'75 Pro Reverb bias problems AA1009

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johnnysomersett
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'75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Fri Jan 14, 2022 9:59 pm

So, my 74 Pro Reverb decided to shit it's pants when arriving at band rehearsal this week. Big hum, excessive blue glow and redplating to hell on the power tubes.

It's on the bench at the moment and I'm struggling to locate the source of the issue. I've done a bunch of continuity tests and checked component values and so far everything seems in order. I've borrowed the 6L6GCs from my HRD as I know those work fine.

It gets wild when I'm testing the pins. 3 and 4 are seeing the correct 470-ishVDC however both Pin 5's are measuring at nothing like the -50mVDC I'd expect to read. In fact, they're skyrocketing up well above -100 and beyond just on standby, flicking standby off sends the voltage into scary numbers both + and - and constantly moving. In order not to cook the tubes or anything else I've been shutting it down as soon as I've ascertained it's going crazy.

Ive seen an invoice from its last service (August last year) and it has two new tube sockets, plus some other general maintenance stuff.

I would appreciate any suggestions as to other things I can look for as I'm truly stumped.

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With the known good tubes, on standby V7 glows noticeably brighter and with standby off has an obvious blue glow. I'm not leaving them on long enough to get too hot and redplate like the other tubes so as not to damage them more than possible

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Fri Jan 14, 2022 11:02 pm

With load, the voltages across the 1ohm resistor to ground from pin 8 were:

V8: +37mV
V7: +65mV (and still rising until I turned it off for safety)

Also noticed the voltage to the screen is being fed directly from the rectifier via a board eyelet, whereas on the layout it should be from the doghouse/choke area one eye to the right...

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Re: '75 Pro Reverb bias problems AA1009

Post by Scout » Sat Jan 15, 2022 6:20 am

I’m not an amp tech just an electrician, but that schematic shows the red wire from the socket is misplaced.

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Sat Jan 15, 2022 7:41 am

Scout wrote:
Sat Jan 15, 2022 6:20 am
I’m not an amp tech just an electrician, but that schematic shows the red wire from the socket is misplaced.
Yep, whoever has worked on this before has fucked around with things a fair bit for what I can tell. There was something going on with V7 pin 6 where he has connected up the grid resistor and the B+ lines so I lifted those as a floating connection to eliminate that as a fault

Whatever is going on seems to be isolated to V7 as I can get the right numbers off the pins on V8. Well, I could for a bit, it's all gone a bit wonky since. I barely slept last night and working on amps this tired is not a good idea. I nearly went in with the iron twice with it still switched on :fp:
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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sat Jan 15, 2022 8:49 am

Bias circuit appears to have old selenium rectifier.

I’d replace that for starters. A simple 1N4007 will do

If you are red plating tubes, you’re probably losing bias, and that would be where I’d start since it’s a common failure point and about 25 cents and about 60 seconds to replace

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Sat Jan 15, 2022 9:02 am

øøøøøøø wrote:
Sat Jan 15, 2022 8:49 am
Bias circuit appears to have old selenium rectifier.

I’d replace that for starters. A simple 1N4007 will do

If you are red plating tubes, you’re probably losing bias, and that would be where I’d start since it’s a common failure point and about 25 cents and about 60 seconds to replace
There's lots of weird shit going on at the moment. Voltages dropping coming off standby, bias voltages flying all over the place.

I forgot entirely about the rectifier, good spot. I'll put it on the parts list. The grid resistors are getting overly hot (especially on V7) so I'm going to swap in some big cement 5w ones. Annoyingly I have entirely larger sizes than I need. I have 680ohm but not 470ohm. I'll probably just rewire the entire power valve sockets from scratch so I know all the connections are solid and correct to the schematic.

Filter caps are still pretty new by the looks of it too, one less thing to worry about.
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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sat Jan 15, 2022 9:09 am

470 ohm are the screen resistors, not the grid resistors (which would be 1k5, IIRC)

If it were my amp, I’d go through with a schematic and a highlighter and inspect each connection in the power supply and output section (after the phase inverter), highlighting each one as its verified good.

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Sat Jan 15, 2022 9:20 am

øøøøøøø wrote:
Sat Jan 15, 2022 9:09 am
470 ohm are the screen resistors, not the grid resistors (which would be 1k5, IIRC)

If it were my amp, I’d go through with a schematic and a highlighter and inspect each connection in the power supply and output section (after the phase inverter), highlighting each one as its verified good.
Sorry, getting myself jumbled up...I'm running on 2 1/2hrs sleep today. Whatever is across pins 6 and 4 is what I meant.

The grid resistors are the little ones from 5 to 1, right? I can never remember the names properly tbh. Just the numbers.

*EDIT: I just tested the diode in place and it reads as good. I'll change it out anyway though seeing as it looks original
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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sat Jan 15, 2022 10:03 am

Yeah, worth changing out the Se diode regardless. A properly rated Si diode in there will probably outlast any of us, so it’s never a bad idea

Looking for open connections is a good idea, but also look for shorts—anything that’s grounding out the C- supply will also cause you to lose bias, obviously

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Sat Jan 15, 2022 10:10 am

øøøøøøø wrote:
Sat Jan 15, 2022 10:03 am
Yeah, worth changing out the Se diode regardless. A properly rated Si diode in there will probably outlast any of us, so it’s never a bad idea

Looking for open connections is a good idea, but also look for shorts—anything that’s grounding out the C- supply will also cause you to lose bias, obviously
I think I have some time tomorrow morning so I'll go through methodically. Fortunately there's no rush, I have plenty of other amps to use - but I like to get things sorted and find it frustrating when I can't spot something that's probably very obvious
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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sat Jan 15, 2022 10:16 am

Fender amps of the era were famously untidy, and this one doesnt look as though it’s been made any tidier, so it’s easy to miss “obvious” things

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Sat Jan 15, 2022 10:18 am

øøøøøøø wrote:
Sat Jan 15, 2022 10:16 am
Fender amps of the era were famously untidy, and this one doesnt look as though it’s been made any tidier, so it’s easy to miss “obvious” things
Hahah, yeah, I always sigh when I see the mess of white cables. I actually have a week off to recover from a vasectomy at the end of the month....maybe I'll rewire the sucker for a nice low-impact project
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Re: '75 Pro Reverb bias problems AA1009

Post by Scout » Sat Jan 15, 2022 2:28 pm

So does the wire coming from pin 6 is connected to the correct point? It appears to be misplaced.

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Re: '75 Pro Reverb bias problems AA1009

Post by øøøøøøø » Sat Jan 15, 2022 6:37 pm

Looking at pics a bit more closely—

Screen resistors are definitely cooking which is a sign something is wrong (and not necessarily with the screen resistors)

A resistor has been added between 6L6 cathodes and ground (presumably 1 ohm for easy bias current measurement)

The grounding of these resistors looks a bit dodgy at best… as though whomever added them did not have a proper high-heat-transfer iron for doing a good chassis ground.

I’d look there. I’m not overly impressed with what I see there.

By the way, relative differences in brightness of filament glow is normal and not a concern here

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Re: '75 Pro Reverb bias problems AA1009

Post by johnnysomersett » Sun Jan 16, 2022 12:05 am

øøøøøøø wrote:
Sat Jan 15, 2022 6:37 pm
Looking at pics a bit more closely—

Screen resistors are definitely cooking which is a sign something is wrong (and not necessarily with the screen resistors)

A resistor has been added between 6L6 cathodes and ground (presumably 1 ohm for easy bias current measurement)

The grounding of these resistors looks a bit dodgy at best… as though whomever added them did not have a proper high-heat-transfer iron for doing a good chassis ground.

I’d look there. I’m not overly impressed with what I see there.

By the way, relative differences in brightness of filament glow is normal and not a concern here
Yep, screen resistors are overheating. On V7 it gets considerably hotter too. Those are being changed out. I've removed those 1ohm resistors and hardwired them to the chassis proper now, with a decent iron.

I'm now just waiting on a few parts to arrive then I can have at it properly and try to solve whatever is happening.
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