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The Need To Bleed...........and conversion voltages

Hello,

In earlier posts I reported I had not yet experimented with bleed resistors on the two supplies in Greg's 245 amp. Well, that has been resolved somewhat this evening.

A high wattage bleed resistor, across the B+ to ground, inserted at the location after C2, in a L1/C1/L2/C2 filter chain, can be helpful in drawing more current through the chokes, and it is beneficially, a stable constant current. The main downside is the sonic signature of the bleed resistor itself, infringing upon the transparency and purity of the music's presentation. ( You can hear the R !! )

We are very fortunate today, because the Mills MRA-12 line of resistors is available from Mike Percy, and THAT specific type R has the least negative artifacts, of any I've tried, in this type of service. The MRA-12 is, IMHO, indespensible !!

We tried bleeder R's across Greg's front end supply and also, across the 245 output stage, a separate B+ supply. The front end supply powers a Type 76, one for each channel, two total, at 7 mA. each tube as I recall. The output stage draws only about 30 or 31 mA. per globe 45, so its a total draw of about 60 mA, versus 14 mA. total draw for the Type 76 input stage.

Bottom line, the bleeder R was VERY helpful to the Type 76 stages, normally drawing only 14 mA., and not too noticeable with the output stage B+ , when it was drawing 60-62 mA.

I experimentally listened to different values of bleed R's across the type 76 stage, but I didn't have enough " non -grungy sounding Rs " to pull all the current I would have liked to pull. For now, we have a 13.33 K ohm power resistor added across the Type 76 supply, drawing an aditional 12.4 mA. So, I've increased this stage's total draw from 14 mA. to 26.4 mA. This added draw decreased the B+ voltage a bit, and I simply addded another 10 VAC to the power trandformer, ( those Signal 500 VA DU-1/2s are flexible !! ) to get back my original B+ voltage, which we liked the best, sonically as a transformer-loaded Type 76 op point. The added bleed resistor sounds better to us, gets the choke drawing more current ( its rated at 600 mA. ) and the resulting music's presentation was both far more dynamic and more subtle in nuance, pretty cool stuff to listen to.

When going to the Triad C-40Xs as L1 and C1, I'd previosly estimated your final B+ will become : VAC on the PT secondary under load times about 1.35 , minus the voltage drop in the rectifier. I also promised I'd measure this supply in AC and DC terms, here goes, using Fluke 8060A:

Line was 120.9 VAC

Output Type 245 stage, two tubes drawing 31 mA each is 62 mA total draw. Power tansformer secondary, under load, 249.6 VAC. Final stage's B+ was 269.7 VDC with 0.43 VAC of ripple.

Input Type 76 stage, 14 mA. from two tubes and 12.4 mA. from an added shunt / bleed resistor, or 26.4 mA. total draw. Power transformer secondary, under load, is 156.3 VAC. Input stage's B+ was 165.7 VDC after temp stabilizing, with 19 mA. ripple. Rectifiers in both supplies were 5U4Gs as a Graetz ( sp? ) bridge. C1 and C2 were 40 uF ASCs in the output stage, and 50 uF oils, in round cans, in the Type 76 stage. Use nothing larger than 50 uF BTW.

I have posted this in an effort to be helpful to any who may want to experimentally duplicate a similar supply in their own gear, which I believe you will really enjoy hearing., Do NOT get hung up on ripple "numbers", but rather, listen to how this supply sounds in dynamic terms - playing music - which is its forte.

Oh, Dennis and I mentioned using heavy gauge wire as being critical. In my experiments with Greg's 245 SEer, I had to use three Radio Shack clip leads in parallel for each wire run in the supply, from the rectifier forward through C2. I can only imagine what we will next hear with good wire, shorter lengths, and Wonder soldered ........ Greg's next DIY project, two mono amps !!

Hope this helps someone out there, let me know please. Thanks a lot.

Jeff Medwin


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