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Topic: Tubes for Vibrosonic |
Danny Naccarato
From:
Burleson, Texas
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Posted 27 Nov 2002 7:58 am |
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Been doing a bunch of boning up on this amp. I'm thinking of replacing the tubes and have read much about it. Seems the Jan Phillps 7581's would be a bit tougher, but cleaner (increased headroom) than the Svet 6L6GC's. Any thoughts??? Got a K-130 on the way, to replace the EV....
Danny |
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Ken Fox
From:
Nashville GA USA
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Posted 27 Nov 2002 8:57 am
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| JJ/Telsa 6L6GC are a great and tough tube. Also their preamp tubes are great. I have been using both in my repairs as of late. Just finished an output transformer in a 135 watt Twin and put in the JJ tubes, they were great! I would avoid Sovtek 6L6WXT+ tubes, in the past they had a high failure rate. This may have been resolved by now, but I have heard no claims to that as of yet. We had a long discussion about their reliablibity on the Fender Forum just recently! I was amazed at the number of people who reported problems with that particular tube. Stay away from current production 5881 tubes as thay do not have the plate dissipation wattage and voltage capability that your amp needs. The Twins and Silverfave Vibrosonics have a plate voltage close to 500 WVDC and are tough on power tubes. |
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Danny Naccarato
From:
Burleson, Texas
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Posted 27 Nov 2002 9:12 am
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Thanks Ken. Any idea how they compare to the 7581A? I checked out their price (Telsa) and they are real reasonable. Also, thinking about the 12AT7 vs. the AX7 for the preamp tube(s). From what I've read, they lower the gain, resulting in less breakup. Am I reading that right? I'd like to be able to get as much headroom as possible out of this amp (1980 Vibosonic), with absolute minimum breakup......
Thanks inadvance,
Danny |
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Ken Fox
From:
Nashville GA USA
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Posted 27 Nov 2002 10:15 am
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| The 12AT7 has a gain of 70 and the 12AX7 has a gain of 100. You want the max gain and headroom, stay with then 12AX7 in position V1,V2,V4. Leave the inverter as a 12AT7 (V6, next to power tubes). You have the infamous 135 watt model amp, if it is a 1980. Be careful to get a power tube that can handle 500 volts of plate voltage. I am unfamiliar with the other power tube you are referring to. I'll try to do a little research on it. |
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Ken Fox
From:
Nashville GA USA
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Posted 27 Nov 2002 10:35 am
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I found the following info:
7581A/6L6-GC Philips ECG/Sylvania (USA). 7581A is the same as 6L6-GC but has additional ratings for use as a modulator tube. Same tone and quality as the famed Sylvania/Philips "STR387" 6L6-GC tube that were used by Mesa and Fender in the 70's & 80's.
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Ken Fox
From:
Nashville GA USA
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Posted 27 Nov 2002 10:42 am
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Here is a link to the tube date sheet for a 7581A. Looks like it will do the job well. It can handle 35 watts plate dissipation and 500 WVDC in the pentode application. I have an old GE tube manual and it touts the same specs!
http://www.audiohot.com/frank/sheets/135/7/7581A.pdf [This message was edited by Ken Fox on 27 November 2002 at 10:43 AM.] [This message was edited by Ken Fox on 27 November 2002 at 05:55 PM.] |
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Danny Naccarato
From:
Burleson, Texas
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Posted 3 Dec 2002 7:22 am
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I read an article where it stated if you don't use Channel 1, you can leave out the tube in V1, resulting in some gain in channel 2. Also, I read that there is a bit of gain by changing V6 to an AX7, from an AT7. Any thoughts on this?
Thank sgain,
Danny |
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Ken Fox
From:
Nashville GA USA
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Posted 3 Dec 2002 8:35 am
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| The voltage supply is unregulated in the Fender tube amps of that era. Removing V1 reduces B+ load and causes an increase in plate voltage across the board. From what I have been reading lately, the Silverface amps had their gain reduced at V6 in comparison to the Blackface amps. This done by changing the plate load resistors to 47K as opposed the original higher values in the Blackface amps. The gain of a 12AX7 is 100 and the gain of the 12A77 is 70. The 135 watt amps puts out as much power as the 6L6GC tubes are already rated for. I have seen this on my bench using a scope and sine wave generator. I would suspect that a 12AX7 in V6 position (inverter or technically a long tailed pair) would push the amp towards power tube distortion. Increasing the plate voltages will also tend to change the load line of all the class A preamp tubes, hence possibly preamp distortion as well. The mods you are speaking of are favorites amongst guitar players striving for more warm and distorted sound. An interesting note, the 90's Vibrasonic had a steel channel. The big difference in it was reduced gain, by eliminating the cathode bypass cap in the preamp stage! This was in fact an attempt in fact to reduce the stage gain, to compensate for the high output of the steel guitar pickup! One of my favorite mods for 135 watt Twins is to install a bias control, as the power tubes tend to run a bit cold in the Silverface amps. Proper bias current will tighten up the Bass in the amp and clean up the sound considerably. Typically these amps are running in the low 20 millampere range! With a plate voltage of 495 volts, I find about 32 millamperes is best. I remove the 33K bias resistor off of the back of the power tube balance pot and replace it (epoxy in place) with a 15 turn, 10K cermet pot and a series connected 27K resistor. That way I can adjsut the bias range up and dowm from the original 33K resistor value that was there. That way you still have power tube balance as well! You can really tweek the tubes right on the money with both a balance and bias adjust. [This message was edited by Ken Fox on 03 December 2002 at 08:37 AM.] |
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Danny Naccarato
From:
Burleson, Texas
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Posted 3 Dec 2002 8:39 am
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| Great info. Thanks much Ken. |
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J Fletcher
From:
London,Ont,Canada
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Posted 4 Dec 2002 7:44 am
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I think the trick of pulling the first preamp tube to increase gain works because V1 and V2 share a common 820 ohm cathode resistor,in a lot of 60's and 70's Fenders.
If you pull V1, the current through this resistor is halved, so the voltage drop and therefore the bias voltage also drops. This bias voltage change causes an increase in gain for V2. The current draw of V1 is about 1 ma, so pulling it isn't going to make much difference to the plate voltage. You need a large plate voltage increase to see any change in gain, but a small grid to cathode voltage change results in a large increase in gain, and current....Jerry |
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Ken Fox
From:
Nashville GA USA
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Posted 4 Dec 2002 8:30 am
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| Thanks and yes you are right there, my mistaske. Most l;ikely there would only be a small gain in B+, the more I thunked 'bout it! I read in Gerald Webers books about changing that common cathode resistor out to two each 1500 ohm resistors. That would be a good mod to increase gain there. I do not think you need much more gain with a pedal steel, but it is agood mod for the guitar player. Another nice mod is to add reverb to both channels, as they did in the 90's Vibrasonic. |
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Bob Metzger
From:
Waltham (Boston), MA, USA
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Posted 4 Dec 2002 4:16 pm
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I have a early 70's Fender Vibrosonic Reverb amp and I like 6550s best of all in this amp when I'm using it for steel guitar. Due to filament demands, you must run an aux heater transformer but that's not difficult to accomplish. The ultra clean and ultra full sound, tons of headroom plus the 6550 tone is well worth it.
I recommend you stick with 12AX7As where they are now used in you amp and you can create additional headroom from the stock circuit by manipulating the cathode resistor of the preamp stage in question. Many techs feel that a 1K unbypassed is the way to go, depending on what type of pickup you're using (and the amount of output it has) on your 1st gain stage.
Removing the preamp tube from the channel you're not using will lessen the heater current demands on the power transformer but will at the same time increase gain on the preamp you are using (for reasons explained above) and lessen headroom as well. It's a trick guitarists like to get the amp to give it up easier but a bad idea for steel player looking for a cleaner sound. Remember, gain can be the enemy of headroom.
If you have a silverface Vibrosonic Reverb, you'll want to look at the phase inverter and possibly, change it to the more music friendly blackface configuration. When I do this on a steel amp, I sometimes increase the tail resistor for additional headroom. Most steel players who try both SF and BF PI circuits feel the the blackface version sounds better for both guitar and steel, YMMV.
Bob M.[This message was edited by Bob Metzger on 05 December 2002 at 02:37 AM.] |
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