next project: rebuild using fisher 500c trannies

[zobsky]

just a quick update:

1. placed an order for a matched quartet of 6bg6 ($32)
2. my oscilloscope came in today. .... assuming it works (no trust in UPS / ebay ...), are there any tests i can perform on the output PP transformers to guage their performance?

thanks

[SY]

Sure. Once you've established continuity and no shorts, load the secondary with an appropriate value resistor (you don't need a lot of power here!), then drive the primary with a 1kHz square wave. See what the waveform looks like across the test resistor. Do the same with 100 Hz and 10kHz. Look for ringing and excessive tilt or excessively slow rise time.

A frequency sweep from 10 Hz to 100kHz using sine waves is also a good idea so that you can get an idea of small-signal frequency response. If your transformer has a HF peak followed by a rolloff, you want to know this so that you can design your feedback network and compensation appropriately.

[zobsky]

Hi again,

how can I find out which leads of my OPTs correspond to what connection? All I have is a schematic ( http://oldtech.net/Fisher/500C/Schem.pdf ) but it the transformer connections are not colour coded.

Also, I notice the B+ on the tubes is around 450V . Does this limit the operating point of my 6BG6 tubes (which should be arriving today, BTW)?. I still need to decide a circuit, or at least an operating point, so I can pick up a power transformer this weekend, but work has kept me busy .

Thanks

[SY]

A few minutes with an ohmmeter should help you sort things out. The primary will have something like 100-300 ohms of DC resistance (the CT being in the middle, of course!). The secondary will have a much lower DCR, a couple of ohms at most.

You can confirm the ID of the leads (especially in the secondary) by driving the primary with a signal generator, then looking at the voltages at the secondary.

[zobsky]

still researching on the topic of interstage, .. i came across the concept of using the output tubes as their own phase inverter, as explained in this article ( http://www.triodeel.com/compact.html ) and also implemented here ( http://www.diyparadise.com/simpleel84.html ).

1. i don't get it, .... this sounds like "free lunch",.... what is the con of this method .



2. the only condition imposed upon the choice of tube is that it should be inherently easy to drive. the 6bq5 mentioned in the circuit needs a max of 22v / 2 = 11v (in this case) . I'm not sure how this figure was obtained (being a newbie), .. but I'm assuming (in case of the 6bq5) the 22V was the largest grid voltage number on the pentode curve multiplied by 2.(tube curve at http://www.retrovox.com.au/STC6BQ5.pdf )
for the 6bg6-ga tubes (which i have already recieved, shipped in 2 days, ...amazing ) with curves at http://www.mif.pg.gda.pl/homepages/f.../6/6BG6GA.pdf, this would correspond to a drive requirement of (30 x 2 = 60V) .. does that mean that this idea won't work for my tube

thanks,

[SY]

No free lunch, it's just that the balance, distortion, and gain are compromised. The earlier in the circuit you do a single-ended to balanced conversion, the better when you consider distortion, noise, and power supply rejection.

[zobsky]

Originally posted by SY
No free lunch, it's just that the balance, distortion, and gain are compromised. The earlier in the circuit you do a single-ended to balanced conversion, the better when you consider distortion, noise, and power supply rejection.

Hi,

How do you best suggest to address the severe demands placed on the preamp by the jensen you suggest. I was thinking a 3 stage amp thus (given my limited knowledge):

1. I was thinking to run a driver tube parafeed through the input splitter (still considering some edcor iron, .. based on encouraging posts at AA).

Limitation of this would be the amount of swing I can feed and get through the suggested 10k:150 or 10K:600 trannys

2. Feed the opposing signals out of the secondary into a dual triode (say a pair of 5687) ==> is this necessary or can I get away using a high amplification driver in step 1??

3. These will be feeding the power stage. Is it possible / desirable to do this direct coupled?

Thanks,

[SY]

10Kohm is not too severe load except for really weedy preamps. If that's what you've got, either deweed it or use a 100K load on the Jensen with the RC compensating network they suggest. The 100K load is really two 50K resistors in series across the secondary, with their junction returned to ground. From there, you've got a phase-split signal (i.e., balanced) which can drive a diff amp. No need for anything like a paraphase, concertina, SYclotron, or suchlike whoopdeedo.