I had an ST70 in need of new caps lying around my workshop so I decided to do a holiday vacation quickie on it. Thanks to planet10’s link to EDCORP I picked up a couple of their 10k/150 interstages for about $15 and some 500V/100uF caps off of
eBay for $3, the rest was from the junk box. I stripped it down to almost nothing but metal and glass, trioded the outputs with 100 Ohms, and mounted the new trannies on a nice piece of mahogany where the circuit card was. My line stage is a Jung AD823/AD815 composite amp so it has no problem with the 150 Ohm input, I just moved my attenuator to before the line stage and lowered the resistance in the bias network to 10k.
I checked all the voltages to prevent the magic smoke and fired it up. First impressions were very favorable. A quick frequency response (w/speaker load) showed only a little bumpiness due to imperfect damping and at a comfortable level (my office is small) the 1kHz distortion was .025% thirds and .02% seconds. The most surprising thing was how quiet it became, hum too (even though I worried about the unshielded trannies you need to put your ear up to the speaker).
Now for the questions; I got 427V plate supply and ran the outputs at the original 100mA. Not being a valve guy, is there a better operating point? Most of the stuff I found in the archive was for SE amps. Any other comments?
eBay for $3, the rest was from the junk box. I stripped it down to almost nothing but metal and glass, trioded the outputs with 100 Ohms, and mounted the new trannies on a nice piece of mahogany where the circuit card was. My line stage is a Jung AD823/AD815 composite amp so it has no problem with the 150 Ohm input, I just moved my attenuator to before the line stage and lowered the resistance in the bias network to 10k.
I checked all the voltages to prevent the magic smoke and fired it up. First impressions were very favorable. A quick frequency response (w/speaker load) showed only a little bumpiness due to imperfect damping and at a comfortable level (my office is small) the 1kHz distortion was .025% thirds and .02% seconds. The most surprising thing was how quiet it became, hum too (even though I worried about the unshielded trannies you need to put your ear up to the speaker).
Now for the questions; I got 427V plate supply and ran the outputs at the original 100mA. Not being a valve guy, is there a better operating point? Most of the stuff I found in the archive was for SE amps. Any other comments?
Ciscokid said:
I guess I left out my original purpose. I wanted to try the ST70 as a no feedback PP triode amp. I bought what were actually preamp output transformers (10k/150) and used them backwards as step-ups directly coupled to the grid bias resistors, the added benefit of no capacitors in the signal path. My SS preamp has no trouble with the 150 Ohm input impedance that the amp now has. So what I have is an 8.16 to 1 step-up directly coupled to the EL34's and the output transformer and nothing else in the signal path. The sensitivity is a lot lower than normal but that particular line stage puts out 20V p-p into even 50 Ohms easily.
BTW the transformers were obviously made to order, they were wrapped in plastic before the Q-dope had cured. Now I know why its called dope, I almost passed out when I opened the box. I tried using a 20k load hoping to get a slightly higher input impedance but the highs started to roll off so I stuck with 10k. I just split it into 2 - 5k sections and fed the -32V bias into the center tap.
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