Hello all,
In the process of rebuilding a 38-38 for a freind, I've found the input transformer is open. Does anyone know what the rating is and if there is a replacement available?
Thanks!
Jon
Tough one! That set has Class B output, which means the driver transformer needs to be able to handle some power. Here's what you need: Ratio 2 to 1 (primary to HALF secondary). In a similar set, the transformer measured Primary: 998 ohms, secondary 1222 ohms CT.
Note this is the OPPOSITE of the typical A52C, A63C etc (1:3CT). These simply will not work well (distortion).
In most cases I used a single ended OUTPUT TRANSFORMER back to back with a UNIVERSAL PP Output Transformer (connect voice coil to voice coil) and set the taps so that the ratio is about 2:1 primary to half secondary. I use an audio oscillator set to about 400 hz and select the tap on the universal transformer that gives me the result closest to the appropriate ratio. Unfortunately, you have to be able to hide one of the transformers, and have room for two.
The only other way is to find a universal input transformer such as the Halldorson HVAT-3 driver transformer, which has 2:1, 1.5:1, or 1:1 ratios, single or push-pull. I found one at PTOP, but it was expensive and also big.
In a real pinch you could try a PT-K2 (available from AES) which are 10K Primary to 2.5K CT Secondary. These used to be available, and were very cheap. But they are 4:1CT so gain suffers. Also, not enough inductance, and very limited frequency response (I think they don't go below about 300hz).
Dave
Correction on the AES PT-2K alternative.
Looks like they are NO LONGER SOLD. There were a close-out item awhile back. I have also seen these sold on eBay. Apparently they were used in a vacuum tube transmitter to drive a class B modulator stage. I have a handful of them. Let me know if you need one - but be advised - your radio will not sound as good as it should (BUT IT WILL AT LEAST WORK!) Here's the complete specification:
PRIMARY: Z = 10K Ohms*, 10 ma. DC; Res. = 266 Ohms; Leads: Blue to driver plate; Red to B+ line
SECONDARY: Z = 2.5K Ohms* with CT; DC Res. = 170 Ohms, Leads: yellow to modulator tube 1 grid; black (CT) to bias supply, green to modulator tube 2 grid
IMPEDANCE RATIO: Full Pri. to Full Sec. = 4 : 1, step-down
TURNS RATIO: Pri. to ½ Sec. = 2.8 : 1, step-down (NOTE: should be 2:1 in your radio - so some loss of audio will result)
POWER RATING: 8 Watts, max.
FREQUENCY RESPONSE: 150 - 15,000 Hz., -3.8\+0.1 dB. (unloaded)
DC OPERATING VOLTAGE: 340 Volts (recommended)
HI-POT TEST: Winding-to-winding and windings-to-core @ 2 KV DC: Pass
CORE: silicon steel, E-I, interleaved laminations
MOUNTING CENTERS: 1.75 inches
*(Impedances verified with General Radio 1650B RLC Bridge
Thanks for the helpful information, Davemc!
I may be interested in one of your transformers and will pm.
Jon