Harvard 5F10 issue

[SleepLess]

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:



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!

[SILVERGUN]

It looks like you have the White wire from the PT primary hooked incorrectly to the bias supply 

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?

[Willabe]

Link for the wiring of your On/Off/Standby switch.

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


                  Brad   

[Willabe]

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   

[SleepLess]

It looks like you have the White wire from the PT primary hooked incorrectly to the bias supply 

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?

[SleepLess]

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   

OK Brad. I used a big tip but only a 30w soldering iron. Will do.

[Willabe]

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     

[SILVERGUN]

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. 

[SleepLess]

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. 

OK, thanks a lot! Oops... After more than 30 builds I sometimes think I'm still a damn noob... 

[SleepLess]

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     

Got it Brad! Will do so on my later builds!

[Willabe]

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.   

Here's a high lighted drawing of the R/B bias wire.


                      Brad     

[SILVERGUN]

OK, thanks a lot! Oops... After more than 30 builds I sometimes think I'm still a damn noob... 

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.
 

[Willabe]

Here's the PT primary color code from the Merc Mag spec sheet you posted.


                      Brad   

[SILVERGUN]

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:

[SILVERGUN]

Checked his profile and it appears he's somewhere between Germany and Spain.....

Remember Sleepless "À mauvais ouvrier point de bon outil"
 

[SleepLess]

Yes guys, I'm in France. 230V here... 227V at my wall.
Thanks, will keep you posted!

[Willabe]

Yes guys, I'm in France. 230V here... 227V at my wall.

Doh!          

[sluckey]

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

[Willabe]

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     

[SleepLess]

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. 

[Willabe]

You need to ground the cathode on V3(?) red circle.


               Brad     

[SleepLess]

You need to ground the cathode on V3(?) red circle.


               Brad   

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.

[Willabe]

Ok, I see it now.

[sluckey]

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.

[SleepLess]

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!

[SleepLess]

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...? 

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:





Thank you!!!

[sluckey]

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.

[SleepLess]

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.

[sluckey]

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.

[SleepLess]

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...

[SILVERGUN]

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
 

[SleepLess]

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? 
 

It looks like he has the PT secondary CT attached to his off/stanby/on switch 
 
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!

[SILVERGUN]

Does it connect to chassis ground with a good solid connection?

[SleepLess]

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.

[SILVERGUN]

HT CT = heater center tap, am I wrong?
So the heaters are the filament winding, right?   
 

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.

[SleepLess]

HT CT = heater center tap, am I wrong?
So the heaters are the filament winding, right?   
 

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.

[sluckey]

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.

[SleepLess]

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!

[sluckey]

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.

[SleepLess]

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)... 

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.

[SleepLess]

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 

[sluckey]

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.

[SleepLess]

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?

[Willabe]

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.

[SleepLess]

I will take these 100Ω out. Sorry for misreading... Will check DC voltage again without them then.

[sluckey]

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

[SleepLess]

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?

[sluckey]

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.

[SleepLess]

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!

[sluckey]

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.