My 36W Project

[TriodeLuvr]

Also, I want to sincerely thank all of you on these forums who provided comments and advice as I went through the design and build of this amp. It paid off in the end!

Jack

[TriodeLuvr]

My 36W amp has now been requisitioned by my daughter, who started playing last year. She's thrilled by the tone and the classic British sound. In exchange, she has given me the little Acoustic G10 practice amp we bought for $20. The case is in perfect condition, so I think it will make a good foundation for a future low power tube combo. Not planning any frills in this one, no reverb or FX loop, just lots of high energy gain and a good speaker. Oh, and maybe tremolo. There are so many choices for the output stage, I don't even know where to start. Will probably restrict it to a single-ended design, but even so there are dozens of candidates for the output stage. 6BL7, 6V6 pentode/triode, 6AQ5, 6CL6, 5763, 5686, etc., etc. I'll start a new thread for this when the work starts.

Jack

[Bieworm]

Wow..wait a little.. Maybe this 1 watter I'm building turns out great, and a good option for low power amp build for you.
Lucky you! I have 3 Kids and none of them play music. They only nag when I'm playing

[TriodeLuvr]

Wow..wait a little.. Maybe this 1 watter I'm building turns out great, and a good option for low power amp build for you.
Lucky you! I have 3 Kids and none of them play music. They only nag when I'm playing

I'm sure your 1W WILL turn out great! I need to do this one on my own though, a one-off mongrel instead of a copy. Also, I have all the basic building blocks on hand for a single ended design - OPTs, power xfmrs, 6X4/6X5 rectifiers, etc. For low power push-pull, I would have to buy at least the OPT. I'm thinking along the lines of two or three high-gain stages driving a James tone stack with a 2W to 4W output stage. Tubes are likely to be types not usually seen in guitar amps. My biggest expense for a project like this will be the chassis and a new speaker. Almost everything else is on the shelf.

Jack

[TriodeLuvr]

I was reviewing some of my docs today and realized that I hadn't posted schematics for this amplifier. The front end is a Marshall clone, but all the other areas - including the active effects loop and reverb circuits - were specifically designed and integrated into the amplifier in ways that might be helpful for others to see. This amplifier is dead quiet and everything works like it should, so all of this circuitry is vetted.

Jack

[JMPGuitars]

Hi Jack! I was just thinking about you discussing tubes yesterday with another EE on here. He asked about some fixed bias stuff, which is not something I know much about.

I see a lot of cool things going on in that design. But the question remains: how does it sound? Got a demo?

Thanks,
Josh

[JMPGuitars]

Hey Jack, I just noticed something. You have a lot of resistors assumed to be 1/2W at or well above ~250V. Do you know a specific series of 1/2W resistors rated for that voltage? Most 1/2W I know of are rated at 250V or less, and the better 1W resistors rated up to 500V.

[TriodeLuvr]

Hi Josh, I'm not a fan of online audio demos (for several reasons). In this case, I'm simply too poor a guitar player to pull it off. OTOH, I would be glad to allow any decent player in the Austin area who might be reading this to review the amp and use it to make a demo.

Which 1/2W resistors are you looking at regarding the voltage rating? This circuit is fairly complex, so maybe I've overlooked it somewhere. FWIW, a 1/2W resistor would need to have a value of 125KΩ or less to dissipate its rated power with 250V across it. I occasionally purchase Stackpole resistors with higher voltage ratings, but I don't remember that being required for this project.

Jack

[JMPGuitars]

Hi Josh, I'm not a fan of online audio demos (for several reasons). In this case, I'm simply too poor a guitar player to pull it off. OTOH, I would be glad to allow any decent player in the Austin area who might be reading this to review the amp and use it to make a demo.

I often feel the same way about my stuff, but I do it anyway.

Which 1/2W resistors are you looking at regarding the voltage rating? This circuit is fairly complex, so maybe I've overlooked it somewhere. FWIW, a 1/2W resistor would need to have a value of 125KΩ or less to dissipate its rated power with 250V across it. I occasionally purchase Stackpole resistors with higher voltage ratings, but I don't remember that being required for this project.

R4, R7, R12, R25, R26 are all under 125kΩ and all of them are being fed 340 to 350 VDC. There are others too, I didn't look that thoroughly. For example, R53 is also not noted as a higher wattage despite a 320VDC working voltage.

I personally use CMF resistors for my builds, and the 1/2W are 250V and 1W are 500V. Those have been generally better rated and far more reliable than anything else I've tried.

Thanks,
Josh

[TriodeLuvr]

R4, R7, R12, R25, R26 are all under 125kΩ and all of them are being fed 340 to 350 VDC. There are others too, I didn't look that thoroughly. For example, R53 is also not noted as a higher wattage despite a 320VDC working voltage.

Didn't we have a similar discussion about voltage ratings in the past? I believe you're looking at the supply voltages, not the voltage drop across the resistors. R4 only has 37V across it. R53 only has millivolts (screen current X 100Ω). If R53 really had 320V across it, dissipation would be 1,024 watts and it would burn up instantly.

Jack

[JMPGuitars]

R4, R7, R12, R25, R26 are all under 125kΩ and all of them are being fed 340 to 350 VDC. There are others too, I didn't look that thoroughly. For example, R53 is also not noted as a higher wattage despite a 320VDC working voltage.

Didn't we have a similar discussion about voltage ratings in the past? I believe you're looking at the supply voltages, not the voltage drop across the resistors. R4 only has 37V across it. R53 only has millivolts (screen current X 100Ω). If R53 really had 320V across it, dissipation would be 1,024 watts and it would burn up instantly.

Jack

Maybe? The voltage ratings are typically referred to as "working voltages" not "dissipation voltage." Even Stackpole refers to them as such. They also say to make sure to derate the resistors in higher heat: https://seielect.com/news/20170821_-_Re ... mation.pdf

Thanks,
Josh

[TriodeLuvr]

The voltage ratings are typically referred to as "working voltages" not "dissipation voltage." Even Stackpole refers to them as such. They also say to make sure to derate the resistors in higher heat: https://seielect.com/news/20170821_-_Re ... mation.pdf

"Working voltage" has always meant the voltage applied across the component. The same is true of capacitor voltage ratings. Stackpole states that, "Maximum working voltage is the maximum amount of voltage the resistor can withstand constantly without arcing." An extreme example to make the point might be a resistor with one end unterminated. It won't arc even if 5KV is applied to the other end.

Jack

[TriodeLuvr]

I just want to add that the definition of "maximum working voltage" in the Stackpole paper doesn't agree with all others. That's because Stackpole is relating it to arcing, while some companies define it as the maximum allowable voltage that can be applied without exceeding its dissipation rating.

For example, Allied uses the phrase "maximum voltage across a resistor" and the term "maximum working voltage" interchangeably. There's nothing wrong with this, but neither involves breakdown due to arcing.

https://www.alliedcomponents.com/blog/m ... s-resistor

The Stackpole paper instead refers to the voltage that exceeds the dissipation rating as the “critical resistance value," and it's a function of both the wattage rating and the value of the resistor.

Regardless of this discrepancy, the bottom line in all these definitions is that "working voltage" refers to the voltage across the resistor. It is never intended to mean the voltage applied to one end without regard for the voltage on the other.

Jack

[JMPGuitars]

Regardless of this discrepancy, the bottom line in all these definitions is that "working voltage" refers to the voltage across the resistor. It is never intended to mean the voltage applied to one end without regard for the voltage on the other.

Thanks for the clarification. To also quote SEI: "The power rating and voltage rating of a resistor are one common source of confusion."

They weren't kidding.

[TriodeLuvr]

Just to add, up-sizing resistor dissipation in order to achieve a greater voltage rating might not be technically necessary, but it surely results in a very high quality product. I would never argue against overrating these parts for that purpose.

Jack