Harmony 420 Need some advice/help again please

[Show Me Guitars]

Hi everyone, I am looking for a new power transformer for a Harmony H420.
 I ordered one but it's not correct even though the manufacture assured me it was.
Last advice helped me finish a Kalamzoo Reverb 12 which came out stellar, much appreciated everybody!!
Thanks

[sluckey]

That's not enough info to even make a wild ass guess! What transformer did you order? Show link to spec sheet. Why do you say it's not correct?

BTW, there is a dangerous error on the schematic, possibly a PT killer.
 

[Latole]

That's not enough info to even make a wild ass guess! What transformer did you order? Show link to spec sheet. Why do you say it's not correct?

BTW, there is a dangerous error on the schematic, possibly a PT killer.

You are right

[scstill]

this might be better (haven't done a complete comparison)
https://www.angelfire.com/mech/beansamps/supro/harmony420.html

Might be a good idea to fix the schematic library, or at least add a footnote
I'll bet the dot should be a jump over

[sluckey]

I'll bet the dot should be a jump over

Both of those dots should be a jump over. That's not the fatal error I see.

[scstill]

I rarely (never) use full wave bridge ss diodes for rectification
But the ones I've seen do not have a HV center tap

[sluckey]

I rarely (never) use full wave bridge ss diodes for rectification
But the ones I've seen do not have a HV center tap

Yep. As drawn those two diodes that are connected to ground will fail in about 1/60 second. Hopefully they will just blow apart. Otherwise, the PT will be in trouble.

[Latole]

As already discussed in 2017

https://el34world.com/Forum/index.php?topic=22725.0

[Show Me Guitars]

Hi All, I was told a Hammond 290PAZ would work, not really close.
This amp is the same circuit a Supro Thunderbolt.
I was hoping someone may have had done this replacement before. thx

[sluckey]

Hi All, I was told a Hammond 290PAZ would work, not really close.
This amp is the same circuit a Supro Thunderbolt.
I was hoping someone may have had done this replacement before. thx

The amp is nothing special. That 290PAZ will work just fine if it will physically fit the chassis (or you can make it fit). But you cannot use a bridge rectifier with the 290PAZ. Most anyone can handle this simple task.

[scstill]

Seems that the 290PAZ has a lot more windings than you need so might cost a bit more (see HawkUSA.com)

Check out this site, section called "Spare Parts and Cloning" Quote below...
https://www.angelfire.com/mech/beansamps/supro/supro.htm

"The original power transformers supplied to Valco were made by Triwec and carried one of these three part numbers: 330-3738, E-3738 or E-3738A.
The output transformer was also supplied by Triwec and carried four (or more) part numbers: 3778, T-3778, E-3778A and 340-3778.
Reproduction power transformers for the solid state rectified model are available from Heyboer.  Contact them and ask for a quote on part number HTS-4664.

The Heyboer transformer has a non center tapped secondary rated 325VAC @ 200mA, the filament winding is center tapped and is rated 6.3Vac @ 4A, there is no heater winding.

Magnetic Components Inc., who operates www.classictone.net  apparently is a decendant of Triwec and has started offering original spec vacuum rectified power transfomers as well as the output transformer which was used on both models. The power transformer is their part number 40-18060, and the output transformer number is 40-18061.

This power transformer has a center tapped secondary rated 700VAC @ 125mA, the filament winding is center tapped and is rated 6.3Vac @ 3.4A, the heater winding is rated 5.0Vac @ 3A."

[WimWalther]

As already discussed in 2017

https://el34world.com/Forum/index.php?topic=22725.0

While "don't connect.." fixes the whole blowing-up issue, I'm not sure why they're bothering to use a bridge in the first place when a pair of diodes (or a tube rect, if voltages / current allow) will serve just as well.

Dump the bridge and install a pair of HFA08TB-60 HexFRED diodes, for tube rectifier sound without any of the downsides of an actual tube.

[bmccowan]

Magnetic Components Inc., who operates www.classictone.net  apparently is a decendant of Triwec and has started offering original spec vacuum rectified power transfomers as well as the output transformer which was used on both models. The power transformer is their part number 40-18060, and the output transformer number is 40-18061.

Classictone has been gone for about 3 years. But you can still find the spec sheets on the web.
I used on to replace a 420 PT. Fortunately I did not use that schematic as a guide.
I agree that it would be good to note the errors on the schematic in the library. There are also errors on the commonly followed Thunderbolt schematic.

[sluckey]

The problem with the H 420 schematic is the PT center tap ***AND*** the FWB are ***BOTH*** connected to ground. YOU CANNOT CONNECT BOTH TO GROUND! So, if the PT is supposed to have a CT and secondary voltage is high, you must disconnect the bridge ground or use a simple 2 diode FW rectifier. On the other hand, maybe the PT has a low voltage secondary that requires a FWB to get proper B+. It this is the case, then the PT CT should not even have a CT, and if it does, it should not connect to ground.

But we really don't know if the grounded PT CT is the drawing error, or the grounded FWB is the error. And Show Me Guitars ain't talking!

Bottom line... You cannot connect both to ground. If so, the grounded diodes will fail and may take out the PT as well.

As for the pic that Latole posted, it is taken out of context and doesn't clearly explain the note I put on it. You must read the short thread to fully understand.

[Show Me Guitars]

Thanks everyone for the input

[scstill]

Thanks everyone for the input

Since you have this amp, could you please look at the amp and let us know if Harmony grounded the FWB or if they grounded The PT HV center tap (or even if it has a HVCT, thinking not)? Thanks.

[Show Me Guitars]

I am happy to look but its been messed with prior to my ownership, purchased it dead and trying to revive it.
Ill post asap...