Marshall 8080 - Lack of volume on boost channel.

[g1]

How are they dead? Do they still light up? Have you tried them in another amp?

[david.ohyama]

Ordered a new one and it lasted 1 gig, order 2 more matched JJ's and the second one I put in maybe lasted 2 weeks at home, now I'm on my last JJ and wondering how long it will last.
The original Marshall tube still works but doesn't sound as good. Not sure if they light up but when I did try to look at the original tube didn't see much light coming off it anyway.

Was just wondering what the actual Marshal tube part number is, maybe their not 12AX7's?

Think I would have to have maybe 3 of these 8080's to get the volume level I need and a manageable sound. At about 4 on the master, Gain half on OD1 or Gain 1/4 on OD2 thing seems like its going to self destruct. Running the parallel out of my floor effect-or into the effects return to balance the harsh boost channel with what is the original sound outta of the floor effector.
Don't have a parametric to put in there but it might work. By the way this EQ is setup think I might need a parametric and 6-10 band EQ.

Have a Peavey Ultra 1-12 on hold all tube at a local shop over here I'm getting for 150 bucks. Going to run these in tandem for a better stage sound. Don't really think the 8080 can handle stage volume well. Becomes to unmusical and the gain and EQ doesn't quite get it right.

I think maybe I might resolder the tube mount. Maybe its vibrating and shorting the tube? I bought it and had it rather tame around the house, didn't crank it up but turned it up at the gig and sure enough by the end of the gig I was all effects circuit. The boost column just kind of went out.

dunno

[J M Fahey]

Just from memory: the 8080 does not have a dedicated 6.3V or 12.6V transformer winding for the filament but connects it straight to 30+30VAC or +/-40VDC dropping extra voltage with one or two large power resistors (which get very hot by the way); **maybe** there's a problem there, or somebody replaced one with the wrong value or something and the 12AX7 does not get proper voltage.
The resistors are R112/R113 180 ohms 7W coming from the roughly 30+30VAC windings, check what you actually have at the tube pins 4&5 , pin to pin; not pin to ground.
That with the tube plugged in; without it you will measure some 60VAC between those tube socket legs.
A very crude way to get filament voltage, by the way.

Too low it sounds like cr*p, if at all; too high and it dies.

Somebody check the schematic (I'm not at home now) and suggest what voltages should be expected, and on which pins.

As a side note, tube used should not really matter, nor should it change sound in an appreciable way; in the 8080 , same as in JCM900 the tube is practically cosmetic, it does not distort (duh!!), has a local feedback network, R95/R68 220k/22k , limiting tube stage gain to 10 X .
NO WAY that tube will clip, the ones doing the job are Red Leds Led6 , Led 7 .
That amp is a fraud, the only tube present is designed not to distort.
We are talking "a Marshall" of course.
In fact in the modern MG version they dispensed with the tube, going full SS .... it sounds the same or better.

Only audible difference may exist between a dead tube (or not having +HV) or a working one; worrying about brand does not cut it here.

EDIT: just got home and checked the cleaner Ampix schematics , so here's (in red) the corrected version.

[Grounded]

You do not have any high voltage.
The tap coming off of the power transformer should be 170 Vac.
Goes through diode (D3) then R111 , R110 & R109.
I have attached the schematics.

Hi. I hope you're still in this thread. I have a similar problem. Channel 1 works fine, while the Boost Channel only produces a mild output at high gain settings.

12AX7 seems to have the customary glow.

Pin voltages (DC) grounded to chassis as follows:

1: 0.00V
2: 0.00V
3: 0.00V
4: 6.5V
5: Small fluctuating value <1V
6: Small fluctuating value <1V
7: 0.00V
8: 6.5V
9: 7.1V

Component readings. "North" = end pointing toward rear of cabinet. "South" = Front of cabinet, knobs, switches, etc.

D3
N 7.45V
S -.38V

Both sides of D3 read 24VAC to ground.

R109
N .6V
S 4.7V

R110
N 7.25
S 4.75

R111
N 7.22
S 7.25

While switched to quiet Boost Channel, touching the center post of the red RCA reverb tank connector immediately produces a normal volume buzz through the speaker.

[g1]

There should be more than 24VAC at D3. Have you checked R114 ?

[Grounded]

There should be more than 24VAC at D3. Have you checked R114 ?

R114
33 Ohms

N Fluctuating around 5V DC
S Fluctuating around 3V DC

N 36.5 VAC
S 18.6 VAC

I have 36.5VAC to ground coming from both white transformer terminals, measured at the transformer.

[52 Bill]

Before you guys get too deep into this repair, why not start a new thread. This one's multiple pages, multiple amps and multiple years old.

[Grounded]

Nah, this thread is fine just as it is.

[callai]

another valvestate without overdriven chanel

Hi guys, I know this thread is a little old, but as is the most similar to my problem iŽll give it a shot.

I got my hands on a Valvestate 8080 that my friend had lying around for some time. It had a problem on the overdrive chanel. Almost no sound. the clean is working fine. I plugued my guitar and confirmed it.

After that I open it and found out the 12AX7 missing. Thanks to the previous technician who worked on it.

I will put a working tube on it, but first i decided to take a look on the circuit. Found on leg of R113 loose. I re-solder it and took some mesurments. Here is what I found.

on D3 diode i found 335Vac and 474Vdc.
refering to the ground (chassis) the tube conections reading were the following:
1 - 0V
2 - 0V
3 - 0V
4 - 465 Vdc
5 - 36 Vac
6 - 36 Vac
7 - 0V
8 - 465 Vdc
9 - 470 Vdc

between pins 5-8 and 3-4 i found 456 Vdc on each pair.


I intend to conect the tube soon, but I am worried about the voltages, it seems to me that they are higher than usual. Anyone can confirm that? Some direction to proceed?

Remembering that I took all readings without the tube.

Thanks in advance,

Callai

[nickb]

Hi guys, I know this thread is a little old, but as is the most similar to my problem iŽll give it a shot.

I got my hands on a Valvestate 8080 that my friend had lying around for some time. It had a problem on the overdrive chanel. Almost no sound. the clean is working fine. I plugued my guitar and confirmed it.

After that I open it and found out the 12AX7 missing. Thanks to the previous technician who worked on it.

I will put a working tube on it, but first i decided to take a look on the circuit. Found on leg of R113 loose. I re-solder it and took some mesurments. Here is what I found.

on D3 diode i found 335Vac and 474Vdc.
refering to the ground (chassis) the tube conections reading were the following:
1 - 0V
2 - 0V
3 - 0V
4 - 465 Vdc
5 - 36 Vac
6 - 36 Vac
7 - 0V
8 - 465 Vdc
9 - 470 Vdc

between pins 5-8 and 3-4 i found 456 Vdc on each pair.


I intend to conect the tube soon, but I am worried about the voltages, it seems to me that they are higher than usual. Anyone can confirm that? Some direction to proceed?

Remembering that I took all readings without the tube.

Thanks in advance,

Callai

I think you got the pin numbering in reverse order If that is true then the voltages are OK but the HT is very high. When you put the tube in it will come down. The problem I see is that C53 and C59 are only rated for 350V so I hope they have survived. Poor design IMHO.

It's better to start a new thread than tack onto an old one.