Gallien Krueger 800RB with weak output

[TheJames]

New patient on the bench...

Good news...It works.  Signal passes, sounds good.  No distortion, no smoke.

Bad news....It's "weak."  I haven't put a load on it and measured, but I'm guessing it's maybe a third of it's rated 300W output, if that.

Checked the voltage rails...Should have:
+/- 85V
+/-60V
+/-15V

Actually have:
+75.7/ -75.8V
+59.7/ -59.7V
+14.3/ -15.5V

So obviously, the power rails for the 300W amp are off.

Power transformer has dual secondaries, one for the +/-85 and another for the rest of the voltages.  Secondary AC voltage for the 85V rails reads 113VAC, and post rectifier, +151.8 VDC.  Disconnecting the 85V rails from the power amp doesn't change things.

The schematic has a few voltages to check...They all look fine (within a tenth of a volt).  The bias was off a tad according to the schematic...So I tweaked that up but no difference.  I would think with this topology that one bad output device would take them all out, so it wouldn't pass anything...Am I missing something?

 

[TheJames]

Here's the PSU...

 

[PRR]

> guessing it's maybe a third of its rated

75 of 85 gives 78% power, not 33%.

Time to stop guessing. Change the battery in your meter(). Do what it takes to dummy-load this beast. Put a 'scope on the output.

 

[TheJames]

Okay...Some numbers...

100hz sinewave in, preamp set to maximum input before distortion, output cranked ("boost" cranked as well) gets me to 92V P2P across a 4ohm load.

Convert peak-to-peak V to RMS  92 * 0.3535 = 32.5Vrms

P=IV

Need current...

I=V/R  32.5/4 = 8.13A

Solve for power... P =IV,  8.13 * 32.5 = 264W

It's a little soft...The amp is rated for 300W RMS into 4 ohms.  The missing 36W could very well be the low power amp rails.  Definitely no clipping on the output.

There doesn't seem to be enough preamp drive to get the amp moving.  I can get the preamp to start to distort with the input gain.  Maybe two issues...

Insufficient preamp drive.

Low amp rails.

 

[TheJames]

Okay...I got the correct preamp schematics..(attached)

Everything is fine up until "boost" circuitry. There's a DG419 CMOS SPDT switch that turns the "boost" on/off (which is really just selecting two points of a voltage divider).  Right after that is a J113 FET.  The signal hitting the CMOS switch should be 93mv (mine is 100mv), the signal out of Q89 should be 1.1V, but is actually 40mv.  Pin1 of the DG419 and the Gate of J113 both read the same signal level as the input of the switch.

I initially swapped the J113....But that didn't change anything.  Next thought...Caps C92 and C93 are bad.  Swap those...BAM!!!  Now I'm getting proper signal to the power amp.  Guess it's time to recap the amp.

 

[rjb5191]

Hello, did you ever figure out why your +/- 85V voltages were 10V low. I have the same issue. If it was reading this way with the power rails disconnected The only things it could be are the rectifier, the transformer, low ac voltage from the wall, or possibly leaking caps? Did you find anything in this regard?

 

[TheJames]

Hello, did you ever figure out why your +/- 85V voltages were 10V low. I have the same issue. If it was reading this way with the power rails disconnected The only things it could be are the rectifier, the transformer, low ac voltage from the wall, or possibly leaking caps? Did you find anything in this regard?

I honestly don't recall if I ever sorted the 85V rail issue. The main thing for me was the signal loss before the power amp that was caused by the bad caps. I did recap the whole amp. Preamp board, power amp board, all the big power supply caps, and that resolved the "weak" output. It's currently racked up. I'll pull it out and see what's going on inside.

One thing to note...If I recall correctly, these things had a tendency to go up in flames when pushed really hard. It's possible that GK reduced the secondary voltage on the 85V rails later in production to resolve this issue. One amp having 75V rails indicates a potential problem. Two or more, indicates a production flaw or design change that's not documented in the available schematics.

 

[5v333]

U^2 / R = P

 

[rjb5191]

I honestly don't recall if I ever sorted the 85V rail issue. The main thing for me was the signal loss before the power amp that was caused by the bad caps. I did recap the whole amp. Preamp board, power amp board, all the big power supply caps, and that resolved the "weak" output. It's currently racked up. I'll pull it out and see what's going on inside.

One thing to note...If I recall correctly, these things had a tendency to go up in flames when pushed really hard. It's possible that GK reduced the secondary voltage on the 85V rails later in production to resolve this issue. One amp having 75V rails indicates a potential problem. Two or more, indicates a production flaw or design change that's not documented in the available schematics.

Thanks, Let me know if you are able to check the voltages. This would be the 3rd 800 RB I've heard with their being 75 VDC vs 85 VDC, so it would make sense that there was a change not noted on the schematic. This makes sense especially with the dissappation being quite high in the 300 watt section at thre maximum swing. Power transistors failing might have been more common then they liked.

 

[TheJames]

Thanks, Let me know if you are able to check the voltages. This would be the 3rd 800 RB I've heard with their being 75 VDC vs 85 VDC, so it would make sense that there was a change not noted on the schematic. This makes sense especially with the dissappation being quite high in the 300 watt section at thre maximum swing. Power transistors failing might have been more common then they liked.

I've confirmed, +/-75V on mine. If I recall correctly, the earlier ones had horizontal power switches. Mine is a vertical switch from the mid 90's I think.

 

[rjb5191]

Thanks a bunch. It ain't science, but that settles it for me.

 

[5v333]

Woops... Old thread...

 

[TheJames]

Thanks a bunch. It ain't science, but that settles it for me.

Yeah...I think it's safe to say that there was a design change that lowered the output stage voltage rails at some point.

Lowered output voltage for less smoke.

 

[moamps]

A supply voltage of +/- 75V is more than enough to obtain a nominal power of 200W at 8ohm and 300W at 4 ohm. The fact is that the power supply has a small capacity of smoothing capacitors (only 3mF per rail) and that the power supply voltage sag is large. I guess it was a design choice.