Fender 300ps

[Smokebreak3]

Got one of these on the bench. I need to recap it first, but I can't seem to find any info on how to bias this one. 700VDC plates and 4x 6550. 6V6 driver and xformer PI
Owner provided tubes, Sino branded that I hope can handle that. Screens at 350VDC or so.

Amp appears to be quasi cathode biased also. I'm thinking set the "bias" pot to -42.5 and making sure it doesn't red plate and call it a day?

schematic https://cdn.shopify.com/s/files/1/0604/ ... 1758728571

[martin manning]

The cathodes are grounded so no mixed bias here. It has both bias level and balance pots. I would set bias in the usual way, use OT primary resistance to measure current if you don't have current sense resistors installed.

Epic 300 PS repair video: https://www.youtube.com/watch?v=V16vBl7poh0

[Stevem]

You might want to do a search for this web page about the ps 400.

It’s basically the same class AB2 amp but with a 6L6 driver , two more 6550s and a special OT.

[Smokebreak3]

The cathodes are grounded so no mixed bias here. It has both bias level and balance pots. I would set bias in the usual way, use OT primary resistance to measure current if you don't have current sense resistors installed.

Epic 300 PS repair video: https://www.youtube.com/watch?v=V16vBl7poh0

Oh jeez, I misread the 6550 cathodes on the schematic at a glance, thank you for the reply Martin. I'll put a 1 W 1R 1% and shoot for 120mA out of all 4, setting the balance pot midway also.

Steve I did find the wild repair checklist you speak of for the 400, if I read it correctly it basically involved setting bias based on how many amps the amp is pulling from the wall. The 400 reaches full power when aaaalll speaker connections are used. There's also a 160 with 2x 6550. This thing weighs 90lbs. the iron is comically large.

[mhuss]

I have a 300PS, I got the 'computer grade' caps to re-cap it awhile ago, but just haven't been able to get together the gumption to wrestle it onto the bench and do the work. The transformers in these are beastly and they are a bit of a rat's nest inside.

[nuke]

You might want to do a search for this web page about the ps 400.

It’s basically the same class AB2 amp but with a 6L6 driver , two more 6550s and a special OT.

Yeah, that's Rich Koerner. Great guy, met him in person many years ago. He knew his stuff and especially about those big PS series amps.

I think the only tubes that will reliably survive are the actual, real-deal GE 6550A. Back when I first started working on amps, you could still just buy brand new ones off the shelf at the local electronic supply house. How I wish I could have stocked up on those and every other kind of USA-made tubes back then, stored them all in military-grade dry storage under nitrogen.

[Stevem]

These days I think your dead on right nuke.

I think only a JJ 6550 would survive for a normal time in the 400, in fact it might be best to just skip a 6550 and pop in KT88s for a good added margin of safety and even a lighter wallet.

[mhuss]

Yes, I put KT88s in mine, and so far, so good.

[Smokebreak3]

Thanks so much for the replies guys, I put the amp down for a month while the strain in my lower healed from the first time I put it up on the bench myself, and I am not kidding.
I've got it all recapped, and fired it up just now with no tubes and all voltages good .
The owner provided the 6550A's and they are the cheapest ones you can get on tube depot, they appear to be the Shuguang ones.
Now that I've done all this work, and am at the finish line, the last thing I want is for this thing to come back.

However from what I understand, these chinese 6550A are more robust than JJ, to handle higher Vp ? Is this correct?

[Stevem]

I don’t know if that’s true or not.

I have used the JJs a ton in SVTs and I believe the Groovetubes version had added heat dissipation ribs added to the plate structure.

[mhuss]

The original GE 6550-A specs says 660 max plate volts (which is why Fender used "selected" 6550 tubes). KT88s are rated for 800 max plate volts. Just sayin'

[Roe]

The original GE 6550-A specs says 660 max plate volts (which is why Fender used "selected" 6550 tubes). KT88s are rated for 800 max plate volts. Just sayin'

Yes but this is running voltage, not idle voltage, according to the design max system. https://www.drtube.com/datasheets/6550a-ge1972.pdf

[nuke]

Been a long time since I've seen a 300ps or 400ps, but I've worked on a few long ago. I don't know that I could still lift one anymore.

They are brutes.

I know straight from Fender's tech support that they always insisted that their selected GE6550A tubes be used in this amp, orderable with a Fender part number. That's going back to the early 1980's when I was first working in an amp shop while I was in high school.

Also, later on in engineering college, I re-furbed estate sale tube hifi stuff as a side gig. I had a pair of William Z. Johnson modified Dynaco MK-III's mono-block amps. He also insisted the GE6550A was the tube of choice for those. I have to say, a pair of those feeding Magnepan speakers was a glorious thing. I still have a blue print schematic Mr. Johnson sent me for those amps.

That was back when you could still get them and most other vacuum tubes, new in the box, made in USA from any electronics supply house in town.

[Roe]

Yes they were designed around the 6550A by Ed Jahns. Are there any reports about which new kt88s and 6550s that work well in these amps?

I've only heard a 400PS once (it seems to be the only amp here in Norway). On guitar it was very bold and unforgiving indeed

[mhuss]

I have JJ KT88s in my PS300, but it doesn't get used much. I had one early failure, but no problems since.