Fender Excelsior (newly acquired) Shifts Btwn Garbled/Distorted and Clean

stormsedge

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The thread title pretty much sums it up...my newly acquired Excelsior occasionally shifts from a nice clean tone to an unusable garbled/distorted mess...then a bit later (if I stay with it) it goes clean again. I had some pre-amp tubes, so I switched them out--no change. I'm thinking about ordering new power tubes. I also tightened all the screws. I really do not want to send it back.

Thoughts?
 

Dacious

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The thread title pretty much sums it up...my newly acquired Excelsior occasionally shifts from a nice clean tone to an unusable garbled/distorted mess...then a bit later (if I stay with it) it goes clean again. I had some pre-amp tubes, so I switched them out--no change. I'm thinking about ordering new power tubes. I also tightened all the screws. I really do not want to send it back.

Thoughts?

This amp is a typical Tweed amp circuit with a cathodyne phase inverter. I think I worked out it's a 5E9 from memory with a MOSFET driving the trem oscillator.

That tube (closest to the power tube) needs to be tough. Either a Ruby/Shuguang or JJ12AX7 is needed in modern tubes. Many tubes that are otherwise fine are not good. Cathodyne arrangements like this and the Tweed Deluxe are very hard on a 12AX7. There's a significant current between the grid and cathode which overheats it. Mesas and some Marshalls will do the same thing with the EQ recovery or effects loop driver which is a cathode follower.

Some NOS tubes do this too. That's why Fender went away from cathode followers in Bassmans after the Tweeds but not before it was bequeathed to Marshall and Vox.

I got one fitted with old 12AX7s and it would do this after 20 minutes. I had one new Shuguang which did the same thing. But ordinary $10 Shugs which have the metal E spacer and come branded Marshall should be ok.

12AT7s work fine as they have higher current capability. They will drop a bit of volume though.
 
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stormsedge

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That tube (closest to the power tube) needs to be tough.

Thanks, I have an ECC83S...rough duty. I'll try that in the V2.

I will order a new set of tubes (preamp and power) for it too...I do not have a pair of matched 6V6 in my stash.
 

Dacious

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Thanks, I have an ECC83S...rough duty. I'll try that in the V2.

I will order a new set of tubes (preamp and power) for it too...I do not have a pair of matched 6V6 in my stash.
Mine had an old, old 12AX7 with writing burned off, it played 20 minutes then cut out with a pzzzzzft. Then after a few minutes off it would okay again. As I had a couple 5E3s I built I remember I had to be careful what tube was there.
 

stormsedge

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That tube (closest to the power tube) needs to be tough.

I had to be careful what tube was there

I replaced the V2 with the ECC83S and V1 with an ECC83MG. The amp has held its tone for an hour and a half, so far. So, it looks like a win.

I ordered another ECC83S and will place it in the V1 when I get it.
 

Dacious

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I replaced the V2 with the ECC83S and V1 with an ECC83MG. The amp has held its tone for an hour and a half, so far. So, it looks like a win.

I ordered another ECC83S and will place it in the V1 when I get it.
ECC83S the -S might indicate a spiral wound heater filament. These are usually a low noise design and can be more fragile due to the longer filament. But it sounds like it's hanging in there ok.
 

stormsedge

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Update: the garbled garbage popped up again. Sent it to the tech and he went thru it…pronounced it “good” after replacing an IRF820 and a 100kohm 1 watt resistor. Okay then.

Played it off and on for nearly a month and darn if it doesn’t do it again only a little different. Sent it back. A week later, he says “can’t find anything, it is good”. Okay then.

Slapped an Eminence 1518 in it. Man, it is a loud dude now! Ran it today on the looper for over an hour at 2 oclock on the knob—-double hearing protection and 120dB on my meter. No failures. I guess while he wasn’t finding anything it healed itself. Time will tell. Cheers!
 

Dacious

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Get a JJ or Shuguang (often branded Ruby or Marshall) - it'll handle it no problem. It has no influence on tone on that position. Mesa actually fit and recommend Mesa-badged JJs for that reason - short plate and grid, very tough. On Mesas it's usually V3.

Voxes do the same thing it's usually V2 on them. Ditto Marshalls. My Mini Jubilee came with a Sov LPS in V1 and two Marshall branded Shuguangs.

I accidentally transposed V1 and V2, the EQ recovery cathode follower - bingo, twenty minutes, cut out.
 




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