven characteristics of this particular topology may not result in a particularly pleasing distortion.

EL34 tubes may be different enough than the 6CA7 to require significantly different bias, as well as other possible anomalies, I don’t know if there is enough range of this bias circuit.

This is a different enough power amp topology that I wouldn’t assume anything about the performance except that I don’t recall any traction of this amp in the power amp overdrive/distortion world.

 

[agedhorse]

An additional comment… most techs are going to choke on these amps unless they truly understand how tubes really work at the fundamental level.

 

[Stylee Mousse]

My original power trio band had my V-4b and 215 “against” an hd -130 twin guitar amp. It worked fine. Two twins? If they aren’t restrained you may need an SVT. Just saying…

 

[ahdw]

An additional comment… most techs are going to choke on these amps unless they truly understand how tubes really work at the fundamental level.

Thank you Wasnex and Andy for the sage wisdom, as always. Truly a privilege to have such expertise in the forums. This quoted bit is also something I have heard before (just exploring other threads), that it’s worth finding a tech who’s highly experienced with Music Man specifically. That may be difficult to do locally but I’ll definitely be looking into it.

My original power trio band had my V-4b and 215 “against” an hd -130 twin guitar amp. It worked fine. Two twins? If they aren’t restrained you may need an SVT. Just saying…

I have an early-mid 70s SVT and I absolutely love it, it’s just such a pain to carry to and from gigs. Our drummer is a weightlifter and doesn’t seem to find it troublesome, and has offered to move it for me before in order for me to use it more often. Maybe it’s time to swallow my pride.

The V4B/HD-130 exploration has primarily been in the name of finding something lighter anyway. I have SS stuff too but I am happy to admit that the mystique and mojo of tube gear is the primary reason why I mess with it in the first place.

 

[ahdw]

EL34 tubes may be different enough than the 6CA7 to require significantly different bias, as well as other possible anomalies, I don’t know if there is enough range of this bias circuit.

I have heard that the high plate voltage of these amps specifically is (relatively) within spec for contemporary 6CA7s of the era but EL34s need to be pretty hardy to handle it. The ones in this amp are relatively new Tung Sols.

There is another HD-130 for sale not far from me at a used gear shop, and it has 6550s Don’t know if anything has been modified/converted or if they just were stuck in.

 

[Wasnex]

I have heard that the high plate voltage of these amps specifically is (relatively) within spec for contemporary 6CA7s of the era but EL34s need to be pretty hardy to handle it. The ones in this amp are relatively new Tung Sols.

There is another HD-130 for sale not far from me at a used gear shop, and it has 6550s Don’t know if anything has been modified/converted or if they just were stuck in.

The Shuguang EL34B was supposed to be good for 800V plate .

Supposedly Shuguang has started making tubes again. I don't know if the new EL34B are as good as the old ones.

I have modern Mullards in my Hiwatt DR201 and the plates run 711V . So far so good, but it's basement queen that I play for a few hours every years.

6550s in high-powered bass amps are typically run about 695V plate. Spec is typically 660V. However, 6550 pulls significantly more heater current, which may cause the OT to burn up.

 

[agedhorse]

Overdriving with guitar may be more benign. Bass makes some negative artifacts really stand out.

The high plate voltage might be ok because the screen voltage is held to about 1/2 the plate voltage. Will of course depend on the tubes used.

I would not recommend 6550’s, the heater current is higher and could damage the power transformer. The bias required will be quite different too.

 

[Deleted member 271037]

Like others have already pointed out, 6CA7 tubes MIGHT be a better choice for this amp. I don't have anywhere near the technical knowledge of some of the previous posters in this thread, but I do have some personal experience with a similar amp, the Music Man Sixty-Five 212 combo.

A friend of mine had purchased a matched pair of TAD/Shuguang EL34s that he wanted to install in this amp (against my advice). I agreed to install the tubes and bias the amp for him, at his own risk. This was 10-15 years ago, so I don't remember the exact details, but the amp didn't sound right at all. I don't remember the exact details, but I vaguely recmember a rather harsh distortion at lower output volumes than expected. It was bad enough to know something was very, very wrong. He agreed to try 6CA7s. I don't remember which brand we got, but they were definitely not NOS. Probably something rebranded/matched by TAD. With the 6CA7s the amp sounded and behaved exactly like these Music Man amps are supposed to, and it still does, after 10-15 years of regular use. Maybe the EL34s we initially installed were faulty or out of spec somehow, but they tested and biased just fine, were brand new, matched by TAD, and I ended up using them in a different amp with no issues. Whatever it was, the tubes were definitely the culprit in THIS specific case.

Having said that, there's of course a chance that there might be some other issue with your amp which isn't related to the tubes at all. Bringing it to a tech seems like the most sensible choice IMO.

 

[coreyfyfe]

The drive circuit on the 130HD uses a unique cathode pull-down drive circuit with the grid undriven except for POSITIVE DC bias. The current flow through the cathode raises the cathode voltage across the driver transistor. The grid then becomes negative with respect to the cathode. The transistor functions as a current source.

I would expect the real world power to be around 100W (maybe a little more) but the overdriven characteristics of this particular topology may not result in a particularly pleasing distortion.

EL34 tubes may be different enough than the 6CA7 to require significantly different bias, as well as other possible anomalies, I don’t know if there is enough range of this bias circuit.

This is a different enough power amp topology that I wouldn’t assume anything about the performance except that I don’t recall any traction of this amp in the power amp overdrive/distortion world.

There are two versions of the HD130, the earlier 12AX7 driver (2275-130) have 6CA7 or EL34 on the schematic, but the later cascode models (2475-130) as you describe only reference 6CA7.

https://www.thetubestore.com/lib/thetubestore/schematics/MusicMan/Musicman-2475-130-2275-130-Schematic.pdf?srsltid=AfmBOopGyAF83_z0hMO4jOi1kXBvgOCJQCfQVGOw5Dx7wAbDGXLybegA

I had one of the earlier 12AX7 models, harsh would be an apt description of the preamp on these in my limited experience, but I also didn’t own the amp long.

 

[agedhorse]

Most of the builds were the later model, I don’t recall ever seeing the earlier model.

 

[ahdw]

Mine has the 12ax7 phase inverter. It does say “6CA7 / EL34” on the tube chart.

 

[rickdog]

For what it's worth - my 1971 RCA Receiving Tube Manual has one set of specs for "6CA7 / EL34".

Heaven knows what current manufacturers are stamping those numbers on....

 

[logdrum]

I hope you get it resolved, but with 2 twin reverbs it is a losing battle for you, either upgrade to a 300 watter all tube or get another cab and slave one of the amps (after you get the MsuicMan fixed)s o the other and get yourself 200+ watts.

You said 12AX7 inverter. I had distortion issues in the past caused by faulty phase inverter. Try replacing that and then off to the tech if it doesn't.

 

[ahdw]

I hope you get it resolved, but with 2 twin reverbs it is a losing battle for you, either upgrade to a 300 watter all tube or get another cab and slave one of the amps (after you get the MsuicMan fixed)s o the other and get yourself 200+ watts.

You said 12AX7 inverter. I had distortion issues in the past caused by faulty phase inverter. Try replacing that and then off to the tech if it doesn't.

I hear you. I am just looking for power without weight which is hard in the tube world. The sensible thing would be to practice through my SVT (so I don’t have to move it) and gig with the V4B RI since stage volumes are pretty quiet these days anyway. I just like the idea of one head to do it all so that I know exactly what tone to expect on stage.

 

[Wasnex]

I hear you. I am just looking for power without weight which is hard in the tube world. The sensible thing would be to practice through my SVT (so I don’t have to move it) and gig with the V4B RI since stage volumes are pretty quiet these days anyway. I just like the idea of one head to do it all so that I know exactly what tone to expect on stage.

There are powerful tube amps with relatively low weight.

Mesa Prodigy and Strategy.

Peavey VB-3 and VB-MA

Depending on what you are looking for, a hybrid like the Jule Monique 700 might do the trick.