Pre-Amp Tube Choice In Twin?

Brian McGaughey · From: Orcas Island, WA USA

I'm working to eliminate that "ice pickiness" high end that comes with hard pickin'. I wondered if I could reduce that quality by using a different preamp tube in my '76 Vibrosonic.

I'm a high end kinda guy as demonstrated by my use of BL 710s and a JBL K130. But I'm finding myself fiddling with the tone controls more often lately. When I dial it in to remove the "ice pick", it's not transparent anymore. I've got Electro Harmonix 12ax7s in there now.

I'm also wondering if it's time for tone caps. My tech said it would be due soon a couple years back. Could be that? I plan to take it in for new caps and am wondering about preamp tube choice.

Ideas? Thanks all.

John Groover McDuffie · From: LA California, USA

Your '76 Vibrosonic has a "bright" switch. Do you turn it on? In my experience a lot of what I call "ice pick" highs are emphasised in the tone when the bright switch is on. If you use the bright switch consider that the lower the volume control is set, the more pronounced the bright switch treble boost will be. If you roll the master volume down you can then raise the volume control and the bright boost will be less pronounced. Rolling the master volume down may also introduce a slight HF attenuation because of interaction in the ciruit. Also while the treble control and the bright switch are both similar HF boosts they effect the tone differently and are somewhat interactive. Playing with those 4 factors (bright on/off, volume setting, master volume setting, treble setting) can introduce more subtlety to adjusting the treble response of the amp.

Trying to fix a tone problem by substituting different brands of 12ax7 tube may be hit or miss. The "men of science" in the tube amp world seem to indicate that tonal control lies in circuit design. You may have more luck trying different capacitor values in the tone stack and bright circuit. There are also anode bypass capacitors that can roll off high end, if you want to add them. Is your amp stock? "Blackfacing" the phase inverter stage is reputed to make the amp sound "softer". (not very scientific wording I admit, but hey it's late!)

Changing the power tubes to a different type (ie different Groove Tubes hardness rating, 5881 vs 6L6, Sylvania type 6L6 vs GE type) might have more pronounced results on the overall tonality of the amp than different preamp tubes.

The speaker choice also makes a BIG difference, as you apparently are aware of already. After all, the speaker is the part that makes the sound we hear.
How about a Weber beam blocker http://www.webervst.com/blocker.html (or a piece of duct tape)? disclaimer - I haven't tried either one.

Brad Sarno · From: St. Louis, MO USA

Brian,

Instead of going to the preamp to address the ice-pick issue, I'd first focus more on what's happening up front. The BL710, under certain conditions, can be about the most brutally ice-picky pickup there is. It really doesn't like a high impedance load and prefers either a pot pedal or a buffered input with an adjustable impedance load (Black Box, Revelation, Tonic, Telonics VP, etc.) so you can tame the ice-picky treble peak right there at the pickup long before it ever reaches the preamp. I find that by addressing the treble peak issue right out of the gate, you then don't have to struggle to use the preamp to fix the problem, and then the preamp is freed up to shape tone for the stage, the speakers, the cabinet, and the overall situation.

If you have that notorious BL710 ice-pick issue happening, I don't think that just selecting various preamp tubes will necessarily address that. I say nip it in the bud, right there at the very first device the guitar plugs into, the first thing the pickup "sees". A LOT of pickup tone shaping happens right there, and primarily it's the strength or power of the pickup's inherent treble (resonant) peak that can be either boosted or attenuated.

And keep in mind that the BL710 is a tricky pickup to use for a lot of people. It's fairly aggressive and unforgiving with regards to pick dynamics. Lloyd Green doesn't seem to have a problem with it though. Notice he uses a plain old pot pedal with his, as the pickup was designed for.

Brad

Scott Swartz · From: St. Louis, MO

-New signal caps will likely be brighter than originals, so that is probably going in the wrong direction

-What is the distance from the strings to the 710? I would use 3/16 inch clearance for a 710

-The Fender reverb circuit has a treble boost starting around 3 kHz with increasing boost as the frequency rises

Two possible mods

1. You can clip the 10pf cap

2. Or, keep the 10pf cap and change the 3.3 meg to 1 meg and the 220k to 82k, thereby moving the treble boost up in frequency so its not in the icepick range, its more in the "air" or "sparkle" range.

If you know your way around tube amps the above will be clear, if its not you should have a tech do the work.
_________________
Scott Swartz
Steeltronics - Steel Guitar Pickups
www.steeltronics.com

Walter Killam · From: Nebraska, USA

Hi Brian,

I tried several preamp tubes in my ultralinear twin and finally settled on 12AY7 (Raytheon I think) that I salvaged from an old HiFi amp I pulled out of my neighbors trash! (I tried several other AX's AT's & AY's). At the time I was playing an acoustic electric 6 stringer with a peizo saddle PU. Moving to the lower gain tube helped with the harsh attack and what I think you are referring to as the "ice pick" qualities. I like the configuration with my stock Magnum SD10 also, now I dont have to kill treble to get a sound I like. Preamp tubes are easy to experiment with, get a few and try them out.
_________________
Mostly junque with a few knick-knacks that I really can't do without!

Brian McGaughey · From: Orcas Island, WA USA

Thank you for your responses fellows.

I'll have to experiment with turning down the master volume like you mention, John. I never use the bright switch at home for practice although I have been known to use it at gigs. The ice pick isn't as noticeable with the band and actually it may help the steel to not be buried without me being too loud.

Brad, it makes good sense to take a look at things ahead of amplification like you mention re the p/u's resonant frequency. I took your advice and did away with my goodrich L10K buffered vp and went back to a standard pot pedal. It did indeed help. In another guitar with a s/c p/u the L10K contributed positively to tone, but not with these components.

Scott, I've got the p/u about 1/4" if not 5/16ths below the strings. After I mounted it, I ran it all the way down within the first couple minutes. Thanks for the cap and resistor values for the tone mod. I won't do that myself but will print out this thread and take it to my tech with the amp when next I go in.

Walter, if I had access to some tubes without having to outlay cash I'd try a bunch, but otherwise I'll use reports I've seen on guitar amp pre-amp tube round ups to help me in choosing the right tubes, which if I hear you all correctly really isn't where I need to be working to fix the ice-pick anyway.

Brian

Ken Fox · From: Nashville GA USA

JBL speakers are infamous for the ice pick tone. I would look at a different speaker, or at least borrow one and do some comparisons.

Larry Robinson · From: Peachtree City, Georgia, USA

JBL speakers reproduce whatever sound is sent. If you can't get rid of the piercing highs without adjusting the presence, bass, mid, and treble controls, you might want to replace the JBL with one with less sensitivity and frequency response. Many steel players use bass speakers for this reason. I have used the "beam blocker" with the Weber Cali and didn't notice any quieting of the highs. I sold the Cali and went back to JBL. I would hesitate to start changing components in the amp unless they are defective. You might want to lower your PUP to decrease the signal.

Forum Index > Electronics		Next oldest topic :: Next newest topic

All times are GMT - 8 Hours	Jump to: