High Plate Voltage in '72 SFDR

[mcasey1]

I've got a '72 Silverface Deluxe Reverb with a plate voltage of 467.  The AC coming from my wall is 123V.  I've read that some silver face amps of this particular time period had higher voltages due to a change in PT specs in order to use 5u4 rectifiers.  I recently recapped the amp with all new power supply filtering caps and cathode resistors/caps.  In doing so I also "blackfaced" the amp, changing a few values here and there to get the amp to AB763 specs without the silverface era tweaks.  I am running a 5u4gb rectifier tube.  I can get my power tubes in the acceptable dissipation range, but honestly the amp sounds very stiff and unpleasant, not nearly as sweet or forgiving as a lot of blackface amps I have played.  My suspicion is that undesirable quality emanates from the high voltages, but I'm no expert.

My question is, if I wanted to get plate voltages down to the 415V ballpark, would the best course of action be to swap the PT with one that will provide lower voltages?  If that is the case, is there anything else i would need to consider changing in the amplifier?  Lastly, are there any other measurements I might be able to take to be more informative? 

Thanks for reading!

matt

[Willabe]

I had a BFSR that I gigged with for years and the plate voltage was ~465, and it sounded warm and not stiff at all. Mine used a GZ34/5AR4 but a 5U4 should be a little spongier.

Our friend Ed is well acquainted with SR's, as he has (and had) a number of them, hopefully he'll give some thought to this.

I think it's something else.


                     Brad   

I'm sorry, somehow I saw/read SR, and I was sober as a judge. This is not good.    

[eleventeen]

It's not the biggest of big deals to change out the 10 or so connections for an alternative PT but it *could* be a PITA to deal with the work of the possible mechanical changes of a different sized tranny. If you have one or can borrow one, I would try running the amp off a variac in your actual playing situation and see if you get the tonality you would want off a 10-15% line voltage reduction. The issue is, in my mind, that it could be difficult to assess the change in tone on a bench versus on a gig or at a jam, at higher volume in a bigger room.


And maybe your P/S choke isn't choking.

[2deaf]

How about the good old zener diode(s) on the center tap thing.

[sluckey]

How about the good old zener diode(s) on the center tap thing.

I believe that will affect the bias supply also?

[2deaf]

I believe that will affect the bias supply also?

Of course.  If you don't like what the bias decreased to when B+ decreased, then change it.  Easier to change the bias than to change the transformer.  Heaters stay at 6.3V.

[mcasey1]

How can I check to see if the choke is functioning properly?  If I could find a PT that fits the mounting holes then I would rather go that route as opposed to dropping with diodes.  I wish I had a variac that I could borrow.

[PRR]

465V is not wrong.

Best course of action is to check EVERYTHING else before dumping money on heavy iron and shipping.

Check all other voltages against a same/similar schematic. If you have somehow got a wrong resistor or a sick tube, this *may* give clues. An orange-stripe resistor where should be a red-stripe resistor can give unhappy performance yet evade many eyeball-checks. (How would I know?) Finding 8V at a cathode that should be 2V might lead you to re-read that cathode resistor ("Ah, 15K aint right.") Finding 0.1V at a cathode that should be 2V, and near-full B+ at that plate, suggests that tube is past its prime (though this would probably not sound "stiff").

[sluckey]

It's a Deluxe Reverb. I think 467V is kinda high for 6V6s.

[mcasey1]

Here are the voltages I measured.  Sorry for the chickenscratch writing.  Most look fairly high in comparison to this schematic, but V5 looks totally wrong.  I know its just for the tremolo circuit, but I wonder whats up with the voltages on it. 

[sluckey]

Enable the tremolo with your footswitch or put a ground clip on the vibrato footswitch jack and V5 voltages will come alive. Looks like the oscillator is turned off right now.

I think your high line voltage accounts for the high voltages. But your filament voltages should be a little high too if that's the case.

[mcasey1]

I'll get a clip on the jack and measure V5.  I might want to go ahead and drop a little cash on a PT that pumps out less voltage.  I guess the way I could describe the sound I dislike about the amp now is its overwhelming punchiness.  Every note seems to wallop me with a very strong attack and a powerful tonality that just sounds flat out wrong for my playing style.  It sounds very hi-fi and strident.  I prefer a spongier attack, somewhat rounder sound with a looser bass.  Possibly more compressed.  The 5u4 is already inducing some sag theoretically but I can hear none all the way through the volume settings.  I know it won't every sound quite as sweet as my old Princeton, but it is so overbearing right now.

[sluckey]

How did it sound before you fixed it?

Which speaker?

[mcasey1]

It sounded pretty close to how it does now.  Now its perhaps marginally smoother.  There really wasn't anything in particular that was wrong with the amp, I just felt like recapping it once I got it.  One of those "bought it but haven't played through it" eBay things.  Weber 12F150B 25W.

[HotBluePlates]

I've got a '72 Silverface Deluxe Reverb with a plate voltage of 467.  The AC coming from my wall is 123V.

Schematic says the power transformer secondary is 330v (maybe with 110v input?). You have 123vac. Your measurements indicate 377vac input to the rectifier tube.

330 / 110v = 3
123v * 3 = 369v

Looks like high wall voltage, relative to the originally intended wall voltage, has voltages high throughout the amp.

2deaf's zener suggestion seems reasonable. A replacement power transformer may not be any better (if it's built to mimic the stock transformer), unless it has a primary expecting 120v+ (like a 125v tap) with a secondary of 330vac. A 110, 115 or 117v primary with a 330v secondary puts you right back where you are now.

[SILVERGUN]

Why aren't we recommending the 'vintage voltage' idea of a bucking transformer on the input? 
http://www.geofex.com/article_folders/vintvolt/vintvolt.htm
For $12 and a trip to radio shack he can drop a bunch of volts in about an hour.

mcasey, there are some simple mods that you can do to the amp to create more sag and soften the response.
Use the bucking transformer, replace your choke with a power resistor (or just put one in series with your choke), go with larger value screen resistors (what value is there now?), and you should notice a difference.

Double check the value of your "mid" resistor (the 6.8K that comes off of your bass pot to ground)
If you accidently used a 68K or 680K then you have lifted the tone stack and your not getting the lovely mid-scooped blackface softness.

[HotBluePlates]

Why aren't we recommending the 'vintage voltage' idea of a bucking transformer on the input? 

Yes, that would be a very good thing to try! Relatively cheap, non-invasive and directly addresses the root problem.

[mcasey1]

The 6.8K resistor is correct.  I'll have to make a trip to Radioshack soon and try the bucking transformer method.  Thanks for all the input guys, I will report back when I've had time to try something out.

[SILVERGUN]

I've used this method on several occasions with great success, and I've been lucky that every time I've needed one, the Radio Shack I walked into just happen to have one in stock.
 
Here's the one you'll want:
http://www.radioshack.com/12-6v-ct-3-0a-chassis-mount-transformer-with-leads/2731511.html#start=4
 
You'll definitely want to test it before running your amp into it to be sure that you didn't reverse the connections and wind up with a 'boost' transformer.

[mcasey1]

Radioshack was fresh out of transformers so I'll have to order something to drop the voltage.  What value of zener should I put on the center tap?  12v 5watt maybe?  Pair of them or just one?  Wouldn't there be a lot of heat to dissipate through the diode(s)?  I'm assuming the diode would be in between the center tap and ground.  I'm looking for the absolute simplest and most effective way to drop voltages across the amp (minus heaters) and the zener method seems easiest of all.  Do I need to heatsink?

I've read about using a wire wound resistor to accomplish the same thing.  Would it be placed on the center tap as well?

If the bucking transformer is the most effective method I'll have to order one!

Thanks for all the suggestions and Merry Christmas + Happy Holidays!

[DummyLoad]

you have very COLD bias: -46V where there should be -35V. that could explain the punchy sound as well. increasing the bias current will probably bring down the plate voltage significantly. your output tubes are probably running at near cut off.


what's the part number of the power transformer?


--pete

[mcasey1]

Its the Fender PT that came with the amp.  J025130 606-1-14 are the markings I can see. 

[sluckey]

Adjust the bias pot to give approx. -35vdc at pin 5 of BOTH 6V6s. (If the pot won't go low enough, replace the 470Ω resistor on the bias board with a larger resistor, maybe 560, 680, 820, or 1K.) Watch the 6V6s for any sign of red plating. What does the plate voltage read now?

[DummyLoad]

according to the back panel of a 72 deluxe, the wall power rating is 117V 60Hz. the PT crosses to a period correct part. sooo...sorry, but i've nothing more to contribute. :-\
5.13% higher wall power will be 452VDC at the OT supply. if i'm reading the schematic correctly, then 430VDC is the target B+ at 117VAC at the source.

--pete

[Ed_Chambley]

Sorry, been away a few days.

My 73 Super Reverb has basically the same voltages.  The 6L6GC will have no problem and the amp is brighter.  Silver SR's are brighter because of the voltage.  Easily fixed.

Get your bias correct for the voltage and it you want to get a better "blackface tone" increase you last dropping resistor to get the plate voltages on your preamp in the 170 range.  This is why it is really not possible to blackface these amps, but if you get the preamp voltages the same, you will simply have a little more clean headroom.  To lower the headroom simply disconnect the negative feedback wire from the board and the amp will breakup the same as a blackface.  It will also make the amp shimmer more.

You can also grab the PI values from the brown super and change to a 12Ax7 in the PI if you want it to be even more dirty sounding.

The price on early silver amps is still holding so keep everything you remove.