Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on May 13, 2021, 10:31:56 am
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I just finished this - it sounds great, and is relatively quiet. I have implemented Robenette's idea for a 3-way cathode resistor for the cold clipper, so when I switch to the higher gain settings, obviously the noise floor increases. I'd appreciate any input regarding my lead dress - if there are areas I can improve.
Thanks
Humbly,
Mark
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...incidentally...the rectifier diodes are mounted to the right side wall on that terminal strip. I've followed Doug's document for the grounds, also.
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I don’t think that’s a suitable chassis safety ground point - how much torque can the fastener take before the plastic of the IEC bezel splits?
Not enough, I suspect.
If you don’t fancy drilling a hole to for a dedicated fastener, I suggest to use a tagstrip mounting fastener and lug.
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I don’t think that’s a suitable chassis safety ground point - how much torque can the fastener take before the plastic of the IEC bezel splits?
Not enough, I suspect.
I can certainly tie it to one of the PT mounting bolts if you think that's a real consern.
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PT bolts are a very poor choice.
Without wanting to come across as being overly preachy or didactic, the electrical safety of mains powered equipment is a big deal.
Details matter, it’s not a freestyle thing.
Sorry, I edited my previous post with a suggestion.
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Thanks. That was my thought, to use the terminal strip lug bolt.
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Looks good to me. One thing I would have done differently would be to mount the C2 node cap on the board at the V1 end. And ground that cap at the same place as V1 cathode resistors.
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The 68k input grid stopper will probably be the noisiest thing in the amp. It’s great that you used a nice resistor type there, but I suggest 10k to 33k to get a lower hiss.
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Ah. Thanks, Slukey. The next rev I'll have to move that C2 node cap onto the board. Is it less noisy that way? To fit it on there now I'd have to move some parts over....but if it's a substantial change, maybe worth the extra work?
Thanks for the tip on the 33k vs 68k resistor. The Vishay Dale R65's are pretty quiet, but if a 33k is quieter, I'll change that out. That's an easy fix.
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The biggest ‘reward for effort’ will depend on the most significant components of the noise floor, at the typical settings used.
eg if 60Hz then humdinger, 120Hz then Sluckey’s filter cap move, if hiss then my (well, Merlin’s :icon_biggrin:) input grid stopper.
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What are the brown resistors, and where do you get them?
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What are the brown resistors, and where do you get them?
Vishay Dale RN65D 1%. You can get them at Mouser, that's where I get them. A lot of guys who build Dumble clones use these.
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What are the brown resistors, and where do you get them?
Vishay Dale RN65D 1%. You can get them at Mouser, that's where I get them. A lot of guys who build Dumble clones use these.
Thanks. Are those the 1/2 watt resistors? They do not appear to have them in 1 watt....
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Thanks. Are those the 1/2 watt resistors? They do not appear to have them in 1 watt....
Those are 1/2 W
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The 68k input grid stopper will probably be the noisiest thing in the amp. It’s great that you used a nice resistor type there, but I suggest 10k to 33k to get a lower hiss.
Would it be quieter to parallel in another 68K with the current one vs replacing it with a 33K?
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The 68k input grid stopper will probably be the noisiest thing in the amp. It’s great that you used a nice resistor type there, but I suggest 10k to 33k to get a lower hiss.
Would it be quieter to parallel in another 68K with the current one vs replacing it with a 33K?
I’m not sure; contact noise may be a bit lower with the parallel resistor. But any difference might be minimal, it’s the magnitude of the value that’s probably most significant.
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I changed out that resistor to 33k (Dale 1% 1Watt). I have 3 gain settings (3 cathode resistors selectable for the cold clipper). On the lowest gain it's pretty quiet. Obviously as you change to the higher gain settings there is a higher noise floor. Hiss, really, not 60 or 120 hz. I don't know what to compare it to, though. I do have a friend with several vintage Marshalls - I can try comparing with his amps for reference. And with the MV turned below 10 it's certainly much quieter. In normal use, I'm thinking a person would have the MV down, maybe below 1/2, and the pre volume up to get Marshally overdrive. I'm more of a blues/jazz player, so what do I know.... :)
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I swear by Vishay-Dale MILSPEC resistors. Many to choose from.
https://www.vishay.com/docs/49851/49851_sg2116.pdf
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... I'd appreciate any input regarding my lead dress - ...
Very tidy looking build.
Obviously, finding an appropriate grounding method will help minimise noise floor issues. I highly recommend Merlin Belcowe's grounding article and following his recommendations to the letter. http://www.valvewizard.co.uk/Grounding.html
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Thanks. Are those the 1/2 watt resistors? They do not appear to have them in 1 watt....
Those are 1W
RN60 are 1/4W
RN65 are 1/2W
RN70 are 3/4W
RNxx types listed above are the military designation for CMFxx type metal film resistors.
commercial/industrial versions CMFxx series have a looser derating for power specifications with the same construction.
CMF50 are 1/4W
CMF55 are 1/2W
CMF60 are 1W
CMF65 are 1.5W
CMF70 are 1.75W
notice in the RNxx datasheet attached that the CMFxx equivalents are de-rated considerably for mil acceptance.
use the CMF series, they are just as quiet and are available with same temperature drift coefficients and tolerance.
CMF types will typically have a better value selection carried by most distributors.
--Pete