Would a tube bass head be more inclined to feedback if 2 of the 4 power tubes had been pulled?

Jim C

Jim C

I believe in the trilogy; Fender, Stingray, + G&L
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Nov 29, 2008
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#1
I played through my buddies Plush Royal 1060-S bass head. This amp is very Fenderish with 4 676GC power tubes and 12AX pre amp tubes.
The outside 2 tubes had been removed and I played a P bass through an Ampeg 4x10 cabinet.

The tone was beautiful but the amp wanted to feed back even with moderate treble settings. Is this any indication of an amp problem?

Otherwise it was the noise floor was very low, the tone very good, and it sounded like a good 50 watt amp.
 
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#3
Jim C said:
I played through my buddies Plush Royal 1060-S bass head. This amp is very Fenderish with 4 676GC power tubes and 12AX pre amp tubes.
The outside 2 tubes had been removed and I played a P bass through an Ampeg 4x10 cabinet.

The tone was beautiful but the amp wanted to feed back even with moderate treble settings. Is this any indication of an amp problem?

Otherwise it was the noise floor was very low, the tone very good, and it sounded like a good 50 watt amp.
Click to expand...
Plush(and the later Earth brand)heads were essentially Fender Showman knock-offs. Any removal of power tubes changes the amp’s output ohm rating.
 
#7
Five or Six said:
What was the point of removing the tubes?
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It’s done to half the wattage output, ostensibly to get a “cranked” sound at a lower volume. But it should be outer two(which had been done), not two in a row, as that plays havoc with the transformer I believe, then there’s the ohm change. I’m wondering if there was an issue with the preamp tubes in the OP’s scenario as well.
 
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#11
Sounds like the amp was being run in an unintended configuration. All bets are off. Just because some armchair internet guru says it is possible to run it that way, doesn't mean it is a good idea.

Without the amp being run in its intended state, no one can tell you what, if anything, is wrong with it.
 
#14
Acoustic feedback between the instrument and cab? Shouldn't affect that unless (to compensate for/ take advantage of the reduced power) you're running the preamp gain high and into overdrive and squealing like happens with a distortion pedLOL.

Microphonic tube feedback of preamp tubes? Shouldn't cause that, either, but if you have one+ tubes that are microphonic, things can get squirrely in a hurry. Had a microphonic PI tube once take out a freshly reconed jbl d120 in an Ampeg guitar amp. That was fun.

Has this head been run at full power?
 
#15
That's a common guitar amp trick - removing half the power tubes (usually but not always the outside ones) to halve the output power - that allows you to turn the amp up to get to distortion quicker. I've seen it and done it on Twins and other 4x6L6 and 5881 and also 4xEl34 amps. I can't recall if all 1/2 power switches take power tubes out of the circuit but I believe most 4x power tube amps do - if it's a 2x power tube amp it obviously works differently.

But consider, if the output power is halved and thus lower, is the preamp (if that amp has one) gain knob being turned up? Or even the master or single volume?

In other words, halving the power might result in turning the amp up to feeding-back level.
 
#16
The amp was at a local shop. They put 2 new matched JJ 6L6's in to get to 4 power tubes but there is still some static and hum that needs to be addressed by a tech. I think the original tubes are partially the culprit.
The amp was $300 and I'm thinking I'll have another $250 in it in repairs.
But the tone... That good. - One interweb article suggested the man behind Marantz amps actually tweaked this basic Bassman circuit for Plush.
I'll report back. Pretty clean.
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#17
Oh wow! I haven't seen PLUSH in decades. Back in the day one of the local stores carried them early on, but dropped them within a year. The story matters little, what does is they had a unique sound when working as intended. I think the "Tuck and Roll" was a weak choice as people were getting wise to Kustom by that time. Even though it still has an appeal today.
 
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#19
Some amps have a reduced power switch that disables two of the four power tubes. It’s good to ensure that the amp bias is correct when pulling tubes. Sometimes it is as simple as plugging into a different transformer tap. For instance, in reduced power mode, plug an 8 ohm cab into the 4 ohm transformer tap. But reducing power shouldn’t cause feedback.

I’d check the power tubes, are they the correct type, are the two tubes badly matched, is the bias optimal, is the speaker impedance matched to the output transformer. Operating with an impedance mismatch could be alright although the amp will be tonally a bit different. It’s not as simple as pulling two tubes.
 
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