Lafayette LA-55 conversion?

[Jess13]

Hello

The LA-55 is a mono amp that runs two EL84's in push-pull, with a 6BL8 driver, a 6AV6 ( not sure how its setup in the chain) and a 12AX7 on the input, and a 6CA4 rect tube.

The PT has 270-0-270 @ 117mA, 6.3v@2.5A, 6.3v@1A and 5v@0.6A. The OT has taps for 4,8 and 16ohms, with the primary having a center tap at 50% ( no other taps).


I have only begun to start leaning about tube amps and so far its pretty hard for me to get a full understanding about it all. I can see if ever want to do anymore than just paint by numbers its going to take a lot and I really struggle with this type learning until I reach a point I can get hands on with it. Even though I am just starting into this I do know enough to understand how to keep myself safe.

Anyway I did get the LA-55 up and running enough to play music through it. I then started taking it apart to build into some type of amp just to use at the house. I wish I had learned more before I took the amp apart so I could get more info for those that may help with a new build. I did get a reading of 355v on HT at the two EL84's when it was under a load but IDK if that help's in know how much power I have to work with or not.

My budget is not good, but I do have a lot of preamp tubes (many new and some even matching), a matching pair of Sovtek EL84's new, several old amps,radio's and such for parts like sockets, tunner pots, shielded wiring. I also have a pretty good stock of caps and resistors new a friend helped me with. I don't have any caps such as the PIO and other such high regarded parts but can get them as I need.


What would be a good amp to build out of these parts? With the research I have done it looks like something along the 18watt amps I am reading about. Not really what I want but I have to get my feet wet somehow and I hope this to only be the beginning of something that so far, I am really enjoying. I would really like something clean and blue type amp but it doesn't seem to be the type amp that gets built with EL84 output tubes. I do have a lot of 6L6 tubes but if I am understanding right my PT may not have enough power & the OT not right for such tubes either (but its not like I know).


What would you guy's suggest. I am welling to take it more as a learning thing than to worry about what type amp I get out of it. I can build number two when the funds get better, all though a nice blackfaces sounding amp would please me.

Thanks Jess

[olddawg]

I would gut it. Use a SS bridge for the power supply and make an 18 watt Marshall Lite variant using all 12AX7s for the preamp and PI. It's a good idea to replace the tube sockets with new ceramic ones and build a new turret board with fresh new components. You have the expensive stuff. Save yourself a headache in the long run and have a very good amp.

[Justin Thomas]

Spitfire anyone?

Title says it all... I'm very happy with my tweaked one (thanks, Chuck H!) It's a guitar amp, it's a practice bass amp, it's even a PA on occasion! I would call it clean and blue. I think either the Marshall or Matchless would be great. And definitely scrap the sockets; I would start with fresh pots, filter caps, & small parts too. I'll second the turret board - stick with an established layout for a first build. I did my Spitfire point-to-point. Built the amp in a day; spent a month fixing noises.

I'd think you could use 6L6s, but only with a VERY simple preamp - as in, like a Spitfire. Or 6V6s. But give the EL84s a chance - you might really like them. And it's a lot less worry about burning up a tranformer, which are the really expensive parts.

Take your time - patience, and don't sweat the small stuff. I think you could end up with a great home AND gigging amp. Happy building!

Justin

[olddawg]

Title says it all... I'm very happy with my tweaked one (thanks, Chuck H!) It's a guitar amp, it's a practice bass amp, it's even a PA on occasion! I think either the Marshall or Matchless would be great. And definitely scrap the sockets; I would start with fresh pots & filter caps, too.

You can put the bridge rectifier, the filter caps, and all the rest of the circuitry on one smallish turret board nowadays. There are lots of variations. I gig with my recycled 18 watt clone a lot. Many times I have to use a Weber Mini Mass attenuator. These amps scream. Just don't expect a lot clean overhead. If I need that I a/b to a Fender amp.

[Jess13]

Thanks guys,


I have researched the 18watt amps more and I think the 18watt light would be a good way for me to go. I don't expect to leave the amp along after the first build so I can add/mod it after I have better understanding of watt's what. In fact I think my PT may have more power than the light version needs and after I try to do some figuring I may even have enough power to do something like three preamp tubes. The PT powers a rectifier tube plus two & half preamp tubes now so I will run the numbers and figure it out if I can. I believe Lafayette was using the one 6AV6 (its like half a 12AX7) to power the tone stack in the original setup but I am not sure.

I am going to have to use the old chassis, and I don't think there is room for one of the turret boards. I have enough smaller boards around to work around this though. Believe me, I do understand the advantages of using a tried and tested layout, but I will have to work with what I have. I have plenty of shielded coaxial wire so hopefully I can get a layout that works.


I have read much info on grounding, to the point of confusion even. My PT has a Earth shield for tying to the chassis. Now I think I understand that this shield would tie to the chassis, plus the third wire of my three wire 120 line coming in, with the star grounding also going to chassis. But my question is should the ground side of the two 120v lines also go to chassis? Or will it only get taken to ground through the different circuits and there star grounds? BTW I do have a good stock of the X1-2 & Y1-2 types safety caps which I think really helps answers my own question here.

Thanks Jess

[loudthud]

But my question is should the ground side of the two 120v lines also go to chassis?

No!!! Never, under any circumstance!!! Where did you get that idea? Tell me so I can have the moderator remove it.

[olddawg]

Jess... It's frightening that you should even ask about the AC mains in that way. I suggest that you do a lot of reading before you go much further. You could seriously hurt yourself. I'm not being rude. It happens. Please learn basic electricity fundamentals and know how to safely service tube amplifiers before you start. There should be room in that unit for a turret or a tag board. I would not suggest trying to do a point to point rebuild with no experience.

[Jazz P Bass]

Just to clarify the OP's mistake, the two 'legs' of the ac mains go to the power transformer primary.
The third 'leg' of the ac is the green ground wire that goes to chassis.
Code specifies that the chassis ground wire be by itself & 'bonded' to the chassis.

[Jess13]

LoL guys, thanks for jumping on that. I guess the way I stated that it did sound like I was asking if I should run the mains straight to ground. And really after rereading, it is the way I described it. I guess my confusion is coming from the way they wired this old amp. I don't see how they ever expected it to have any safety about it. But it is 1950's tech.

They have multiple star groundings through out the amp, and of course no third wire on the mains, but the shield of the PT transformer was not run to ground. They had it tied into 0v side of the HT circuit, right at the capacitor. This shield of the PT needs to go to the chassis ground ? This is what I meant to be getting at for the question, and I hope I am understanding the use of that shield right. I don't see why they had it tied into the circuits the way they did, but maybe running it into the big cap worked to drain off any unwanted noise. I guess I need to remember that I did see reason to think someone had already been working on the amp. But the amp is wired exactly the way I have seen other examples of the same model. So again, the shield tap on the PT transformer should go to chassis for grounding ? And if this is correct, this is for noise rejection, or draining, and if not what is its exact purpose?



BTW if we need to remove part of the post above by all means please take care of it.

Jess