• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

5AR4 questions

C

chetgallery

Member
Had a 5AR4 blow on a Pilot 240 after about 30 hours after a complete restoration. I normally knock the line voltage down due to the higher line voltages these days. On this one I used a CL90 thermistor and a 15 ohm 60 watt resistor. I would like to hear the voices of experience on my voltages. Pin 4 and pin 6 both have 235 volts ac and that produces 270 volts dc out. Is this voltage too small and has it expedited the 5AR4’s life? There are no minimum voltages that I can find. Hope to get a better insight on this. I have restored many tube amps with the voltage knocked way down without any issues.
 
Register to hide this ad
270V out is too low, not for a 5AR4 but for a circuit where the output voltage is spec'd at 340V. Either the rectifier was about to go to begin with or there are some circuit issues that are pulling the voltage down that far and possibly stressing the 5AR4. Was the 5AR4 known to be strong?
 
Not clear where you have the CL90 and 15 Ohm, 60 watt resistor installed. The schematic says you want 320-0-320 AC going to the 5AR4. I wonder why you are getting only 235 Vac on pins 4 & 6? If you have only 235 Vac what is your heater voltages on the 5 V as well as 6.3 V lines?

I’m with the previous posts. It is important to know if the rectifier was a good NOS Mullard, a tired old warhorse, or a modern replica 5AR4

However, looking at the Pilot 240 schematic I see that the first cap is 80 uF, whereas 60 uF is the max 5AR4s are spec’ed for. Good old Mullard 5AR4s probably survived the 80 uF, but modern 5AR4s will not. Additionally, modern 5AR4’s require the series resistance in the transformer secondaries specified on the datasheets in the Rs formula. Many classic amplifier designers ignored that (presumably because the NOS 5AR4 exceeded the specifications), including the Pilot 240.
 
If there is too much drop on the primary side, the heater voltage ends up low and the rectifier won't like it. Easier to measure the 6v heater line to confirm its in spec but if that's 6.3 vac +/- 0.3v its fine.
 
Selmerdave said:
First cap on a 240 is 40uF
Click to expand...
I stand corrected - if you have the original schematic. I was looking at Sams that showed 80uF for the first cap and 640 Vac CT for the PT secondary. It won’t be the first mistake by Sam’s.
 

Attachments

  • IMG_2995.png
    IMG_2995.png
    610.5 KB · Views: 26
Wow. I am grateful for all who have responded. I was on AK a lot during this build. The CL90 and resistor are right after the fuse.
The first electrolytic is a 60 like the data sheet and AK have said. The tube was a bugle boy. It measured 925/650 on both, on my calibrated Hickok 752. I used the values right off the Sams. With the voltage dropped I had 275-281 volts on the 6bq5’s and the rest of the AX7’s all were within 10% of what the schematic called for. This 5AR4 had taken a fall from the bench to a carpeted floor from a curious cat. Retested and it looked good. Should I invest in an older 5AR4 and hope the tube was injured during the fall?
Again very gracious for all the interest and help here. Thanks again..
 
chetgallery said:
Wow. I am grateful for all who have responded. I was on AK a lot during this build. The CL90 and resistor are right after the fuse.
The first electrolytic is a 60 like the data sheet and AK have said. The tube was a bugle boy. It measured 925/650 on both, on my calibrated Hickok 752. I used the values right off the Sams. With the voltage dropped I had 275-281 volts on the 6bq5’s and the rest of the AX7’s all were within 10% of what the schematic called for. This 5AR4 had taken a fall from the bench to a carpeted floor from a curious cat. Retested and it looked good. Should I invest in an older 5AR4 and hope the tube was injured during the fall?
Again very gracious for all the interest and help here. Thanks again..
Click to expand...
In my opinion, I would not invest in another NOS GZ34 until I was sure there was no other unknown big current draw in the power supply or output stage. You do not want to blow another $$$ GZ34 to find out it's not a rectifier tube issue.

How recent was this restoration? How old are the power supply caps and/or cap cans? What brands are they? What is the idle bias of your output tubes? Is it a stable bias? Are the output transformers running hot?
 
Restoration was about 90 days ago. Transformers were not hot. Hottest tube was about 230-250 Fahrenheit. Filter caps are new nichicon. Could pull a 5AR4 from another amp to get readings on
Mr Boochie said:
In my opinion, I would not invest in another NOS GZ34 until I was sure there was no other unknown big current draw in the power supply or output stage. You do not want to blow another $$$ GZ34 to find out it's not a rectifier tube issue.

How recent was this restoration? How old are the power supply caps and/or cap cans? What brands are they? What is the idle bias of your output tubes? Is it a stable bias? Are the output transformers running hot?
Click to expand...
 
chetgallery said:
Restoration was about 90 days ago. Transformers were not hot. Hottest tube was about 230-250 Fahrenheit. Filter caps are new nichicon. Could pull a 5AR4 from another amp to get readings on
Click to expand...
Failure of an NOS Mullard (UK) GZ34 can happen; I may have seen it once. Usually, the fault lies elsewhere in the circuit. Can you have the guy who did the restoration check your amp before you put in another NOS tube?

Thanks!
 
The bugle boy came in the amp. Most likely original.

So, is it over 30 years old? Consistent use? Then yes, you may need a new tube.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom