"Kingfisher" P-P Amp using 7591A Outputs and Fisher 500B Iron

[wrenchone]

This is the start of a thread for a P-P amp using the power and output iron from a Fisher 500B receiver. I got the transformers a couple of years ago on Ebay, intending to do a 7591A amp with radically different input/driver stage than the original design, which used 12AX7 for input and concertina phase splitter.

In short, I'm using a hybrid differential input stage featuring a pair of matched J309 JFETs cascoded with the small triode each from a pair of 6CS7 dual dissimilar triodes. The rationale behind this is to get some decent first stage gain and allow easy close matching of the input differential piar - a snap fro me with JFETs, not quite so easy (or cheap) with tubes. I'll be using a similar scheme with a pair of JFETs and a 12AU7A to drive a pair of 6P14P-EVs for another project.

So that the second, beefier triode does not feel left out, the remaining two triodes are used as source folowers to drive the 7591A control grids. This will allow some excursion into Class AB2 operation without blocking and other antisocial behavior.

Actually the biggest pain so far has been to put together the dropping strings to generate all the fiddly intermediate voltages for the 7591A screens, a cleaner supply for the input stage, and a ~200V supply for the cathode followers. I could have cheated and built a dedicated switching supply with all those outputs available, but I was determined to use the original power transformer, which has a single HV winding that drove a doubler in the original receiver schematic, generating a +400V supply. In the original receiver, all the B+ voltages were derived from this supply. Same for the new amp.

The circuit and picture will follow when I'm happy with everything. Hopefully the amp will make an appearance at the 2009 Burning Amp this weekend.

[SpreadSpectrum]

Be careful with that Fisher power transformer. My X-101-B draws 208mA quiescent. The Sam's folder shows the power transformer as rated at 200mA.

One day my wife said, "What's that smell?" I ran in to check and noticed wisps of white smoke coming out of that transformer, amp still playing music. I killed the power and listened to the insulation inside the transformer boil for the next few minutes. I guess it never shorted because the fuse never blew. I'll be building an outboard supply as I don't care to have a transformer rewound that was inadequate to begin with. Plus, I've recently fallen in love with Maida regulators for P-P AB power stages.

[wrenchone]

If it comes to that, I'll gin up a switcher that will run as cool as a cucumber and allow me to strip out all the lousy dropping resistors. I plan to use standard 7591A biasing to start with, 30-35 mA per tube, 400V plate, 350V screen.

[SpreadSpectrum]

I love those 7591s. They are just beautiful in the dark.

[wrenchone]

That's what I'm hoping, too. Right now, I have a matched quad of the EH 7591As. I had an old Scott LK-72B years ago (never should have sold it). The 7591As in that beast glowed rather fetchingly as well. I forget whether they were Scott branded or Sylvanias. The glow would comoplement my chassis nicely - metallic cast-iron gray for outer chassis and transformers, deep metallic blue-green for the front anf top deck, with blue-green LED in front.

IF the EH tubes poop out on me, I'm pretty sure I have enough latitude in bias and drive to make do with some 6P3S-Es.

[SpreadSpectrum]

I found the fisher amp in my mother-in-law's attic. All of the 7591s in the amp had the blue glow. I think two were GE with a regular base, one was a GE with a coin base, and one was marked 'Fisher'. (maybe one of the original tubes?) Unfortunately, the GE coin base 7591 would glow red as well after about 60 seconds on.

The tubes in the picture are new EH 7591s. In my limited experience, I've actually never seen a 7591 that didn't have a nice blue glow.

[Eli Duttman]

Avoiding blocking distortion is fine. Serious g1 current in 7591s is not a good idea. The type is high gm, which means a tight g1 helix. If substantial g1 current is present, the heat generated has no place to go.

[wrenchone]

Thats why I'm using just cathode followers (and not particularly rugged ones) to drive the outputs rather than a mosfet. I wasn't planning on hammering the snot out of the poor 7591s, but to be able to take light dips into AB2 without serious audible consequences.Actual depth of AB2 operation depends on the drive stage, and I'll be taking steps to limit that. I may also consider putting more stopper resistance on the grids to help limit grid current. Right now I have 120 ohms.

Edit -I have the drive capability, so if I feel like hammering, I can always drop some other 6L6 variant in the output sockets and rebias. However, since I have 35W transformers and only 400V B+, there won't be a whole lot of payback from getting too frisky.

[rickl]

I just picked up a X-101-B so I'll be watching this thread with great interest.

I haven't even powered it up yet.

[Eli Duttman]

Originally Posted by rickl

I just picked up a X-101-B so I'll be watching this thread with great interest.

I haven't even powered it up yet.

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