I need to know something....

[JKD]

Is it possible there's something wrong with the amp load selector switch...maybe a short or something?

I've heard these are quite fragile...am wondering if it's not set on the load/tap you think it is so the amp heats up/eats the tubes...

 

[Julia]

Is it possible there's something wrong with the amp load selector switch...maybe a short or something?

I've heard these are quite fragile...am wondering if it's not set on the load/tap you think it is so the amp heats up/eats the tubes...

Is this the impedance selector switch? But wouldn't it do that all the time then? Would it take 3 months to eat a set of tubes or would they go quicker than that?

We have one Randall repair shop in the state. It's an all day event to go there and back. If it's intermittent it'll be impossible to diagnose. And to get a shop to actually replace it or do anything but plug it in, replace the tubes, and charge me for tubes and a bias and send me home after keeping the amp for a month would be a worse experience than having teeth pulled. If they find something they'll fix it. If they don't I get charged $100. Basically service in this area sucks unless you have one particular brand of amplifiers that are made about 90 minutes from here.

I am coming to the conclusion that I need to learn how to repair amps.

 

[JKD]

Is it possible there's something wrong with the amp load selector switch...maybe a short or something?

I've heard these are quite fragile...am wondering if it's not set on the load/tap you think it is so the amp heats up/eats the tubes...

Is this the impedance selector switch? But wouldn't it do that all the time then? Would it take 3 months to eat a set of tubes or would they go quicker than that?

We have one Randall repair shop in the state. It's an all day event to go there and back. If it's intermittent it'll be impossible to diagnose. And to get a shop to actually replace it or do anything but plug it in, replace the tubes, and charge me for tubes and a bias and send me home after keeping the amp for a month would be a worse experience than having teeth pulled. If they find something they'll fix it. If they don't I get charged $100. Basically service in this area sucks unless you have one particular brand of amplifiers that are made about 90 minutes from here.

I am coming to the conclusion that I need to learn how to repair amps.

Yeah...the impedance switch...no idea on the tubes tho'

AMEN to your last sentence though

 

[Julia]

I'm supposed to pull the amp out and inspect the board visually for any burn marks. I doubt I have any on the board but you never know. Other than this I'll just periodically check the bias on it just to see if it's drifting hot and adjust to even it out.

The amp did get a few good bumps during initial shipping. I wish I'd brought my camera to take a photo of the box when it arrived at the store. And the E34Ls probably took a good shot or two or three. Gotta love UPS. They've got this guy working there -- caught on video:

UPS Amp Handling Department

 

[Julia]

I pulled the amp out of the chassis yesterday to look for possible scorch marks on the PCB around the power valves sockets. None. Clean. The only discoloration seems to be some surface dirt around the preamp valve nearest the guitar input jack.

This seems to indicate that I've just been very unlucky with tubes. Alas, I'm going to take advantage of the design of this amp and check bias about every couple weeks just to make sure that the tubes don't start drifting too much. Kind of like checking the oil in your car periodically. And how long does this take to do? Warm up for 20 minutes, then check? At least you don't have to pull the amp out of the chassis every time like you do with most amps.

Just another thing to do.

But just a little question here.....

I've been doing a little "math" and it seems like when you apply the transformer plate voltage numbers and do the formula, in the Randall manual they consider the 6L6/5881 a 25W max plate dissipation (mpd) tube , and the EL34 as a 30W mpd, and everywhere I've read considers EL34s a 25W mpd and the 6L6 a 30W mpd. So is this a misprint in the manual? Or wrong everywhere else?

 

[kc2eeb]

Julia
A great book you can download at http://136.159.225.44/GE1951.html
Title: GE Essential Characteristics is a great reference.
Also, RCA RC-30 tube manual, the last in the series has great theory.
Go to the JJ tubes website. All the data is there for plate dissipation (graphs) at various voltages.
Jack Darr's Guitar amp repair book has been reprinted.
Any of the Kendrick, (Gerald Weber) books or videos, and Barnes and Noble has a very inexpensive reprint of the Navy's Basic Electronics book.
Also, try building a tube amp kit or find a really beat Fender tube amp and restore it.

 

[Julia]

Thanks for the link and the resources. And I went to

http://www.svetlana-tubes.com/svetlana.htm for the info on the 6550C.

I'll bookmark them. So far everything looks consistent with the numbers I've gotten.

Randall Amps says the transformer puts out 530V. So I did the calculations from there and I got a range of 33 mV to 46.2 mV. The manual gives 35 mV - 45 mV for the tube. I'll be biasing at 40 mV anyway for a 60% value just to start and give a little wiggle room for the burn in.

The 6550s are in. Damn. Now this is some serious glass. I had to remove the bear trap brackets since the base is too big and the fact that the tubes are a metal base which means it could cause a short. I think Eurotubes has something in stock for their KT66s and KT88s that would fit.

Now Ruby Tubes goes and tests them pretty well, but Doug's Tubes tested them again, and wrote down the actual matching numbers.

And speaking of KT88s. I don't know how you guys get them to fit. They'd have maybe 1/2" between the glass?

I've thought about building an amp kit at one point. Maybe I'll get a kit for a JCM 800 2204 since that's one I'd use and it's a MV amp. Ceriatone makes a decent clone kit but the shipping is a killer from Singapore, and then there are the Weber kits for the 1987 or JTM45.

 

[Rod]

Julia, It sounds like you've just had some bad mojo with tubes...Granted, the JVM's can be POS, but not the Lynchbox...FWIW,,,amp kits can be a bag of worms gerttng them to sound correct sometimes....1 wire out of place, and the amp develops high pitched squeals, or feedback...or it just may sound crappy when you're done...then what do you do??? .The Ceriatones aren't bad, but they're not particulary great amps....Certainly not in the same tone park as a Lynchbox...you'd kind of be going backwards...just my opinion.....FWIW...I've been using JJ 6L6's the past few years for gigs and touring because of the issues the newer EL34's have.....Haven't had 1 failure in 5 years

 

[Julia]

No I'm not complaining about tone. I've got the tone with the LB. It's killer. The amp is quiet. No squealies. The 6550Cs in it now are killer tonewise. The SL+ module sounds like a bloody chainsaw. I'm keeping an eye on the bias and monitoring it about once a week just to make sure I don't have one socket where the trimpot isn't holding.

The idea of Ceriatone or Weber kit was just to build an amp. Not for replacing the LB. To be honest I don't have the money to do this right now. Probably not the patience either.

I think I've just had some bad luck. If the numbers are right, the E34Ls were biased way hot in the beginning -- at least acc'd to the plate dissipation numbers from JJ. But the 6L6GEs should have been fine, although Eurotubes sources have indicated that they have a pretty high failure rate. SED is supposed to make some good glass, so I'm hoping I have some good luck with the 6550C's. I'll burn them in through 10/1 then rebias back to 60%.

 

[Rod]

Those 6550's are pretty rugged tubes....Originally used in military radar amps....I think you'll find they hold up well for you.....Good luck from the East coast!!

 

[aeroic]

No I'm not complaining about tone. I've got the tone with the LB. It's killer. The amp is quiet. No squealies. The 6550Cs in it now are killer tonewise. The SL+ module sounds like a bloody chainsaw. I'm keeping an eye on the bias and monitoring it about once a week just to make sure I don't have one socket where the trimpot isn't holding.

The idea of Ceriatone or Weber kit was just to build an amp. Not for replacing the LB. To be honest I don't have the money to do this right now. Probably not the patience either.

I think I've just had some bad luck. If the numbers are right, the E34Ls were biased way hot in the beginning -- at least acc'd to the plate dissipation numbers from JJ. But the 6L6GEs should have been fine, although Eurotubes sources have indicated that they have a pretty high failure rate. SED is supposed to make some good glass, so I'm hoping I have some good luck with the 6550C's. I'll burn them in through 10/1 then rebias back to 60%.

I'll tell ya man...you'll love those 6550s even more at 70%. Running them at 47-48 is the sweet spot! Just tried mine cold...and albiet great tone...70% was even better!

 

[JKD]

My 6550Cs sound great anywhere from 38 to around 50...things start to get a little mushy above 50 tho

 

[aeroic]

My 6550Cs sound great anywhere from 38 to around 50...things start to get a little mushy above 50 tho

agreed above 50 also. highest I go is 48mV...which between 46-47 is my personal sweet spot!

 

[Julia]

Yeah, but 50 mV on a RM100 is pushing 80%. I don't have Cesar Diaz around to keep my amps running.

46.2 mV is 70% according to the information from SED on these tubes, and Randall on the transformer.

I initially biased at 60% at around 40 mV. They've dropped to around 37 mV during the "cook-in". I'll bring them back up 40 mV around 10/1. I'm not pushing them harder since I want to see if I can get a quad of tubes to last more than 100 days.