V-man

Just picked up this '81' 1992 Bass Head, my first "new" Marshall purchase since (oddly enough,) the year 1992.

The power switch did not illuminate starting it up. A couple of the pots were somewhat scratchy and one was clearly replaced (a prominant gold-faced knob amidst its faded near-silver brethren). It powered up and played, but the first thing I realized is there was NO "oomph" (volume) to the amp at all. I rolled the volume to 8 and it felt about as loud as 4 on my JCM 900, which told me right off there was something up with the amp. I looked in back and discovered that 2 of the 6550s were firing (#2 & #3). This model doesn't have a 50/100 watt switch, and one fuse powers all 4 power tubes, so the 2 dead tubes suggested a pair of blown tubes or something more sinister. I pulled #1 & #4 and dropped #2 & #3 in their places, and they fired up again, so I am hoping this was a matter of 2 blown tubes.

The outer chassis looked pretty clean for a 28 year old amp, Both the top and bottom inputs worked fine as well. The impression I got was that the seller either didn't know his way around a Marshall (or he knew plenty, including all the boogums I will be facing when I drop this off with the tech). He claims it's been his practice amp for 10 years (with the same tubes in it for 10 years). He said he never changed tubes and acted like I was teaching him about the two tubes being fired out, then 5 min later he mentioned some possibility about a capacitor or resistor (forget which). That's the bad. The reason why I didn't walk away from it is because of the "good". I took it off his hands for a little more than $400.



So I am figuring I practically stole the amp, or I got a fair price (depending on what my tech has to say this week), but I figured some of you guys may provide me with some spoilers...

1. Does the entire power switch need replacing, or is it a little bulb inside?

2. What do you think about the volume situation? The difference between 50-100 watts is like 5-10dB, so I am a little concerned that 5-10dB louder than what I heard today (compliments of a new set of tubes) is still on the low side of performance. Does the volume issue suggest anything to you?

3. Putting stock into the story... used weekly for 10 years, w the same tubes, God knows how long ago tubes #1 and #4 died, what if any damage do you think may/could have happened? FWIW I checked the fuses and they're good.

4. Assuming the worst I'm out is the cost of a matched quad, a set of filter caps, a pot-cleaning, and maybe some resistor somewhere, what do you think I should drop in? I was thinking of trying out some KT88s. This is primarily used for guitar (metal/punk/rock/blues), and I already have the EL34 sound covered with my JCM 900.

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Gibson: '88' Explorer 90, Flying Vs - '02' V-Factor, '06' RI, '07' RI
Jackson: '92' Rhoads Tribute, '92' Rhoads PCS, '02' KV2
'92' ESP/Hill King V
'04' Epiphone Goth V

'84' Marshall 1992
'93' Marshall 4100
'97' Crate Blue Voodoo BV-60 Combo

Marshall Amps

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V-man

No love for the 1992 around here?

__________________
Gibson: '88' Explorer 90, Flying Vs - '02' V-Factor, '06' RI, '07' RI
Jackson: '92' Rhoads Tribute, '92' Rhoads PCS, '02' KV2
'92' ESP/Hill King V
'04' Epiphone Goth V

'84' Marshall 1992
'93' Marshall 4100
'97' Crate Blue Voodoo BV-60 Combo

Wilder Amplification

It is just the bulb inside. However, there's no way to remove it since those switches are sealed. So the entire switch will need to be replaced.

Yes, the volume suggests worn out valves. They're 10 years old...replace 'em. The plate resistance of the valves has probably drifted up so high that they just aren't as responsive to the input signal amplitude as they were when they were new.

Just FYI as well, there's only 3dB acoustical volume difference between 50-100 watts. 10dB difference in volume is what the typical human ear perceives as "twice louder", and you'd have to have 10 times the power to get 10dB more. The power/acoustical dB is not linear (more like logarhythmic) and other factors come into play such as speaker sensitivity, frequency curve of the amp vs. frequency curve of your own ears...just to name a few of the variables involved with power vs perceived acoustic volume phenomenon. The difference between 50-100 watts is mainly headroom, and a 50 watt amp will break up more at full crank than a 100 watter will.

The screen resistors are more than likely blown on those sockets. They're relatively cheap...your tech should know what to do. Worst case scenario is a blown output transformer, but this isn't as common as some would like you to think.

From what I've read elsewhere SED Winged C valves are supposed to be the balls. They claim to be a reissue of the General Electric 6550 valves, although I've never personally heard them.

Keep us posted and let us know what the outcome is. Great score BTW.

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Jon Wilder
Wilder Amplification

sales@wilderamplification.com

tech@wilderamplification.com