Hello all, I recently picked up 3 modded Stereo 70. All were mods from the 1980-1990 era. One is a Sutherland, one a GSI (with 8417 in triode mode) and the topic of the moment, a VTA with the deluxe board from 1990. Having some trouble getting this unit to operate correctly. The previous owner and modder added a 1K 10 watt resistor between the rectifier tube and the 470 uf cap stack. I don't have a proper schematic, but it appears he was trying to limit surge on startup, as 235 uf would definitely hurt a GZ34. B+ at pins 3/4 of the EL34 is only 390, and with the bias scheme as he set it up, I barely got to 40 mv across the 10 ohm resistors. He had the bias resistors at 12k - bias pot - 30k - ground. Was measuring -42v on the lowest setting of the bias pot at pin 5 of the EL34. Changed the 30k to 15k and was able to get the reading across the cathode resistor to about 100 mv, and the voltage at pin 5 was down to ~ -29. Put a 22k in place of the 12k to reposition the voltage divider and while it got better, I was still in the 200 mv range for max bias, and began to notice a slight burning smell. With each bias attempt I also checked a 1kHz sine wave and in each case the sine wave was stepped mid wave. Max power at clipping was anemic at best. I suspect the prior owner took this amp on an unfinished journey. I don't have any documentation to help me return this to the intended state of mod, so I would be grateful for any comments from the group. Have tried several times to load a pic of the chassis. Hopefully it displays. If not, I will try to add. https://i.servimg.com/u/f74/20/46/71/27/pxl_2011.jpg
Last edited by pickindoug on Sun Dec 04, 2022 10:10 am; edited 1 time in total (Reason for editing : adding photo)
