18-watt TMB Build

Pete Farrington

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Raised 150R cathode bias resistor to 180R
It would be interesting to know your cathode voltage, at idle and when cranking out fully overdriven.
As mentioned, a large increase has consequences on tone (and power output). I characterise the tone as becoming more ‘reedy’, dunno if that might align with what you’re hearing.
Electrically, at power outputs below overdrive, the amp alters a high power sine wave to make it somewhat more triangular.
 
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william vogel

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Y’all are going to hate me for this. The best sound I get out of my 18 watt tremolo is to use a 100 ohm cathode resistor which over dissipates the tubes. I use Russian 6p14p output tubes and I have a bunch of them. I’ve pushed the resistor value around many ways to get the dissipation safe and it looses power and tone. I use what Vox used on the AC30 and I’ve been playing the amp for over a year with the same tubes. Heavy crossover distortion is still present and I’ve used clamping diodes which eliminated the notch but it didn’t sound right. My circuit now is completely stock except for the 100 ohm cathode resistor.
 

neikeel

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I may have missed it but what is the HT in your PT, are you ss or tube rectified?
What does that give you on your EL84 anodes?
Edit that , I see the EZ81 base!!
 
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Pete Farrington

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Yes, the discussion has been rather hypothetical so far; a comparison of the voltages noted on the schematic to those of your amp, along with the mains voltage, for the different cathode resistor values mentioned, would be beneficial.
 

Pete Farrington

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Mains voltage running 120 , not sure if dropping it 5 volts would compensate enough.
I find it’s more helpful to think of these things in terms of ratios; 10% off the primary voltage will also reduce secondary voltages by 10%. As power is related to voltage squared, that will reduce dissipation by nearly 20% (0.9 x 0.9 = 0.81)
So feeding an amp 115 instead of 120 should, all else being equal, bring down dissipation by over 8% -
115/120 = 0.958
0.958 x 0.958 = 0.918
 

william vogel

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Do you mean zeners on the EL84 control grids, to clamp very large negative signal excursions (and so mitigate blocking distortion)?
Have you tried the zener on the cathode idea (I haven’t yet)?
Yes, strap 12 volt zeners across the 470k grid resistors. Something like that. I’m thinking I had to series connect a regular 1n400x diode with the zener. It was a year ago and when I wasn’t pleased I don’t remember everything now. I’ve never tried a zener on the cathode.
 

william vogel

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Another thing to think about is that while the dissipation is hot at idle, when we’re playing the excursion moves it colder and the valve is cutoff for about half the cycle. As long as you’re playing it it’s much safer than just idle. Play or cut it off.
 

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Yeah, I've heard from others that it's just how these amps are, and not to be worried about 125% idle dissipation (which is where mine sits at 150R cathode bias resistor).
 

william vogel

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Yeah, I've heard from others that it's just how these amps are, and not to be worried about 125% idle dissipation (which is where mine sits at 150R cathode bias resistor).
Parallel a 270 ohm with your 150 ohm and play it quickly. That’ll make right at 100 ohms and let me know if it’s better. Just give it a try.
 

william vogel

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What I notice is that moving the resistor value up increases the plate voltage and really doesn’t have a dramatic effect on dissipation. As per the schematic the design is set to have 129% dissipation and you’ve increased to 150 ohms but only shifted 4% dissipation. In this you’ve probably raised the cathode voltage 2 volts and increased the crossover distortion and bias excursion about 16%. I don’t know what plate voltage you have, it’s never been published. With the lower resistance value, the plate voltage drops and creates a more efficient power section. You’ll see the rms clean output increase. This increase in power indicates improvement. A low light check for redness on the center of the plate structure will indicate if it’s too hot or “within possible safety”. This is why I prefer fixed bias but these EL84 amps sound so good when they run on the ragged edge.
 

BenTobith

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What I notice is that moving the resistor value up increases the plate voltage and really doesn’t have a dramatic effect on dissipation. As per the schematic the design is set to have 129% dissipation and you’ve increased to 150 ohms but only shifted 4% dissipation. In this you’ve probably raised the cathode voltage 2 volts and increased the crossover distortion and bias excursion about 16%. I don’t know what plate voltage you have, it’s never been published. With the lower resistance value, the plate voltage drops and creates a more efficient power section. You’ll see the rms clean output increase. This increase in power indicates improvement. A low light check for redness on the center of the plate structure will indicate if it’s too hot or “within possible safety”. This is why I prefer fixed bias but these EL84 amps sound so good when they run on the ragged edge.
To clarify, I was at 150R before, but just swapped to 180R. After this change — plus the others noted in my recent post — I came away with some improvements, but one key downside. The downside being, I’m hearing a bit more “ice pick” edge when really overdriving the amp, and it feels a bit less beefy. I don’t like it. So I’m going to try to walk back and figure out what did that.

I’m also getting a bit of DC noise on pots and through my guitar, need to figure that out. Could be one of the silver micas.

EDIT: My plate voltage with the 150R resistor was 354 VDC. Dissipation was around 42mA. I haven’t measured since upping to 180R, haven’t had a chance yet.
 

BenTobith

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For a bit more background, when I first built the amp and tested it out, it had the original 120R bias resistor and 100R screen grids. One of the (new production) EL84s was smoking, and not in a good way. Way too hot. So then I upped to 150R and 470R screen grids. That seemed to make the amp fairly stable. However, when I measured my plate voltage (354) and bias at idle (~42mA), I was thinking: “Holy cow, 125% dissipation!”

But I’ve also been told that’s normal in these amps.

So if it truly is normal, I’m happy to revert to 150R and assume everything is okay at those readings.
 

_Steve

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I had similar trouble with changes to power section making it sound thinner and more trebbly. My conclusion was that those changes to the power section were affecting the voltages in the preamp negatively. I found that if the PI node (thinking of it as the first node of the preamp, even though it really isnt) dropped too much below 330VDC I would notice it get thinner. I solved it with a very small dropping resistor at that node, and zeners to drop the screens. That's just my experience and might not be applicable to you. Also my diagnosis may have been incorrect, although the outcome was good.

Does your normal channel sound thin now too? Or just the TMB/Plexi channel? If the latter then maybe consider the above.
 

BenTobith

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I had similar trouble with changes to amp making it sound thinner and more trebbly. My conclusion was that changes to the power section was affecting the voltages in the preamp negatively. I found that if the PI node (thinking of it as the first node of the preamp, even though it really isnt) dropped too much below 330VDC I would notice it get thinner. I solved it with a very small dropping resistor at that node, and zeners to drop the screens. That's just my experience and might not be applicable to you. Also my diagnosis may have been incorrect, although the outcome was good.

Does your normal channel sound thin now too? Or just the TMB/Plexi channel? If the latter then maybe consider the above.
The normal channel sounds mostly the same. Although, I actually do wish it was opened up a bit more, not as “wooly” or dark.
 

BenTobith

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So I reverted a few of the cap changes and kept the 180R bias resistor. Not much change. Still had some of the “ice pick” I don’t like.

Decided to swap output tubes. Went from Valvos to RFTs. Bam. Ice pick gone. RFT EL84s just seem to be magic for my ears. I loved them in a Matchless, as well.
 

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