• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL34 Triode Mode Resistors

Monjul

Member
2010-01-22 5:09 pm
Hi. About 2 months ago I converted my Cary Audio SLA 70A Signature EL34-based amp to triode mode tying back the UL tap and adding 100 ohm 1/2W metal films and I like what I heard in the conversion very much.

But to play it safe, I decided to replace these resistors with Radio Shack 100 ohm 1W metal oxide resistors. The problem is that the resultant sound changed. There's now a slight tilt in the upper mids and a grain in vocals that wasn't there previously. After about a month of listening, I decided to convert back to UL and the symptoms disappeared.

Could the resistors cause the change in sound that I heard? Also, what resistors would you recommend for this conversion? Thanks.
 

kevinkr

Administrator
Paid Member
I'm not a fan of metal oxide resistors in the signal path - what you describe sounds a lot like what I hear. Subjectively to me they sound brighter and harsher, perhaps they generate distortions that other types don't since there is no such thing as a perfect passive component, but some are much better than others. I have never bothered to measure one, but below is the story of a distortion generating metal film that caused me some grief.

Something over 20yrs ago I had a problem with the subjective and measured performance of a solid state pre-amplifier design I was working on at the time. It sounded quite bad and seemed to have unexpectedly high levels of distortion, and I was very puzzled. It turned out that the feedback resistor in one of several compound complementary amplifier stages was generating significant amounts of odd order distortion - these resistors were made by Mepco Electra and were MF types. Unused samples drawn from my inventory all generated > 0.15% in a series connection, know good reference resistors gave results very close to the generator residual which was around 0.01%.. Replacing those resistors solved the problem.
 

Monjul

Member
2010-01-22 5:09 pm
I'm not a fan of metal oxide resistors in the signal path - what you describe sounds a lot like what I hear. Subjectively to me they sound brighter and harsher, perhaps they generate distortions that other types don't since there is no such thing as a perfect passive component, but some are much better than others. I have never bothered to measure one, but below is the story of a distortion generating metal film that caused me some grief.

Something over 20yrs ago I had a problem with the subjective and measured performance of a solid state pre-amplifier design I was working on at the time. It sounded quite bad and seemed to have unexpectedly high levels of distortion, and I was very puzzled. It turned out that the feedback resistor in one of several compound complementary amplifier stages was generating significant amounts of odd order distortion - these resistors were made by Mepco Electra and were MF types. Unused samples drawn from my inventory all generated > 0.15% in a series connection, know good reference resistors gave results very close to the generator residual which was around 0.01%.. Replacing those resistors solved the problem.

Thanks Kev. What resistors brands should I look for? Vishay's maybe? The shipping will probably cost more than the resistors, lol.
 

Monjul

Member
2010-01-22 5:09 pm
Mills, Kiwame or better Shinkoh.

What design though? Carbon comp, metal film or metal oxide? CC's get rather noisey quickly don't they?

It's a shame because I have a bunch of NOS CC's from Allen-Bradley and Wirt from back in the 60's but with resistive values that I can't use.

But I did find some RShack 1/2W metal film 150 and 220 ohm resistors. Will these suffice or should I seek out better resistors at the 100 ohm value?
 
What design though? Carbon comp, metal film or metal oxide? CC's get rather noisey quickly don't they?

It's a shame because I have a bunch of NOS CC's from Allen-Bradley and Wirt from back in the 60's but with resistive values that I can't use.

But I did find some RShack 1/2W metal film 150 and 220 ohm resistors. Will these suffice or should I seek out better resistors at the 100 ohm value?

Mills: non inductive, non magnetics, Nickel-chromium or nickel-copper
alloys used

Kiwame: carbon film

Shinkoh: tantalum

If possible avoid, metal oxide, metal film, generic ceramics, etc
 

Monjul

Member
2010-01-22 5:09 pm
It's my subjective opinion a question of personal taste, it's easy try metal film & after try non inductive, non magnetic & you can hear the difference.

I might have to agree with you.

I didn't even like what I heard in triode mode using 100 ohm 1/2 RShack metal film resistors let alone the metal oxides. After speaking with Jim McShane, he thought the amp might be oscillating. So upon his recommendation I bought some 272 ohm Vishay/Dale's and amazingly, all the problems I heard disappeared and it's now obvious that this Cary amp in triode mode is the way to go. The sound is smooth, sweet, open and less electronic sounding although the added warmth from 2nd harmonic distortion can make some recordings sound slightly heavy.

So the advice here is if you don't like the way your EL34 amp sounds in triode mode, give it another chance by trying better manufactured resistors with an increased resistance (>100 ohms), to be on the safe side.
 
Given that some metal oxides are used for distinctly non-linear resistors (such as varistors or thermistors) one might expect that other metal oxides are not as linear as plain metal film. However, Vishay claim a voltage coefficient of better than 10^(-7) for some of their metal oxide Rs. Even with a few 100V swing this is unlikely to be audible, but are some resistors not as good as this?

Two explanations: some people can hear distortion far below what any reasonable person would assume (given the simultaneous presence of much larger distortions from other components), some resistors are significantly less linear than is typical for their type. (Third explanation: it is our old friend the placebo effect). (Fourth explanation: RF instability due to inductance of the spiral - likely to be worse for low values).

We need to tread cautiously, because some people prefer carbon resistors and they can easily give distortion in the 0.1-1% range i.e. in some cases preference may favour higher distortion!
 

Monjul

Member
2010-01-22 5:09 pm
Given that some metal oxides are used for distinctly non-linear resistors (such as varistors or thermistors) one might expect that other metal oxides are not as linear as plain metal film. However, Vishay claim a voltage coefficient of better than 10^(-7) for some of their metal oxide Rs. Even with a few 100V swing this is unlikely to be audible, but are some resistors not as good as this?

Two explanations: some people can hear distortion far below what any reasonable person would assume (given the simultaneous presence of much larger distortions from other components), some resistors are significantly less linear than is typical for their type. (Third explanation: it is our old friend the placebo effect). (Fourth explanation: RF instability due to inductance of the spiral - likely to be worse for low values).

We need to tread cautiously, because some people prefer carbon resistors and they can easily give distortion in the 0.1-1% range i.e. in some cases preference may favour higher distortion!

Don't lose site of the fact that the resistance was upped from 100 to 272 ohms. Maybe my amp was truly oscillating as suggested as that would certainly explain the strange overall sound I experienced.