High Plate voltage

[Rich_C]

Hello,

I build a Deluxe Reverb clone (kit) and all and all it went off without a hitch. But, my plate voltages on the 6v6's is pretty high 490VDC, is the highest I've seen. Lowest is around 468VDC. I do have high wall voltage (highest is 125V at times). Could the power transformers voltage fluctuate that much because of a few volts more at the wall socket? I was wondering if changing the power transformer which is a 354-0-354 to a 330-0-330 would lower the voltage a bit. I'm a little afraid to use the amp until I get my head around what's going on.

[shooter]

what's the bias set at?
did you install 1 ohm resistors from PA tube cathode to ground for easy current monitoring?
the PT is a step-UP transformer, ~~ 3:1 so for every 1 volt increase in the wall you have 3 on the secondary side (simplistic explanation)

[Rich_C]

I've got the Bias set slightly over 60%, around 17mA. I have not installed a 1 ohm resistors...yet. I've been using the Eurotubes bias probe.

[shooter]

so what's 480vdc X .017A = ?
what's the tube data sheet show for Max plate dissipation?

[Esquirefreak]

354 - 0 - 354 sounds like a lot, for a DR. Something like 325-330 (650-660CT)VAC @ 140mA is probably what you want.

[Rich_C]

 480vdc X .017A = 8.16W. Maximum plate dissipation for the JJ 6v6s is 14W.

[Rich_C]

Esquirefreak, Not sure why they went with a higher voltage transformer in the kit? I guess they had some reason.

[HotBluePlates]

... my plate voltages on the 6v6's is pretty high 490VDC ... I was wondering if changing the power transformer which is a 354-0-354 to a 330-0-330 would lower the voltage a bit. ...

354 - 0 - 354 sounds like a lot, for a DR. Something like 325-330 (650-660CT)VAC @ 140mA is probably what you want.

I used to own a 1964 Deluxe Reverb.

With exactly 120vac input from the "wall" (via a variac) I got 6.3vac on the heater winding, 334-0-334v at the PT high voltage winding, and 394v on the 6V6 plates when drawing 24.9mA of plate current (70% of 14w) and using a British GZ34 rectifier. I didn't record the voltage at the first filter cap (which would be a few volts higher than at the 6V6 plates).

Anything significantly above 330v per side on the PT is too much.

Esquirefreak, Not sure why they went with a higher voltage transformer in the kit? I guess they had some reason.

IMO, "They" don't have reasons & never encountered the vintage amp.

I tend to find that power transformers have lower winding resistance, which delivers higher voltages & impacts dynamic compression when playing the amp (the modern transformers sound "stiffer" or "harder").

I also find that some vintage amps have voltages very-different from what is printed on their schematics.  One wouldn't know that without measuring the original article.

[Esquirefreak]

Who knows 

I've never bought a kit build, but I guess a kit is not automatically a guarantee that voltages will be right on spec. Or the company simply could have sent the wrong PT?

What's your 5v and 6.3v voltages look like? If those are high, that could indicate that your wall voltage is too high for that PT.

480VDC looks (to my eyes) more like something you'd see in a 6L6 amp.

Although I've heard that the JJ 6V6s is sturdy, I probably wouldn't wanna run the plates higher than 450VDC.

But then again, I'm in no way an expert.

/Max

[shaun]

As others have said, seems the PT is putting out too much voltage, assuming everything else is in order. I'd go for the lower rated PT, or I might consider adding a dropping resistor in the B+ line before the first node. Probably have to be a 10watter thought, at least, and rated from 100ohms to 300ohms. Expect it to get pretty warm fairly quickly, and watch for it overheating. You can lower B+ using diodes, too, but the resistor may be simpler. Ultimately, you're fighting the PT though, so if you have the new PT available and have the patience to undo your recent build - which I always hate - that would be the ticket.

[bmccowan]

I do not see that you have contacted the kit supplier? Of course if it was a China direct deal - that may be of no help, but I think all of the reputable US and European suppliers will offer advice, and may offer to swap the PT.
Also, the BFDR used a GZ34 rectifier. If that is what you are using, you can drop a lot of volts changing to something like a 5U4GB.

[sluckey]

I build a Deluxe Reverb clone (kit)

Who made the kit? Any schematic, layout, or instructions available?

[Lectroid]

480VDC looks (to my eyes) more like something you'd see in a 6L6 amp.

Although I've heard that the JJ 6V6s is sturdy, I probably wouldn't wanna run the plates higher than 450VDC.
/Max

If you have built a 'standard' AB763 circuit then your amp would be pretty close to a Pro Reverb if you just swapped in 6L6s and dropped the B+ voltage a little.  Or not.  If your preamp tubes are running equally hotter then you might have extra clean headroom as well, not a bad thing.

If you do keep the 6V6s, go with the JJ 6V6s.  They can take a lot.

[Rich_C]

Yeah, I'm in talks with the company. I suspect they get a lot of calls about this issue from guys like me that aren't professional amp builders. The immediate response was to blame my house voltage. Yes, the amps got a GZ34 rectifier. I think the transformer voltage that came with the kit is rated with an unloaded amp and draws down with a load. I think     

I do not see that you have contacted the kit supplier? Of course if it was a China direct deal - that may be of no help, but I think all of the reputable US and European suppliers will offer advice, and may offer to swap the PT.
Also, the BFDR used a GZ34 rectifier. If that is what you are using, you can drop a lot of volts changing to something like a 5U4GB.

[sluckey]

I build a Deluxe Reverb clone (kit)

Who made the kit? Any schematic, layout, or instructions available?

[HotBluePlates]

... I think the transformer voltage that came with the kit is rated with an unloaded amp and draws down with a load. I think     

You said the amp is built & working.  If true, then that is the "load" to pull the voltage down to its rated value.

I do not see that you have contacted the kit supplier?

Who made the kit?

If we knew who you got the kit from, we might be able to figure out the source of the error.

I have been surprised recently by the number of people buying "amp kits" off Amazon, and getting who-knows-what from vendors who very obviously don't know about the amps they're supposedly cloning.  Also true of some eBay vendors.

[Rich_C]

It's a mojotone kit. Yes, the amp has been completed and running. Other than voltage issue, I had a crackling 12ax7 that I replaced with and old one I had kicking around. No big deal. Schematic file is attached.
                                             

... I think the transformer voltage that came with the kit is rated with an unloaded amp and draws down with a load. I think     

You said the amp is built & working.  If true, then that is the "load" to pull the voltage down to its rated value.

I do not see that you have contacted the kit supplier?

Who made the kit?

If we knew who you got the kit from, we might be able to figure out the source of the error.

I have been surprised recently by the number of people buying "amp kits" off Amazon, and getting who-knows-what from vendors who very obviously don't know about the amps they're supposedly cloning.  Also true of some eBay vendors.

[HotBluePlates]

It's a mojotone kit.

Yeow!!  Their "125P23B/MOJO761EX" says 354-0-354v.  That's crazy-high.



FWIW, my 1964 Deluxe Reverb had a 125P23B power transformer.
By 1965, the Deluxe Reverb had a 125P23C power transformer with slightly higher voltages.
By 1966 the part #s changed to the 6-digit system:  025130 (and voltage may have gone up again)


Mojo may have been selling the PT for a long time, but the voltages are way higher than a vintage unit.

[Rich_C]

That's the one. I've been digging around and I see that a few of the transformers from Mercury Magnetics meant for the Deluxe Reverb have very similar voltages to Mojotones. Not sure why they build them that way. As far as plate dissipation goes...the math works.  Hammond's got a 660Vct @ 138 mA for the deluxe reverb.

It's a mojotone kit.

Yeow!!  Their "125P23B/MOJO761EX" says 354-0-354v.  That's crazy-high.



FWIW, my 1964 Deluxe Reverb had a 125P23B power transformer.
By 1965, the Deluxe Reverb had a 125P23C power transformer with slightly higher voltages.
By 1966 the part #s changed to the 6-digit system:  025130 (and voltage may have gone up again)


Mojo may have been selling the PT for a long time, but the voltages are way higher than a vintage unit.

[sluckey]

It's also my opinion that mojo is using the wrong PT. The AB763 schematic clearly shows 330-0-330.

     https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf

[Esquirefreak]

The fact, alone, that they use 475v E.caps for the reservoir with that PT (and those voltages) is a major design flaw.

I think that rather than using another rectifier tube (like a 5U4) you should get the appropriate transformer. The GZ34/5AR4 also has the benefit of "slow starting", which makes life alot easier on everything in the power supply. 

/Max

[Esquirefreak]

I've mostly used Hammond iron for my builds and I generally end up with voltages pretty darn close to the schematic. YMMV.

[scstill]

It's also my opinion that mojo is using the wrong PT. The AB763 schematic clearly shows 330-0-330.

Could a first dropping resistor of about 250ohm/5w get the voltage of the existing PT in line?

[scstill]

IMO, "They" don't have reasons & never encountered the vintage amp.

I tend to find that power transformers have lower winding resistance, which delivers higher voltages & impacts dynamic compression when playing the amp (the modern transformers sound "stiffer" or "harder").

When possible, is it then better to use vintage PT over modern ones?

[Rich_C]

I think I'm going to give a 330-0-330 try. They're not crazy expensive.

It's also my opinion that mojo is using the wrong PT. The AB763 schematic clearly shows 330-0-330.

     https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf

[sluckey]

It's also my opinion that mojo is using the wrong PT. The AB763 schematic clearly shows 330-0-330.

Could a first dropping resistor of about 250ohm/5w get the voltage of the existing PT in line?

Maybe. 15 years ago I used a pair of 250Ω/7W resistors between the PT secondary and rectifier to lower the voltage in my November amp. Two weeks later I changed the PT for a lower voltage secondary. I prefer using a proper PT.

[Rich_C]

I think that's the direction I'm headed.

The fact, alone, that they use 475v E.caps for the reservoir with that PT (and those voltages) is a major design flaw.

I think that rather than using another rectifier tube (like a 5U4) you should get the appropriate transformer. The GZ34/5AR4 also has the benefit of "slow starting", which makes life a
lot easier on everything in the power supply. 

/Max

[Rich_C]

Yeah, I'll get the PT. Wow. That's a nice looking build. Mines not a sloppy mess, but it's not as neat and tidy as that! 

It's also my opinion that mojo is using the wrong PT. The AB763 schematic clearly shows 330-0-330.

Could a first dropping resistor of about 250ohm/5w get the voltage of the existing PT in line?

Maybe. 15 years ago I used a pair of 250Ω/7W resistors between the PT secondary and rectifier to lower the voltage in my November amp. Two weeks later I changed the PT for a lower voltage secondary. I prefer using a proper PT.

[scstill]

15 years ago I used a pair of 250Ω/7W resistors between the PT secondary and rectifier to lower the voltage in my November amp. Two weeks later I changed the PT for a lower voltage secondary. I prefer using a proper PT.

I was thinking a DC drop after the rectifier rather than AC drop. Would that make any difference?

It seems that 24 additional PT volts is rather small. For me its not always possible to find the exact PT. If not building to a existing design always seems best is to draw the load lines, make output tubes happy. But I've also noted that the resolution on the old tube charts is often difficult to see a 24v variation on the load line.

Having said that I agree with others that Mojo should provide the correct parts.

And I also agree that the "November Amp" is very nice to look at. How did it get the name?

[sluckey]

I was thinking a DC drop after the rectifier rather than AC drop. Would that make any difference?

Probably not if you put the resistor between the rectifier and first filter cap.

And I also agree that the "November Amp" is very nice to look at. How did it get the name?

The name and amp came from ax84.com. In 2007 the November, which is a Plexi front end and a fixed bias EL84 power amp, was on the front page. It lives in the archives now...

     https://ax84.com/archive/ax84.com/oldprojects.html

[tubeswell]

480vdc X .017A = 8.16W. Maximum plate dissipation for the JJ 6v6s is 14W.

So what happens to the plate voltage if you increase to 70% of 14W?

[PRR]

> Could a first dropping resistor of about 250ohm...

It is "fixed bias". The idle is cooler than the full-roar current. Large "sag". This may be good, or not, depending.

Roughing 100mA, 250r drops 25V, which is hardly worth mentioning. Idled cold might be 60mA which is 15V which is about nothing. FWIW, beating it to 140mA makes it 35V and 4.9W so your 5W resistor might last a month.

[Rich_C]

I had bumped the bias up last night to about 65%. It appears the voltage went up from 460 to 470. I didn't check to see what my wall voltage was at that time. There may have been a correlation. I went to recheck voltages again, but my meter was set to measure current and I blew the fuse. I'm now out of action until I get a new fuse. 

480vdc X .017A = 8.16W. Maximum plate dissipation for the JJ 6v6s is 14W.

So what happens to the plate voltage if you increase to 70% of 14W?

[PRR]

I have not installed a 1 ohm resistors...yet.

...my meter was set to measure current and I blew the fuse. I'm now out of action until ...

[Rich_C]

 

I have not installed a 1 ohm resistors...yet.

...my meter was set to measure current and I blew the fuse. I'm now out of action until ...

[ac427v]

Mojo should fix that. If they won't, I like the 5R4GYB to drop 40-50 volts.

[Rich_C]

Haha, yeah...sure. They got paid.

Mojo should fix that. If they won't, I like the 5R4GYB to drop 40-50 volts.

[Rich_C]

Follow up on what happened with this situation. I change the power transformer to a Hammond 290BX. Last I checked the plate voltage was around 448VDC with the bias set to around 65%. I think I'm going to call it a success.

[HotBluePlates]

Follow up on what happened with this situation. I change the power transformer to a Hammond 290BX. Last I checked the plate voltage was around 448VDC with the bias set to around 65%. I think I'm going to call it a success.

I looked up my data on the 1964 125P23B transformer.  The DC Resistance on the High Voltage winding (Red to Red) was 220.4Ω.

Hammond's data on the 290BX shows a winding resistance of 112.5Ω.

IMO, the 108Ω of difference is significant, and could explain the lower output voltage & softer attack of the vintage part.  If I were building a Deluxe Reverb for myself using the 290BX, I would likely use a 50Ω 15w chassis-mount resistor between the PT Red wire and the rectifier's Pin 4, and another between Red and Pin 6.  This would mimic the vintage part's winding resistance.

It could be that 15w is moderate overkill, but 140mA through 50Ω is 7w dissipation.  Note the resistor I linked assumes bolting to the chassis as part of its power-handling rating.