New 36W build, guidance needed

Double-Bubble! Place for discussing the 36W version...

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JMPGuitars
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Re: New 36W build, guidance needed

Post by JMPGuitars »

TriodeLuvr wrote:
Tue 12/15/20 8:36 pm
I just want to mention that the design represented in the schematic regarding the grid stoppers is incorrect. I suspect a misprint in the schematic, but regardless, there's no valid reason for the existence of stoppers on only two tubes. Also, to be effective at eliminating oscillation, stoppers need to be soldered directly to the grid pins with the shortest possible leads.

Don't be tempted to use cathode bias as mentioned in your first post. Cathode bias applied to amplifiers operating in other than Class A causes an increase in DC bias with increasing output. This isn't always obvious when a large signal is present, because the loudness of the material masks lower level artifacts. However, when the signal suddenly goes quiet, the output tubes are left in an under-biased state for some brief period of time. The result can be a short term presence of audible crossover distortion.
Huh?? Are you saying cathode bias is only good on class A amps? That's ridiculous. None of these 18 watters are class A, and they're basically all cathode bias. There's a plethora of amp designs with cathode bias that aren't class A.
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Re: New 36W build, guidance needed

Post by crgfrench »

I believe TriodeLuvr is making a "hifi amp" type of statement here. He is saying cathode bias has more distortion than fixed bias, except in the case of a class A push-pull. We don't care about that here, since we are seeking to generate a certain distortion, specifically the distortion from a cathode biased, class AB guitar amplifier.
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Re: New 36W build, guidance needed

Post by TriodeLuvr »

JMPGuitars wrote:
Wed 12/16/20 7:06 am
TriodeLuvr wrote:
Tue 12/15/20 8:36 pm
I just want to mention that the design represented in the schematic regarding the grid stoppers is incorrect. I suspect a misprint in the schematic, but regardless, there's no valid reason for the existence of stoppers on only two tubes. Also, to be effective at eliminating oscillation, stoppers need to be soldered directly to the grid pins with the shortest possible leads.

Don't be tempted to use cathode bias as mentioned in your first post. Cathode bias applied to amplifiers operating in other than Class A causes an increase in DC bias with increasing output. This isn't always obvious when a large signal is present, because the loudness of the material masks lower level artifacts. However, when the signal suddenly goes quiet, the output tubes are left in an under-biased state for some brief period of time. The result can be a short term presence of audible crossover distortion.
Huh?? Are you saying cathode bias is only good on class A amps? That's ridiculous. None of these 18 watters are class A, and they're basically all cathode bias. There's a plethora of amp designs with cathode bias that aren't class A.
Cathode bias is popular because it's cheaper to produce. In addition, the distortion it causes is easily hidden in RMS specifications due to its transient nature.

Some tubes - including the 6BQ5 - suffer less from this phenomenon because they are typically biased hot. The difference in average cathode current between idle and full output at each tube is often 10mA or less. This means the tubes operate closer to Class A then AB1. In comparison, a 6L6GC is usually biased such that full output nearly doubles the average cathode current. The crossover distortion this creates in a self-biased PP stage is anything but minor and easily observable on a scope.

The extent to which this distortion is evident depends on other factors as well, including the time constant of the cathode RC. And it's worth noting that the term "cathode bias" can include schemes that apply a fixed, positive DC voltage to the cathode, rather than relying on a series RC. "Self-bias" is a more accurate way way to describe the RC approach.

In any event, after seeing the problem for myself a number of years ago, I stopped using self-bias for AB1. If you like it, that's great, no one is stopping you. It's just not a technique that I can recommend unless there's a compelling reason to do so other than cost or convenience.
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Re: New 36W build, guidance needed

Post by JMPGuitars »

TriodeLuvr wrote:
Wed 12/16/20 1:12 pm
Cathode bias is popular because it's cheaper to produce. In addition, the distortion it causes is easily hidden in RMS specifications due to its transient nature.
You do understand that this is a guitar amp site, yes? We like distortion. Even a small amount of crossover distortion can be desirable. If there's too much crossover distortion, there are a number of things that affect that which can be adjusted.

Cathode bias also improves the touch-sensitivity of the amp, which is another thing we like.

Guitar amps are nothing without distortion. Even "clean" guitar sound generally has some distortion. Otherwise it would sound sterile like a HiFi amp.

If you're using your scope to get rid of all traces of distortion in your amp, you're not building a guitar amp.
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Re: New 36W build, guidance needed

Post by TriodeLuvr »

It's just a matter of opinion. I don't like the characteristic sound of crossover distortion. I also don't like the way it degrades low-level nuance, and that doesn't happen only with hi-fi. And on this point, not everyone wants to play through a distorted amp all the time. Sometimes loud and clean is a good thing. If an amp is self-biased and adjusted down into true AB1, it probably can't accomplish this. You know, I lack the specific experience that many of you have here when it comes to these types of amps. On the other hand, I know what sounds good to me and what doesn't. This type of distortion doesn't, so I don't use or recommend self-bias unless there's a compelling reason to do so. You don't have to agree with me, but that doesn't mean I'm wrong or that this is bad advice. It only means we might expect different things in the equipment we use.
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Re: New 36W build, guidance needed

Post by JMPGuitars »

TriodeLuvr wrote:
Wed 12/16/20 3:07 pm
It's just a matter of opinion. I don't like the characteristic sound of crossover distortion. I also don't like the way it degrades low-level nuance, and that doesn't happen only with hi-fi. And on this point, not everyone wants to play through a distorted amp all the time. Sometimes loud and clean is a good thing. If an amp is self-biased and adjusted down into true AB1, it probably can't accomplish this. You know, I lack the specific experience that many of you have here when it comes to these types of amps. On the other hand, I know what sounds good to me and what doesn't. This type of distortion doesn't, so I don't use or recommend self-bias unless there's a compelling reason to do so. You don't have to agree with me, but that doesn't mean I'm wrong or that this is bad advice. It only means we might expect different things in the equipment we use.
I certainly agree that too much crossover distortion sounds terrible.

However, there's nothing wrong with cathode bias, and it isn't cheaper than other methods. Any method is only as cheap as the components you use. Some people may spend $5 on a fixed or adjustable bias set of parts, and others might spend $30 on a single cathode bias resistor.

In any case, you're entitled to your opinion, but when you come on a website which primarily uses cathode biases circuits, with people who enjoy those circuits, you shouldn't tell people to avoid cathode biasing because you don't like it. I'm a member on a number of forums, but I don't go on a F*nder forum and tell them their circuits sound like plastic doodoo....even though that IS my opinion. ;)
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Re: New 36W build, guidance needed

Post by TriodeLuvr »

JMPGuitars wrote:
Wed 12/16/20 5:02 pm
I certainly agree that too much crossover distortion sounds terrible.

However, there's nothing wrong with cathode bias,
It can suffer from the specific technical shortcoming that I described. That's not merely my opinion.
and it isn't cheaper than other methods. Any method is only as cheap as the components you use. Some people may spend $5 on a fixed or adjustable bias set of parts, and others might spend $30 on a single cathode bias resistor.

Marshall didn't spend $30 on cathode bias resistors and bypass caps. No large production manufacturer that I'm aware of ever did.
In any case, you're entitled to your opinion, but when you come on a website which primarily uses cathode biases circuits, with people who enjoy those circuits, you shouldn't tell people to avoid cathode biasing because you don't like it.

The OP's amp currently uses fixed bias, and it was reasonable to let him know that changing it could risk an anomaly he might not be aware of. You're reading much more into this than I intended. Let's just let it go, OK? I'm not the enemy. :D
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Re: New 36W build, guidance needed

Post by JMPGuitars »

TriodeLuvr wrote:
Wed 12/16/20 5:34 pm
It can suffer from the specific technical shortcoming that I described. That's not merely my opinion.
Anything improperly done suffers something.


Marshall didn't spend $30 on cathode bias resistors and bypass caps. No large production manufacturer that I'm aware of ever did.
Not really the point, is it? You can spend as much or as little as you want on any bias method. The large manufacturers tend to cheap out on whatever method they're using. That's why the average amateur clone sounds better than the big name stuff off the shelf.

The OP's amp currently uses fixed bias, and it was reasonable to let him know that changing it could risk an anomaly he might not be aware of. You're reading much more into this than I intended. Let's just let it go, OK? I'm not the enemy. :D
lol, fair point. Just don't hate toooo much on our cathode biasing. ;)
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Re: New 36W build, guidance needed

Post by Bieworm »

I think most people that choose for cath bias do it for sonic reasons rather than saving money. After building some kits we all want to build OUR amp.. not saving penny building and trying to achieve the best possible tone in their head. The money saving part is often the first build and the scrap builds.
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Re: New 36W build, guidance needed

Post by Henrik76 »

FWIW, it will be adjustable fixed bias, at least as a starting point. The PS shows an unloaded voltage of about 425v, which is crazy high for el84's. Fixed bias will at least allow me to fine tune the dissipation to a safe(ish) level.
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Re: New 36W build, guidance needed

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TriodeLuvr wrote:
Tue 12/15/20 8:36 pm
I just want to mention that the design represented in the schematic regarding the grid stoppers is incorrect. I suspect a misprint in the schematic, but regardless, there's no valid reason for the existence of stoppers on only two tubes. Also, to be effective at eliminating oscillation, stoppers need to be soldered directly to the grid pins with the shortest possible leads.
If there's a mistake in the mesa schematic, I wouldn't know. What I can tell you, though, is that the peavey does indeed sport 100R 5W resistors on the screens of two out of 4 output tubes. I can trace that out on the circuit board I have on the bench. It might have something to do with the fact that the peavey allows you to pull two tubes to reduce power. I'm not sure, but I will wire all 4 tubes equal.
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Re: New 36W build, guidance needed

Post by Henrik76 »

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Re: New 36W build, guidance needed

Post by crgfrench »

Henrik76 wrote:
Tue 11/10/20 3:28 am
...in the Peavey schematic, only one out of two paralleled EL84s has a screen resistor on it! This seems very dubious to me and I cannot see why that would be, unless it's a misprint.
Are you referring to R46 and R55? I agree, it looks like R46 should be shifted down and R55 should be rotated 90 degrees counterclockwise, pivoting at its upper end. Is the circuit actually wired that way? maybe the diagram has errors.
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Re: New 36W build, guidance needed

Post by Henrik76 »

Yup, and it is indeed wired like that. Makes no sense to me, but I guess they must have had their reasons.

Cheers,
Henrik
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Re: New 36W build, guidance needed

Post by crgfrench »

Huh... well, it's not like 100R does a lot anyway.
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Re: New 36W build, guidance needed

Post by Henrik76 »

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Channel one, input and first stage complete. DC30-ish. Grid load switchable between 68k and 1M.

Cheers!
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Re: New 36W build, guidance needed

Post by Henrik76 »

Progress!
CH1 stage 2, CF and tone stack plus LTP phase inverter.
CH2 5879 first stage also completed.
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Re: New 36W build, guidance needed

Post by Henrik76 »

It's filling up in here. Hope to hear some noise later this week, if the lady of the house let's me work... :wink:
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Re: New 36W build, guidance needed

Post by Henrik76 »

She's alive! hurray
Both channels functional right out of the can. Word of advice: I went in my den to work after nailing a beautiful bottle of red. Had to redo most of that work :? ..... Glad I caught it before I powered her up. Had the pinout on the 5879 all messed up.
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I had to make some significant adjustments in the PS to get the screen voltage down below 300, but now we're at 280, with 395 plate volts. Biased to dissipate 15W per side. Sounds sweet now, but I have yet to add a master volume so I'm very limited as to how far I can go on the volume.
To be continued....
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Re: New 36W build, guidance needed

Post by Henrik76 »

I've tweaked the PS to where the voltages are sensible throughout and I'm happy to report that it sounds excellent, and LOUD!
However, I have more hiss and static than I'm willing to accept, so now starts the tedious process of trying to iron this out without sacrificing tone. It'll be a tough one.
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