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12AX7 tube ?


Posted on Thursday, June 06, 2002 - 09:57 am:   

hi! I got a hd-150 bass amp (chassis number:2100-150) and on harmony central I read that the models with the "one fifty" have a 12AX7 driver tube? am i right? when do I have change this tube?
ps:the amp really rocks...
Steve Kennedy
Posted on Friday, June 07, 2002 - 12:24 pm:   

Not quite... the amps with "One Thirty" or "Sixty Five" on the bottom silver grill badge (as opposed to the "130" and "65" badges) typically have the 12AX7 phase splitter tube.

The HD150 series wasn't made until 1980 which is several years after the Music Man design was changed to a solid-state phase splitter.

Therer shouldn't be a 12AX7 tube in your amp if it is truly an HD150... and the badges should be black instead of silver.

Posted on Saturday, June 08, 2002 - 06:14 pm:   

ok thanks!! actually, after i posted i just downloaded the message archives and then found some things about this. and yes the badges are black! can you tell me how much i would get for this amp?? (just wanna know ;-) ) everything is original and its in a extremly good condition.

Posted on Saturday, June 08, 2002 - 06:45 pm:   

ok thanks!! actually, after i posted i just downloaded the message archives and then found some things about this. and yes the badges are black! can you tell me how much i would get for this amp?? (just wanna know ;-) ) everything is original and its in a extremly good condition.

Steve Kennedy
Posted on Sunday, June 09, 2002 - 03:55 am:   

Is this just a "head" or does it have a matching speaker cabinet too? If so, which speakers?

If not, is it a combo amp? Which speaker(s)?

The Music Man HD-150 amp chassis was used in a variety of different models so you would have to be specific.


Posted on Monday, June 10, 2002 - 08:43 am:   

hi! it's only the head! it looks exactly like this one here! (not my auction)
http://cgi.ebay.com/ws/eBayISAPI.dll?Vie wItem&item=882562284

Mine is the export version with a diffrent back panel. It still has the original phillips tubes in it(ECG STR 387). i think before it was in a church and probably used very little.
Posted on Tuesday, July 02, 2002 - 12:48 pm:   

Ha! No wonder I couldnt find the 12AX7 tube on my
HD130...How long can the original tubes last?
Steve Kennedy
Posted on Friday, July 05, 2002 - 03:01 pm:   

I have bought amplifiers that were 20 years old and had the ORIGINAL tubes in them and they played fine!

Time is relative to how hard the amp was used throughout its life and how much abuse (physical and electrical) the output tubes saw during that time.


Posted on Thursday, August 15, 2002 - 08:39 pm:   

Mine's an HD-210 One*Fifty combo.

No 12AX7.

Have yet to see another like it!
Clayton Choate
Posted on Wednesday, November 27, 2002 - 12:24 pm:   

I have a MM212-HD 130. Silver name badge with the words "One Thirty". There are no tubes outside of the four 6CA7 power array. There is a space for an additional tube but it appaears to have been modified so that no tube is required. Was this a change to the design of the amp or has it been modified after-market? What tube should go there and what was it's purpose? The bypassed tube would be located next to a transformer with the transformer being between it and the power array.
Steve Kennedy
Posted on Wednesday, November 27, 2002 - 02:47 pm:   

It appears that your amp was either modified at the factory during repair or was a "transition" period amp.

The earliest series of Music Man amps were made between 1974 & 1977. These are identifiable by the "script" "One-Thirty" as opposed to the "HD130" of later models.

Originally, this design had a 12AX7 phase splitter (the circuit that derives the 180 degree phase shift and drives the output tube pairs in push-pull).

These amps had a very warm overdrive sound when pushed to the max (very loud, though), partially due to this tube. Sometime in 1976 or so, it was decided to replace this tube circuit with a solid-state design that was more reliable.

Apparently, if the 12AX7 tube itself shorted (due to mechanical abuse primarily) it could take out the ouput tubes AND output transformer if not immediately shut down. The solid-state circuit minimizes this particular problem.

The solid-state phase-splitter circuit doesn't sound as warm at or near output tube overdrive (again, VERY loud with an HD130 model), but it is a much more reliable circuit that can take the physical punishment typically applied to guitar amplifiers.

At anything less than max output, I am hard pressed to notice any difference (your mileage may vary). Apparently, many amps that were returned to the factory for repairs were upgraded to the solid-state circuit and many of the later "old-style" amps were modified on the assembly line while retaining all the external cosmetic attributes usually used to identify those with the 12AX7 circuit.

The modified "old-style" amps and the first units of the newer series all had the 12AX7 tube socket hole plugged with a plate and a couple of screws (until the supply of pre-punched chassis ran out). The solid state circuit was a retrofit circuit board inside the chassis.

Posted on Monday, December 02, 2002 - 04:09 pm:   

Hi Steve.
I have one of the "transitional" units mentioned in the last paragraph above. A 2475-75 chassis, 75 watt solid state splitter with covered 12ax7 hole.
My problem is as follows:
With twin bias probes (precision 1 ohm resistor reading current through pin 8 of power tubes )I do not get close to balanced idle currents. When I test the output tubes they match for bias and transconductance. The bias pot adjusts the bias current fine but I don't get balanced currents.
All passive components in the driver circuit test ok as do the driver transistors. The output transformer is ok also. Any ideas?
Dick Billups
Steve Kennedy
Posted on Sunday, December 08, 2002 - 05:07 pm:   

The only other components you haven't specifically mentioned would be the 220 & 470 ohm grid resistors.

The current through the tube is ALSO dependent on having the grids properly biased. The resistors may be "good" but if they are mismatched then the bias current could be different in each tube.

carl from AUS
Posted on Thursday, January 16, 2003 - 06:06 am:   

hi carl from australia here.
i have a MM 210HD130(script), with a preamp valve in the metal protector cover which is spring loaded - does this mean my preamp section is valve - i am to beleive it is half valve and this is why MM are nice and clean???
excuse my ignorance as i am only young but my love of MM is big!
thankyou, you will here from me alot im guessing - so stoked to have found the site.
Steve Kennedy
Posted on Monday, January 20, 2003 - 06:11 pm:   

All Music Man amplifiers basically have a solid-state preamp and tube output sections. The early "script" versions had a 12AX7 tube in the phase-splitter circuit (the circuit that drives the output tubes in push-pull fashion).

The later models (after 1976-77) replaced the 12AX7 with a solid-state transistor driver board.

The models with the 12AX7 tend to sound "warmer" & "bluesier" when the output stage is overloaded. This is done by turning the Master volume control all the way up then increasing the Gain drive until the output stage starts sounding "bluesy".

Posted on Thursday, July 17, 2003 - 02:50 am:   

The end-around to warming up your MM (read Steve's
comment above);

Classic tube distortion. I play through a vintage box (original Chandler Tube Driver [MK Butler/San Francisco] but from all I've heard, seen and read, the new Radial Tone Bone will "warm up" your distortion as well or better.
Martin Raymon
Posted on Wednesday, August 13, 2003 - 06:31 pm:   

I mus concur with Steve K. I have two 2-10 65's, one with and one without. The tube amp is definitely more bluesy/rocking, while the solid state is the cleaner of the two. When I gig - which is not too much these days - I'll set up both, the older model loaded with Pyle Drivers & a 2-12 JBL D-120 extension & the newer one with 2 JBL D-110's & a 2-12 Black Widow extension cab. You can get just about anything from this rig. Looks cool too!
Doug Elick
Posted on Saturday, October 02, 2004 - 11:10 am:   

Just as a curious note, My HD-130 head (silver plates, "HD-130", not script) has a 12AX7 in it.

So did Music Man throw together whatever they had lying around? There seems to be no consistancy to their SN's and schematics to real world examples.

Steve Kennedy
Posted on Monday, October 04, 2004 - 04:35 pm:   

Yes, there seems to be only a gross correlation between features and cosmetics and almost NO match up to serial numbers!

You will find "script" logo units with solid-state phase-splitters and as you have found out, "non-script" logo units with 12AX7 phase-splitters! While these feature combinations are generally true, there are "cross-over" units that happen for a variety of reasons:

1. There were more 12AX7 chassis left than "script" logo badges (or vice versa).

2. A chassis failed test on the production line and a different series chassis was substituted.

3. The unit was sent back to the factory for warranty repair and got an "exchange" chassis installed for some reason.

There are probably a few other possibilities I didn't immediately think of as well!


John Drexler
Posted on Monday, January 31, 2005 - 11:01 pm:   

How much does the 12AX7 tube effect the tone of the amp? And which would be the better tube to use of these two (I have both)? A NOS Mullard 12AX7A or a Sovtek/EH 12AX7LP (long plate)?
Mike Algozino (mike_a)
Username: mike_a

Registered: 10-2012
Posted on Monday, October 15, 2012 - 09:52 pm:   

Please explain about the high/low gain 12ax7 v. 5751 tube. Does the low gain tube have less breakup than the high gain 12ax7 tube? I use a '75 HD 130 as a bass amp, so i'm assumming I should be using a 5751 for a cleaner signal.

You say?
Lars Verholt (lmv)
Username: lmv

Registered: 11-2009
Posted on Tuesday, October 16, 2012 - 10:15 pm:   

Use a 12AX7/7025 as it was intended. The MM design was to allow the preamp board to mildly overdrive the phase inverter tube if you turn down the master and turn up the channel volume. Using a different tube with less gain will not allow you to drive the output amp properly in turn not allowing you to reach the maximum clean headroom.

'dime' the master volume, run it on 'hi' and adjust the channel volume to taste. Don't forget, in the tube PI amps the 'deep' filter is right before the 12AX7 tube so it has an influence on the 'cleanness' of the sound too.

Lars Verholt