JCM900 4500

[TopOfTheMountain]

I have been working on a JCM900 Model 4500 for the last few days. I can not get it right. The complaint was low volume. Yes, there is low volume. I have isolated the problem (I think) to valve 1.

In the attached picture I have a good signal at "1" driving another amp that I'm using to figure out where the gain is dropping in the Marshall. After "1" the gain dropped way too much. So I isolated the next gain stage by cutting at points "A", "B", and "C" then feeding the test amp from point "2" through a .1uf cap. I get no signal at all. I'm just using the first stage of the tube eliminating the cathode follower with the circuit cut like I have it.

I have checked all the voltages going to the tube the components, and the continuity from the board to the tube side of the socket. Everything checks OK.

R13 (a 220K resistor) was the only thing letting the signal through with no gain from the tube stage I guess. Why am I not getting a signal at point "2"? This is driving me nuts and of course the customer is in a hurry.

[Enzo]

You cut at B? SO the plate signal of the first triode has nowhere to go?

Hi and welcome to the forum. Since I don;t know you, I assume nothing, so...

Did we try a different tube, I hope?

Cutting traces is not really a very good way to go about this, especially since you wind up without a circuit.

Are both side of the tube conducting? In other words, current flowing? That means there will be a couple volts at the cathode of V1a, and the plate of V1a is sitting at 100-150v lower than the B+ for that stage. Yes/no? And the other half of the tube is a cathode follower, so B+ will be directly on its plate, and there should be 200v or something on its cathode. You'll have to reconnect B for that side to work. Oh, and C as well. Why did we cut C in the first place?

If either is not conducting, turn out the room lights and verify BOTH heaters glowing in the tube. No voltage on either cathode would mean that triode is not conducting. Also, on V1a, if the plate voltage is the same as B+ - ie no drop across the plate resistor - then that means no conduction. No conduction = no signal passing.

If you are losing tons of signal, don;t worry about numbers so much as just signal or no signal. Put a nice reasonably strong signal at 1, then look for a substantially larger signal at 2. V1a amplifies or not.

With B and C connected so V1b can fuction, if no signal is getting to its grid, because no signal appears at the plate of V1a, then inject a strong signal at V1b grid through a cap. (remember there is large DC there) Does the signal come through the amp? For that matter, if you inject a signal at the treble wiper, does that come on out the amp?

Any DC voltage sitting on 1, the grid of V1a? As if C22 was shorted.

You are aware of cathode followers? They have no gain. Well they have a slightly less than unity gain. In other words whatever is at the grid is about what will be at the cathode. It is not a "gain stage" then. SO don;t expect gain there.

[TopOfTheMountain]

Thank you so much for getting back with me. I was kind of hoping it would be you Enzo. I'm not really new on here. I've lurked for years...just never posted.

First off, yes, I tried other tubes and when I say I cut the circuit I was really just removing wires from the tube socket. I wanted to isolate that one stage to see why it wasn't working.

I went down your list of suggestions. I was not getting any voltage on the cathode. The cathode resistor was good as well as the connections. Still no conduction. Hmmmm. You mentioned heaters. I've been doing this for 15 years. I know that means nothing but I should have known better. The heater wasn't glowing!!! ARRRRGGG! I didn't even think to check the tension of the socket. It works! Thank you very much for the advice and revealing my own mortality I'll be sitting in the corner now with my tail between my legs.

In my defense, it did have one of the IC chips bad in it which I thought would cure it and probably would have if the heater was working.

Thanks again. Now I can get this customer off my back.