Questions about blackfacing my Twin

Hamilton Barnard · Post by **Hamilton Barnard** » 8 Jul 2000 3:04 pm

I have one Marshall that came with 6550s (USA only) and since headroom is anything BUT what I want in those amps, I replaced them with EL34s.

My point, paralleling to what Dan said, is that the difference in headroom and gain in the 6550s (KT88) vs EL34 (6CA7)is huge.

Bill Terry · Post by **Bill Terry** » 9 Jul 2000 6:16 pm

Well I fiddled around with the Twin some more the last couple of days and I yanked the big cathode resistors. Seemed to tighten up the whole amp a bit, but not a noticeable change in gain or power that I could tell. Mostly just a more solid tone and a little less 'give' when you hit it hard. As Bob suggested, it's probably better for steel use with the Cathode bias gone.

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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<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Bill Terry on 09 July 2000 at 07:18 PM.]</p></FONT>

Hamilton Barnard · Post by **Hamilton Barnard** » 9 Jul 2000 9:04 pm

Bill, KT88s came in the "Pigs" and Marshall Majors, but were dropped from the rest of the line in favor of the 6550s because of their high cost.

Marshall was having trouble with the EL34s getting damaged during shipping, hence the reason for USA models receiving 6550s.

Dan Tyack · Post by **Dan Tyack** » 9 Jul 2000 9:35 pm

According to my tube source (who knows), the tubes that are sold as KT-88s are not the same as the tubes that were sold in the 60s (but then again nothing is). I do know that they are capable of very high output (90 watts for a pair) with very low distortion. You will not get this by re-biasing a twin, the amp will need to be designed to take advantage of this tube.

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 <name of film star deleted>).

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Bob Metzger · Post by **Bob Metzger** » 10 Jul 2000 12:51 am

Re-biasing can afford one lower distortion and more headroom, regardless of what tube is being used. KT-88's, as 6L6's have their characteristic sound, and it's a good sound for alot of musical instruments and Hi-Fi applications (including steel guitar).

Bob

Dan Tyack · Post by **Dan Tyack** » 10 Jul 2000 8:49 am

You are right, Bob, the KT-88s do have a great sound for steel. What I meant was that if you rebiased a Twin and put in 4 KT-88s, it probably wouldn't put out 180 watts, as my VHT amp does. The low end on this amp is awesome. The low C on the C6th sounds like the bottom C note on a Steinway.

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Bob Metzger · Post by **Bob Metzger** » 10 Jul 2000 11:30 am

A Fender Twin Reverb is not going to sound like a VHT amp but neither is a VHT amp going to sound exactly like a Twin Reverb; they both have their own sound and constituencies. But will using KT-88's or 6650's improve a Twin Reverb for steel guitar use? Absolutely. However, there are some severe technical details that must be addressed. A Twin Reverb cannot support the heater/filament current demands of these aforementioned tubes and YOU WILL DAMAGE YOUR POWER TRANSFORMER (the most expensive single part in the amp) if you drop them in without modification. This is tech work, not hobbyist stuff. The warning is necessary. If you have a Twin and it sounds ok to but want better for steel, consider what has been discussed here then take your Twin to a tech who has successfully performed this tube mod/swap before. Dan, most players know that if were as easy as just changing a few tubes that everyone would be walking around with 1000 watt Champ amps with more lows than the 1929 New York Stock Exchange.

Bob

Dan Tyack · Post by **Dan Tyack** » 10 Jul 2000 10:28 pm

Great advice, Bob. My only point about the VHT amp is that my tube guy here in Seattle tells me that you would need different output transformers and other parts to get the maximum power from these tubes (the high power KT-88s).

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....

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Bob Metzger · Post by **Bob Metzger** » 11 Jul 2000 12:53 am

Red was a great guy and a real great musician and quite the amp guru. He worked on my 5F6-A.
He understood big power in a small package and had the only red-tolexed Twin Reverb (from Leo). There's a good thread going now in Bar Chatter on Red.

I'm sure the Twin Reverb circuit can be looked at and re-designed to optimize any particular tube compliment and, of course, the output transformer was chosen with 6L6GC in mind. However, keeping in mind the number of units in the field and the present enthusiasm for vacuum tube devices, one could do much worse than a 6550 driven Twin Reverb for steel guitar. The point is that it's about tone, not watts.

Steve Hinson · Post by **Steve Hinson** » 11 Jul 2000 7:37 am

I have 2 Blackface Deluxe Reverb amps that Red Rhodes built for me...he changed both transformers(power and output)and the rectifier(from tube to solid-state).They were built to use 6550 power tubes and sounded awesome...back when you could still get GE 6550's.The Chinese and Russian versions just can't take the higher voltage they are subjected to(in these particular amps,700+ volts).GE 6550's can still be had at a premium price-I think I'm going to have these amps switched back to stock.I used them when I played guitar-didn't really care for them for steel.I think the preamp section was too sensitive-I could't get them clean enough to suit me.Best guitar tone I ever got,though!

Donny Hinson · Post by **Donny Hinson** » 11 Jul 2000 3:27 pm

Great thread, and some GREAT info guys! Thanks to Bob, and everyone else who's contributed to the "Twin-Mysteries". I may dig my old one out and try a couple more things!

Dan Tyack · Post by **Dan Tyack** » 11 Jul 2000 9:31 pm

Steve, don't modify those Deluxes yet, you should try these tubes they are selling as 'KT-88's. I can't remember the manufacturer, but they are increadible sounding.

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

I am trying to remember what I didn't like about the Red modified Princetons I used to hear. Seems to me it might have been the fault of the guitar players, not the amp guy. At least one of them that I used to hear had a Gauss 12", which has to be one of the harshest sounding speakers with a Fender.

Steve Hinson · Post by **Steve Hinson** » 12 Jul 2000 4:17 am

Dan-when Red first built these amps for me,he put Groove Tubes KT-88's in them-they didn't last very long at all.I don't know if it was because of poor quality or just because of the high plate voltage on the power tubes,but the GE 6550 seemed to work much better.Maybe you're right-I might try some of the newer tubes-I guess they could be better now.If I remember correctly,the GT tubes were Chinese-maybe the Russian versions are better.Thanks-Steve PS-I never liked those Gauss speakers either!

Hamilton Barnard · Post by **Hamilton Barnard** » 12 Jul 2000 6:40 am

Here is some KT88 info...uhhh...hold on to your wallet.

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

Kevin Post · Post by **Kevin Post** » 12 Jul 2000 4:56 pm

Bob,
Thanks for all the great info. Are the new Vibrasonics cathode biased? If not, can they be modded?

Hamilton Barnard · Post by **Hamilton Barnard** » 13 Jul 2000 6:34 am

Bob, I just received the Kevin O'Connor book (not cheap). Lot's of mods in there...except what we are talking about here. I was hoping for the schem on redoing silverface phase inverters and fixed vs balanced bias circuit.

Bob Metzger · Post by **Bob Metzger** » 13 Jul 2000 10:41 am

Hamilton,
Kevin O'Connor has 4 books that I know of. One I recommended is called 'The Ultimate Tone' which is in two volumes. In Volume One, there is a bias discussion (6-33 thru 6-35). In Volume Two there is a more complete discussion (3-13 thru 3-28). Phase inverter discussions in Vol 1 are found in sections 6-9 thru 6-16 but I think the exact thing you're looking for is covered step by step in 'A Desktop Reference Of Hip Vintage Guitar Amps' by Gerald Weber, pp 165-169. This is more of a how-to article whereas the O'Connor stuff presents more of the why and wherefors. The Weber book was mentioned on the first couple of posts on this very thread. My initial post on this thread was a critique of the Weber mod in terms of suitability and practicality for the steel guitarist. O'Connors descriptions are more theoretical but the material covers the same areas.

Bob

Bob Metzger · Post by **Bob Metzger** » 13 Jul 2000 10:51 am

Hamilton,
If you're stuck, email me and I'll walk you through it.

Bob

Bob Metzger · Post by **Bob Metzger** » 13 Jul 2000 1:17 pm

Kevin,
Any amp can be modded; the question is how easily. I haven't had my nose inside the new Vibrasonic (incidentally it's called 'Custom Vibrasonic Amp') so I don't know how it's manufactured but I bet it's a circuitboard amp which is generally more time consuming to mod than a point to point wired amp (Blackface and Silverface amps). The Custom Vibrasonic is a fixed bias amp; it is not cathode biased but could easily be converted by a competent amp tech to be cathode biased, although fixed biasing may produce a more appealing sound to the majority of steel guitarists.

More Fender Trivia/History: The first Fender Vibrasonic (5G13) was made in the early 60's, brown tolex, a single 15' speaker (usually a JBL), 2x 6L6GC power tubes, presence control, solid state rectifier and no reverb. It has the rare distinction of having it's front panel Volume control not adjacent to the input jacks (like almost evey other Fender amp) but 'two knobs down' between the 'speed' and 'treble' controls! It was superseded soon after by the 6G13-A, which changed to 2x 5881 power tubes and added an extra vibrato (tremelo) tube. These brown amps are not a bright as later Fenders and without reverb and two 2x 6L6GC power tube compliment, they are probably not the first choice amp for steel players. However, they are incredibly sought after as guitar amps and collectors pieces and undoubtedly trade for stratospheric prices due to their unique features, age, and how few of them were made.

Next came the Vibrosonic Reverb (silverface), notice no 'a' in the name, which is the'Twin Reverb with a 15" speaker' that has been mentioned before in this thread. With 4x 6L6GC, reverb and usually, a big orange JBL 15" speaker (does anyone know the letter designation of these orange JBL 130 15"speakers?) This was a good steel amp, a variation on the Twin Reverb theme. Does anyone have of of these without a master volume control; I've never seen one without that and that would, indeed, be rare!

Then came the 135 watt Vibrosonic Reverb in the mid/late 70's. (My files show 2/78 but I wouldn't bet my life on it). You can tell it by: 4 amp fuse, no choke, and line out/recording jack on the back panel. It has a ultra-linear taps on the output transformer and is very similar to the corresponding Twin Reverb of its era. Another good steel amp. This amp could be modded to use a choke for improved performance.

And, lastly, is the Custom Vibrasonic Amp (back to the'a' in the name), a blackface re-issue of sorts but not of any particular amp. It has 4x 6L6GC, reverb (in both channels!), no negative feedback and a 4 ohm 15" speaker. These amps actually have a channel marked 'steel' but, except for a little extra headroom in the preamp and a few bells ('sweet switch'), they bear a great resemblance to the usual Fender circuitry we seen since the beginning of the Blackface era (AB763). The 15" speaker is of no special pedigree. (I know how particular, or is that peculiar, steel players can be about their choice of speaker!) Has anyone put a Black Widow in one of these amps? Or any other speaker? Steel players, let's hear how you like these amps for steel!

Does anyone know the story on how the 'A' got changed to an 'O' and back again?

Bob

Bob Metzger · Post by **Bob Metzger** » 13 Jul 2000 3:26 pm

About cathode biasing, I found this on a bbs and I thought it applied to our discussion:

"Just yesterday, I took one of
my 2-EL84 amps with no negative feedback and a fixed/cathode bias
switch, adjusted the bias for equal plate currents, and asked a friend
of mine, who is one of the best guitarists I know, to do a blind A/B
test. He picked fixed bias *every* time as sounding the best, using
terms like "fuller", "better definition", "better tone", "my sound", etc.
Most people automatically assume an EL84 amp should be
cathode-biased, but sometimes fixed bias does indeed sound better."

The speaker is a knowledgeable amp builder named Randall Aikens.

Bob

Hamilton Barnard · Post by **Hamilton Barnard** » 13 Jul 2000 3:48 pm

The Custom "reissue" Vibrasonic was designed by the Custom Shop, but built on the regular production line with pcb. Adding the name "custom" was just more marketing, I suppose. To add what was said about the original Vibrasonic, it was Leo's first amp with the tone controls on the front.

I believe that Fender came out with their "master volume" gimmick in '72, which was near the time that they released the Vibrosonic. So I don't know if there is such a thing as a "none" master Vibrosonic.

In the new Vibrasonic, they used a 50 pF cap for the "sweet" switch instead of the normal 120pF bright cap. The speaker is the "heavy magnet" Eminence 4 ohm which sounds amazing with Tele, but is a bit brittle and lacking in the mids for steel guitar, IMO. For a 100 watt amp, that thing is not all that loud. I suspect they held the gain back a bit to keep 'er clean.

Bob, Gary Brandin came by today and he is still singing praises about his "Rick". On a Supro Thunderbolt, what would be the ballpark for the cathode current (6L6)?

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Make mine old...and country.

Bob Metzger · Post by **Bob Metzger** » 13 Jul 2000 4:20 pm

I have to know alot of stuff to allow for even an educated guess:
B+ voltage
Power tube plate voltage.
Power tube cathode voltage.
Power tube screen grid voltage. Screen resistors present? At what value and wattage rating?
What type of power tube compliment, in push pull configuration?
What type of phase inverter/splitter? With what plate voltages? What type of tube is used?
What is the value of the resistor on the cathodes of the power tubes now? What is it's wattage? Is it bypassed (by a cap)? Of what value and voltage?

I have to evaluate the whole power amp section, really.
It's a complex bouillabaisse. Want to bring it by? Or email me?

Bob

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Michael Brebes · Post by **Michael Brebes** » 14 Jul 2000 4:51 am

Regarding the cathode resistors, they are probably there in the mod to be able to get the fixed bias in the range of the negative voltage available from the power supply. From what I gather from your post, the mod is neither a true fixed-bias or cathode-bias design. One thing you definitely will hear, by removing the resistors, is a small level change which can sometimes be perceived as a better sound. The only way to match the gain levels in the test is to use electrolytic caps across the resistors because the gain is a function of the cathode/plate resistors and the plate voltage. By using the electrolytic the cathode resistor is bias only and doesn't factor into the gain. Also remember that removing the resistors changes the biasing on the amp, which will affect the tone. Are you rebiasing each time you do a different mod? It will change everytime.

Bill Terry · Post by **Bill Terry** » 15 Jul 2000 2:45 pm

Michael,
I think you're comment was directed at me, I can't tell this thread has wandered all over... but has been a wealth of information.

In my case, I re-biased after every modification, the last one being the removal of the cathode resistors. The actual bias voltage didn't change more than a volt or so from where it was before the resistors were removed. I've exchanged quite a few emails with Bob Metzger regarding the biasing subject, and have found a bias value that works for me. I think I ended up at about -48.5V.

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Jerry Hedge · Post by **Jerry Hedge** » 15 Jul 2000 10:30 pm

I use a "Blackfaced" Silverface Twin for steel and I love the tone!!! I've modded the preamp a bit for less distortion. I replaced the cathode resistor in the first gain stage with an 820 ohm without a bypass cap, hooked up a separate cathode resistor and bypass cap for the second gain stage (in the stock amp the second gain stage cathodes for both channels are tied together).I replaced the 12AX7 preamp tube with a 5751(the gain is between a 12AX7 and a 12AT7) and I use a 47pF cap for the bright switch (similar to a Deluxe Reverb). The second gain stage uses a .047uF coupling cap. The biggest improvement in my Twin came when I replaced the power supply caps with larger values.I also replaced the dropping resistors with smaller values so the preamp plates receive more voltage. The amp plays cleaner and has more bottom end. I also hooked the normal channel to the reverb so I have a stock channel for my Tele and a steel channel. It's like a twin 12 Custom Vibrasonic,but it's hand wired with a lot more bottom end depth because of the power supply mod.