6L6 Replacements?

Dan Robinson · From: Colorado, USA

The power tubes in my Fender Hot Rod Deluxe have become microphonic. Sometimes the amp breaks into a nasty parasitic oscillation. It settles down after it gets hot, but it's getting worse. Purchased new in 96 or 97, so I think it's time for a new pair of 6L6 tubes.

Looking for a recommendation what to buy. Hopefully not too spendy, but this will be my working amp. Need them soon. I have a gig June 2nd. Just enough time to get the new pair, install them and set bias

Chris Johnson · From: USA

Get a set of JJ's from eurotubes and you'll be set.

Steven Paris · From: Los Angeles

Here's a nice chart for comparing 6L6s:
http://www.thetubestore.com/Resources/Product-Reviews/6L6-Tube-Reviews-Chart
There's also a buncha reviews on different brands on this website. Read and come to your own conclusions.
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Emmons & Peavey

Dave Meis · From: Olympic Peninsula, Washington, USA

I've had good luck with the Tung Sol 6L6GC STRs and the 7581As (large bottle..make sure you have room), and the Svetlanas in my SF Twin.

ajm · From: Los Angeles

Are you sure it's the power tubes?
Have you tried swapping them out?

Keep in mind that swapping the power tubes should also include a rebiasing to be safe.

If you are gigging with this amp and you don't have any spares as backups, you need a pair anyway. But, if you haven't tried replacing them, this still may not be the problem.

Tim Whitlock · From: Colorado, USA

I went to Lord Valve here in Denver to buy an expensive set of Winged C's for my Twin. He actually downsold me a set of Rubys. They proved to be good sounding, reliable and affordable.

That being said, I'd be willing to bet the oscillation is coming from elsewhere in the circuit.

Dan Robinson · From: Colorado, USA

Thanks for the replies.

This may be a case of "necessary but insufficient." I don't KNOW that replacing the power tubes will fix everything, but something is definitely amiss with them.

As ajm points out, I'm depending on this amp for gigs, so best to have spares anyway. I ordered a pair of JJ/6L6GCs from Eurotube. They burn-in, test and match the sets.

A next step may be finding a lower-noise 12AX7 or lower gain tube for the first pre-amp stage.

The HR Deluxe has a layer of foil on the back panel for shielding. That does assume that the foil is making proper contact with the chassis. There are several ribbon cables that connect the PCB to the controls mounted on top. They run very close to the back. I have some suspicion about coupling with the layer of foil. It seems better when there is more distance between these flat ribbons and the foil. Could be other interactions, such as running straight/parallel runs are bad, but I don't know enough to sort this out yet.

David Gertschen · From: Phoenix, Arizona

If the amp worked OK before, I would bet money that it is a bad tube and nothing more. Have you tried the chopstick test? Turn on amp with nothing plugged into input and turn the volume way up. Take a wooden stick and tap lightly on the glass envelope of each and every tube. You will most likely hear nasty noises when you tap on the bad tube.

Michael Brebes · From: Northridge CA

When power tubes start ringing it is usually the heater coils. If you hear the oscillation change when you tap on the power tubes, then that is probably the problem. Otherwise it could be a number of other places in the amp.
_________________
Michael Brebes
Instrument/amp/ pickup repair
MSA D10 Classic/Rickenbacher B6/
Dickerson MOTS/Dobro D32 Hawaiian/
Goldtone Paul Beard Reso

Mesa Boogie Studio Pre/Hafler 3000
RP1/MPX100

Dan Robinson · From: Colorado, USA

John Billings · From: Ohio, USA

Dig into your retirement accout and buy some Mullard EL37s. You will be amazed!
_________________
Dr. Z Surgical Steel amp, amazing!
"74 Bud S-10 3&6
'73 Bud S-10 3&5(under construction)
'63 Fingertip S-10, at James awaiting 6 knees
'57 Strat, LP Blue
'91 Tele with 60's Maple neck
Dozen more guitars!
Dozens of amps, but SF Quad reverb, Rick Johnson cabs. JBL 15, '64 Vibroverb for at home.
'52 and '56 Pro Amps

Paul Sutherland · From: Placerville, California

I've been told that whenever you replace the power tubes you should also replace the phase inverter tube, also known as the driver tube. It should be the 12at7 next to the first of the power tubes. I like TAD short bottle power tubes for pedal steel.
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It don't mean a thing if it ain't got that swing.

Dan Robinson · From: Colorado, USA

John Billings · From: Ohio, USA

Oh! They'll fit! They are the best 6L6 types ever made! Abdo=lutely amazing sound. But, yeah, crazy expensive. I really love the two that I have! They're in a '56 Fender Pro Amp.
_________________
Dr. Z Surgical Steel amp, amazing!
"74 Bud S-10 3&6
'73 Bud S-10 3&5(under construction)
'63 Fingertip S-10, at James awaiting 6 knees
'57 Strat, LP Blue
'91 Tele with 60's Maple neck
Dozen more guitars!
Dozens of amps, but SF Quad reverb, Rick Johnson cabs. JBL 15, '64 Vibroverb for at home.
'52 and '56 Pro Amps

Dan Robinson · From: Colorado, USA

Dan Robinson · From: Colorado, USA

Chris Johnson · From: USA

I've done the power/preamp tone quest. Tried almost all of em'. JJ's from eurotubes are reliable and sound great. TAD 6L6GC sounded good too but had some reliability problems

Dan Robinson · From: Colorado, USA

Chris, I took your suggestion. The JJ 6L6GC seems like a nice product.

The comparison chart on thetubestore was helpful. Thanks, Steve.

This HR Deluxe was originally unbearable for steel. With a few circuit mods and component value changes it really came to life. The stock speaker was NOT the problem.

I drag this amp around a lot. My band rehearsals are in a basement studio. I practice on the second floor at home. Sorry JB, don't mean to be a heretic. If there's going to be a quest, it's going to be a quest for a lighter amp.

Then again, we have 4 gigs booked in June. And I've had my eye on the Eminence TT-12, 8-ohms, great for steel, it would save a few pounds.

Tommy Huff · From: Texas, USA

hi Dan, just a thought, but because I myself have several tube amps I searched e-bay and found many tube testers from the 50's and 60's.Many needed work,but I found many that were functioning perfectly. They are usually priced in the $50-$100 range. To me if a tube amp is giving problems the tubes are the first thing I check. Since tubes are so expensive this could save money in the long run......just a thought.....Tommy

Dan Robinson · From: Colorado, USA

Dan Robinson · From: Colorado, USA

The new JJ 6L6GCs have arrived!

Before replacing the tubes I checked the bias on the old set. I saw 71 mV at the test point, but it was drifting like crazy. When the amp became microphonic and broke into oscillation it spiked to 102 mV! Enough of that nonsense. I killed the power and removed the old tubes.

The deposit at the glass/base junction is further evidence that this tube is bad. I won't even use this as a spare.

Onto the good newsâ€¦ the JJ6L6GC is a nice product. They were thoughtfully packed, and promptly shipped by Eurotubes. They included printed instructions on handling of the tubes, and steps for cleaning the tube pin-sockets (strongly recommended)... apply contact cleaner to the pins of a tube, insert it, then work it in-and-out a few times. I used one of the old tubes for this task. That's all they are good for now Laughing

An insulated screwdriver is essential for safely setting bias. I found this nice set for $18 at my local Lowe's hardware store.

I wore nitrile gloves when handling the tubes to avoid getting any oil from my skin onto the glass. With the new bottles installed, I powered them up, allowed some warm-up time, then set bias to 74.8 mV.

I left the amp on for a few hours, then sat down to play my pedal steel. These new tubes exhibit no microphonics. The amps sounded clean, but somewhat lackluster. Another bias check revealed some drift, 81 mV at the test point. After adjusting down to 72 mV I tried the steel guitar again. Much better! This is counter-intuitive, but there ya' go. I'm happy, and lower idle current means less plate dissipation, which should extend the service life of these nice valves. I'm back in business, ready for Tuesday night band rehearsal.

The next "opportunity" is replacing the 3 original 12AX7s with better tubes. Several products have lower noise, which would be a big help in the first pre-amp stage.

I believe it is OK to substitute a lower-gain 12AT7 for any 12AX7. I might try this first in the reverb stage. If any of you have experience doing this I would like to hear about it.

Jim Sliff · From: Lawndale California, USA

You can sub 12**7 tubes al you want in the V2 socket to vary the gain; a 12AX7 can be tried as a phase inverter but it's often a bit harsh. The reverb tube values are best left as-is, and the tremolo tube is not an audio tube - as long as the4 oscillator works it's fine.

FWIW tube testers are useless for guitar amps except as a way to eliminate bad tubes - except for very expensive, rare models they do not hit the tubes with nearly the same plate voltage a guitar amp does making tests invalid in a guitar amp context.

They also must be properly calibrated periodically, which rarely happens. There are very few tech that perform calibration and it usually costs more than the purchase price of a used tester!.

Also - I have a concern about the amp in the original post. It's 20 years old - the filter capacitors have lived long beyond their usual lifespan unless it's been serviced in the last 10 years or so. Older amps had much better quality ones that sometimes were OK for 20 years use (sometimes more, although it's not a good idea).

If one fails it can cause serious damage to the amp. There's no way to visually or audibly tell if one or more is going to fail - it often just happens suddenly. They should be replaced asap along with the bias cap.

I'm hoping when you had the back of the amp open that you understand amp safety, as filter caps can store deadly voltage even when the amp is off and unplugged. They need to be tested and probably discharged safely before changing them. If you're not familiar with this have a tech do it. Again, I'd suggest doing this soon before the amp is damaged.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Steven Paris · From: Los Angeles

Jim Sliff · From: Lawndale California, USA

The 12AT7 does not see 8 ohms - it goes to the transformer, which has an input impedance of 25 ohms. The transformer converts that to 8 ohms.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Rex Thomas · From: Thompson's Station, TN

Staying the course with 6L6 replacements, I wanted to give a "thankew" to John Billings for steering me to the Mullard brand. Now no, I didn't get NOS, but I was having "bad batch" issues with EL34/6CA7's, saw his mention about Mullard's, ordered me up a pair of EL34's, WOW. The amp is awake. Shocked

Thanks, John. Cool

Now, I am indeed looking to have a 6L6 type amp built for me in the near future. Honestly, once I have it I might save up & take John's suggestion. Meantime, if they're still available, I'll probably go with Winged "C".
http://www.thetubestore.com/Tubes/6L6-5881-Tube-Types/Winged-C-SED-6L6GC
There's my 2 cents.

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