Randall RH150 low output

[LarBal]

Hi there,

I've been repairing tube amps for a few years, and have recently started taking in SS amps. Up until now they have been simple repairs, that I've been able to take care of without no transistor knowledge. Now I have this amp that is going to require me to do some learning, and I'm hoping to get some help and guidance from this forum.

Amp is a Randall RH150. It has one tube, a 12at7, that appears to be a driver tube of some sort. Power section schematic is attached. I should say that I'm still staring at the schematic and figuring out what everything is...

Also, when the amp came to me it had a 12AX7 in it....

The amp has really low, farty , kinda crackly output, but I can hear the tone of the amp buried in there, and it sounds pretty good . I'd say its about as loud as a Champ right now, at full bore.

I've done all the normal cleaning of the fx jacks and ins/outs, and jumpered them to see if that worked, but it didn't. I took the fx send and plugged it into a different, working power amp, and the preamp is fully functional. I took a working preamp I built (MKIIC+), and plugged it into the return, and the amp had the same farty low output.

So that's as far as I've made it. I've narrowed down the problem to the power amp, which I know nothing about.

Any thoughts on where I should start, and what I should do, along with possibly a beginner's explanation?


G3Poweramp_large.pdfG3Poweramp_large.pdf

[Enzo]

Don't know that I would call that a driver tube, but if you like, go ahead. I just see it as a pair or triodes in series, like any tube amp. The signal hits the left grid, and the resulting plate signal is fed to the second grid. The second plate signal feeds a source follower to feed the power amp proper at SIGNAL A.

So it is low and farty. First check power. Looks like it runs on 70v rails, so are they both up close to that and clean? The op amps no doubt run on +/-15v or maybe 12. Are those all up to voltage and clean? The op amp ICs with 8 legs have outputs on pins 1 and 7, is there any DC offset on any of those? Looking for volts here, not a few millivolts. The tube supply is +375vDC, is that close to voltage and clean?

You know tube amps? OK, so are the voltages around the tube normal? With 375v B+, is the plate voltage on each triode a couple hundred volts more or less? Are the cathodes sitting at a few volts? Did you try a different tube?

Check out the signal path. It enters on page 2 through IC2A. From there through IC2B. All that stuff left of and below IC2B is basically a graphic EQ circuit that is not adjustable. They set it up for a tine and hard wired it.

The outputs on the 14 leg IC are the corners.

So apply a signal to the FX return and scope it through the amp. What does it look like at IC2A? IC2B? At grids and plates of the tube, and at SIGNAL A. That is page 2 and the lead-in to the power amp. If you don't have a clean signal at SIGNAL A, then you won't have one at the speaker.

Page 1 is the power amp. We'll look at that later.

[LarBal]

Thank you very much for the response! This gives me a great starting point. I'll report back when I get a chance.

[LarBal]

Ok, sorry for the delay.
As for voltages, the plate of the first triode of the 12at7 was 220VDC or so, and the plate of the next one was only 110VCD, which seemed odd given similar setups. The supply was fine at ~375VDC.
For the 70V supplies, I get nothing at QC1, or on the drain or source(pins 2 + 3) of TR1 and TR2. So there's something wrong there obviously.

For the -V supply, I get -60VDC at QC4, but I'm getting 0V on pin 2(drain) , and -60V on pin 3, of both TR4 and TR5. (I believe the schematic is for the 300W PA, so I have no TR3 or TR4)
This makes no sense to me, unless I have my pinout wrong. I would expect -60 V on the drain,pin2, that connects to the rail. I would also expect this same -60V to be on the other side of the big 1W resistors, the sources of the +rail MOSFETS, but again, I got nothing.

I've attached some more schemes I got from them today, including the power supply, and the 75W version power ampG3Powersupply[1].pdfG3_75-150PA[1].pdf

[g1]

That 75W PA file didn't work, here's a zip that works: http://music-electronics-forum.com/a...68-g3files.zip

Which pin of the 12AT has 110VDC, pin1 or pin6 ?
Have you checked the associated plate and cathode resistors for both halves?
Which schematic matches, the one from post #1, or from post #4 ?

[LarBal]

Thanks! The AT7 pin 1 has 110 plate, 1.7V cathode , pin 6 had 230V plate, 5V cathode
The plate and cathode resistors read within spec for the 6,7,8 side, and I cant get an accurate reading on the wonky side for the plate resistor when it's in circuit, but I don't know why. I'm getting 16V and climbing.

As far as the PA schematic, the one I posted in post #1 has 6 MOSFETs, for 300W, and the one in post#4 has 2 MOSFETs, for 75W.
Mine is a 150W, and has 4 MOSFETs, but I don't have a schematic for that. I'm inferring that they schematics are nearly identical apart from that, with the supply voltage being a bit different.(70V for 300W vs ~55 for 75, and mine is showing -60 for 150W, but as I said I have no +V 60V rail.

I'll try and reattach, not sure what happened

[g1]

Lift one side of the plate resistor to pin1 and check it's resistance.
There are fuses shown for the + and - 60V rails, is the fuse for the + side good?

Don't worry about the schematic in post #4 not working, it's a forum glitch. It's included in the zip file in post #5.

[LarBal]

Ok the plate resistor for pin 1 reads good when lifted.

The fuses for the +60V rail are the tiny brown box variety with 1/4" leads. There are 2 in series, 3.15A each, for the 6.3V on the schematic.
One of them is open, and the other is fine.

I'm trying to figure out which fuse this is exactly, as I'm unfamiliar with these types. Can you help me out with the part? The fuse itself says T3.15A, which I'm assuming means time/slow , but the schematic says 6.3AF, which I would think means fast, and I know tube amps usually do fast for B+ fuses.

This is as far as I've gotten : Axial Radial Thru Hole Fuses | PICO Fuses - Littelfuse
But I'm not sure if I need TE5 or TE7

[g1]

T3.15 should be close enough, if you want the exact one, what other numbers are on it? The TE5 and TE7's have series numbers that are printed on the fuse. They are in series and not in parallel?

Can you recheck the tube voltages, one probe to ground, pins 1,3,6,&8? Then one probe on 3 other probe on 2, then one probe on 8 other on 7.

[LarBal]

Turns out they are neither in parallel or series, even though continuity tests on the sockets they were in are seriously confusing.
With the fuses out and amp on, I am getting +60V on 1 side of F1, and -60V on the same side of F2. So it seems the voltage source is good!
We've been looking at the "75-150W" power section, which has the fuse labeled at 6.3A for both F1 and F2 . Turns out this one uses 3.15A ?? Again, my amp is the 150W vesrion. Maybe a misprint? 3.15A for each side seems plenty. Or maybe it isn't, and that's the problem?

I've attached a picture of the fuse. There aren't any other designations that would lead me to a part #, that I can see.

The voltage readings for the AT7 are:
pin 1 :113V
pin 3: 1.77V
Pin 6: 236V
pin 8: 5.11V

Pin 3>2 :1.75V
Pin 8>7 :5.11V

[The Dude]

Presumably, the amp has worked with that fuse value since it was built, so I'd assume the value is correct. There's likely a short causing them to blow. Any T3.15A solder in fuse should work. I wouldn't be concerned about an exact part number.

[g1]

There must be something on the +60V line that made the fuse blow, as The Dude said, you will need to fix that or the fuse will just blow again (order a few).
Aside from that issue, the voltages at pin 1 & 3 of the tube don't look right. 1.77V on the cathode says there is little current flow through that half of the tube (compared to the other half). Is it really a 3K3 resistor at pin3 ? If so, there should be much more than 100V at the plate, is the resistor to pin1 really a 100K ?

[LarBal]

Thanks guys. Fuses are on the way, and I'm with you, I know the fuse blowing is a symptom of whatever else is wrong with the amp.
I'll sub in another tube and see if the readings are different. Maybe a bad triode. I did that when I first got the amp, but it sounded no different, and didn't "fix" it. I didn't take readings at the time...
The plate R for pin 100 is definitely 100K, I measured. I need to locate the 3K3 Rk still.

So back to the fuse blowing, what are some common failures downstream in these types of power amp circuits. I know in a tube amp, if the B+ fuse goes, I'll be looking for bad tube, screen R, grid R, etc., but I'm not sure how to go about troubleshooting this. I don't have a lot of these components lying around like I do for tube amps

So where do I start?

[g1]

The fuses are shown downstream of the rectifier and caps, so the fault should be after those.
A good place to start would be to check the .1R 5watt resistors, one per output device.

[LarBal]

Ok great. What is the purpose of those .1R resistors, and how to check? I mean what am I looking for, a meter reading that runs away? I've never measured something that small of resistance.

Also, can you help me understand the signal flow, starting with signal A, on the power amp sheet?