Big bottle power amps

[Zozobra]

I might add a sag/valve rectifier emulator switch in as I cant see this amp sagging a great deal. I was thinking that 150R 50W resistor (maybe 120R) between the bridge rectifier and the reservoir cap should do it?

Assuming I have something like 500 mA max draw at full tilt then that would result in a drop 75V and the resistor would dissipate ~38W. It could be bypassed with a switch.

[mooreamps]

Ya, it could... seems to me it would be just as easy to use an actual rec tube instead.....

-g

[Steve Conner]

Well, we are talking a couple of hundred watts "big bottle" amp here, aren't we? So it would need a couple of big rectifier tubes in parallel, and then the Antek transformers don't have heater windings for tube rectifiers. So no, I think a resistor would be easier.

The rectifier tube was the first thing to go, from about 1960 on, as soon as the new-fangled silicon diodes could be trusted.

[Zozobra]

The attraction of doing it this way is the ease of switching. I could perhaps put a few resistors in series and then select how many are bypassed with a rotary switch, assuming they can take the abuse, to select the amount of sag.

[Steve Conner]

Careful now as Father Ted would say. High voltage DC is difficult for any switch. I once had quite a scare seeing a switch arc internally, blow smoke and melt into a lump before my eyes when trying to switch a couple of amps at 600V DC.

In this case, if you do it right you're just switching the voltage drop across the resistors, not the full HT voltage, so you should get away with it. But I'd still recommend the heftiest rotary switch you can find, and avoid twiddling it while the amp is blasting at high volume.

[R.G.]

You're right Steve. Once a self-supporting arc starts, it's going to eat things up. On top of that, running extra wires holding the full high voltage around the chassis is prone to other failures and repair hazards too.

It would make a lot of sense to make the "sag" switch(es) not be a front panel control so it can't be switched under load. Or better, to experiment with resistor values and pick out one you like and then permanently wire it in.

[Chuck H]

But I'd still recommend the heftiest rotary switch you can find,

= $$$

The cost of HV capable rotory switches would definitely be prohibitive for me.

Not speaking of high voltage now, but current. I always cringe when I see chassis shots of the Ceriatone builds and they're using a plain Jane Alpha $2.00 rotory switch for the OT secondaries !!! Whenever I have cause to post to someone building one of those kits I always suggest (at least) doubling up the switch poles (there is usually enough poles on the provided switch to do this).

Switch abuse has been happening for a long time though. How many under rated power and standby switches were installed in vintage amps ?!? And of course, they usually don't fail, but sometimes they do.

[Steve Conner]

Building a large tube power amp = $$$ to start with. If you want to save money, buy a solid-state one. I've seen some fairly hefty ceramic switches on Ebay, intended for use in ham antenna tuners and the like.

Any standby switch that switches the DC rail is underrated, because the ones that are usually used don't have a DC rating at all, or it's only 48V or 110V or whatever. I've seen proper DC rated switches, and they are monstrous things with spring-loaded contacts, blowout magnets and so on. If you fitted one of these in your amp, there wouldn't be room for any knobs!

I use a double-pole switch and put the two sets of contacts in series for some extra arc-breaking power.

Sometimes the arc breaking problem is avoided by using a MOSFET or IGBT as the switch, which RG will surely recommend. But these can just as easily fail to break a high current in an inductive DC circuit. A catch diode is required to fix that, but the fact of the matter is that the catch diode helps a mechanical switch to break the arc too.

[km6xz]

813's were used in the famous ART-13 transmitter used in the B-17. They were designed by Collins for the navy found most use in big bombers. The transmitter had a complex mechanical auto-tune system that would tune all the tank circuits and VFO selected from either the transmitter panel or a remote head. In that service, they ran 100watts out with 1625 modulators got MCW and AM. I had one as a pre-teen and used it for a few years.
When I saw the tread title I though the subject was Big Bottles, like the 750TH or 4-1000a. Those are big bottles, all glass envelopes. Of course any serious power applications were the task of the metal ceramic exterior anode tubes that are very compact for their ratings. I built some 3CX300ae1 amps for audio, the tubes are about the size of a door knob but have 300 watts of dissipation. Normally they are forced air cooled but one series I built for a high end hi-fi company used a pair with all the circuits including the driver tubes and 3CX300's
were submerged in a glass column filled with clear transformer oil for convection cooling. For the same company, I built a 833a triode Class A amp, where each of the 4 chassis needed for a 2 channels was 55kg. It was impressive when that big bottle fired up, the glow from each would light the room. They sold them for $45,000 a pair which must have been for their appearance because they did not sound that good. 833s are cheap, relatively, because they were common in old broadcast transmitters and modulators. A pair in Class B modulators could generate 1500 watts in CCS so maybe more in guitar application. Each heater was 100 watts and about as bright as a 60 watt bulb.

[Zozobra]

Perhaps then just one sag setting will do. I'd probably only ever use one setting anyway! I think for the sake of the switch it would best if it were only thrown when the amp is off.

[Zozobra]

Hi guys,

I've had a little time to revisit a KT88 poweramp/supply design thats been sitting on my hard disk since November now that I'm back home. I've now added the bias supply and heater elevation circuits to it which has thrown up a few uncertainties for me.

Here is the design:
http://dl.dropbox.com/u/8802910/poweramp.png

Firstly I'm a little bit unsure of the bias supply and the reservoir caps as I'm not using a transformer with a CT. Are R20 & R21 poorly chosen? Looking at Merlins power supply book it suggests that the bleeder resistance should be less than 10xR27 and I've over shot that! Does the fact that I'm using series/parallel caps change anything? Would 100k resistors be more suitable? Would this be detrimental to their function in balancing the caps? Do I need a pair for each pair of caps?

Next the heater elevation. Assuming I have something like 420V idle at A then I should get something like 70V elevation. does that look sensibly arranged?

Finally the preamp I'm thinking of using has an EF86 as the second stage which requires a lower voltage than the rest of the preamp and looks something like this:
http://dl.dropbox.com/u/8802910/pre_2.png

It looks like splitting the power supply into two branches to feed the input and EF86 stages is a sensible way to do this but I'm slightly concerned about the grounding scheme. I'm using a local star bus layout so should the ground also split too? I'd guess so otherwise it could form a loop?

Thoughts, opinions and your patients appreciated as ever!

[imaradiostar]

I wouldn't bother dropping the EF86 voltage that much. There's a lot to be said for getting it to draw some current, which is a good thing. I don't see resistor values so it's tough to estimate the operating point of the pentode. I like the preamp design- by varying different elements you should be able to achieve a lot of different tonal variations. You could even try some different pentodes if you felt the need. I've had good results with 6au6's and they're quite inexpensive for good quality NOS tubes.

A note on your phase inverter- it looks like this would be an ideal amp to use the 12dw7. Use the low gain half as the phase splitter and lower your resistor values- perhaps to 47k or even less. That way you can always decrease the output grid leak resistors if you decide you need to.

R20 and 21 look fine. I have several amps in service with those values and they work fine. The thing merlin is referring to is a climb of voltage if there is no load because the bias caps charge through the load. With no standby switch you'll have a load within 15-30 seconds, hopefully applied gradually enough that voltage shouldn't build up too badly. Perhaps someone with more experience could lend their opinion.

Also mentioned on another thread is the option of adding a bias winding to the antek toroid. This could give a lower impedance bias supply which you may find is easier to deal with.

Have you started building it yet? I have a similar but lower power amp on my bench right now, about 85% completed. It'll be a fixed bias 15 watter with JJ 6v6's and a 6U8 for the input triode and pentode rather than an EF86 and dual triode. It should be fun to compare notes when we're both done. My theory was to build a lower power amp for low volume gigs and get the pentode mojo happening out front so that I can control the volume a little better. I like your approach- the cathode follower should be a neat addition to the sound.

jamie

[Zozobra]

Thanks. I'll have a play with the dropping. Main thing I wanted to achieve was to nerf the gain and make it stable. I may well add local negative feedback to the next stage anyway as my AOR does something like that and I like how it takes dirt pedals. I want to make this a dirt pedal platform and for me at least a lot of how well that happens is in how the amp gets dirty and this arrangement should allow a fair bit of tweaking to get there. I'm sure it'll get pretty damn dirty on its own too though!

Good tip on the 12DW7. I may well audition a few different PI tubes out. Interesting that I could use the ECC83 to then drop the grid leak resistors.

Thanks. I'm not planning on using a standby switch. This is definitely a part of the circuit I'm wary of.

I've not gotten too far into the build. Mainly just chassis prep and cutting out boards. I have a big bag of screw mount reusable turrets so I can be quite flexible in trying things out. Next thing really is to finalise the poweramp/supply board so I can lay it out.


EDIT: also there is an error in the bias circuit as it happens. I'll correct the schematic.

[mooreamps]

= $$$

The cost of HV capable rotory switches would definitely be prohibitive for me.

Not speaking of high voltage now, but current. I always cringe when I see chassis shots of the Ceriatone builds and they're using a plain Jane Alpha $2.00 rotory switch for the OT secondaries !!! Whenever I have cause to post to someone building one of those kits I always suggest (at least) doubling up the switch poles (there is usually enough poles on the provided switch to do this).

Switch abuse has been happening for a long time though. How many under rated power and standby switches were installed in vintage amps ?!? And of course, they usually don't fail, but sometimes they do.

Not required... Stand-by is handled by the electronic power braking circuit...

-g

[Zozobra]

I've updated the schematic with a few tweaks:
Corrected the bias circuit
Decreased the bleeder/balancing to 220k such that the sum of R20/21 is less than 10xR27 as per Merlins suggestion.
Added the power switching/fusing to the diagram including an inrush reduction standby.

http://dl.dropbox.com/u/8802910/poweramp.png
This is the same location as the old diagram so any previous links will be updated too