info on old telefunkin tube power amp

[donald stringer]

      My mother in law had[a few years back] an telefunkin stereo in falling apart cond. She wanted it moved to the dump. This was a right now type of thing,no time to save the falling apart cab. etc. but I took the occasion to plug it in and saw the tubes glow,seized the poweramp, and the main control front with the tube sockets on it. This thing has been haunting me for a couple of years as I have been wanting to turn it in to a guitar amp. But working with mains voltage has made me put on the brakes. Are there any ideas on working with this particular power amp. I need pictures and info. Its in a tight little cluster and personally whoever put thing together in the factory in the first place deservs an award.

[Dan N]

I need pictures...

So do we!

[sean k]

Its a kinda educated decision as to whether you work on the chassis you have or take out all the bits you need and start again. The eductated decision revolves around getting to know whats in there and how it is positioned with what you want to do with it. First stage is getting some info on what the various tubes are and what they are there for.

Once you decide which tubes are going to do it for you, voltage amps (preamp stages) and the current amp (power amp) you'll want info on the tubes pinouts and from there make a schematic of whats going on, with the bits you want to retain.

At this point you've become eductated so you can make your decision.

All that said it might have an auxillary input... boost the guitar to line level and use that.

Check out the articles at audio express by Norman H Crowhurst and the tube amp stuff at AX84 for some info on how va (opps, showing my nationality) tubes work.

[donald stringer]

   Thanks for the reply. A picture would be a good start. I will get some uploaded in the next couple of days. In the meantime I will try to do some basic circuit sketching.

[donald stringer]

I "ve started taking the cluster apart so I can see how everything is connected.Just some basic info. One pre-amp tube 12ax7/ two output el 84. It seems like your basic setup so far. There is some sort of volt. bias on the back it reads 220/ 240/110/ 25. I set that on 110

[Ripthorn]

The 12ax7 is probably the phase inverter to feed the el84's which are probably in push/pull configuration.

[sean k]

Seeing that it's a stereo, well you say it is 'cause alot of the oldies where dual mono and one can't be sure, then it's gotta be two single ended amps with a triode of the 12AX7 as the voltage amp and a EL84 as the driver... or its a dual mono and it's as rip said. How many OT's?

Output transformers... follow the single wire that leaves the 84 and goes into a hole...

[donald stringer]

<a href="http://smg.photobucket.com/albums/v172/troublerat/?action=view&current=Apr2130.jpg" target="_blank"><img src="http://img.photobucket.com/albums/v172/troublerat/Apr2130.jpg" border="0" alt="Photobucket"></a>    Here are some pictures. There is one with my  finger on the left of the photo, behind my finger are the wires that came from the push button control front. These go into the 12ax7/  on the right side are where the speakers connect. I will try to get a clearer picture.                                                   

[donald stringer]

 The rainbow colored wires came the control panel to control other functions.

[donald stringer]

 There are three transformers one very large one, two small one, one of these has to be for the speakers I suppose.

[donald stringer]

http://www.schmidt-sarra.de/TC/GermanTubeHifi/Poweramps/Telefunken%202500/2500.htm     What luck, found a picture and a schematic here. This is what I have 

[head_spaz]

By the schematic it looks like a single ended, ultra linear, stereo amp section.
It appears to be both preamp and power amp together. Funky layout though!!!
I can't really see the schematic well enough to determine whether the transformers
are interstage, or output types.
If they are output transformers, I would guess maybe 3 to 5 watts per channel.
There's obviously several control stages missing, along with half of the power supply.
The power transformer, tubes and output transformers could be useful in building a
small guitar amp. But it would be a challenge for anyone other than Doug Hammond.
If might be interested if you're willing to part with it. Maybe we can work out a trade
or something.

[petemoore]

  Doesn't look too complicated..
  If it's working..
  Might not take too much to make it safe and then voice it for guitar, might be voiced nicely for 'X' volume, or perhaps just need a little bass tightening' [such as reduce an input or a few cap values].
  Nice compact dual amp thing !
  It would take some gawking and schematic tracing through the convoluted physical layout, but I'd guess there's some logic [like the preamp is 'over on here']..
  and you can get the guts loaded 'in your head' well enough to go through the signal path and view values as you go.
  I've had good luck sticking with mostly the original values, especially for bias etc.
  Old transformers of 'select origin' such as these have always proven fun and rewarding when I work with them.
 

[donald stringer]

  I"m going through the schem now, on that page for possible improvements it says to remove eq pcb from sig path conn 1-2 and 5-6 [of course this is for hi-fi but if theyre talking about tone controls this would also be where I would install my own tone improvements. This amp does not have an push pull output.

[petemoore]

This amp does not have an push pull output.
  Viva SE amplification !
 

[donald stringer]

 What is SE amplification?

[petemoore]

   
  Doesn't require a balanced PI, shoot, no Phase Inverter needed...
  And the output tube[s don't have a mid-swing crossover point to worry about.
  Less effecient than push-pull, Single ended is sweet, doesn't tend toward being powerful like Push Pull, they're generally <15 or 20 Watts, so I had to use PP amps so I could have enough power.
  I think a Tele would sound Funken through your amp, it kind of impresses me as a 'luxury' type amp, being dual single ended outputs.

[sean k]

I'm looking at the schematic and the B+ goes to a tap on the primary of the OT then on one end is to the plate of the EL84 and the other end goes to a 1k resisistor, meets the other 1k resistor from the other OT then connects to the EL84 screen + supply. Ok, its kinda ultralinear but not as I remember ultralinear which would see the B+ to one end of the OT primary and the plate on the other end and the screen to a tap.. but, okay I suppose it works. I think theres using the end of the OT as a kind of choke to smooth out the B+ alittle and get it down for the screen supply and the then the preamp tube. Plus it looks like theres feedback to the cathodes of the triode.

I think if this where mine I'd run the EL84's without that OT tap and just use resistors to drop the B+ somewhat for the screens and lift that feedback meanwhile changing a few resistors and caps in the signal path which would basically mean building a new box for and remounting everything. Then I'd get another 12AX7 and make the propietary front end of a fender or marshall then split the signal into the two drivers for the EL84's. I wouldn't even worrow about a set of tone controls and use the FX before it to control what goes in.



Thats your basic old fender champ from the late forties but then they put more gain in and the schematic at the top of this page has the high and low inputs which basically, by cutting out the low input switch is two volatge amps. Below that is a bunch of mods to make it like a train wreck
http://yeomansinstruments.blogspot.com/2008/06/schematic-for-my-champ-to-wreck-mods.html

[donald stringer]

I am digesting the info and so far its pretty clear. This circuit is like having two different  circuits to two speakers. A dual mono block? Hence the name stereo concerto not true stereo but you know what I mean. I am understanding that according to the schematic I can put your standard fender or marshal pre- amp or what have you where the triodes inputs are on the schem 2-7

[petemoore]

  Screen taps ...Fancy !
  Personally I would consider the output section special, and wouldn't change a value in it.
  The preamp...well looks pretty tight, perhaps you can just re-arrange a few things to press a wise tone control of in there, it doesn't look like it'd be easy to find room for adding components.