Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SleepLess on September 05, 2014, 09:30:16 am
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Hi guys.
I'm coming for a little help before I fire this pup up. I couldn't exactly follow the Harvard layout you can find everywhere because I have an OFF/Stdby/ON switch instead of the simpler OFF/ON switch. Here's the PT schematic:
(http://img15.hostingpics.net/pics/906514Scan0001.jpg)
And three pictures to show you what I did. I hope you can clearly see. It's wired for 230V/240V operation. I added two 100Ω resistors on the 6V6 sockets (between pins 2 and 8 on V4 and between pins 7 and 8 on V3) because the PT didn't have any HT CT (perhaps I was wrong there...).
What I'm really hesitating with is the fuse holder and switch connections. So far the neutral power cord wire is left unsoldered, I don't know what to do with that one!
Thanks for your help!
(http://img15.hostingpics.net/pics/877811DSC36924512x3012.jpg)
(http://img15.hostingpics.net/pics/876606DSC36934512x3012.jpg)
(http://img15.hostingpics.net/pics/293711DSC36944512x3012.jpg)
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It looks like you have the White wire from the PT primary hooked incorrectly to the bias supply :huh:
That wire should be one of your PT input connections, and partially explains why you have an unsoldered white (blue?) wire from your power cord...
Your switching scheme is difficult to see, and I'm thinking a diagram of your intentions would be helpful to anyone else who's having a look
Where do you have the red/blue bias tap wire going?
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Link for the wiring of your On/Off/Standby switch.
http://www.el34world.com/charts/SwitchDPST.htm (http://www.el34world.com/charts/SwitchDPST.htm)
And link for the thread started by Sluckey on it.
http://el34world.com/Forum/index.php?topic=17125.0 (http://el34world.com/Forum/index.php?topic=17125.0)
Brad :icon_biggrin:
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Your power cord safety ground solder connection to the chassis looks ify to me.
Takes a BIG iron to get that soldered to the chassis, at least 90+w, 100+w even better, with a BIG tip.
That's a very important connection and it is most times done (now a days and for good reasons) either with a solder lug that is attached/bolted to 1 of the PT mounting bolts or is given it's own bolt in the chassis.
I would not trust it as it is now.
Brad :icon_biggrin:
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It looks like you have the White wire from the PT primary hooked incorrectly to the bias supply :huh:
Yes, the white PT wire is going there. Why, where should it go? Crap, I'm lost on this amp!
That wire should be one of your PT input connections, and partially explains why you have an unsoldered white (blue?) wire from your power cord...
Your switching scheme is difficult to see, and I'm thinking a diagram of your intentions would be helpful to anyone else who's having a look
Where do you have the red/blue bias tap wire going?
The bias tap wire is going nowhere, it's heat shrunk and set aside. Why? Should I use it?
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Your power cord safety ground solder connection to the chassis looks ify to me.
Takes a BIG iron to get that soldered to the chassis, at least 90+w, 100+w even better, with a BIG tip.
That's a very important connection and it is most times done (now a days and for good reasons) either with a solder lug that is attached/bolted to 1 of the PT mounting bolts or is given it's own bolt in the chassis.
I would not trust it as it is now.
Brad :icon_biggrin:
OK Brad. I used a big tip but only a 30w soldering iron. Will do.
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1 more thing, it looks like your just poking the wire through the solder lug hole on tube sockets, switches, etc, and soldering them in place.
That can have problems later on, the wire should be 'wrapped' around the solder lug (for a mechanical connection) before soldering the wire in place. Solder is not glue.
Brad :icon_biggrin:
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The red/blue wire is your correct bias supply feed and needs to go where you mistakenly put the white wire.
The blue wire from your power cord should connect to that white wire that feeds the primary of the PT.
There could be more errors that I haven't seen, but we needed to get that over with first. :wink:
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The red/blue wire is your correct bias supply feed and needs to go where you mistakenly put the white wire.
The blue wire from your power cord should connect to that white wire that feeds the primary of the PT.
There could be more errors that I haven't seen, but we needed to get that over with first. :wink:
OK, thanks a lot! Oops... After more than 30 builds I sometimes think I'm still a damn noob... :sad2:
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1 more thing, it looks like your just poking the wire through the solder lug hole on tube sockets, switches, etc, and soldering them in place.
That can have problems later on, the wire should be 'wrapped' around the solder lug (for a mechanical connection) before soldering the wire in place. Solder is not glue.
Brad :icon_biggrin:
Got it Brad! Will do so on my later builds!
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OK Brad. I used a big tip but only a 30w soldering iron.
When I say BIG solder iron tip, I mean the size of your thumb in length and diameter. :laugh:
Here's a high lighted drawing of the R/B bias wire.
Brad :icon_biggrin:
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OK, thanks a lot! Oops... After more than 30 builds I sometimes think I'm still a damn noob... :sad2:
EVERYONE makes mistakes....luckily that was an obvious one...
It's the unobvious ones that make my hair gray....and make me WEAR SAFETY GLASSES when I fire up a new amp.
:icon_biggrin:
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Here's the PT primary color code from the Merc Mag spec sheet you posted.
Brad :icon_biggrin:
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I'm assuming Sleepless is overseas because he mentioned he has it connected for 230/240 VAC input...
Here's another way to see it:
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Checked his profile and it appears he's somewhere between Germany and Spain.....
Remember Sleepless "À mauvais ouvrier point de bon outil"
:undecided:
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Yes guys, I'm in France. 230V here... 227V at my wall.
Thanks, will keep you posted!
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Yes guys, I'm in France. 230V here... 227V at my wall.
Doh! :w2: :l2:
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Today a good friend of mine gave me his 1957 Harvard 5F10 amp. I had restored it for him back in 2002. That's what got me restarted in this hobby. I've got a few pics from the rebuild on my website if you want to see what the original looked like.
http://home.comcast.net/~seluckey/amps/harvard/harvard.htm (http://home.comcast.net/~seluckey/amps/harvard/harvard.htm)
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Good friends like that are hard to find and worth their weight in gold, IMO.
(It's not the $$ value of the amp it's what it means to both of you and he put it back in your hands.)
Brad :icon_biggrin:
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Hi guys.
So, I've redone the PT/power cord wiring as Silvergun suggested. The PT bias tap is going to the board in place of the white wire. The PT white wire is soldered to the power cord neutral wire and both are tied and heat shrunk.
Fired up the amp and took some measures first without any tubes. Filaments all had 3.2V. I then place the 6AT6 and 12AX7 and the rectifier. Everything looked good too, had -33v at pins 5 of the 6V6GTs. Then installed the 6V6GTs and fired up. Had some crackling that ceased after one or two seconds. Took my measures and had 375V on pin 3 of V4 and when I placed the red probe on pin 3 of V3 the multimeter turned off and had some sparks. Turned it back on and measured at the board instead just to check, at the eyelet between the 470Ω and 22kΩ resistors, where a wire going to pins 4 of V3 and V4 is soldered too, along with the 16uF cap. Same thing happened: sparks and the MM turned off. Now it's broken, I opened it up, the fuse is ok but the MM won't turn back on... And the amp doesn't emit any sound. If you need any pics, I can take some. I don't get it, I've checked the layout again and everything looks ok. Do you think a bad 16uF cap could have caused that?
Crap... Need to buy a new MM... Time to upgrade, I had a $20 Velleman MM. :sad2:
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You need to ground the cathode on V3(?) red circle.
Brad :w2:
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You need to ground the cathode on V3(?) red circle.
Brad :w2:
Brad, it is grounded: the black wire you see running above the 6V6 sockets links both pins 8 and 1 and goes from V3 to V4 where it eventually goes to ground.
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Ok, I see it now.
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Did you use the stock Fender layout? Where does the red wire from the OT primary physically connect?
Disconnect the NFB look at the speaker jack until the amp is working. The OT secondary needs to have one wire grounded. How did you do that?
I would reflow all the solder joints on both 6V6 tubes. Since you just poked the wires thru the pin holes it's possible you have a bad (resistive) connection. I highly recommend wrapping the wires and components one turn on those sockets. Just temporarily connect the OT leads to pins 3 though. There's a good chance you will need to reverse those leads.
Since this amp uses a RCA jack for speaker connection, it's important to have a speaker connected. At this point I'm kinda suspecting a NFB phase problem. Disconnecting the NFB wire will tell us for sure. You're gonna need another meter. And we need some more hi rez pics too.
EDIT... Show us the ground connection for the HT center tap. The soldering iron you described above is not capable of producing a reliable solder joint to that steel chassis. I recommend a ring lug bolted to one of the PT bolts using a kep nut or star washer.
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Did you use the stock Fender layout? Where does the red wire from the OT primary physically connect?
Hi Sluckey!
No I didn't use the stock layout, I used an OFF/Stdby/ON switch that needed some changes made. Other than that, yes I pretty much followed the stock alyout, with teh addition of 100Ω resistors for an artificial HT CT. The red OT wire goes to that three-position switch and from there another wire goes to the rectifier, pin 8.
Disconnect the NFB look at the speaker jack until the amp is working. The OT secondary needs to have one wire grounded. How did you do that?
The OT secondary is grounded straight on the OT metal frame. I didn't do that, Mercury Magnetics did.
I would reflow all the solder joints on both 6V6 tubes. Since you just poked the wires thru the pin holes it's possible you have a bad (resistive) connection. I highly recommend wrapping the wires and components one turn on those sockets. Just temporarily connect the OT leads to pins 3 though. There's a good chance you will need to reverse those leads.
Since this amp uses a RCA jack for speaker connection, it's important to have a speaker connected. At this point I'm kinda suspecting a NFB phase problem. Disconnecting the NFB wire will tell us for sure. You're gonna need another meter. And we need some more hi rez pics too.
OK. I'm investigating on what MM to get. I'd like a Fluke but it's too expensive for me. If you guys have any reference in mind, don't hesitate, there are so many models it's crazy. I will purchase one today or tomorrow so I can have it as soon as possible.
More pics coming! Thanks!
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Alright, some more news. I grounded the power cord right with a hole and nut/ground tab.
Fired up the amp and got some smoke coming after less than ten seconds. The 100Ω resistor on V3 was burning, saw the smoke coming straight from it. I turned the amp off, disconnected the NFB wire at the speaker jack, replaced the 100Ω resistor and turned the amp back on again. This time the 100Ω resitor on V4 started burning... Maybe I shouldn't use those resistors, they're not on the Fender layout but...? :dontknow:
As a reminder the 100Ω resitor on V3 is soldered between pins 7 and 8, and the 100Ω resistor on V4 is soldered between pins 2 and 8. I chose that method instead of soldering them at the indicator lamp.
Pics:
(http://img4.hostingpics.net/pics/732974DSC37544512x3012.jpg)
(http://img4.hostingpics.net/pics/606567DSC37584512x3012.jpg)
Thank you!!!
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About the only thing that will burn those resistors is a short (or resistive) circuit between pins 2 and 3 of either or both 6V6 sockets. Pull the 6V6s and examine the sockets for signs of a carbon trace or burn marks between pins 2 and 3 of both sockets. Look at the socket from both sides of the chassis. Really need to measure resistance between pins 2 and 3 also.
If you find evidence of an arc between those pins then you will need to replace the sockets. Your OT may be shot too. This is all related to my questioning about the OT secondary, speaker connection, and NFB. Leave the NFB disconnected until this is resolved. And measure the resistance from the speaker jack to chassis. It's important for the OT secondary to be securely grounded and making good electrical contact with the chassis. The original 5F10 also had one of the secondary leads soldered to the shell of the OT and I'm sure that's why MM did it. IMO, that's just a poor arrangement.
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About the only thing that will burn those resistors is a short (or resistive) circuit between pins 2 and 3 of either or both 6V6 sockets. Pull the 6V6s and examine the sockets for signs of a carbon trace or burn marks between pins 2 and 3 of both sockets. Look at the socket from both sides of the chassis. Really need to measure resistance between pins 2 and 3 also.
No signs of burn on the sockets on both sides. The sockets look brand new. Didn't find any piece of wire that could have ended up between them. I need to buy a new mm to check the resistance. Will do so.
If you find evidence of an arc between those pins then you will need to replace the sockets. Your OT may be shot too. This is all related to my questioning about the OT secondary, speaker connection, and NFB. Leave the NFB disconnected until this is resolved. And measure the resistance from the speaker jack to chassis. It's important for the OT secondary to be securely grounded and making good electrical contact with the chassis. The original 5F10 also had one of the secondary leads soldered to the shell of the OT and I'm sure that's why MM did it. IMO, that's just a poor arrangement.
Thanks Sluckey. I'll come back here when the new MM gets home.
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Both of your OT plate leads are laying on your filament leads. Separate those and keep them separated. Look for an arc puncture in the insulation.
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Both of your OT plate leads are laying on your filament leads. Separate those and keep them separated. Look for an arc puncture in the insulation.
Will do. Thanks Sluckey. I only have one 100 ohms resistor left after having burnt two so I don't want to fire the amp up before I'm sure it won't burn again...
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Show us the ground connection for the HT center tap.
I think I have identified a communication breakdown....
I have heard Sleepless describe the 2- 100 ohm resistors he is using to create a filament winding CT ,,,and calling them the HT CT
EDITED ---looked closer and saw where it was connected
Sorry for the false alarm Sleepless
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Show us the ground connection for the HT center tap.
I think I have identified a communication breakdown....
I have heard Sleepless describe the 2- 100 ohm resistors he is using to create a filament winding CT ,,,and calling them the HT CT
HT CT = heater center tap, am I wrong? So the heaters are the filament winding, right? :think1:
It looks like he has the PT secondary CT attached to his off/stanby/on switch :huh:
Sleepless, where is the red/yellow that comes off of the secondary of your PT connected?
The red/yellow PT wire is going to the first 16uF cap negative side which goes to ground.
Thanks for your help!
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Does it connect to chassis ground with a good solid connection?
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Does it connect to chassis ground with a good solid connection?
Yes it does, There's a ground tab below the pots that hosts it.
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HT CT = heater center tap, am I wrong?
So the heaters are the filament winding, right? :think1:
The term HT is usually reserved for the high voltage winding.
In your case the HT CT is the red/yellow wire.
Yes, the heaters are also called the filaments.
Go back to listening to Sluckey,,,if you follow all of his questions he will lead you to the problem..
I don't have enough time to do this justice today.
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HT CT = heater center tap, am I wrong?
So the heaters are the filament winding, right? :think1:
The term HT is usually reserved for the high voltage winding.
In your case the HT CT is the red/yellow wire.
Sorry for confusing you. I call the heaters center tap the HT CT. And the high voltage center tap (the red/yellow wire) I call the HV CT. It may be a wrong calling though, I don't know.
Yes, the heaters are also called the filaments.
Go back to listening to Sluckey,,,if you follow all of his questions he will lead you to the problem..
I don't have enough time to do this justice today.
No problem Silvergun, thanks for your help! I will buy a new MM tomorrow so I can check resistance and answer Sluckey's questions.
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I only have one 100 ohms resistor left after having burnt two
Just leave them out until this issue is resolved. You can use your voltmeter to check for dc B+ voltages on the heaters rather than keep burning them up. I'm hoping now that you have the NFB wire disconnected They wont burn up again.
BTW, check your speaker and be sure the voice coil is not open.
When talking about guitar amps, HT generally means HIGH TENSION and refers to the HV or B+ secondary winding. HT was probably borrowed from the cross country AC distribution grid which is real high tension. I don't really know, just guessing. It is confusing. I had never heard of the B+ winding being called HT until I started following guitar amp forums. I just accept it even though I think the term is misapplied.
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Hi!
So, I'm back to this debug thread. The new MM has arrived so I can follow your tips now. Just to make sure, the NFB wire is the wire that goes from the board to the speaker jack, right?
Sluckey you mentioned checking resistance between the speaker jack to ground, do I have to take this measure with the amp fired up or leave it on the OFF position? Any other things I need to check?
Thanks!
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the NFB wire is the wire that goes from the board to the speaker jack, right?
That's right. Leave it disconnected until the amp is fixed.
Sluckey you mentioned checking resistance between the speaker jack to ground
Yes. I want to be sure the speaker and OT secondary are not open. Turn the amp off for these resistance checks. Unplug the speaker and check voice coil resistance ( several ohms) across the RCA plug. Then check resistance across the RCA jack on the amp to verify the OT secondary DCR is very low, probably only a couple ohms.
I'm still thinking about the burnt 100Ω filament resistors and I'm suspecting an arc between pins 2 and 3 on one or both of the 6V6 sockets, or some other short from B+ to filament string.
Also check for B+ voltage present on the filament string by measuring DC volts between pin 2 of 6V6 and chassis. Pull all tubes and leave them out until you are sure there is no B+ on the filament string.
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Yes. I want to be sure the speaker and OT secondary are not open. Turn the amp off for these resistance checks. Unplug the speaker and check voice coil resistance ( several ohms) across the RCA plug. Then check resistance across the RCA jack on the amp to verify the OT secondary DCR is very low, probably only a couple ohms.
Across the RCA plug: 6.6Ω (8Ω speaker)
RCA jack: No reading (black probe on chassis RCA jack ground tab and red probe on chassis RCA jack tip, NFB wire disconnected, only the OT secondary wire remains)... :think1:
I'm still thinking about the burnt 100Ω filament resistors and I'm suspecting an arc between pins 2 and 3 on one or both of the 6V6 sockets, or some other short from B+ to filament string.
Also check for B+ voltage present on the filament string by measuring DC volts between pin 2 of 6V6 and chassis. Pull all tubes and leave them out until you are sure there is no B+ on the filament string.
Measured 0.1V on both 6V6 sockets between pins 2 and chassis.
What I don't get is why my previous MM blew up when I touched pin 3 of V3 with the red probe. It was set on the 1000VDC setting. Don't get it.
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Sluckey,
I fired up the amp again with the NFB disconnected and it still burns... Smoke came up... I see aboslutely no signs of burn at pins 2 and 3 of the 6V6s :BangHead:
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RCA jack: No reading (black probe on chassis RCA jack ground tab and red probe on chassis RCA jack tip, NFB wire disconnected, only the OT secondary wire remains)...
I'm assuming when you say 'no reading' you mean infinity, an open circuit, the same indication you get with the probes hanging in the air? If so, that ain't good. That says the secondary of the OT is open. I'll bet it's probably good and the solder connection of the wire attached to the frame of the OT is bad. I highly recommend clipping that wire from the OT frame and soldering it directly to the ground lug on the RCA jack. Then recheck the resistance on the RCA jack.
Measured 0.1V on both 6V6 sockets between pins 2 and chassis.
You did have the power turned on, right?
What I don't get is why my previous MM blew up when I touched pin 3 of V3 with the red probe. It was set on the 1000VDC setting. Don't get it.
My suspicion was wrong phase on the NFB causing the amp to oscillate/squeal loudly. That's why I wanted the NFB disconnected. And since the OT secondary is open this loud squeal caused a high voltage spike to be reflected back to the primary (plates). This is what killed your other meter. This is just a theory and has not been proved yet.
it still burns... Smoke came up...
What is burning? You don't have the 100Ω resistors back on the filament circuit do you?
I'm beginning to suspect another wiring error. Pull the rectifier tube and insure that all tube filaments light up.
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RCA jack: No reading (black probe on chassis RCA jack ground tab and red probe on chassis RCA jack tip, NFB wire disconnected, only the OT secondary wire remains)...
I'm assuming when you say 'no reading' you mean infinity, an open circuit, the same indication you get with the probes hanging in the air? If so, that ain't good. That says the secondary of the OT is open. I'll bet it's probably good and the solder connection of the wire attached to the frame of the OT is bad. I highly recommend clipping that wire from the OT frame and soldering it directly to the ground lug on the RCA jack. Then recheck the resistance on the RCA jack.
Done. Now I get a 1Ω reading. I was getting infinity before that yes.
Measured 0.1V on both 6V6 sockets between pins 2 and chassis.
You did have the power turned on, right?
Yes, all tubes pulled out, ON position, I get about 1V.
What I don't get is why my previous MM blew up when I touched pin 3 of V3 with the red probe. It was set on the 1000VDC setting. Don't get it.
My suspicion was wrong phase on the NFB causing the amp to oscillate/squeal loudly. That's why I wanted the NFB disconnected. And since the OT secondary is open this loud squeal caused a high voltage spike to be reflected back to the primary (plates). This is what killed your other meter. This is just a theory and has not been proved yet.
Problem is I didn't get any squeal at all... Just a slight noise when flipped ON and then smoke after about ten seconds. No noise at all. I know what a wrong phase does and it really squeals loudly, I didn't get that at all.
it still burns... Smoke came up...
What is burning? You don't have the 100Ω resistors back on the filament circuit do you?
I'm beginning to suspect another wiring error. Pull the rectifier tube and insure that all tube filaments light up.
I have 100Ω resistors back in place. I have just checked their resistance, both are 66Ω.
What do you mean with the last test? Install all tubes exept the rectifier, connect to speaker and turn the amp ON to see if all tubes glow?
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I only have one 100 ohms resistor left after having burnt two
Just leave them out until this issue is resolved. You can use your voltmeter to check for dc B+ voltages on the heaters rather than keep burning them up. I'm hoping now that you have the NFB wire disconnected They wont burn up again.
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I will take these 100Ω out. Sorry for misreading... Will check DC voltage again without them then.
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Problem is I didn't get any squeal at all... Just a slight noise when flipped ON and then smoke after about ten seconds. No noise at all. I know what a wrong phase does and it really squeals loudly, I didn't get that at all.
You could not have heard any squeal because the OT secondary was open due to that bad connection to the OT frame. Same as not having a speaker plugged in.
What do you mean with the last test? Install all tubes exept the rectifier, connect to speaker and turn the amp ON to see if all tubes glow?
yes
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OK, done the test. Sluckey all the tubes glow fine with the rectifier out and the 100Ω resistors out too.
Should I swap the OT wires going to either pins 3 or is it still too soon?
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Should I swap the OT wires going to either pins 3 or is it still too soon?
Hold off on that for now. When you finally reconnect the NFB you'll know if you need to swap plate leads.
So, filaments are all glowing. Let's go in small steps since there has been or still is a problem. Next step, pull all tubes and install only the rectifier. Turn it on watching for smoke and be ready to turn it off if any problems. Now measure dc volts on both 6V6s pin 3. Should be a little high since the B+ is unloaded. Now measure dc voltages on both 6V6s pin 2. It should be 0v.
Report back and if everything looks OK, we'll take a big step.
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Should I swap the OT wires going to either pins 3 or is it still too soon?
Hold off on that for now. When you finally reconnect the NFB you'll know if you need to swap plate leads.
So, filaments are all glowing. Let's go in small steps since there has been or still is a problem. Next step, pull all tubes and install only the rectifier. Turn it on watching for smoke and be ready to turn it off if any problems. Now measure dc volts on both 6V6s pin 3. Should be a little high since the B+ is unloaded. Now measure dc voltages on both 6V6s pin 2. It should be 0v.
Report back and if everything looks OK, we'll take a big step.
OK, so didn't get any smoke...
VDC both pins 3: 448V
VDC both pins 2: -1.4V (yes, negative...)
Thanks Sluckey, really!
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OK, big step. Plug in all tubes. Connect speaker. Turn the volume to zero. DO NOT INSTALL 100Ω RESISTORS YET! DO NOT RECONNECT NFB YET!
Turn the power on and be ready to turn off at the sign of any problems. Hopefully you'll have a working amp. Measure dc voltage on both 6V6s pins 3. I'm hoping that voltage will not be so high now. If no apparent problems, try a guitar. Report back.
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OK, big step. Plug in all tubes. Connect speaker. Turn the volume to zero. DO NOT INSTALL 100Ω RESISTORS YET! DO NOT RECONNECT NFB YET!
Turn the power on and be ready to turn off at the sign of any problems. Hopefully you'll have a working amp. Measure dc voltage on both 6V6s pins 3. I'm hoping that voltage will not be so high now. If no apparent problems, try a guitar. Report back.
Done. No smoke.
Both 6V6 pins 3: 375VDC
I tried a guitar, I get the sound and volume of the amp but some crackling noise, I suppose the noise is the consequence of not having the NFB wire connected?
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OK, you're getting there!
I tried a guitar, I get the sound and volume of the amp but some crackling noise, I suppose the noise is the consequence of not having the NFB wire connected?
Probably not related to NFB. How much crackling? Maybe just needs to run a little while.
It's time to reconnect the NFB. Be ready to turn off if you get a squeal. Sometimes phase reversal is not an obvious squeal. I would just use a gator clip lead to temporarily connect. Listen to the guitar sound without the NFB and get an ear reference. Now connect the NFB. If the phase is correct, the guitar sound level should drop slightly and sound more refined. If the phase is reversed, you may get a squeal or the volume level may only increase a little and sound more raspy, or something in between. Swap the OT plate leads if the phase is reversed.
If you still have crackling sounds after 15 minutes, get a chop stick and start tapping tubes, components, and wires.
DON'T CONNECT THE 100Ω RESISTORS YET!
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OK, you're getting there!
I tried a guitar, I get the sound and volume of the amp but some crackling noise, I suppose the noise is the consequence of not having the NFB wire connected?
Probably not related to NFB. How much crackling? Maybe just needs to run a little while.
It's time to reconnect the NFB. Be ready to turn off if you get a squeal. Sometimes phase reversal is not an obvious squeal. I would just use a gator clip lead to temporarily connect. Listen to the guitar sound without the NFB and get an ear reference. Now connect the NFB. If the phase is correct, the guitar sound level should drop slightly and sound more refined. If the phase is reversed, you may get a squeal or the volume level may only increase a little and sound more raspy, or something in between. Swap the OT plate leads if the phase is reversed.
If you still have crackling sounds after 15 minutes, get a chop stick and start tapping tubes, components, and wires.
DON'T CONNECT THE 100Ω RESISTORS YET!
OK, I don't think the phase is wrong. The crackling got away after about one minute. I don't get any crackling even when I turn the volume up and down all the way. The amp sounds good and loud.
Does it mean that adding the 100Ω resistors on this amp was not only useless but dangerous? That I blew up my MM because of those two resistors I decided to add? :w2:
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Does it mean that adding the 100Ω resistors on this amp was not only useless but dangerous? That I blew up my MM because of those two resistors I decided to add?
Those 100Ω resistors are a good thing. I think they burned up due to a problem caused by the bad ground connection on that OT secondary ground wire. Now that the OT secondary is grounded properly it should be OK to install the 100Ω resistors again. The method you used should be fine. I don't think they will burn again. Give it a go and report back.
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Does it mean that adding the 100Ω resistors on this amp was not only useless but dangerous? That I blew up my MM because of those two resistors I decided to add?
Those 100Ω resistors are a good thing. I think they burned up due to a problem caused by the bad ground connection on that OT secondary ground wire. Now that the OT secondary is grounded properly it should be OK to install the 100Ω resistors again. The method you used should be fine. I don't think they will burn again. Give it a go and report back.
This time I had a flash inside V4 and both 100Ω resistors smoked and burnt about 1 second after I switched the amp ON... I think they are both dead. :BangHead:
Both still read 95Ω though...
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Install another set of resistors. The value is not critical. Use anything between 47Ω and 330Ω, if you don't have another pair of 100Ω. Pull both 6V6s and turn the power on. Do the resistors burn now? If not, I suspect you have one or more shorted 6V6s or you have some non standard 6V6 that will not allow you to ground pin 1.
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Install another set of resistors. The value is not critical. Use anything between 47Ω and 330Ω, if you don't have another pair of 100Ω. Pull both 6V6s and turn the power on. Do the resistors burn now? If not, I suspect you have one or more shorted 6V6s or you have some non standard 6V6 that will not allow you to ground pin 1.
I found another pair of 100Ω resistors and soldered them in (the previous ones cracked in two when I grabbed them with my needle pliers, I wonder how they could stiull test good before!). Took the 6V6s out and switched the amp ON. Everything seems to be ok, no smoke, no burning.
The tubes I have are a new set of matched EH 6V6s
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Everything seems to be ok, no smoke, no burning.
The tubes I have are a new set of matched EH 6V6s
Put another set of 6V6s in. Meanwhile, with the EH tubes laying on the bench, check resistance between pins 2 and and all other pins of the EH tubes. You should have resistance between pins 2 and 7 ONLY. All other pins should show infinity ohms to pin 2.
I suspect a damaged tube at this point.
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Everything seems to be ok, no smoke, no burning.
The tubes I have are a new set of matched EH 6V6s
Put another set of 6V6s in. Meanwhile, with the EH tubes laying on the bench, check resistance between pins 2 and and all other pins of the EH tubes. You should have resistance between pins 2 and 7 ONLY. All other pins should show infinity ohms to pin 2.
I suspect a damaged tube at this point.
Hi Sluckey!
One tube has resistance between pins 2 and 3 (about 400Ω), the other one is fine. I will install another set and report back once the amp is fired up.
Thanks!
UPDATE: The amp works perfectly with a new set of power tubes. Played it for about 15 minutes and it sounds great. I have a 355V B+, pins 5 show -34V. I don't know if that's a good bias point, but the amp sounds good. Once again Sluckey (and all the others who helped!) I can only say a million thanks, I wouldn't have been able to pull it through without you. I'm 35 and I hope I can have all the knowledge you have in a few years, or decades... Debugging an amp when it's on your bench is already tricky, but when it's 5000 miles away, it's just plain impressive. THANKS! I have another amp to build now and then I'll be able to get back to my Revibe build! :icon_biggrin:
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Good news! Just to summarize, here's what happened...
The OT secondary was open due to the common wire being attached to the OT frame. That open circuit reflected some high voltage pulses back to the primary plate leads. These high voltages were created when any sound tried to reach the speaker. Could have been just the crackling noise you heard, or could have been a guitar sound. These high voltage pulses arced inside the tube between pins 2 (filament) and 3 (plate) causing that 400Ω resistance between those pins. Now your plate voltage has a 400Ω resistance path between plate and filament AND on to ground thru the 100Ω resistors. The half watt 100Ω resistors were the weak link so they popped.
So, in a way, those 100Ω resistors acted like a fuse and probably saved your OT, and/or other expensive parts. MM attached the OT wire to the frame of the OT because that's how it was on the original. That's just a bad idea IMO. Doing that means you not only have to rely on a good solder connection on the OT frame, but the bolts that attach the OT to the chassis must also make a good electrical connection. You may want to contact MM and discuss your experience with them.
The good news is... Now you have a single 6V6 that's probably good. You did mention Revibe. :wink: Now break the glass on that bad 6V6 and throw it away. Otherwise it will work it's way into another amp at some later time.
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:thumbsup:
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Thanks a lot for the explanation Sluckey and yes, this bad 6V6 has been thrown away! I have built a Hoffman bias checker. Do you think Ic an use it on this Harvard? With a 355V B+ I should aim at a 23mA bias point. If the bias checker shows that the current is lower than that, how can I raise it? I suppose I should change the 6.8K resistor at the far end of the board? Should I raise or lower that value? That's my last question about this amp!
Oh, I actually also have a spare 10KL bias pot at hand... But I don't know if and how I could use it in this build... Hmmm... THANKS!
I can't wait to return to the Revibe build!
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I didn't mess with the 6.8K. I replaced the 56K sitting above the diode with a 50K pot in series with a 24K resistor. That gave plenty of bias voltage swing above and below the stock value. Here's a pic...
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I didn't mess with the 6.8K. I replaced the 56K sitting above the diode with a 50K pot in series with a 24K resistor. That gave plenty of bias voltage swing above and below the stock value. Here's a pic...
Crap, I only have a 10KL bias pot...
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Hi.
I'm having problems with this amp. It lacks bass and when I push it a little it kind of farts. Very ugly distortion. I have replaced all the tubes with know good ones and nothing changed. Do you think the OT could have suffered and could be responsible for this bad behaviou and sound?
I actually don't know where to start as for tests... B+ is 377V which is a bit high compared to the Harvard schematic that says 302V... Pins 5 of the 6V6s read -34V.
Thanks for your help.
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Here are the voltages for my 5F10. The PT is TP25 from Allen Amplification so my voltages are high too.
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Hi again Sluckey.
Well then it means that my B+ is not responsible for these problems. Do you think the OT could be bad? Is there a way to know for sure before I buy another one uselessly?
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I doubt the OT is bad. It will likely either work or not work.
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Thanks HotBlue. I will try and shoot you a video tonight. You'll be able to hear the farting bass notes and ugly distortion when the amp is pushed a bit... I don't know if it's the OT or not but it's definitely not normal.
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How do your voltages compare with Sluckey's schematic?
Farting bass notes usually equals grid-blocking at the output tubes. Are you using any pedals when this happens (especially boost pedals or distortion/fuzz pedals with high output)? Is it only when you're playing very loud?
Usual cure for grid-blocking is to reduce the size of the coupling caps from phase inverter to output tubes (0.1uF originally in this amp). Yes, that will reduce bass.
The total system responsible for grid-blocking (if that is actually causing the bad sound in your amp) includes the coupling caps, the 220kΩ bias feed resistors and the 1.5kΩ grid stoppers on the 6V6's. Grid-blocking happens when you slam the output tube grids with a very large signal, such that the positive peak of the signal is greater than the bias voltage (or a peak greater than your 34v of bias). The grid signal overcomes the 34v of bias so that the net grid voltage is positive. That causes grid the grid to attract electrons from the cathode, which then flow out through the bias feed resistors and bias supply to ground. But large-ish bias feed resistors tend to slow that drain of grid current, which charges the coupling cap plate closest to the output tube grid negative. The grid is now more-negative than it would be due to the bias alone. When the signal swings negative, it will cut-off faster than it ordinarily would because the net grid voltage is more-negative than it would be due to bias and signal alone. The whole cycle builds up as long as there is a huge input signal and causes some pretty ugly clipping.
Interestingly, output tube grid current also causes distortion in the phase inverter. Normally, the grid looks like an open-circuit so the phase inverter's load is the 220kΩ bias feed resistor. While grid current flows, the output tube grid looks like a much smaller resistance to the phase inverter. The end effect is a momentary reduction of the phase inverter's load, which can clamp the peak output of the inverter for an instant. One part of the phase inverter output wave gets changed, which amounts to distortion (signal out not same shape as signal in).
Once the too-big input signal is removed, it takes some time for the cap charge to drain away, which is determined by the time constant created by the coupling cap and the bias feed resistance. Make either of those parts larger-values, and the time constant increases as well as time to recover from grid blocking. Reducing their value speeds blocking recovery, but making the bias feed resistor smaller may load the phase inverter too much (depends on the particular amp design). So typically the coupling caps are reduced.
But we do have those grid stoppers. Making them bigger slows the electrons coming out of the grid, which is the same as reducing current (definition of current is 'rate of movement of electrons'). You could try inserting bigger & bigger grid stoppers (start with, say, 10kΩ) to nix the grid current, but large-enough values will reduce treble response at the output tubes.
All-in-all, smaller coupling caps or bigger grid stoppers. You could try 100kΩ bias feed resistors, but they may not be enough to fix the problem and I certainly wouldn't go lower than 100kΩ in this amp (lower needs a phase inverter redesign).
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Hi.
Thanks HotBluePlates for this lengthy explanation. It seems though that you suggest the amp needs tweaking whereas it needs repairing. I have already had a Harvard in the past and I can tell you this one is not working properly. No I don't use any pedals.
Here's a video. At the beginning the amp is on 3, tone on 7 and it already sounds bad. Then I turn it up to ten and it sounds awful and you'll clearly hear the ugly distortion and farty bass. It's weird because the amp actually lacks bass. The amp really sounds thin. There's no body to the sound. Sorry for the bad image but the sound is all that matters... I hope you can help me from there. Thanks!
http://youtu.be/gF2ivqUBsGo (http://youtu.be/gF2ivqUBsGo)
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A full set of voltage readings for the three filter caps and all tube pins would be helpful.
Your OT has definitely been stressed with that open secondary problem you had earlier. Do you have another OT you could try?
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A full set of voltage readings for the three filter caps and all tube pins would be helpful.
Your OT has definitely been stressed with that open secondary problem you had earlier. Do you have another OT you could try?
Nope, I don't have another OT at hand, I would need to buy one. Unfortunately I don't know of another maker of the Harvard OT besides Mercury magnetics and they're pricey...
I will take all the measures you asked and report back.
Thanks for your help.
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Any OT that's suitable for P/P 6V6s or EL84s will work. You could pull one from another amp just to temporarily hook up to the Harvard. I'd certainly do that before buying another OT.
I don't recall that you ever positively proved that the NFB was phased correctly. Disconnect the NFB wire between the speaker jack and the board. Does the amp sound better? It's a slim chance that this is related to your problem but it's easy to do.
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So here's my voltage chart:
PINS 1 2 3 4 5 6 7 8 9
V1 0 1.8 3.07AC 3.27AC / / 179
V2 198 0 1.49 3.07AC 3.07AC 251 18 49.8 3.25AC
V3 0 3.06AC 358 364 -34.4 -34.4 3.27AC 0 0
V4 0 3.04AC 356 359 -34.1 -34.1 3.25AC 0 0
V5 363 324AC 326AC 363
Sluckey I think I tried to swap the OT wires back then but I will do it again.
Thanks!
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Those voltages look fine.
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Got a different speaker/cabinet handy? To me it sounds like the speaker is rubbing. Either internally (meaning blown) or against something like the baffleboard.
Don't laugh at the latter guess; I once bought a cabinet for a 5E3 Deluxe build. Had a strange distortion only on the low E string or at high volume. Only when direct sunlight fell on the grill cloth could I see the speaker cone from the front of the amp and that it was not concentric in its baffle cutout. The speaker had been buzzing against the baffleboard.
I would try eliminating simple causes like the speaker/cabinet first before figuring out tests for the OT.
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OK, I will try the amp with another cab. I have a big 1X12 Marshall cab at 8ohms.
Thanks!
Update:
So I swapped the OT wires on pins 3 and the sound was worse with added noise. So they are not reversed.
I tried the amp through my marshall cab, same problem.
I think the problem is narrowing down to the OT. I have a tweed Tremolux amp, I'm gonna take out its OT and install it in the Harvard to see waht it does. I'll let you know. I must take it out right, there's no other way to discard the current Harvard OT?
Thanks!
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Update above...
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I think the problem is narrowing down to the OT. I have a tweed Tremolux amp, I'm gonna take out its OT and install it in the Harvard to see waht it does. I'll let you know. I must take it out right, there's no other way to discard the current Harvard OT?
You mean an actual 50's Tremolux? If so (and maybe even if not), don't remove the OT from the Tremolux.
I haven't tried this; wait for confirmation from someone who has. I wonder if you can put the chassis of the two amps next to each other, with a wire from each of the Harvard's 3 primary connections (2x output tube plates, 1 B+ connection) over to the Tremolux's primary & out to the Tremolux's speaker for a test. You'd need to completely unhook those 3 primary connections at the Harvard's existing OT, and use some high-voltage rated wire. For that reason, I'm hesitant to even suggest lashing this up due to the potential danger to the amp & you.
However, it would allow testing using the Tremolux OT without removing it from the chassis, and confirm/deny the OT as the problem.
If it's a copy of a tweed Tremolux, the safer thing is to just remove the Tremolux OT and attach the wires directly to the relevant circuit points.
Silly Question: Do you have more than 1 speaker jack; if so, is the problem the same at both jacks?
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You mean an actual 50's Tremolux? If so (and maybe even if not), don't remove the OT from the Tremolux.
NO... It's a clone of course, I wouldn't do that to an original!
I haven't tried this; wait for confirmation from someone who has. I wonder if you can put the chassis of the two amps next to each other, with a wire from each of the Harvard's 3 primary connections (2x output tube plates, 1 B+ connection) over to the Tremolux's primary & out to the Tremolux's speaker for a test. You'd need to completely unhook those 3 primary connections at the Harvard's existing OT, and use some high-voltage rated wire. For that reason, I'm hesitant to even suggest lashing this up due to the potential danger to the amp & you.
However, it would allow testing using the Tremolux OT without removing it from the chassis, and confirm/deny the OT as the problem.
If it's a copy of a tweed Tremolux, the safer thing is to just remove the Tremolux OT and attach the wires directly to the relevant circuit points.
Silly Question: Do you have more than 1 speaker jack; if so, is the problem the same at both jacks?
I only have one speaker RCA jack...
Thanks!
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Hi.
I installed the Tremolux OT inside the Harvard and checked. The problem is gone.
Looks like I'm buying another OT...
Thanks!
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Wow! That's a first-time hearing of OT damage like this (without smoke, etc). Sorry to hear you'll have the added expense...
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Hi.
Sorry for taking such a long time in getting back to this topic. I hope you won't take as long as I did to help me out! lol
So if you remember, the MM OT seemed to be responsible for the bad sound and wrong bass notes.
I installed the new Classictone OT and it turns out the same and I now have some high-pitched squeal when I turn the volume knob, basically between 1 and 3 and then 9 and 12, nothing in the middle. I just don't get it, the problem seemed to be solved when I tried the amp with the Tremolux OT. So right now, it turns out that the MM OT was maybe good after all. Could a component be responsible for this mess? A cap? A resistor?
I'm totally at a loss with this amp, I really need you guys to help me find out what's wrong. Here's a video I just made of it:
Harvard issue 2 (http://www.youtube.com/watch?v=x-LS4P1ngsI#ws)
I pulled V1 out and the problem remained identical so at least this takes V1 and its surroundings out of the equation. I also discarded the tubes and the speaker, I swapped both for known good ones and got very bad results as well...
A million thanks already!
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Reverse the OT plate leads. Does that help?
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Reverse the OT plate leads. Does that help?
Done. It's worse...
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Unplug that guitar cable. Is the problem still there?
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Thanks for your help Sluckey. It's the same thing without any cable plugged in...
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The negative feedback wire from the speaker jack is still disconnected? If not, disconnect. Still have the squeal?
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The negative feedback wire from the speaker jack is still disconnected? If not, disconnect. Still have the squeal?
It was back in. I have just disconnected it. The squeal is gone. Thanks Hotblue!
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So, totally disconnecting the NFB wire killed the squeal but reversing the OT plate wires made it worse?
Hmm, very interesting!
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All my apologies Sluckey, I had read your last postr too fast. I reversed the plate wires and the amp now sounds awesome. I'm gonna put the MM OT back in to check how it sounds.
Thanks and sorry.
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All my apologies Sluckey, I had read your last postr too fast. I reversed the plate wires and the amp now sounds awesome. I'm gonna put the MM OT back in to check how it sounds.
Thanks and sorry.
Well, then you'll go through the whole possible-squeal situation again...
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All my apologies Sluckey, I had read your last postr too fast. I reversed the plate wires and the amp now sounds awesome. I'm gonna put the MM OT back in to check how it sounds.
Thanks and sorry.
Well, then you'll go through the whole possible-squeal situation again...
I did... Can you guys believe that I think the problem was coming from the RCA speaker jack (along with the reversed plate leads on the Classictone)? I have to make sure 100% that it was coming from there. I'll get back to you and report. Thanks!
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So, conclusions:
The MM OT was bad, I was right in buying a new one.
The Classictone OT was good but there were three mistakes:
- the OT was grounded to its frame which was causing noise and hum. I unsoldered it, extended the wire and grounded it at the speaker jack.
- The plate leads were reversed.
- The RCA jack was providing a poor ground, I installed a regular 1/4" jack and the amp now sounds awesome.
Thanks guys! :worthy1:
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It's odd that the new OT has the same issue as the MM OT with the secondary wire being attached to the frame. I wonder if the frame has some clear coating or plating that does not allow a good electrical ground connection when bolted to the chassis. I also wonder the same about your chassis.
Anyway, glad you got it figured out.
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I wonder if the frame has some clear coating or plating that does not allow a good electrical ground connection when bolted to the chassis. I also wonder the same about your chassis.
The OT's for my 5E3 and 5G9 both had the common secondary soldered to the U channel. And both looked like the channel was varnished to me.
I unsoldered the common from the channel and ran it to the speaker jacks just to be sure it made good contact. I guess I could have just sanded the bottom of the channel's feet and the chassis a little where they mount together but they could tarnish over the years and slowly loose good ground contact?
Brad :icon_biggrin: