Deluxe Reverb power transformer change?

[amplifryer]

My second tube noob question. I'm putting it in the Fender amp section as it seems (to me) to be more historical than it is technical.

Does anyone know if the PT turns ratio changed when Fender Deluxe Reverb changed from AB763 to AB868 circuit? I am aware of the heater winding center tap removal and the rectifier tube change but the published schematic voltages did not change.

I have a vintage '65 Deluxe Reverb that has a K025130/606334 ('73 Xfrmr). The AC output to the rectifier seems high. I made these measurements with the amplifier operating but PT bias cut way back to 15mA. It looks like some voltage sag on the output is expected.

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[Stevem]

The main difference between the two amps is the responce time of the two different rectifiers used once the output stage starts to really load the power supply hard.

The 5U4 has a much higher internal impedance then the GZ and there for will respond a bit slower under high current draw conditions.

Note that the Ac voltage input difference is only 10 volts ( 300. Vs 305. ) and in noted in the 868 schematic as being 120 input.
In the earlier schematic we don’t know exactly what the input voltage was, and in fact even in the early 60s the local wall voltage supply could be all over the place !

A big tone and response changing difference between both amps is the change in the second node power supply dropping resistor.

Another big response difference between the two amps is the double MFD of the first filter node in the 868 amp over the 763 amp.
This will effect voltage sag under high current draw conditions.

Also in terms of your main question when you look close at each schematic you will find that the part number for each power transformer has not changed.

[amplifryer]

Thanks Stevem, I did not see a part number for the Xfrmr on my schematics. I'm tuning this amp up after some repair work. My initial concern was the B+ voltage(s) seem high. At 120VAC, B+ is 478VDC/plates are 476VDC with 17mA bias current. This is with the grid voltage dialed back as far possible -59V.

I know the voltages on the schematic are "ball park". As they did not change with the rectifier change and the xfrmr on my amp is not original, I wanted to make sure there wasn't a winding change that might account for these high measured voltages.

[sluckey]

B+ is high because the output tubes are biased pretty cold. Set the bias pot for about 24mA current per tube and recheck the plate voltage.

[amplifryer]

Thanks Sluckey, Makes sense. I'll do that when I get to my workshop tomorrow. My B+ dropping resistors have increased to 11K and 11.5K. Is it worth replacing them at this time?

[sluckey]

Thanks Sluckey, Makes sense. I'll do that when I get to my workshop tomorrow. My B+ dropping resistors have increased to 11K and 11.5K. Is it worth replacing them at this time?

no

[amplifryer]

This thing wants to run hot. with 120VAC line, the plate V is 455, when tubes are idling 26/30 mA with bias V -50.8V.

With the price of tubes throught the roof these days, I'd really like to make them last. Is there a relatively painless way to lower B+ down some?

[JD0x0]

This thing wants to run hot. with 120VAC line, the plate V is 455, when tubes are idling 26/30 mA with bias V -50.8V.

With the price of tubes throught the roof these days, I'd really like to make them last. Is there a relatively painless way to lower B+ down some?

Just use JJ's. Datasheets spec the plates at 500VDC max and the screens at 450VDC.
In addition to that, cool the bias back down to ~65% dissipation and use 1k-2.2k screen grid resistors to limit screen current.

Fender always ran the 6V6's in those amps at bonkers voltages.

[didit]

Perhaps a read through of this semi-related thread would help 1974 Deluxe Reverb Restoration.

..

[amplifryer]

Update: I have been unable to sign in to this forum for a while but have been reading. I took the amp out to the gig last night and absolutely love how it sounds. My band mates were amused that I kept looking in the back of it (to be sure the plates weren't glowing).

Perhaps a read through of this semi-related thread would help 1974 Deluxe Reverb Restoration.
..

Thanks. I have been reading that with interest. So it looks like resistance can be added to the B+ line coming from the transfomer? I realize the resistor(s) would need to be properly sized and located for power dissipation. I didn't quite follow some of the other approaches mentioned.

I have been reading on another forum that substituting a 5V4 for the rectifier might also work?

[pjd3]

Hi Amp,

Just wanted to mention that using a 5V4 helped my amp B+ come down 12 or 15 volts or so over a GZ34. This amp has a Plexi preamp but the power output section is nearly identical to a Deluxe reverb - a pair of JJ 6V6's with around 435vdc on the plates, 20mA or so bias current. Its working out very nice. I've heard to be careful when using 5V4's with 6L6's or EL34"s due to the increased current potential or AC input voltage but, with 6V6 pair type amps the 5V4's work out very well. Also, you get maybe an added bonus of being able to use a NOS 5V4 (they all are I think) should you think they are more reliable and robust.

My amp is gigged every weekend and working like a charm. - good luck.

Best,

Phil D.

[amplifryer]

Thanks Phil,
It looks like I may be able to use a 5U4 also. My power transformer should be able to provide the extra amp of filament current.

[amplifryer]

It looks like changing the rectifier is a relatively painless way to lower the B+ in this amp. I took the following measurements without changing bias.
Line=120VAC
GZ34 B+=462V Icathode 24/28mA
5U4GB B+=450V Icathode 22/26mA

I may try the MOSFET B+ reduction circuit described in this current thread but for now I'm just going to continue to play the amp.
https://ampgarage.com/forum/viewtopic.php?f=8&t=37046

Thanks for the help guys.

[pjd3]

Since mention of the B+MOSFET reducer:

Just this week I installed a B+MOSFET reducer into my Blackvibe 6L6 upgrade. I was seeing 470vdc on the plates of some expensive 6L6WGB phillips tubes. While many claim that they will take that kind of voltage for a while, I was inclined to see about lowering it a bit so, I build one with a 20volt zener. It brought the plate voltage down to around 443vdc which I slept much better on. The price of those tubes went from $100 a matched pair to $220 each! I wanted to see if I could help them last longer!

Placed the Mosfet reducer right next to power ground bolted to the chassis with a healthy shot of thermal paste.

Best,
Phil D

[Colossal]

I ran some 6L6GWB/5881s in a Bassman at about 460V for a little less than a year before one flashed over. They were new production and decent-ish, but they weren't up for the challenge in the long run. I installed a pair of EL34s and increased the screen resistors and haven't looked back.