Urk. Musicman 210 broken - has SS pre.

I hate this kind of stuff but again, it's a longtime friend who's been gigging in Baltimore (poor thing) since the days when my cousin ran the Bluesette and Grin played there.

Her rhythm guitarist uses this Musicman 210 which has an all transistor preamp which goes to a 12 AX7 splitter and two EL34 outputs. It blows fusses fast after some odd noises and fading (oscillation?). I pulled the splitter to try and isolate if the problem was in the pre or power stage... hoping for a tube problem in the power stage but the same exact thing happened... sat there for 45 seconds then popped the 5 Amp fuse. Here is one of those times I wish I knew more about SS repairs. A google search doesn't find any common "this always goes wrong with theses suckers" problems. Each hat style transistor is socketed at least. What I wonder is how a pre with a low voltage PS can pull enough current to blow a 5 amp fuse just like that, unless I'm just wrong about pulling the splitter. Anyone got any ideas or is there anything else I can do to provide more info? All I could think of to do is pull each transistor and nake sure it was accurately socketed. SS guitar amps. Bleh.

Kiira

No short between PSU rails and ground ? I would check the electrolyt capacitors just in case
(although the fuse would blow right away after power on in case of short, I don't think there will be a 45sec delay)

Kiira, I've just fixed a Musicman HD130 (the model with 4X EL34s) this week.

First off, grab the schematic.

I believe it's more or less the same as the 130, and if it has high and low power modes, then the valves will be seeing ~700V in high (!), and ~350 in low. The valves can end up failing quite badly, and I found the amp I looked at recently had developed an internal short.

Pull the phase splitter and power valves and power it up. Fuse still blow? Now add the phase splitter. Still ok? Try another pair of power valves. Check for scorch marks on the power valve bases and on the valve bases themselves.

I'm still getting my head around the hybrid transistor/valve power section. I'm not sure why Leo Fender did this.

The preamp is simple; just several opamps, simple eq, and bypass caps for bright/deep switches if it has them fitted.

I'd recommend running this amp in the low power setting unless it sounds worse. Note that the heaters run cooler in low power mode too. Nice sounding amp by the way, and great quality components. Just shows you what effect the power section as on the overall sound. The preamp section is not designed to be overdriven in my opinion.

As Rody said, grab the schematic at:
http://www.music-man.com/techinfo/old_amps/

Here's another great source of information for MusicMan amps :
http://www.pacair.com/cgi-bin/discus/discus.cgi?pg=topics
A whole forum dedicated to it!

BTW, I have an HD130 waiting to climb on my bench :-[ :-\ :'(

Axel

> sat there for 45 seconds then popped the 5 Amp fuse

There's no way this half-watt preamp is popping 600 watt fusing.

Yanking the 12AX7 cuts signal, but the big bottles can still pull all the power they want.

Pull the EL34s. Will the fuse hold? (You really should own a Lamp Limiter.)

If so:

Check the bias at the junction of the two 330K resistors going to the 1K5 at EL34 grids. This MUST be negative 30V or 40V. If dead-zero, replace the 150uFd 50V cap near here. If still ill, check for nominal +46V and -46V DC at PCB terminals A and C. Zero suggests trouble on the rectifier board. 32V DC points to dead-open in the 150u caps on rect board.

When bias pot can push neg 30V-45V on to the 330K grid resistors, leave it set -45V. Now check the voltage directly at each EL34 pin 5 (grid). It will be around -43.5V; -45V minus meter-loading through the 330K. If it is positive, or not very negative, the 0.047u grid caps are leaking and must be replaced.

Got good grid voltage? Put a voltmeter across the 10 ohm resistor in EL34 cathodes. Put in one good EL34. With this initial mis-bias, a good EL34 will rise to less than 0.1V across the 10 ohms. Let it sit that way for a while and watch for drift. Stable? Trim bias pot so that you get 0.2V across 10 ohms. Watch a while. Stable? Smack it. Still stable?

Note this position of the bias pot.

Now turn it back to the -45V extreme. Remove the one EL34 and insert the other. Repeat: bomb-check for very low voltage with mis-bias, then sneak up toward 0.2V across the 10 ohms, and let it cook. Smack. Stable?

Is the "good" bias-pot setting fairly similar for each tube? If not, the best answer is to find a better pair of EL34.

If happy so far, trim the bias back half-way. Insert both tubes. Fire-up, watch the voltage on the 10 ohms. Since we turned-back a bit, you expect both tubes together to pull roughly 0.3V. A bit more or less is OK. A lot more is cause for panic.

At this point the amp should "play". It should play LOUD very fine. The 10 ohm test voltage will jump when you pump it, but when you rest it should fall right back to a few tenths volt.

Because we've biased "cold" for conservative testing, the amp will sound "rough" or "slow" at very low volumes. That's OK. The next step is to play LOUD for an hour. As rodabod says, this amp abuses its tubes, and you want to find out if your tubes short-out with heat and vibration. Take occasional breaks and watch the meter come back to tenths-volt. It won't be dead-steady, but it shouldn't stray far.

Still AOK? Let the amp idle a minute, then trim bias for 0.5V across 10 ohms. This is MM's specified idle point. This hotter bias takes most of the turn-on kink out, but still does not overheat the tubes at idle. For "perfect" linearity we want to idle much hotter, but that WILL overheat the tubes. 0.83V across 10 ohms exceeds the rated dissipation, 0.58V is 70% rated Pd and probably as hot as you should accept, so 0.45V to 0.55V is your target.
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> the hybrid transistor/valve power section.

Kiira has the 12AX7 driver. This is a very basic all-tube power stage, albeit at obscene voltages. Ampeg had a similar over-volted 60W amp. Mine grew a G2 short. The damage was impressive.

The one with BJTs pumping the cathodes of the power bottles gives strict control of tube current. Maybe they thought that was a good idea. Maybe they did it just because they could (falling price of BJTs and op-amps now allowed such frivolity). IMHO, this one is a funny kind of solid-state amp, but with vacuum buffers. The tubes don't add sound, they must do exactly what the silicon tells the tubes to do.

PRR said:

> the hybrid transistor/valve power section.

Kiira has the 12AX7 driver. This is a very basic all-tube power stage, albeit at obscene voltages. Ampeg had a similar over-volted 60W amp. Mine grew a G2 short. The damage was impressive.

The one with BJTs pumping the cathodes of the power bottles gives strict control of tube current. Maybe they thought that was a good idea. Maybe they did it just because they could (falling price of BJTs and op-amps now allowed such frivolity). IMHO, this one is a funny kind of solid-state amp, but with vacuum buffers. The tubes don't add sound, they must do exactly what the silicon tells the tubes to do.

Click to expand...

That makes more sense; I assumed the version with the valve driver also had the hybrid power section, but it doesn't do as you point out.

What I don't understand is, this amp here (HD130 with hybrid power) definitely reminds me of a good valve amp. It's that same difference you hear when playing a "proper" valve amp compared to solid state - almost like light compression, with none of that DI-like sound that one gets from SS amps. Could it be the coupling of the valves to the output transformer? Maybe just a very well designed SS amp! The other thing I noticed when recording this amp with bass was, it growls (in a nice way) on bass. Again, not what I'd have expected from a SS amp.

Roddy

PRR, Rod,

Yeah, it was biased too hot and had fried an output tube. I set it to put -35 on the two 330ks and about 680 volts on the plates of a new set of JJs. THANKS so much for the help. I guess I am just too used to my Deluxe Reverb and JCM800... it sounds kind of characterless and sterile to me with a strat. I banged on it hard through a Fulltone Fulldrive and my strat and it seems happy.

I just don't see the sense in mixing SS with a tube output, must be a money thing.

best,

Kiira

Just for reference if anyone searches here for how to bias the Musicman amps with the solid-state phase-splitter, you want to aim for ~25mV across the 3R9 resistors sitting under the power valve cathodes.

PRR said:

The one with BJTs pumping the cathodes of the power bottles gives strict control of tube current. Maybe they thought that was a good idea. Maybe they did it just because they could (falling price of BJTs and op-amps now allowed such frivolity). IMHO, this one is a funny kind of solid-state amp, but with vacuum buffers. The tubes don't add sound, they must do exactly what the silicon tells the tubes to do.

Click to expand...

I'm not sure I understand this fully. How exactly do the transistors control current? Does the power valve not try to pass more current as the grid-to-cathode voltage shifts?

rodabod said:

Just for reference if anyone searches here for how to bias the Musicman amps with the solid-state phase-splitter, you want to aim for ~25mV across the 3R9 resistors sitting under the power valve cathodes.

Click to expand...

Thanks, Sir! I have two of those in custody, needing some therapy... ;D

/Dave

Look at this schematic
http://www.music-man.com/techinfo/old_amps/2100-b.pdf

basically the plate and screen currents sum up and "go thur" the transistors E to C (yes a little out of the base).  The tube currents have to match the transistor current because each side is a series circuit of a tube and transistor
G1 looks like it is at 22VDC
G2 at 350VDC
Q3 and Q4 do not show the collector voltage so we don't know the G1 to cathode voltage difference.

> I just don't see the sense in mixing SS with a tube output, must be a money thing.

Vacuum-tube preamps are nothing but trouble. They hiss, they humm, the sockets get scratchy, the tubes go noisy in several ways.... solid-state, once mastered, is so much more reliable, predictable. And by this point, an SS preamp could do more for the same price.

> it sounds kind of characterless and sterile

Great New Sound! "Clear". Not fat and flabby like old-fashioned tube preamps.

This seems to a stage in Leo's search for "better". His early amps use circuits from a 1937 tube manual. Going into the Space Age he started using NFB, although in limited amounts. 6dB NFB can actually increase audible distortion. It drops the sweet 2nd and raises an IM haze. New Sound! Widely used but not widely considered better. The MusicMan amps actually got enuff NFB happening to clean-up the sound.

True, the clear sound of a naked Strat can get boring.

And if "clean" is your goal, post-MM transistor amps got that done very well.

The pendulum swings, and swings back.
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> How exactly do the transistors control current?

Is there a third variant? The early ones were simple tube power amps. The last ones had BJT-tube cascode. Was there one in between with a "twin triode grid driver", only with BJTs?

For the final variant... What Gus said. It is a Cascode. The upper device current -must- equal lower device current (neglecting small Base and G2 leaks). The lower device is a BJT with large emitter resistor, a voltage to current source. The BJT collector and tube voltages don't matter (less than 1% influence), strictly base-ground voltage.

> we don't know the G1 to cathode voltage difference.

We can estimate. We know G2 is 350V. We know G1 is 22V. We know that G1 is surely negative of cathode, or that cathode is positive of G1. For ab-max current, cathode voltage is equal to grid voltage; for ideal cut-off cathode voltage is Vg2/Mu where Mu is about 10 (for EL34, 7027, 6550) and "Vg2" is the G2-Cathode voltage. So ideal cut-off is 1/11 of 350V-22V or 30V. These tubes do not have hard cut-off, so we may need a bit more or less. But I'd expect cathode 30V higher than G1 or near +50V.

And indeed, as you say, it does not matter. The BJT flows XX mA up to cathode. The tube cathode assumes whatever voltage makes XX mA happen. G1 is set at some positive voltage so that the BJT always has C-E bias. Or perhaps a positive voltage so that the tube can be pulled some desired amount into class AB2. We might like to know that the "off" condition will not exceed the BJT's Vce rating; although the BJT would not be harmed, if it breaks-down before the "off" tube gets really low flow, this subtracts from the "on" tube and reduces test-power.

PRR  Do you think the cascode type output was built because it might be a simpler way to interface the ICs to the drive tubes and be more easily driven by an IC output with it's limited current output and less than +- 16 peak to peak output because of the IC power supply.

  If one wanted to use an IC in a conventionally way to mimic a tube PI the output tube G1 grid would be - and the IC output would be cap coupled to G1.

For others look at the schematic I linked, check the cool limiting feedback network around some of the ICs.

MM sound OK they are what they are.  A friend of mine seems to like the ones with lf353s over the tl072 and 4558 ones.

Thanks PRR and Gus. I'd only seen cascodes used as high gain preamp stages before. I will look again.

I went to see a band last night which had these amps for bass and guitar and they sounded great.