'65 DRRI Keeps Blowing 5AR4 Rectifier Tubes

[Stevem]

To the OP.
In the technical section on this site there’s a post called
“Blowing fuses” debugging.

I suggest you read it !

Also if you do not understand how a simple tube rectifier works then you’re going to have a hard time understanding how even ( the next step up ) a triode works .
And troubleshooting a gain stage will be way more aggravating then what your dealing with in this recto / power supply deal.

[B Ingram]

Failed 5AR4 Rectifier tube verified with Fluke 117 Multi-meter diode check between pins 4 and 6 and between pins 6 and 8. 4 and 6 were fine but 6 and 8 were open.

The heater has to be hot for a GZ34 to work.

But setting that aside, just fit backup diodes to the rectifier socket, in the manner shown in the Princeton Reverb Reissue. If you're convinced the "amp is eating rectifier tubes" then this modification will make that moot (although I've never had a rectifier tube from the 1970s or earlier fail, but many report modern rectifier failing frequently).

The owner brought it in for resolution of a single symptom: an incredibly loud and random crackle when the amp is brought out of Standby.

I agree with xtian: it's probably the Standby switch itself failing, and your customer is best off not even using it. If you monitor B+ volts in the amp after turning the amp On and with Standby in "Play" then you'll see the DC Volts gradually ramp up after a warm-up period. A GZ34's natural slow-start is better than any Standby switch.

[sssmoka]

I bypassed the Standby switch to no avail - the loud, random, intermittent crackle is still present.

There is no sign of leakage or bulging of the A/C Filter Caps, but they are the original Illinois Capacitor caps, and I fear they've gone south.

I remember a similar scenario with a HRD IV and an intermittent crackle which went away when she was re-capped.

I've got F and T caps en route to re-cap this amp which I do hope alleviates the customer's complaint.

[B Ingram]

... single symptom: an incredibly loud and random crackle when the amp is brought out of Standby. ...

I bypassed the Standby switch to no avail - the loud, random, intermittent crackle is still present.

The original presentation was "crackle when the amp is brought out of Standby," but now it seems the Standby was just an unrelated detail.

Many things causes popping/crackling.

• Try pulling the Reverb Driver 12AT7. Does the crackling stop?

One of the classic issues in a vintage-style Fender amps with reverb is very high voltage to the 12AT7 reverb driver, and that tube starts arcing internally in a way that makes random noise.

Try pulling tubes to isolate the section of the amp causing the crackle/popping. The amp might still benefit from new filter caps, but it would be unfortunate for a customer to pay for parts swapped that were not the problem that brought the amp in.

[sssmoka]

I concur. Agreed, indeed. I'm not in this business to rip off the amp owner - I'm in it to provide an affordable service to those in my community and make a modest profit whilst doing so.

- sssmoka

[sssmoka]

Pulled both 12AT7 reverb driver tubes and the strange noises ceased. I've ordered two new 12AT7 Groove Tubes to replace the factory originals as per the customer's request. Lesson learned: always check your consumables, first.

Thank you, ALL, for your guidance and admonition. It is GREATLY appreciated!

- sssmoka

[B Ingram]

Pulled both 12AT7 reverb driver tubes and the strange noises ceased. ...

Both?

The 12AT7 closest to the output tubes is the phase inverter. Pull that, and all noise should cease.

The 3rd tube from the right (looking in the back of the amp) is the 12AT7 that drives the reverb transformer.

There are several protocols for isolating a noise-source. Pulling tubes is one, but a similar job can be done with a scope or a signal tracer.

[sssmoka]

Pulled both 12AT7 reverb driver tubes and the strange noises ceased. ...

Both?

The 12AT7 closest to the output tubes is the phase inverter. Pull that, and all noise should cease.

The 3rd tube from the right (looking in the back of the amp) is the 12AT7 that drives the reverb transformer.

There are several protocols for isolating a noise-source. Pulling tubes is one, but a similar job can be done with a scope or a signal tracer.

I realize the noise source is likely isolated to one tube - the reverb driver. I'll replace the phase inverter and swap it for the reverb driver and report back with my results. Thank you for your continued support and patience.

- sssmoka

[Stevem]

The 12AT7 reverb driver can not add any sound or noises to the down stream signal chain unless the reverb is turned up, on the other hand ( the next tube in the signal chain) the reverb recovery / mixer tube ( 12AX7 ) can do such.

[sssmoka]

I replaced the Reverb Driver with a new 12AT7, replaced OEM Phase Invertor and all static and crackle ceased. I let the customer's '65 DRRI warm up for a few, cranked the Reverb from 1 to 10 and everywhere in-between, and pumped test frequencies through all four inputs. No abnormal sounds from this amp, anymore. I do hope I have a happy camper on my hands. Thank you all for your assistance.

- sssmoka

[ampdan]

To the OP...I just read this entire thread. You don't seem to grasp even the most basic understanding of
tube amplifier diagnostics, and yet you start a Amplifier repair business? Seriously?

[sssmoka]

To the OP...I just read this entire thread. You don't seem to grasp even the most basic understanding of
tube amplifier diagnostics, and yet you start a Amplifier repair business? Seriously?

I came for assistance, not criticism, sir. Am I subject to both? Certainly. However, if you're here to provide the latter, save your keystrokes and your time.

- sssmoka

[pjd3]

is it plausible to place a one ohm resistor in series with the OT primary center tap and say, look across the resistor on a scope or meter with peak hold to see if there are any bad flyback or high voltage spikes?

pjd3

[B Ingram]

is it plausible to place a one ohm resistor in series with the OT primary center tap and say, look across the resistor on a scope or meter with peak hold to see if there are any bad flyback or high voltage spikes?

No.

Ohm's Law (Volts = Current x Resistance) means the 1Ω resistor can tell you something useful about the current in that part of the circuit.
However, we mostly care about the current through individual tubes, and placing the 1Ω resistor between Cathode and Ground (or non-grounded side of the Cathode Resistor, if cathode-biased) minimizes Volts present.

"Flyback Voltage" is not something that can be easily measured without tools that can safely operate in the 5-10,000 volt range. Flyback voltage happens when current through an inductor is stopped suddenly: the inductance creates a voltage-spike to attempt to re-start the current (here, stop by an output tube driven to shut-off).

If you're concerned about flyback voltage damaging the output transformer, then just go ahead and fit diodes to the ends of the output transformer primary, as shown by D2 and D3 in the 65 Super Reverb Reissue schematic.

[pjd3]

I'm not concerned about anything, or necessarily measuring anything specific, just a way to indicate if something extreme is occurring, such as some sudden huge voltage dropping across a one ohm resistor that was induced in the primary of the OT.

Best,

Phil