weak preamp overdrive on 2555 silver jubilee

[mocklah]

i just received my marshall 2555 that i won on ebay to replace the 2553 i had stolen from me last november.

the first thing i did was to dial in the settings i'd been using for almost 2 decades and give it a try.
VERY disappointed! the preamp overdrive was weak and thin. i popped in some EH preamp tubes that i had used in my old 2553, but it barely increased the gain.

so i opened it up to see if there was any obvious mods, like bypassing the clipping diodes & LEDs. but it looks pretty clean to me. but i'm just going by the ugly mods i've performed in the past.

while i had it open, i went ahead and checked the power tube bias. it was at 25ma. so i thought i'd do a quick bump to 35...but the bias pot was maxed out!
i even tried a new set of jje34ls w/ the same results. the bias resistor is 56k, but a schem i found says it should be 47k.

any ideas?

does cold bias on the power tube affect the preamp tubes?

pin3=430volts

here's some pics (click for larger versions):


thanks,
-mike

[drewl]

Check the f/x send/return jacks.
Run a short cord between the jacks.
if the volume comes up, the jacks are the problem.
try cleaning or re-tensioning them first.

[Andy]

colder power tube bias could raise the overall voltage in the amp (due to less voltage drop across the PT secondary) and give the preamp more headroom, but I don't think it would be as pronounced as what you're describing. on the other hand, cold power tube bias would make the power stage stay cleaner and make it sound thin and sterile. I'd try upping the bias to 35 and see if that helps.

[mocklah]

thanks guys.

the bias pot is maxed out to 25ma ish. i've tried the GTs that came w/ the amp & a set of JJ e34ls.

the bias resistor is 56k (looks stock), if i change it to 47k (like the schem says). would that put it in range?

i'm hoping you guys can help me figure out why its outta range in the first place.

thanks again,
-mike

[Ray Ivers]

Mike,

As a guess, perhaps the amp originally came with 5881's, in which case a stock value of 56K would make sense for R23; if so, it could be that someone just plugged in a set of EL34's without making any circuit changes at all.

Ray

[Ampclutz]

Hi,

I looked at the schematic you posted and between the two caps from V1a there is a wire going to ground off one end of the gain pot. (Edit... Actually both ends of the gain pot.) Can this really be? How could you get any signal to the next stage? Is this an error on the schematic?

clutz

[Matt T.]

I posted a response on the Vintage Amps forum but I wanted to add that that little blue electrolytic cap between the gain control and the input jack is a cathode bypass cap for V1b. If it has gone bad, that would reduce the gain of that stage and basically castrate the amp.

[mocklah]

ray, that's what i was thinking, but i remember these amps being promoted on the return of the el34. so i don't think the jubilees ever came with 6550s from the factory. if this was a jcm-800 i would have assumed that right away.

ampclutz, i have no idea if that's an error or not. i have some 2550 schems that i can post later...the preamp section should be the same as the 2555.

matt, would that be "C4" on my photos?

i have a sister thread running here: http://vintageamps.com/plexiboard/vi...asc&highlight=
(lemme know if this link isn't cool to post here)

thanks alot guys!
-mike

[Ray Ivers]

Mike,

You're absolutely correct - AFAIK these amps never came with 6550 output tubes, although the 5881 tubes I

[Ray Ivers]

Mike,

You're absolutely correct - AFAIK these amps never came with 6550 output tubes, although the 5881 tubes I mentioned in my post could still be a distinct possiblity; they would use roughly the same bias-voltage range as 6550's. It's no big deal anyway; if that 56K R23 was stock it could have been a simple mistake, I was just thinking out loud about some kind of better explanation.

According to the Doyle Marshall book, these amps were just about the last new model of the JCM 800 line, and were discontinued in 1989. The JCM 900's - introduced at the 1990 N.A.M.M., great show! - were when Marshall started phasing in the use of 5881 output tubes and scrapping the EL34's, so it's conceivable (to me, at least) that one of the later runs of 2555's could have used 5881 outputs - but as always, YMMV.

If it were my amp, I'd just replace R23 with a 47K, or whatever value gives you a bias range you're comfortable with.

Ray

[mocklah]

cool. at the very least, i'm gonna swap those resistors...couldent hurt.

i'm just having a hard time with that being the reason for the lo-gain on the preamp.

wouldn't that be great i just did a simple resistor change and everything sounded just like i remember with my old 2553!

[Ray Ivers]

Mike,

I'd recommend getting the output stage squared away first (as you're doing) as it will obviously 'color' everything the preamp puts out.

I didn't realize the preamp was actually low on gain. On your bottom-center chassis pic, between the orange and black pot switch wires - what's going on with that diagonally-connected resistor apparently replacing two missing PCB components? If it's R30 than it's just a series dropping resistor for LED1 (probably the channel-selection indicator), but I can't read the other component designations so I don't really know what's going on. It might possibly be a fruitful avenue of investigation, or maybe nothing at all.

Ray

[Ray Ivers]

Mike,

On that same center-bottom photo, R29 appears to be jumpered across. This resistor works with the clipping diodes to create overdrive/sustain - I'd definitely replace the jumper with a 47K resistor as per the print, and I would also check to make sure the clipping LED's haven't been fried by exposure to the full AC voltage swing of V1b. At this point, I'd also suggest going over every component value on the PCB against the schematic, just in case.

I misread your original post - I thought you were getting 2.5mA output tube idle current, not 25mA. Marshalls are often biased cold from the factory, and 25mA seems very much ballpark to me, although you may prefer biasing it to a different level. I don't think you'll hear a night-and-day difference between 25mA and 35mA, although from 2.5mA to 35mA you would have.

Ray

[Matt T.]

Ray, many of those numbers on the schematic don't match up with the circuit card. That 47k resistor is R28 on the circuit card, right next to the LEDs. I actually reduced this resistor to 22?K on my Jube so that's how I know they don't match up...some of the numbers do match up, some don't.

[Ray Ivers]

Matt,

OK, thanks for the info - that certainly adds a whole new 'fun 'n games' aspect to the troubleshooting procedure, doesn't it?

Ray