EL34-PP building questions

[tiagor]

Hello,

I'm again in need for some good advice. Fortunately, most of the questions I had could be found by searching the forum, but because this is my first amp, I still have a few that for the sake of my safety I thought I'd rather ask them anyway. So, any help will be greatly appreciated

1. I was thinking of building the whole chassis on wood. I do not have the tools to punch holes on metal, so it's more a need than a preference. Ventilation and heat dissipation should not be a problem, but I'll still need to have a ground plane. What are the ways I can acomplish this? Can I have a copper plate along one side of the chassis, or will I have problems? I read I should have no more than 2 grounding points, one near the splitter valve(ECC82) and the output stage grounded near the EL34's(is this mandatory?) All in all, I'm looking for a solution that avoids needing holes in metal.

2. I had soldered the power supply components before reading this forum(probably a big mistake). I used 2 small perforated boards for the components and hardwired them(one for the plate voltage and one for the negative voltage), though i did not clean the copper from around the holes in the board(note the copper is only around the holes and not connected all over the board). I need advice on this, will it be prone to RFI? If this is problematic I'll redo all the power supply. I now realize I'd probably have done better with no mounting boards at all.

3. Probably the dumbest question on earth.. My mains transformer(a toroidal from Amplimo) comes wrapped in a plastic envelope wound around the trannie. Should I mount it on the chassis as is or strip the plastic cover away(doh.. do i feel stupid).

Thanks for the patience. As soon as I solve this problems I'll post some pictures of the building as it progresses.

All the best,

Tiago

[analog_sa]

1. You don't need a ground plane. For star or bus grounding you may find useful strips of copper foil. I'd first glue them to a known good insulator (teflon, ceramic) and then attach this to the wooden chasis.

2. Certainly no probs with RFI.

3. It depends on how much nicer it looks with the wrapping If the transformer gets seriously hot it may melt down...

[mcs]

1. I was thinking of building the whole chassis on wood. I do not have the tools to punch holes on metal, so it's more a need than a preference

You can buy cheap hole saws normally used for cutting round holes in wood - they also work for aluminium if it's not too thick.

but I'll still need to have a ground plane.

You may not need any shielding at all. My current main amp (2A3 SE) is built on a piece of wood - no noise or hum.
I have simply fitted a small solder tag at each stage/tube where all the ground wires are connected. Then just make a "master" ground tag somewhere and connect all the ground tags to that point.

3. Probably the dumbest question on earth.. My mains transformer(a toroidal from Amplimo) comes wrapped in a plastic envelope wound around the trannie. Should I mount it on the chassis as is or strip the plastic cover away(doh.. do i feel stupid).

Who knows? Some toroidal transformers come in a plastic case, and if that's the case you're not supposed to take it off. If it is just packing I guess you should take it off. Could you post a picture of the transformer?

Best regards,

Mikkel C. Simonsen

[fdegrove]

Hi,

Who knows? Some toroidal transformers come in a plastic case, and if that's the case you're not supposed to take it off. If it is just packing I guess you should take it off. Could you post a picture of the transformer?

It's just a plastic wrapping that serves to protect the enamel wires against damage from scraping against rough surfaces.

Leave it in place.

Cheers,

[tiagor]

yep, that's it fdegrove. What was worrying me is that the wrapping is kept is place with normal tape. Wondering if wouldn't melt with the heat. I'll try keeping it and see how it handles.

I'll also, try star grounding for a start, and deal with eventual problems later. When you say "master ground", does it mean I can connect it to earth ground? or just one point in the chassis?

Thanks,

Tiago

[richwalters]

Auch....toroids.....I've replaced so many of them in equipment because they have been screwed down too tight, thus nipping the turns "over each other".....Treat them with respect...winding turns are often pile-wound. . The final tape should be compliant variety...if no spec'd on labêl.... I always assume tranny has limited temp rise ratings. (Typ 25°C above amb)
Hint...For mounting toroids use a semi-hard mastic or rubber (from hardware stores) to provide an even mounting base load, and tranny not mounted on hard surface.
There are so many classes of insulation and specifications for toroids that I cannot elaborate on any type.. however, the mechanical treatment (unless epoxied) is basically the same.

As for chassis.....metal or < wood it > sound good ? I still have my first <starvation circuit> Mullard 3watt..botched up on wooden sub frame. I made in 1959; it looks horrid, but sounds good.
Do entirely what suits you best.......bear-in-mind important grounding tips.
good luck

rich

[Jax]

My very first tube design was a stereo amp with EL90's SE and ECC83 input+driver. Built it in the late 60's. On plywood.

Screening is good but not essential. Heat can be a problem but if the holes for the tube sockets are slightly oversized it should not be much of a problem.

[eeka chu]

Tiagor, while you where looking at output transformers for the amp, did you ever come across anything that might suit a single ended EL34?

50 watts is way too much for me, I want something smaller that I won't get complaints about. I like EL34's but everyone seems to only want to run them in PP.

I guess I am looking for 2 - 2.5k ohms, which isn't tricky to find. The only problem is, I am having trouble finding anything in that rating that is designed for the slightly higher currents I wish to use. Most of them are rated for around 90ma, I believe I need closer to 120ma.

I am currently having a look round, but have yet to find something.

All the best,
John

[dhaen]

Last time I checked, Sowter will wind you a pair for the same cost as supplying a stock pair. Have a browse around the site.
If you email them, you'll probably get a reply from Brian Sowter himself. Be's very helpful.

[eeka chu]

Will do, thanks for the suggestion!

John - 2