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Thread: So here’s a fun one (AD-100 clone woes)

  1. #36
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    Also, I took a closer look and it’s actually 1n4005, but these are only for the screen supply. There is a string of 8x 5408’s tied to the primaries, does this make a difference in the acceptability of the 1n400x series?
    Oh, this is new info. So you have separate screen and HT supplies? Please try to post a drawing of both.

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    Click image for larger version. 

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    Here is HV supply- the red wires attached to the diodes lead to HV secondary transformer wires. The red lead from the filters lead to OT B+.

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    If these pictures don’t do it for you I can sketch the supply! Those caps are 2x250m in series

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    I can sketch the supply
    Yes, please sketch both (screen and plate) supplies showing filter caps and voltages.

    (Please try to not mix up m (=milli) and µ (=micro). 1mF equals 1000µF.)

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    Chuck I missed this earlier (my phone is not the most effective forum reading device), but thank you very much for your input! Great stuff here.

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    Here are the supplies, sketched. There is a diagram of the KT88 too from earlierClick image for larger version. 

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    Quote Originally Posted by Mr_bibbles View Post
    Here are the supplies, sketched. There is a diagram of the KT88 too from earlierClick image for larger version. 

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    Ok, thanks.
    The 6A screen fuse and 200µ screen reservoir capacitance make absolutely no sense. Add to this a missing HT fuse.
    This all shows that the "designer" had no idea what he was doing.

    Nevertheless it gives no clue to the failures.

    I still suspect an intermitting load connection or plate pin contact.

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    Thank you for your reply- after your comment about the fusing system it led me to examine the three fuses... one is a 8a mains attached to primaries, one is a 6a screen fuse.... and one is a 2a fuse attached to the ground through the 1 ohm resistors.. I had already measured the rest of the voltages for vibrational disconnect under duress.... but I never measured the ground connection! It was being disconnected at the fuse holder with even slight vibration. His has to be the issue, right???


    I’m considering replacing that fuse holder and converting it to a HV fuse, hardwiring the ground to avoid any further issues. Can anyone tell me how to determine fuse ratings via the voltage, or if this is a fools errands?

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    I thought about a way to calculate the appropriate rating of the screen supply fuse and came up with the following:

    The (absolute) max screen dissipation of a KT88 is 8W. Times 8 (tubes) makes 64W. Dividing by screen supply voltage (400V) gives the max allowable total screen current as 0.16A. So a 200mA fuse should be fine.

    Comments welcome.

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    Last edited by Helmholtz; 01-24-2020 at 09:57 PM.
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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    I thought about a way to calculate the appropriate rating of the screen supply fuse and came up with the following:

    The (absolute) max screen dissipation of a KT88 is 8W. Times 8 (tubes) makes 64W. Dividing by screen supply voltage (400V) gives the max allowable total screen current as 0.16A. So a 200mA fuse should be fine.

    Comments welcome.
    My only consideration might be the tendency for overdissipation to be balanced with periods of much lower dissipation WRT clipping circuits. Which tubes handle rather gracefully. We see this a lot in guitar amps. Since fuses in these locations tend to be regular fast blow type I might be concerned about popping fuses whenever the amp is clipping.?.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Lifetime Member Enzo's Avatar
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    It was being disconnected at the fuse holder with even slight vibration. His has to be the issue, right???
    Actually, don't ask that. Clearly it needs to be corrected, so correct it. THEN tell us if it made the difference.

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    Member HaroldBrooks's Avatar
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    Quote Originally Posted by g1 View Post
    Multiple arcing issues make me immediately suspicious of too high cab impedance or speaker cable going open.
    Did the 1st replaement PT fail or was the 2nd replacement just to try with tamer voltages?
    Working my way through this thread so I will see how it all ended, but G1 your intuition sounds good to me in any event.

    So far I've been lucky with OT transformers or tube shorts, but my bandmate had a scare when we were playing a gig and the crimp speaker connection fried at the speaker wire connection, and he played for a perhaps 10-15 seconds that way full tilt, as he could barely hear himself over the drums. Turns out no carnage, he dodged the bullit.

    Since then I have been using 625v Varistors between each leg of my push pull plates and center tap, just incase a thing like that happens to me. It's supposed to dump voltage and current back to ground and protect things, for a while at least, and may buy you time before things light up.

    You have the best guys helping you out on this, but I think you know that. Hope the ground was the issue !

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    Last edited by HaroldBrooks; 01-25-2020 at 07:19 AM.

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    How many amps have a fused screen supply? I can't recall one right now. Fusing the entire tube set off one fuse gives a problem in identifying if a single tube blew the fuse. If so, which one? I would rather use individual fusible resistors (FF type) for each tube screen as Fender does. It's always easy to determine if a single tube or location is problematic. The peak screen current can sometimes be momentarily higher than the specified maximum in some amps (as Chuck points out) which leads builders to either increase the wattage of screen resistors or use carbon comp screen resistors for their better surge characteristics. Usually these amps run higher screen volttges than I would like. Dropping screen voltage can give prolonged service life for tubes without a huge impact on output.

    Fusible resistors are quite useful. Marshall uses FF1R resistors between cathode and ground in some amps, as well as overall fuse protection for the HT circuit. A benefit is a 4-tube amp will keep running in the event of a single tube failure. The downside is the same.....

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    Lifetime Member Enzo's Avatar
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    I don't recall ever seeing a fused screen supply except in cases where they were separate supplies, such as a 700v plate supply and a 300v screen sup-ply. generally screens are just a node on the B+ string.

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    Quote Originally Posted by Chuck H View Post
    My only consideration might be the tendency for overdissipation to be balanced with periods of much lower dissipation WRT clipping circuits. Which tubes handle rather gracefully. We see this a lot in guitar amps. Since fuses in these locations tend to be regular fast blow type I might be concerned about popping fuses whenever the amp is clipping.?.
    I don't think the balancing of overdissipation works for screens as they have much lower thermal inertia than plates and heat up very fast.

    My proposal above already considers some abuse:

    - The individual screen resistor reduce actual screen voltage and dissipation at screen current peaks.
    - Peak screen currents are supplied from the large filter cap and the fuse only "sees" total average current.
    - I was going to propose a slow blow fuse, because a fast fuse might trigger by the the turn-on inrush current of the filter cap. A SB fuse opens after 2 minutes of 2.1 times rated current

    This said, the question remains, what a common screen fuse is actually meant to protect. It is not able to protect individual screens, but may provide some protection for the PT screen winding.

    I did this exercise mainly to demonstrate that a 6A screen supply fuse is ridiculous and to get a feeling what realistic (averaged) max screen currents could be expected.

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    Last edited by Helmholtz; 01-25-2020 at 04:48 PM.
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    So the ground disconnect was verifiable, I removed the ground fuse and hardwired it to ground. I guess before I was asking if a disconnected ground reference would account for all the previous failures- I’m fairly certain 8 kt88’s with no ground reference would experience some ‘slight’ issues .

    Although it seems that a ‘global’ screen fuse is not the right way to go, I’m probably going to leave it in rated slightly above Heimholtz’s great recommendation, if not only for protection against direct shorts from the way-too-close-together tube rails. After all it’s already installed...

    So if you can calculate screen fusing using screen voltage and screen dissipation, you could so the same for a Hv fuse yes? 8 kt88’s at 40w a piece, (320) divided by he plate current of 500? .64 amps at maximum dissipation?... corrections appreciated

    And lastly, I have some 270 ohm 10w on hand, this should work for dummy loads, yes? At least work better than the 4.7ks?

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    So if you can calculate screen fusing using screen voltage and screen dissipation, you could so the same for a Hv fuse yes? 8 kt88’s at 40w a piece, (320) divided by he plate current of 500? .64 amps at maximum dissipation?... corrections appreciated
    Plates are much more rugged than screens and can take (and probably see) more abuse. So although tube data suggests something like a 1A HV fuse, I would go with a 3A fuse HV fuse as used in similar high power amps. Also as said before a supply fuse will not protect single tubes but rather the PT.

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    Thanks! 3A seems reasonable and I will go with that. Is there an accompanying explanation of WHY they see abuse above the chart ratings?

    Also, can you give any insight about the 270 ohm dummy loads, or point me to a reference on how to calculate and select dummy load protection?

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    Bent Member Chuck H's Avatar
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    I think Helmholtz sort of covered this responding to me earlier. The screens, both ideally and in their circuits, don't see the same variation in current that the plates do. So the concession I was suggesting for the screen fuse rating, while not ideally applicable for screens, is applicable for plates. That is, you want to bump the rating a little for those peaks of higher dissipation.

    And 270 ohms should be fine. But I believe your 4.7k resistor safety loads in there now are wired in parallel, right? So it's actually a 2.35k safety load. Still too high, but my point is that if you were to wire the 270 ohm resistors the same way the you'll have a 135 ohm safety load. Which seems a bit low and might noticeably affect performance. If you just used one 270 ohm at 10W I think the rating would be a little low as that resistor will probably dissipate a little more than that intermittently if no speaker is used. That could open the resistor and render the safety load moot. So, to wire two resistors like it is now you would replace the 4.7k resistors with 470/10W resistors or a single 220 ohm 20W resistor.

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    Last edited by Chuck H; 01-25-2020 at 09:09 PM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Aha! So it’s a matter of ‘peaks’ through normal
    operation? The screens will remain more constant while the draw of the plates will vary with a wider degree? This is honestly beyond my understanding but this project has been illuminating.

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    Is there an accompanying explanation of WHY they see abuse above the chart ratings?
    Tube data is always for linear unclipped operation and specified optimum load. Clipping and deviation from optimum load can strongly increase tube dissipation.


    Also, can you give any insight about the 270 ohm dummy loads, or point me to a reference on how to calculate and select dummy load protection?
    I think what you call dummy loads are base loads that protect the OT from arcing over in case of load-drop (open output). AFAIK, values of 470R and lower work ok.
    Too low values dissipate and steal a lot of power. I don't know of a way to calculate an optimum resistance.
    Such base loads will not prevent melting of tube screens if the amp is operated without speakers connected.

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    This was very helpful. Thank you all for your knowledgeable replies. Chuck, good catch! With no jack plugged in, it would be in parallel, however with the jack’s shorting lug lifted, it only engages one, so I think 470r would be a great fit because as y’al mentioned, 220-470 is the acceptable range.

    EDIT: at no time are these resistors paralleled, with zero, one, or two jacks plugged in the resistance is always 4.7k.


    I’ll update you if anything else happens, but I feel much more confident that this will run smoothly and have better protection against failure and misuse in the future. Thank you all for your help, as always.

    Long live the forum.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    With no jack plugged in, it would be in parallel, however with the jack’s shorting lug lifted, it only engages one,..
    I'm trying to logic this out and coming up short. I think the idea must be to prevent loading when a speaker is in use. But this fails to provide equal protection should a speaker system fail. Like an open cord/jack, blown speaker, etc. Which seems just as likely as anything else. I would absolutely wire it up for a single 220 ohm 20W resistor parallel to the output at all times. The amp will suffer a loss of about 7W peak performance. Your guy would never notice any difference between 200W and 193W. I promise.

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    Why 200W?
    With 8 (!) KT88s and 500V plate voltage, 400V screen voltage this amp should deliver in excess of 400W (provided that PT and OT support that power).

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    The largest matched assembly of 10watters I have on hand is 2x350ohm, for a combined 175 ohm 20watt. I also have a 2.7k/270r pair of 10 Watts I could use, for a total of 245r, 20w. Would his be safe considering the imbalanced resistance of the pair? That would change the handling of each individual resistor, correct?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Why 200W?
    With 8 (!) KT88s and 500V plate voltage, 400V screen voltage this amp should deliver in excess of 400W (provided that PT and OT support that power).
    Right. Thank you. The guy still won't notice

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    The largest matched assembly of 10watters I have on hand is 2x350ohm, for a combined 175 ohm 20watt. I also have a 2.7k/270r pair of 10 Watts I could use, for a total of 245r, 20w. Would his be safe considering the imbalanced resistance of the pair? That would change the handling of each individual resistor, correct?
    Well since Helmy was good enough to illuminate the wattage issue (not sure why I got 200W stuck in my head) you're going to need a higher rating on the resistance. 220 ohms will draw about 16W and 470 about 8W. So if you land in between a 20W resistor is probably ok. I'd go 25W or a pair of 20W in parallel.

    So what do you have for 20W? 25W? You can stack rating by putting resistors in series too. That is, two 150ohm 10W resistors in series is 300ohms at 20W. I prefer not to use series arrangements for this circuit if only because it's just another possible weak link in a safety chain. That said, I would do it if it's what I happened to have on hand. So using lower values in series could be an option for your inventory too.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Mr_bibbles View Post
    The largest matched assembly of 10watters I have on hand is 2x350ohm, for a combined 175 ohm 20watt. I also have a 2.7k/270r pair of 10 Watts I could use, for a total of 245r, 20w. Would his be safe considering the imbalanced resistance of the pair? That would change the handling of each individual resistor, correct?
    A 400W amp produces around 57 Vrms across an 8 Ohm load before clipping (clipping increases RMS output voltage). Square the voltage and divide by load resistance to find resistor dissipation.

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    Unfortunately the hugest I have is 10w. 270, 330, and 350 at the only low ohmage ones. This leaves me with very little options, so perhaps I should just mouser the correct parts.

    Also, you can stack wattage handling with a series assembly?!? Would the amperage cross each resistor with its full power in sequence? I trust your expertise but am just surprised.

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    Maximum load resistance for this amp is 8, so (57x57)/8 = 406, increase it a sprinkle to account more a maximum clipped load and somewhere around 425-470 is appropriate?

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    Quote Originally Posted by Mr_bibbles View Post
    Maximum load resistance for this amp is 8, so (57x57)/8 = 406, increase it a sprinkle to account more a maximum clipped load and somewhere around 425-470 is appropriate?
    Clipped output power and voltage are hard to predict. It strongly depends on power supply sag. Marshall states 170W clipped power for their 100W amps.

    My advice above was meant for calculating the dissipation of the base load resistors.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    Also, you can stack wattage handling with a series assembly?!? Would the amperage cross each resistor with its full power in sequence? I trust your expertise but am just surprised.
    I'm unclear by the way you worded this I think. I'll say it like this. In a series arrangement the resistors share the current relative to the voltage across their resistance. So it's like I said above about the 150ohm 10W resistors. The two in series would be good for 300 ohms at 20W. If the resistances are different then the load would be shared unequally with the lower value resistor carrying more of the current. In direct proportion actually. So if a 90ohm resistor is in series with a 10ohm resistor the 90ohm resistor will carry 10% of the current and the 10ohm resistor the other 90%.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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