headroom

[Rocketboy]

On a 200S, is it reasonable, after stiffening the p/s, to increase the plate voltage of the second preamp stage (the unbypassed one) to around 250V in an effort to drive the finals a bit harder?  Will the 6AN8 freak?  Anybody tried this?  Seems to me a fresh pair of KT88's at 480V should be easily able to do better than 60W.  195V on the 12AX7 plates seems a little squishy for a bass amp, especially when half the tube has the cathode unbypassed.

This may not be worth the effort; kinda wondering if anyone out there has tried something like this.  Takers?  Thanks.  Ric

[Anonymous]

I don't think the P/S is going to be the problem, I think it is going to be the size of the Output transformer that will dictate the output wattage limit

[Rocketboy]

Thanks for the reply!  The GZ34's gonna go away; I'm not concerned with the p/s either.  Guess I'll need to find specs on the 3-6OT.  It sure looks like a 100W output transformer.  Sure would hate to smoke it.  Thanks again.   Ric

[Greg Z]

OT's on the early Sunns are very adequate for whatever you throw at them.That tranny should
handle 120w no problem. Ditch the GZ-34,and maybe throw another 20uf up front. 90
watts clean should be no problem. Don't forget to rebias.
Greg Z
to thine own sound be true

[Rocketboy]

Mr Greg Z - Thanks for the most useful reply.  I am now about to display ignorance, so please just laugh amongst yourselves.

Back in the '70's we set bias voltage using the oscilliscope/audio frequency generator/dummy-load method.  It's been a few years.  Mr. Gerald Weber has made such an airtight case against using the above biasing method, I now feel like a fool for ever having used it.  He's correct.

So my question is:  Is -55V OK for the KT88 grid bias supplies even with the slightly-increased drive?  I intend to use a matched-pair of Svetlana KT88 tubes, for which I have the curves.  Can I neglect the change in plate current?  Guess if not I can calculate the grid bias based on measurement, but under what condition?  Please excuse my ignorance.  I would like to get this right.  Thanks again!  Ric

[Greg Z]

Mr Rocketboy
Heres my take on biasing,Plate dissapation
is the important thing here. Bias it too hot
and it cooks the tubes, too cold and the amp
sounds lifeless. On the cool side you will get more headroom and the tubes last longer. On the warm side you will
saturate earlier and the amp will sound harmonically richer,however the tubes will wear faster.

As far as the -55v, Good starting point back in those days when tube specs were pretty tight.

Todays line voltages are a bit higher,raising
the b+, the required bias voltage needed changes with this.

Here is a link to probably one of the better
papers on biasing.
http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt

Let your ear be the final judge.

[Rocketboy]

Dear Greg Z -  Got it!  You are rather cool.  So here's where I start: the rectifier tube is history.  Rectification to be accomplished by two pair of 1N5408 diodes connected appropriately.  First filter increased to 47mf/630V.  Replace all other filters at original values but higher voltage.  Plate voltage on the (new JAN 12AX7WA) preamp tube will be increased to 250V.  Yes, I'm going to do this one step at a time.  Bypass and tone cap values will be increased 25% or next higher available equivalent, except driver-final couplers will be doubled (25mf to 47mf).

As for the biasing, -55V is given with line voltage at 117.  I'm just going to assume that Joe's Cheeshead House of Exotic Pleasure up in Wisconsin has lousy regulation.  What else can one do?  -55V it is.

What I am after here is an amp that responds a little quicker and has some more headroom.  I'll keep you posted if you care.

Thank you so much, again.  Ric

[Greg Z]

Yes Ric please keep me posted.
While your in there stick the 1^ cathode resistors (between pin8 and ground)in so you can monitor the dissapation. I'll think you'll find its gonna take more than -55 to get the bias in the zone. Any questions,my e-mail is below.