18Watt.com

Hey all it’s been a long time since I’ve been here (2-3 years I think).. Anyway Ive got a Fender 65’ Princeton Reverb Reissue which I modded a bit back when I first bought it. I’ve had this amp for years, And it’s never once given me a single hiccup. That is until the other day….

I was playing it at lower volumes at home when I began noticing a very uncharacteristic hum, with intermittent crackling sounds and scratching noises. I know this amp well and I was immediately concerned so I took a look at the back panel and immediately noticed a bright orange/reddish glow coming from the rectifier tube. A glow far greater than typical and which looked like a bottle rocket in comparison to all the others. I held my hand close and it was putting out a TON of heat. I quickly flipped off the power switch and figured the tube was kicked. So I ordered a new Tung-Sol GZ34 and left the amp alone. When I got the new tube I put it in, rechecked the bias was still set ok which it was and then used the amp like usual. After 2 weeks the exact same thing I just described happened again. I thought it was strange since I’ve never had a rectifier fail once in the 6-7 years I’ve owned it, But it happens… So again I ordered a new tube, again checked the bias and..Yup AGAIN the same thing happened. Only this time it didn’t take two weeks to redplate it took about 40 minutes!

**Now it’s possible that I’ve just had some real bad luck with rectifier purchases, or more likely my amp has an issue. I’m fairly adept at testing & working with live valve amps but I’m unsure how to troubleshoot this specific problem. What initial steps and measurements would you take in this situation to at least confirm that there’s something wrong with the amp?? I pulled the chassis and closely examined every wire/component and everything checked out visually, So I don’t believe photographs would be helpful in this case. The circuit hasn’t been changed and nothing harmful has happened it, it just started cooking rectifier tubes out of nowhere.
I’ve always done any/all maintenance myself replacing rectifier, preamp & output tubes, re-setting the the bias, the usual stuff. But unfortunately I must admit I’m out of my depth with this and need guidance on what to do. What should I look for, measurements or voltages to check?? Bottom line the best way to proceed!

-Thanks in advance,
Justin

Hey Justin.. troubleshooting time!
Pull all tubes, switch the amp on and check the HT and 5V voltages.
If they're good: put in the rectifier tube, switch on and measure the B+ DC voltage at the1st filter cap. Also the negative bias voltage. Let's see from there...

Maybe first switch the amp off, unload the filter caps from + to ground. If there is a cap that has a very low resistance, it might be shorted. If it's the 1st filtercap it could also be the output transformer that's malfunctioning

I use a solid-state rectifier replacement for this sort of troubleshooting. Pull all the tubes, insert a milliameter just after the rectifier socket, and bring the amp up slowly on a variac. If the filter caps are failing, 85-90V line voltage should be enough to see excess current draw. You can disconnect the caps to confirm.

TriodeLuvr wrote: ↑

Mon 01/24/22 3:09 am

I use a solid-state rectifier replacement for this sort of troubleshooting. Pull all the tubes, insert a milliameter just after the rectifier socket, and bring the amp up slowly on a variac. If the filter caps are failing, 85-90V line voltage should be enough to see excess current draw. You can disconnect the caps to confirm.

That would be the better (pro) way indeed.. but I seriously doubt OP has access to lab equipment, let alone use it
I just handed over some rudimentary DIY troubleshooting methods, which would likely be sufficient to solve the problems OP is facing. My bets are on failing filter cap(s) ...

it sounds like the filter caps might be the issue with the hum that was’nt there like suggested,the schematic shows 2 diodes “BYD33V” between the HT and GZ34 so it might pay to check those as well.are the filter caps the dreaded “IC” type?

Hello everyone, Apologies for a VERY delayed response and absence from the forum! Right after I posted this thread a personal emergency came up.. But thankfully everything’s fine now & back to normal, So naturally I wanna dust off the soldering iron and get into some guitar electronics stuff!!
Obviously, the situation w/the amp remains the same. I haven’t had a chance to get into the chassis with a multimeter yet but I plan on pulling the tubes and taking measurements according to the suggestions in the first reply.

In regards to whether I owned or had access to equipment mentioned in the last post; He’s 100% right.. I don’t own a Variac, Or frankly, any valve amplifier specific test equipment just a couple voltmeter’s.
It’s the type of thing where currently, the cost and time required to learn how to use the majority of tools is prohibitive. If you asked me to use an oscilloscope I wouldn’t have a clue where to begin. I’m sure some of what’s considered essential to many first time DIY kit builders would present a learning curve.

It’s funny I’ve come quite a long way in a short time, without any background or mentoring simply my curiosity and desire to understand & modify my gear. I mention this because I do plan to grow & expand my equipment and what I’m able to do w/tube amplifiers.
I’m considering some things like; building a light bulb current limiter (something which was recommended & heavily stressed I do early on), A dummy load would make things a ton easier so I don’t have to run the chassis from my bench down to a big heavy 2x12 clumsy speaker cabinet just to bias new power tubes! There’s other equipment on websites like antiqueelectronicsupply & TedWeber that sell a ton of equipment which may or may not be anything more than novelty or only useful to the working pro.
Is there any test equipment or tool you guys feel is indispensable, but doesn’t cost the same as a Metropolis amps custom build or a Ltd wood special Suhr guitar w/exhibition grade maple pickup covers..!

I build TONS of fx but the operating principles, parts, and circuit layouts call for distinctly different kinds of equipment. I have specialized meters to test, sort, and match Jfets and germanium transistors.. I have a fairly pricey multi-function digital soldering/rework station. All of which I would never have considered paying for not long ago. Just a couple years back I asked on a forum like this “is a capacitor that has 600v written on it high enough to use in my strat?” Haha

geoff 1965 wrote: ↑

Mon 01/24/22 8:58 am

it sounds like the filter caps might be the issue with the hum that was’nt there like suggested,the schematic shows 2 diodes “BYD33V” between the HT and GZ34 so it might pay to check those as well.are the filter caps the dreaded “IC” type?

Yep they are and I’d change em out if it wasn’t impossible to get underneath the main pcb!