Seeking advice for Plexi revalve (update)

[Guitarman18]

A friend of mine has been pestering me to revalve his 60's 50W Plexi for some time and so far I've resisted because most of my amp building experience revolves around Dumble clones and old Fender amps and I don't want to do anything that would undermine the mojo of this amp.

The amp is sounding a bit dull and has the crackling in the sound, I believe due to EL34's. The pre amp tubes and PI tubes seem to be behaving ATM.

I'm wondering if there is a consensus on what is the best way to proceed. If I replace the EL34's, would you replace all the electrolytic caps, PS, Bias and cathodes also?

Also, which current production EL34 would be best for this amp? I've heard some good reports on new Mullard EL34, but not sure whether that is a reliable source or not.

This amp is gigged a few times a week and is run pretty hard, so I don't want to just re-tube it only to expose other underlying issues.

Any tricks/tips/advice on biasing this thing would also be appreciated. Is it appropriate to put in a 1 ohm resistor on pin 1/8 to ground or would that be considered bad form? Where do they like the bias to be set?

Anyway, enough of my endless questions. If anyone has any thoughts, advice or can point me towards any previous posts or websites that have dealt with this in the past I'd really appreciate it.

Thanks in advance for any help you can offer,

Paul.

[Reeltarded]

It is gigged all the time?

Needs recapped. It's nice. I say JJ 6CA7 is my favorite modern tube, JJ EL34L are also very nice.

Crackle? Hmm.. Let's see after the recap and tubes. Umm you need to rework the polarity switch to disable it and bypass the cap for it. It's not safe.

Umm RS .68uF.. is that just covers on something crappy? If not, I want lots of those.

HEY EVERYONE LOOK OUT FOR THOSE CAPS

[xtian]

If your friend has been playing several times a week for a year, it could certainly be time for new power tubes. But this should be the least stressful thing on your list--just try some new ones, and if they sound worse than the originals, put the originals back. Crackling could indeed be due to bad tube, but also dirty pins, bad solder joints, aging plate resistors.

JJs are my go-to new production tube. But Tung Sol, Mullard, EH, and others are just as good. People seem to like =C= (winged C) a lot.

There is no reason to replace caps when you replace tubes. BUT you may need to replace caps! Treat it as a separate issue. How old are the electrolytics? Is the amp lacking in bottom end? Too much hum? Static (from DC) on the controls?

Use Weber or AX84 bias calculator to ballpark how much bias current you need. I like the 1-ohm resistors on the cathodes, but there are other easy ways to go, if owner does not want any out-of-spec mods.

[Blackburn]

Man, those are some cleeeeeaaaaan guts! Wish I had a 60s Plexi to retube...

[donzoid]

I don't build 'em (yet) but I sure play them a lot, and for what its worth even though I'm not an el34 guy I agree the JJ are the best sounding new ones, or =C=. The others mentioned here (Mullard, Tung Sol, EH) are not at all like the old stock brands...they are all made by Sovtek and not as good as those top two in that they sound flat to me. If this guy likes nice tight low end the el34L is a better choice than the el34. The 6CA's sound good too.

[Roe]

winged C el34s and possibly new F&T filter caps

[Guitarman18]

Thanks for the responses guys, I'm feeling a bit more confident at moving forward with this project now.

Reeltarded wrote,

It is gigged all the time?

He gig's it often, alternating with the ODS clone I built for him.

The problem is that my friend bought it about 10 years ago, whilst he was a guitar student of mine, around the age of 15. He came to me one lesson and said he had seen this ad for a Marshall valve amp, and had gone to have a look. The guy had two of them and said he could have both for $1000. At the time my friend had no idea what he had bought, other than it said Marshall on the box (small box). So he doesn't necessarily treat it with the reverence it probably deserves!

Some people just have the luck I guess

I'm afraid the RS .68uF caps are all original.

xtian wrote;-

There is no reason to replace caps when you replace tubes. BUT you may need to replace caps! Treat it as a separate issue. How old are the electrolytics? Is the amp lacking in bottom end? Too much hum? Static (from DC) on the controls?

Thanks xtian, I will replace the tubes and see if it gets me closer to sorting it out. The caps are all the original Hunts (UK spec), so I wouldn't be surprised if they were in need of a change.

donzoid wrote;-

If this guy likes nice tight low end the el34L is a better choice than the el34

Thanks donzoid, I've seen a listing on the MojoTone website for the JJ E34L, is that the one we are talking about?

Roe wrote;-

winged C el34s and possibly new F&T filter caps

I've had good success with F&T in my own builds, so I'd be pretty happy to use them on this project, and they have direct replacements for the Hunts, which helps.

Thanks once again guys. I think it's important to recognise our limitations when it come to building amps etc, and if in doubt ask, rather than screw up a perfectly good amp!

Have a great weekend,


Paul.

[statorvane]

I've seen a listing on the MojoTone website for the JJ E34L, is that the one we are talking about?

Yes. You can get them wherever they sell EL34s. The E34Ls support a higher grid voltage.
http://www.tubesandmore.com/products/T-E34L-JJ

[donzoid]

Love him or hate him, Pletka at Eurotubes has a new matching machine he had custom built and I've seen it. Its an engineering masterpiece really...I buy from him now but if you are in the UK you may want to go local as the shipping and duties could kill a guy! Anywhere they sell the regular el34 they should also sell the E34L (or el34L) in matched sets.

Yeah, some kids have ALL the luck. I thought I paid a lot when I bought my first 100 watt in 1979...for 350 bucks.

[Guitarman18]

Just received the E34L valves from my supplier and noticed that one of the sockets came apart as I took the old valve out. As you can see from the attached photo someone has epoxied the bakerlite part to the metal, but now that the epoxy is old, it has cracked and is allowing the bakerlite to fall through the base. The metal teeth on the metal mount are just not making a secure contact with the bakerlite.

Given this situation, what would you do to sort it out. Would you a) try to fix the socket or b) replace the socket?

As always any help with this would be greatly appreciated.

Cheers,

Paul.

Also, still getting crackle(frying bacon) with the new valves. However I am getting about 12 VDC on pin 1 of the Presence pot.

[statorvane]

That may be where your bacon is getting fried. I'd dump those sockets (option b), they are only going to deteriorate further, and replace them with some new ones.

[Guitarman18]

statorvane wrote:

That may be where your bacon is getting fried. I'd dump those sockets (option b), they are only going to deteriorate further, and replace them with some new ones.

Thanks statorvane,

I think you are right, I'll replace them and find out where that gets me.



Cheers,

Paul.

[ampmike]

It is gigged all the time?

Needs recapped. It's nice. I say JJ 6CA7 is my favorite modern tube, JJ EL34L are also very nice.

Crackle? Hmm.. Let's see after the recap and tubes. Umm you need to rework the polarity switch to disable it and bypass the cap for it. It's not safe.

Umm RS .68uF.. is that just covers on something crappy? If not, I want lots of those.

HEY EVERYONE LOOK OUT FOR THOSE CAPS

I agree,Mine is a 69 supertrem and it needed a cap job,Its a monster now!
It had everything original in it.You dont ever want to loose a tranny,Just a thought,Mikey

[Guitarman18]

ampmike wrote:

I agree,Mine is a 69 supertrem and it needed a cap job,Its a monster now!
It had everything original in it.You dont ever want to loose a tranny,Just a thought,Mikey

Thanks for chiming in Mikey, did that include the electrolytics in the bias supply?


Reeltarded wrote:

Umm you need to rework the polarity switch to disable it and bypass the cap for it.

This UK amp hasn't got a polarity switch, as far as I can see, but I can see the cap. Is this a 'kill cap', as with Fender amps of old?

Thanks for your help guys, much appreciated.

Cheers,

Paul.

[ampmike]

You know Im not sure,I had this done before I could do anything myself.He installed a master into one of the input jacks,and I believe he took out the tremolo.Everything can be put back to normal if I let it go one day.But this master vol sounds real nice.The guy who did the work was Dave,from Daves sound and repair ,Whippany NJ.I really like this amp now it can sound real plexie like at a lower volume.Thanks Mikey