El84 - resurrected amp project

[kepeb]

Hi guys, It's been a long time

I started a tube amp project around 3+ years ago, it was since abandoned and has been

gathering dust.
There were a couple of things I never managed to figure out and probably more which I

have forgotten.
Anyway...

The build is limited to the parts I have:
El84 Output Transformer
Power Transformer from broken Oscilloscope

A small chassis and cab salvaged from an ex-Orange practise amp
and got the tubes and a bunch of resistors/caps.


I was attempting to follow the tiny terror circuit as much as possible for obvious reasons - 2x 12ax7 and 2x el84.
however I believe the salvaged PTX only has around total 40watts to work with, so standard push/pull el84 setup would be too much. so maybe reducing the maximum the powerstage will try and pull somehow?
less plate volts?(maybe half wave rectified instead of full?)

Any suggestions for me based on what I got so far?
Ideally I would just get it working point to point with what I have, because I'm not set up for any real chassis modding etc.

[kepeb]

So this is the original schematic:

The pre section shouldn't need any changes, thought about separating the dual pot either side of the line driver stage but It looks like that wouldn't offer anything if I'm keeping a master section.

I also looked at the position of the tone control and thought about putting it before the phase splitter but my guess is the current position offers better efficiency with a wire less to solder.
I will also be putting in a lineout but that will just hang off the speaker output

Having only built one tube amp before I'm still unsure of the best steps when I wire this up.
Unless I'm corrected, I'll go ahead with what I have here:

The same but increased the value of the shared cathode and grid resistors.
I guess I'll just wire it up without the tubes and get a volt reading from the plate nodes?

[olddawg]

Have you considered just getting a Heyboer PT and doing it right? They aren't that expensive. With little experience nothing beats a known good component layout.

[kepeb]

Hey, thanks for the reply.
I haven't looked at buying anything so far, or a paint by numbers approach.
The Idea was using parts on hand,
and based on the cabinet I would 'try' not to stray too far from the simple Orange TT design.

I did a search for those guys and don't really fancy $80 minimum. plus UK postage, plus import tax
Particularly when I have a Tube PT already, mounted as in the picture too.

It's OK, i'll carry on as intended,
only posted here on a whim in case someone had experimented with building original designs or had a better Idea.

[olddawg]

Didn't realize you were overseas. If you do a forum search there will be lots of recommendations of where to get a less expensive PT in the EU, if indeed that's your problem. There is a HUGE thread on reverse engineering the TT circuit. I would start with that.

[kepeb]

Thanks mate, weekend coming i'll search for the thread and see if there are any ideas i can pilfer

[g1]

As he said HUGE, this must be the thread he meant :
http://music-electronics-forum.com/t1477/

[Chuck H]

I'm just going to back the idea of abandoning building a 2xel84 circuit down to a 40mA limitation. Voltage limiting would work for that, but it won't sound the same and it might make it difficult to get decent preamp voltages. For the effort and peripheral expenses that go into building an amp, it seems impractical build the whole thing down to the limitations of one component. Are you even certain that little PT can supply the necessary filament current?

OTOH... Taking advice from me about not advancing projects until all your criteria are lined up can lead to a closet full of dusty intentions

[pdf64]

Some info about the salvaged PT is needed to assess whether it's suitable for the amp. eg what windings does it have?

Have you checked what that vintage RS OT would fetch on eg ebay?
It may fund a decent transformer set and more.
Whatever, you could get a good PT from Barry for less than £40 delivered Amp Maker: Guitar amp kits and parts :: Power transformers :: 0-190-275V 20W power transformer
Transformers with exposed external terminals are a electrocution hazard, especially in a combo type cab; it would be bad to take your creation around to a friends place to show it off, and it shock their toddler or pet.
Metal mesh over the the open back could help to mitigate that.

[kepeb]

pdf64:

"Some info about the salvaged PT is needed to assess whether it's suitable for the amp. eg what windings does it have?"

- Already Linked above
Clearly shows the current requirements and puts the Tx in the 40watt range.
Filaments previously supplied 6 ecc series tubes and a rectifier.

"Have you checked what that vintage RS OT would fetch on eg ebay?"

-Why would I? Like always with ebay, depends on the day, then you get those lovely people to deal with, one in every ten is a good experience these days, but at least you don't really make any money after fees and postage. so that's good.

"It may fund a decent transformer set and more.
Whatever, you could get a good PT from Barry for less than £40 delivered Amp Maker: Guitar amp kits and parts :: Power transformers :: 0-190-275V 20W power transformer"

- Indeed pricing up parts to build an amp is a different game, I looked at the parts I already have, and thought I would turn them into something!
As far as parts go, that is a good deal on a Ptx but You can pick up a working tube amp or combo for less than £100 all in.

"Transformers with exposed external terminals are a electrocution hazard, especially in a combo type cab; it would be bad to take your creation around to a friends place to show it off, and it shock their toddler or pet.
Metal mesh over the the open back could help to mitigate that."

Seriously? and miss out on all the fun that would ensue from that scenario?
I feel like you are taking the piss here.

- Am I in the wrong forum?
Is there actually anyone interested in Music Electronics?
I am happy to plod along without the continual advice of custom purchases.
and now common sense safety advice too...
Really?!

[kepeb]

I think I will end this one here and proceed on my own without the dialogue.

Thanks... I think.

[g1]

Your link in post #1 for the power transformer from o-scope is a dead end for us.
Do you have to be signed in to google "bilder" or something?

I don't think there was any malicious intent behind pdf64's post.
Hang in there, with some info on the P.T. (especially voltages), more responses may be possible.

[Chuck H]

You know... Maybe if one individual had only taken this to be resolved via PM all this confusion and animosity could have been avoided

I think we need a "dislike" tag.

[olddawg]

I only suggested sourcing a known adequate PT and using a known and tested workable layout because of this statement. "Having only built one tube amp before I'm still unsure of the best steps when I wire this up." No disrespect was intend. I just thought it would be the most efficient resolution of the problem.