I've only owned my very first tube amp for a couple of days now and my DIY nature is already turning my thoughts to how I can modify/improve it. Although I've got a few electronics/speaker projects under my belt now I've never touched tubes before so I have quite abit of reading to do. That aside, do any of our more experienced tube forum members want to throw a few ideas around where I might makes some gains here?
Regards,
Dan
Regards,
Dan
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
What is it that you're specifically trying to improve? Or is this just a matter of, "I can't keep my hands off this, please tell me something to do which will keep me occupied?"
If you're attacking a commercial design, it first helps to understand it. If you can't point to any resistor at random and say what it does, you ought to keep your hands out of it until such time as you do. Presumably, the guy who designed it had at least some minimal level of competence, so you're not repairing or correcting obvious errors.
MOST importantly, you need to have the right tools (sig generator, scope, voltmeter) and basic knowledge of how to avoid killing yourself while working on high voltage circuits.
I'm not trying to be harsh here, I just don't want you dead or with a smoking pile of expensive parts where there used to be a functioning amp.
If you're attacking a commercial design, it first helps to understand it. If you can't point to any resistor at random and say what it does, you ought to keep your hands out of it until such time as you do. Presumably, the guy who designed it had at least some minimal level of competence, so you're not repairing or correcting obvious errors.
MOST importantly, you need to have the right tools (sig generator, scope, voltmeter) and basic knowledge of how to avoid killing yourself while working on high voltage circuits.
I'm not trying to be harsh here, I just don't want you dead or with a smoking pile of expensive parts where there used to be a functioning amp.
The first bit of advice:
Be careful!!
This amp has a high B+ voltage.
Read up on High Voltage Safety
Be careful!!
This amp has a high B+ voltage.
Read up on High Voltage Safety
SY said:
A commerical product is usually a compromise between cost of components, manufacturing costs and what product price the market will support.
My approach to date has not been to redesign anything but to replace certain components with better quality 'audiophile' grade components that wouldn't have been commericially viable. An excellent example of this is my participation in the extremely popular Marantz CD63 & 67 mods. I followed this thread and performed most of these mods on my CD67.
I have the experience and tools mentioned below. And, I always jokingly say the NICE thing about working with high voltage is that if you make a mistake you know INSTANTLY!!!!!!!!
The two things I would probably like to look at is any reasonable power supply improvements I could make and perhaps some better quality components (ie. replace C103 with a Hovland or Jupiter) in the signal path.
Regards,
Dan
I'm probably not the right one to help you with fashion accessory questions but no doubt lots of others will chime in.
As far as power supply work, what's the inadequacy in the power supply that you're trying to fix? I'll stress again, if you're not VERY experienced with high voltages (there's probably 700V or so on that B+ rail), don't even THINK about messing with this.
As far as power supply work, what's the inadequacy in the power supply that you're trying to fix? I'll stress again, if you're not VERY experienced with high voltages (there's probably 700V or so on that B+ rail), don't even THINK about messing with this.
Not necessarily. Most likely you will be dead.
Probably not a good choice because of the heat (very likely) under the chassis.
But yes caps are the most obvious choice for "fashionable accesoires" However I was suprised (positively) when opening up a Chinese built commercial amp when I saw the good quality parts.
PS. The 300B tubes according to the schematic sure don't look like 300B's in the picture.
Bas Horneman said:
I was sort of motivated by this article on a very similar amp from the same manufacturer. Another motivation is , and these are only my very brief first impressions, this tube amp is very good but my current non-tube setup trounces it in high-end detail, air, and soundstage depth and definition. If I can't improve it it will not be a 'keeper'.
Regards,
Dan
It won't likely do anything for sonics but one recommendation is to drop a couple hundred kilohm from pin 4 of the 6JP4 to ground. As-is that wiper going intermittent will make for an 'illuminating' experience.
Edit: It looks like that schematic might be a little inaccurate as well. Rectified 5 VAC on the 300B filament? And same recommendation for the 845 grid bias pot.
Edit: It looks like that schematic might be a little inaccurate as well. Rectified 5 VAC on the 300B filament? And same recommendation for the 845 grid bias pot.
I noticed that the 300B is being used as a cathode follower, and at what appears to be relatively low currents at that based on the 47K cathode resistor in the CF.
I would not think that a 300B run at such low currents as a CF would be the optimum choice for the job. I'll wager that the cathode current is no more than 7-8mA and is probably even less if that schematic is correct. Frankly at these low currents a 6J5 would probably do a better job. Seems like the design is capitalizing on the market cachet of having a 300B in the circuit.
Unfortunately it is probably the case that the 400V winding on the power transformer cannot handle a reasonable amount of current. I'd check with the manufacturer before bumping up the current in the driver stage.
If you are tube rolling I would measure plate voltages in the first stage with the oem tubes and when using other types modify the cathode bias resistor value on that pentode to assure approximately the same plate current since the 300B is direct coupled to the voltage amplifier stage, and changes in plate voltage will affect the operating point of the CF.
Do the mods rdf suggests as they will prevent unfortunate mishaps should a wiper go intermittent. Get rid of the volume pot altogether if you aren't using it.
I would not think that a 300B run at such low currents as a CF would be the optimum choice for the job. I'll wager that the cathode current is no more than 7-8mA and is probably even less if that schematic is correct. Frankly at these low currents a 6J5 would probably do a better job. Seems like the design is capitalizing on the market cachet of having a 300B in the circuit.
Unfortunately it is probably the case that the 400V winding on the power transformer cannot handle a reasonable amount of current. I'd check with the manufacturer before bumping up the current in the driver stage.
If you are tube rolling I would measure plate voltages in the first stage with the oem tubes and when using other types modify the cathode bias resistor value on that pentode to assure approximately the same plate current since the 300B is direct coupled to the voltage amplifier stage, and changes in plate voltage will affect the operating point of the CF.
Do the mods rdf suggests as they will prevent unfortunate mishaps should a wiper go intermittent. Get rid of the volume pot altogether if you aren't using it.
Modifying a very high voltage tube amp as a first project in tube electronics is not particularly recommended.
However assuming that you follow the high voltage guidelines I would suggest that there are three stages of mods possible.
Stage one :replace existing components with higher quality parts.This could be primarily coupling caps,resistors in the signal path ,vol control,cathode bypass caps etc.
Stage two: power supply mods such as better quality caps,bypass main caps with film caps, change rectifiers to high voltage hexfred or series HV schottky diodes.
Stage three: Rethink the first driver stage.A triode solution would be advised from a purist point of view.This needs more expertise and study.Basically you have a wealth of options here cascade,srpp,mu,ccs loaded.Just study driver stages for 300B and we can talk in a couple of years again
However assuming that you follow the high voltage guidelines I would suggest that there are three stages of mods possible.
Stage one :replace existing components with higher quality parts.This could be primarily coupling caps,resistors in the signal path ,vol control,cathode bypass caps etc.
Stage two: power supply mods such as better quality caps,bypass main caps with film caps, change rectifiers to high voltage hexfred or series HV schottky diodes.
Stage three: Rethink the first driver stage.A triode solution would be advised from a purist point of view.This needs more expertise and study.Basically you have a wealth of options here cascade,srpp,mu,ccs loaded.Just study driver stages for 300B and we can talk in a couple of years again
protos said:Modifying a very high voltage tube amp as a first project in tube electronics is not particularly recommended.
However assuming that you follow the high voltage guidelines I would suggest that there are three stages of mods possible.
Stage one :replace existing components with higher quality parts.This could be primarily coupling caps,resistors in the signal path ,vol control,cathode bypass caps etc.
Stage two: power supply mods such as better quality caps,bypass main caps with film caps, change rectifiers to high voltage hexfred or series HV schottky diodes.
Stage three: Rethink the first driver stage.A triode solution would be advised from a purist point of view.This needs more expertise and study.Basically you have a wealth of options here cascade,srpp,mu,ccs loaded.Just study driver stages for 300B and we can talk in a couple of years again
As I eluded to previously. My current non-tube setup, although not without some weaknesses, has too many virtues above and beyond this tube amp. That may make this not worth pursuing.
Hmmmmmmmmmmmm...........
Regards,
Dan
The two brands you have highlighted are good brands. The US distributor for Lundahl is http://www.kandkaudio.com by the way.
Hi !
First of all: The schematic doesn't match with the amp in the pic.
But a 300B as cathode follower is anyway idiocy. Except for the high voltage and high signal amplitude it can be feeded with.
The pentode as first stage pleases me. A very good approach. This tube makes the basic sound of the amp. The Question is whether it can deliver enough signal voltage to the 845. Last one has µ of 5 ! You didn't give any infos about the signal voltage of the amp. But the schematic mentions 90mA. At a plate dissipation of 100W this gives a voltage ~1100V. We have an inductive load, means the signal amplitude at the anode will increase to a max of 2,2KV. Hence we must be able to drive the tube from Zero to 2,2KV with a pitiful µ=5. This gives a P-P value of 440V or 156V RMS. Of course, these values are now a bit oversized, but it gives a perception.
Furthermore I'm missing negative Feedback. This means additional gain of the driver.
If I were you, I would build a µ-Follower-cascode. A cascode with a active load. Such a stage with 3 tubes "on top of each other" will require nearly as much operating voltage as the 845 itself. But you have the Advantage of very high gain and low output resistance.
Another way is a simple two stage driver. with a EL84, EL34, 6L6 to drive the 845.
regards, manta
First of all: The schematic doesn't match with the amp in the pic.
But a 300B as cathode follower is anyway idiocy. Except for the high voltage and high signal amplitude it can be feeded with.
The pentode as first stage pleases me. A very good approach. This tube makes the basic sound of the amp. The Question is whether it can deliver enough signal voltage to the 845. Last one has µ of 5 ! You didn't give any infos about the signal voltage of the amp. But the schematic mentions 90mA. At a plate dissipation of 100W this gives a voltage ~1100V. We have an inductive load, means the signal amplitude at the anode will increase to a max of 2,2KV. Hence we must be able to drive the tube from Zero to 2,2KV with a pitiful µ=5. This gives a P-P value of 440V or 156V RMS. Of course, these values are now a bit oversized, but it gives a perception.
Furthermore I'm missing negative Feedback. This means additional gain of the driver.
If I were you, I would build a µ-Follower-cascode. A cascode with a active load. Such a stage with 3 tubes "on top of each other" will require nearly as much operating voltage as the 845 itself. But you have the Advantage of very high gain and low output resistance.
Another way is a simple two stage driver. with a EL84, EL34, 6L6 to drive the 845.
regards, manta
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