Stereo el84 se donor amp conversion

[Markusv]

Hi folks.

Its been a while since i last posted.
I just received a free amp out of a radio console.

2x el84. One each for a channel hence single ended operation .
One 12 ax 7 - each half driving respective outputs.
Two se ot's.

Question:
Can i tie the ot's together to form push pull and use the ax7 as a pi?
Of course i would have to install another preamp tube for input

Thanks as always
Markus

[Stevem]

A PP 15 watt RMS rated OT will sound a good bunch better then dicking around with the 2 of them, and they don't cost that much either!

[Phil_S]

SE transformers are air gapped. PP transformers are not. This is a fundamental difference that you can read about online. I'll second what Steve says. Either get a PP tx or raid the chassis for parts and build a couple of SE Champ-like amps. Or remove one and just use the chassis you have.

[Kagliostro]

There was an old italian brand that, missing of adequate OT, used a pair of SE OT for PP power section configuration





one of my friends sostituted the two SE OT with a single PP OT and it was an increase on performances

in my view your chances are

1. Build an SE guitar amp

2. Build a "P"SE guitar amp (each power tube connected to his SE OT - secondaries of the OT paralleled)

3. Build a Stereo guitar amp

for option 1 use the 12ax7 as preamp + 1 x el84 as power tube + 1 x SE OT

for option 2 use the 12ax7 as preamp + 2 x el84 as power tubes + 2 x SE OT

optionally primary and secondary of the OT can be connected in parallel if the power tubes are connected in parallel

or

connect each power tube to one OT primary and the secondaries of the OT in parallel

for option 3 if you want a stereo guitar amp a single 12ax7 (two triodes) is not enough for preamp

you must add one other tube (12ax7 may be) but this require some extra heater current (300mA for 12a*7 tube)

so it will be easier to add a jfet as "V1" for each channel ( plus one triode ot the 12ax7 for each stereo channel), build two separated preamp (one for each stereo channel)

each one with a sligtly different character to add tone opportunities

this is only one example of how you can arrange a jfet or mosfet as "V1"



if you do a search with google you can find some schematics (some use the LND150 mosfet)

K

[pompeiisneaks]

I think I remember hearing somewhere that it's basically massive overkill, and a waste, but works fine. The OT's in SE run a lot more current, due to the inefficiency of SE vs PP, but since you've got them, and they're free, go for it

~Phil

[Kagliostro]

Current handling is better on SE OT, they have a gap to avoid core saturation and a 5W SE OT is a lot bigger than a 5W PP OT

The problem, using a pair of SE OT for a PP Power Section is that being the two winding (the winding related to each tube) on a different

core, there is no hum cancellation, that happens when in a PP OT the hum noise of one section counteracts the hum noise present on the other section

My opinion is that if you have a pair of SE OT and want to use it, don't go to use them PP, use them in Stereo or PSE configuration

K

[jjman]

Wouldn't the hum cancel in the same manner as a HB guitar pickup would? Those have 2 separate windings and poles. 2 single coil pickups can be humbucking if they have inverted magnets and are wired properly. If the PS voltage goes up momentarily (as with hum) both outer ends of the primary string go up in voltage. How can that be transferred to the secondary and would the speaker move in or out at that moment?

[Kagliostro]

I hope someone is able to give you one answer, I'm not, I know nothing at all about PickUp, sorry

K

[statorvane]

Parallel SE using two OTs, single PT? That is the best sounding amplifier I have built. Frankly this started as a sort of kludge, using up a leftover OT. I wanted something bigger than a 10-15 watt amp. So I bought another Hammond 125ESE OT, and a single Hammond 270HX as a PT. Preamp is from a Hiwatt DR-103 design, and Power amp is my take on an AX82 Power Amp x 2, driving two 6CA7s. Sounds nothing like the Hiwatt though. This sounds very much like a Marshall 59 derated to 25 watts.

I say go for it. There's something about a cathode biased SE amp that really rings out. I am guessing it's the even order harmonics or something. I love it. Also since it is cathode biased, it is easy to swap out the octal tubes without having to rebias. I have tried 5881, 6L6GC, EL34 E34L (JJ), 6CA7, and 6550s. The 6CA7 and 6550 sounded the best IMO. Here's a link....
http://s165.photobucket.com/user/JKCavi ... t=2&page=1

[Kagliostro]

I liked your link, NICE AMP and perfectly builded

COOL

The schematic is readable but my poor eyes suffers, have you a higher resolution schematic image to share ?

Thanks

K

[Markusv]

Hello Everyone

Thanks so much for your generous replies.
I think Parallel SE using 2 OT's paralleled at the output only is the way for me.
I believe I may get away with installing another ax7 as I intend using solid state rectification thus removing the Rec tube and using its current allotment.

I remembering building and later selling a 4-4-0 from the ax84 website resource and loving it. Basically using 4 half triodes IIRC.

Thanks again!!

Markus

[Kagliostro]

Oh, I didn't considered to remove the Vacuum Rectifier

that is a feasible and good idea if the rectifier uses 6.3v heaters

if so you can add one other tube to increase the preamp complexity

K

[Kagliostro]

I remembered that some time ago I find a very interesting thread about the use of SS (solid State) components to emulate tube circuits

and there were some Tube Preamp converted into SS preamp

like this

BAJA solid state high voltage Fender Twin input stage 080414.gif

if you want to give a try here is the link with the documentation

http://www.freestompboxes.org/viewtopic ... 13&t=24287

and here a .pdf file with the schematics of that thread

From Tube to SS.pdf

K