200W Steel String Singer Build--What PT?

[hellhound]

Hi, I had a chance to try out the Fender 300PS (300wRMS)with a Strat through a 4x10 bass cab with 98db/w Celestions about a week ago. It sounded really nice when switched to the bass voicing. The guitar voicing wasn´t too pleasing IMHO. The amp had a cool SRV vibe to it and I have to say that it wasn´t as loud as I expected. You have to remember that there´s no distortion present with an amp like that and distortion is a thing that makes an amp feel "louder" than it actually is. But an amp that powerful isn´t nice to carry around. The head weights about the same as my Vox AC30 combo.

HH

[tictac]

I just used some SS diodes off the 260-0-260 winding with 30uF of filtering straight to the screens and the plates of the cathode follower tubes. I put a switch in series that serves as a standby as well.

TT

[David Root]

So you ran the screen taps with their own standby switch? Separate from the power tubes standby switch?

I don't think I am understanding you as that would allow you to put plate voltage to the power tubes without the screens energized?

[tictac]

Not exactly,

I'm only using the screen supply for standby, there is no switch on the plate supply. I didn't elaborate on the standby switch fully. I'm using a DPDT switch for standby. In the down position (standby mode) the screens are grounded (and B+ switched off) which effectively mutes the output of the amp. With the switch in the up position (play mode) B+ is connected to the screens and the ground is lifted.

The reason I chose to use this method is switching the lower voltage circuit puts little stress on the toggle switch which would be severely underrated switching the plate supply at 530VDC. Secondly it is effective as a muting switch which in my mind is the real reason for having a standby switch.

I know there are some who talk about cathode stripping as a reason for having a standby switch on the plate supply but there are a few papers floating around on the net by knowledgeable people explaining that cathode stripping is only a problem with tube circuit that operate over 1kV, which is why some makers (like London Power for example) don't even have standby switches in their amps.

I think thermal cycling is more of a concern than cathode stripping so I do like a standby to keep the tubes warm during a break or to mute the amp while I'm changing cabling etc...

Hope this clears up any misunderstanding...

TT

[David Root]

It does, thank you. I just assumed a plate supply standby switch because it's conventional. You must use pretty good quality power tubes!

[tictac]

Not sure what you mean by using "Good Quality Power tubes" but running the screens at 360V and not having a standby switch on the plate supply certainly doesn't put any extra stress on the power tubes.

I'm using Svetlana 6550's which are rated at 630V plate voltage and 400V screen voltage. My amps operating voltages are 530V/plate and 360V/screen so I'd say at these voltages the power tubes are being babied and should outlast the "conventional" circuits I see these tubes being used in.

Power beam tubes are by design robust and can take abuse which is why you often see them being used in circuits that go beyond the factory specs. Like Jim Kelly amps running 6V6's at 475V on the plates, talk about over-voltage!

Many amps using 6550's seem to ignore the 400V screen rating and the tubes seem to perform well enough, I just happened to have a PT with two secondary windings otherwise I'd have probably done the same...

TT

[David Root]

I do understand the options in actually operating power tubes, and that you can safely run them way above what the datasheets spec.

I guess what is puzzling me is switching HT onto cold plates with a cold cathode and the screens out of circuit too. Isn't that (once the cathode heats up) essentially operating the tube as a diode, ie there is a current flow? Does grounding the screens take care of that?

[tictac]

Actually No, the power tube will not act as a diode during a power up situations without a standby for two reasons. First until the cathode is heated up and an electron cloud has formed, no current flow from cathode to plate is possible. Secondly, the control grid bias voltage is present upon startup which would again prevent current flow.

The screen grid being grounded would prevent current flow too but isn't necessary. There are quite a few amps that don't even have standby switches. (like most Fender tweeds and some blonde & brown models).

The standby switch was a feature Fender added so musicians could mute their amplifier during a break and still keep the power tubes warmed up, not to prevent thermal runaway or prevent cathode stripping which is not a concern in amps running at such low voltages.


TT

[David Root]

OK, I forgot about the bias being on.

[Structo]

The thing about the SSS is, why would you ever need an amp that powerful?

Or is it just curiosity to see what it will sound like.

Or is it like a hot rod, you just gotta have that big block but you know you will never use all that horsepower?

[briane]

The thing about the SSS is, why would you ever need an amp that powerful?

This seems a common comment, and is my take on it as well....

But I have a friend with a 50w #124 I built....Every time I see it he has NFB off, master on 9, and pre-amp vol on 8, plus generally only runs in OD mode with ration on 7 and gain on 9 (wants more volume)....Way too loud, and personally I dont think these amps were intended to run that much wattage.

Punk band, heavy footed drummer, and big shows with no PA. all the makings of wanting more for him.

So he goes ape over my version of the SSS, He wants loud and clean. Mind you when I test an amp for him now, my walls literally shake, and stuff falls off all the shelves in the house.

go figure....I plan on having some hearing left when I am 80...

[talbany]

For me the the added 2 tubes 25+25 is not there so much for the volume( as we all know 100w= 3db louder) it's the added current applied to the primary of the OPT EVEN @ IDLE... This promotes a phenomenon known as flux density...(flux is the term regarding the magnetic field around a wire that is conducting current. Those invisible lines of magnetic force, if you will.)
Obviously this is the fundamental force that excites the secondary.
Second, Flux Density is, roughly, the combined strength of the flux -- that field of magnetic force radiating out. More power on the primary, greater flux density more induced on the secondary..
The increase in density on the secondary even at idle promotes wider bandwidth greater harmonic content and touch sensitivity more dynamic response at any volume level and faster out of the blocks..For the grip it and rip it type players don't pay much attention.. But the feel players especially slide guy's notice it.. The louder you push it the better it gets..Sound familiar!!

Tony

[Structo]

Good description Tony.

[BobW]

+1, Tony.

[Bob-I]

For me the the added 2 tubes 25+25 is not there so much for the volume( as we all know 100w= 3db louder) it's the added current applied to the primary of the OPT EVEN @ IDLE... This promotes a phenomenon known as flux density...(flux is the term regarding the magnetic field around a wire that is conducting current. Those invisible lines of magnetic force, if you will.)
Obviously this is the fundamental force that excites the secondary.
Second, Flux Density is, roughly, the combined strength of the flux -- that field of magnetic force radiating out. More power on the primary, greater flux density more induced on the secondary..
The increase in density on the secondary even at idle promotes wider bandwidth greater harmonic content and touch sensitivity more dynamic response at any volume level and faster out of the blocks..For the grip it and rip it type players don't pay much attention.. But the feel players especially slide guy's notice it.. The louder you push it the better it gets..Sound familiar!!

Tony

Wouldn't the lower impedance on the primaries have an affect on this as well?