New build eating power tubes

[GeorgeG]

New to this site, poking around here for my next build.

My current build is an SEL from AX84. Single KT77 (J&J) power tube biased at 70%. Double checked the OT taps/impedances and they look correct for the load. Plate voltage is a little higher than the schematic but under the tube maximum. I eat the power tube about once a month. I have been trying different pedals with various levels of gain but almost never play above half way up the master volume. Have never seen red plating. What do I need to be looking for?

Thanks,

George

[HotBluePlates]

... Single KT77 (J&J) power tube biased at 70%. ... I eat the power tube about once a month. ... Have never seen red plating. ...

IF you're not redplating, what does "Eat the power tube" mean?

[GeorgeG]

... Single KT77 (J&J) power tube biased at 70%. ... I eat the power tube about once a month. ... Have never seen red plating. ...

IF you're not redplating, what does "Eat the power tube" mean?

After a few weeks of flawless operation, the output starts crackling and the volume starts dropping and then the output usually dies. If I put in a new power tube I'm good for another month.

Thanks

[sluckey]

Exactly how many KTs have you eaten in what period of time.? What resistance does the cathode resistor measure? What is the measured voltage on pins 3, 4, 5, and 8 of the KT? What are the specs (provide link if possible) on the OT?

What kind of pedals are you talking about? Does it work OK with no pedals in front? Has this amp ever performed properly?

Can you provide a schematic or a link to the ax84.com schematic? Have you made any modifications to the ax84 schematic?

[frankenxtein]

Too much heater voltage ?

[GeorgeG]

Schematic and layout in the first attachment. The speaker wiring is according to the layout (25 watt tube, white wire to 16 ohm for 5K) not the schematic (35 watt tube, 2.5K impedance).

Adjustable bias option in the second attachment which I used.

The 125ESE OT is in the third attachment.

The requested voltage measurements will be posted tomorrow. I am on my third KT77 in three months.

Thanks,

George

[GeorgeG]

I forgot to mention that I lifted the connection from one side of the heater circuit to pin 8 and implemented a virtual ground with a pair of resistors as is used on this site.

Voltages:

Pin 3 = 381 VDC
Pin 4 = 363 VDC
Pin 5 = 0     VDC
Pin 8 = 365 VDC
Heater = 6.8 VAC
Cathode resistor = 440 ohms (fixed plus variable)
Bias current from pin 8 to gnd = 42 ma as measured through 1 ohm resistor

Besides the KT77's dying the amp works great. I have many different pedals for delay, distortion, phasing, reverb, etc. Once the tube dies the amp doesn't work with or without pedals.

Thanks,

George

[sluckey]

Pin 8 = 365 VDC

Recheck pin 8. Surely it's not really 365vdc.

[GeorgeG]

Sorry, pilot error, lets try again:

Voltages:

Pin 3 = 381 VDC
Pin 4 = 363 VDC
Pin 5 = 0     VDC
Pin 8 = 27 VDC
Heater = 6.8 VAC
Cathode resistor = 440 ohms (fixed plus variable)
Bias current from pin 8 to gnd = 42 ma as measured through 1 ohm resistor

[John]

Cathode resistor = 440 ohms (fixed plus variable)

I'm a dummy; what does that mean?

[sluckey]

Are you sure about the value of the cathode resistance? Did you actually measure 440Ω between pin 8 and ground?

I ask because the numbers don't calculate. You say you have 42mA cathode current as determined using a series 1Ω resistor (Idle power = 14.9W). But you say the cathode voltage is 27 and the cathode resistor is 440Ω. That calculates to 61mA (Idle power = 21.5W).

Which to believe? Cathode current should be same regardless of how you calculate it.

[sluckey]

Cathode resistor = 440 ohms (fixed plus variable)

I'm a dummy; what does that mean?

Look at page 2 of his "options" pdf.

[John]

Oh, research! 

[GeorgeG]

Your right the math doesn't work,

I measured the cathode resistance from pin 8 to gnd with the amp off which includes the fixed resistor, pot, and precision 1 ohm resistor.

Voltage was pin 8 to gnd, amp at idle.

My stated cathode current was the voltage drop measured across the 1 ohm resistor, amp at idle.

Looks like I have a meter problem, or a "precision" resistor problem. I'll get another meter and figure out why my numbers don't tie out.

"I picked a bad week to stop sniffing glue".

[GeorgeG]

My trusty DVM of 20 years is starting to act up.

Lets try this yet again:

Voltages:

Pin 3 = 386 VDC
Pin 4 = 365 VDC
Pin 5 = 0     VDC
Pin 8 = 27.6 VDC
Heater = 6.8 VAC
Cathode resistor = 651 ohms (total of fixed plus variable)
Bias current from pin 8 to gnd = 43 ma as implied by measuring voltage across a precision 1 ohm resistor

Thanks,

George

[sluckey]

All the numbers line up now. Idling about 15 watts. I don't see any reason to be eating tubes. Unless you are overdriving it hard with lots of square waves all the time or just got some poor grade KT77s. If you have the patience maybe try it with a 6L6 or 6550 or KT88 and see if they last any longer.

[GeorgeG]

All the numbers line up now. Idling about 15 watts. I don't see any reason to be eating tubes. Unless you are overdriving it hard with lots of square waves all the time or just got some poor grade KT77s. If you have the patience maybe try it with a 6L6 or 6550 or KT88 and see if they last any longer.

Thanks. I have been wanting to try the lower wattage 6V6 anyways. Is there any problems with dropping it into my existing octal socket and adjusting the bias appropriately? I have a 6V6S which shows a max plate voltage of 450V.

Thanks,

George

[sluckey]

Give it a try. I run 6V6s at 400+ volts all the time. Your adjustable cathode bias circuit should cover 6V6 or those others I mentioned. It's OK to idle these tubes at 90 to 100%. Lower power should give extra life.

What kind of music do you play? How many hours per month?

[GeorgeG]

I play old rock and blues. Been playing for 55 years including a working band in the 70's but I am just starting back up again after a long layoff. I am really enjoying getting some of the more classic tones at garage practice levels as apposed to the 150 watt stack levels when I was playing live. I am also building pedals from General Guitar Gadgets and speaker cabinets.

[fossilshark]

Its a bit of a stretch, but are you sure about the speaker cab impedance matching the OT?

[GeorgeG]

Its a bit of a stretch, but are you sure about the speaker cab impedance matching the OT?

I did check that. Thanks.

George

[HotBluePlates]

I'm thinking you got a bad batch of KT77's.  That can occasionally be a problem with new-production tubes, unless your vendor does a lot of pre-screening to sort out the duds.

[ac427v]

[quoteI forgot to mention that I lifted the connection from one side of the heater circuit to pin 8 and implemented a virtual ground with a pair of resistors as is used on this site.]

Does this mean you attached one side of the transformer heater tap to the power tube cathode? or did you attach the virtual ground to the power tube cathode?
--Craig

[GeorgeG]

The original schematic and my original wiring had one side of the heater tap connected to the cathode. I removed that connection, created a virtual ground with two resistors and tied that resistor common connection to chassis ground.

[GeorgeG]

I'm thinking you got a bad batch of KT77's.  That can occasionally be a problem with new-production tubes, unless your vendor does a lot of pre-screening to sort out the duds.

That's what I am thinking at this point. The tubes that are dying are my favorites so I bought one from a different source to at least get a tube from a different lot (hopefully). If that doesn't work, I'll change tubes entirely and see what happens.