steve f

Myles, I have a Laney Supergroup Session 50 amp. It's been sitting for about 2 1/2 years and I'd like to bring it up slowly using a Staco variac I have. What is the procedure for doing this? It was checked out by a competent amp tech when I first got it and everything was fine, all resistors, caps, etc were ok, it's just been sitting for awhile.

It is not close to the Sylvania as it does not have crossing grid 1 and grid 2 design. It sounds different than a Sylvania or GE both of which sound much bigger and are much stronger. Myles, thanks for the heads up on the EH 6CA7. Another question. I'm going to be retubing a friend's Marshall DSL50. She doesn't do any type of regular maintenance on her amps, and it's hard to get them away from her to check them out. Basically she waits until one or more of the tubes wears out before replacing them. I read your piece on the phase inverter being the hardest working tube in an amp, and was wondering what you think of putting a NOS 12AT7 as the PI in a DSL50. I know someone with an old Mullard ECC81/12AT7 that is new in the original box that I can get pretty cheap. My friend likes the sound of her DSL50 a lot, and I don't want to do anything that would affect it too much, so, would a 12AT7 be appropriate for a DSL50 in your opinion? I'd like to put something in there that would last as long as possible.

Myles, I understand the Electro-Harmonix big bottle 6CA7 is supposed to be modeled after the original Sylvania big bottle 6CA7. What is your opinion on the EH 6CA7? Is it close to the Sylvania in sound?

Myles, I had questions about Shuguang 12AX7 tubes in amps with DC heaters. 1) Do you know if there are any problems using the 8th & 9th Gen Shuguang 12ax7s in these type amps? 2) Do you know the current draw of the Shuguang 12ax7s (is it in the normal range for 12ax7s?).

Myles, Are matched power tubes as important on cathode biased amps compared to other amps? I've got some nice tubes that are different by 6 to 7 milliamps as far as current draw, about 20%, and I'd like to use them on a couple of Carr amps if the bias circuitry in these amps will adjust individually to each tube. Will cathode biased amps normally adjust for each tube separately?

Myles, I've read in a couple of places where the Fender Bassman doesn't have a true bias adjustment. Can you provide a little info on this, and what do you recommend as far as current draw in tubes, colder, mid-range, hotter?

Myles, I'd like to hear your take on the JAN-Philips 6L6WGB tubes from the 80s. It's been said that these have a similar plate structure to 7581As, but are rated as a 30 watt tube, similar to a 6L6GC. Would you say they can used as a direct substitute for a 6L6GC? Any opinions as to life expectancy in amps with 450-500 plate volts compared to a 6L6GC?

Myles, I've got a few U.S. made 6V6GT tubes, what wattage are these considered to be for biasing purposes?

Myles, I have a tube tester that measures transconductance, which I understand is related to current output vs voltage input. Would a tube measuring higher in current output be better in the phase inverter position versus one of average output? I know that there's probably more to it than that, but just trying to keep it basic. My tester was professionally refurbished and recalibrated a few years back (by the late Chris Haedt), and it's the best I'm going to be able to do since I can't afford real professional gear. I'm just trying to get an idea of what tubes to use in what position relative to each other, not do professional grading etc.

Myles, Since you're on the topic of 30th Anniversary Marshalls. Would NOS Tung-Sol 5881s be ok in one of these? I know some of the older 6L6 class tubes weren't meant for higher plate voltages that modern amps have.

Myles, What's the scoop on the reproduction Mullard preamp tubes GT is developing?

Myles, I've got a NOS pair of the big bottle GE 6CA7s. Will these stand up to plate voltages in the 450-475 range?

Myles, I have a friend with a Dual Rectifier. She plugged it into two 4x12 cabs (Marshall 1960). She went from the two 4ohm connections on the amp to the 4 ohm jacks on the cabs. Of course this resulted in a 2 ohm load, which the amp isn't designed to handle. She said the sound level was very low and "distant". She unplugged after a few minutes and went back to using a single cab (16 ohm to 16 ohm) and the sound was fine. What is the possibility of damage to the amp? She's played the amp for several hours since then (using a single cab with the correct connection), and it seems fine. Is there anything that should be checked?

Myles, What's the advantage of running high plate voltage in an amp? It seems like all the modern high gain amps (Mesa Rectifiers, Peavey, Soldano) are up around 475-500 volts. I know that this isn't necessarily new, I've read about the old MusicMan & Ampeg amps that have even higher plate voltage, but it seems like the current amps all are going in this direction. Would it be possible to have a high gain, heavy sounding amp running something like 350v?

Myles, What are the differences in having a 12AT7 in the phase inverter position compared to a 12AX7? I know the 12AT7 has less gain than the 12AX7, but is gain important in this position?

Myles, How are those new Groove Tubes 6CA7 tubes coming along?

Myles, Assuming an amp with a quad of power tubes, outside tubes paired, inside tubes paired. Suppose I had 4 tubes, 2 reading 40ma and 2 reading 34ma at whatever plate voltage the amp had. If I paired them 40ma+34ma inside, 40ma+34ma outside so I'd have both pairs totaling the same, 74ma in the inside and outside positions, would there be any difference in this setup compared to 4 matched tubes, each at 37ma? In other words would this be a valid method to use when biasing this 4 power tube amp?

Myles, Just to doublecheck my memory, did you say the Groove Tubes KT66-HP should be biased as a 25 watt tube?

Myles, Any experience with Sovtek 6L6WXTs as far as reliability at higher plate voltages? I was told they're not reliable over 450 volts. I know Peavey ships them in their XXX & 5150 and I know Soldano uses the Sovtek 5881WXT in amps that have plate voltages around 500. I'd think the 6L6WXT would hold up at least as well as the 5881. I'm not really a fan of a lot of the Sovtek tubes, but I've been playing around with a Peavey XXX and these actually sound pretty good to my ears in the XXX.

Myles, Are the Groove Tubes KT-66HP considered 25watt or 30watt tubes (or something in between)? Just wondering about biasing them. I couldn't pass them up at the reduced price GT has them at.

Myles, I believe that you have commented on the fact that Marshall amps need to be biased a bit cold to retain the traditional Marshall tone. I'm going to be biasing a friends JCM 600 (2 EL 34s) shortly. Since I don't know the plate voltage yet I'll just speak to the static dissipation figures. Normally I'd figure on approx 17.5 - 18.0 watts for EL34s. How much below that would you recommend for a Marshall? I'm thinking of putting in either Svetlana EL34s or JJ E34Ls. I don't know if you have any recommendations as far as Svets vs JJs in master volume Marshalls, but feel free to state your preference if you have one. I'm just looking for good tone over the entire range, with tight bottom end. Also I think you may have referred to the JJ E34Ls as being a higher powered tube. Would you suggest adjusting upward for this when biasing?

Myles, thanks for giving us the benefit of your experience. I wanted to get your thoughts on the overall quality of Bogner amps (build quality, components used, etc), the Shiva in particular. I recently looked at a Soldano HotRod 100+ and wasn't overly impressed compared to my Rivera M100. It sounded fine, but boy, the M100 just seems to have much more attention to detail and the quality of components seems higher. I don't won't to invest a lot of time trying to track down a Shiva if it's not going to have the quality built in like my M100.