[Phlogiston]

Question for you tube amp gurus:  A bit of background before the question. Normal suggested bias for the 7591A pair is 70ma combined cathode current (35ma/tube). The amp generated fair heat and the trannies would get fairly warm to the touch.  Reduced current to 65ma and noticed a reduced heat factor, without affecting the sound - plenty of power in my studio.  I've now reduced the cathode current to 56 ma (required a bit of circuit change to get the bias voltage where it needed to be for the reduced current), and the amp still sounds great, though I'm sure my max power output is down, but still way more than I need.  How far can one reduce idle current before a degradation in sound at lower listening levels?  I would assume that getting the current too low would put the tube in an operating range where linearity would be affected.  I'm just looking for maximum tube life without sacrificing sound quality.  Power supply voltages run about 15 volts high with the reduced output idle current.  A problem? Well within the voltage ratings of all caps.

I think nobody answered the bias question because it is hard to tell. At some point the sound becomes a bit thin and tinny. A little depends on the listener. I don't think what you have done will be noticed.

As to the B+, how does it compare with the max plate/screen voltage of your output tubes?

Yeah, add a fan.

[rfeenstra]

voltages are well within operating limitations of the tube.  The amp sounds great this way and I don't really push it that hard so I don't run into a power problem because of the lower idle current, so I'll leave it this way.  It runs fairly cool. No fan yet, but it's fully exposed so cooling is good.  I have looked seriously at the bias setup that many are adding to the Fisher amps.  The active control of the bias and screen voltage as a function of the B+ apparently greatly reduces the idle current while maintaining the power and distortion characteristics of a properly biased output.  Looks interesting.  The AudioKarma forum has a good bit of info that Dave Gillespie has put out on the fishers that should also work on the Scott.  It's working so good right now that I'll probably just leave it alone for now.

I've got the matching FM tuner up and running now - A HH Scott Stereomaster 350D.  I have the RF and IF sections aligned using a spectrum analyzer, sweep generator, scope.  I don't have a multiplex generator so getting the multiplex section aligned and adjusted properly is a bit of a problem.  I can't find any Scott info on this unit outside of the schematic.  No procedures, so I have to adapt from other equipment without the proper test equipment.  It works and I stereo is ok, but i think proper adjustment would improve separation.  I seem to have trouble getting the 19khz pilot to peak at the same place as the signal.  I need to be just off peak signal for the stereo to lock in.  I don't know enough about this stuff to know what to do to fix it.  I also am not sure how to adjust the R and L separation controls for best separation. I've learned a lot, though, in working with this thing!

[morzh]

You could try to find the operating curves of the amps' output (or the same for the output tubes with the load you are running) and see what it'll do when the current goes down.

[Phlogiston]

You know, over time the idle current is probably going to drop as the tubes age (assuming bias on a "new" tube is set properly from the beginning). In this aging process the sound usually doesn't degrade much until the end. I have installed old/used tubes that conduct considerably less and they sound fine.

As I recall, I set my Scott up so the idle current was about 10% below spec. I'm happy with it.

Oh, did I mention the fan

Heat causing vaporization and condensation of conductive material on the micas/insulators is a common cause of redplating, tube and transformer failure.

The little 10 ohm cathode resistors are a good way to protect and calibrate the output.

[rfeenstra]

The cathodes for each channel are hooked together and run through a 3.3 ohm cathode resistor.  That's where bias current is measured.  I like the idea of individual 10 ohm resistors as it would allow measuring the current in each tube rather than the combined current.  The Scott has separate bias and balance controls for each channel so setting the bias for each tube would be doable.  I like David Gillespie's bias circuit as it runs the tubes close to cutoff should a bias pot open.  If my bias pot opens, the tubes would redplate.

[Phlogiston]

The 10 ohm resistors are wire wound 1/4 watt. If your tube redplates they will open. I highly recommend this for most tube amps.

[rfeenstra]

Do you have a source? Mouser is at about $12 each! Digikey, not much better if at all (non stock - 1000 minimum).

[morzh]

https://www.ebay.com/sch/i.html?_from=R4...m&_sacat=0

https://www.mouser.com/Passive-Component...z3Z1z0wljo

[rfeenstra]

Thanks.  I have no problem finding the metal film resistors. In fact I have those.  Russ, for some reason, specified wire wound,  1/4 watt resistors.  I don't know the significance of the WW as opposed to the metal film.  I'm just not that familiar with resistor characteristics.

[Ron Ramirez]

I think I used metal film in my Fisher TA-600. It has the IBAM mod as well.

[morzh]

In this particular case, with low power dissipation and not in the grid circuit where the noise characteristics are not important, I'd say it's probably all the same.

https://www.mouser.com/Passive-Component...9iZ1z0z819

Mouser wirewounds: they are not $12 each.

[Phlogiston]

I remember getting WW at one time. They may not be available now at a reasonable price. The foil one will likely open  if a tube redplates as long as they are only 1/4 watt. The ones I have been using are Vishay Dale. I probably have a part number on the bag - try to remember to look.

Being able to use them as a shunt for calculating idle current is the 2nd best reason to add them.

>>>Bag is gone but the resistors say Dale 1%t1 1535. I know I got these in the last year from Mouser.

[rfeenstra]

Thanks.  I'll check it out again or I'll just use metal film.  Better than what's there.

[rfeenstra]

Been working on the 350D HH Scott FM tuner that's the mate to the LK-72-B.  I was having a dickens of a time getting the thing aligned.  Couldn't get the multiplex unit to work worth a crap.  Mono was good.  Finally borrowed the right equipment to get the ratio detector set right.  Still problems with the multiplexer unit.  Had beautiful 19KHz signal and I thought a good 38KHz signal.  I could line the phase up perfectly.  Still, stereo was very touchy. Then I accidentally shorted the grid of the 12AX7 38Khz osc. to the plate with a clip lead. Ouch!  Smelled it. It took out the tube and 2 germanium diodes in the stereo switching circuit.  A blessing in disguise.  Replaced the tube with a NOS RCA I had kicking around (the old Telefunken 12AX7 had tested very strong on my Heathkit TT1), replace the 2 diodes.  Powered it up and viola, everything was great.  The 38KHz signal was much stronger, the decoder started working great, and I could set the R and L separation controls  for excellent separation.  It even works very good with a 5% 19KHz modulation instead of the 10% modulation.  Right and left output look great on the scope.  Best part is it sounds great!  I've been fussin with this thing for the last month and was frustrated.  Success at last.  Lesson: don't trust tube testers.  Use substitution if you can.

[Phlogiston]

AND you can probably still get $10 on Pay-Bay for that tube