Princeton Reverb Blowing Fuse - Resolved

[Fresh_Start]

I'm a little embarrassed to ask for help troubleshooting this older build, but better to ask than ruin something.  The amp worked fine for 5 or 6 years.  I moved bringing the amp with me rather than leaving it to the tender mercies of the "professional" movers.  It worked for a while (months).  Then one day I turned it on, the pilot light glowed for almost a minute then went dark.

Not having a workshop and not being able to find my DMM or other amp related tools for over a year, I've been playing something else.  Now the work bench is mostly built and I finally found the DMM, electronic parts, etc. 

I can't see any obvious burn marks or fried components.  The only thing that looks a little odd is the V1 12AX7 - it was an "almost new old stock" tube though and frankly I can't remember what it looked like before.

After checking obvious spots for shorts to ground like heaters, the pilot light, etc. I tried turning it on standby with a new fuse.  Voltage on pin 8 of the rectifier tube went up and stayed steady - for a moment.  Then there was a weird, mild buzz for a moment and I cut the power switch right away.  The replacement fuse had a little spot on the glass but still read continuity.  I tested some more point inside the amp hoping to find something with continuity to ground that shouldn't be there test its again.  The pilot light came on, DC volts went Up and steady, then the pilot light went dead simultaneously with weird buzz, and the fuse was blown.  The noise sounded like it came from the general area of the PT but I'm not sure.

So, I haven't found my liight bulb current limiter yet but need it. 

Could some of you help me out with a procedure to figure out what the short is?  One problem I have is that I only seem to have 1 more 1 amp fuse. 

Apologies for elementary question & thanks in advance.

Chip

[sluckey]

Fuses are disposable. Get some more.

Pull the output tubes. Does fuse hold?

Check negative bias on pin 5 of both output tubes. How much?

I would concentrate on power amp and power supply.

[p2pAmps]

Fuses are disposable. Get some more.

Pull the output tubes. Does fuse hold?

Check negative bias on pin 5 of both output tubes. How much?

I would concentrate on power amp and power supply.

I agree with Steve on this.  Fuses blow from current so check the voltages on pin 5 like he said...  Check that AC cord too just in case. 

[Fresh_Start]

OK - I've done some testing and have run out of 1 amp fuses.

The "range resistor" between the PT secondary and the rectifier diode for the bias supply reads 97K instead of the 82K it's supposed to be.  Don't know if this is a cause or a symptom.

With NO tubes in, the highest (absolute value) I could get the negative bias voltage is about -43 VDC.  Fuse held in standby and no smoke.

I put the rectifier tube in and got the same result in standby.  Found my light bulb limiter (yay!), so I put the power tubes in to test.  Turned amp on.  40 watt bulb stayed lit but did not burn out.  About 10 ma on a power tube.  Pin 5 was around 25 VDC but I ass-u-me that was lowered by the light bulb.  Thought I was making progress.

Plugged straight in, turned on standby and OK.  Turned amp On and less than a minute later I hear that weird mechanical buzz for a moment, pilot light goes out, and fuse is torched.

Pull out all tubes.  Start over.  43 volts on pin 5 of power tube sockets.  Heater voltages look right for amp without load, both 6.3 volt and 5 volt secondaries.  Replace rectifier tube ONLY.  Put amp in standby and in a minute I hear the weird buzz and the pilot light goes out.

Could I have fried the PT with the power tubes running too hot because bias voltage was too low? (closer to zero that is)  This sound seems to come from the PT. 

What tests can I do to check the PT?

Obviously, I need more 1 amp slo blow fuses.  I haven't found the biggest part of my tube collection yet so don't know if I have another 5AR4/GZ34 rectifier tube to check.

Any and all suggestions are appreciated!

Chip

P.S. my troubleshooting approach was not as methodical as it should've been.  I'm rusty at this!

[sluckey]

Pull out all tubes.  Start over.  43 volts on pin 5 of power tube sockets.  Heater voltages look right for amp without load, both 6.3 volt and 5 volt secondaries.  Replace rectifier tube ONLY.  Put amp in standby and in a minute I hear the weird buzz and the pilot light goes out.

Your bias supply is fine.

So, fuse blows with ONLY the rectifier tube plugged in. No need to plug in the other tubes until this is resolved. I would suspect a bad 5AR4 or a bad filter cap. Changing the tube is easy to do. You can use any of the 5 volt rectifiers as a test, even a 5Y3. Or tack solder a couple 1N4007s on the recto socket. If fuse still blows, check for a shorted filter cap.  Be sure caps are discharged and measure resistance of each cap to ground. Any resistance less than 10K would indicate a failed cap. If you are using a multi cap can you will need to replace the whole can.

PS, I would not be afraid to use a 2 amp fuse while troubleshooting. Heck, if I didn't see 1A slow on the schematic I would be running the amp on a 2 amp slow blow anyway.

[Fresh_Start]

Thanks sluckey!

I already checked all of the sections of the cap can for a short but will do it again, plus the other filter caps.

Good to know that even a 5Y3 would be OK for preliminary testing. Hadn't thought of that.

Chip

[Fresh_Start]

Each node on the power rail showed 5 Meg or more resistance to ground.

Everything is fine with only a 5Y3 in place.  Ordered new rectifier tube plus other odds and ends from Doug. I did not realize that a rectifier tube could fail shorted. Sovtek 5AR4. It looks fine but must've suffered a jolt when I moved it or something.

Thanks,

Chip

[eleventeen]

There sure are increasing numbers of stories about bunko GZ34s.


I dunno. At some point, one has to question the integrity or quality of these Russian GZ34 tubes. Of course any sort of "domestic" NOS 5AR4, not to mention Amperex or Mullards have climbed in price into the silly stratosphere. I think if it were me, I would be trying ebay for NOS for NOS US made 5V4 and sacrifice 10-15 volts of B+ for the $50-$200 it saves. Get straight sided ones, same type of bottle as 6L6GC, not the older bowling pins. There are plenty of Sylvanias or Westinghouse ones under $20.

[Fresh_Start]

Thanks!

Isn't the 5U4GB a viable alternative? (With slightly more voltage loss)

I just discovered that I have no clue how to read a rectifier tube data sheet. Just plugged in whatever the circuit called for.

Chip

[eleventeen]

A 5U4 will "work" in the same socket as 5Y3 or 5AR4/GZ34 but utilizes 3 amps fil current vs 2 amps as does a GZ34. The real differences are: MUCH MUCH higher voltage drop (vs GZ34 or 5V4) due to the direct-heated fil vs indirect as in the 5V4 or GZ34 and faster warmup time.


A 5V4 in place of a GZ34 will cost you 10-15 volts. Big deal. A 5U4 would cost you about 40 volts. Some would still say "big deal". Could matter in a Princeton, I suppose. If you want the big voltage drop and to stay with 2 A fil current, go for 5R4GY. (or stay 5Y3)


Five Volt Fullwave Rectifier Tubes


Tube#   - Base - Fvolt - Famp - Vdrop - MaxPmA - MaxPv - notes
5AR4/GZ34-5DA  - 5.0   - 1.9  - 17    - 250    - 450 - low Vdrop 5U4/5Y3
5AS4-A  - 5T   - 5.0   - 3.0  - 50    - 275    - 450 - improved 5U4
5AT4    - 5L   - 5.0   - 5.5  - 30    - 800    - 550 - higher power 5U4
5AU4    - 5T   - 5.0   - 3.75 - 50    - 325    - 400 - high power 5U4
5AW4    - 5T   - 5.0   - 3.7  - 46    - 250    - 450 - long life 5U4
5AX4-GT - 5T   - 5.0   - 2.5  - 65    - 175    - 350 - high power 5Y3
5AZ3    - 12BR - 5.0   - 3.0  - 44    - 275    - 600 - compactron 5U4
5AZ4    - 5T*  - 5.0   - 2.0  - 60    - 125    - 350 - loctal 5Y3
5BC3    - 9QJ  - 5.0   - 3.0  - 53    - 300    - 500 - compactron
5CG4    - 5L   - 5.0   - 2.0  - ?     - 125    - 400
5DJ4    - 8KS  - 5.0   - 3.0  - 44    - 300    - 600 - redesigned 5U4 (higher volts)
5R4-G/GY- 5T   - 5.0   - 2.0  - 67    - 250    - 750 - high voltage 5U4
5R4-GYA - 5T   - 5.0   - 2.0  - 67    - 250    - 750 - high voltage 5U4 (ruggedized)
5R4-GYB - 5T   - 5.0   - 2.0  - 63    - 250    - 900 - high voltage 5U4 (ruggedized)
5T4     - 5T   - 5.0   - 2.0  - 45    - 225    - 450 - metal 5U4
5U4-G   - 5T   - 5.0   - 3.0  - 44    - 225    - 450 - octal 5Z3
5U4-GA  - 5T   - 5.0   - 3.0  - 44    - 250    - 450
5U4-GB  - 5T   - 5.0   - 3.0  - 50    - 275    - 450
5V3     - 5T   - 5.0   - 3.8  - 47    - 350    - 425 - higher power 5U4
5V3-A   - 5T   - 5.0   - 3.0  - 42    - 415    - 550 - 5V3 reduced filament current
5V4-G/GA- 5L   - 5.0   - 2.0  - 25    - 175    - 375 - octal 83-V
5W4-G/GT- 5T   - 5.0   - 1.5  - 45    - 100    - 350 - low power 5Y3
5X4-G   - 5Q   - 5.0   - 3.0  - 58    - 225    - 450 - 5U4 diff pinout
5X4-GA  - 5Q   - 5.0   - 3.0  - 47    - 250    - 450
5Y3-G/GT- 5T   - 5.0   - 2.0  - 60    - 125    - 350 - octal 80
5Y4-G/GT- 5Q   - 5.0   - 2.0  - 60    - 125    - 350 - 5Y3 diff pinout
5Z3     - 5T   - 5.0   - 3.0  - 58    - 225    - 450 - 4pin 5U4
5Z4     - 5L   - 5.0   - 2.0  - 20    - 125    - 350 - low drop 5Y3
80      - 4C   - 5.0   - 2.0  - 60    - 125    - 350 - 4pin 5Y3
83      - 4C   - 5.0   - 3.0  - 15    - 225    - 450 - 4pin mercury vapour
83-V    - 4C   - 5.0   - 2.0  - 25    - 175    - 375 - 4pin 5V4
5931    - 5T   - 5.0   - 3.0  - 47    - 300    - 600 - industrial 5U4
6004    - 2AJ  - 5.0   - 2.0  - 60    - 120    - 375 - rabbit ear 5U4
6087    - 5L   - 5.0   - 2.0  - 50    - 125    - 350
6106    - 5L   - 5.0   - 2.0  - 60    - 125    - 350
6853    - 8HE  - 5.0   - 1.7  - 60    - 125    - 350

[Ed_Chambley]

There sure are increasing numbers of stories about bunko GZ34s.


I dunno. At some point, one has to question the integrity or quality of these Russian GZ34 tubes. Of course any sort of "domestic" NOS 5AR4, not to mention Amperex or Mullards have climbed in price into the silly stratosphere. I think if it were me, I would be trying ebay for NOS for NOS US made 5V4 and sacrifice 10-15 volts of B+ for the $50-$200 it saves. Get straight sided ones, same type of bottle as 6L6GC, not the older bowling pins. There are plenty of Sylvanias or Westinghouse ones under $20.

Absolutely a problem with the 5Ar4 Sovtek.  I am seeing it a lot.  I traditionally use a 5v4 straight bottles too as they are still cheap and draw less current.  I have stopped using true GZ34's I have horded away and really do not understand why the price has risen so much, but I am glad they have.


Also, I have taken to SS rectification in a lot of instances and use the larger resistor on screens for sag if I want it from an amp.  I guess when I bought that old Blond Bassman and absolutely fell head over heels for the tone my rectifier preference has changed.


Still prefer low voltages tho.

[Fresh_Start]

Eleventeen - I really should've noticed the heater current draw myself. Thanks.

Chip

[sluckey]

eleventeen, when doing a table such as this, if you change the font to Courier, everything will line up as expected. True Type fonts will never line up properly because they are not uniformly spaced. See below...

Five Volt Fullwave Rectifier Tubes

Tube#   - Base - Fvolt - Famp - Vdrop - MaxPmA - MaxPv - notes
5AR4/GZ34-5DA  - 5.0   - 1.9  - 17    - 250    - 450 - low Vdrop 5U4/5Y3
5AS4-A  - 5T   - 5.0   - 3.0  - 50    - 275    - 450 - improved 5U4
5AT4    - 5L   - 5.0   - 5.5  - 30    - 800    - 550 - higher power 5U4
5AU4    - 5T   - 5.0   - 3.75 - 50    - 325    - 400 - high power 5U4
5AW4    - 5T   - 5.0   - 3.7  - 46    - 250    - 450 - long life 5U4
5AX4-GT - 5T   - 5.0   - 2.5  - 65    - 175    - 350 - high power 5Y3
5AZ3    - 12BR - 5.0   - 3.0  - 44    - 275    - 600 - compactron 5U4
5AZ4    - 5T*  - 5.0   - 2.0  - 60    - 125    - 350 - loctal 5Y3
5BC3    - 9QJ  - 5.0   - 3.0  - 53    - 300    - 500 - compactron
5CG4    - 5L   - 5.0   - 2.0  - ?     - 125    - 400
5DJ4    - 8KS  - 5.0   - 3.0  - 44    - 300    - 600 - redesigned 5U4 (higher volts)
5R4-G/GY- 5T   - 5.0   - 2.0  - 67    - 250    - 750 - high voltage 5U4
5R4-GYA - 5T   - 5.0   - 2.0  - 67    - 250    - 750 - high voltage 5U4 (ruggedized)
5R4-GYB - 5T   - 5.0   - 2.0  - 63    - 250    - 900 - high voltage 5U4 (ruggedized)
5T4     - 5T   - 5.0   - 2.0  - 45    - 225    - 450 - metal 5U4
5U4-G   - 5T   - 5.0   - 3.0  - 44    - 225    - 450 - octal 5Z3
5U4-GA  - 5T   - 5.0   - 3.0  - 44    - 250    - 450
5U4-GB  - 5T   - 5.0   - 3.0  - 50    - 275    - 450
5V3     - 5T   - 5.0   - 3.8  - 47    - 350    - 425 - higher power 5U4
5V3-A   - 5T   - 5.0   - 3.0  - 42    - 415    - 550 - 5V3 reduced filament current
5V4-G/GA- 5L   - 5.0   - 2.0  - 25    - 175    - 375 - octal 83-V
5W4-G/GT- 5T   - 5.0   - 1.5  - 45    - 100    - 350 - low power 5Y3
5X4-G   - 5Q   - 5.0   - 3.0  - 58    - 225    - 450 - 5U4 diff pinout
5X4-GA  - 5Q   - 5.0   - 3.0  - 47    - 250    - 450
5Y3-G/GT- 5T   - 5.0   - 2.0  - 60    - 125    - 350 - octal 80
5Y4-G/GT- 5Q   - 5.0   - 2.0  - 60    - 125    - 350 - 5Y3 diff pinout
5Z3     - 5T   - 5.0   - 3.0  - 58    - 225    - 450 - 4pin 5U4
5Z4     - 5L   - 5.0   - 2.0  - 20    - 125    - 350 - low drop 5Y3
80      - 4C   - 5.0   - 2.0  - 60    - 125    - 350 - 4pin 5Y3
83      - 4C   - 5.0   - 3.0  - 15    - 225    - 450 - 4pin mercury vapour
83-V    - 4C   - 5.0   - 2.0  - 25    - 175    - 375 - 4pin 5V4
5931    - 5T   - 5.0   - 3.0  - 47    - 300    - 600 - industrial 5U4
6004    - 2AJ  - 5.0   - 2.0  - 60    - 120    - 375 - rabbit ear 5U4
6087    - 5L   - 5.0   - 2.0  - 50    - 125    - 350
6106    - 5L   - 5.0   - 2.0  - 60    - 125    - 350
6853    - 8HE  - 5.0   - 1.7  - 60    - 125    - 350

[eleventeen]

Thanks, man! Far more readable.

[Fresh_Start]

Thanks sluckey - that's much more usable.

Chip

[Fresh_Start]

There aren't too many things you can always count on in life. However, incredibly fast delivery from Doug Hoffman is an exception to the rule!

New Sovtek 5AR4 fixed the problem.

I got a bunch of fuses and other assorted hardware while I was ordering. The only issue now is the reverb doesn't work, but I'll do my best to sort through that. Spare tank will help.

Thanks guys, and thanks Doug!

Chip

[HotBluePlates]

Isn't the 5U4GB a viable alternative? (With slightly more voltage loss) ...

A 5U4 will "work" in the same socket as 5Y3 or 5AR4/GZ34 but utilizes 3 amps fil current vs 2 amps as does a GZ34. The real differences are: MUCH MUCH higher voltage drop ...

I used to have a '67 Princeton Reverb with a 5U4GB, in the amp and on the tube chart. Apparently, it was the exception rather than the rule. Someone on another forum pointed out that a resistor in the bias supply typically changed depending on whether Fender used the 5U4 or GZ34.

But yes, it boils down to whether your 5v winding can supply 3A; since this is likely a homebrew, you can look that spec up.

[Fresh_Start]

Isn't the 5U4GB a viable alternative? (With slightly more voltage loss) ...

A 5U4 will "work" in the same socket as 5Y3 or 5AR4/GZ34 but utilizes 3 amps fil current vs 2 amps as does a GZ34. The real differences are: MUCH MUCH higher voltage drop ...

I used to have a '67 Princeton Reverb with a 5U4GB, in the amp and on the tube chart. Apparently, it was the exception rather than the rule. Someone on another forum pointed out that a resistor in the bias supply typically changed depending on whether Fender used the 5U4 or GZ34.

But yes, it boils down to whether your 5v winding can supply 3A; since this is likely a homebrew, you can look that spec up.

Hoffman's PT only has 2 Amos specified for the 5 vol secondary.

Chip

P.S. Still haven't found my box of tubes... Don't ever move!