12AX7 Plate Resistor

[TomCarlos]

Hello Friends....

Some time ago, I was repairing a Fender Hot Rod Deluxe. It might have been on this board or somewhere else where members talked about the plate resistors on the 12AX7 tubes and how those resistors were prone to failure. The big issue was the Carbon filled, 1/4 watt overheating and failing. The suggested fix was changing over to Wire Wound, 1/2 watt. I made that change on that amp (and other changes) and it's been working since.

If you look at the attached schematic for a typical 12AX7 application, you will see this in tons of tube amps. So I am curious... are other amps experiencing the same problems with the plate resistors or was this problem limited to the Fender Deluxe and Deville amps?

Thanks, Tom

[52 Bill]

There was a certain time when Fender had a problem with failing plate resistors in the driver circuit of some of their amps. For some reason it was usually the 82K resistor that would go open circuit, but sometimes it would be the 100K. I had seen it mainly on the Deluxe/DeVille amps, but it also occasionally happened to others like the 65 Deluxe and 65 Twin reissues.

But it was only the driver circuit that seemed to have these problems, and Fender has since changed suppliers on these resistors and there doesn't seem to be any high failure rate in the more recent amps.

I have only seen one or two failed plate resistors in the preamp circuits of the Deluxe/DeVille amps. I have not heard of any major problems with these resistors from any sources that I know of. Perhaps somebody else here will have some input.

[Leo_Gnardo]

Yes, seen it several times on Deluxe/Deville Fenders. I just use any competent half-watt resistor, doesn't have to be wire wound. Carbon-film or Metal-oxide-film or corbon-comp all work fine in this application.

[TomCarlos]

I stand corrected.... I believe the 100K resistors are 1/2 watt and the recommendation was to install 1 watt wire wound. Thanks.

[frus]

should not be necesarry, most 1/2W carbon comps are still running strong after 50 years

[Jazz P Bass]

A 1 watt wire wound resistor is most certainly overkill.

[TomCarlos]

Thanks everyone... I appreciate the replies.

[Alan0354]

My Marshall JCM900 Dual Reverb used 1/4W( yes, not a typo) plate resistor, it burned on me. 1/2W carbon comp should be ok. A lot of old Fender BF and SF used 1/2 100K plate resistors. But a lot used 1W. I changed my Bassman 100 to 1W carbon comp.

Old Fender seemed to be built with a lot of margin, not like Marshalls. I had a Plexi and I still have the JCM 900, I can't keep them running. Burned time after time, again and again. It just has no margin.

[Jazz P Bass]

[QUOTE=Alan0354;331309]1/2W carbon comp should be ok. A lot of old Fender BF and SF used 1/2 100K plate resistors. But a lot used 1W..QUOTE]

Not to argue, but Fender did not use 1 watt plate resistors.

Not that I can recall, at least.

1/2 watt resistors from the '60's may appear to be one watt, as compared to current production resistors.

[soundguruman]

I would recommend 1W metal film, wire wound is a good second choice. Bulk metal foil is even better...
The constant cycle of heating cooling heating, tends to crack carbon resistors over time.
carbon resistors are held together with glue, and the leads are glued in.
So, from the standpoint of reliability, noise, and long term stability, 1 watt is not overkill at all.

Fender did use some really cheapo resistors, and they are what I would call undersized. I would have used better ones for sure.
I mean, Carvin is using 1/8 watt in plenty of amps...not terribly reliable.
I routinely change plate and cathode to 1Watt metal film, and changing all the resistors in the signal path is not a bad idea either.

The HRD sounds better if you replace the signal path with better resistors. Fromel is using better grade carbon but I like metal film especially in the front end.

Some people think that carbon sounds better but I have to disagree. I don't see the advantage.
And metal film is relatively cheap so I would rather spend the extra $1.50...

The resistor really fails because the tube is going bad and draws too much current, or it's just that the input stage tends to take a beating anyhow.

[TomCarlos]

Thanks again everyone.... I am going to prototype and experiment. I appreciate the replies!!

[Jazz P Bass]

The resistor really fails because the tube is going bad and draws too much current, or it's just that the input stage tends to take a beating anyhow.

This is exactly why I said 1 watt is overkill.
By going to a (too) high wattage, you are transfering the failure to something else.
You have to be very careful when you reengineer a circuit.
There are consequences that should be taken into account.

[soundguruman]

This is exactly why I said 1 watt is overkill.
By going to a (too) high wattage, you are transfering the failure to something else.
You have to be very careful when you reengineer a circuit.
There are consequences that should be taken into account.

I see, you are using the plate resistor as a fuse.

I still use 1 watt, like so many other builders.

[Alan0354]

[QUOTE=Jazz P Bass;331331]

1/2W carbon comp should be ok. A lot of old Fender BF and SF used 1/2 100K plate resistors. But a lot used 1W..QUOTE]

Not to argue, but Fender did not use 1 watt plate resistors.

Not that I can recall, at least.

1/2 watt resistors from the '60's may appear to be one watt, as compared to current production resistors.

A lot of the early 70s use 1W. My 74 Twin is 1W. 1/2W should be ok. It's not the power, it's the voltage standoff. 1/2W resistor has long enough body to stand off 400V. It's the 1/4W that cut very close to the power limit and definitely violate the voltage stand off.

[Alan0354]

I would recommend 1W metal film, wire wound is a good second choice. Bulk metal foil is even better...
The constant cycle of heating cooling heating, tends to crack carbon resistors over time.
carbon resistors are held together with glue, and the leads are glued in.
So, from the standpoint of reliability, noise, and long term stability, 1 watt is not overkill at all.

Fender did use some really cheapo resistors, and they are what I would call undersized. I would have used better ones for sure.
I mean, Carvin is using 1/8 watt in plenty of amps...not terribly reliable.
I routinely change plate and cathode to 1Watt metal film, and changing all the resistors in the signal path is not a bad idea either.

The HRD sounds better if you replace the signal path with better resistors. Fromel is using better grade carbon but I like metal film especially in the front end.

Some people think that carbon sounds better but I have to disagree. I don't see the advantage.
And metal film is relatively cheap so I would rather spend the extra $1.50...

The resistor really fails because the tube is going bad and draws too much current, or it's just that the input stage tends to take a beating anyhow.

If you go to surplus store or on ebay, carbon comp resistor is a lot more expensive than MF. I agree with you that I don't think carbon comp sounds better at all, it's all because of the hype by people that don't know electronics. It is stupid that a 100K 1W cost over $2 and 120K 1W is cheap. People can't design, they copy blindly. I went to the surplus store, the 100K bin was empty, and they had plenty of 91K, 110K!!!! Don't they know they can design around to use that??!!