All of this is pretty straightforward, my question concerns the big honker 150 ohm / 7W resistors on the cathode pin in this particular Twin circuit (AC568 I think). As far as I can tell from the schematics I have, none of the other Twin Reverbs use cathode bias, any suggestions or experience on whether to leave these guys in or not? The Kendrick book does not mention these resistors but they are certainly not on the AB763 schematic (the holy grail of Twins ).

Seems that some people prefer the tone of the cathode biased amps, but you lose some power. I guess I could try both ways and see, just wondering if anybody else has tried it.

Thanks in advance,
Bill

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bterry.home.netcom.com

I think I'm just gonna yank the big resistors and ground the cathode as per AB763 and see what happens.

Bill

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bterry.home.netcom.com

[This message was edited by Bill Terry on 05 July 2000 at 07:14 AM.]

Quote:
my question concerns the big honker 150 ohm / 7W resistors on the cathode pin in this particular Twin circuit.
End quote:

Eventho I do not know a lot about the brand (of amps) you have or are trying to diddle with, I wonder about the cathode resistor....

1) At 7 watts rateing, it must be a really BIG device to dissipate that kind of power...

2) At 150 ohms, (and I assume the resistor is to ground) then there must be BIG (large) current flow across the tube, and the gain be very low.

3) as I think that the tubes in a amp is usually 12AX7's, 12AXU7's, 12AT7's or equal, (Excludeing the power tubes) I wonder if this resistor is the correct value????

The 12Axx tube series isn't designed for that kind of current flow and of course, depending upon the plate voltage and other peremiters(sp?), are generally devoted to gain ampification and EQ type of applications. 12AT7 and 12AU7 are often used as Current drivers for reverb tanks,phase inverters and such.

I belive the amp has been "HACKED" already and, at this point, isn't at the optimium performance level. Anything you do to it now,in order to enhance it's performance level should start at the beginning (read this as getting the original schmatics) and then restart your upgrade of this amp.

Again, this is only my opinion of this thread,and if you feel that I am wrong here, please feel free to e-mail me about it and I will delete this post as I do not wish to step on any toes.

Looks like nobody (musicians that is) liked the sound of the cathode bias versions and they (CBS) removed that mod shortly later (AC769? I think) because it doesn't show up in any of the later schematics.

Gerald Weber's point of reference for amp modding is centered in electric guitar playing, and if you've read any amount of what he's written, I'm sure you've come across the descriptions of the guitars sounds of the band ZZ Top. Now ZZ Top is a very good band but I'm not so sure that I'd like my steel guitar to sound like his electric guitar (not to say that the 'blackface mod' yields instant ZZ Top tone). So let's look at the mods and see what seems to apply to the steel guitar and what seems to pointed more toward getting the amp to'give it up' for electric guitar tone. There is a certain amount of compression distortion that Fender amps naturally do that I find somewhere between tolerable and desireable for steel but I usually like alot more of it on electric guitar, even if I'm playing country music. Alot of steel players don't like any of this, want it clean as a whistle, and we steel players have a word for this: Peavey. Cathode biasing only enhances this compreesion, singing quality of tube amps.

The Weber Blackface Mod seems to be divided into 4 areas: 1) Bias mods, 2) Phase inverter mods, 3) Power amp mods, and 4) Power supply mods. The first mod is so one can adjust the bias range rather than just balance the bias for the two sides of the push/pull circuit. This a good mod for steel, as you (or your tech) can then set your bias for a little cleaner response (and Gerald can set his for a little more hotter/overdriven response). Of course, the ideal circuit would have both a range control and a balance control but that's usually more work than most people want to undertake. (In my twin, I have 4 individual bias pots, one for each power tube but then again, no one ever said that sanity and tube amp design are mutually inclusive). There's alot that has been written about setting bias for tube amps so I wont repeat it here but this is one method to clean up the sound a bit for steel.

The phase inverter mod is designed to provide more drive to the output tubes and changing the input cap to the phase inverter which determines how many lows enter this stage. I would say that the verdict is still out on this portion of the blackface mod, from a steel guitarists standpoint. This is one area that you can say that the silverface amps are cleaner and that the blackface amps have more compression distortion. You might want to tweak values in between the Silverface and Blackface values, to get a little more drive but not quite as much as the blackface. Of course, if you've played thru a blackface and absolutely love it, then go for it. I, myself, would play the amp and then tweak a little, play some more, and re-tweak, over a period of a few months. It will become obvious, if one pays attention. But this silverface phase inverter circuit favors the steel guitar, if left stock; it's the last thing I would change. About the Phase Inverter input cap, the change to the .001 will make the amp a bit more 'shimmery' on top and the stock .01 will voice it a little lower (let thru a bit more mids/bass). C6 players might want to consider a .02 for this cap to fatten things up a bit. Me, I'm in the shimmery school.

Next, there are some caps on the power tubes designed to prevent oscillation and removal of these will increase upper level harmonics. The amp will seem to breathe more. I like them outta there but Fender amps have alot of highs so experimentation is the best thing again. In some instances, the amp will oscillate without them and without a lead dress mod, done by an expert, they could be necessary. Fender didn't want to have to deal with that so they appeared in all silverface amps, after a certain date. There is also a cap that bleeds some highs off the reverb. To remove or not; listen, then decide. The tremelo ticking cap is next (some tremelos make a ticking noise without it). Again, it's trial and error but I leave it in. On all four of these caps, you can disconnect one side only and leave them in the amp, neatly tucked away, not touching anything else. Please leave the ground side connected and disconnect the other side!

Lastly, the power supply mod will give more headroom and make the amp a very little brighter. I like it, however, if the amp seems extra harsh and midrangy, maybe you'll want to leave it (or only change the 2.2K to 1K for the phase inverter headroom). Weber recommends checking the filter caps at this juncture; excellent sound advice (pun intended). Also, if the 2-12 configuration isn't yielding enough lows in the amp, you could up the value of the first filter cap(s). I'd double or so what's in there (2x 70uf@350 volts change to 2x 220uf@300 volts).

Steel guitar pickups have more output than electric guitar pickups in many instances. You may want to pay attention to the preamp, first stage. You can lower the gain of the stage and create some additional headroom by raising the value of the the cathode resistor (from pin 3 of the channel in question) from 1.5K to 2.2K ~ 3.3K. I like 2.7K but this depends on how hot your pickup is. Also, the cathode bypass cap (25uf@25volts) located at the same pin could be increased for addition low end (to 100uf @25volts) but I have found that increasing the value of the first filter caps (previously mentioned) is a more effective methodology. Another headroom trick is to try a different preamp tube. You could try a lower gain 12AX7A (check with you tube amp guru/supplier) or more probably, a 12AY7 or a 12AU7 tube. The tone of the tube becomes an issue along with the headroom so be prepared for some critcal listening tests. The 12AY7 has less gain and the 12AUY has even less.

Having said all this crap, I'm sorry to inform all of you that probably the biggest voicing changes of all are contained in the choice of speakers and output tubes. I'm a confirmed 6550 fan for my twin but, even though the pinout is the same, they are not drop in replacements. You have to have a tech do this conversion work or risk burning out your power transformer; it's tricky as noise/hum is involved. 6550's draw alot of current and generate a ton of heat but sound glorious for steel guitar.

Many steel players will not get past the 2-12 configuration of the twin reverb and will want to play through a separate 15" cabinet. Some will think of changing the speaker/ baffleboard to a single 15 - There is a word (actually, two) for this also; it's called: Vibrosonic Reverb. It's basically a twin with a 15' speaker (and the proper output transformer). Sell the twin and look for the one true steel amp that Fender ever made.

Experiment away! Please report your findings to this outstanding forum, and if you have any questions, feel free to email me directly. Have any others come up with great ideas to make the Twin Reverb into an even greater steel guitar amp? Let's hear 'em!

I also intend to play guitar through this amp so I'm going to try to find a good compromise for both. I expect some trial and error but that's half the fun .

Once I've gotten a chance to try some of this stuff out I'll post the results.

Thanks.
Bill

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bterry.home.netcom.com

Addendum to my above post: When I said, "from pin 3 of the channel in question" I really meant: "from pin 3 of the preamp tube of the channel in question". Sorry for being imprecise; It's a bad idea when electricity is involved.

Good luck, and post your results,

In true low tech fashion I adjusted the bias voltage to -45 volts for a starting point and just went with that.

I also decided to leave the cathode resistors in for now, I'd really like to make a 'before and after' comparison with just that change.

The results were dramatic. I had used the amp with steel and was fairly pleased with the tone, although it had a little upper mid-range honk I couldn't quite lose. But, I had never gotten a guitar tone that seemed quite 'Fenderish' to me. It just seemed to lack the sparkle and depth that Fender Twins are known for.

After the mods I tried the amp with my '62 Strat and it had really opened up, the missing high end was back and the reverb seemed noticeably brighter and smoother, in fact I turned it down a notch or two from where I had been setting it. Fortunately, there didn't seem to be any tendency for oscillation after removing the 2000pF caps.

Guitar players will know what I mean when I say it now had that bite that you get when a tube amp is just beginning to compress a little, nothing like it in the solid state world IMO. I could never quite seem to get that before the mods but it's definitely there now.

Regarding steel: I didn't get a chance to play my Emmons through it yet, I was so knocked out with the improved guitar tone I couldn't put it down , it was a TOTALLY different amp. I'll try the steel this weekend, I have a low volume gig where I think I can use it, but I expect I'll like it better. I may end up having to take Bob's suggestion and back off of the drive in the phase inverter for steel, but I'm going to give it a go first.

Overall at this point, I'd give the BF mod the thumbs up for sure. After I play it a while and get a feel for it, I'll probably remove the cathode resistors and re-evauluate. Stay tuned......

Bob, how much work to put reverb on both channels? That sounds like a great idea, I always thought they should have done that from the factory.

Thanks,
Bill

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bterry.home.netcom.com

I think the above posts say it all !!!!

They covered any and all that I might have responded to your e-mail to me.

Bill

[This message was edited by Bill Crook on 06 July 2000 at 09:02 AM.]

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Ricky Davis
http://hometown.aol.com/sshawaiian/RickyHomepage.html
http://users.interlinks.net/rebel/steel/steel.html
sshawaiian@aol.com

Glad to hear the BF conversion worked out. It sounds like it is a really worthwhile thing to do. I have always thought that the old Fender amps could make good steel amps, just based on all of the classic recordings made with them.

You sound like you have a little background in electronics already. So just a couple of thoughts......

If the power tube cathodes have resistors to ground, you can use these to make rebiasing the amp easier by measuring the current through them. You could probably also change them to 1 ohm or 10 ohm to make the readings easier to calculate in your head than the 150 ohms that are in there now.

I believe that adding reverb to both channels is a simple mod, something like two resistors. Also, there is a way to take the output of the reverb tank and send it to the "normal" channel. By doing this you will pick up a mix control and tone controls for the reverb. I've never heard it so I can't say how it sounds. I know that Gerald Weber has written up how to do this, but I don't know if it's in the first book or not.

Regarding the bias adjustment, I think I read somewhere (might have been in the Weber book) of a similar method. I talked with one of his guys the other day and they actually sell a kit with connectors that are mounted on the back of the chassis, you just plug in a meter and make the adjustment. No need to remove the chassis. Pretty slick if you don't mind drilling holes.

Yo Ricky,
I'm going to try to get the reverb to the normal channel 'how to' from Bob Metzger, (thanks Bob...) who has graciously offered to fax me the info. I think we can work something out.... I'm thinking I need free steel guitar lessons, an oil change and my house painted. How's that sound Hey, I quit my band, you quit yours, what else do we have to do? I'll give you a call.

BT

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bterry.home.netcom.com

Anyway, what are your thoughts on the power tube cathodes being lifted above ground
(silverface). I think that "bias balancing" is a bad setup for yet another reason, we are now forced to buy European and Russian tubes (not talking NOS). I had a silverface Vibrosonic over here to set the bias after a fella bought some new Svetlana 6L6s. I could not get the current under 50 mA.

Afterward, I got to thinking that the reason for this might be that these amps were manufactured with American 6L6s and the import tubes obviously now require less resistance in the bias circuit and/or more negative voltage to the grids.

Your thoughts.

We are undergoing a tube renaissance right now. This is a great time to have a tube amp. Is it the best time that there's ever been for tube amps? Who cares! Let the historians argue that. There are a number of good tubes to use now and it is increasing all the time, after years of junk new tubes. People are caring about this again and there's money to be made by the capitalists (ironically, sometimes in conjunction with the Russians or the Chinese!) The problem, which I run into all the time, is people say stuff like, "I hate all Russian tubes" or "Such and Such Brand didn't sound good in my buddy's Floating Flatus!" (No tube made can make a Floating Flatus sound good; doesn't he know that???) Different tubes sound different in different circuits. I have about 30 or so tube amps, and I'm pretty happy with all the selection of tubes available (now that there appears to be a reasonable 6V6 being made). Of course I'd love some more but considering the technology, I think we're really lucky. Fifteen to twelve years ago, I wasn't so optomistic about the future of tube amps/tube technology. Thank Hank I was wrong. Good tubes are out there; look harder, read more!

About Biasing/bias circuitry: in my above post in this thread I have stated my biasing preference.
When I work on any tube amp, biasing is one of the first things I look at. And it's that last thing I look at before the amp goes back in the wood. If I know a guy likes to tweak his own bias, I'll design his bias circuit so he can't hurt his amp or power tubes and so he can put it where he thinks it sound best to him. If you have only one bias control, I think having a bias range control is perferable but I think having a balance control and a range control is better.

It's hard for me to say what going on with your Vibrosonic, as it isn't in front of me, but when you say,"could not get the current under 50 mA", do you mean 50 ma per tube or 50ma per side of the push -pull circuit (these are two very different conditions)? If that is per side of the circuit, and there are two power tubes per side (a Vibrosonic has 4x 6L6GC) you would be averaging 25ma per tube and that's probably ok, depending on what your plate voltage is, ect, ect. Now if you can't get the current less than 50ma per tube, you'll have to redesign you bias circuit which is basically Tube Electronics 101. May I recommend Kevin O'Connor's excellent book, "The Ultimate Tone".

Lifting those cathodes above ground sound like heavy work; Don't do it too long or your arms will get tired. Seriously, if you want to experiment with cathode biasing, take your amp to a qualified tech who knows about this and have it correctly set up. It's much less invasive than a tattoo and can be easily reversed to it's stock condition. I just think that most steel players would prefer the sound of fixed bias, if one were commited to a tube steel amp. As I 've said before, if you're looking for some kind of tube magic for steel guitar, try 6550's!

As far as Fender Silverface amps with the combined cathode/fixed bias circuit, most people prefer the sound of these amps when they've been Blackfaced (has this discussion gone full circle, or what?) but, as I like to keep this tube amp thing in a steel guitarists' perspective, I guess we'll have to ask the people who have played these combined bias amps with their steels and did they like them better or not once the amps were converted to fixed bias only? (as you'll recall, Bill Terry, who started this thread, has, for the time being, left the cathode resistors in his amp, making it, still, a combined bias amp). I don't believe I've ever used a combined bias Fender for steel guitar, so I'm not really qualified to comment.

On my buddy's Vibrosonic, I was using a bias probe reading one at a time and each tube was drawing 50 mA of current. That circuit has since been changed to fixed bias.

I'm in agreement, there are very good tubes to be bought now. However, I am going to miss Ei 12AX7s. Those things wailed in Marshalls.

I have no interest in experimenting with cathode biasing. To me, the wheel has already been perfectly invented. My favorite amps, black face Fenders and anything valve Marshall, are the benchmarks.

Did Fender raise Twin Reverb output from 80 watts to 100 watts or was it that the style of music being played on these amps was changing. It called for alot of volume, turned up, cranked out distortion which was beyond anything Leo envisioned when the amps were designed and tested. Certainly, those CBS suits didn't envision what was happening in music even though the other hand (Clive Davis) was snatching up anything that had a distorted guitar and a pulse. There were alot of returns on blown speakers under warranty. CBS hated that, hence, the infamous "Paley Mod". I don't think you'll find anything in the documentation to support the Twin Reverb (or Showman) jumping from 80 watts to 100 watts, i.e., same power/output transformers, same circuit w/ some minor changes. The "Paley Mod" was the first full line change to most Fender amps since AB763. Some plate voltages were increased on certain amps in the Silverface era (phase inverter) but after or at the same time as the combined bias circuit. (Most notable exception : the Bassman, but that's a whole different story, and it's not really a steel amp, anyway).

Bob

Tone-wise this Twin is really singing now, KILLER tone.... total investment at this point is about $3.00 and a little time. Highly recommended mods IMO. Best thing is it still works great for guitar too.

I mentioned an early 70's 50W Marshall I had in your other thread Bob. If I remember it came with KT88's stock. In fact I had two of those amps at different times and both had KT88s. Did Marshall ship those models both ways? (6550 or KT88, for some reason I thought there was a difference) I remember vividly because at the time KT88's were pretty expensive compared to the 6L6s all the Fender amps were using.

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bterry.home.netcom.com

[This message was edited by Bill Terry on 09 July 2000 at 07:18 PM.]

I will add the following observation. I think that 99% of steel players would prefer the amp with the KT-88s. However, for most gigs I prefer the amp with El34s. I do a lot of blues and rock and roll, and for this the EL34s kick it. I prefer the KT-88 based amp for strictly country or Jazz gigs. And if I were offered a gig with Megadeath, I would bring the KT-88 based amp (the bottom end of this amp is bigger than ).

Hey Bob, are you old enough to remember those Princetons that Red Rhodes modified that were running around LA in the 70s? Talk about small and loud. But the ones I heard didn't sound that great....

I agree with you on many of the 6550s on the market, some of them are a little harsh.

http://www.newtube.com/
http://store.yahoo.com/thetubestore/kt8890100types.html