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Thread: So hereís a fun one (AD-100 clone woes)

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    So hereís a fun one (AD-100 clone woes)

    So, there is a customer of mine with a boo-teek ad-100 copy. It has never really worked properly.

    When he received it, blew fuses, blew tubes, it had been through 3 shops before it arrived here. The builder has assured me it is Ďnot a design issueí, and has sent two different transformers to my shop for install, and has now washed his hands of it. (He also seemed upset the customer was placing the amplifier on top of his cabinet...) The physical construction was solid, and his company seems to do well. This was his first amp with 8 kt88ís though.

    Iím seeking ideas to make this functional for this poor customer. My only real thought it to run 4 kt88ís instead of 8, or to build a second supply rail for the screens as Doug from eurotubes suggested after only a minute or two or discussion about this.

    History:

    The first time in the shop, was brought in with a new transformer, the builder had mailed it and said to install, no other real information so I did a careful job and sent it out.

    Second time, came back shortly after with a huge carbon arc spot between plate and heater on the TOP (tube side) of the socket. The tubeís 1k screen was heat scorched, the physical metal rails that connected the sockets together had melted their clear heat shrink, and basically it needed major surgery. Cleaned and replaced everything, mounted discreet wires instead of rails...

    To fix this, designer sent me another transformer, one with lower plate voltage, that was closer to the screens..

    Two tubes had blown out, so I advised the customer to run with 6 for the time
    being until we could prove it was stable. Now that the plate was closer to the screen and with 2 less kt88ís worth of draw, we should have an easier time right?

    Amp worked for a month before he had crackling issues, but no outright failures. Brought it back, and it seemed like the screens have all taken an intense amount of heat damage for the short period they were out. I decided to up the value to 1.5k and change the 2w Metal Oxides to 5w cements. Customer really wanted 8 tubes worth of juice, so installed a new set of jjís and biased them as conservatively as I could.

    Everything tested fine In the shop, sounded good, thermally tested good for 8 hours without issue.

    Went back to customers space, blew within ďfirst minute of playtimeĒ. Everything looks %100, except the first KT88 in line with the HV OT leads went white-topped, and had a small heat mark on the socket, Tube-side, around pin 3.

    SO

    What do I do? What can I do to help this poor guy get a working amp? I canít attach the actual schematic, but I can answer any questions you have. Really would like to help this guy, probably not charging him as at this point I really feel for the guy.

    Some basic info:

    8 jj kt88ís
    I.5k, screens 2.2k grids.
    Pt500 heyboer
    250w OT heyboer?
    No tube voltage:
    Plate: 496
    Screen: 396
    Grid: -47

    Voltages measure great in sockets 1-8.

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    Bent Member Chuck H's Avatar
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    What is an AD-100? And if it has 8 kt88's why don't they call it an AD-200?

    How is the customer using it? Are they really hitting the grids hard? What sort of speakers are plugged in and have there been any reports of sound cutting out when in use?

    The last amp I built with big bottles over 450Vp blew through two pair before I landed a pair that would survive. I don't think they make 'em like they use to.

    Have you clipped your meter on the grid pins to read bias and slapped the amp around a bit to see if it might be intermittent with vibration?

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    Last edited by Chuck H; 01-11-2020 at 11:54 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

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

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    Chuck, thanks for your reply!

    Maybe Iím misremembering, itís possible the designer said Ad200! I know thatís not incredibly helpful...

    Customer uses two bass distortion pedals , seems like he is cranking the volume/pre a bit, but not like an insane person would. This is the third set of KT88ís having issues, these are JJs rated well above these plate voltages. Iíll test for vibration but that seems unlikely if it was running for a month at one point, right?

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    Bent Member Chuck H's Avatar
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    Have you noticed any pattern regarding WHICH tube/s are failing? If, for example, it's the same two all the time that could be part of a diagnosis. And though the JJ's may be rated for higher voltages, in my experience it doesn't mean they'll actually take it. There's also the matter of screen voltage and screen circuit impedance. I've had plenty of trouble with new tubes and I incinerated a pair of JJ's in the above mentioned amp. There are some things that can be done but it's better to know the cause before affecting a solution since unnecessary repairs can complicate diagnostics. Do you have the amp on the bench now?

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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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    Lifetime Member Enzo's Avatar
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    And what is the nature/mode of the failure?

    I maintain it isn't voltage that kills the tubes anyway.

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    Education is what you're left with after you have forgotten what you have learned.

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    Enzo,

    Both times (three including the failure before I got it) itís blown the main fuse, but once it arced from heater to plate on the first tube fed from the filter caps, and once in a separate socket it white-topped a tube and left a burn mark on the tube side of pin 3 plate. This tube was tied directly to the OT lead.

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    Last edited by Mr_bibbles; 01-12-2020 at 03:24 AM.

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    Chuck! See my response to Enzo about the different failures. Not the same each time, the only difference between operation was the heateró> plate arc happened with 6 tubes and 1k screens, the white-topper happened with 8 tubes and 1.5k screens.

    I want to prove a single failure mode for sure, and with the knowledge that this amp has NEVER worked correctly, even when brand new, I want to examine the working parameters as if they may be faulty. This is a bit above my pay grade, but itís on the bench (when Iím back Tuesday) and Iím happy to include any measurements and information!

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    Don't forget the joker g1's Avatar
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    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?

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Bent Member Chuck H'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?
    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    With failures to both the main fuse AND the tubes themselves I'll wager there's shenanigans in this amps daily operations. Maybe an MOV and some protection diodes on the OT primary??? Your customer is probably abusing this amp. Both in being cavalier about plugging everything in properly, possibly the AC wiring where he plays and maybe with how hard he's driving it relative to it's intended function (read that too much grid drive in a wide open circuit will exploit a low G2 circuit impedance every time!).

    I think you and your client could benefit from such things as an MOV on the AC primary, protection diodes across the OT, a 220 ohm dummy load resistor across the output (A big one, obviously. Maybe chassis mount) and a significant increase in screen circuit resistance. Gotta keep the tubes alive!!!

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    Last edited by Chuck H; 01-13-2020 at 04:43 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

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

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    A white-topped tube is perhaps an unrelated failure to the others - it's caused by a vacuum leak will cause the tube to break down and short. I had a problem KT88 amp in a while back. It all measured OK on the bench even when driven very hard. So, the first time a tube had shorted (JJ). Screen resistors were 470R so maybe the amp was designed originally to take 6L6. I changed these to 1K and fixed some issues with wiring dress. It went back out and after a few weeks the owner reported it was glowing brightly and hot so had to turn it off. All 4 tubes had the logo bleached white - a sure sign of overheating. I reworked the bias circuit to fail-safe, replaced the scorched screen resistors and fitted a new set of tubes. Tested it brutally. It went out and came back a few weeks later with the same fault.

    Had it on the bench again - no fault. Two hours at maximum output (constant sine input) into a dummy load until I thought it would self-destruct. So my final action was to install a large 220R resistor across the speaker output wiring and the owner supplied me with another set of tubes - this time Genalex rather than his usual JJ.

    This is now coming up to 14 months of hard gigging by a full-time pro with no further problems. Another change is the amp is now running off a power conditioner and I supplied the speaker cable. B+ is 580v, screen voltage 320v

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    So my final action was to install a large 220R resistor across the speaker output wiring and...
    OOOooh! Good one that I missed. I edited and added it to my post above

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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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    Hey G1, just to be sure I had him bring the cab in and it measured ok and was the proper impedance. He uses the cab with another head, no issues.

    No PT has failed in this amp, I believe it was he designers attempt to alter voltages. This first time bumped up the heater voltages I believe, the second time lowered the B+.

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    Thanks for getting back Chuck! Let me address hear one at a time:

    MOV: the amp failed at a show one f the times it went down, so while I agree theyíre a good idea to install, Iím not convinced the AC is causing these issues. Same story with the protection diodes!

    As for the dummy resistor, there are 2x 10W 4.7k cements hooked
    From ground ó> speaker terminal positive lug.

    Considering all the different failures, in different environments, is there a way to look into the screen supply/draw to find an inherent fault?

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    Bent Member Chuck H's Avatar
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    You need to know what the screen circuit is. Is it choke fed after the first HV node? What is the inductance (so that the impedances can be calculated)? Is it resistor fed? What value? Is it some combination? of these? Are individual screen grid resistors in place on every socket? What value?

    What I've found is that if you're really hammering the grids then if the screen circuit impedance is low, peaks and spikes try to find a way through the screens instead of the plates, which are almost always at a higher impedance than the screens. At least that's the perception as I've seen it (corrections accepted). Increasing screen circuit impedance (and sometimes that can just mean resistance) seems to help greatly.

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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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    Mick! Thank you.

    My initial thought after the designer ďwashed his handsĒ of the amp was to increase the value of the screens from 1k to 1.5k to cool it down. Right before I did, it was running but you could tell every screen resistor in there was getting hit with a lot of heat. Unfortunately, right after this is when the tube popped.

    As I said above, the amplifier has 4.7k 10w resistors from ground to the speaker +. Since you suggests 220, is it possible these dummy loads are so large that theyíre causing issues??

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    Chuck I really appreciate your sage advice, let me try and relay that Info as best I can:

    1. Thereís no choke that I can see.
    2. The screens are fed B+ from 2x paralleled 100uf caps. Two leads connect to pin 6 from each half of the power section, and an additional lead
    connects two separate tubes together at pin 6 (in case of failure?)
    3. Each of the 8 screen resistors connected from pin 6 ó> 4 is a 1.5k 5w cement.
    4. The transformer leads feeding the screen supply connect to the center of a 1n4007 bridge, one half leading to the standby, the other half leading to to secondary fuse and then the aforementioned filters.
    5. Screen Voltages:
    A. At transformer leads: 194/189VDC
    B. After bridge (filter caps/screen resistor pin 6): 400vdc
    C. Pin 4 after screen resistor- 400vdc

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    Don't forget the joker g1's Avatar
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    200uF at the screen node is really stiff. I'd drop that to the point where he complains about it.
    I've seen screen nodes as low as 10uF to basically function as screen current limiting.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    As I said above, the amplifier has 4.7k 10w resistors from ground to the speaker +. Since you suggests 220, is it possible these dummy loads are so large that they’re causing issues??
    I think that a base load of 4.7k is too high resistance to provide any protection. 220R will at least protect the OT in case of load-drop. But not the tube screens. Even doubling nominal speaker impedance typically causes screen dissipation to exceed its limiit with large signals.


    ...and I supplied the speaker cable.
    To me this is maybe the most important statement/hidden advice in Mick Bailey's post.

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    Last edited by Helmholtz; 01-14-2020 at 10:51 PM.
    - Own Opinions Only -

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    Word! If I were to disconnect one of the parallel caps, would that drop the current to a more acceptable point? Is there any reading youíd recommend to help me understand the capís effect on a screen node?

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    If I were to disconnect one of the parallel caps, would that drop the current to a more acceptable point?
    Most probably not. Can you post a drawing of the complete screen supply?

    With a screen supply voltage of around 400V and 1.5k screen resistors I don't think that screen dissipation is an issue with nominal plate load.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Most probably not. Can you post a drawing of the complete screen supply?

    With a screen supply voltage of around 400V and 1.5k screen resistors I don't think that screen dissipation is an issue with nominal plate load.
    But what is the plate load? And are we sure this amp isn't sometimes plugged into too high an impedance? If there are two jacks on the back of the amp for speaker cabinets, how are they wired? Are they being used or if two cabinets are plugged in is the player using in/out hacks on the speakers themselves? How are those wired?

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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 Chuck H View Post
    But what is the plate load? And are we sure this amp isn't sometimes plugged into too high an impedance? If there are two jacks on the back of the amp for speaker cabinets, how are they wired? Are they being used or if two cabinets are plugged in is the player using in/out hacks on the speakers themselves? How are those wired?
    All valid questions. But I am not the person who could give answers.
    I think I gave the relevant hints, though.
    Too high an output impedance is risky for the screens in most amps.

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    Bent Member Chuck H's Avatar
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    Oh I know. I wasn't asking you. I was just using your post as a segue for the questions. Usually I cut and paste the quote to a general response but I guess I got lazy.

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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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    Don't forget the joker g1's Avatar
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    A couple sets of schematics here, are either close to what you have?

    AD200B MKIII (only one pair power tubes shown) https://music-electronics-forum.com/...l=1#post541579

    AD200 https://music-electronics-forum.com/...l=1#post541731

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    One thing to look out for with KT88 tubes is how pin 1 is connected. With many tubes this connects to the shell - even if the spec sheet doesn't show it or refers to it as NC. It's best to measure and check to find out. Many custom builders will then use pin 1 as a tie point for the grid stopper. So, now all the tube shells are at bias potential and there's a metal ring hanging off each grid. I don't have any evidence to support this, but my conjecture is that this can cause stability issues. I've had mysterious faults where removing the grid circuit off the shells has resolved problems. My fix is to install new grid stoppers without tying them to pin 1 and then connect this pin to ground.

    Another issue with having the shells connected to the grids is that anything that shorts any one of the shells to ground immediately kills the bias to all tubes. A spring retainer could make contact with the shell, for example. Ok, it kills the drive as well but the tubes will very quickly red plate.

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    Thank you all for your continued interest! Tomorrow I will sketch the screen supply, and give more detail
    About the plate circuit.

    G1:

    For now I can say itís closer to AD200 (4 tubes). But the grids are 2.2kís and the screen filters are 200uf rather than the 110 total of two 220ís in series.

    Chuck:

    The 2 shorting jacks are paralleled to the OT selector with 2 additional speakon connectors. The 1/4 have a 4.7k 10w to ground from the shorting terminal, not he positive as I previously stated.

    Mick:

    Pins 1 and 8 are tied together on each tube with 1 ohm 2w MO to ground, with a 2afuse in between.

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    Apologies for my lack of artistic vision:
    Click image for larger version. 

Name:	F80CD084-9490-49EF-BFC7-7C89D35D9E06.jpeg 
Views:	16 
Size:	1.84 MB 
ID:	56681

    This is the screen supply- note that the 6a slo blow fuse is usually Ok, itís the mains that blow.

    I only drew the first set of tubes but theyíre tied pin to pin in a line, with the screen filters hooked directly to 1.5kís.

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    Bent Member Chuck H's Avatar
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    While I think g1 is right about the screen filter, I have seen high value screens before in bass amps. Granted, they weren't typically overdriven, but I still have to go with the possibilities of:

    1) An intermittent fault in the circuit causing a loss of bias or load or a condition of shorted load

    2) Wrong load, no load or a shorted load on occasion due to user error or a bad speaker cable

    3) The preamp design can deliver too much signal to the power tube grids and the owner is hitting the grids too hard with signal

    Or some combination of the above. I think installation of 220ohm safety resistors is a good start. Also, clip your meter to each grid pin and smack the amp to see if bias is intermittent. Monitor bias at each grid for a time while the amp is running just to see if anything weird happens. Test the output jack while smacking the amp too. This time with the amp off while monitoring a cable plugged into the output jack. Any changes in resistance?

    NOTE: Checking bias at the bias supply is not sufficient. You need to know what's happening at each and every tube socket under simulated playing conditions. That is, amp is on for a time and there are vibrations.

    Get as much info as possible from the customer about the failure. How does the amp behave before a failure? Crackling? Cutting in and out" Gets quiet? Louder? More distorted? etc.

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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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    Chuck, I will do all of those vibration tests today. One question, could a loss of bias cause a heater to plate short though?
    Here’s a picture of the speaker jacks, are these already installed dummy load resistors you’re suggesting? Click image for larger version. 

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    2. The PI caps are .1, I could try dropping hem to .047 if I don’t find a vibration fault?

    3. What changes would you recommend to the filters? Just disconnect one of the 100’s?

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    Last edited by Mr_bibbles; 01-18-2020 at 08:49 PM.

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    note that the 6a slo blow fuse is usually Ok, it’s the mains that blow.
    I think the 6A HT fuse is rated too high. A Marshall Model 2001, 375W, 8x6550 bass amp has a 3A HT fuse. Typically the HT fuse should blow before the mains fuse. Otherwise the HT fuse doesn't provide additional protection.


    A. At transformer leads: 194/189VDC
    Do you mean AC?


    BTW, 1N 4007s are only rated at 1A continuous average current. Not sufficient for outputs >200W.

    Did you clean and retension power tube socket contacts?

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    Also, where does the OT get it's supply from, the same node as the screens? With no common dropper resistor between the OT and screen feeds?
    As you mentioned, disconnecting one of the 100uF's would be the quickest method of dropping that node capacitance.

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    As you mentioned, disconnecting one of the 100uF's would be the quickest method of dropping that node capacitance.
    Yes, but I don't think less capacitance will noticeably increase sag and relieve the screens. KT88s should be safe with a stiff screen supply voltage of 400V and individual screen resistors >1.5k.

    This said, a reservoir cap of 100Ķ should be completely sufficient.

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    Quote Originally Posted by Mr_bibbles View Post
    could a loss of bias cause a heater to plate short though?
    For my small experience a plate to heater short is usually caused by a bad tube. But it's hard to say what will happen in a tube if it's stressed and overheated. A short in a power tube is usually an arc. That requires a high differential in voltage. So I would imagine a short from plate to anywhere else requires a voltage spike. This can happen under a few different conditions, but a load that's too high or an open load would seem the most likely scenario.


    Quote Originally Posted by Mr_bibbles View Post
    Here’s a picture of the speaker jacks, are these already installed dummy load resistors you’re suggesting?
    Those would be "dummy load" resistors... If they were 220 ohms! At 4.7k they won't drop enough current across their load to affect the voltage through the OT and on the plates significantly at all. My personal amp (I have several, but the one I play through most), for example, is 20W. It has a "line out" circuit that includes a 2.7k 1/2W resistor and a 500 ohm pot rated about the same I'll guess. There has been more than one occasion when testing and other bench shenanigans that I've run the amp into an open load WITH signal at the input and the amp cranked. Fortunate for me I used a burly OT and there was no damage, BUT... The only load on the amp would have been the line out circuit at something like 3k total and rated for less than 1W with both components in series (considering the pot was probably at five?). That circuit did not roast. Which it should have were it dropping more than a couple of watts of the amps total power across it's relatively high resistance components. Those resistors are doing basically nothing. You should change them out for the suggested 220 ohm value.

    Quote Originally Posted by Mr_bibbles View Post
    The PI caps are .1, I could try dropping hem to .047 if I don’t find a vibration fault?
    That depends on a couple of things. For one, with eight kt88 tubes the grid load resistance is probably quite small. A small resistance here would require a larger cap value for an acceptable knee frequency. But even if your grid loads are something like 100k a .047u coupling cap value would have a -3dB of something like 25(ish)Hz. So your customer isn't likely to hear a difference or notice at all. Unless he's trying to communicate with submarines . A reduction in extreme LF can't hurt since it's just useless current affecting the power tubes.

    Quote Originally Posted by Mr_bibbles View Post
    What changes would you recommend to the filters? Just disconnect one of the 100’s?
    g1 already covered this, but I'll back him here in the interest of completing my response. Disconnecting one cap should be fine.

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    Helmholtz,

    Thanks for your reply!

    I’ve meticulously cleaned the sockets and replaced the one that arced, it had melted a bit.
    The sockets are almost new do not look like they need retensioning. Each socket showed healthy current draw with a bias probe last time it was on the bench.

    I actually do mean DC for the transformer leads, they’re tied to the rectifier. That measurement is with standby off.

    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?

    Finally, Good thinking on the fuse i’ll drop it down.

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    Last edited by Mr_bibbles; 01-18-2020 at 09:57 PM.

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    The OT is supplied by 2x 250m cap cans in series, tied to 8 5408ís (is this a dual rectifier??)
    Pics:
    Click image for larger version. 

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    Click image for larger version. 

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    The red leads from the cap go to B+ of the OT, and the 5408ís as seen in the pic.

    The black lead from the diodes is tied to ground via standby switch, and the two red leads in the center are HV secondary from transformer

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