Weak reverb, or...what's weak this week

[bassistbobby]

Thanks guitician and g1,...
I live in an apartment, and have an appointment,...so have to wait until mid-afternoon before I can crank my amp a little to check how equal those levels are to my guitar. I'll report back around 3:30 or so PDT. Hopefully you'll be available then.
Here's a question,... somebody said the reverb circuit is part of the effects loop in this amp. Is that's true?
If yes,... then does the effects loop potentiometer have an effect on the level of effect I'm getting from the reverb?
I couldn't find a IC1 on the schematic, just a IC2, and IC 3 (a & b). Then there's IC7 (a & b) in the recovery unit.

Thanks

[guitician]

No the effects loop is before the reverb circuit, and when not being used, doesn't change the signal to the reverb stage. IC1 is the input circuit. I'll be around until about 5:00pm PST.

[bassistbobby]

The hum and splash are about the same level, but neither is as loud as strumming my guitar is.
Just got back from Guitar Center, and got to play a couple used Marshall DSL 40s, and their reverb's weren't any more of an effect then mine is.
Now I don't know what to think.

[g1]

If the DSL doesn't sound any better, then I think yours is working as it should. Compared to classic Fender, the 900 gives a very slight reverb effect.
Are you running the amp distorted or clean? You will probably notice more reverb when using cleaner settings.

[bassistbobby]

I normally use my amp on the drive channel, and with effects all in the loop,... but I unplugged my effects loop, and tried both channels, while making an evaluation of the reverb effect.
I think you're probably right. I guess Marshall just doesn't give you any more reverb then they think you'll ever need.
I still wish I could squeeze a little more out of it though.
Thanks for all the help.
CHEERS

[guitician]

Increasing R20 may lower the dry signal and bring up the wet, if you want to squeeze some more reverb out.

[bassistbobby]

wow, I didn't realize this amp had resistors that high. How much would I increase it by, 10%? To 2m42 ?

[guitician]

I would think more like 33%, add a 1M ohm in series with the 2M2 and see what happens. Doing a impedance calculation to find just the right value would require analyzing the whole circuit for the equivalent resistances.

[bassistbobby]

Radical ! I just changed out my out of spec grid resistors (R32 and R33) to the proper 2K2, 5watt, 5% resistors about 10 days ago. I'm going to see if I like the reverb a little longer before doing that. I live where I can't turn it up, but there's a jam at my brother's place, w/his band, this weekend. Then I'll probably try that.
Thanks for the great information so far. Great stuff to know, for sure.

[bassistbobby]

So, that signal path with R20 is the reverb bypass circuit, right? I gotta tell you guitician,... I'd definitely need help solving the modded circuit using ohm's law. I haven't had to do anything like that since electronics class. Do you know how to do it? I guess I could look it up, but I'd want somebody to check me before I started doing experimental mods to my amp. This is stuff I need to learn though. I should just take an advanced electronics class over at Renton Vo-Tech.

BTW,... I'm in a Seattle suburb,... where are you guys? Jimi Hendrix is laid to rest less then 1 mile from here. There's always people from all over the world over at the grave site, paying their respects. There's commonly many many guitar picks and joints left on the headstone, for Jimi to enjoy.

Either of you ever own a JCM 900 Dual Reverb?

[guitician]

The calculation really isn't necessary, it's not like having the wrong value is going to harm anything. The dry signal runs through R20, so if it was open then all there would be is wet reverb signal, and if it was shorted the reverb signal would be way too low to blend with the now huge dry signal.

I visited Seattle once, and saw Bill Gates from Microsoft at a restaurant I was at in Redmond. I live in sweltering Phoenix,AZ Jimi did some home movies driving on I-10 back in the day.

[bassistbobby]

Well yea, all of that is true, but I wouldn't want to just solder a resistor in there without at least checking to see if it might result in the desired effect. The odd grid resistors I took out were sets of two resistors, in series. It had two 470ohm 2 watt resistors, where one 2k2ohm 5 watt resistor was supposed to be, and it worked. Although I absolutely would not recommend anybody try this. Anyway, I'd want to check wattages of the resistors with ohm's law before adding another resistor to the reverb bypass. I've looked around for an answer about this wattage question, and nobody seems to agree. Isn't the wattage rating of a resistor just how much power the resistor can handle? So I'd just need another 1/2 watt resistor, and it wouldn't effect the resulting wattage of the circuit? So, if replacing a single resistor with two resistors in series, then each series resistor would need to have a wattage rating of at least 1/2 of the original resistor, and probably shouldn't exceed the wattage of the original resistor...? Is that correct?

We here in Western Washington are getting a summer to beat all this year. The driest June ever recorded. This is SeaFair week, so we've got a big hydroplane race on Lk Washington this weekend, complete with a big air show featuring The Blue Angels. Then pre-season NFL football starts.

Seattle's real nice (but don't tell anybody)

I'll check back later.

CHEERS

[guitician]

This "reverb bypass" (R20) is not a power circuit. The wattage of the resistor could be a lot less, but the common 1/2 watt type is used everywhere they need a resistor in the amp. A 940 ohm resistor(2x470) will drop half the power of the 2K2, so at 4 watts(2x2) it is more than enough. You wouldn't solder it in, you "tack" it in just to see if your good. You don't fill the hole, just touch the lead with the iron that has a tiny bit of solder on it and it should stick to the solder pad around the hole. The wattage of the resistors adds together series, or parallel. And adding a bigger wattage resistor doesn't change a thing, except take up more space.

[g1]

I think the "5881" version of these amps used 470R screen resistors? If so, maybe someone just added an extra in series with each of them and threw some EL34's in there.
Have a close look at the schematics for the 2 power amp versions and see if you can figure out which version is was stock. If it was 5881, it would be best to do the complete change over.
The following conversion manual shows how to go from EL34 to 5881, but from the info you should be able to figure out how to do the reverse.

(also note that some revisions of the EL34 versions used 1K screen R's, some used 2K2.)

[bassistbobby]

I suspected that too, so I checked the serial number date code, and it's January of 1993. Then I got all of the Marshall engineers schematics for my model and type amp, and got all the parts lists of the different common mods (to also see if it had been otherwise partially or fully modded),.... then matched the components on my boards to the drawings,.. and determined that way that it's an early 1993 JCM 900 4500 EL34 Power Tube amp (probably one of the last), and had just the one grid resistors mod. If Marshall or somebody else modded just the grid resistors in order to be able to bias a set of 6L6GC tubes onto it, then they sure did a crappy job, and it was totally unnecessary. You can bias either type of tube into this (or any, I think) EL34 power amp (but not the other way around), and who ever altered those grid resistors left long flying leads coming up from the board at both ends of the splice. Grid resistors are supposed to be wired as close to the tube as possible, so I don't think Marshall LTD. would have done that. The other way I confirmed it was by matching the amp schematics drawing numbers, PCB 0012, issue 7, with a file number of JMP 50C, and (preamp) PCB 0189, JMP 52C... with the stamps and numbers my power and preamp boards have on them,.. and they match that last EL34 design. So, that kind of cinched it for me. I'll post the power amp schematic, along with the photos of my circuit boards that show the stamps on the boards.

Power amp schematic...
http://www.drtube.com/schematics/mar...0112-iss10.gif

Photos of the power amp circuit board's stamps, and the preamp board's stamps,...