Taming a REALLY bright JCM800

Re: Taming a REALLY bright JCM800

ErikH said:
That cab is wired series/parallel which is correct. All 4 speakers are rated at 16 ohms and all of them wired series/parallel makes 16 ohms.

See post #32 for actual solution to the problem. ;)
Click to expand...

Called them up and asked... they won't let me!!!

They said I can keep it for the entire three weeks and deal with it or I can give it back now and eat the other 2 weeks worth of rental money. And they claim the thing's in top shape and sounds great.

FUUUUUUUUUCCCCCCCC.....

Ok... ahm. Well I guess if they wont give me my money back I think I'll play with the bias some and see if I can warm this thing up.
 
Re: Taming a REALLY bright JCM800

Sounds like a shietty deal Brendon. Not good customer service from the rental place. At least now you know, and hopefully won't give them your business in the future.

If you need some JJ tubes, I can root around in my boxes and see how many I have. I think it's 2 or 3. I'd be happy to send you whatever I have if it would help.
 
Re: Taming a REALLY bright JCM800

B2D said:
Called them up and asked... they won't let me!!!

They said I can keep it for the entire three weeks and deal with it or I can give it back now and eat the other 2 weeks worth of rental money. And they claim the thing's in top shape and sounds great.

FUUUUUUUUUCCCCCCCC.....

Ok... ahm. Well I guess if they wont give me my money back I think I'll play with the bias some and see if I can warm this thing up.
Click to expand...

What????.. who did you rent the amp from???....I'm assuming you agreed to this....

Jeff Seal

p.s..... when's your next gig? I know lots of people in OC....
 
Last edited:
Re: Taming a REALLY bright JCM800

Jeff Seal said:
What????.. who did you rent the amp from???....I'm assuming you agreed to this....

Jeff Seal

p.s..... when's your next gig? I know lots of people in OC....
Click to expand...

Ahhh I'd rather not say the name of the biz because they've been good to me otherwise. If you really wanna know I'll tell you though.

And unfortunately I did agree to it... I didn't actually try the amp out before renting it but I did make them plug it in to make sure the tubes were looking healthy. Sure enough the one step I skip is the one I can't afford to. :yell:

My next gig is on the 27th at Hogue Barmichael's in Newport Beach.
 
Re: Taming a REALLY bright JCM800

Instead of keeping the midrange high, turn the mids DOWN then add just a little treble and presence, say T-4, P-2.

JCM800s usually have a "bright cap" connected to the input gain--to add brightness and definition on your clean settings. You will want to run the gain up to at least 6 or seven to negate this feature. (I clipped my out of the circuit, but of course--you wouldn't want to do that with a rental.)

I always liked the RECORDED sound of my Marshall when I plugged into the LOW GAIN input and cranked it. I would often add a little clean boost. This created a low volume Marshall with the most unbelievable FAT tone. MIne was a 50-watt JCM 800, so there wasn't quite enough power to gig with--but, but those old recordings still sound great.

Otherwise a graphic or parametric EQ (as others have suggested) is about your only other option.

And with a Marshall, louder IS better...mine always sounded best after it got hotter than Hades!

Good luck.

Bill
 
Re: Taming a REALLY bright JCM800

Well it's the head and not the speakers. Does this head have 6550's? A lot of 6550 amps were never biased correctly, and they did have a bright cap from hell. Dolye once commented that the 6550 equiped JCM800's were among the brightest amps ever.

50watt amps have a hard time pushing high wattage speaker cabs, and they almost always sound thin, when they try. The head isn't right and it's further disadvantaged by the high wattage cab. The speaker cab is rated at 270 watts RMS, so the ratio of amp output (RMS) to the speakers is like 1:5.5 This is why the T75's (300 watts in a 4x12) usually sound better with a 100 watt amp too.


6550's can give a strong solid bass when biased properly. Perhaps you can get some help from a brother in such a large population area, and check the tube type and the bias.
 
Re: Taming a REALLY bright JCM800

Have you put tape over the speaker grill cloth yet? That is your simple fix to cut out high end. Simple and non-invasive.
 
Re: Taming a REALLY bright JCM800

Speaking of JCM800s, i'm getting back mine from the repairs this weekend.

I agree that its bit bright in a way or so .. the pre amp tubes just changed to some svetlana and I think I would wanna replace them soon though
 
Re: Taming a REALLY bright JCM800

Lake Placid: He already stated that it has EL34's.

Since they won't let you return it, do the tape on the grill cloth and try the low input. That should help. If you can borrow an EQ pedal from a buddy or if you have an overdrive pedal with a tone knob on it, try one of those before the input. On the overdrive just turn the gain down if you don't need much and adjust the tone to taste.
 
Re: Taming a REALLY bright JCM800

ErikH said:
Lake Placid: He already stated that it has EL34's....

.
Click to expand...
Oops! missed that. Sorry.:smack:

If he can find some one that knows their way around Marshall innards, he could possibly get the bright cap temporarily fixed, and the bias checked though.

I agree with putting some tape over the speaker centers too.
 
Re: Taming a REALLY bright JCM800

Boogie Bill said:
Instead of keeping the midrange high, turn the mids DOWN then add just a little treble and presence, say T-4, P-2.

JCM800s usually have a "bright cap" connected to the input gain--to add brightness and definition on your clean settings. You will want to run the gain up to at least 6 or seven to negate this feature. (I clipped my out of the circuit, but of course--you wouldn't want to do that with a rental.)

I always liked the RECORDED sound of my Marshall when I plugged into the LOW GAIN input and cranked it. I would often add a little clean boost. This created a low volume Marshall with the most unbelievable FAT tone. MIne was a 50-watt JCM 800, so there wasn't quite enough power to gig with--but, but those old recordings still sound great.

Otherwise a graphic or parametric EQ (as others have suggested) is about your only other option.

And with a Marshall, louder IS better...mine always sounded best after it got hotter than Hades!

Good luck.

Bill
Click to expand...

Tried the eq trick you mentioned already. No dice. Now that I think about it... the only controls that really seem to work at all on this amp is the volume controls. The other controls ARE operational but their range seems like its almost nil.

The low gain input seems to be broken... I can't get any sort of competitive volume out of it and theres a lot of noise.

Cranking the amp without the attenuator doesnt really help round it out... it just gets that same annoying ear-shatteringly bright tone... just LOUDER.

I checked the pre tubes again... turn out they're GT 7025s. I found a listing for them here: http://www.groovetubes.com/tubes_list.cfm?ObjectGroup_ID=107 Are these real 12AX7s or not? I have some Sovtek 12AX7s lying around, I'll pop those in and see if it makes a positive difference. I'm not going to go clipping bright caps unless I've exhausted my beam-blocker/bias by ear options first.

Stay tuned...
 
Re: Taming a REALLY bright JCM800

That thing just plain needs service. The JCM800's are bright by nature but still, they shouldn't be as bright as you're saying. And they fact that the low input isn't working is an issue in itself. I'd be raising some hell.
 
Re: Taming a REALLY bright JCM800

If the low input isn't working, the high gain input can't work correctly either. That amp is broken in such a case.

On the JCM 800 2204's the high gain input goes directly to V1A, then it goes to the low gain input through an isolation cap (cap3) before going through the gain sensitivity pot and on to V1B. In other words, the low gain channel shares the same signal path as the high gain channel, but just plugs in later, by passing the amplification of the first preamp tube triode.

If the low isn't hardly working , then there's a possibility that there's a short, or a bad cap, or something shunting the lows and mids to ground. Perhaps V1 (first 7025) is bad. The possibility exists that the RC network between the cathode of V1B and ground may be bad. This can cause crackling, static, and awefull metalic tone. I've seen this happen in older Fender amps, with worn out caps and resistors.

7025's are a military grade American 12AX7. There were ofton used in Fender amps in noise sensitive positions. It wouldn't hurt to try some other 12AX7 tube in V1 and V2 agian.
 
Re: Taming a REALLY bright JCM800

B2D said:
Tried the eq trick you mentioned already. No dice. Now that I think about it... the only controls that really seem to work at all on this amp is the volume controls. The other controls ARE operational but their range seems like its almost nil.

The low gain input seems to be broken... I can't get any sort of competitive volume out of it and theres a lot of noise.

Cranking the amp without the attenuator doesnt really help round it out... it just gets that same annoying ear-shatteringly bright tone... just LOUDER.

I checked the pre tubes again... turn out they're GT 7025s. I found a listing for them here: http://www.groovetubes.com/tubes_list.cfm?ObjectGroup_ID=107 Are these real 12AX7s or not? I have some Sovtek 12AX7s lying around, I'll pop those in and see if it makes a positive difference. I'm not going to go clipping bright caps unless I've exhausted my beam-blocker/bias by ear options first.

Stay tuned...
Click to expand...
The Problem could be in pots!!
Try to scoop the mids if it doesn't work, the pots aren't grounded very well. The biggest difference is in the middle(am I right?) 'cos it's wired nearest to ground

Erm if the eq pots don't work you should mention it to the rental-place and they have to repair it! :chairfall
 
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