Changing B+ via dropping resistors

[TweeDLX]

I think I know this one, but I like to be sure. I have an amp (AB763 type with GZ34) with the PT from Magnetic Components (330-0-330) @ 120ma advertised as a drop-in replacement for a Deluxe Reverb. I've been running the amp for several months with 6L6GC's, but today decided to try some 6V6GT's just to see if I could "loosen it up" a bit. I was a bit surprised to find a PV of almost 475VDC! A bit high for any NOS glass I have, so I tried some JJ's. They sounded OK, but no headroom.
Now to the crux of the matter:
Will changing dropping resistor values drop enough B+ to run some NOS 6V6's? I have 1K5/5W, 22K/2W, 22K/2W in it now. Would a better solution be a 5U4GB? The PT rectifier tap is rated 5V @ 3A. Thanks in advance for yer wunnerful hep.

Mike

 

[VanR]

I believe the 5U4 will drop some voltage. You could also try the 5R4.
Look here for voltage drop info
http://www.300guitars.com/articles/rectifier-tube-voltage-drop-chart/

 

[schmidlin]

Nor sure what tree you're shaking there, Mike: some think loosening-it-up means lowering headroom. Lowering voltage will always do that anyway, so maybe you are just looking to play around and see what you can find.

That said, 5U4s are cheap, and you could even go 5Y3 if you are using 6V6s for even more drop. Both are a better route than messing with power resistors that will change your B+ to your plates, which is what you need to protect your old 6V6s. The 22K altering is really off into the "power scaling" land: no opinion here for that.

You really could use almost any rectifier here. Easy and non-intrusive.

 

[TweeDLX]

Nor sure what tree you're shaking there, Mike: some think loosening-it-up means lowering headroom. Lowering voltage will always do that anyway, so maybe you are just looking to play around and see what you can find.

You really could use almost any rectifier here. Easy and non-intrusive.

Schmidlin,
Thanks for your insight. I was pretty sure that the rectifier change would be the recommended path. As far as shaking trees goes, with the 6L6's, it had too much headroom, but with the JJ's, not enough. I'm constantly looking for that nice middle ground. "Playing around" is the most apt description of what I do!
What I wound up doing was to decrease the plate load resistors in the PI to 56K & changed the tail resistor to 33K. Sounds clean and very BIG, just not as loud. I added a 100K variable voltage divider after the 3rd gain stage that lets me "loosen things up" if I want. It works better with the lower plate resistors in place. Thanks again,

Mike

 

[Blue Strat]

What they said, plus, messing with existing resistors won't have any effect on the B+ voltage...because they're all downstream.

The only way to affect B+ voltage would be to ADD a dropping resistor between the rectifier cathode and the center tap of the OT. Using a lower rated rectifier would be a better way to go.

 

[Jerry Glass]

I've noticed over the years that most, if not all replacement power transformers for the DRs end up producing more B+ than the originals. Its almost as though they are all working from the same flawed specs.

I typically will offer two solutions to my customer. The first, which I term the economical choice, is to go to a rectifier with more envelope. The issues I see with this choice is that, in the future, someone could decide to use the original rectifier type in the amp and have excessive voltage related issues.

The second recommendation is to drop the voltage down to expected levels and use the original type rectifier. I use a 50w zener of the correct voltage to achieve the desired voltage drop between the rectifier and the OT center tap. The zener drops the voltage and, unlike a resistor, doesn't induce a sag of its own. Using a 50w zener seems like overkill but I also don't have to worry about heat sinking it.

 

[JJman]

The higher voltage on the 6v6s should be providing more headroom potential, when properly biased, than a "normal" 6v6 plate voltage. What is the idle current and grid/bias voltage? Maybe they are running too cold (which would also bring the plate voltage higher,) and are running into premature cutoff and therefore crossover distortion?

 

[TweeDLX]

I had the JJ's biased around 21ma (somewhere in the 60-65% range) with a plate voltage of around 450VDC. The PT has a -50V bias tap that I'm using. They sounded like... JJ's. I just wanted a bit more headroom so I can keep up with our drummer.
With my previous set-up in this amp, I could rarely get the volume past 3. It's loud (good!). But...to use my nifty variable divider (thanks Mike!) I had to damage the eardrums of everyone in the band. I'll play with it some more today, try the 5U4 and report back. It was actually sounding pretty good when I left off yesterday. Thanks for all of your suggestions!

Mike

 

[hasserl]

I've noticed over the years that most, if not all replacement power transformers for the DRs end up producing more B+ than the originals. Its almost as though they are all working from the same flawed specs.

I typically will offer two solutions to my customer. The first, which I term the economical choice, is to go to a rectifier with more envelope. The issues I see with this choice is that, in the future, someone could decide to use the original rectifier type in the amp and have excessive voltage related issues.

The second recommendation is to drop the voltage down to expected levels and use the original type rectifier. I use a 50w zener of the correct voltage to achieve the desired voltage drop between the rectifier and the OT center tap. The zener drops the voltage and, unlike a resistor, doesn't induce a sag of its own. Using a 50w zener seems like overkill but I also don't have to worry about heat sinking it.

This is a chassis mount zener? If so the heat sink is the chassis itself. It is still a good idea to use heat sink compound between the zener and the chassis to improve heat transfer between the two.

 

[TweeDLX]

Did a little rectifier swapping this morning. Tried an RCA 5U4GB, 5R4, and wound up with a Mullard GZ37 sounding the best. Even with the 5R4 and 5U4, voltages were still a bit high for my vintage 6V6GT's (over 440VDC), so I stayed with the 6L6GC's.
Many thanks to all who replied!

Mike

 

[wizard333]

My '73 DR runs around 475 on the plates with the stock PT. JJ 6v6s handle it just fine.

And yeah the dropping resistors are all downstream so they wouldnt affect it anyway.

FWIW, a good NOS 6v6 should be able to handle it. I've worked on other DRs with PV that high as well and they worked for 40 years that way just fine.

 

[Zelja]

The second recommendation is to drop the voltage down to expected levels and use the original type rectifier. I use a 50w zener of the correct voltage to achieve the desired voltage drop between the rectifier and the OT center tap. The zener drops the voltage and, unlike a resistor, doesn't induce a sag of its own. Using a 50w zener seems like overkill but I also don't have to worry about heat sinking it.

I was going to suggest something like this. I'm pretty sure you can wire in the zeners so it affects just the plate voltages on the power tubes (this leaving pre-amp & PI unaffected) or wire them such that it drops all the subsequent voltages as well.

 

[Jerry Glass]

This is a chassis mount zener? If so the heat sink is the chassis itself. It is still a good idea to use heat sink compound between the zener and the chassis to improve heat transfer between the two.

You can't do that in this application; the body of these diodes is the anode. If you choose to heatsink it, you must isolate the heatsink from ground. The typical use for these high wattage zeners is as a voltage regulator where the anode would be grounded.

 

[Blue Strat]

A common zener approach is to connect it in series with the CT of the PT and ground. Chassis mounting with no insulation/isolation works fine.

 

[KindaFuzzy]

I've tried this before and it works great

Here's the link to the website: http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm