amplifier for unknown output transformers?

[tonskulus]

Okay, I just got two really old output transformers (1950's). I was planning some kind of amplifier around these. Trafos has two primarywindings for push-pull outputstage and two secondary windings. I used 12VAC psu to measure impedance ratio.
input: 12VAC in single sec.winding caused 2x115V in "primary windings".
230/12=19.1
19.1*19.1=365:1 impedance ratio.
So if I use 8ohm speaker, there is 2900ohm load impedance for tubes. Is that correct?

Physical sizes of trafo:
10x12cm, cross section area is 12cm2, enough for 50W output?

Anyway, that is good starting point. Now I just need some tubes which can handle 2.9khom load. Any ideas? 5881, EL34 or similar would be nice.

[Tom Bavis]

Let's see.. that might be a 1.3" stack of EI50? Might be good for 50W with 6550s, or parallel push-pull 6L6. What is DC resistance? Are you sure they are output transformers, not power?

[tonskulus]

These transformers lies in old tube amps. Sad thing is that there is no tubes(except preamp stage, EF40) and another one has no power transformer. Only chassis and components and both output transformers are still there.
I'm not 100% sure but I think this amp has used pair of EL51 pentodes. No other tubes has matching pinouts.
There is also phase shifting transformer for output tubes. preamp stage has EF40 and EL41. As much as possible, I want to use original circuit with transformer coupling. Power supply will be external.
Output transformers DC resistance: pri - 2x154ohm(308 p-p), sec: 2x1.3ohm.

Picture:
amps

[Tom Bavis]

I had never heard of EL51 - but I found a data sheet at tubedata.org. Closest current tube is 6550 or KT88.

[refference]

Hi tonskulus ,

If you see the EL51’s datasheet , you will conclude that your
output transformer , was built regarding the old 12 ohms spea-
kers from old-time cinemas , so the primary impedance was 4300 ohms plate to plate , enough for a 65 Watts push-pull class AB cathode bias amplifier , BUT if you will use an 8 ohms speaker (no less ), the primary impedance will be 2900 ohms plate to plate as you had calculated .

In being so , IN MY OPINION , you have some options , but the
“best” ones are :

1st ( so/so good ) – KT88 , push-pull , triode connected , class AB1,
cathode bias , +B = 450 V , Pout ~ 30 W

2nd ( good ) – EL34 , single-ended , pentode connected , class A ,
cathode bias , +B = 270 / 300 V , Pout ~ 10 W

3rd ( best ) – EL34 , push-pull , pentode connected , class AB1 ,
fixed bias ( - 36V ) , +B = 380 / 400 V , Pout ~ 45 W

As I said above , these options are MY OPINION , other people
may has different one , BUT , I believe that , this is a good starting point for your job .

Regards ,

Carlos

[ray_moth]

If it's a PP tranny, forget a single-ended solution because it won't be designed to have net DC in the primary and it will saturate.

[Shoog]

If you want to use the interstage phase splitter, back in the old days they seemed to have fairly limited frequency response(12Khz response was considered fairly good). You might need to redesign the front end to lower driving impedence. This should extend their useful response somewhat. Think about rapping some loop feedback around the interstage. The pentode driver doesn't seem like a good idea.

Shoog

[tonskulus]

Originally posted by refference
Hi tonskulus ,

3rd ( best ) – EL34 , push-pull , pentode connected , class AB1 ,
fixed bias ( - 36V ) , +B = 380 / 400 V , Pout ~ 45 W


Carlos

That sounds good but I'm not sure about that low load impedance for two EL34 tubes. Under 3000ohm impedance gives 45W out? I think this will need four tubes, two in parallel to match 3000ohm load impedance. Well, I have test bench ready so I can test this configuration. Is that 380V for G2 and 400V for plate? Well, there should be at least 300V for G2 to get as low plate resistance as possible. However, let's see how it works.
Glowing plates and grids..Not so unusual for my tubeamp prototypes.
I still can't figure out everything necessary what comes to tube theory.. but practice makes makes champion.

[refference]

Hi tonskulus ,

Sorry for the delaied reply .

The optimum plate to plate impedance for 2 x EL34 in the
push-pull way is 3500 ohms , BUT with 2900 ohms they
still work properly .

Don’t try to use 4 x EL 34 , being paralleled 2 by 2 , becau-
se the total current will rise A LOT , and the output transfor-
mer will “suffer” A LOT , perhaps blow out .

But if you have any doubt about my previous suggestion , START FROM A BIT LOWER POINT , then I recommend you to do the following experiment :

Class AB1 , EL 34 push-pull pentode operation , CATHODE BIAS

+ B = 350 Volts able to supply at least 190 mA
Supressor grids (G3) – Connect each one with the respecti-
ve cathode
Screen grids (G2) : Connect each one to the + B , using a
470 ohms x 2 watts resistors , obviously ,
one resistor for each grid .

FORGET ABOUT FIXED BIAS : Use cathode bias , strap
the two cathodes togheter , then connect a 135 ohms x 10 W
( two paralleled 270 ohms x 5 W ) wirewound resistor , paralle-
led with a 220 uf x 50 V cap , between the two cathodes and
Ground . The bias voltage at the cathode , must to stay around
22.5 ~ 25 Volts ( optimum 23.5 Volts )

Control grid (G1) : Connect each one to the earth , by a 220 K
x ½ Watt resistor .

If you build a convenient phase inverter , able to produce a maxi-
mum 15 Vrms of audio signal , at each output , you can do your
1st listening test . Be careful to choose the speaker , the impe-
dance value have to be 8 ohms , NO LESS . This assembly can
produce a maximum 35 Watts of power output , with only 1.5%
THD without feedback . With global feedback loop , this value
will lower to aprox. 0.15% or less .

Any doubt , feel free to ask .

Carlos

[tonskulus]

Originally posted by refference
Hi tonskulus ,

Sorry for the delaied reply .

The optimum plate to plate impedance for 2 x EL34 in the
push-pull way is 3500 ohms , BUT with 2900 ohms they
still work properly .

Don’t try to use 4 x EL 34 , being paralleled 2 by 2 , becau-
se the total current will rise A LOT , and the output transfor-
mer will “suffer” A LOT , perhaps blow out .

But if you have any doubt about my previous suggestion , START FROM A BIT LOWER POINT , then I recommend you to do the following experiment :

Class AB1 , EL 34 push-pull pentode operation , CATHODE BIAS

+ B = 350 Volts able to supply at least 190 mA
Supressor grids (G3) – Connect each one with the respecti-
ve cathode
Screen grids (G2) : Connect each one to the + B , using a
470 ohms x 2 watts resistors , obviously ,
one resistor for each grid .

FORGET ABOUT FIXED BIAS : Use cathode bias , strap
the two cathodes togheter , then connect a 135 ohms x 10 W
( two paralleled 270 ohms x 5 W ) wirewound resistor , paralle-
led with a 220 uf x 50 V cap , between the two cathodes and
Ground . The bias voltage at the cathode , must to stay around
22.5 ~ 25 Volts ( optimum 23.5 Volts )

Control grid (G1) : Connect each one to the earth , by a 220 K
x ½ Watt resistor .

If you build a convenient phase inverter , able to produce a maxi-
mum 15 Vrms of audio signal , at each output , you can do your
1st listening test . Be careful to choose the speaker , the impe-
dance value have to be 8 ohms , NO LESS . This assembly can
produce a maximum 35 Watts of power output , with only 1.5%
THD without feedback . With global feedback loop , this value
will lower to aprox. 0.15% or less .

Any doubt , feel free to ask .

Carlos

I'll try that. So let's see if it works..