Q: Blackfacing a '69 SF Vibrolux Reverb?

[Humbucker]

I just got myself a 1969 Vibrolux Reverb and I was wondering if the two 47K plate load resistors on the phase inverter, (instead of the usual 82K/100K arrangement) is anything to concern myself with?

The amp sounds fine IMHO but I've heard some people comment that these lower value plate load resistors on the PI is a bad thing. I just put in a fresh GZ34 and the amp is pretty loud and clean when the volume is set at 5 or so but going beyond 6 you start to get more dirt, which sounds about right for a VR.

Also there's a pair of 100K bias feed resistors instead of the typical 220K/220K, these are fed by a bias balance circuit. I added an extra 20K of resistance in series with the stock 15K bias resistor because I needed to up the bias voltage, everything sounds great, I was just wondering about those two PI 47K plate load resistors though.

[stokes]

If you are satisfied with the sound,then there is no reason to modify it.People do the Blackface mod because they dont like the sound of the Silverface,and try to get it closer to the BF sound.If you are happy with the way the amp sounds,then there is nothing "bad" about the 47k plate resistors.A practical mod would be to change the bias scheme from a bias "balance" to a true bias control.

[Humbucker]

Please excuse my ignorance here but what's the difference between a silverface Vibrolux Reverb and one that's been blackfaced, soundwise?

I know that Fender was going for a cleaner sound during the SF era, is that why they went with lower plate load resistors in the phase inverter on these amps?

FWIW, I wouldn't want my Vibrolux Reverb any dirtier than it already is, it's pretty clean right up to 5 on the volume knob but quickly starts getting dirty beyond that. Would putting in 82K and 100K plate load resistors in the phase inverter make the amp any less clean? Or would it make the amp cleaner, with more headroom?

BTW, I took Bruce Collins' advice on leaving the bias balance circuit intact and following his advice, I was going to add a trimpot for bias level but I just ended up using a temporary external 50K pot and adjusted it until I got the bias voltage just right, I then installed a permanent resistor. I'm glad that I left the bias balance circuit alone, it's actually pretty handy, I now wish that I would have left the bias balance circuit alone in my Twin Reverb, oh well.

[hasserl]

I'd leave it alone as well, and I have on my sf Super Reverb. On the old Ampage there was a discussion about this very thing, Doc and Ray Ivers both opined that the 47k plate load resistors are better suited to the 12AT7 driver tube used for the PI; that 82k/100k values are better suited to a 12AX7 tube, not used in Fender amps. The higher values may cause an increase in voltage levels, but restrict current. The results are a dirtier OD, but according to a couple fellows that tried the different values it was more squashed and compressed, they all liked the lower values better.

In a different thread John Kelley Brown was experimenting with all the different PI arrangements in his Trans Atlantic amp, his findings were a little different, he prefered the higher values, in fact he went up to 100k/120k. But he might have used an AX7 in that amp. I thought I had a schematic of the Transatlantic but I can't find it in my files, and I can't find it online either. Seems like it used to be on the Blue Guitar website or on Keely's website, but neither one of them have it now. Maybe Kelley prefers not to have it out in public anymore.

[mimicthis]

hi
i have a bassman ten which has the bias balance control. i would really like to make it true bias but i do not think i am ready for that yet. i was wondering how i would be able to boost the mA into my tubes.
the guy below said he put more resisitors in series with the 15k pot resisitor to boost his mA

"I added an extra 20K of resistance in series with the stock 15K bias resistor because I needed to up the bias voltage"

here is the schematic
http://www.fenderholic.com/schem/bassman_10_schem.gif

i believe i see the 15k resistor attached to the 10kl balance pot. is this the resistor the other guy was speaking of?

my tubes are running at 29 mA right now. i would like to have them at 42 mA give or take a mA

thanks for the help
Greg

[Bruce / Mission Amps]

hi
i have a bassman ten which has the bias balance control. i would really like to make it true bias but i do not think i am ready for that yet. i was wondering how i would be able to boost the mA into my tubes.
the guy below said he put more resisitors in series with the 15k pot resisitor to boost his mA

Not sure I quite followed that but more resistance then the fixed grounded 15K will cause the bias voltage to go more negative and that will cool the tubes down with less ma of idle current....
Depending on the actual tubes and your plate voltage, you'd have to experiment with different resistors, but here's a mod I do to many of these amps:
Take the 15K resistor off and get a 22K trim pot, now solder a new 4.7K to 6K resistor in series with the 22K trim pot... (use only the center lug and one of the end lugs of the pot so it will act like a variable resistor).
Replace that 15K fixed resistor with the trim pot/resistor combo.
Now you have a variable resistor in series with a fixed resistor so that with the trim pot set to zero resistance, the whole mess is 4.7K to 6K and with the trim pot turned up to max resistance the total resistance is 26.7K to 28K.
That makes the bias supply "voltage variable" so you can set that bias voltage higher or lower to get the power tubes idling at a current level you want.
By the way 42ma is meaningless without knowing the plate voltage.
You need to read up a little on setting idle current and power tube bias before setting the idle current to some arbitrary value such as "42ma".
Typically in those Bassman 10 amps the power tubes sound very good at around 15- 17 watts each.
That 15-17 watts would be the measured idle current of the tubes times actual plate to cathode voltage.

[slidincharlie (Carlo P)]

I have a '79 Vibrolux Reverb.
I've never modded it to blackface specs (although I often think to do such mod).
This is what I did in my amp (leaving out some other minor mods):
- adjustable bias
- replace all tone and coupling caps with Mallory M150's, trying to get a mellower tone
- use power tubes according to my needs: Philips 6L6WGB for a runchier midrang-ish tone and earlier breakup; RCA 6L6GC for cleaner tone.

There is no end to experimenting, but for now I am perfectly happy with my Vibrolux.