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Messages - g1

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Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« on: March 10, 2019, 09:32:40 PM »
Yep.  Notice the 5205 schematic still has 25V caps?  Not possible if that were really a 21-0-21 secondary.
This may sound crazy, but some companies label the transformer or schematics with what the rectified DC voltage will be.  (Roland?)
That must be the case with this 5205 schematic.  So a 15-0-15 secondary like Phil mentioned will give a rectified +/-21V which will drop a little bit under full load and give the 12W at 16ohms that the 5205 specs. 

The 5203 is completely designed around a certain voltage supply.  If you change it, even though you put in higher voltage caps, you would need to adjust the dropping resistors R35 & R36 so that the 15V zeners don't burn.  Then you will probably have to increase the size of the heatsink, which was spec'd for 30W output.  Now you can hope that the power transistors can handle the extra current, and that the speaker can handle the extra power.  :D

Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« on: March 09, 2019, 03:39:06 PM »
No don't do that as you will exceed the voltage rating of the main filter caps which you say is 25volts.
Your transformer should have close to 15-0-15ACV which will reap 15 x 1.4 = 21VDC.
That sounds better.  Solving backwards from 30W into 4ohms, rails would be approx.  +/-15.5V fully loaded.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 08, 2019, 01:34:19 PM »
Reverb circuits are normally wired with only the pickup side grounded to the Tank Case. (Not always but most often)
I think you are right that something has changed with regard to the tank.
Other possibilities are that the tank was replaced with same type but different jack grounding scheme (5th character of accutronics tank code), or there may have been rubber mounts insulating the reverb tank from the aluminum plate it is mounted on.

Anyway, good to hear smackoj found a workaround and it's all working fine now.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 06, 2019, 01:41:30 PM »
All 3 pieces touch metal to metal.
And you have all these contacts made when testing on the bench?
Does the reverb control affect the hum at all?
If all electrical connections remain the same as on the bench, then it is more likely some kind of EMF due to positioning of components or lead dress.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 05, 2019, 04:07:46 PM »
Still have no idea how you could have had -30VDC on that pot if Q26 and it's circuit is ok.
Have you tried the amp now to see if it works?

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 04, 2019, 10:26:44 PM »
That sounds right.  Now on the schematic there is a straight line down from the E of Q26 to the pot.
So if there is -9V at Q26 E, it must be on the pot as well, or there is a missing connection.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 03, 2019, 10:15:53 PM »
If you are getting -30V on the pot, Q26 has likely shorted.
Q26 is being used as a voltage regulator.  It should have a constant -9V on the emitter (H) no matter what the pot setting.  The zener D3 on Q26 base is what determines the emitter voltage.
All the current limit pot does is put some resistance between the supply node H and the R4/R5 junction in the power amp.
The only way to get -30V on the current limit pot is if Q26 is shorted from C to E.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 03, 2019, 08:56:23 PM »
Yes, in this case with the OT a load should always be connected.  Better safe than sorry.

Have you checked Q26 with your meter?

The Newcomer's Forum / Re: What kind of capacitors to use where??
« on: March 03, 2019, 08:41:50 PM »
You mentioned ripple current and lifespan, for electrolytics also look at the temperature rating and opt for the higher temp if you can.  It will usually further extend the lifespan.

  Did you remove the bridge rect. to test it?
If not, you may be reading a shorted power transistor across the supply rail.
Shorted power transistor would be the most likely suspect, and this model can be very difficult to repair if that is the case.
How many power transistors are fitted and what type?

I have a feeling you are going low enough now with that resistor that it is affecting the bias of V1b (because it is in parallel with the 1K5 cathode resistor).
So it may be that what you are hearing is the change in V1b bias, rather than the change in NFB.
You can check this by measuring the DC voltage at V1b pin8.
With no signal, if the DC changes for the different switch positions, the bias change may be what you are noticing.
Ideally, the NFB should not affect the DC conditions of V1b.
If you really wanted to do a definitive test, you would then open the NFB loop, and put the resistors directly in parallel with the 1K5 cathode resistor, then compare the audible result.

Your meter readings sound about right.
You will always have R7 there, and it is basically in parallel with R15 because of the low resistance of the OT secondary.
Feedback can be very subtle and hard to notice.
Try open loop (no R15 installed), compared to a lower value like 5K for R15.
You should notice some difference, at least a bit of volume drop with a 5K for R15.  All other values between 5K and open, will be even less noticeable.

Also, with 5K installed for R15, it should read around 1K with your meter. (5K paralleled with 1K5)

The Newcomer's Forum / Re: Amp repair?!
« on: January 25, 2019, 03:13:48 PM »
Reading the heater voltage without the tubes installed is in this case not much help.
You need to check with the tubes installed.
Post your results.

The Newcomer's Forum / Re: Fender bass cab questions
« on: January 17, 2019, 02:31:39 PM »
The two 10's are wired in series.  So that part is simple addition, if they are 8 ohms each then the two of them in series is 16ohms.
Now you have 16ohms wired in parallel with 8ohms (the 18).
The equation for parallel is product over sum.  So (16x8) divided by (16+8)

Maybe your meter is reading high, disconnect one of the speakers and measure it by itself to get an idea of what your meter reads for an 8 ohm speaker.  Also, DC resistance readings on a meter are usually lower than the impedance rating.  So for a calculated 5.33ohm impedance, we expect the meter to read a little less.
Also note what your meter reads with the probes shorted together.  This should be zero, if it is 1 or 2 ohms or something, you must subtract that from your measurements.

The Newcomer's Forum / Re: Gear Suggestions
« on: January 15, 2019, 01:53:52 PM »
Have a look at the Aneng 8002 meter.  You won't find similar features and specs for that price anywhere else.

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